A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: The case of the highly active metal/CeOx/TiO2(110) catalysts

Calcium-Poison-Resistant Cu/YCeO2-TiO2 Catalyst for the Selective Catalytic Reduction of NO with CO and Naphthalene in the Presence of Oxygen

The pollution from industrial processes based on biomass combustion is still an ongoing problem. In the present contribution, the selective catalytic reduction of NO with CO and naphthalene is carried out in the presence of 10% oxygen. The accumulation of alkaline and alkaline earth metals during biomass combustion is here simulated by the addition of calcium to a Cu-impregnated YCeO2-TiO2 support. The results show that a high dispersion of copper is obtained, which is resistant to the accumulation of calcium. Full conversion of CO and naphthalene is achieved above 200 °C, whereas NO conversions of 80, 90, and 87% are obtained for the catalysts with Ca loadings of 2.6, 5.2, and 13%, respectively, at 350 °C. It is proposed that the high activity of the catalysts is ascribed to the formation of Cu-Ox-Ce species and that the accumulation of Ca acts as a barrier to avoid copper sintering. It was found that different forms of carbonate and nitrite/nitrate species form during reaction, coexisting as adsorbed species during the SCR reaction. The selectivity to N2 was almost 100% in all cases, due to the small presence of NO2 in the reactor outlet (no N2O was detected in any conditions).

Recent Progress of Transition Metal Compounds as Electrocatalysts for Electrocatalytic Water Splitting

Hydrogen has enormous commercial potential as a secondary energy source because of its high calorific value, clean combustion byproducts, and multiple production methods. Electrocatalytic water splitting is a viable alternative to the conventional methane steam reforming technique, as it operates under mild conditions, produces high-quality hydrogen, and has a sustainable production process that requires less energy. Electrocatalysts composed of precious metals like Pt, Au, Ru, and Ag are commonly used in the investigation of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Nevertheless, their limited availability and expensive cost restrict practical use. In contrast, electrocatalysts that do not contain precious metals are readily available, cost-effective, environmentally friendly, and possess electrocatalytic performance equal to that of noble metals. However, considerable research effort must be devoted to create cost-effective and high-performing catalysts. This article provides a comprehensive examination of the reaction mechanism involved in electrocatalytic water splitting in both acidic and basic environments. Additionally, recent breakthroughs in catalysts for both the hydrogen evolution and oxygen evolution reactions are also discussed. The structure-activity relationship of the catalyst was deep-going discussed, together with the prospects of current obstacles and potential for electrocatalytic water splitting, aiming at provide valuable perspectives for the advancement of economical and efficient electrocatalysts on an industrial scale.

Oxidation of Methanol and Dichloromethane on TiO2-CeO2-CuO, TiO2-CeO2 and TiO2-CuO@VUKOPOR®A Ceramic Foams

The application-attractive form of TiO₂, CeO₂ and CuO-based open-cell foam supported catalysts was designed to investigate their catalytic performance in oxidation of two model volatile organic compounds-methanol and dichloromethane. TiO₂-CeO₂, TiO₂-CuO and TiO₂-CeO₂-CuO catalysts as thin films were deposited on VUKOPOR^®A ceramic foam using a reverse micelles-controlled sol-gel method, dip-coating and calcination. Three prepared catalytic foams were investigated via light-off tests in methanol and dichloromethane oxidation in the temperature range of 45-400 °C and 100-500 °C, respectively, at GHSV of 11, 600 h^-1, which fits to semi-pilot/industrial conditions. TiO₂-CuO@VUKOPOR^®A foam showed the best catalytic activity and CO₂ yield in methanol oxidation due to its low weak Lewis acidity, high weak basicity and easily reducible CuO species and proved good catalytic stability within 20 h test. TiO₂-CeO₂-CuO@VUKOPOR^®A foam was the best in dichloromethane oxidation. Despite of its lower catalytic activity compared to TiO₂-CeO₂@VUKOPOR^®A foam, its highly-reducible -O-Cu-Ce-O- active surface sites led to the highest CO₂ yield and the highest weak Lewis acidity contributed to the highest HCl yield. This foam also showed the lowest amount of chlorine deposits.

