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At present, carbon dioxide (CO2) is the largest contributor among greenhouse gases. This article addresses the potential application of photocatalysis to the reduction of CO2 emissions from industrial flue gas streams. Not only does this process remove CO2, but it can also convert CO2 into other chemical commodities such as methane, methanol, and e...
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... makes use of semiconductors to promote reactions in the presence of light radiation. 8 Unlike metals, which have a continuum of electronic states, semiconductors exhibit a void energy region, or band gap, that extends from the top of the filled valance band to the bottom of the vacant conduction band when exposed to light radiation (Figure 1). The generation of electron-hole pairs (e --h + ) and its reverse process are shown in eqs 1 and 2, respectively where hV is the photon energy, e -represents a conduction band electron, and h + represents a hole in the valence band. ...
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... the past decade, low-cost supported catalysts consisting of photoactive materials that deposit on transparent and inert bodies have been developed significantly. This improves not only the surface area of the fixed-bed reactor design but also the UV light penetration (Figure 10). 87,88 The proposed design is a bubble-flow fixed-bed column that is cylindrical and contains a bed of catalytic particles and a coaxial UV-lamp radiation source. ...
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... addition to the above reactors, a new photoelectrochemical cell has been designed that integrates three techniques: pho- tochemical, electrochemical, and membrane processes ( Figure 11). One half-cell of this reactor is a proton anode that splits water into one proton (H + ) plus diatomic oxygen and consists of sol-gel-deposited TiO 2 coated on a Ti metal substrate. ...
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... As well as in FT-IR analysis, the catalysts selected were the ones prepared with the HA/P ratio of 30 mol/mol for TDB, TIP, TBO, and TEO. Subsequently, from the spectra obtained, the absorption thresholds (λ lim ) were estimated, and the band gap energies were calculated as usual [2,27,53]. The results presented in Figure 10 show that the catalysts have absorption thresholds between 400 and 420 nm for all the precursors; however, the maximum values vary as follows: TIP > TDB = TBO > TEO. ...
The synthesis of TiO2 nanoparticles (NPs) in supercritical media has been reported over the last two decades. However, very few studies have compared the physicochemical characteristics and photoactivity of the TiO2 powders produced from different precursors, and even fewer have investigated the effect of using different ratios of hydrolytic agent/precursor (HA/P) on the properties of the semiconductor. To bridge this knowledge gap, this research focuses on the synthesis and characterization of TiO2 NPs obtained in a supercritical CO2 medium from four different TiO2 precursors, namely diisopropoxytitanium bis (acetylacetonate) (TDB), titanium (IV) isopropoxide (TIP), titanium (IV) butoxide (TBO), and titanium (IV) 2-ethylhexyloxide (TEO). Further, the effect of various HA/P ratios (10, 20, 30, and 40 mol/mol) when using ethanol as a hydrolytic agent has also been analyzed. Results obtained have shown that the physicochemical properties of the catalysts are not significantly affected by these variables, although some differences do exist between the synthesized materials and their catalytic performances. Specifically, photocatalysts obtained from TIP and TEO at the higher HA/P ratios (HA/P = 30 and HA/P = 40) led to higher CO2 photoconversions (6.3–7 µmol·g−1·h−1, Apparent Quantum Efficiency < 0.1%), about three times higher than those attained with commercial TiO2 P-25. These results have been imputed to the fact that these catalysts exhibit appropriate values of crystal size, surface area, light absorption, and charge transfer properties.
... With the development of industrialization, the increasing CO2 emission from human activities has caused serious greenhouse effects 1,2 . It is urged to exploit efficient and environmentalfriendly methods to reduce atmospheric CO2 levels. ...
