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Photocatalytic Properties of Nanosized Bi2WO6 Catalysts Synthesized Via a Hydrothermal Process
Abstract
Nanosized Bi2WO6 was synthesized by a hydrothermal crystallization process. The as-prepared samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and porosity measurements, transmission electron microscopy, Raman spectra, and diffuse reflectance spectroscopy. The photoactivities of the as-prepared samples for the rhodamine-B photodegradation were investigated systematically. As a result, the sample prepared at 180 °C exhibited the highest photochemical activity under visible-light irradiation. The further experiments revealed that the catalyst was active in a wide spectral range. Density functional theory calculations suggested that the visible-light response was due to the transition from the valence band formed by the hybrid orbitals of Bi 6s and O 2p to the conduction band of W 5d. The photoactivity of the catalyst in relationship with the hydrothermal temperature, the crystal and band structure were also discussed in detail.
... As a result of the layered structure, Bi 2 WO 6 own several advantages in photocatalysis over other semiconductors, particularly its visible light absorption, high photocatalytic activity and stability [Zhang, 2014]. Regarding the band structure, the valence band of the semiconductor is composed of hybrid O 2p and Bi 6s orbitals; and the conduction band is composed of W 5d orbital with small contribution of Bi 6p, as reported in the literature determined by DFT method [Fu, 2006[Fu, , 2005. The special electronic structure (hybrid of O 2p and Bi 6s) helps the valence band to be widely dispersed and results narrow band gap of the semiconductor (2.8 eV) able to absorb visible light. ...
... Bi 2 WO 6 can be synthesized from various Bi and W containing precursors using different preparation methods; such as, solid state reaction [Tang, 2004], hydrothermal treatment [Fu, 2006], ultrasonic assisted hydrothermal [Kaur, 2016], microwave hydrothermal [Yao, 2009], solvothermal [Mi, 2012], sonochemical , sol-gel and so on. Among those methods, hydrothermal synthesis is widely used due to its simplicity of experimental process, effective to control the size and morphology, low cost, high yield, low temperature and large-scale method. ...
... Such high conversion is in agreement with that reported for the dye in the literature Hu, 2006;Takizawa, 1978]. It is well-known that RhB absorbs in the range of 450-600 nm ( Figure 3.7, Chapter 3), with the formation of ground state and excited state [Fu, 2008[Fu, , 2006[Fu, , 2005. Thus, electron transfer from the excited state of RhB to the conduction band of the semiconductor could occur (photosensitization); these electrons would react with water and/or oxygen molecules to generate radicals that would attack RhB cation [Wei, 2016]. ...
Au début des années 1970, Fujishima et Honda ont rapporté la première réaction induite par la lumière sur une électrode de TiO2 sur le dégagement d'oxygène et d'hydrogène dans une solution aqueuse. Depuis, l'application des réactions photocatalytiques s'est étendue dans différents domaines, en particulier dans l'assainissement de l'environnement en phase aqueuse et gazeuse. Les réactions photocatalytiques peuvent être utilisées pour la dégradation de polluants organiques (par exemple, COV, aromatiques, colorants) et inorganiques (par exemple, NO3-, Cr6+), pour la décomposition de l'eau en hydrogène et oxygène, la réduction du dioxyde de carbone, la désactivation des micro-organismes, ces exemples étant parmi les plus représentatifs.Les photocatalyseurs les plus courants sont des matériaux semi-conducteurs tels que TiO2, ZnO, Bi2WO6, WO3, Fe2O3, SnO2, CdS, ZnS, etc. Le TiO2 est un semi-conducteur de type n avec une bande interdite optique de 3,2 eV, aujourd'hui considéré comme le matériau de référence pour les applications photocatalytiques dans l'assainissement de l'environnement. D'autre part, Bi2WO6 est également un semi-conducteur de type n qui présente de bonnes caractéristiques d'absorption de la lumière visible, avec une bande interdite de 2,9 eV, une activité photocatalytique élevée et une bonne stabilité.Malgré le fait que les processus photocatalytiques présentent de nombreuses applications, leur mise en œuvre à grande échelle est limitée en raison de la faible absorption généralement de la plupart des semi-conducteurs sous lumière visible, des taux de recombinaison des porteurs de charge élevés et des phénomènes de photocorrosion qui affectent leur stabilité sous un éclairage à long terme.Afin d'augmenter la photoefficacité des semi-conducteurs, plusieurs approches ont été proposées; parmi elles, l'incorporation de matériaux carbonés comme additifs aux semi-conducteurs s'est révélée être une alternative intéressante. Une grande variété de matériaux carbonés a été utilisée à cet effet pour plusieurs réactions photocatalytiques, avec d'excellentes performances photocatalytiques des composites semi-conducteur/carbone. Cela a été attribué à plusieurs aspects : une meilleure absorption de la lumière visible du composite quand l’additif de carbone est incorporé, l'effet bénéfique du nanoconfinement des polluants ciblés dans la porosité des matériaux en carbone, les forts effets électroniques interfaciaux entre le carbone et le semi-conducteur, une séparation électron-trou efficace en raison de la faible longueur de diffusion à travers la porosité du carbone, ou la capacité de l'additif de carbone à agir comme accepteur des porteurs de charge photogénérés, réduisant ainsi le taux de recombinaison.Plus récemment, l'activité photochimique intrinsèque des matériaux carbonés a été démontrée, en raison de leur capacité à former des espèces réactives (par exemple, des radicaux hydroxyles, des superoxydes, des radicaux centrés sur le carbone) lorsqu'ils sont exposés à une illumination dans des environnements contrôlés. Lorsque les carbones sont illuminés, il est proposé que l'excitation électronique se produise dans les transitions π-π* et σ–π* impliquant des sites en zig-zag libres et des sites de type carbène. Une telle activité photocatalytique est influencée par la structure des pores (c'est-à-dire les effets de nanoconfinement), la chimie de surface et la composition du matériau carboné (c'est-à-dire la présence de groupes chromophores).Dans ce contexte, l'objectif principal de cette thèse était d'explorer l'application de matériaux carbonés d'origines et de caractéristiques différentes comme additifs aux semi-conducteurs pour la dégradation photocatalytique des polluants environnementaux. Une série de carbones issus de diverses sources (charbon bitumineux, polysaccharides, lignocellulosiques, nanotubes de carbone) ont été sélectionnés comme additifs en raison de leurs différentes caractéristiques. Le dioxyde de titane disponible dans le commerce et le tungstate de bismuth synthétisé en laboratoire ont été utilisés comme semi-conducteurs. Les photocatalyseurs semi-conducteur/carbone ont été préparés en incorporant une faible charge de l'additif de carbone (2% en poids) suivant deux approches: (i) mélange physique des deux composants, (ii) synthèse hydrothermale en un seul pot du semi-conducteur en présence de l'additif de carbone. Selon la nature des matériaux carbonés et la méthode de préparation, le mélange présentait des caractéristiques physico-chimiques, une réponse photoélectrochimique et une activité photocatalytique différentes.Tout d'abord, les spectres de réflectance diffuse des photocatalyseurs préparés ont été mesurés, pour étudier leurs caractéristiques optiques. Les données ont montré la présence de groupes fonctionnels optiquement actifs (c'est-à-dire impliquant des hétéroatomes O-, N-, S-) dans certains des matériaux carbonés, ce qui permet une augmentation de la capacité d'absorption de la lumière des composites dans le domaine visible. Les composites préparés par synthèse hydrothermale étaient de couleur claire et présentait une réflectance légèrement plus élevée que les composites obtenus par mélange physique des deux composants. Cette tendance était plus prononcée pour les composites impliquant des additifs de carbone hautement fonctionnalisés. La valeur énergétique de la bande interdite des mélanges déterminée par la méthode d'ajustement linéaire double est légèrement plus élevée quelle que soit la nature du carbone et la méthode de préparation que pour le semi-conducteur nu. Les valeurs de bande interdite peuvent être corrélées à la réflectance de l'échantillon: des valeurs de réflectance plus faibles représentent des bandes interdites plus élevées, et vice versa.Concernant la porosité des composites semi-conducteur/carbone, et quelles que soient les différentes caractéristiques poreuses des additifs de carbone utilisés, tous présentent des isothermes d'adsorption d'azote de type II(b), caractéristiques de matériaux à faible porosité. Ceci est associé à la faible teneur en additif carboné, comme mentionné ci-dessus. Néanmoins, la présence de l'additif de carbone a augmenté les paramètres de texture du composite semi-conducteur/carbone résultant; cet effet était plus remarquable dans le cas de ces matériaux de carbone avec une structure de pores bien développée. Concernant la dispersion des additifs de carbone dans les composites obtenus par mélange physique des deux composants, les particules des additifs se sont révélées être bien dispersées au sein des nanoparticules dominantes du semi-conducteur (typiquement de plus petites tailles), indiquant un bon contact entre le carbone et le semi-conducteur. Lorsque les composites ont été préparés par un procédé hydrothermal (par exemple, pour les composites Bi2WO6/carbone), les nanoparticules de semi-conducteur Bi2WO6 étaient plus grosses, en raison de la présence des additifs de carbone pendant la synthèse. Quoi qu'il en soit, une bonne dispersion de l'additif carboné a également été observée, suggérant un bon contact entre les deux phases. Les méthodes de préparation ont montré un effet sur la structure cristalline, avec des tailles de cristaux plus faibles des composites Bi2WO6/carbone synthétisés par méthode hydrothermale que le Bi2WO6 nu, soulignant une légère distorsion du réseau du semi-conducteur en raison de la présence de carbone lors de la synthèse. D'autre part, la nature acide/basique du carbone a également influencé la nature des composites semi-conducteur/carbone résultants. Tous les composites préparés par méthode hydrothermale présentent un caractère plus acide quelle que soit la nature du carbone, que les composés correspondants préparés par mélange physique.La réponse photoélectrochimique des matériaux semi-conducteurs/carbone a été évaluée en solution aqueuse. À cet effet, des électrodes ont été préparées sur un support conducteur (par exemple, un substrat en verre revêtu d'oxyde d'indium et d'étain) et analysées par voltamétrie cyclique, potentiel de circuit ouvert et chronoampérométrie. Les voltammogrammes des composites semi-conducteur/carbone mesurés dans le noir ont montré la forme typique du semi-conducteur, avec des densités de courant variables en fonction du type d'additif au carbone. Les densités de courant plus élevées dans toute la fenêtre électrochimique des voltammogrammes sont associées à la contribution de la double couche électrique formée dans la porosité des additifs carbonés. De plus, la méthode de préparation des composites a montré un effet sur les voltammogrammes. Les composites préparés par l’approche hydrothermale affichent des courants cathodiques plus élevés que leurs homologues obtenus par prélèvement physique mixte. Sous illumination, la photogénération de paires électron-trou dans la région de charge d'espace des composites a été confirmée par les photocourants anodiques enregistrés. L'amplitude des courants enregistrés dans le noir et du photocourant variait pour les différents matériaux. En général, les composites TiO2/carbone présentaient des valeurs de photocourant plus élevées que les composites Bi2WO6/carbone.Le potentiel de circuit ouvert des électrodes constituées de semiconducteurs uniquement a montré des résultats chargés positivement, indiquant le transfert d'électrons du semi-conducteur à la solution d'électrolyte lors de l'immersion dans l'électrolyte. Pour la plupart des électrodes à semi-conducteur/carbone, le potentiel de circuit ouvert s'est déplacé vers des valeurs négatives par rapport au semi-conducteur, indiquant le transfert d'électrons du carbone vers le semi-conducteur. Ceci était en accord avec le résultat observé dans le voltammogramme cyclique. Lorsque les électrodes ont été éclairées, le potentiel a chuté à des valeurs plus négatives en raison de l'augmentation de la population d'électrons dans la bande de conduction du semi-conducteur due à l'injection des électrons photogénérés. La goutte photopotentielle était reproductible sur plusieurs cycles marche/arrêt consécutifs. L'ampleur et la vitesse de la goutte photopotentielle dépendaient de la nature du carbone et de la méthode de préparation des composites. Toutes les électrodes TiO2/carbone présentaient une amplitude de chute photopotentielle plus élevée que le TiO2 nu, tandis que toutes les électrodes de Bi2WO6/carbone présentaient une amplitude de chute photopotentielle plus faible que le Bi2WO6 nu. En termes de séparation de charge, les semi-conducteurs seuls ont montré des valeurs de constante de vitesse plus rapides que les composites, quels que soient le type de semi-conducteur, la nature des additifs de carbone et le procédé de préparation des composites.Les transitoires de photocourant des électrodes à semi-conducteur/carbone enregistrés lors de l'éclairage marche/arrêt présentaient la forme typique des semi-conducteurs de type n, avec une augmentation du photocourant