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Exploring the Size Effect of Pt Nanoparticles on the Photocatalytic Nonoxidative Coupling of Methane
Abstract
The high inertness of the C-H bond makes the photocatalytic methane conversion a significant challenge. The platinum nanoparticle is a promising cocatalyst for CH4 activation, while the study of its structure characteristics and functionality remains in its infancy. Herein, the size effect of Pt on the photocatalytic nonoxidative methane conversion efficiency was systematically investigated over x-Pt/Ga2O3 with the particle size (x) ranging from 1.5 to 2.7 nm, where a volcano-shaped relation was observed. The smaller size is beneficial to the formation of Ptδ+ species, which is mainly distributed on the terrace sites according to the DFT calculation. The corner Pt atom is the geometric active site for the CH4 polarization, and the terrace Ptδ+ helps promote C-H activation since the activity is decreased on reduced x-Pt/Ga2O3 with a lower Ptδ+ content. Meanwhile, Ptδ+ species favors the oxidation of adsorbed -CH3 group to ·CH3. The volcano-shaped size effect on the NOCM activity was finally rationalized by the balance between C-H activation and C2H6 desorption from the corner sites on different sized Pt.
... Pt nanoparticles were subsequently photodeposited onto the surface of the Ga2O3 microrods to achieve well-controlled sizes of 1.5, 1.9, 2.2, 2.5, and 2.7 nm, resulting in highly dispersed Pt nanoparticle distribution ( Figure 10C,D). The particle size of the Pt nanoparticles extended the absorption range into the visible-light region and increased the band gap of the composite material [118]. For instance, in methane oxidation, Ni/CeO 2 nanoparticles, nanorods, and nanocubes were utilized in methane oxidation [116]. ...
... Pt nanoparticles were subsequently photodeposited onto the surface of the Ga 2 O 3 microrods to achieve well-controlled sizes of 1.5, 1.9, 2.2, 2.5, and 2.7 nm, resulting in highly dispersed Pt nanoparticle distribution ( Figure 10C,D). The particle size of the Pt nanoparticles extended the absorption range into the visible-light region and increased the band gap of the composite material [118]. (5)). ...
... Figure 10E,F illustrate the selectivity and yield to ethane, hydrogen, and propane. Ga 2 O 3 microrods exhibited lower values, while the incorporation of Pt nanoparticles strongly influenced the photocatalytic activity, leading to improved performance [118]. ...
This study investigates the utilization of controlled nanocatalysts in methane conversion reactions, addressing the pressing need for the efficient utilization of methane as a feedstock for valuable chemicals and clean energy. The methods employed include a comprehensive review of recent advancements in nanocatalyst synthesis, characterization, and application, as well as the critical analysis of underlying mechanisms and controversies in methane activation and transformation. The main findings reveal significant progress in the design and synthesis of controlled nanocatalysts, enabling enhanced activity, selectivity, and stability in methane conversion reactions. Moreover, the study highlights the importance of resolving controversies surrounding metal–support interactions for rational catalyst design. Overall, the study underscores the pivotal role of nanotechnology in shaping the future of methane utilization and sustainable energy production, providing valuable insights for guiding future research directions and technological developments in this field.
... In the following sections, we will review nanoscale engineering strategies of Pd-based electrocatalysts for improving the catalytic performance toward formic acid/formate products, including size control, morphology and shape control, alloying, heteroatom doping, surface-strain engineering, and phase control. To aid readers in quickly capturing valuable and recapitulative information from related literature addressed in the following sections [31,34,40,[65][66][67][68][69][70][71][72][73][74] , we outlined the performances (potential, selectivity, current density, and stability) of various Pd-based related catalysts and their measurement conditions, including reactors and electrolyte solution, as shown in Table 1. ...
... In recent years, researchers have devoted tremendous efforts to finely designing highperformance Pd-based electrocatalysts through alloying. A variety of bimetallic Pd alloys with other transition metals, such as tin (Sn) [31,97] , silver (Ag) [33,40,68] , gold (Au) [73,98,99] , copper (Cu) [67,100,101] , bismuth (Bi) [34,74] , lead (Pb) [102] , etc., have been synthesized and displayed enhanced catalytic performance for CO 2 -to-formic acid/formate conversion. ...
... Jia et al. synthesized Pd 3 Bi intermetallic alloys (IMAs) with a fixed stoichiometry of 3/1 and uniform sizes by adopting a facile solvothermal method [74] . It is well accepted that a solvothermal method provides a versatile approach to synthesizing intermetallic compounds. ...
Electrochemical conversion of carbon dioxide (CO2) into high-value chemicals and fuels driven by electricity derived from renewable energy has been recognized as a promising strategy to achieve carbon neutrality and create sustainable energy. Particularly from the viewpoint of the product values and the economic viability, selective CO2 reduction to formic acid/formate has shown great promise. Palladium (Pd) has been demonstrated as the only metal that can produce formic acid/formate perfectly near the equilibrium potential; yet, it still suffers from CO poisoning, poor stability and competitive CO pathway at high overpotentials. Herein, recent progress of Pd-based electrocatalysts for selective CO2 electroreduction and their mechanistic understanding are reviewed. First, the fundamentals of electrochemical CO2 reduction and the reaction pathway of formic acid/formate on Pd are presented. Then, recent advances in the rational design and nanoscale engineering strategies of Pd-based electrocatalysts for further improving CO2 reduction activity and selectivity to formic acid/formate product, including size control, morphology and shape control, alloying, heteroatom doping, surface-strain engineering, and phase control, are discussed from the perspectives of both experimental and computational aspects. Finally, we discuss the pertinent challenges and describe the future prospects and opportunities in terms of the development of electrocatalysts, electrolyzers and characterization techniques in this research field.
... In addition, some other studies have found that an increase in CH4 pressure in the reaction system (from 0.27 kPa to 0.49 KPa) can reduce the catalytic activity [36]. [46], copyright 2021, American Chemical Society. (c) C 2 H 6 yield over TiO 2 catalysts supported by different noble metals in NOCM [53], and (d) IR spectra over Au/TiO 2 and P25 [53]. ...
... Zhang et al. [46] studied the influence of the size effect of the active metal Pt. The catalytic activity showed a volcanic trend when the crystal size of Pt increased from 1.5 nm to 2.7 nm, and the maximum selectivity of C 2 H 6 reached 90%. ...
... (a) Production rates on Pd/Ga2O3 with different light intensities[32], copyright 2011, Elsevier. (b) Photocatalytic reaction conversion and selectivity of Pt/Ga2O3[46], copyright 2021, American Chemical Society. (c) C2H6 yield over TiO2 catalysts supported by different noble metals in NOCM ...
Methane is the fundamental raw material of the C1 chemical industry, with abundant reserves. Its direct conversion into high-value-added chemicals has great scientific significance and broad commercial potential for the efficient use of methane resources. However, it is difficult to convert methane into more useful hydrocarbons and hydrogen, as the reaction usually requires external energy to overcome thermodynamic limitations. Non-oxidative coupling of methane to produce ethane and hydrogen is a promising supply technology. Catalysts which can be adapted to various energy sources are key to this technology. In recent years, considerable progress has been made in the design and application of these thermal and photocatalysts. This review outlines some typical catalysts, and reviews the progress in the understanding of reaction mechanisms. Finally, suggestions for the development of high-selectivity and high-stability catalysts for the future are presented.
... Recent studies have focused on controlling the shapes and sizes of metal nanoparticles, including Ag. Under certain experimental conditions, the size of Ag nanoparticles can be controlled [45,46], influencing the LSPR effect, similar to other metals such as platinum [47], aurum [48], and ruthenium [49]. ...
... From previous research by Flores-Rojas et al. [63], given a certain dose irradiation, lower dose rate induces larger particle size of Ag NPs because the production rate of reducing free radicals is slower than the association of ions with atoms. As shown in TEM image (Figure 3a-c), Ag NPs are formed in the shape of sphere, and the reduced particle sizes lead to bigger specific surface area and contact area with RhB and greatly enhance the surface-to-volume ratio of photocatalysts, which hence would improve the photocatalytic activity of Ag/P25 nanocomposites [40,47]. ...
Titanium dioxide (TiO2) has garnered significant attention among various photocatalysts, whereas its photocatalytic activity is limited by its wide bandgap and inefficient charge separation, making the exploration of new strategies to improve its photocatalytic performance increasingly important. Here, we report the synthesis of Ag/P25 nanocomposites through a one-step gamma-ray radiation method using AgNO3 and commercial TiO2 (Degussa P25). The resulting products were characterized by powder X-ray diffraction, UV-Vis diffused reflectance spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The effect of free radical scavengers, feed ratios of Ag/P25, and dose rates on the photocatalytic activity of the Ag/P25 nanocomposites were systematically investigated using rhodamine B under Xenon light irradiation. The results showed that the Ag/P25 photocatalyst synthesized with a feed ratio of 2.5 wt% and isopropyl alcohol as the free radical scavenger at a dose rate of 130 Gy/min exhibited outstanding photocatalytic activity, with a reaction rate constant of 0.0674 min−1, much higher than that of P25. Additionally, we found that the particle size of Ag could be effectively controlled by changing the dose rate, and the Ag/P25 nanocomposites doped with smaller size of Ag nanoparticles performed higher photocatalytic activities. The synthesis strategy presented in this study offers new insight into the future development of highly efficient photocatalysts using radiation techniques.