Structure, stability and water adsorption on ultra-thin TiO2 supported on TiN

Interfacial metal-oxide systems with ultra-thin oxide layers are of high interest for their use in catalysis. The chemical activity of ultra-thin metal-oxide layers can be substantially enhanced compared to interfacial models with thicker oxide. In this study, we present a Density Functional Theory (DFT) investigation of the structure of ultra-thin rutile layers (one and two TiO₂ layers) supported on TiN and the stability of water on these interfacial structures. The rutile layers are stabilized on the TiN surface through the formation of interfacial Ti-O bonds. Charge transfer from the TiN substrate leads to the formation of reduced Ti³⁺ cations in TiO₂. The concentration of Ti³⁺ is proportionally higher in the ultra-thin oxide, compared to interfacial models with thicker oxide layers. The structure of the one-layer oxide slab is strongly distorted at the interface while the thicker TiO₂ layer preserves the rutile structure. The energy cost for the formation of a single O vacancy in the one-layer oxide slab is only 0.5 eV with respect to the ideal interface. For the two-layer oxide slab, the introduction of several vacancies in an already non-stoichiometric system becomes progressively more favourable, which indicates the stability of the highly defective interfaces. Isolated water molecules dissociate when adsorbed at the TiO₂ layers. At higher coverages, the preference is for molecular water adsorption. Our ab initio thermodynamics calculations show the fully water covered stoichiometric models as the most stable structure at typical ambient conditions. This behaviour is similar to that observed on thicker oxide in TiO₂-TiN interfaces or pure TiO₂ surfaces. In contrast, interfacial models with multiple vacancies are most stable at low (reducing) oxygen chemical potential values. The high concentration on reduced Ti³⁺ introduces significant distortions in the O-defective slab. Whereas, a water monolayer adsorbs dissociatively on the highly distorted 2-layer TiO_1.75-TiN interface, where the Ti³⁺ states lying above the top of the valence band contribute to a significant reduction of the energy gap compared to the stoichiometric TiO₂-TiN model. Our results provide a guide for the design of novel interfacial systems containing ultra-thin TiO₂ with potential application as photocatalytic water splitting devices.

An electronic perturbation in TiC supported platinum monolayer catalyst for enhancing water-gas shift performance: DFT study

The water-gas shift (WGS) reaction behaviors over the TiC(0 0 1) supported Pt monolayer catalyst (Pt_ML/TiC(0 0 1)) are investigated by using the spin-unrestricted density functional theory calculations. Importantly, we find that the Pt_ML/TiC(0 0 1) system exhibits a much lower density of Pt-5d states nearby the Fermi level compared with that for Pt(1 1 1), and the monolayer Pt atoms undergo an electronic perturbation when in contact with TiC(0 0 1) support that would strongly improve the WGS activity of supported Pt atoms. Our calculations clearly indicate that the dominant reaction path follows a carboxyl mechanism involving a key COOH intermediate, rather than the common redox mechanism. Furthermore, through the detailed comparisons, the results demonstrate that the strong interactions between the monolayer Pt atoms and TiC(0 0 1) support make Pt_ML/TiC(0 0 1) a highly active catalyst for the low-temperature WGS reaction. Following the route presented by Bruix et al (2012 J. Am. Chem. Soc. 134 8968-74), the positive effect derived from strong metal-support interaction in the metal/carbide system is revealed.

Understanding the Photocatalytic Properties of Pt/CeOx /TiO2 : Structural Effects on Electronic and Optical Properties

Ceria-titania interfaces play a crucial role in different chemical processes but are especially promising for the photocatalytic splitting of water using light in the visible wavelength region when Pt is added to the system. However, the complexity of this hierarchical structure hampers the study of the origin of its outstanding properties. In this article, the structural, electronic and optoelectronic properties of CeO₂ /TiO₂ systems containing 1D, 2D, and 3D particles of ceria are analyzed by means of density functional calculations. Adsorption sites and vacancy effects have been studied to model Pt adsorption. Density of states calculations and absorption spectra simulations explain the behavior of these systems. Finally, these models are used for the screening of other metals that can be combined with this heterostructure to potentially find more efficient water splitting photocatalysts.