lors de l'activation de l'éclairage qui se rétractait à sa valeur d'origine lorsque la lumière était éteinte. Les amplitudes de photocourant des électrodes semi-conductrices/carbone impliquant des carbones faiblement fonctionnalisés présentaient des valeurs plus élevées que celles mesurées pour le semi-conducteur seul; en revanche, le carbone hautement fonctionnalisé a montré des valeurs inférieures ou égales à celles du semi-conducteur nu. La conductivité électrique et la mobilité électronique de la matrice de carbone se sont avérées être une force motrice pour augmenter l’amplitude du photocourant. Ceci était plus remarquable dans les composites TiO2/carbone que dans les composites Bi2WO6/carbone. Toutes les électrodes TiO2/carbone ont montré un photocourant anodique quel que soit le potentiel appliqué. D'autre part, la plupart des électrodes Bi2WO6/carbone présente un photocourant anodique pour des potentiels appliqués positifs et des photocourants cathodiques pour des potentiels appliqués négatifs. Cela suggère que les différents types de réactions photoélectrocatalytiques se sont produits à l'interface électrode/électrolyte en conditions d'éclairage, ce qui s'est avéré dépendant du potentiel appliqué pour le même matériau, en particulier pour Bi2WO6/carbone. Le photocourant anodique correspond à l'oxydation de l'eau, puisque l'eau est le seul donneur d'électrons au trou de l'électrolyte aqueux, tandis que le photocourant cathodique est attribué à la photoréduction de l'oxygène dissous dans l'électrolyte.Enfin, l'activité photocatalytique des matériaux a été évaluée pour la dégradation de la rhodamine B choisi comme polluant modèle. Le colorant a été imprégné sur les films photocatalytiques coulés sur un substrat en verre à partir de solutions aqueuses, et laissé sécher dans l'obscurité. Avant l'illumination des films photocatalytiques, leur capacité d'adsorption a été évaluée. Les données ont montré qu'une faible quantité (environ <5 μg) du colorant était adsorbée dans tous les composites étudiés. Cette faible capacité d'adsorption des catalyseurs est en accord avec la structure poreuse mesurée. Ensuite, le test photocatalytique a été réalisé en éclairant les films à l'aide d'une lumière solaire simulée. La décomposition photolytique du colorant en l'absence d'un photocatalyseur a montré une conversion non négligeable (environ 80%) après 60 minutes d''illumination. En présence d'un photocatalyseur, l'élimination complète du colorant a été observée après 10 minutes d'illumination; une légère différence de performance a été observée parmi les photocatalyseurs à des temps d'illumination courts. Tous les composites TiO2/carbone ont montré des performances légèrement meilleures que le semi-conducteur nu. D'autre part, les composites Bi2WO6/carbone préparés à partir de mélanges physiques ont montré des performances légèrement inférieures à celles du Bi2WO6. En revanche, les composites Bi2WO6/carbone synthétisés par un procédé hydrothermal ont montré une activité photocatalytique plus élevée que leur composite correspondant préparé par mélange physique. Des expériences réalisées en présence d'agents chimiques désactivateurs ont confirmé la formation de radicaux hydroxyles et de trous lors de l'irradiation des composites, vraisemblablement impliqués dans la dégradation photocatalytique du colorant. En raison de la nature acide de la plupart des catalyseurs, une forte interaction avec la forme cationique de la rhodamine B est attendue; en conséquence, la dégradation photocatalytique s'est réalisée par la voie de N-déséthylation, ce qui conduisait à un grand nombre d'intermédiaires. La différence de performance photocatalytique des composites était plus prononcée pour la formation et la dégradation de ces intermédiaires. Dans la plupart des cas, le semi-conducteur/carbone a montré une dégradation photocatalytique améliorée des intermédiaires par rapport au semi-conducteur nu. Cette amélioration était plus prononcée pour les catalyseurs comprenant des additifs de carbone acides ou/et microporeux.Les matériaux composites ont également montré une bonne performance photocatalytique pendant les cycles consécutifs (jusqu'à cinq cycles) avec une conversion rapide et presque complète du colorant dans tous les cycles. Ceci a confirmé l'absence de perte d'activité significative des catalyseurs après des cycles consécutifs, soulignant que les phénomènes de photocorrosion sont négligeables. La stabilité des photocatalyseurs a également été confirmée par l'absence de changements dans les photocatalyseurs après des cycles consécutifs et une irradiation à long terme (en termes de texture et de nature hydrophobe/hydrophile).L'activité photocatalytique des matériaux a également été évaluée pour la photoréduction du dioxyde de carbone (CO2) en milieu gazeux. Le CO2 gazeux a été introduit dans le photoréacteur contenant le film catalytique, suivi d'un éclairage à l'aide d'un simulateur solaire. La photoréduction de CO2 a été analysée par GC/MS et capteur numérique, montrant la production de gaz CO même si la quantité de CO est restée trop faible par rapport à la concentration initiale de CO2.
... The kinetic limitations of the photocatalysts, such as the surface area, size and the charge diffusion lengths are considered easier to manipulate, by e.g. synthesis method, additives, or calcination temperature [9,10]. Some studies have succeeded to enhance photocatalytic degradation of contaminants by considering kinetic aspects [11][12][13][14][15][16]. ...
... The lattice parameters and the crystallite size of the samples (based on the half-width of their (131) peak via the Scherrer formula) are calculated by the Rietveld method using the FullProf program and the structure refinements are presented in Table 2. is attributed to an anisotropic growth along the {0 1 0} planes [17,18,41]. 10 [15,42]. For Bi(NO3)3·5H2O, the vibration mode at 1200 -1700 cm -1 is a characteristic for the NO3 -1 group and the broad peak at 700 -400 cm -1 may be ascribed to the metal-oxygen (Bi-O-Bi) vibration [42]. ...
... . 10 ...
Bi2WO6 is considered an effective photocatalyst even under the visible part of the solar spectrum and recent advances in the modification of its structure leave promise for their enhanced effectiveness. Our experimental E. coli disinfection results have shown the flower-like morphology has brought considerable enhancement to the overall performance over the nanoparticle form. To clarify the photocatalytic mechanism of Bi2WO6, a combination of experimental and computational methods has been employed to investigate the surface properties of Bi2WO6 nanosheets self-assembly flower-like architecture. Although experimental evidence has determined the band edge positions of the as-synthesized samples to address the photocatalytic properties, the mechanistic basis of this concept remains unclear. The calculation results demonstrated here deepen our understanding and indicate the potential of surface configuration to considerably alter the electronic structure and related photocatalytic properties of Bi2WO6 nanosheets. Firstly, we consider a range of surface slab models of Bi2WO6 (010) facet and calculate their surface Gibbs free energies. Having determined that the Bi-termination is energetically more favorable by ab initio atomistic thermodynamics, hydrogen passivated of this termination was proved to be most stable. Through the analysis of electronic band structure within the DFT-1/2 scheme and work function of the most stable termination, excellent agreement of experimental and theoretical predictions provides a meaningful understanding of the kinetic dependence of photocatalytic bacterial inactivation.
... Another example of doped Bi 2 WO 6 is the creation of Bi 2 W x Mo 1-x O 6 solid solutions [29,35]. It is believed that the layered structure of the obtained solid solutions [36] as well as the presence of the [Mo/WO 6 ] octahedral layers may promote the migration of photoinduced electrons and holes after doping with Mo [37]. ...
... (3) As previously shown, the layered structure of perovskites [36] as well as the presence of the [Mo/WO 6 ] octahedral layers promotes the generation and separation of photoinduced charge carriers [37]. These layers are formed due to doping with molybdenum. ...
In this work, Bi2WxMo1–xO6 solid solutions and Bi2WO6/g-C3N4 composites were synthesized by a simple hydrothermal method, characterized by a complex of physicochemical methods and used in the decomposition of organic pollutants under visible light to comprehensively compare different modulating techniques. The highest efficiency was detected for Bi2W0.5Mo0.5O6 in the degradation of methylene blue (conversion of 52.9%, TON = 0.022), indicating the attractiveness of the doping strategy. Excellent results were achieved with small amounts of H2O2 in the photodegradation of methylene blue (conversion of 97.9%, TON = 0.040) and phenol (conversion of 81.2%, TON = 0.056).
... solid solutions is primarily attributed to their ability to absorb more visible light than Bi2WO6 and Bi2MoO6 due to a narrower band gap (Table 2). Moreover, as has been shown previously, the layered structure of perovskites [36] as well as the presence of the corner-shared [Mo/WO6] octahedral layers may promote the generation and separation of photogenerated charges [37]. In addition, the use of the solid solutions with x = 0.5-0.75, which partially retain a hierarchical 3D flower-like structure, allows the complete conversion of DBT to be achieved after 120 min. ...
... The enhanced photocatalytic activity of the Bi 2 W 0.25 Mo 0.75 O 6 , Bi 2 W 0.5 Mo 0.5 O 6 , and Bi 2 W 0.75 Mo 0.25 O 6 solid solutions is primarily attributed to their ability to absorb more visible light than Bi 2 WO 6 and Bi 2 MoO 6 due to a narrower band gap (Table 2). Moreover, as has been shown previously, the layered structure of perovskites [36] as well as the presence of the corner-shared [Mo/WO 6 ] octahedral layers may promote the generation and separation of photogenerated charges [37]. In addition, the use of the solid solutions with x = 0.5-0.75, which partially retain a hierarchical 3D flower-like structure, allows the complete conversion of DBT to be achieved after 120 min. ...
Photocatalytic oxidative desulfurization has attracted much attention in recent years due to the continuous tightening of the sulfur content requirements in motor fuels and the disadvantages of the industrial hydrodesulfurization process. This work is devoted to the investigation of the photocatalytic activity of Bi2WxMo1−xO6 solid solutions (x = 1, 0.75, 0.5, 0.25, 0) in the oxidative desulfurization of hydrocarbons under visible light irradiation using hydrogen peroxide as an oxidant. The synthesized photocatalysts were characterized in detail using XRD, SEM, EDS, low-temperature nitrogen adsorption–desorption, and DRS. It was shown that the use of solid solutions Bi2WxMo1−xO6 with x = 0.5–0.75 leads to the complete oxidation of organosulfur compounds to CO2 and H2O within 120 min. The high photocatalytic activity of solid solutions (x = 0.5–0.75) is attributed to their ability to absorb more visible light, the presence of the corner-shared [Mo/WO6] octahedral layers, which may promote the generation and separation of photogenerated charges, and the hierarchical 3D flower-like structure. The reaction mechanism of the desulfurization was also analyzed in this work.
... Bi 2 WO 6 has attracted considerable attention as an Aurivillius oxide semiconductor with a 2.66 eV narrow bandgap. Bi 2 WO 6 forms with different morphology can be synthesized by various approaches, like a cetyltrimethylammonium bromide-assisted bottom-up route, hydrothermal processes and solidstate reactions [265][266][267]. It was used in several applications, including the decomposition of pollutants [266,267]. ...
... The selective photocatalytic 4-NP reduction on blank nanocomposites BWO, rGO and BWO/rGO after 30 min of irradiation is shown in Figure 10. forms with different morphology can be synthesized by various approaches, like a cetyltrimethylammonium bromide-assisted bottom-up route, hydrothermal processes and solid-state reactions [265][266][267]. It was used in several applications, including the decomposition of pollutants [266,267]. ...
Photocatalysis is a classical solution to energy conversion and environmental pollution control problems. In photocatalysis, the development and exploration of new visible light catalysts and their synthesis and modification strategies are crucial. It is also essential to understand the mechanism of these reactions in the various reaction media. Recently, bismuth and graphene’s unique geometrical and electronic properties have attracted considerable attention in photocatalysis. This review summarizes bismuth-graphene nanohybrids’ synthetic processes with various design considerations, fundamental mechanisms of action, heterogeneous photocatalysis, benefits, and challenges. Some key applications in energy conversion and environmental pollution control are discussed, such as CO2 reduction, water splitting, pollutant degradation, disinfection, and organic transformations. The detailed perspective of bismuth-graphene nanohybrids’ applications in various research fields presented herein should be of equal interest to academic and industrial scientists.
... The conduction band (CB) is mainly comprised of W 5d orbitals while the valence band (VB) is formed by Bi 6 s and O 2p orbitals. This structure results in increased photogenerated electron mobility in the VB (Fu et al., 2006;Liu & Yu, 2008Zhu et al., 2020). Under visible light irradiation, Bi 2 WO 6 's photogenerated electrons struggle to reduce O 2 to •O − 2 , which limits the production of reactive substances and results in incomplete organic pollutants mineralization. ...