... We compared the reaction performance for NOCM of our Fe 3 + -containing photochemical system with the reported representative works ( Figure 2c and Table S1). [7][8][9][10][11][12][13][14][15][16][17][18][19] Our system achieved a significantly higher C 2 + formation rate and utilized less expensive material and an energy-efficient LED lamp. ...
... c) Comparison of reaction performance for NOCM of the Fe 3 + -containing photochemical system with the reported representative works. [7][8][9][10][11][12][13][14][15][16][17][18][19] d) Photochemical performance of Fe 3 + for hydroxylation of benzene and conversion of cyclohexane to cyclohexanol and cyclohexanone. ...
Photo‐driven CH4 conversion to multi‐carbon products and H2 is attractive but challenging, and the development of efficient catalytic systems is critical. Herein, we construct a solar‐energy‐driven redox cycle for combining CH4 conversion and H2 production using iron ions. A photo‐driven iron‐induced reaction system was developed, which is efficient at selective coupling of CH4 as well as conversion of benzene and cyclohexane under mild conditions. For CH4 conversion, 94 % C2 selectivity and a C2H6 formation rate of 8.4 μmol h⁻¹ is achieved. Mechanistic studies reveal that CH4 coupling is induced by hydroxyl radical, which is generated by photo‐driven intermolecular charge migration of an Fe³⁺ complex. The delicate coordination structure of the [Fe(H2O)5OH]²⁺ complex ensures selective C−H bond activation and C−C coupling of CH4. The produced Fe²⁺ can be used to reduce the potential for electrolytic H2 production, and then turns back into Fe³⁺, forming an energy‐saving and sustainable recyclable system.
... [16][17][18] However, when the particle size is reduced to the nanoscale, the electronic and geometrical structure of the metal particles will be quite different from those of bulkphase metals and change significantly with the particle size, which is the so-called size effect. 19,20 Taking Ru as a typical example, the adsorption of hydrogen on Ru exhibits a certain dependence on the size of Ru. 21,22 Unlike bulk Ru, the adsorption of hydrogen on Ru nanoclusters is too strong, resulting in poor HER activity. To improve the HER performance of Ru clusters, various supports have been developed such as carbon nanomaterials, 23,24 metal carbides, 25 metal oxides, 26 metal hydroxides 27 and so on to regulate the electronic structure of Ru clusters. ...
Regulating the metal–support interaction (MSI) is an effective strategy to enhance the catalytic activity of electrocatalysts. Herein, taking Ru clusters as an example, we report a hybrid electrocatalyst with ultrafine Ru nanoclusters anchored on sulfur and nitrogen co-doped carbon (Ru/SNC) hollow spheres for efficient hydrogen evolution reaction (HER) in an alkaline electrolyte and real seawater. The optimal Ru/SNC hollow spheres on a glassy carbon electrode exhibit superior HER activity, with small overpotentials of only 12 and 30 mV to reach 10 mA cm⁻² in alkaline media and alkaline real seawater, respectively. When loaded on carbon paper, the Ru/SNC hollow spheres only need small overpotentials of 171 (in alkaline solution) and 205 mV (in alkaline real seawater) to deliver an industrial current density of 1000 mA cm⁻². Furthermore, the assembled Ru/SNC||RuO2 electrolysis cell displays a high current density of 1000 mA cm⁻² at a cell voltage of 2.3 V and impressive stability up to 100 h at a current density of 1000 mA cm⁻² in alkaline real seawater at an elevated temperature of 80 °C. Density functional theory (DFT) calculations suggest that S-doping can induce a strong MSI between Ru clusters and the carbon support to boost the HER activity and stability. S-doping triggers the downshift of the d-band center, weakening the adsorption of H* on Ru clusters and thereby enhancing the hydrogen spillover.
... 58 The Wang research group has conducted systematic studies on the size effect of Pt particles with diameters (x) ranging from 1.5 to 2.7 nm on x-Pt/Ga 2 O 3 for PNOCM. 59 Our research group has developed a ''single-atom Pt collection'' modified black TiO 2 (Pt@BT-O) using a ''reductionoxidation-reconstruction'' strategy, as depicted in Fig. 9(A). 60 The surface of this black TiO 2 is densely populated with subnanometer Pt collections, each averaging 0.6-0.8 ...
This review critically assesses advances in photocatalytic non-oxidative methane conversion, offering deep insights and guiding future studies in this vital, evolving field.
... Pt-based cocatalysts were more effective than Rh, Au, Pd and Ni for enhancing the photocatalytic performance of β-Ga 2 O 3 . Later, Ma et al. [115] investigated the size effect of Pt cocatalysts on the efficiency of photocatalytic nonoxidative CH 4 conversion over Pt-loaded Ga 2 O 3 . Pt/Ga 2 O 3 was prepared via the annealing treatment of GaOOH followed by the photo-deposition of Pt (Figure 4a, 4b). ...
Photocatalysis, which performed under mild conditions by utilizing solar energy, has become a desirable technology to convert methane into highly valuable chemicals, such as methanol, ethane, and other hydrocarbons. However, pristine photocatalysts still suffer from the low utilization efficiency of solar light and the high recombination rate of photogenerated charge carriers, which exhibit the low activity and selectivity for photocatalytic methane conversion. Loading cocatalysts on photocatalysts is an attractive strategy to manipulate the products’ yield and selectivity of photocatalytic methane conversion due to the enhanced charge carrier separation efficiency, extended light absorption and promoted reactant adsorption/desorption kinetics. This review discusses the recent achievements of the cocatalysts for photocatalytic methane conversion reactions. Moreover, the challenges and perspectives for the development of efficient cocatalysts are presented. This review provides considerable guidelines for the design and construction of efficient cocatalysts for photocatalytic methane conversion reactions.
... Methane is an important source of energy as well as a chemical raw material, which comes from vast renewable and fossil resources [1][2][3][4] . Methane is a greenhouse gas (GHG) with its warming potential on the climate 30 times more significant 5,6 than carbon dioxide. ...
... In addition to the types of cocatalysts, the sizes also play an important role in the efficiency of methane conversion. Through a photodeposition method, Ma et al. synthesized different-sized Pt-loaded Ga 2 O 3 by simply regulating the amount of H 2 PtCl 6 ·6H 2 O. 109 With the particle size ranging from 1.5 to 2.7 nm, the methane conversion rate showed a volcano-shaped trend, which reached the highest when the size is 1.9 nm. The corner sites of Pt nanoparticles are inferred to be the main active sites according to the normalized TOF of different geometric sites. ...
With 28-34 times the greenhouse effect of CO2 over a 100-year period, methane is regarded as the second largest contributor to global warming. Reducing methane emissions is a necessary measure to limit global warming to below 1.5 °C. Photocatalytic conversion of methane is a promising approach to alleviate the atmospheric methane concentrations due to its low energy consumption and environmentally friendly characteristics. Meanwhile, this conversion process can produce valuable chemicals and liquid fuels such as CH3OH, CH3CH2OH, C2H6, and C2H4, cutting down the dependence of chemical production on crude oil. However, the development of photocatalysts with a high methane conversion efficiency and product selectivity remains challenging. In this review, we overview recent advances in semiconductor-based photocatalysts for methane conversion and present catalyst design strategies, including morphology control, heteroatom doping, facet engineering, and cocatalysts modification. To gain a comprehensive understanding of photocatalytic methane conversion, the conversion pathways and mechanisms in these systems are analyzed in detail. Moreover, the role of electron scavengers in methane conversion performance is briefly discussed. Subsequently, we summarize the anthropogenic methane emission scenarios on earth and discuss the application potential of photocatalytic methane conversion. Finally, challenges and future directions for photocatalytic methane conversion are presented.
... On the other hand, with the continuous discovery of abundant methane (CH 4 ) resources, especially shale/natural gas, the direct CH 4 conversion into value-added chemicals such as methanol, formaldehyde and formic acid offers considerable economic and environmental benefits [3][4][5][6][7] . However, due to the high C-H bond dissociation energy (439 kJ·mol −1 ), CH 4 serves as the most stable and inert industrial feedstock among alkanes [8][9][10][11][12] , and its industrial utilisation through indirect steam-reforming and subsequently Fischer-Tropsch synthesis is usually energy-intensive due to the high operating temperature (700-1100°C) [13][14][15][16] . Therefore, sustainable CH 4 utilisation under mild conditions is highly desirable. ...