Activation of Water on MnOx-Nanocluster-Modified Rutile (110) and Anatase (101) TiO2 and the Role of Cation Reduction

Surface modification of titania surfaces with dispersed metal oxide nanoclusters has the potential to enhance photocatalytic activity. These modifications can induce visible light absorption and suppress charge carrier recombination which are vital in improving the efficiency. We have studied heterostructures of Mn4O6 nanoclusters modifying the TiO2 rutile (110) and anatase (101) surfaces using density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. In our study we have considered partial hydroxylation of the rutile and anatase surfaces and the role of cation reduction, via oxygen vacancy formation, and how this impacts on a variety of properties governing the photocatalytic performance such as nanocluster adsorption, light absorption, charge separation, and reducibility. Our results indicate that the modifiers adsorb strongly at the surface and that modification extends light absorption into the visible range. MnOx-modified titania can show an off-stoichiometric ground state, through oxygen vacancy formation and cation reduction spontaneously, and both modified rutile and anatase are highly reducible with moderate energy costs. Manganese ions are therefore present in a mixture of oxidation states. Photoexcited electrons and holes localize at cluster metal and oxygen sites, respectively. The interaction of water at the modified surfaces depends on the stoichiometry and spontaneous dissociation to surface bound hydroxyls is favored in the presence of oxygen vacancies and reduced metal cations. Comparisons with bare TiO2 and other TiO2-based photocatalyst materials are presented throughout.

Surface reconstruction of cobalt phosphide nanosheets by electrochemical activation for enhanced hydrogen evolution in alkaline solution

The surface reconstruction of cobalt phosphide nanosheets is investigated by an in situ electrochemical strategy for enhanced hydrogen evolution.

Transition metal phosphides exhibit promising catalytic performance for the hydrogen evolution reaction (HER); however their surface structure evolution during electrochemical operation has rarely been studied. In this work, we investigate the surface reconstruction of CoP nanosheets by an in situ electrochemical activation method. After remodeling, CoP nanosheets experience an irreversible and significant evolution of the morphology and composition, and low-valence Co complexes consisting of Co(OH)_x species are formed on the surface of CoP nanosheets, and they largely accelerate the dissociation of water. Benefiting from the synergistic effect of CoP and Co(OH)_x, the working electrode shows a remarkably enhanced HER activity of 100 mV at 10 mA cm^–2 with a Tafel slope of 76 mV dec^–1, which is better than that of most transition metal phosphide catalysts. This work would provide a deep understanding of surface reconstruction and a novel perspective for rational design of high performance transition metal phosphide electrocatalysts for water related electrolysis.

Improving the activity of gold nanoparticles for the water-gas shift reaction using TiO2-Y2O3: an example of catalyst design

In the last ten years, there has been an acceleration in the pace at which new catalysts for the water-gas shift reaction are designed and synthesized. Pt-based catalysts remain the best solution when only activity is considered. However, cost, operation temperature, and deactivation phenomena are important variables when these catalysts are scaled in industry. Here, a new catalyst, Au/TiO2-Y2O3, is presented as an alternative to the less selective Pt/oxide systems. Experimental and theoretical techniques are combined to design, synthesize, characterize and analyze the performance of this system. The mixed oxide demonstrates a synergistic effect, improving the activity of the catalyst not only at large-to-medium temperatures but also at low temperatures. This effect is related to the homogeneous dispersion of the vacancies that act both as nucleation centers for smaller and more active gold nanoparticles and as dissociation sites for water molecules. The calculated reaction path points to carboxyl formation as the rate-limiting step with an activation energy of 6.9 kcal mol-1, which is in quantitative agreement with experimental measurements and, to the best of our knowledge, it is the lowest activation energy reported for the water-gas shift reaction. This discovery demonstrates the importance of combining experimental and theoretical techniques to model and understand catalytic processes and opens the door to new improvements to reduce the operating temperature and the deactivation of the catalyst.

Ab Initio Study of the Atomic Level Structure of the Rutile TiO2(110)-Titanium Nitride (TiN) Interface