In the work, bismuth tungstate (Bi2WO6, orthorhombic system) photocatalyst nanomaterial was synthesized by hydrothermal method. The photocatalytic ozonation oxidation synergistic degradation on organic pollutants in coal chemical phenol-ammonia wastewater by Bi2WO6 was studied. The effects of ozone (O3) concentration, catalyst dosage, pH and O3 flow rate on the degradation efficiency of wastewater were investigated, respectively. The study found that the degradation processes of these four single factors were fitted kinetically and aligned with the pseudo second order kinetics model, and the maximum chemical oxygen demand (COD) removal rate of the coal chemical phenol-ammonia wastewater was 56.34%, 79.03%, 78.63%, and 79.66%, respectively. The COD removal rate reached 80.37% under the optimum reaction conditions. Additionally, the degradation process was optimized, which conformed to the pseudo second order kinetics model. Density functional theory (DFT) calculations showed that the adsorption energies of O3 at the Bi, W, and O atomic sites were -0.477 eV, -2.604 eV, and -0.421 eV, respectively, on the exposed crystalline surface of (131), indicating that O3 had a stronger interaction force with W, which was easy to be activated by the surface-transferred electrons to form reactive oxygen species and mineralize the organic pollutants. The catalytic mechanism indicates that the photocatalytic ozonation oxidation is primarily accomplished by producing •OH, ¹O2, and ∙O2-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bullet O_2^-$$\end{document}, which results in the degradation on organic pollutants in coal chemical phenol-ammonia wastewater. The analysis of water quality and GC–MS indicated that most pollutants present in coal chemical phenol-ammonia wastewater had degraded upon treatment. Furthermore, the BOD/COD ratio of coal chemical phenol-ammonia wastewater was increased from 0.25 to 0.32. Moreover, the COD removal rate only decreased to 70.25% after five cycles of the experiment, which demonstrated that the Bi2WO6 catalysts had a high stability and reusability, implying that it has great potential for application in coal chemical phenol-ammonia wastewater treatment.
... The valence band of Bi 2 WO 6 is formed through hybridization between the Bi 6 s and O 2p orbitals, whereas the W 5d orbital contributes to the conduction band. According to a previous study [52], the material exhibits a high absorption peak at wavelengths ranging from 440 to 500 nm. The utilization of transmission electron microscopy (TEM) has proven to be a valuable methodology in the assessment of the particle size and shape distribution of Bi 2 WO 6 . ...
Bismuth tungstate (Bi2WO6) has received extensive research in a numerous area, including degradation, CO2 reduction, organic transformations, etc. Due to their wide range of applications, the discovery and development of effective, environmentally safe, gentle, and affordable techniques for the synthesis of bismuth tungstate are critical in organic transformations. There have been reports on variety of multicomponent reactions employing the heterogeneous catalysts Bi2O3, BiVO4, and as well Bi2WO6 nanoparticle. Among other materials, Bi2WO6 nanoparticles are perceived for their high reactivity at ambient temperature in an aquatic medium. The main objective of this study is to emphasize the mechanistic considerations, scope, benefits, and limits of recent catalytic improvements in the process of oxidation and other reactions. Consequently, the use of Bi2WO6 catalyst offers many advantages, including high yields, an ecologically friendly process, quick reaction times, and a straightforward work-up technique. It has been created to use a Bi2WO6 catalyst in an aqueous medium in a versatile, simple, one-pot, multi-component technique. This process offers easy-to-find, inexpensive reagents, quick reaction times, great yields, and high atom economy. In this review, we have elaborated how Bi2WO6 nanomaterials can be employed as effective and reusable catalysts for organic transformation.
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... The valence band of Bi 2 WO 6 consists of a hybridization of O 2p and Bi 6p orbitals with a minor contribution from Bi 6s orbitals, while the conduction band is primarily composed of 5d tungsten orbitals with a slight contribution from Bi 6p orbitals, as revealed by theoretical studies on its electronic band structure (Fig. 8). 6,95 This results in a widely dispersed valence band that enhances the mobility of photon-excited holes and facilitates oxidation reactions. Bi 2 WO 6 has demonstrated great potential in selective organic synthesis and bacterial inactivation in aqueous solutions under visible light, such as benzyl alcohol oxidation to aldehydes and glycerol oxidation to dihydroxyacetone, according to previous. ...
This review paper provides a comprehensive overview of the recent trends in bismuth tungstate (Bi2WO6) research, covering its structural, electrical, photoluminescent, and photocatalytic properties. The structural characteristics of bismuth tungstate are explored in detail, including its different allotropic crystal structures with respect to its isotypic materials. The electrical properties of bismuth tungstate, such as its conductivity and electron mobility, are also discussed, along with its photoluminescent properties. The photocatalytic activity of bismuth tungstate is a particular focus, with recent advances in doping and co-doping strategies with metals, rare earth and other elements summarized. The limitations and challenges of using bismuth tungstate as a photocatalyst are also examined, such as its low quantum efficiency and susceptibility to photodegradation. Finally, recommendations for future research directions are provided, including the need for further studies on the underlying mechanisms of photocatalytic activity, the development of more efficient and stable bismuth tungstate-based photocatalysts, and the exploration of new applications in fields such as water treatment and energy conversion.
... Photocatalysis can solve the growing problem of environmental pollution by converting solar energy into active radicals that can degrade organic pollutants into CO 2 , H 2 O and other non-toxic products [1][2][3][4][5][6][7]. In past decades, Bi-based semiconductors, such as Bi 2 WO 6 [8], BiVO 4 [9], Bi 2 O 2 CO 3 [10], BiOX (X = Cl, Br, I), [11] and Bi 2 SiO 5 [12][13][14] have attracted a considerable attention in photocatalysis due to their unique energy band structure and chemical stability. Bi 2 SiO 5 is a newly discovered compound in the Aurivillus family, whose layered structure is formed by the alternating growth of (Bi 2 O 2 ) 2? layers and (SiO 3 ) 2pyroxene layers along the c-axis. ...
AgBr/Bi2SiO5 photocatalyst with Z-scheme structure was obtained via a hydrothermal reaction, calcination, and in situ precipitation process. The as-prepared AgBr/Bi2SiO5 composite photocatalytically degraded 96.2% of RhB under visible light irradiation within 30 min. Moreover, the photodegradation efficiency remained above 81.7% after being reused five times. Trapping experiments revealed that ·O²⁻ was the main reactive species during RhB photodegradation. The degradation mechanism study of AgBr/Bi2SiO5 showed that the Z-scheme structure between AgBr and Bi2SiO5 facilitated the separation of charge carriers, thereby improving the photocatalytic degradation with the assistance of a dye sensitization effect. This work provides a reference for developing other Z-scheme photocatalysts for the removal of organic pollutants.
... In this process, individual ethyl groups are successively removed from tetraethylated RhB to form tri-, di-, mono-, and non-ethylated molecules, and the corresponding absorption peak positions are 539, 522, 510 and 498 nm, respectively. Therefore, the absorption peak appears to undergo a blueshift during the degradation process [39], and the solution will gradually change from red to yellow and finally colourless. Figure 10 exhibits the evolution of the optical absorption spectrum in RhB degradation by the 120°C BWO photocatalyst to further clarify the change in RhB during degradation. ...
The morphology-controlled synthesis of nanostructured photocatalysts by an environmentally friendly and low-cost method provides a feasible way to realize practical applications of photocatalysts. Herein, Bi2WO6 (BWO) nanophotocatalysts with mulberry shape, sheet-like, and round-cake morphologies have been successfully synthesized through a highly facile solvothermal process by simply adjusting the solvothermal temperature or utilizing selective addition of ethylene glycol as an orientation agent without using strong acids and bases and/or hazardous chemicals. The ratio of ethylene glycol and glacial acetic acid can affect the morphology and oxygen vacancy content of BWO, thereby influencing the photocatalytic performance. The photocatalytic activity of the as-prepared samples was evaluated by degradation of rhodamine B (RhB) and tetracycline under visible-light irradiation. The results indicated that all the BWO samples exhibited morphology-associated photocatalytic activity, and the sheet-like structure of BWO obtained via solvothermal treatment at 120 °C with ethylene glycol and glacial acetic acid ratio of 1:3 achieved the maximum specific surface area and possessed abundant oxygen vacancies, exhibiting outstanding photocatalytic activity for degradation of RhB and tetracycline. The degradation rate of RhB reached 100% within 20 min. To the best of our knowledge, this value is one of the most remarkable values for pristine BWO photocatalysts. Radical capture experiments demonstrated that hydroxyl radicals (·OH) play major roles compared with electrons (e⁻) and holes (h⁺) in the photocatalytic degradation process. A possible mechanism for the photocatalytic degradation of pollutants was proposed to better understand the reaction process. We believe that the more economical, efficient and greener methodology can provide guidance to develop highly efficient photocatalysts with favourable morphology and structure.
... The major visible region absorption of the films is responsible for enhanced photoactivity. The steep shape of absorbance curve indicates absorption is due to band gap transition and not because of impurity transitions [32]. The optical absorption spectra was analyzed by using following relation [33]. ...
In this work, visible region active Bi2WO6 thin films with variable thickness are successfully deposited by simple and effective spray pyrolysis technique. The structural, optical, morphological and photoelectrochemical properties of synthesized thin films were studied. Rietveld refinement of the structural data tested purity and degree of crystallinity of Bi2WO6 thin films synthesized with variable solution quantity. Optical study revealed visible light absorption of films and small variation in band gap energy. A thin film deposited with 70ml spraying solution quantity exhibit higher photocurrent density (460 mA/cm2) and same film with large area (10 × 10 cm²) was used for photocatalytic and photoelectrocatalytic degradation of rhodamine B (RhB) dye under solar radiation. The photoelectrocatalytic removal of RhB exhibit higher degradation efficiency (94%) as compared to photocatalytic process (23%). This study can trigger potential application of Bi2WO6 photoelectrode in solar energy conversion, wastewater treatment and energy production.
... Among those, the bismuth-based semiconductor photocatalysts exhibit potential applications in degradation of water pollutants due to their low band gap, lower valence band (VB) position, and high chemical stability. Among the bismuth photocatalysts, bismuth tungstate (Bi 2 WO 6 ) is an aurivillius oxide family, and potential candidate has attracted the researcher due to its unique physical and chemical properties and exhibit exceptional degradation behavior in the presence of visible light irradiation (Fu et al. 2006;Zhang et al. 2011;Zhang and Zhu 2012;Zhao et al. 2014). Nevertheless, the photocatalytic degradation ability of bare Bi 2 WO 6 does not fulfill the practical requirements because of their low adsorption capacity and poor separation rate of photo-generated electron-hole pairs. ...
A visible light-responsive redox-mediator-free calcium indium sulfide (CaIn2S4) and bismuth tungstate (Bi2WO6)–based direct dual semiconductor nanocomposites were prepared by combination of hydrothermal and wetness impregnation methods. The results substantiated the formation of CaIn2S4 marigold flowers, the flower-like structure of the Bi2WO6, and CaIn2S4/Bi2WO6 nanocomposites. UV–Vis-DRS analysis confirmed that visible light response of Bi2WO6 was considerably enhanced after combining with CaIn2S4. The inter-cross-sectional charge carrier transfer during photocatalysis using synthesized catalysts was studied by determining the band position of Bi2WO6 and CaIn2S4, and photoluminescence analysis. The adsorption of rhodamine B (RhB) dye study revealed that after coupling with CaIn2S4, the adsorption capacity of Bi2WO6 was significantly improved. For 15%-CaIn2S4/Bi2WO6, 38% of RhB dye was adsorbed, whereas it was 27% for bare Bi2WO6. The 15%-CaIn2S4/Bi2WO6 nanocomposite showed higher percentage degradation for RhB (82%) dye as compared to the other percentage loaded composites, bare Bi2WO6 (59%) and CaIn2S4 (72%). Photoluminescence results revealed that photo-generated electron–hole pair recombination rate was drastically suppressed after coupling with CaIn2S4 which may be presumed to inter-cross-sectional transfer of photo-generated charge carriers. The synergic effect of enhanced adsorption capacity and efficient separation of electron–hole pairs were accountable for the enhanced photodegradation efficiency under visible light.
... Bi 2 WO 6 is one of the most commonly used visible-light catalyst with band gap of 2.75 eV and good photostability. Bi 2 WO 6 has the simplest structure of Aurivillius family, the layer structures and unique properties offer great ability for the decomposition of organic compounds (Dong et al., 2017;Fu et al., 2006). In this work, three morphologies vis-à-vis nanoplate, flower-like and swirl-like structures of Bi 2 WO 6 were prepared by hydrothermal methods and applied for the degradation of NAs. ...