Direct solar-driven methane (CH4) reforming is highly desirable but challenging, particularly to achieve a value-added product with high selectivity. Here, we identify a synergistic ensemble effect of atomically dispersed copper (Cu) species and partially reduced tungsten (Wδ+), stabilised over an oxygen-vacancy-rich WO3, which enables exceptional photocatalytic CH4 conversion to formaldehyde (HCHO) under visible light, leading to nearly 100% selectivity, a very high yield of 4979.0 μmol·g⁻¹ within 2 h, and the normalised mass activity of 8.5 × 10⁶ μmol·g⁻¹Cu·h⁻¹ of HCHO at ambient temperature. In-situ EPR and XPS analyses indicate that the Cu species serve as the electron acceptor, promoting the photo-induced electron transfer from the conduction band to O2, generating reactive •OOH radicals. In parallel, the adjacent Wδ+ species act as the hole acceptor and the preferred adsorption and activation site of H2O to produce hydroxyl radicals (•OH), and thus activate CH4 to methyl radicals (•CH3). The synergy of the adjacent dual active sites boosts the overall efficiency and selectivity of the conversion process.
... A strong polarization environment is the prerequisite for efficient C-H activation 24,26,27 . However, the polarization sites based on adjacent lattice atoms of semiconductor photocatalysts are generally incompetent to activate the C-H bond according to the previously reported low conversion rates [28][29][30] . ...
Photocatalytic methane conversion requires a strong polarization environment composed of abundant activation sites with the robust stretching ability for C-H scissoring. High-density frustrated Lewis pairs consisting of low-valence Lewis acid Nb and Lewis base Nb-OH are fabricated on lamellar Nb2O5 through a thermal-reduction promoted phase-transition process. Benefitting from the planar atomic arrangement of lamellar Nb2O5, the frustrated Lewis pairs sites are highly exposed and accessible to reactants, which results in a superior methane conversion rate of 1456 μmol g⁻¹ h⁻¹ for photocatalytic non-oxidative methane coupling without the assistance of noble metals. The time-dependent DFT calculation demonstrates the photo-induced electron transfer from LA to LB sites enhances their intensities in a concerted way, promoting the C-H cleavage through the coupling of LA and LB. This work provides in-depth insight into designing and constructing a polarization micro-environment for photocatalytic C-H activation of methane without the assistance of noble metals.
... Ma and colleagues studied the effect of Pt nanocrystal size on the efficiency of the photocatalytic nonoxidative conversion of methane to ethane. 42 They studied Pt particle sizes ranging from 1.5 to 2.7 nm on a Ga 2 O 3 support and observed a maximum in the CH 4 conversion rate as a function of particle size. Xiao et al. studied the copper-catalyzed deoxygenation reaction of aromatic epoxides catalyzed by various Cu nanomaterials. ...
A significant challenge in the development of functional materials is understanding the growth and transformations of anisotropic colloidal metal nanocrystals. Theory and simulations can aid in the development and understanding of anisotropic nanocrystal syntheses. The focus of this review is on how results from first-principles calculations and classical techniques, such as Monte Carlo and molecular dynamics simulations, have been integrated into multiscale theoretical predictions useful in understanding shape-selective nanocrystal syntheses. Also, examples are discussed in which machine learning has been useful in this field. There are many areas at the frontier in condensed matter theory and simulation that are or could be beneficial in this area and these prospects for future progress are discussed.
... Methane, the main component of natural and shale gas, gas hydrate, and biogas is a promising feedstock for the chemical industry but, at the same time, an extremely inert molecule. [1][2][3][4][5][6] The chemical stability of methane is closely related to high C-H bond energy (439 kJ mol À1 ) and its symmetric tetrahedral molecular geometry, which lead to low polarizability, weak acidity, and low affinity for electrons and protons. [7][8][9] As a result, methane is currently burned for energy production and accounts for 20%-25% of global carbon dioxide emissions into the atmosphere. ...
Direct conversion of methane into fuels and chemicals remains a major challenge in modern science. Formic acid is one of the most
promising platform molecules. Photocatalysis proposes an attractive
route for methane partial oxidation under mild conditions.
The radical mechanism of methane photocatalytic oxidation restricts
the selectivity to target products. In this article, we propose
a strategy to break conventional limitations of methane photocatalytic
oxidation by adding a thermocatalyst and conducting the
process in a one-pot reactor. In this strategy, the methane selective
conversion into formic acid proceeds first over cesium salt of
phosphotungstic acid on titania, which photocatalytically oxidizes
methane into a mixture of C1 oxygenates. These oxygenates are
then selectively converted into formic acid over a heterogeneous
alumina-supported ruthenium catalyst. All reactions occur at room
temperature in the same reactor. A selectivity to formic acid of
85% and a productivity of 5 mmol/g photocatalyst are achieved.
The employment of single atoms (SAs), especially Pt SAs, as co‐catalysts in photocatalytic H2 generation has gained significant attention due to their exceptional efficiency. However, a major challenge in their application is the light‐induced agglomeration of these SAs into less active nanosized particles under photocatalytic conditions. This study addresses the stability and reactivity of Pt SAs on TiO2 surfaces by investigating various post‐deposition annealing treatments in air, Ar, and Ar‐H2 environments at different temperatures. It is described that annealing in an Ar‐H2 atmosphere optimally stabilizes SA configurations, forming stable 2D rafts of assembled SAs ≈0.5–1 nm in diameter. These rafts not only resist light‐induced agglomeration but also exhibit significantly enhanced H2 production efficiency. The findings reveal a promising approach to maintaining the high reactivity of Pt SAs while overcoming the critical challenge of their stability under photocatalytic conditions.
Breakage of the C–N bond is a structure sensitive process, and the catalyst size significantly affects its activity. On the active metal nanoparticle scale, the role of catalyst size in C–N bond cleavage has not been clearly elucidated. So, Ru catalysts with variable nanoparticle sizes were obtained by modulating the reduction temperature, and the catalytic activity was evaluated using 1,2,3,4-tetrahydroquinoline and o-propylaniline with different C–N bond hybridization patterns as reactants. Results showed a 13 times higher reaction rate for sp3-hybridized C–N bond cleavage than sp2-hybridized C–N bond cleavage, while the reaction rate tended to increase first and then decrease as the catalyst nanoparticle size increased. Different concentrations of terrace, step, and corner sites were found in different sizes of Ru nanoparticles. The relationship between catalytic site variation and C–N bond cleavage activity was further investigated by calculating the turnover frequency values for each site. This analysis indicates that the variation of different sites on the catalyst is the intrinsic factor of the size dependence of C–N bond cleavage activity, and the step atoms are the active sites for the C–N bond cleavage. When Ru nanoparticles are smaller than 1.9 nm, they have a strong adsorption effect on the reactants, which will affect the catalytic performance of the Ru catalyst. Furthermore, these findings were also confirmed on other metallic Pd/Pt catalysts. The role of step sites in C–N bond cleavage was proposed using the density function theory calculations. The reactants have stronger adsorption energies on the step atoms, and step atoms have d-band center nearer to the Fermi level. In this case, the interaction with the reactant is stronger, which is beneficial for activating the C–N bond of the reactant.
Photocatalytic oxidation of methane to value-added chemicals is a promising process under mild conditions, nevertheless confronting great challenges in efficiently activating C–H bonds and inhibiting over-oxidation. Herein, we propose a comprehensive strategy for the selective generation of reactive oxygen species (ROS) by regulating the sizes and facets of Au nanoparticles loaded on ZnO. For photocatalytic methane oxidation at ambient temperature, a high oxygenates yield of 36.4 mmol·g−1·h−1 with a nearly 100% selectivity has been achieved over the optimized 1.0% Au/ZnO-9.6 (1% Au with (111) facet and 9.6 nm size on ZnO) photocatalyst, exceeding most reported literatures. Mechanism investigations reveal that 1.0% Au/ZnO-9.6 with the medium size and Au (111) facet guarantees the favourable formation of superoxide radicals (·OOH) through mild oxygen reduction, ultimately leading to excellent photocatalytic methane oxidation performance. This work provides some guidance for the delicate design of photocatalysts for efficient photocatalytic methane oxidation and oxygen utilization.
We propose a universal, green, and surfactant-free strategy to synthesize noble metal particles with high monodispersity using gases H2 as a reducing agent in solution at 60℃. The prepared Pt...
Unique morphological structure and tunable physicochemical properties have made cluster-catalysts promising candidates to effectively drive artificial solar fuel generation. However, such catalysts still suffer from instability during reaction, while durability...