Titanium nitride (TiN) is widely used in industry as a protective coating due to its hardness and resistance to corrosion and can spontaneously form a thin oxide layer when it is exposed to air, which could modify the properties of the coating. With limited understanding of the TiO₂-TiN interfacial system at present, this work aims to describe the structural and electronic properties of oxidized TiN based on a density functional theory (DFT) study of the rutile TiO₂(110)-TiN(100) interface model system, also including Hubbard +U correction on Ti 3d states. The small lattice mismatch gives a good stability to the TiO₂-TiN interface after depositing the oxide onto TiN through the formation of interfacial Ti-O bonds. Our DFT+U study shows the presence of Ti³⁺ cations in the TiO₂ region, which are preferentially located next to the interface region as well as the rotation of the rutile TiO₂ octahedra in the interface structure. The DFT+U TiO₂ electronic density of states (EDOS) shows localized Ti³⁺ defect states forming in the midgap between the top edge of the valence and the bottom of the conduction band. We increase the complexity of our models by the introduction of nonstoichiometric compositions. Although the vacancy formation energies for Ti in TiN (E_vac (Ti) ≥ 4.03 eV) or O in the oxide (E_vac (O) ≥ 3.40 eV) are quite high relative to perfect TiO₂-TiN, defects are known to form during the oxide growth and can therefore be present after TiO₂ formation. Our results show that a structure with exchanged O and N can lie 0.82 eV higher in energy than the perfect system, suggesting the stability of structures with interdiffused O and N anions at ambient conditions. The presence of N in TiO₂ introduces N 2p states localized between the top edge of the O 2p valence states and the midgap Ti³⁺ 3d states, thus reducing the band gap in the TiO₂ region for the exchanged O/N interface EDOS. The outcomes of these simulations give us a most comprehensive insight on the atomic level structure and the electronic properties of oxidized TiN surfaces.

Trends in Adhesion Energies of Metal Nanoparticles on Oxide Surfaces: Understanding Support Effects in Catalysis and Nanotechnology

Nanoparticles on surfaces are ubiquitous in nanotechnologies, especially in catalysis, where metal nanoparticles anchored to oxide supports are widely used to produce and use fuels and chemicals, and in pollution abatement. We show that for hemispherical metal particles of the same diameter, D, the chemical potentials of the metal atoms in the particles (μ_M) differ between two supports by approximately -2(E_adh,A - E_adh,B)V_m/D, where E_ad,i is the adhesion energy between the metal and support i, and V_m is the molar volume of the bulk metal. This is consistent with calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces where the metal grows as 3D particles, which proved that μ_M increases with decreasing particle size below 6 nm and, for a given size, decreases with E_adh. Since catalytic activity and sintering rates correlate with metal chemical potential, it is thus crucial to understand what properties of catalyst materials control metal/oxide adhesion energies. Trends in how E_adh varies with the metal and the support oxide are presented. For a given oxide, E_adh increases linearly from metal to metal with increasing heat of formation of the most stable oxide of the metal (per mole metal), or metal oxophilicity, suggesting that metal-oxygen bonds dominate interfacial bonding. For the two different stoichiometric oxide surfaces that have been studied on multiple metals (MgO(100) and CeO₂(111), the slopes of these lines are the same, but their offset is large (∼2 J/m²). Adhesion energies increase as MgO(100) ≈ TiO₂(110) < α-Al₂O₃(0001) < CeO₂(111) ≈ Fe₃O₄(111).

Ceria-based model catalysts: fundamental studies on the importance of the metal–ceria interface in CO oxidation, the water–gas shift, CO2 hydrogenation, and methane and alcohol reforming

Model metal/ceria and ceria/metal catalysts have shown to be excellent systems for studying fundamental phenomena linked to the operation of technical catalysts.

Effects of strong interactions between Ti and ceria on the structures of Ti/CeO2

The effects of strong interactions between Ti and ceria on the structures of Ti/CeO2(111) are systematically investigated by density functional theory calculation. To our best knowledge, the adsorption energy of a Ti atom at the hollow site of CeO2 is the highest value (-7.99 eV) reported in the literature compared with those of Au (-0.88--1.26 eV), Ag (-1.42 eV), Cu (-2.69 eV), Pd (-1.75 eV), Pt (-2.62 eV) and Sn (-3.68 eV). It is very interesting to find that Ti adatoms disperse at the hollow site of CeO2(111) to form surface TiOx species, instead of aggregating to form Ti metal clusters for the Ti-CeO2 interactions that are much stronger than those of Ti-Ti ones. Ti adatoms are completely oxidized to Ti4+ ions if they are monatomically dispersed on the next near hollow sites of CeO2(111) (xTi-NN-hollow); while Ti3+ ions are observed when they locate at the near hollow sites (xTi-N-hollow). Due to the electronic repulsive effects among Ti3+ ions, the adsorption energies of xTi-N-hollow are slightly weaker than those of xTi-NN-hollow. Simultaneously, the existence of unstable Ti3+ ions on xTi-N-hollow also leads to the restructuring of xTi-N-hollow by surface O atoms of ceria transferring to the top of Ti3+ ions, or oxidation by O2 adsorption and dissociation. Both processes improve the stability of the xTi/CeO2 system by Ti3+ oxidation. Correspondingly, surface TiO2-like species form. This work sheds light into the structures of metal/CeO2 catalysts with strong interactions between the metal and the ceria support.