Bitumen extraction from oil sands produced large quantities of oil sands process water (OSPW), which contains highly recalcitrant naphthenic acids (NAs). In this study, three different morphologies of bismuth tungstate (Bi2WO6) photocatalysts were prepared by hydrothermal method. The prepared catalyst was characterized to obtain its structural, textural and chemical properties and tested for the degradation of model NAs and real OSPW under simulated solar irradiation. Nanoplate, flower-like and swirl-like Bi2WO6 were prepared with the flower-like structure exhibited highest specific surface area and total pore volume. Highest photocatalytic activity for the degradation of NAs was also demonstrated by the flower-like Bi2WO6, achieving complete degradation of cyclohexanoic acid (CHA) at fluence-based rate constant of 0.0929 cm²/J. O2•− and holesare identified as the major reactive species generated during the photocatalytic process. The effect of metallic ions on the degradation rates of S-containing and N-containing NAs differs and heteroatom was the main reactive site. The by-products of heteroatomic NAs were identified and degradation pathways were reported for the first time. The concentration changes of each byproduct were further estimated by mass balance. This research will provide valuable information for the treatment of NAs by engineered passive solar-based approaches.
Photocatalysis leverages light energy for chemical transformations, presenting an environmentally friendly approach to mitigating pollution. We have developed Bi 2 WO 6 /Ag/ZnO composite nanomaterials that demonstrate enhanced photocatalytic efficiency. These nanomaterials have been characterized through advanced techniques such as x‐ray diffraction and x‐ray photoelectron spectroscopy. A key challenge in photocatalysis is the effective separation of electron–hole pairs, and our Bi 2 WO 6 /Ag/ZnO composites achieve an impressive degradation rate of 96.93% for methyl orange. This result notably exceeds previous performance standards and highlights the crucial role of Ag in broadening the light response spectrum of zinc oxide (ZnO), and the addition of Bi 2 WO 6 in amplifying the electron–hole pair separation efficiency within these composite nanomaterials. These findings underscore the significant potential of Bi 2 WO 6 /Ag/ZnO composites in environmental detoxification and establish a new benchmark in photocatalytic material performance. The advancement of such nanomaterials could be transformative for environmental remediation strategies, offering efficient, sustainable, and cost‐effective solutions to ecological challenges. This study underscores the pivotal role of materials science in advancing environmental sustainability.
The present time nanocomposites show highly improved photocatalytic performance in water pollution remediation technology. Herein, BMZ (Bi2MoZnO7) nanocomposite photocatalyst has been synthesized via the co-precipitation method. The structural and morphological studies were interpreted by analytical techniques including X-Ray Diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM), and Energy Dispersive X-ray (EDX) analyses. The accomplishment of photocatalysis was determined by the mineralization of toxic azure B dye using a light-emitting diode (LED) as a source of radiation. The characterization studies revealed that the average particle size of BMZ nanocomposite was 28 nm with a flake-like structure having a smooth surface. An EDX spectrum shows the characteristic peaks of Bi, Mo, Zn, and O elements. The parameters such as pH, dye concentration, catalyst weight, reusability of photocatalyst, Fenton’s reagent, CO2, COD, NO3− and SO42− evaluation, and N2 and O2 purging were studied for optimum conditions. Fenton’s reagent test is one of the green approaches in which radical dotOH is generated that is utilized for the degradation of toxic azure B dye. The photocatalytic degradation in Fe2+/H2O2/BMZ/Azure B system, high value of the rate constant was recorded as compared to the experiments, performed without BMZ nanocomposite photocatalyst. The eco-friendly LEDs irradiation was applied as a light source material throughout the experiment. The 98 % degradation of toxic azure B dye was observed in 120 min with the highest rate constant (k = 6.1 × 10−4 s−1) in LEDs irradiation.
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Photocatalysis is proven as a desirable technology for elimination of the tetracyclines pollutant from wastewater. Herein, a novel core-shell Z-scheme heterojunction In2O3@BiFeO3 was, for the first time, fabricated via facile hydrothermal method to effectively eliminate tetracycline in wastewater. The prepared photocatalysts were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), UV–vis spectroscopy and so on. The photocatalytic performance of the prepared samples was evaluated by photo-degradation of tetracycline (TC) under visible light. TC degradation results demonstrated that the as-prepared In2O3@BiFeO3 obeyed the pseudo-first-order kinetics and exhibited a higher photocatalytic rate of 0.01173min⁻¹ that was approximately 2.97 and 14.4 folds those of the raw BiFeO3 and In2O3, respectively. Additionally, In2O3@BiFeO3 possessed high stability during five consecutive cycles. The core-shell structure and the photosensitization of BiFeO3 significantly improved light-absorption in the entire visible region. The enhanced photocatalytic activity is attributed to the improved light harvesting and the effective separation of photogenerated electron-hole pairs due to the formation of core-shell Z-Scheme heterojunction. Due to the well-matched band position of In2O3 and BiFeO3, •OH, •O2⁻ and h⁺ all act as primary reactive species in TC degradation. This research provides a new strategy to construct the promising visible-light-driven photocatalyst for environmental remediation.
This article reports preparation of Bi2WO6 using solid-state and solution combustion methods. Different physicochemical characterization techniques such as XRD, SEM, TEM, BET-N2 adsorption study, UV–vis, XPS and photoluminescence spectroscopy were used to examine crystalline phase, optical and morphological properties of synthesized Bi2WO6 samples. The evaluation of prepared photocatalyst for methyl orange degradation, Cr(VI) reduction under visible light and rhodamine B degradation under direct sunlight was carried out. The sample prepared by a solution combustion route, exhibited superior photocatalytic activities than that prepared by a traditional solid-state route.
The step-scheme (S-scheme) heterojunction of Bi2Sn2O7 quantum dots (QDs)/TiO2 nanotube arrays (NTAs) photoanodes were prepared for efficient and stable photoelectrocatalytic (PEC) degradation of sulfamethazine (SMT) at low potential and light-emitting diode (LED) illumination. In the Bi2Sn2O7/TiO2 NTAs system, the synergistic factor of photocatalysis (PC) and electrocatalysis (EC) reached 10.1. The elevated performance was attributed to improved light-harvest capability by the decorated TiO2 NTAs with Bi2Sn2O7 QDs, lower charge transfer resistance, more efficient photoexcited charge separation, and higher redox ability to produce photogenerated holes (h⁺) and reactive oxygen species (ROS) with great efficiency. The possible PEC mechanism of S-scheme Bi2Sn2O7/TiO2 NTAs was elucidated by experimental studies and density functional theory (DFT) calculations, which confirmed the establishment of the internal electric field (IEF) promoted the carrier separation and transport. This research provides a feasible approach for fabricating superior PEC photoanodes as well as new insights into heterojunction design.
The global evolutional changes towards the use of renewable energy source for transportation purposes is on the increase in an attempt to mitigate the environmental hazard and the proposed depletion of fossil fuel supply. Pyrolysis and Hydrothermal processes of biomass conversion into renewable biofuel resulted into the production of biocrude with high oxygen content due to the presence of large amount of oxygenates in biomass materials. The presence of oxygen content in bio-oil causes corrosiveness, low heating value, instability and high viscosity in bio-oil. These including others challenges have necessitated the application of upgrading techniques such as catalytic hydrodeoxygenation process among techniques. The presence of several oxygenated compound made the mechanisms of bio-oil synthesis difficult and model bio-oil were reviewed to understand the effect of process parameters and catalyst on aromatic selectivity and conversion. The selectivity of aromatic hydrocarbons was affected by deactivation of catalysts’ active sites. Coke formation has been identified as one of the common and notorious causes of catalyst deactivation which is dependent on the nature of feedstock, condition of operation and the nature of catalyst. Therefore, the need to develop, evaluate a structurally and thermally stable catalyst with high catalyst recovery and reusability are of importance in the quest to depict hydrodeoxygenation process as an excellent technique for bio-oil upgrading.
A novel sillenite, Bi12CoO20, is reported to effectively utilize a large portion of the solar spectrum up to the Near-IR region (1000 nm), and exhibits excellent photothermal degradation. The degradation is evaluated by using phenol as the model pollutant and correlated with a temperature-induced structure change through in-situ photoelectrochemical and spectroscopic characterizations. The degradation rate on Bi12CoO20 is ca. 3.0 times higher than that on P25-TiO2 under simulated sunlight irradiation and the best by comparing with other reported photothermal catalysts. The thermal effect is demonstrated to cause the conversion of Co³⁺ to Co²⁺ at the octahedral sites of the Bi12CoO20, increase the internal electric field, and facilitate charge separation. The conversion also positively shifts the band potential, increasing the oxidative reactive species. The photothermal activity is newly found to be enhanced by increasing the IEF and band edge potential, which may provide strategies for designing more effective photothermal catalysts.
Biotic and abiotic oxidation of Mn(II) in aqueous environments is an important process for the cycling of many elements. However, the mechanism involved in photocatalytic oxidation of Mn(II) has not been clearly elucidated yet. In this study, the photocatalytic oxidation of Mn(II) on the surface of self-doped Bi2+xWO6 (Bi2.15WO6) under visible light was conducted. Kinetics results show that visible light apparently accelerates the oxidation of Mn(II) to Mn(III, IV) oxides on Bi2.15WO6. The average oxidation states (AOS) of manganese reach 2.18 after 80 min of reaction under visible light at pH 8.50. Characterizations indicate the formation of Bi(III)-O-Mn(II) surface complexes between Mn(II) and surface Bi(III) on Bi2.15WO6, which then decreases the bandgap of [Bi2.15WO6 + Mn(II)]light (2.53 eV) compared with those of [Bi2.15WO6 + Mn(II)]dark (2.72 eV) and pure Bi2.15WO6 (2.86 eV), suggesting the contribution of the ligand-to-metal charge transfer (LMCT) pathway to the photocatalytic oxidation of Mn(II). Moreover, the addition of inorganic oxidants with strong oxidizing capacities (such as Cr2O72-, NO3- or NO2-) significantly increases the oxidation rate of Mn(II), further verifying the contribution of the LMCT pathway to Mn(II) oxidation. We therefore suggest that the LMCT pathway is one of the important oxidation routes for Mn(II) oxidation on Bi2.15WO6 under visible light.
Superoxide anions (.O2⁻) and hydroxyl radicals (.OH) are vital reactive oxide species (ROS) in the photocatalytic process. In this study, a novel triple-doped bismuth tungstate (Yb/Er/Pr-Bi2WO6) superior photocatalyst was constructed using hydrothermal method. The yield of .O2⁻ and .OH are concurrently boosted by the synergistic effect for Yb/Er/Pr triple-doping. The X-ray photo-electron spectra confirmed the existence of oxygen vacancy and Pr⁴⁺/Pr³⁺ redox center, which can act as trap electron center to produce more .O2⁻ and promote electron-holes pairs separation. Meanwhile, Yb and Er doping observably regulated the valence band position of Bi2WO6 from 2.39 V to 2.63 V, which is beneficial to produce more .OH. Comprehensive characterization illustrate that Yb/Er/Pr-Bi2WO6 shows superior performance in photocatalytic activity, photocarrier separation and light absorption efficiency compared with the pristine Bi2WO6, single-doped Bi2WO6 and co-doped Bi2WO6. The research provides a simple and valid method for enhancing the visible-light-responding photocatalytic property and exploring the possible enhanced photocatalytic performance mechanism.
The sillenite phaseBi12.7Co0.3O19.35 was prepared successfully by pyrolysis of the BiCo-based metal coordination polymer (BiCo-MCP) as precursor. The preparation was studied under varied temperatures from 600 to 700 °C and different reaction time. SEM images showed the diverse morphologies including amorphous, spindle and flake particle shapes obtained under different conditions. Among them, the spindle particles of pure phase Bi12.7Co0.3O19.35 displayed superior catalytic activity for degradation of imidacloprid and RhB under visible light. The pH of the wastewater was found to have significant influence on the photocatalytic performance of Bi12.7Co0.3O19.35. When pH < 1.91 or pH > 11.0, the photocatalytic degradation rate of imidacloprid pesticide wastewater reached more than 96% for 4 h and when pH < 2.4, the degradation effect of RhB dye could achieve 100% degradation within 30 min. The degradation products of imidacloprid changed with different pH values of waste water. Two different catalytic mechanisms are proposed according to the capture experiment. Bi12.7Co0.3O19.35 has potential application prospects in the degradation of organic pollutants with high catalytic efficiency and repeatability.