Achieving high photocatalytic activity with the lowest possible platinum (Pt) consumption is crucial for reducing the cost of Pt-based cocatalysts and enabling large-scale applications. Bimetallic Ni–Pt cocatalysts exhibit excellent photocatalytic performance and are considered one of the most promising photocatalysts capable of replacing pure Pt for hydrogen evolution reaction (HER). However, the synergistic photocatalytic mechanism between bimetallic Ni–Pt cocatalysts needs to be further investigated. Herein, we deposit highly dispersed Ni–Pt bimetallic cocatalysts on the surface of TiO2 by radiolytic reduction. We study the dynamics of photogenerated charge carriers of the Ni–Pt-comodified TiO2 and propose their underlying electron transfer mechanisms, in which Pt acts as an electron trap, whereas Ni serves as an electron supplier. The synergistic effect is Ni/Pt ratio-dependent and can confer bimetallic Ni–Pt to pure Pt-like photocatalytic activity in HER. The Ni2–Pt1-comodified TiO2 is optimized to be the most cost-effective photocatalyst with robust stability, which exhibits about 40-fold higher performance than bare TiO2.
Methane (CH4) is the world's second most potent greenhouse gas after carbon dioxide (CO2). Removal of low concentrations of CH4 from the environment by an advanced oxidation process (i.e., total...
Hollow structure hybrids have gained considerable attention for their ability to reduce CO2 owing to their rich active sites, high gas adsorption ability, and excellent light utilization capacity. Herein, a template-engaged strategy was provided to fabricate copper sulphide@cerium dioxide (CuS@CeO2) p-n heterojunction hollow cube photocatalysts using Cu2O cubes as a sacrificial template. The sequential steps of loading of CeO2 nanolayer, sulfidation, and etching reaction facilitate the formation of CuS@CeO2 p-n heterojunction hollow cubes. Compared with the single CuS, CeO2, and their physical mixture, the CuS@CeO2 p-n heterojunction hollow cube photocatalyst expresses a higher performance toward photocatalytic CO2 reduction under solid-gas reaction conditions due to the faster separation of photogenerated charges. The further enhanced performance of CuS@CeO2 p-n heterojunction hollow cubes was achieved by decorating pt nanoparticles due to the fact that Pt nanoparticles had a high electron affinity and CO2 adsorption capacity, and the highest CO and CH4 yields of the optimized hybrid reached 195.8 μmol g-1 h-1 and 19.96 μmol g-1 h-1, respectively. This work might provide a strategy for designing and synthesizing efficient hollow heterostructured photocatalysts for solar energy conversion and utilization.
In this paper, a series of Au-Ru-CeO2 aerogel-based photocatalysts are constructed for the nonoxidative methane coupling reaction. A modified epoxide-adding method is employed to incorporate Au and Au-Ru nanoparticles into the CeO2 aerogel framework. While the pristine CeO2 shows negligible activity, it is found that the incorporation of Au nanoparticles can greatly enhance the photoactivity, which is attributed to the improved charge separation efficiency and the surface plasmon resonance effect of Au. The nonoxidative methane coupling reaction then becomes feasible in the visible light region. The C2H6 yield rate on Au-CeO2 aerogel photocatalyst can reach 3.17 μmol/g/h under full-spectrum light and 0.48 μmol/g/h under visible light. Adding an appropriate amount of Ru to the above composite during the synthesis can further improve the activity. By screening several Au-Ru-CeO2 aerogel photocatalysts, an optimized Au-to-Ru ratio (10:1) is found. The C2H6 yield rate on Au10Ru1-CeO2 aerogel photocatalyst can be further increased to 7.09 and 1.55 μmol/g/h, under the full-spectrum light and visible light, respectively. Theoretical calculation shows that a small amount of Ru can lower the reaction barrier of H2 evolution and facilitate the overall reaction.
In recent years, the use of single atoms (SAs) has become of a rapidly increasing significance in photocatalytic H2 generation; here SA noble metals (mainly Pt SAs) can act as highly effective co-catalysts. The classic strategy to decorate oxide semiconductor surfaces with maximally dispersed SAs relies on "strong electrostatic adsorption" (SEA) of suitable noble metal complexes. In the case of TiO2 - the classic benchmark photocatalyst - SEA calls for an adsorption of cationic Pt complexes such as [(NH3 )4 Pt]2+ which then are thermally reacted to surface bound SAs. While SEA is widely used in literature, in the present work we show by a direct comparison that reactive attachment based on the reductive anchoring of SAs, e.g. from hexachloroplatinic(IV) acid (H2 PtCl6 ) leads directly to SAs in a configuration with a significantly higher specific activity than SAs deposited with SEA - and this at a significantly lower Pt loading and without any thermal post-deposition treatments. Overall, the work demonstrates that the reactive deposition strategy is superior to the classic SEA concept as it provides a direct electronically well-connected SA-anchoring and thus leads to highly active single-atom sites in photocatalysis. This article is protected by copyright. All rights reserved.
Photo‐driven CH 4 conversion to multi‐carbon products and H 2 is attractive but challenging, and the development of efficient catalytic systems is critical. Herein, we construct a solar‐energy‐driven redox cycle for combining CH 4 conversion and H 2 production using iron ions. A photo‐driven iron‐induced reaction system was developed, which is efficient at selective coupling of CH 4 as well as conversion of benzene and cyclohexane under mild conditions. For CH 4 conversion, 94 % C 2 selectivity and a C 2 H 6 formation rate of 8.4 μmol h ⁻¹ is achieved. Mechanistic studies reveal that CH 4 coupling is induced by hydroxyl radical, which is generated by photo‐driven intermolecular charge migration of an Fe ³⁺ complex. The delicate coordination structure of the [Fe(H 2 O) 5 OH] ²⁺ complex ensures selective C−H bond activation and C−C coupling of CH 4 . The produced Fe ²⁺ can be used to reduce the potential for electrolytic H 2 production, and then turns back into Fe ³⁺ , forming an energy‐saving and sustainable recyclable system.
Surface plasmon resonance (SPR) photocatalysts have attracted considerable attention because of their strong absorption capacity of visible light and enhanced photogenic carrier separation efficiency. However, the separate production of metal nanoparticles (NPs) and semiconductors limits the photogenic charge transfer. As one of the most promising organic photocatalysts, porphyrin self-assemblies with a long-range ordered structure-enhance electron transfer. In this study, plasmonic noble metal-based porphyrin hexagonal submicrowires composites (M-HW) loaded with platinum (Pt), silver (Ag), gold (Au), and palladium (Pd) NPs were synthesized through a simple in situ photocatalytic method. Homogeneous and uniformly distributed metal particles on the M-HW composites enhanced the catalytic or chemical properties of the organic functional nanostructures. Under the same loading of metal NPs, the methyl orange photocatalytic degradation efficiency of Ag-HW [kAg-HW (0.043 min−1)] composite was three times higher than that of HW, followed by Pt-HW [kPt-HW (0.0417 min−1)], Au-HW [kAu-HW (0.0312 min−1)], and Pd-HW [kPd-HW (0.0198 min−1)]. However, the rhodamine B (RhB) and eosin B photocatalytic degradations of Pt-HW were 4 times and 2.6 times those of HW, respectively. Finally, the SPR-induced electron injection, trapping, and recombination processes of the M-HW system were investigated. These results showed that M-HW plasmonic photocatalysts exhibited excellent photocatalytic performances, making them promising materials for photodegrading organic pollutants.
Material design is one of the major driving forces to enhance the intrinsic light harvesting ability of a photocatalyst. Herein, we report the design of black TiO2‐x/CuxO with chiral‐like structures where Pt single atom (SA) are successfully deposited on both black TiOx and CuOx for visible‐light H2 generation. Pt are well dispersed in the TiOx surface but also at the surface of CuOx, forming single atom alloys (SAAs). Oxygen defects stabilize the SA deposition and Pt SAs significantly enlarge the space charge layer, facilitating both the separation and transfer of photogenerated charge carriers. This structural optimization endows the black TiO2‐x/CuxO/Pt film excellent light harvesting ability in the visible region, making it a promising visible‐light responsive photocatalyst. This work is expected to provide insights into the rational construction of nanostructured materials with chiral nematic structure exhibiting improved light harvesting ability. This article is protected by copyright. All rights reserved.
The surface plasmon resonance (SPR) effect of oxygen-deficient WO3-xhas been utilized for photocatalysis in the literature. A unique feature of WO3-xis that its plasmonic peak can be adjusted to the visible light range, which is close to that of typical plasmonic metals like Au. In this work, we have demonstrated the coupling effect of the SPR of WO3-xand Au can significantly alter the photocatalytic activity toward the gas phase nonoxidative methane coupling reaction in terms of the C2product rate and selectivity. A photocatalyst composed of Au and WO3-xnanoparticles of comparable size is prepared by the simple Pechini method and thermal reduction in a H2containing atmosphere. The SPR of Au can mainly promote C2H6production, while the SPR of WO3-xcan mainly increase the selectivity to C2H4. Combining two such strategies can give a composite photocatalyst with a higher C2production rate and almost 100% selectivity to C2H4under visible light. Detailed characterization reveals that the coupling of Au and WO3-xcan effectively prolong the charge lifetime and the charge generation rate, and improves the hydrogen evolution activity, which results in much higher photocatalytic activity.