Photocatalytic CO2 reduction to renewable hydrocarbon fuels provides a feasible protocol for alleviating the greenhouse effect and addressing energy shortage. However, the CO2 reduction activity of a single-component photocatalyst is very low because of two problems. One is the fast recombination of photogenerated charge carriers, which leads to low photon efficiency, while the other is the large energy barrier to CO2 activation. There have been considerable research efforts to develop photocatalysts with improved CO2 reduction performance. For example, step-scheme (S-scheme) heterojunctions have been developed to improve charge carrier separation and enhance the redox abilities of photocatalysts. Single-atom metals have also been applied cocatalysts to optimize the reaction thermodynamics. Thus, the synergy between S-scheme heterojunctions and single-atom metal cocatalysts is anticipated to promote both charge carrier transfer and CO2 reduction reaction processes. In this study, a Pt-C3N4/BiOCl heterojunction photocatalyst is modeled, composed of single-atom Pt-loaded g-C3N4 and BiOCl, and its photocatalytic properties are studied using density functional theory calculations. Its structure and electronic property are explored, and the process of CO2 conversion is also simulated. The charge density difference results show that electrons in g-C3N4 are transferred to BiOCl owing to the higher Fermi level of g-C3N4 than that of BiOCl. Therefore, an interfacial electric field from g-C3N4 to BiOCl is established at the g-C3N4/BiOCl interface. Under light irradiation, charge carrier transfer in the g-C3N4/BiOCl composite is consistent with the S-scheme mechanism. Specifically, the photogenerated electrons in the CB of BiOCl recombine with the photogenerated holes in the VB of g-C3N4, while the photogenerated electrons in the CB of g-C3N4 and the photogenerated holes in the VB of BiOCl are retained. After the loading of Pt atom at each sixfold cavity of g-C3N4, the work function of g-C3N4 decreases, thereby enlarging the difference between the Fermi levels of the two semiconductors. Consequently, more electrons are transferred from Pt-C3N4 to BiOCl, and the strength of the interfacial electric field is increased. This enhanced electric field is beneficial to the S-scheme charge transfer in Pt-C3N4/BiOCl heterojunctions. Besides, based on the calculated variation in reaction energy, the rate-limiting step involved in CO2 reduction on g-C3N4/BiOCl heterojunction is the hydrogenation of CO2 to COOH, which has an energy barrier of 1.13 eV. After Pt loading, the hydrogenation of CO to HCO is the rate-limiting step and the corresponding energy increase is 0.71 eV. These results manifest that the introduction of Pt single-atom cocatalysts improves the CO2 reduction performance of g-C3N4/BiOCl S-scheme photocatalysts by strengthening the interfacial electric field and reducing the energy barrier. This study provides guidance for constructing metal-atom-incorporated S-scheme heterojunction photocatalysts to realize efficient CO2 reduction.
... The reaction mechanism of CO 2 reduction is different from Eqs. (1)- (11). Since CO 2 is a highly inert molecule, the single electron reduction path is highly unfavorable and proton-assisted multi-electron steps are the more favourable way [85]. ZnO has a good efficiency of CO 2 photoreduction compared to TiO 2 [86]. ...
... Phenols are organic pollutant compounds that are harmful to the environment and human health [22,23]. The photocatalytic process is the process most often used for phenol degradation because it can degrade phenols into more environmentally friendly compounds namely CO2 and H2O at lower costs and less energy consumption [24,25,26]. The purpose of this article is to understand the basic aspects of spreading CNT with various surfactants (CTAB and cocoPAS) to form composite bonds with TiO2 to increase composite photoactivity in degrading phenols. ...
A series of CNT-TiO2 composite photocatalysts was successfully prepared by dispersing carbon nanotubes using surfactants (CTAB and cocoPAS). Composites are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared absorption spectroscopy (FTIR), Breuneur Emmet Teller (BET) and diffuse reflectance UV-vis spectroscopy. The purpose of this study were synthesize TiO2-CNT composite using surfactant and evaluate its performance for photodegradation phenol. The catalytic activity of this composite material was investigated by composite applications for phenol degradation. The as-prepared nanotubes, with surfactant contents, showed high- photocatalytic activities with increase degradation.
... Doping can modify the structure of the energy levels and improve catalytic activity [33,34]. The separation, migration and recombination of electrons and holes are the keys to the catalytic process, and the photocatalytic reduction of CO 2 is a surface catalysis reaction involving electrons and holes [35]. Based on traditional photothermal catalysis, TAS was used to further understand the difference between LCMO, LMNO and LNCO from the kinetic perspective. ...
... Doping can modify the structure of the energy levels and improve catalytic activity [33,34]. The separation, migration and recombination of electrons and holes are the keys to the catalytic process, and the photocatalytic reduction of CO2 is a surface catalysis reaction involving electrons and holes [35]. Based on traditional photothermal catalysis, TAS was used to further understand the difference between LCMO, LMNO and LNCO from the kinetic perspective. ...
To gain insight into photocatalytic behavior, transient absorption spectroscopy (TAS) was used to study LaCoxMn1−xO3, LaMnxNi1−xO3 and LaNixCo1−xO3 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) on a microsecond time scale. The results show that the electron lifetime is key to determining the photocatalytic reduction of CO2. This is the first time that the photogenerated electron lifetime in perovskite has been proposed to express the performance of the photocatalytic reduction of CO2 with H2O into CH4. In all cases, the decay curve can be well explained by two consecutive first-order kinetics, indicating that the electron exists within two major populations: one with a short lifetime and the other one with a long lifetime. The long-lived electrons are the rate-limiting species for the photocatalytic reaction and are related to the activity of the photocatalytic reduction of CO2 with H2O to produce CH4. For different photocatalysts, we find that the longer the electron decay lifetime is, the stronger the electron de-trapping ability is, and the electrons perform more activity. In this paper, TAS can not only detect the micro-dynamics process of carriers, but it is also demonstrated to be an easy and effective method for screening the most active catalyst in various catalysts for the photocatalytic reduction of CO2 with H2O accurately and quickly.