Pollution of surface and groundwater resources with antibiotics creates problems for human health, meanwhile, because of development of antibiotic resistance in bacteria. Chlortetracycline (CTC) is a tetracycline antibiotic, for which different removal methods were developed. Therefore, this study evaluated the removal efficiency of CTC from aqueous solutions using a photocatalytic process. In this study, Bi2WO6 was synthesized by microwave method and Ce loaded on Bi2WO6 by wet impregnation method. The prepared photocatalysts were characterized by XRD, FTIR, SEM, BET/BJH, DRS analysis. The degradation of CTC was investigated by the photocatalysis under solar light. Among the photocatalysts, Ce/Bi2WO6 (8%) had the highest CTC degradation efficiency. Response surface methodology (RSM) with central composite design (CCD) was used to investigate pH, time (min), mass of catalyst Ce/Bi2WO6 (8%) (g), and concentration of CTC (mg/L). In optimal conditions at pH 3.19, time 201.07 min, mass of Ce/Bi2WO6 (8%) of 0.05 g, and concentration of CTC 24.99 mg/L the photocatalytic degradation percentages of CTC were found to be 99.99%. Isothermic studies have shown that Fritz–Schlunder isotherms provide the best fit with the least error with experimental data.
The development of single phase photocatalyst is expected to realize clean energy and pollution treatment. Herein, we reported a novel Tremella-like Bi2WO6 catalyst which was obtained by facile hydrothermal technique. The formation of Tremella-like Bi2WO6 strongly depended on introduction of Bi2O3. Based on the Kirkendall effect, Bi2O3 induced Bi(NO3)3·5H2O to form biscuit-like Bi6O6(OH)3(NO3)3·3H2O particles which provided templates and reacted simultaneously with WO4²⁻ to synthesize Tremella-like Bi2WO6. The Tremella-like Bi2WO6 exhibited remarkable visible-light catalytic performance. The degradation rate of RhB dye reached 100% with 10 min, the reduction rate of CO2 was 5.5 times higher than pure Bi2WO6. Moreover, the Tremella-like Bi2WO6 catalyst displayed excellent stability during the recycle experiments. The high catalytic activity makes single phase Bi2WO6 catalyst great potential in environmental protection field.
Scale-up of Fischer-Tropsch (F-T) synthesis using microreactors is very important for a paradigm shift in the production of fuels and chemicals. The scalability of microreactors for F-T Synthesis was experimentally evaluated using 3D printed stainless steel microreactors, containing seven microchannels of dimensions 1000 µm × 1000 µm × 5cms. Mesoporous silica (KIT-6), with high surface area, containing ordered mesoporous structure was used to incorporate 10% cobalt and 5% ruthenium using a one-pot hydrothermal method. Bimetallic Co-Ru-KIT-6 catalyst was used for scale-up of F-T Synthesis. The performance of the catalysts was evaluated and examined for three different scale-up configurations (stand-alone, two, and four microreactors assembled in parallel) at both atmospheric pressure and 20 bar at F-T operating temperature of 240 ˚C using a syngas molar ratio (H2:CO) of 2. All three configurations of microreactors yielded not only comparable CO conversion (85.6% to 88.4%) and methane selectivity (~14%) but also similar selectivity towards lower gaseous hydrocarbons like ethane, propane, and butane (6.23% to 9.4%) observed in atmospheric F-T Synthesis. The overall selectivity to higher hydrocarbons, C5+ is in the range of 75% to 82% at 20 bars.
A CFD model was used to investigate the effect of different design features and numbering up approaches on the performance of the microchannel reactor. The effect of the reactor inlet, the mixing internals and the channel designs on the dead zone %, the quality index factor, the cooling requirement and the maximum dimensionless temperature within the microreactor were quantified. There is no significant effect of increasing the channel width on the microreactor performance and operation of the microchannel reactor at lower Nusselt number that results in higher CO conversion. Increasing the channel width reduced the maximum temperature exhibited in the channel. Finally, the effect of increasing the y/x stacking ratio, i.e. having more reactor units in parallel compared to series, was investigated. Increasing the y/x ratio increased the cooling requirement and the maximum dimensionless temperature increase within the unit will decrease the productivity. To minimize the productivity losses, numbering up in series is the better approach; however further analysis must be done to delineate heat removal requirements.
Nearly 100% exposure of π-conjugated planes, whose structure inherently exhibits large electron delocalization and fast charge transfer, has been achieved in perylene diimide (PDI) supramolecular photocatalysts by a solvent-induced self-assembly method. The high exposure ratio of π-conjugated planes is found to cause a larger surface potential and higher surface charge density by experimental data, and higher electron distribution by DFT calculations, relative to π-stacked planes exposed on PDI nanorods or (020) planes exposed on PDI particles, resulting in a strong internal electric field. This gives π-conjugated PDI ca. 8-17 times higher activity on phenols photodegradation than reported PDI, and 4-6 times higher activity than well-known photocatalysts like Bi2WO6 or g-C3N4. The successful control of PDI to preferentially expose π-conjugated planes may not only boost the photocatalytic activity in this system, but also give some guidelines in the design and development of more efficient organic photocatalysts with wide spectrum response.
A novel Yb³⁺/Nd³⁺-Bi2WO6 photocatalyst with oxygen vacancies was synthesized through one-step hydrothermal method. The 1.5%Yb³⁺/0.5%Nd³⁺-Bi2WO6 exhibited an outstanding performance, whose decomposition efficiency of Rhodamine B was 3.7 times higher than that of bare Bi2WO6. The crystal structure, morphology, chemical state and optical properties were systematically investigated to reveal the mechanism of its enhanced photocatalytic performance. Its excellent visible light absorption and an absorption peak at 584 nm due to the excitation of Nd³⁺ were confirmed by the UV-Vis diffuse reflectance spectra of composite samples. Meanwhile, not only the existence of dopants was confirmed by the X-ray spectroscopy, but the in-built Yb³⁺/Yb²⁺ redox center was also revealed. Moreover, the formation of oxygen vacancies and Yb³⁺/Yb²⁺ redox center could effectually restrict the recombination of photo-generated electron-hole pairs, which could be demonstrated by photoluminescence characterization. Consequently, the mechanism of an enhanced photocatalytic performance was speculated about the extension range of visible light and the synergistic effect of oxygen vacancies as well as the redox center. This study provides a new concept, a rational design and the acquirement of a high-efficiency visible-light-driven photocatalyst.
The development of energy-harvesting and energy-storage devices based on renewable sources will be fundamental for the deployment of autonomous or isolated systems like emerging internet-of-things (IoT) applications.
This chapter will focus on the use of bismuth nanomaterials in energy-related applications. These materials fulfill the requirements of being composed of abundant and non-toxic elements and therefore can be considered environmentally friendly functional materials.
The specific applications that will be covered are: energy-harvesting devices including solar cells and thermoelectrics, electrochemical energy storage devices such as batteries and supercapacitors, and photocatalysts for solar hydrogen production. For each technology, we will discuss the state of the art, challenges, and the focus areas for current research.
Bi2WO6: Er, Yb nanofibers prepared via solvothermal-electrospinning method have high-efficiency photocatalysis performance and superior temperature feedback. Combining crystal integrity and polymer processability, polyacrylonitrile composite fibers are commendable in many aspects, such as large specific surface area, excellent flexibility, controllable structure and high solar energy utilization. The boost in photocatalysis activity can be attributable to non-radiative energy transfer between Er³⁺ ions with bismuth tungstate and doping effect of rare earth ions in Bi2WO6, where this doping enhances separation efficiency of electron-hole pairs and promotes generation of highly oxidative species. The electrospun nanofibers also can accurately perform contact-free temperature monitoring and real-time thermal feedback on the degradation system through ²H11/2 → ⁴I15/2 and ⁴S3/2 → ⁴I15/2 radiation transitions of Er³⁺ ions. In addition, the absolute sensitivity SA and the relative sensitivity SR are as high as 0.868% K⁻¹ and 1.548% K⁻¹ at 313 K, respectively, which indicates nanofibers have good thermal sensitivity. This dual-function nanofiber provides new application prospects for the development of innovative multifunctional semiconductor materials to cope with the ecological and energy crisis.
Photocatalytic ozonation technique for wastewater treatment has received much attention for their efficient capability in the mineralization of persistent organic pollutants. In this study, nanostructured Bi2WO6 was prepared by hydrothermal method and applied in the photocatalytic ozonation process for tetracycline hydrochloride (TCH) degradation under simulated solar light irradiation. Bi2WO6 triggered an effective synergy between photocatalysis and ozonation, and it showed a good activity and adaptability in the degradation of organic compounds. Besides, the influence of experimental factors on the (total organic carbon) TOC removal (including catalyst dosage, ozone concentration, initial pH, reaction temperature and coexisting ions) was also investigated comprehensively. Spin-trapping electron paramagnetic resonance measurements and quenching experiments demonstrated that O2⁻, OH, ¹O2 and h⁺ contributed to TCH degradation. The possible degradation pathways of TCH were proposed by identifying the intermediates with liquid chromatography-mass spectroscopy.
Photocatalytic oxidation method is a promising technology for solving flue gas mercury (Hg) pollution from industrial plants. Semiconductor photocatalysts have been widely applied in energy conversion and environmental remediation. However, key issues such as low light absorption capacity, wide energy band gap, and poor physicochemical stability severely limit the application of photocatalysts in practical industrial plants. In recent years, bismuth-based (Bi-based) photocatalysts, including bismuth oxide halide BiOX (X = Cl, Br or I), bismuth salt oxymetal BiVO4, and BiOIO3 etc., have increasingly aroused scientists’ attention due to their peculiar crystalline geometric structures, tunable electronic structure and high photocatalytic performance. In present review, we firstly review the photocatalytic reaction mechanism and main photocatalytic oxidation mechanism of mercury. Secondly, the synthetic methods of Bi-based photocatalysts are summarized. Then, according to the mechanism of mercury removal, the experimental modifying approaches including heterojunction making, external atoms doping, defect creating, and crystal face regulating to promote the photocatalytic oxidation of mercury removal are summarized, as well as the determination of the band gap and electronic density of states (DOS) of Bi-based photocatalysts to elucidate the photocatalytic oxidation mechanism via density functional theory (DFT) calculation. Furthermore, constructing electronic transmission channels is an efficient way to improve the photocatalytic activity. Finally, challenges and perspectives of Bi-based photocatalyst for photocatalytic oxidation of mercury removal are presented. In addition, the excellent performance photocatalysts and efficient pollution removal equipment for mercury removal in industrial plants are still required in-depth study.
Although much attention has been recently focused on Bi2WO6 and doping materials as highly efficient visible-light photocatalysts, the selection of an appropriate element as dopant and our understanding of the photocatalytic mechanism from the electronic level still need to be further investigated. In the current work, electronic band structures, density of states, optical properties, and effective mass for Bi2WO6, Ag-doped Bi2WO6 and Y-doped Bi2WO6 semiconductors have been studied by density functional theory calculations. We find that, while both the doped systems show little change in the band gap from Bi2WO6, Ag-doped and Y-doped Bi2WO6 produce more dispersive bands compared to the original Bi2WO6, implying that the doped systems can facilitate the spatial separation of photo-induced electron–hole pairs, and thus enhance the photocatalytic performance. In addition, for Ag-doped Bi2WO6, the presence of an Ag impurity energy band just above the valence band is expected to act as electron-trapping sites leading to a reduction of the recombination rate of photogenerated carriers. Our findings could provide an explanation for the experimental observations reported in the literature.
This chapter is a description of methodologies for the conversion of carbon dioxide to products of value using catalytic processes. Accumulation of carbon dioxide in the atmosphere is a major problem and an optimal solution is to capture and convert it to useful products. In this chapter, first, the climate challenges due to increasing carbon dioxide emissions, the earth’s carbon cycle, sources of carbon dioxide emissions and carbon dioxide capture are discussed. Second, carbon dioxide utilization processes such as enhanced oil recovery, and conversion methodologies including photo-conversion, bio-conversion and catalytic conversion processes are discussed. Third, the importance of scale for carbon dioxide utilization is discussed, and the catalytic conversion pathway is identified as a potential route for utilization in large scales. Wet and gas phase aerosol synthesis methods for catalysts are discussed followed by characterization methods. And finally, a futuristic application on the potential for utilization of carbon dioxide via reforming processes on Mars is presented.