With recent advances in the field of single-atoms (SAs) used in photocatalysis, an unprecedented performance of atomically dispersed co-catalysts has been achieved. However, the stability and agglomeration of SA co-catalysts on the semiconductor surface may represent a critical issue in potential applications. Here we describe the photoinduced destabilization of Pt SAs on the benchmark photocatalyst, TiO2. In aqueous solutions within illumination timescales ranging from few minutes to several hours, light-induced agglomeration of Pt SAs to ensembles (dimers, multimers) and finally nanoparticles takes place. The kinetics critically depends on the presence of sacrificial hole scavengers and the used light intensity. DFT calculations attribute the light induced destabilization of the SA Pt species to binding of surface-coordinated Pt with solution-hydrogen (adsorbed H atoms), which consequently weakens the Pt SA bonding to the TiO2 surface. Despite the gradual aggregation of Pt SAs into surface clusters and their overall reduction to metallic state, which involves >90% of Pt SAs, the overall photocatalytic H2 evolution remains virtually unaffected.
This article is protected by copyright. All rights reserved
The development of direct routes for catalytic functionalization of methane would accelerate the chemical utilization of this abundant carbon resource to produce value-added chemicals or easily transportable fuels. However, the selective transformation of methane remains highly challenging due to the dilemma between the activation of inert C−H bond and the keeping of vulnerable target products from over-oxidation or deep dehydrogenation. A number of studies have been devoted to selective methane transformations by heterogeneous photocatalysis. Unlike a comprehensive review of the field, this article focuses on the insights into the catalytically active sites/species, which are usually overlooked in photocatalysis, and offers the strategies that may facilitate selective methane conversions by manipulating the active sites/species at surface/interface regions. The key factors that determine not only the C−H activation but also the selectivity control are discussed. The future opportunities for photocatalytic methane conversions by rationally controlling catalytic active sites/species are analyzed.
Catalytic combustion is promising in removing trace amounts of CH4 to address serious environmental concerns. Supported Pd‐based catalysts are most effective but often suffer from low stability in applications owing to the water‐vapor‐induced sintering. Herein, we develop a universal strategy to prepare irreducible‐oxide‐modified Pd/MgAl2O4 catalysts which show high activity and excellent stability against both hydrothemal aging at elevated temperatures and deactivation in long‐term reaction under wet conditions. The addition of irreducible oxides inhibited the deep oxidation of Pd in the oxygen‐rich conditions, which preserved not only the epitaxial structure but also a suitable active phase of Pd‐PdOx on MgAl2O4, thus promoting both activity and stability. This work provides new insights into the effect of metal–oxide interaction on CH4 combustion and offers an avenue to design hydrothermally stable and active combustion catalysts for industrial applications.
Unraveling how reactive facets promote photocatalysis at the molecular level remains a grand challenge, while identification of the reactive facets can provide guidelines for designing highly efficient photocatalysts and unravelling the microscopic mechanisms behind them. Recently, a series of polytriazine imides (PTIs) was reported with highly crystalline structures; all had a relatively low photocatalytic activity for overall water splitting. Here, high-angle annular dark-field scanning transmission electron microscopy, energy dispersive spectroscopy mapping, and aberration-corrected integrated differential phase contrast imaging were used to study PTI/Li+Cl− single crystals before and after in situ photodeposition of co-catalysts, showing that the prismatic {101¯0} planes are more photocatalytically reactive than the basal {0001} planes. Theoretical calculations confirmed that the electrons are energetically favourable to transfer toward the {101¯0} planes. Upon this discovery, PTI/Li+Cl− crystals with different aspect ratios were prepared, and the overall water splitting performance followed a linear correlation with the relative surface areas of the {101¯0} and {0001} planes. Our controlling of the reactive facets directly instructs the development of highly efficient polymer photocatalysts for overall water splitting. Unlike with inorganic photocatalysts, the facet-dependent reactivity of conjugated polymers remains elusive. Now, the authors provide molecular-level insights on the reactive facets of crystalline poly(triazine imide) intercalated with LiCl and achieve a remarkable improvement in its overall photocatalytic water splitting activity.
Methane activation and utilization are among the major challenges of modern science. Methane is potentially an important feedstock for manufacturing value-added fuels and chemicals. However, most known processes require excessive operating temperatures and exhibit insufficient selectivity. Here, we demonstrate a photochemical looping strategy for highly selective stoichiometric conversion of methane to ethane at ambient temperature over silver–heteropolyacid–titania nanocomposites. The process involves a stoichiometric reaction of methane with highly dispersed cationic silver under illumination, which results in the formation of methyl radicals. Recombination of the generated methyl radicals leads to the selective, and almost quantitative, formation of ethane. Cationic silver species are simultaneously reduced to metallic silver. The silver–heteropolyacid–titania nanocomposites can be reversibly regenerated in air under illumination at ambient temperature. The photochemical looping process achieves a methane coupling selectivity of over 90%, a quantitative yield of ethane of over 9%, high quantum efficiency (3.5% at 362 nm) and excellent stability. Activating methane at ambient temperature is challenging due to its stability, but could ultimately give access to a variety of other fuels and chemicals. Here, the authors present a photochemical looping strategy based on silver chemistry that converts methane to ethane under illumination at room temperature.
Photocatalytic methane conversion is attractive for utilization of renewable biogas and solar energy to directly produce useful compounds. In the present study, gallium oxide (Ga2O3) photocatalyst was examined for non-oxidative coupling of methane (NOCM) around room temperature in a flow reactor. It was found that ethane and hydrogen were continuously yielded at constant rate from methane upon photoirradiation around room temperature, confirming that NOCM can be promoted photocatalytically over Ga2O3. In addition, Pd cocatalyst was found to improve the activity of the Ga2O3 photocatalyst for NOCM and achieve more than three times higher formation rate of ethane such as 0.22 μmol h⁻¹ in a flow of 10% methane at 30 mL min⁻¹ with 0.8 g of photocatalyst. The methane conversion achieved to 0.006% within a short contact time of 0.8 s, which is higher than the thermodynamically equilibrium conversion.
Dry reforming of methane usually affords low-quality syngas with equimolar amounts of CO and H2. Here we report the high conversion of CH4 and CO2 to syngas and solid carbon through simultaneous pyrolysis and dry reforming of methane in a bubble column reactor using a molten metal alloy catalyst (65:35 mol% Ni:In). The H2 to CO ratio can be increased above 1:1 using feed ratios of CH4:CO2 greater than 1:1 to produce stoichiometric solid carbon as a co-product that is separable from the molten metal. A coupled reduction–oxidation cycle is carried out in which CO2 is reduced by a liquid metal species (for example, In) and methane is partially oxidized to syngas by the metal oxide intermediate (for example, In2O3), regenerating the native metal. Moreover, the H2:CO product ratio can be easily controlled by adjusting the CH4:CO2 feed ratio, temperature, and residence time in the reactor. Dry reforming of methane can so far afford syngas with equimolar CO and H2, which is suboptimal for Fischer–Tropsch chemistry. Now a process is reported based on a Ni–In molten metal alloy catalyst that is capable of producing syngas with practically relevant H2/CO ratios together with separable carbon.
Stimulated cells and cancer cells have widespread shortening of mRNA 3’-untranslated regions (3’UTRs) and switches to shorter mRNA isoforms due to usage of more proximal polyadenylation signals (PASs) in introns and last exons. U1 snRNP (U1), vertebrates’ most abundant non-coding (spliceosomal) small nuclear RNA, silences proximal PASs and its inhibition with antisense morpholino oligonucleotides (U1 AMO) triggers widespread premature transcription termination and mRNA shortening. Here we show that low U1 AMO doses increase cancer cells’ migration and invasion in vitro by up to 500%, whereas U1 over-expression has the opposite effect. In addition to 3’UTR length, numerous transcriptome changes that could contribute to this phenotype are observed, including alternative splicing, and mRNA expression levels of proto-oncogenes and tumor suppressors. These findings reveal an unexpected role for U1 homeostasis (available U1 relative to transcription) in oncogenic and activated cell states, and suggest U1 as a potential target for their modulation. U1 snRNP is a key regulator of mRNA biogenesis through its roles in splicing, and transcription and 3’-end processing. Here the authors show a tumor suppressor-like function of U1 snRNP using in vitro cell migration/invasion assays and transcriptome profiling.
Syngas is a very important intermediate in chemical industry for energy chemicals production through F–T synthesis. Methane steam reforming (MSR) and dry reforming (MDR) reactions are two extensively studied approaches for syngas production. Developing sintering-resistant catalyst for syngas production from the reforming reactions is a hot topic because high-temperature requirement for the reactions always deactivates catalyst due to sintering and carbon deposition. In this study, we synthesized sintering-resistant Ni@SiO2 catalyst for stable performances of MSR and MDR. Characterization of TEM, XRD, etc., revealed that the Ni@SiO2 catalyst could maintain original core–shell structure and preserve Ni nanoparticle size at temperature as high as 1123 K. The excellent sintering resistance was attributed to encapsulation of thermally stable SiO2 nanospheres, which confined Ni nanoparticles migration and thus avoided aggregation. The work provided a potential sintering-resistant catalyst for heterogeneous reactions.