... This figure demonstrates how the addition of CdS to ZnO nanofiber increases its solar absorbance, and how the ZnO-xCdS samples' absorption intensities get better as the concentration (x) goes up.This is due to the fact that CdS has better photocatalytic activity thanZnO and a lower energy band gap than ZnO, which increases the samples' absorption intensity. Light with a wavelength below a threshold is absorbed by semiconductor materials (fundamental absorption edge )[21]. Pure ZnO has an absorbing edge below 330.8 nm, whereas ZnO-0.25CdS ...
In recent years element doping has been extensively employed to modulate the physicochemical properties of semiconductors. In this work, new Pure ZnO and ZnO-xCdS composite nanofibers synthesized through the electrospinning technique, where x= (0.25, 0.5, and 1). Investigated were the effects of doping a ZnO nanofiber composite with varying concentrations of CdS nanoparticles.
... Finally, the treated gas passes through a catalytic conversion device for catalytic conversion. This structured photothermal reactor is 10-100 times more efficient than other catalyst carriers in terms of catalyst exposed area, catalytic reaction activity, and light usage 110 . Recently, Aldo Steinfeld et al. designed a tower production unit using H2O, CO2, and solar energy to make aviation kerosene 111 . ...
... [53][54][55] As a result, it has been noted in numerous studies that the degradation rate at low levels of light intensities increases in direct proportion to the intensity of light, while the rate at moderate levels of light intensity relies on the square root of the light intensity, and the degradation rate at high intensities of light remains unaffected. 53,56 The complicated mechanisms that the CRR employs are still being fully understood. As a result of insufficient CO 2 conversion, a variety of products are produced, which may have low purity and concentration. ...
Emission of greenhouse gases (GHG) by fossil fuels surged at a fast pace ensuing rise in Earth’s temperature, altogether cause the most dangerous impact, i.e., global warming. Endless efforts are...
... Photo-catalytically converting CO 2 into valuable fuels is a promising approach, since it could mitigate the climate issues caused by greenhouse gas CO 2 and store the renewable solar energy as chemical energy simultaneously [6][7][8][9]. The products of CO 2 photocatalytic conversion reactions include CO [10][11][12], CH 4 [13,14], CH 3 OH [15,16], C 2 H 5 OH [17][18][19][20], HCOOH [21,22], etc. Noteworthily, C 2 H 5 OH (ethanol) is a chemical with wide applications in the chemical industry, medical and healthcare industries, food industry, agriculture production, and so on. Therefore, photocatalytic CO 2 conversion to ethanol has recently become a research hotspot. ...
Photo-catalytically converting the greenhouse gas CO2 into ethanol is an important avenue for the mitigation of climate issues and the utilization of renewable energies. Catalysts play critical roles in the reaction of photocatalytic CO2 conversion to ethanol, and a number of catalysts have been investigated, including semiconductors and plasmonic metal-based catalysts, as well as several other catalysts. In this review, the progress in the development of each category of catalysts is summarized, the current status is reviewed, the remaining challenges are pointed out, and the future research directions are prospected, with the aim being to pave pathways for the rational design of better catalysts.
... Fossil fuels are a vital source of energy for human beings [1]; however, the use of fossil fuels releases CO 2 to accumulate in the atmosphere, leading to a serious greenhouse effect [2,3]. Finding efficient ways to reduce CO 2 emissions or capture CO 2 and convert it into chemical commodities is the key problem to solve [4][5][6][7]. ...
Nonmetallic co-doping and surface hole construction are simple and efficient strategies for improving the photocatalytic activity and regulating the electronic structure of g-C3N4. Here, the g-C3N4 catalysts with B-F or B-S co-doping combined with nitrogen vacancies (Nv) are designed. Compared to the pristine g-C3N4, the direction of the excited electron orbit for the B-F-co-doped system is more matching (N2pz→C2pz), facilitating the separation of electrons and holes. Simultaneously, the introduced nitrogen vacancy can further reduce the bandgap by generating impurity states, thus improving the utilization rate of visible light. The doped S atoms can also narrow the bandgap of the B-S-Nv-co-doped g-C3N4, which originates from the p-orbital hybridization between C, N, and S atoms, and the impurity states are generated by the introduction of N vacancies. The doping of B-F-Nv and B-S-Nv exhibits a better CO2 reduction activity with a reduced barrier for the rate-determining step of around 0.2 eV compared to g-C3N4. By changing F to S, the origin of the rate-determining step varies from *CO2→*COOH to *HCHO→*OCH3, which eventually leads to different products of CH3OH and CH4, respectively.