Fossil fuel combustion is often considered as one of the major threats to the environment, because of the carbon dioxide (CO2) release in the atmosphere. Such an accumulation of CO2 in the atmosphere leads to drastic climate change in the environment. The control in the discharge of CO2 into the atmosphere and the effective utilization of CO2 are great global challenges behind us. The recent research works show there are reasonable technologies developed on the CO2 capture, and utilization leaves us to relieve little. The recent progresses in the organometallic chemistry and catalysis afford the effective chemical transformation of CO2 and its incorporation into synthetic organic molecules under mild reaction conditions. The catalytic conversion of CO2 into small and beneficial molecules such as carbonates, methylamines, methanol, formic acid, etc., by molecular catalysts, is an interesting topic that has significantly developed in recent years. The aim of this chapter is to reveal the recent advancement in the CO2 capture and its utilization in the synthesis of commodity chemicals. In addition, this also converses various homogenous metal complexes, catalyzed fine chemicals synthesis, and their challenges.
In recent years, environmental challenges have led to a focus on the production of clean synthetic fuels from different carbon sources using Fischer-Tropsch (FT) synthesis. Catalyst development and reactor improvements are the major points of interest to obtain high selectivity toward desired hydrocarbons in FT synthesis. The first part of this chapter summarizes the fundamentals of FT synthesis, catalysts, and possible reaction mechanisms, the drawbacks of present synthesis reactors, and how microchannel microreactor (specified as microreactor in this chapter) technology addresses them with its unique characteristics. Two case studies are presented to describe catalyst screening for FT synthesis in two types of microreactors: Silicon (Si) microreactors are fabricated using conventional microfabrication techniques with dimensions 1.6 cm × 50 μm × 100 μm. Stainless steel (SS) 3D printed microreactors of dimensions 2.4 cm × 500 μm × 500 μm are fabricated by direct metal laser sintering method. The FT studies with Si and SS microreactors coated with different catalysts/supports and temperature-programmed reduction (TPR) experiments with H2 not only provide insight into metal-support interactions but also catalyst performance in terms of kinetics, selectivity, CO conversion, and stability. Conversion of syngas enriched with CO2 and CO2 utilization in FT synthesis are the key factors in the production of next-generation biofuels. A case study on the effect of silica and alumina promoters on Co-Fe-K precipitated catalysts in a lab-scale reactor to enhance CO2 utilization in FT synthesis is also included.
Solid-state reaction method was opted for the preparation of bismuth tungstates (Bi2WO6) in the stoichiometric ratio. The structural characterization related that the material has got orthorhombic symmetry. The high-energy ball milling did not show any structural change, but a reduction in grain size was observed from 100 to 34 nm after 5 h. The higher activity for the decolourization of rhodamine B (RHB) and methylene blue (MB) in the presence of UV light has been studied by employing Bi2WO6 as a catalyst. The dye degradation was observed by a decrease in the absorption spectrum and decolourization in the presence of UV irradiation. The degradation efficiency was found to be dependent on the size of the catalyst added in the dye solution, which may be due to increased surface area that increased the number of active sites for the reaction. The degradation efficiency of the unmilled and 5-h ball milled (Bi2WO6) catalyst was observed to be 32 and 90% in RHB, respectively. While in MB, 24 and 49% degradation efficiency was achieved by unmilled and 5-h ball milled (Bi2WO6) catalyst. The degradation rate coefficient was found to be in the decreasing order of RHB > MB, which pursued the first-order kinetic mechanism. Therefore, Bi2WO6 can act as a catalyst for the treatment of noxious and imperishable organic pollutants in water.
Carbon dioxide (CO2) photoreduction is a complex process and, despite significant efforts in photocatalyst development, an investigation of the influence of process parameters on photoreduction is still necessary. In this work, a Bi2WO6 photocatalyst was used for the photocatalytic reduction of CO2 in a continuous-flow differential photoreactor and a rotational central composite design (RCCD) was employed for a systematic evaluation and optimization of the operational parameters. The experimental parameters considered were CO2 flow rate, light intensity, partial pressure of H2O, and amount of photocatalyst. The synthesized Bi2WO6 was characterized by XRD, XPS, Raman, SEM, BET measurements and UV–vis spectroscopy. The results showed that the CO2 flow rate had an impact on CO2 photoreduction; the light intensity, partial pressure of H2O, and amount of photocatalyst significantly influenced the CO cumulative production. At optimum conditions (120 mW cm⁻², 2.7 kPa, and 15 mg), it was possible to obtain a 60% enhancement in photocatalytic efficiency, compared to the results obtained before the experimental design. Characterization studies carried out after the use of the photocatalyst showed that Bi2WO6 experienced changes on its surface, which may explain the deactivation observed during the photoreduction under gaseous flow.
This book is part of a two-volume work that offers a unique blend of information on realistic evaluations of catalyst-based synthesis processes using green chemistry principles and the environmental sustainability applications of such processes for biomass conversion, refining, and petrochemical production. The volumes provide a comprehensive resource of state-of-the-art technologies and green chemistry methodologies from researchers, academics, and chemical and manufacturing industrial scientists. The work will be of interest to professors, researchers, and practitioners in clean energy catalysis, green chemistry, chemical engineering and manufacturing, and environmental sustainability.
This volume focuses on catalyst synthesis and green chemistry applications for petrochemical and refining processes. While most books on the subject focus on catalyst use for conventional crude, fuel-oriented refineries, this book emphasizes recent transitions to petrochemical refineries with the goal of evaluating how green chemistry applications can produce clean energy through petrochemical industrial means. The majority of the chapters are contributed by industrial researchers and technicians and address various petrochemical processes, including hydrotreating, hydrocracking, flue gas treatment and isomerization catalysts.
Absorption, reflection and emission polarization spectra of CdWO4 crystals have been studied in the region 3.5–30 eV in order to distinguish the excitonic and electron–hole processes in the vicinity of the band gap. The following parameters have been defined from the Urbach tail study at 6–300 K: E0=5 eV, σ0=0.31, hω=70 meV (565 cm−1). Excitonic processes have been shown to dominate at excitation of tungstate crystals in the lower part of the conduction band. Excitons are formed due to the transitions into the tungstate W5d states hybridized with O6p and possess a very strong tendency for self-trapping. Free electrons and holes can be created at the energies 1–2 eV (depending on the crystal) higher than the bottom of the conduction band due to the transitions into cationic states.
The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. Problems with hazardous wastes at military installations are related in part to the disposal of chemical wastes in lagoons, underground storage tanks, and dump sites. Typical wastes of concern include heavy metals, aviation fuel, military-vehicle fuel, solvents and degreasing agents, and chemical byproducts from weapons manufacturing. In the civilian sector, the elimination of toxic and hazardous chemical substances such as the halogenated hydrocarbons from waste effluents and previously contaminated sites has become a major concern. General classes of compounds of concern include: solvents, volatile organics, chlorinated volatile organics, dioxins, dibenzofurans, pesticides, PCB's, chlorophenols, asbestos, heavy metals, and arsenic compounds. Advanced physicochemical processes such as semiconductor photocatalysis are intended to be both supplementary and complementary to some of the more conventional approaches to the destruction or transformation of hazardous chemical wastes such as high-temperature incineration, amended activated sludge digestion, anaerobic digestion, and conventional physicochemical treatment. 441 refs.
To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light.
Films and powders of TiO2-xNxhave revealed an improvement over titanium dioxide (TiO2) under visible light (wavelength < 500 nanometers) in optical absorption and photocatalytic activity such as photodegradations
of methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface. Nitrogen doped into substitutional sites
of TiO2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations
and x-ray photoemission spectroscopy.
The photocatalytic splitting of water into hydrogen and oxygen using solar energy is a potentially clean and renewable source for hydrogen fuel. The first photocatalysts suitable for water splitting, or for activating hydrogen production from carbohydrate compounds made by plants from water and carbon dioxide, were developed several decades ago. But these catalysts operate with ultraviolet light, which accounts for only 4% of the incoming solar energy and thus renders the overall process impractical. For this reason, considerable efforts have been invested in developing photocatalysts capable of using the less energetic but more abundant visible light, which accounts for about 43% of the incoming solar energy. However, systems that are sufficiently stable and efficient for practical use have not yet been realized. Here we show that doping of indium-tantalum-oxide with nickel yields a series of photocatalysts, In(1-x)Ni(x)TaO(4) (x = 0-0.2), which induces direct splitting of water into stoichiometric amounts of oxygen and hydrogen under visible light irradiation with a quantum yield of about 0.66%. Our findings suggest that the use of solar energy for photocatalytic water splitting might provide a viable source for 'clean' hydrogen fuel, once the catalytic efficiency of the semiconductor system has been improved by increasing its surface area and suitable modifications of the surface sites.
Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of layered structure with perovskite slabs interleaved with Bi2O2 layers showed photocatalytic activities for H2 evolution from an aqueous methanol solution and O2 evolution from an aqueous silver nitrate solution. Bi2WO6 with the Aurivillius structure and a 2.8 eV band gap was active for the O2 evolution reaction under visible light irradiation (λ > 420 nm).
The complex oxides, Bi2MoO6 and Bi2WO6, were hydrothermally prepared at 120–180°C for 2–4 days and characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), IR spectroscopy, transition electron microscopy (TEM), EDAX analysis and electric conductivity measurements.
Heterogeneously dispersed semiconductor surfaces provide both a fixed environment to influence the chemical reactivity of a wide range of adsorbates and a means to initiate light-induced redox reactivity in these weakly associated molecules. Upon photoexcitation of several semiconductors nonhomogeneously suspended in either aqueous or nonaqueous solutions or in gaseous mixtures, simultaneous oxidation and reduction reactions occur. This conversion often accomplishes either a specific, selective oxidation or a complete oxidative degradation of an organic substrate present. The paper discusses the following: survey of reactivity (functional group transformations and environmental decontamination); mechanism of photocatalysis (photoelectrochemistry, carrier trapping, inhibition of electron hole recombination by oxygen, involvement of the hydroxy radical, adsorption effects, Langmuir-Hinshelwood kinetics, pH effects, temperature effects, and sensitization); and semiconductor pretreatment and dispersion (photocatalytically active semiconductors, photocatalyst preparation, and surface perturbation). 215 refs.
The paper examines a wide range of oxides for use as anodes in photoelectrochemical cells for the conversion of solar energy into electrical power of hydrogen. The Schottky barrier model of the semiconductor-electrolyte interface is used; type (a) oxides, not containing partly filled d-levels, conform to a relationship between flat band potential and band gap; this essentially rules out the possibility of finding type (a) oxides with simultaneously small band gap and large negative flat band potential required for efficient operation in the unbiased photoelectrolysis of water. Incorporation of this relationship into the Schottky barrier formula for photocurrent makes possible the calculation of efficiencies of conversion for type (a) oxides; for power cells with redox operation, type (a) oxides are predicted to give 5-6% efficiency for a band-gap of 2.4 eV, with a redox couple of standard potential not less than 0.8.
Sodium hexatitanate, Na2Ti6O13, with a tunnel structure, combined with oxidized Ru, was found to be an efficient photocatalyst for the complete decomposition of water.
Square Bi2WO6 nanoplates have been successfully synthesized by simple hydrothermal process. The effects of hydrothermal temperature and reaction time on morphologies and sizes of the nanoplates were investigated. These nanoplates are square geometric shapes having their basal plane as the (001) plane of orthorhombic Bi2WO6. On the basis of results of morphologies observation and selected area electron diffraction of series samples, a possible growth mechanism of the nanoplates is revealed. The square laminar shape could be attributed to anisotropic growth along the (001) plane, which is parallel to their intrinsic layer structure. UV−visible diffuse reflection spectra of the prepared Bi2WO6 nanoplates indicate they had absorption in the visible region, but a blue shift appeared compared to their bulk counterparts. Their photocatalytic activities are determined by rhodamine B degradation under visible light irradiation (λ > 400 nm). The reaction constant (k) of the best quality Bi2WO6 nanoplates is three times that of the sample prepared by solid-state reaction, which indicates much higher photocatalytic activities of the nanoplates performed under visible light irradiation.
Phase-pure solid solutions with the composition of Sr2NbxTa2-xO7 (SNT, x = 0−2) were prepared at 900 °C for 5 h by the Pechini-type polymerizable complex (PC) technique, based upon polymerization between citric acid and ethylene glycol. The two end compounds, Sr2Ta2O7 (x = 0) and Sr2Nb2O7 (x = 2), produced H2 and O2 in a stoichiometric ratio from pure water under UV light irradiation without a NiO cocatalyst. The photocatalytic activity of SNT for the water decomposition was greatly improved by loading NiO as a cocatalyst for a whole range of x. The photocatalytic activity was dramatically decreased approximately by 1 order of magnitude once Ta has been replaced by Nb, even when the amount of Nb was small. For all of the NiO-loaded SNT samples, water was stoichiometrically decomposed into H2 and O2. While samples prior to the complete crystallization showed very low activities despite their high surface area, the corresponding photocatalytic activities of well-crystallized samples depended primarily on their surface area. The low photocatalytic activities of such premature samples were interpreted as a consequence of the increased number of lattice defects acting as inactivation centers. The maximum photocatalytic activity was obtained for NiO (0.15 wt %)/Sr2Ta2O7 prepared by the PC method at 800 °C for 48 h; the photocatalyst having a specific surface area of 10.4 m2·g-1 produced H2 and O2 from pure water with specific rates of 3517 and 1733 μmol·h-1·g-1, respectively, 3.5 times larger than the best result for a sample prepared by the conventional solid-state reaction method.