Graphic abstract
Methane activation under moderate conditions and with good selectivity for value-added chemicals still remains a huge challenge. Here, we present a highly selective catalyst for the transformation of methane to methanol composed of highly dispersed iron species on titanium dioxide. The catalyst operates under moderate light irradiation (close to one Sun) and at ambient conditions. The optimized sample shows a 15% conversion rate for methane with an alcohol selectivity of over 97% (methanol selectivity over 90%) and a yield of 18 moles of alcohol per mole of iron active site in just 3 hours. X-ray photoelectron spectroscopy measurements with and without xenon lamp irradiation, light-intensity-modulated spectroscopies, photoelectrochemical measurements, X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra, as well as isotopic analysis confirm the function of the major iron-containing species—namely, FeOOH and Fe2O3, which enhance charge transfer and separation, decrease the overpotential of the reduction reaction and improve selectivity towards methanol over carbon dioxide production. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
Wave-particle duality is an inherent peculiarity of the Quantum world. The double-slit experiment has been frequently used for understanding different aspects of this fundamental concept. The occurrence of interference rests on the lack of which-way information and on the absence of decoherence mechanisms, which could scramble the wave fronts. Here, we report on the observation of two-center interference in the molecular-frame photoelectron momentum distribution upon ionization of the neon dimer by a strong laser field. Postselection of ions, which are measured in coincidence with electrons, allows choosing the symmetry of the residual ion, leading to observation of both, gerade and ungerade, types of interference.
Many studies have shown how pigments and internal nanostructures generate color in nature. External surface structures can also influence appearance, such as by causing multiple scattering of light (structural absorption) to produce a velvety, super black appearance. Here we show that feathers from five species of birds of paradise (Aves: Paradisaeidae) structurally absorb incident light to produce extremely low-reflectance, super black plumages. Directional reflectance of these feathers (0.05-0.31%) approaches that of man-made ultra-absorbent materials. SEM, nano-CT, and ray-tracing simulations show that super black feathers have titled arrays of highly modified barbules, which cause more multiple scattering, resulting in more structural absorption, than normal black feathers. Super black feathers have an extreme directional reflectance bias and appear darkest when viewed from the distal direction. We hypothesize that structurally absorbing, super black plumage evolved through sensory bias to enhance the perceived brilliance of adjacent color patches during courtship display.
Zika virus (ZIKV) has recently caused a pandemic disease, and many cases of ZIKV infection in pregnant women resulted in abortion, stillbirth, deaths and congenital defects including microcephaly, which now has been proposed as ZIKV congenital syndrome. This study aimed to investigate the in situ immune response profile and mechanisms of neuronal cell damage in fatal Zika microcephaly cases. Brain tissue samples were collected from 15 cases, including 10 microcephalic ZIKV-positive neonates with fatal outcome and five neonatal control flavivirus-negative neonates that died due to other causes, but with preserved central nervous system (CNS) architecture. In microcephaly cases, the histopathological features of the tissue samples were characterized in three CNS areas (meninges, perivascular space, and parenchyma). The changes found were mainly calcification, necrosis, neuronophagy, gliosis, microglial nodules, and inflammatory infiltration of mononuclear cells. The in situ immune response against ZIKV in the CNS of newborns is complex. Despite the predominant expression of Th2 cytokines, other cytokines such as Th1, Th17, Treg, Th9, and Th22 are involved to a lesser extent, but are still likely to participate in the immunopathogenic mechanisms of neural disease in fatal cases of microcephaly caused by ZIKV.
An efficient and direct method of catalytic conversion of methane to liquid methanol and other oxygenates would be of considerable practical value. However, it remains an unsolved problem in catalysis, as typically it involves expensive1-4 or corrosive oxidants or reaction media5-8 that are not amenable to commercialization. Although methane can be directly converted to methanol using molecular oxygen under mild conditions in the gas phase, the process is either stoichiometric (and therefore requires a water extraction step)9-15 or is too slow and low-yielding16 to be practical. Methane could, in principle, also be transformed through direct oxidative carbonylation to acetic acid, which is commercially obtained through methane steam reforming, methanol synthesis, and subsequent methanol carbonylation on homogeneous catalysts17,18. However, an effective catalyst for the direct carbonylation of methane to acetic acid, which might enable the economical small-scale utilization of natural gas that is currently flared or stranded, has not yet been reported. Here we show that mononuclear rhodium species, anchored on a zeolite or titanium dioxide support suspended in aqueous solution, catalyse the direct conversion of methane to methanol and acetic acid, using oxygen and carbon monoxide under mild conditions. We find that the two products form through independent pathways, which allows us to tune the conversion: three-hour-long batch-reactor tests conducted at 150 degrees Celsius, using either the zeolitesupported or the titanium-dioxide-supported catalyst, yield around 22,000 micromoles of acetic acid per gram of catalyst, or around 230 micromoles of methanol per gram of catalyst, respectively, with selectivities of 60-100 per cent. We anticipate that these unusually high activities, despite still being too low for commercial application, may guide the development of optimized catalysts and practical processes for the direct conversion of methane to methanol, acetic acid and other useful chemicals. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
We report the direct splitting of pure water by light-excited graphitic carbon nitride (g-C3N4) modified with Pt, PtO
x
, and CoO
x
as redox cocatalysts, while pure g-C3N4 is virtually inactive for overall water splitting by photocatalysis. The novelty is in the selective creation of both H2 and O2 cocatalysts on surface active sites of g-C3N4via photodeposition triggering the splitting of water for the simultaneous evolution of H2 and O2 gases in a stoichiometric ratio of 2 : 1, irradiated with light, without using any sacrificial reagents. The photocatalyst was stable for 510 hours of reaction.
The strong metal-support interaction (SMSI) is of great importance for supported catalysts in heterogeneous catalysis. Here we report the first example of SMSI between Au nanoparticles (NPs) and hydroxyapatite (HAP), a non-oxide. The reversible encapsulation of Au NPs by HAP support, electron transfer and changes in CO adsorption are identical to the classic SMSI except that the SMSI of Au/HAP occurred under oxidative condition-the opposite condition for the classical SMSI. The SMSI of Au/HAP not only enhanced the sintering resistance of Au NPs upon calcination but also improved their selectivity and reusability in liquid-phase reaction. It was found that the SMSI between Au and HAP is general and could be extended to other phosphate supported Au systems such as Au/LaPO4. This new discovery may open a new way to design and develop highly stable supported Au catalysts with controllable activity and selectivity.
The CASTEP code for first principles electronic
structure calculations will be described. A brief, nontechnical overview will be given and some of the features
and capabilities highlighted. Some features which are unique
to CASTEP will be described and near-future development
plans outlined.
ConspectusThe abundance of cheap, natural gas has transformed the energy landscape, whereby revealing new possibilities for sustainable chemical technologies or impacting those that have relied on traditional fossil fuels. The primary component, methane, is underutilized and wastefully exhausted, leading to anthropogenic global warming. Historically, the manipulation of methane remained "clavis aurea," an insurmountable yet rewarding challenge and thus the focus of intense research. This is primarily due to an inability to dissociate C-H bonds in methane selectively, which requires a high energy penalty and is an essential prerequisite for the direct conversion of methane into a large set of value-added products. The discovery of such processes would promise an energy gainful use of natural gas benefiting several essential chemical processes associated with C1 chemistry. This first C-H bond dissociation step of the methane molecule appears in numerous catalytic mechanisms as the rate-determining step or most essential barrier sequence for all subsequent steps that follow in the production of C-C, C-O, or C
x
-H
y
-O
z
bonds found in value added products. A main goal is to catalytically reduce the energy barrier for the first C-H bond dissociation to be able to achieve the activation of methane at low or moderate temperatures. As such there is great value in understanding the fundamental nature of the active sites responsible for bond breaking or formation and thus be able to facilitate better control of this chemistry, leading to the development of new technologies for fuel production and chemical conversion. Surface science studies offer enhanced perspectives for a careful manipulation of bonds over the last layer atoms of catalyst surfaces, an essential factor for the design of atomically precise catalysts and unravelling of the reaction mechanism. With the advent of new surface imaging, spectroscopy, and in situ tools, it has been possible to decipher the surface chemistry of complex materials systems and further our understanding of atomic active sites on the surfaces of metals, oxides, and carbides or metal-oxide and metal-carbide interfaces. The once considered near impossible step of C-H bond activation is now observed at low temperatures with high propensity over a collection of oxide, metal-oxide, and metal-carbide systems in a conventional or inverse configuration (oxide or carbide on metal). The enabling of C-H activation at low temperature has opened interesting possibilities for the specific production of chemicals such as methanol directly from methane, a step toward facile synthesis of liquid fuels. We highlight the most recent of these results and present the key aspects of active site configurations engineered from surface science studies which enable such a simple reactive event through careful manipulation of the last surface layer of atoms found in the catalyst structure. New concepts which help in the activation and conversion of methane are discussed.