Low Pressure CO2 Hydrogenation to Methanol over Gold Nanoparticles Activated on a CeO(x)/TiO2 Interface

Capture and recycling of CO2 into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO2 is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO(x)/TiO2 substrate generates active centers for CO2 adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO2 hydrogenation.

Highly dispersed CeO₂ on TiO₂ nanotube: a synergistic nanocomposite with superior peroxidase-like activity

In this report, a novel nanocomposite of highly dispersed CeO2 on a TiO2 nanotube was designed and proposed as a peroxidase-like mimic. The best peroxidase-like activity was obtained for the CeO2/nanotube-TiO2 when the molar ratio of Ce/Ti was 0.1, which was much higher than that for CeO2/nanowire-TiO2, CeO2/nanorod-TiO2, or CeO2/nanoparticle-TiO2 with a similar molar ratio of Ce/Ti. Moreover, in comparison with other nanomaterial based peroxidase mimics, CeO2/nanotube-TiO2 nanocomposites exhibited higher affinity to H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB). Kinetic analysis indicated that the catalytic behavior was in accordance with typical Michaelis-Menten kinetics. Ce(3+) sites were confirmed as the catalytic active sites for the catalytic reaction. The first interaction of surface CeO2 with H2O2 chemically changed the surface state of CeO2 by transforming Ce(3+) sites into surface peroxide species causing adsorbed TMB oxidation. Compared with CeO2/nanowire-TiO2, CeO2/nanorod-TiO2, and CeO2/nanoparticle-TiO2, the combination of TiO2 nanotube with CeO2 presented the highest concentration of Ce(3+) thus leading to the best peroxidase-like activity. On the basis of the high activity of CeO2/nanotube-TiO2, the reaction provides a simple method for colorimetric detection of H2O2 and glucose with the detection limits of 3.2 and 6.1 μM, respectively.

When ruthenia met titania: achieving extraordinary catalytic activity at low temperature by nanostructuring of oxides

Nanostructured ruthenia shows strongly modified properties compared to the pure oxide thereby becoming a low-temperature high-activity catalyst.

TiO2@CeOx core-shell nanoparticles as artificial enzymes with peroxidase-like activity

The Ce4+↔Ce3+ redox switch is at the basis of an all-inorganic catalytic cycle that is capable of mimicking the activity of several natural redox enzymes. The efficiency of these artificial enzymes (nanozymes) strongly depends on the Ce4+/Ce3+ ratio. By capitalizing on the results obtained on oxide/oxide model systems, we implemented a simple and effective procedure to obtain conformal TiO2@CeOx core-shell nanoparticles whose thickness is controlled with single-layer precision. Since the Ce3+ species are stabilized only at the interface by the electronic hybridization with the TiO2 states, the modulation of the shell thickness offers a simple method to tailor the Ce4+/Ce3+ ratio and therefore the catalytic properties. The activity of these nanoparticles as artificial peroxidase-like enzymes was tested, showing exceptional performances, even better than natural horseradish peroxidase enzyme. The main advantage with respect to other oxide/oxide nanozymes is that our nanoparticles, having a tunable Ce4+/Ce3+ ratio, are efficient already at low H2O2 concentrations.

Rationalization of the Hubbard U parameter in CeO(x) from first principles: unveiling the role of local structure in screening

The density functional theory (DFT)+U method has been widely employed in theoretical studies on various ceria systems to correct the delocalization bias in local and semi-local DFT functionals with moderate computational cost. We present a systematic and quantitative study, aiming to gain better understanding of the dependence of Hubbard U on the local atomic arrangement. To rationalize the Hubbard U of Ce 4f, we employed the first principles linear response method to compute Hubbard U for Ce in ceria clusters, bulks, and surfaces. We found that the Hubbard U varies in a wide range from 4.3 eV to 6.7 eV, and exhibits a strong correlation with the Ce coordination number and Ce-O bond lengths, rather than the Ce 4f valence state. The variation of the Hubbard U can be explained by the changes in the strength of local screening due to O → Ce intersite transitions.