Sol-gel prepared mixtures of silica or zirconia with titania are shown to have significantly higher activities than pure titania for the complete photocatalytic oxidation of ethylene. These higher activities are only apparent when the respective catalysts are stabilized by sintering. The differences become even more pronounced when the catalysts are used in a tubular reactor. Optimum mixture concentrations are found to be 12 wt % zirconia and 16 wt % silica in titania. Both catalyst types exhibit activity maxima with respect to sintering temperature. It is hypothesized that the maxima arise from opposing effects of densification and phase transformation versus beneficial sintering. A comparison of our catalyst compositions with literature in this area suggests that the increase in activity due to the addition of silica or zirconia may be a result of higher surface acidity. However, isoelectric point measurements employing the unsintered and sintered catalysts show no conclusive increase in surface acidity. The effects of sintering temperature on the surface area, porosity, and crystal structure of the catalysts are also presented.
A new approach has been developed for the fabrication of visible light photocatalysts. Nanoclusters of MoS2 and WS2 are coupled to TiO2 by an in situ photoreduction deposition method taking advantage of the reducing power of the photogenerated electrons from TiO2 particles. The photocatalytic degradation of methylene blue and 4-chlorophenol in aqueous suspension has been employed to evaluate the visible light photocatalytic activity of the powders. The blue shift in the absorption onset confirms the size quantization of MS2 nanoclusters, which act as effective and stable sensitizers, making it possible to utilize visible light in photocatalysis. Quantum size effects alter the energy levels of the conduction and valence band edges in the coupled semiconductor systems, which favors the interparticle electron transfer. In addition, the coupled systems are believed to act in a cooperative manner by increasing the degree of charge carrier separation, which effectively reduces recombination.
Nitrogen doping was recently shown to extend the absorptivity of TiO2 photocatalysts into the visible. We find that N-doped TiO2 materials fail, however, to catalyze the oxidation of HCOO- into CO2•-, or of NH3OH+ into NO3-, under visible illumination. By N-doping anatase at ambient or high temperature according to the literature we obtained yellow powders A and H, respectively, that absorb up to 520 nm. Aqueous H suspensions (pH 6, 1 atm O2) photocatalyze the oxidation of HCOO- into CO2•- radicals at λ 330 nm, but the quantum yield of CO2•- formation at λ > 400 nm remains below 2 × 10-5 and is probably zero. A is similarly inert toward HCOO- in the visible region and, moreover, unstable in the UV range. Thus, the holes generated on N-doped TiO2 by visible photons are unable to oxidize HCOO- either by direct means or via intermediate species produced in the oxidation of water or the catalyst. Reports of the bleaching of methylene blue (MB) on N-doped TiO2, which may proceed by direct oxidative or reductive photocatalytic pathways and also by indirect photocatalysis (i.e., induced by light absorbed by MB rather than by the catalyst) even under aerobic conditions are, therefore, rather uninformative about the title issue.
The structure of nickel-loaded K4Nb6O17 pholocatalyst in an overall water splitting reaction was studied by means of XPS, EXAFS, TEM, and XRD. K4Nb6O17 has an ion-exchangeable layered structure which possesses two different kinds of alternating interlayer spaces, i.e. interlayers I and II, where K+ ions are located. The interlayers are hydrated in an aqueous solution. It was revealed that in the active catalyst which was pretreated by H2 at 773 K for 2 h and reoxidized by O2 at 473 K for 1 h, loaded nickel is predominantly located in interlayer I as ultrafine metal particles (ca. 5 Å). In contrast, only a very small amount of nickel was observed over the external surface of K4Nb6O17. On the basis of the structure, a novel mechanism for the photodecomposition of H2O into H2 and O2 is proposed; i.e., intercalated water is reduced to H2 in interlayer 1 and is oxidized to O2 in interlayer II. Therefore, each niobate macroanion sheet is regarded as a “two-dimensional” photocatalyst where H2 and O2 evolve at different sides of the layer.
Photocatalytic activities for the decomposition of distilled water into H2 and O2 were investigated on various tantalates. In the alkali and alkaline earth tantalates, LiTaO3, NaTaO3, KTaO3, MgTa2O6, and BaTa2O6 showed photocatalytic activities for water decomposition without co-catalysts. Among them, BaTa2O6 in the orthorhombic phase was the most active. The addition of a small amount of Ba(OH)2 into the water and supporting NiO drastically enhanced the photocatalytic reaction on the BaTa2O6 catalyst. On the other hand, in the transition metal tantalates, NiTa2O6 produced both H2 and O2 without co-catalysts.
The oxidation power of the TiO2-xNx powders with low nitrogen concentrations (<0.02) was evaluated by the decomposition of gaseous 2-propanol (IPA) under the same absorbed photon number, 1.4 x 10(14) quanta.cm(-2).s(-1), of visible (Vis) or ultraviolet (UV) light. Regardless of the x value, the quantum yield values from irradiating with Vis light was lower than with UV light, which suggests that the isolated narrow band formed above the valence band is responsible for the Vis light response in the present nitrogen doped TiO2. In addition, increasing the nitrogen concentration when irradiating with UV light lowered the quantum yields, indicating that the doping sites could also serve as recombination sites.
We prepared a new series of rare earth photocatalysts, Bi2RNbO7 (R = Y, rare earth), and found that the variation of R ion radius rR3+ in Bi2RNbO7 led to a change in the band structure and the activity of photocatalytic decomposition of water. The band gap Eg increases with increasing rR3+, while the photocatalytic activity AR3+ decreases with increasing rR3+. The analysis of the E(rR3+) dependency showed that the position of the 4f band of the R ion in Bi2RNbO7 determines the band gap Eg, resulting in the difference in photocatalytic activity.
A number of transition metal incorporated MCM-41 mesoporous molecular sieves with silicon=80 have been synthesized by a hydrothermal methods. The study demonstrated that the presence of transition metal salts in the gel during synthesis hinders the action of the template. The catalytic performance are correlated with the UV vis spectrum of each synthsized catalysts to reveal the specific role played by each metal ions. The characterization results shows enhancement of the absorption in visible light by some composite materials.
Various preparations of nanostructured TiO2 starting from Ti(iso-OC3H7)4 or TiCl4 are reported. The samples were characterized by X-ray diffractometry, specific surface area and porosity determinations, scanning and transmission electron microscopy, and diffuse reflectance spectroscopy. 4-Nitrophenol photodegradation in aqueous medium was employed as a probe reaction to test the photoactivity of the catalysts. The photoactivity of some samples derived from Ti(iso-OC3H7)4 was found comparable with that of commercial powders. Calcination after the hydrolysis process was necessary to achieve crystallization of the particles before using them as photocatalysts for the reaction studied. The samples deriving from TiCl4 were the most photoactive among the home-prepared catalysts, and neither filtration nor calcination was needed to obtain a highly photoactive anatase phase.
Titanium dioxide (TiO2) nanoparticles of both anatase and rutile phases were synthesized by hydrothermal treatment of microemulsions, and their photocatalytic activity for wet oxidation of phenol was studied. The only difference between the two syntheses used was that different acids were added to the microemulsions, making direct comparison of the catalytic activity of the two polymorphs possible. If hydrochloric acid was used, the rutile structure formed, and if nitric acid was used, anatase formed. The phase stability of the microemulsion was studied and according to conductivity and turbidity measurements the idea of a direct template effect could be discarded during the hydrothermal treatment. However, an initial size-templating phenomenon is possible during the mixing step. The particles, which were in the size range of a few nanometers were characterized with N2-adsorption, XRD, SEM, and XPS. The activity of the two polymorphs for the photocatalytic oxidation of phenol in water was examined. It was shown that the rutile phase initially decomposed phenol much faster and follows a first-order process reasonably well (k = 4 × 10-5 s-1). The photodecomposition process using the anatase phase led, however, to a much more rapid overall degradation following an initial slower rate of phenol oxidation. The results indicate that the observed difference of the photodecomposition process for the two TiO2 phases is due to the formation of different intermediates.
The highly crystalline nanoparticles of In0.9Ni0.1TaO4, with a wolframite-type structure, were prepared by a solid-state reaction method. The band gap Eg of In0.9Ni0.1TaO4 is about 2.3 eV, and particles with sizes in the range of 300−500 nm were observed by TEM measurements. We loaded 1.0 wt % partially oxidized nickel as electron-trapping and hydrogen-evolution sites onto the In0.9Ni0.1TaO4 surface from an aqueous Ni(NO3)2 solution. The TEM measurements show that nearly spherical nano NiOx particles with a size of about 15 nm are distributed on the surface of In0.9Ni0.1TaO4. Stoichiometric amounts of oxygen and hydrogen were evolved under visible-light irradiation using NiOx/In0.9Ni0.1TaO4 with a quantum yield of about 0.66%. This system behaves as a short-circuited microphotoelectrochemical cell. The surface of NiOx is the cathode, and the surface of In0.9Ni0.1TaO4 is the anode.
Nanocrystalline anatase titanium(IV) oxide (TiO2) particles were synthesized by hydrothermal crystallization in organic media (HyCOM) followed by calcination at various temperatures up to 1273 K, and they were characterized by analysis of surface adsorption of the substrates, as well as by X-ray diffraction (XRD) and Brunauer−Emmet−Teller (BET) surface area measurements. These HyCOM TiO2 samples were used for three kinds of photocatalytic reactions: mineralization of acetic acid (AcOH) in aerated aqueous suspensions, dehydrogenation of 2-propanol (2-PrOH) by in situ platinized powders, and silver-metal deposition from silver ions (Ag+) in deaerated aqueous suspensions of bare TiO2 samples. Dependence of the photocatalytic activities on calcination temperature (Tc) and on the amount of adsorbed substrates in each reaction and correlations with the physical properties of HyCOM TiO2 were examined. In the case of mineralization of AcOH, the activitiy of each sample was almost proportional to the amount of surface-adsorbed AcOH in the dark, and the uncalcined (as-prepared) HyCOM TiO2 showed the highest activity, which was monotonically reduced with Tc, that is, with decrease in the amount of surface-adsorbed AcOH. On the other hand, in the case of silver-metal deposition, the photocatalytic activity was enhanced by calcination at higher temperature, despite the simultaneous decrease in the amount of surface-adsorbed Ag+ in the dark. Overall, the effects of calcination on the photocatalytic activities for several reaction systems strongly suggested that photocatalytic activity depends on two significant factors, adsorbability and recombination probability, corresponding to the specific surface area and crystallinity, respectively, and that the balance of these two factors determines the Tc dependence.
The effect of metal ions (Cu2+, Fe3+, Zn2+, Al3+, and Cd2+) on the photodegradation of several dyes: sulfo-rhodamine B (SRB), alizarin red (AR), and malachite green (MG) has been investigated in aqueous TiO2 dispersions under visible irradiation (λ > 420 nm). Trace quantities of transition metal ions such as Cu2+ and Fe3+ having suitable redox potentials alter the electron-transfer pathway involving the dye, O2 and TiO2 particles, and markedly depress the photodegradation of all three dyes under visible irradiation. Other metal ions, such as Zn2+, Cd2+, and Al3+, have only a slight influence on the photoreaction by altering the adsorption of dyes. Photogeneration of H2O2 and reactive radicals, and the changes in fluorescence emission of SRB in TiO2 aqueous dispersions were examined to elucidate the role of the metal ions. Addition of Cu2+ or Fe3+ decreases the reduction of O2 by the conduction electrons, subsequently blocks the formation of reactive oxygen species (O-•, •OH), and depresses the degradation of dyes under visible irradiation. We deduce that the reduction of O2 is essential for the photodegradation of dyes under visible irradiation.
BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or a hybrid orbital of Bi6s and O2p to a conduction band of V3d. The photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under UV irradiation (300 < λ < 380 nm) on the tetragonal BiVO4 was comparable to that on the monoclinic BiVO4. The monoclinic BiVO4 also showed the high photocatalytic activity for the O2 evolution under visible light irradiation (λ > 420 nm). When the monoclinic BiVO4 was calcined at 700−800 K the activity was increased. The activity of this monoclinic BiVO4 was much higher than that of BiVO4 prepared by a conventional solid-state reaction. The quantum yield at 450 nm for the O2 evolution on the monoclinic BiVO4 was 9%.