A water boost for methanol synthesis
Model catalysts based on metals and metal oxides can dissociate methane (CH 4 ) at room temperature, converting it directly to methanol (CH 3 OH). Liu et al. show that for one of these catalysts, an “inverted” CeO x -Cu 2 O oxide on Cu(111), water tunes the selectivity from forming CO and CO 2 to forming surface CH 3 O groups, as revealed by ambient-pressure x-ray photoelectron spectroscopy. Theoretical modeling showed that adsorbed water blocks O 2 dissociation and O 2 instead oxidizes the reduced catalyst. Hydroxyl groups from water generate the CH 3 O species from dissociated CH 4 , and water then goes on to form and displace CH 3 OH to the gas phase.
Science , this issue p. 513
Pt/Ga 2 O 3 exhibited high activity for dehydrogenative coupling of CH 4 into ethane (2CH 4 → C 2 H 6 + H 2 ) in a fixed-bed flow reactor at 25 °C under 254-nm UV irradiation. The C 2 H 6 ...
Metal nanoparticles stabilized on a support material catalyse many major industrial reactions. Metal-support interactions in these nanomaterials can have a substantial influence on the catalysis, making metal-support interaction modulation one of the few tools able to enhance catalytic performance. This topic has received much attention in recent years, however, a systematic rationalization of the field is lacking due to the great diversity in catalysts, reactions and modification strategies. In this review, we cover and categorize the recent progress in metal-support interaction tuning strategies to enhance catalytic performance for various reactions. Furthermore, we quantify the productivity enhancements resulting from metal-support interaction control that have been achieved in C1 chemistry in recent years. Our analysis shows that up to fifteen-fold productivity enhancement has been achieved, and that metal-support interaction is most impactful for metal nanoparticles smaller than four nanometres. These findings demonstrate the importance of metal-support interaction to improve performance in catalysis.
Photodriven nonoxidative coupling of CH4 (NOCM) is a potential alternative approach to clean hydrogen and hydrocarbon production. Herein, a Mott-Schottky photocatalyst for NOCM is fabricated by loading Pt nanoclusters on a Ga-doped hierarchical porous TiO2-SiO2 microarray with an anatase framework, which exhibits a CH4 conversion rate of 3.48 μmol g-1 h-1 with 90% selectivity toward C2H6. This activity is 13 times higher than those from microarrays without Pt and Ga. Moreover, a continuous H2 production (36 μmol g-1) with a high CH4 conversion rate of ∼28% can be achieved through a longtime irradiation (32 h). The influence of Ga on the chemical state of a surface oxygen vacancy (Vo) and deposited Pt is investigated through a combination of experimental analysis and first-principles density functional theory calculations. Ga substitutes for the five-coordinated Ti next to Vo, which tends to stabilize the single-electron trapped Vo and reduce the electron transfer from Vo to the adsorbed Pt, resulting in the formation of a higher amount of cationic Pt. The cationic Pt and electron-enriched metallic Pt form a cationic-anionic active pair, which is more efficient for the dissociation of C-H bonds. However, the presence of too much cationic Pt results in more C2+ product with a decrease in the CH4 conversion rate due to the reduced charge-carrier separation efficiency. This study provides deep insight into the effect of the doping/loading strategy on the photocatalytic NOCM reaction and is expected to shed substantial light on future structural design and modulation.
Direct methane functionalization and, in particular, the selective partial oxidation to methanol, remains an eminent challenge and a field of competitive research. The conversion of methane to methanol over transition-metal-containing zeolites using molecular oxygen is a promising and extensively studied process. Herein, we scrutinize some oft-cited assumptions in this topic—which include the labelling of the process as biomimetic, the debate regarding the industrial viability of direct methane-oxidation systems and the claim that methane is difficult to activate—and delineate the extent to which these are scientifically robust. We highlight both the merits and pitfalls of such statements and point out the hazards associated with their improper use. By examining these misconceptions, we build an outlook for future research, highlighting the need to optimize materials and process conditions for the stepwise approach and to further explore catalytic processes that explicitly employ strategies for the preservation of methanol.
Solar CO2 photoreduction into hydrocarbons is promising and significative. However, many conventional catalysts reported usually suffer from poor photocatalytic activities. Herein, ultrathin Bi2WO6 nanosheets with a thickness of about 4.8 nm have been synthesized by hydrothermal method, which exhibited a CH4 production rate of 19 ppm g⁻¹ h⁻¹ under a low CO2 concentration of 400 ppm. PtOx nanoparticles with a size of about 2 nm were then loaded on the Bi2WO6 nanosheets as excellent co-catalysts by photoreduction in aqueous solution, and an optimal CH4 yield of 108.8 ppm g⁻¹ h⁻¹ was achieved, which was about 5.7 times than that of pristine Bi2WO6 nanosheets. Further analyses of photocurrent curves, electrochemical impedance spectroscopy and polarization curves of water oxidation indicated that the improved photocatalytic activity was suggested to result from the enhanced carrier separation and accelerated water oxidation by PtOx nanoparticles. The work will likely give a deeper insight of PtOx nanoparticles and provide a new idea to design catalysts for CO2 photoreduction to CH4.
Hot‐electron‐driven chemical transformation (HEDCT) represents an emerging research area in utilizing photoresponsive nanoparticles to enable efficient solar‐to‐chemical conversion. The unique properties of quantum‐sized metal nanoparticles (QSMNPs) make them a class of photocatalysts that can generate hot electrons to drive surface chemical reactions with high quantum efficiency. Compared to the conventional thermal‐driven chemical reactions, HEDCT offers the advantages of accelerating reaction rate, improving reaction selectivity, and possibly enabling the occurrence of thermodynamically endergonic reactions. Despite its embryonic stage of development, using QSMNPs for HEDCT shows great promise. Herein, a timely overview on the research progress is provided with a focus on the fundamental quantum processes involved in the photoexcitation of hot electrons and the following HEDCT on the surface of QSMNPs. The last section discusses the challenges, which also represent the opportunities for the materials research community, in designing robust QSMNP photocatalysts and understanding the fundamental quantum phenomena in HEDCT.
A radical route from methane to methanol
The conversion of methane into chemicals usually proceeds through high-temperature routes that first form more reactive carbon monoxide and hydrogen. Agarwal et al. report a low-temperature (50°C) route in aqueous hydrogen peroxide (H 2 O 2 ) for oxidizing methane to methanol in high yield (92%). They used colloidal gold-palladium nanoparticles as a catalyst. The primary oxidant was O 2 ; isotopic labeling showed that H 2 O 2 activated methane to methyl radicals, which subsequently incorporated O 2 .
Science , this issue p. 223
Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (∼97%) over a copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH3OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at CuII oxide active centers, followed by CuI reoxidation by water with concurrent formation of hydrogen. © 2017, American Association for the Advancement of Science. All rights reserved.
We report that the coupling of methane dehydroaromatization (MDA) and methanol methylation over a Mo/HZSM-5 catalyst can realize the direct conversion of methane to benzene, toluene, and xylene (BTX) with long-time steady state (60 h), higher activity (26.4%), and selectivity of BTX (>90%) at atmospheric pressure and 973 K. Based on characterization, it was confirmed that the formed benzene can be effectively methylated by methanol, leading to high activity and stability, which proves that the coke from polycondensation of the formed benzene results in rapid deactivation of MDA.
Excess electrons from intrinsic defects, dopants and photoexcitation play a key role in many of the properties of TiO2. Understanding their behaviour is important for improving the performance of TiO2 in energy-related applications. We focus on anatase, the TiO2 polymorph most relevant in photocatalysis and solar energy conversion. Using first-principles simulations, we investigate the states and dynamics of excess electrons from different donors near the most common anatase (101) and (001) surfaces and aqueous interfaces. We find that the behaviour of excess electrons depends strongly on the exposed anatase surface, the environment and the character of the electron donor. Whereas no electron trapping is observed on the (101) surface in vacuo, an excess electron at the aqueous (101) interface can trigger water dissociation and become trapped into a stable surface Ti(3+)-bridging OH complex. By contrast, electrons avoid the (001) surface, indicating that oxidation reactions are favoured on this surface. Our results provide a bridge between surface science experiments and observations of crystal-face-dependent photocatalysis on anatase, and support the idea that optimization of the ratio between {101} and {001} facets could provide a way to enhance the photocatalytic activity of this material.