It was found for the first time that the photocatalytic decomposition of pure water proceeded over ZrO2 powder without any loaded metals under UV irradiation. The rate of H-2 and 02 evolution increased upon addition of Na2CO3 and NaHCO3. Moreover, the evolution of CO (the photocatalytic reduction product of CO2) was observed from NaHCO3 solutions. The special characteristics of ZrO2 semiconductor are believed to be associated with its highly negative flat-band potential and wide bandgap. In the can of Cu(1 wt %)-ZrO2 catalyst suspended in NaHCO3 aqueous solution, the rates of ps evolutions were 19.5 mumol/h of H-2, 10.8 mumol/h of O2, and 2.5 mumol/h of CO. These mass balances were indicative of a stoichiometric and catalytic reaction.
A surfactant-templated approach was used to synthesize phosphated mesoporous titanium dioxide by incorporating phosphorus from phosphoric acid directly into the framework of TiO2. The resulting materials were characterized by XRD, nitrogen adsorption, TEM, XPS analysis, UV−vis spectroscopy, FT-IR spectroscopy, and isoelectric point measurements. The surface area of phosphated mesoporous TiO2 exceeded 300 m2/g after calcination at 400 °C. It was found that the incorporation of phosphorus could stabilize the TiO2 framework and increase the surface area significantly. This stabilization is attributed to two reasons: the more complete condensation of surface Ti−OH in the as-prepared sample and the inhibition of grain growth of the embedded anatase TiO2 by the interspersed amorphous titanium phosphate matrix during thermal treatment. Both pure and phosphated mesoporous TiO2 show significant activities on the oxidation of n-pentane. The higher photocatalytic activity of phosphated mesoporous TiO2 can be explained by the extended band gap energy, large surface area, and the existence of Ti ions in a tetrahedral coordination.
An oxide photocatalyst Bi2WO6 with corner-shared WO6 octahedral layered structure was synthesized. Its band gap was determined to be 2.69 eV from UV–vis diffuse reflectance spectra. The photocatalyst showed not only the activity for photocatalytic O2 evolution with the initial evolution rate of 2.0 mol/h but also the activity of mineralizing both CHCl3 and CH3CHO contaminants under visible light irradiation. Meanwhile, wavelength dependence of CH3CHO decomposition was observed, which indicated that the photocatalytic activity of the photocatalyst was in good agreement with its light-absorption ability.
Photocatalytic decomposition of liquid water into H2 and O2 over various semiconductor catalysts, such as TiO2, Ta2O5 and ZrO2, suspended in pure water, was studied with a special interest on the effect of Na2CO3 addition. It was found, for the first time, that both RuO2Ta2O5 and NiOxTa2O5 photocatalysts were able to decompose liquid water into H2 and O2 in pure water as well as with the addition of Na2CO3. It was speculated that a negative flat-band potential and wide band gap of the photocatalysts are very important for the stoichiometric decomposition of liquid water. Furthermore, the addition of Na2CO3 is very effective for the photocatalytic decomposition of water for PtNa2Ti6O13, PtK2Ti6O13 and Pt−K4Nb6O17 catalysts which did not show photocatalytic activity in pure water. The significant effect of Na2CO3 addition was observed commonly for both the TiO2 catalyst system and the Pt-loaded catalyst system.
From a theory of Hohenberg and Kohn, approximation methods for treating an inhomogeneous system of interacting electrons are developed. These methods are exact for systems of slowly varying or high density. For the ground state, they lead to self-consistent equations analogous to the Hartree and Hartree-Fock equations, respectively. In these equations the exchange and correlation portions of the chemical potential of a uniform electron gas appear as additional effective potentials. (The exchange portion of our effective potential differs from that due to Slater by a factor of 23.) Electronic systems at finite temperatures and in magnetic fields are also treated by similar methods. An appendix deals with a further correction for systems with short-wavelength density oscillations.
Raman spectra of chillagite, wulfenite, stolzite, scheelite and wolframite were obtained at 298 and 77 K using a Raman microprobe in combination with a thermal stage. Chillagite is a solid solution of wulfenite and stolzite. The spectra of these molybdate minerals are orientation dependent. The band at 695 cm-1 is interpreted as an antisymmetric bridging mode associated with the tungstate chain. The bands at 790 and 881 cm-1 are associated with the antisymmetric and symmetric Ag modes of terminal WO2 whereas the origin of the 806 cm-1 band remains unclear. The 4(Eg) band was absent for scheelite. The bands at 353 and 401 cm-1 are assigned as either deformation modes or as r(Bg) and (Ag) modes of terminal WO2. The band at 462 cm-1 has an equivalent band in the infrared at 455 cm-1 assigned as as(Au) of the (W2O4)n chain. The band at 508 cm-1 is assigned as sym(Bg) of the (W2O4)n chain.
We present an initial experience of creating an extramural continent valve in the ileal pouch in 4 patients who required continent urinary diversion. Using the appendix or a tapered ileal segment, the continent valve was created by the extramural tunnel technique along the anterior suture line of the pouch to facilitate the umbilical anastomosis. All patients were continent postoperatively with easy catheterization. This technique can provide a simple and effective continent pouch formed entirely from the ileum.
We propose a simple analytic representation of the correlation energy εc for a uniform electron gas, as a function of density parameter rs and relative spin polarization zeta. Within the random-phase approximation (RPA), this representation allows for the r-3/4s behavior as rs-->∞. Close agreement with numerical RPA values for εc(rs,0), εc(rs,1), and the spin stiffness alphac(rs)=∂2εc(rs, zeta=0)/deltazeta2, and recovery of the correct rslnrs term for rs-->0, indicate the appropriateness of the chosen analytic form. Beyond RPA, different parameters for the same analytic form are found by fitting to the Green's-function Monte Carlo data of Ceperley and Alder [Phys. Rev. Lett. 45, 566 (1980)], taking into account data uncertainties that have been ignored in earlier fits by Vosko, Wilk, and Nusair (VWN) [Can. J. Phys. 58, 1200 (1980)] or by Perdew and Zunger (PZ) [Phys. Rev. B 23, 5048 (1981)]. While we confirm the practical accuracy of the VWN and PZ representations, we eliminate some minor problems with these forms. We study the zeta-dependent coefficients in the high- and low-density expansions, and the rs-dependent spin susceptibility. We also present a conjecture for the exact low-density limit. The correlation potential musigmac(rs,zeta) is evaluated for use in self-consistent density-functional calculations.
The photocatalytic degradation of a series of (CH3)nNH(4-n)+ (0 < or = n < or = 4) was systematically studied in the UV-illuminated TiO2 aqueous suspensions at pH ranges of 3-11. By investigating the pH-dependent kinetics and analyzing intermediates and products, we elucidated the mechanistic pathways and the role of OH radicals in the photocatalytic oxidation. The deprotonated neutral species more rapidly degraded than their protonated counterparts for these homologous compounds because the OH radicals favorably reacted with the lone-pair electron on the nitrogen atom. Therefore, the photocatalytic degradation was highly enhanced at alkaline solutions for all substances except (CH3)4N+. The H-atom abstraction (from (CH3)4N+) by OH radicals initiated successive demethylation processes to generate tri-, di-, and monomethylammonium/amine as an intermediate and NH3/NH4+ as a final product. On the other hand, the OH-addition to the N-atom with the lone-pair electron led to NO2-/NO3- whose production was highly favored at alkaline conditions. The photocatalytic degradation rates of (CH3)4N+ were comparable at both acidic and alkaline conditions, which could not be explained by a simple electrostatic surface charge model. By using OH-scavenging tert-butyl alcohol as a diagnostic probe into the mechanism, it is suggested that the photocatalytic oxidation of (CH3)4N+ at acidic conditions proceeds through free OH radicals in the solution bulk, not on the surface of TiO2.
Photocatalytic degradations of alachlor in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Direct photolysis of alachlor was a rather slow process, but the addition of TiO2 enhanced the reaction rates by 12 and 26 times using 300 and 350 nm UV irradiation, respectively. The results showed that a low H2O2 dosage in photocatalysis using 300 nm UV would enhance the rates by 3.3 times, but an overdose of H2O2 will retard the rate due to the hydroxyl radicals are consumed. However, this process is impracticable at 350 nm due to the absorption characteristic of H2O2. A neutral initial pH level was found to favor the H2O2 assisted photocatalysis at 300 nm UV illumination. Eleven major intermediates were identified by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) and MS/MS. The major degradation mechanisms of H2O2-assisted alachlor photocatalysis include dechlorination, dealkylation, hydroxylation, cyclization, scission of C-O bond, and N-dealkylation. Bell-shaped evolution profiles of different intermediates were observed. Degradation pathways were proposed accordingly to illustrate series of degradation steps. The TOC analysis revealed the different stages of the reaction.
A novel ZnIn2S4 catalyst synthesized by hydrothermal method shows high and stable photocatalytic activity for water reduction under visible light illumination.
Novel nanostructured porous fibers of self-supported, radially aligned H2Ti8O17 x 1.5H2O nanorods were prepared from layered H2Ti4O9 x 1.2H2O tetratitanate fibers by novel solvothermal reaction in glycerine at 150-250 degrees C. The H2Ti8O17 x 1.5H2O fibers with diameters of 0.5-1.5 microm and lengths of 10-20 microm consist of multi-scale nanopores and nanostructures. They also are of high crystallinity, large surface area of 127 m2 g(-1), and stable phase up to 350 degrees C. Photocatalytic activity of the H2Ti8O17 x 1.5H2O fibers was evaluated in aqueous photooxidation of an azo dye methyl orange in the presence of UV irradiation and 02, using P-25 as the standard sample. Both the photocatalytic activity and the dispersity-agglomeration property of H2Ti8O17 x 1.5H2O fibers are pH-controllable. Highly photooxidative activity, superior to that of P-25, occurs at pH 6.0-11.0 due to certain distinguishable material characteristics and to large amounts of adsorbed reactants of surface active OH* free radicals, surface hydroxyl OH, O2*-, O*OH, and methyl orange. The agglomeration of H2Ti8O17 x 1.5H2O fibers becomes more serious from pH 2.0 to pH 5.0 and from pH 6.0 to pH 11.0. Well-dispersed H2Ti8O17 x 1.5H2O fibers occur at pH 6.0. Both the total photodegradation of waste chemicals and the entire sedimentation of H2Ti8O17 x 1.5H2O fibers can be timed to end simultaneously at suitable pH value. The photocatalyst-free reaction solution is then easily removed, and the fresh wastewater is added again. Standard unit operation processes of chemical engineering are used to design a continuous, low-cost, large-scale, liquid-phase photocatalysis technique based on the H2Ti8O17 x 1.5H2O fibers.
Green degradation: Organic contaminants such as volatile organic compounds and dyes (e.g. acetaldehyde and methylene blue (MB)) can be photocatalytically degraded over a novel CaBi2O4 catalyst. The reaction is environmentally friendly (O2 oxidant) and proceeds at room temperature under visible-light irradiation of wide-ranging wavelength (see picture: x = conversion of CH3CHO into CO2, A = absorbance).
Visible-light-induced photodegradation of rhodamine B over nanosized Bi2WO6 has been observed. Bi2WO6 exhibited a high photoactivity to photodegrade rhodamine B in the central pH solution under visible irradiation (lambda > 420 nm). After five recycles for the photodegradation of rhodamine B, the catalyst did not exhibit any significant loss of activity, confirming the photocatalyst is essentially stable. The total organic carbon measurement displayed that a high degree of mineralization was achieved in the present photochemical system. The results of density functional theory calculation illuminated that the visible-light absorption band in the Bi2WO6 catalyst is attributed to the band transition from the hybrid orbitals of Bi6s and O2p to the W5d orbitals. The Bi2WO6-assisted photocatalytic degradation of rhodamine occurs via two competitive processes: a photocatalytic process and a photosensitized process. The transformation of rhodamine is mainly via the photocatalytic process. Kinetic studies by using electron spin resonance and the radical scavenger technologies suggest that *OH is not the dominant photooxidant. Direct hole transfers and O2*- could take part in Bi2WO6 photocatalysis. This study provided a possible treatment approach for organic pollutants by using visible light in aqueous ecosystems.
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Band structures of Bi 2 WO 6 photocatalyst. The VB and CB levels of Bi 2 WO 6 are roughly estimated by Eq
- Fig
Fig. 12. Band structures of Bi 2 WO 6 photocatalyst. The VB and CB levels of
Bi 2 WO 6 are roughly estimated by Eq. (2).
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