Photocatalytic CO2 reduction on metal-oxide-based catalysts is promising for solving the energy and environmental crises faced by mankind. The oxygen vacancy (V o) on metal oxides is expected to be a key factor affecting the efficiency of photocatalytic CO2 reduction on metal-oxide-based catalysts. Yet, to date, the question of how an V o influences photocatalytic CO2 reduction is still unanswered. Herein, we report that, on V o-rich gallium oxide coated with Pt nanoparticles (V o-rich Pt/Ga2O3), CO2 is photocatalytically reduced to CO, with a highly enhanced CO evolution rate (21.0 μmol·h−1) compared to those on V o-poor Pt/Ga2O3 (3.9 μmol·h−1) and Pt/TiO2(P25) (6.7 μmol·h−1). We demonstrate that the V o leads to improved CO2 adsorption and separation of the photoinduced charges on Pt/Ga2O3, thus enhancing the photocatalytic activity of Pt/Ga2O3. Rational fabrication of an V o is thereby an attractive strategy for developing efficient catalysts for photocatalytic CO2 reduction.
We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.
Ga2O3 nanomaterials were synthesized from mechanically ground GaN powders with thermal annealing; Ga2O3 nanobelts were formed in a nitrogen atmosphere, while Ga2O3 nanoparticles were formed in an oxygen atmosphere. The structural properties of the Ga2O3 nanomaterials were investigated by X-ray diffractometer (XRD) and high-resolution transmission electron microscope (HRTEM). The study of field emission scanning electron microscopy (FESEM) on the microstructures of nanomaterials revealed that the nanobelts are in the range of about 10–200nm width and 10–50nm thickness, and that nanoparticles are in the range of about 20–50nm radius. On the basis of XRD and HRTEM data, we determined that the nanobelts grow toward a direction perpendicular to the (010) lattice plane and that they are enclosed by facets of the (101̄) and (101) lattice planes. The formation of the nanobelts may be described by the vapor–solid (VS) mechanism, and the supersaturation degree of gaseous phase may play an important role in the formation of Ga2O3 nanomaterials.
The atoms at the surface of a metal crystal can be differentiated according to the number (j) and arrangement of their nearest neighbours. We have determined how, in (different) crystals with f.c.c., b.c.c. and h.c.p. structures, the total number of surface atoms (Ns) and the proportions of the various types of surface atoms depend on the crystallite size.In some cases we have also determined how the number of Bni,j,k,… sites varies with the crystallite size, a Bni,j,k,… site being defined as an ensemble of n surface metal atoms in a certain specified arrangement where the n surface metal atoms have i,j,k etc. nearest neighbours.On the basis of the results we have considered the influence of crystallite size on the sorptive and catalytic behaviour of metal catalysts.
Both unsupported and silica-supported gallium oxide were found to promote the nonoxidative coupling of methane (NOCM) to produce ethane and hydrogen upon photoirradiation at around room temperature. The unsupported gallium oxide demonstrated high methane conversion and high ethane selectivity in hydrocarbon products; however, the amount of hydrogen produced was not equivalent to the amount of ethane produced, because other reactions, such as consecutive coupling and coke/carbon formation, also would proceed. The gallium content on the silica-supported gallium oxide affected both the local structure of gallium species and the photocatalytic property. The sample with the lowest loadings of Ga (0.1 mol%), on which the gallium oxide existed mainly as highly dispersed tetrahedral species, exhibited high specific activity per Ga atom and high selectivity for NOCM, where ethane and hydrogen were produced in almost equal amounts. The gallium oxide clusters or nanoparticles on the silica-supported samples with Ga loadings of 0.5–10 mol% exhibited low selectivity, likely due to the variety of the active sites.
Pt nanoparticles with various sizes of 1, 2, 4 and 6 nm were synthesized and studied as catalysts for gas-phase methanol oxidation reaction towards formaldehyde and carbon dioxide under ambient pressure (10 Torr of methanol, 50 Torr of oxygen and 710 Torr of helium) at a low temperature of 60 oC. While the 2, 4 and 6 nm nanoparticles exhibited similar catalytic activity and selectivity, the 1 nm nanoparticles showed significantly higher selectivity towards partial oxidation of methanol to formaldehyde, but lower total turnover frequency. The observed size effect in catalysis was correlated to the size-dependent structure and oxidation state of the Pt nanoparticles. X-ray photoelectron spectroscopy and infrared vibrational spectroscopy using adsorbed CO as molecular probes revealed that the 1 nm nanoparticles were predominantly oxidized while the 2, 4 and 6 nm nanoparticles were largely metallic. Transmission electron microscopy imaging witnessed the transition from crystalline to quasicrystalline structure as the size of the Pt nanoparticles was reduced to 1 nm. The results highlighted the important impact of size induced oxidation state of Pt nanoparticles on catalytic selectivity as well as activity in gas-phase methanol oxidation reactions.
THE diminishing reserves of petroleum oil have focused attention on the possibility of making more efficient use of natural gas, reserves of which are at present considerably under-utilized. Methane is commonly used as a fuel, but it is also the starting material for the production, by steam reforming, of synthesis gas (carbon monoxide and hydrogen), which acts as a feedstock for the synthesis of ammonia and methanol, and can be converted to higher hydrocarbons, alcohols and aldehydes by Fischer–Tropsch catalysis1. The partial oxidation of methane to synthesis gas is also an established industrial process2 but operates at very high temperatures (> 1,200 °C). Here we report that this reaction can be carried out at temperatures of only ~775 °C by using lanthanide ruthenium oxide catalysts.
The process of Fermi level equilibration in 5 nm ZnO quantum dot−metal nanojunctions has been monitored using changes to both the surface plasmon band of the metal island and the sharp exciton band of the ZnO nanocrystals following photoinduced electron accumulation. In the cases of silver, copper, and gold islands, excess electrons reside on both the quantum dot and the metal, whereas for Pt islands, the excess electrons reside exclusively on the Pt island. Electrons are transferred rapidly from Pt to the solvent ethanol, preventing accumulation on the quantum dots. The combination of exciton bleaching and surface plasmon shifts provides a simple way of probing the efficiency of small metal islands as redox catalysts on semiconductor particles.
Monoclinic Ga2O3 nanowires were grown by DC arc discharge of GaN powders with small amounts of transition metals in a gaseous mixture of O and Ar. High resolution transmission electron microscopic observations clearly revealed the growth mechanism. GaN powder is transformed catalytically into gallium oxides by the oxidation reaction assisted by the transition metal particles. Ga2O3 start growing from the twin defects at the center of the wire. The prematured nanowires exhibit rough step edges, whereas the completed nanowires show a clean surface without steps. Step-growth takes place in association with the transition metals, which enhance the anisotropic diffusion along the direction of the wire axis, resulting in the formation of the nanowires.
The photodeposition of Pt on colloidal CdS and CdSe/CdS core/shell nanocrystals has been demonstrated. It was observed during the study that the photoexcitation of CdS and CdSe/CdS with an organometallic Pt precursor can accelerate the deposition of Pt nanoparticles on the semiconductor surface. The tuning of semiconductor band structure, spatial organization, and surface chemistry can play a significant role for designing photocatlytic nanostructures. The study also found that the saturation of the deposited Pt at higher laser powers can increase the excitation frequency. It was also observed that the photodeposition of Pt on CdS depend on the nature of the amines used in the reaction. UV-vis spectrophotometer was used in the study to characterize the metal-semiconductor heterostructures.
Breaking down methane: A Ga(3+) -modified microporous titanosilicate (ETS-10) exhibits distinct photoactivity for the cleavage of the methane CH bond at room temperature. The highly enhanced activity is attributed to the synergistic effect of gallium-induced CH bond polarization and a titania-based photoredox process.
A matter of size: The particle size effect on the activity of the oxygen reduction reaction of size-selected platinum clusters was studied. The ORR activity decreased with decreasing Pt nanoparticle size, corresponding to a decrease in the fraction of terraces on the surfaces of the Pt nanoparticles (j(k) =kinetic current density, see picture).
Sun bath: A Zn(+) -modified zeolite catalyst showing superior photocatalytic activity for the activation of a CH bond converted methane into ethane and hydrogen upon irradiation of sunlight. Light at wavelengths shorter than 390 nm transferred electrons from the zeolite framework to the zinc centers.
Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al(2)O(3). Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis.
The interaction of photogenerated carries in GaN photocatalyst with Pt cocatalyst for hydrogen evolution under irradiation was investigated from in situ ATR-SEIRAS measurement by following the CO vibrational frequency. After irradiation, the CO frequency shifted higher, indicating that the Fermi level of Pt particles was positively shifted by the photogenerated holes. Thereafter, a lower frequency peak appeared, indicating that the Fermi level of some Pt particles was negatively shifted to the hydrogen evolution potential by photogenerated electrons, which is the essential function of cocatalysis for hydrogen evolution.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Catalytic cobblestones: Monodisperse platinum nanoparticles were prepared with controlled sizes (3-7 nm) and shapes (polyhedron, truncated cube, or cube). The cubic nanoparticles are a much more active cathode catalyst for the oxygen reduction reaction: the current density J from 7 nm cubes is four times that of the other shapes (see picture), indicating great potential for fuel cell applications. (Graph Presented)