Water-gas shift on gold catalysts: catalyst systems and fundamental studies

Since the pioneering finding by Haruta et al. that small gold nanoparticles on reducible supports can be highly active for low-temperature CO oxidation, the synthesis, characterization, and application of supported gold catalysts have attracted much attention. The water-gas shift reaction (WGSR: CO + H2O = CO2 + H2) is important for removing CO and upgrading the purity of H2 for fuel cell applications, ammonia synthesis, and selective hydrogenation processes. In recent years, much attention has been paid to exploration the possibility of using supported gold nanocatalysts for WGSR and understanding the fundamental aspects related to catalyst deactivation mechanisms, nature of active sites, and reaction mechanisms. Here we summarize recent advances in the development of supported gold catalysts for this reaction and fundamental insights that can be gained, and furnish our assessment on the status of research progress.

Structure and special chemical reactivity of interface-stabilized cerium oxide nanolayers on TiO2(110)

Novel interface-stabilized ceria nanophases have been grown on TiO2(110) by physical vapor deposition. At low coverage, dumbbell nanostructures constituted by reconstructed titania and ceria clusters are formed, while long range ordered nanoxides can be obtained by increasing the ceria dose. Scanning tunneling microscopy and photoemission spectroscopy were used to characterize the electronic properties of the films, showing that the TiO2 substrate can effectively stabilize ceria in reduced form over a wide range of experimental conditions. Epitaxial coupling is a very useful tool for tuning the chemical properties of mixed oxide systems. The special electronic properties of the films have a direct counterpart in the chemical activity, which has been investigated by temperature programmed desorption using methanol as a probe molecule. The experimental results indicate an exceptional activity of the ceria-titania interface in the selective dehydration of methanol to formaldehyde at an unprecedented low temperature (330 K).

First-principles prediction of new photocatalyst materials with visible-light absorption and improved charge separation: surface modification of rutile TiO₂ with nanoclusters of MgO and Ga₂O₃

Titanium dioxide is an important and widely studied photocatalytic material, but to achieve photocatalytic activity under visible-light absorption, it needs to have a narrower band gap and reduced charge carrier recombination. First-principles simulations are presented in this paper to show that heterostructures of rutil TiO₂ modified with nanoclusters of MgO and Ga₂O₃ will be new photocatalytically active materials in the UV (MgO-TiO₂) and visible (Ga₂O₃-TiO₂) regions of the solar spectrum. In particular, our investigations of a model of the excited state of the heterostructures demonstrate that upon light excitation electrons and holes can be separated onto the TiO₂ surface and the metal oxide nanocluster, which will reduce charge recombination and improve photocatalytic activity. For MgO-modified TiO₂, no significant band gap change is predicted, but for Ga₂O₃-modified TiO₂ we predict a band gap change of up to 0.6 eV, which is sufficient to induce visible light absorption. Comparisons with unmodified TiO₂ and other TiO₂-based photocatalyst structures are presented.

Density functional study of water-gas shift reaction on M3O(3x)/Cu(111)

Density functional theory (DFT) was employed to study the water dissociation and water-gas shift (WGS) reaction on a series of inverse model catalysts, M(3)O(3x)/Cu(111) (M = Mg, Ti, Zr, Mo, W; x = 1, 2, 3). It has been found that the WGS reaction on Cu can be facilitated by introducing various oxides to lower the barrier of water dissociation. Accordingly, the calculated reaction energy for water dissociation was used as a scaling descriptor to screen the WGS activity of oxide-Cu model catalysts. Our calculations show that the activity towards water dissociation decreases in a sequence: Mg(3)O(3)/Cu(111) > Zr(3)O(6)/Cu(111) > Ti(3)O(6)/Cu(111) > W(3)O(9)/Cu(111), Mo(3)O(9)/Cu(111). It seems that Mg(3)O(3)/Cu(111) is the best WGS catalyst among the systems studied here, being able to dissociate water with no barrier. During the process, both Cu and oxides participate in the reaction directly. The strong M(3)O(3x)-Cu interaction is able to tune the electronic structure of M(3)O(3x) and therefore the activity towards water dissociation. Further studies of the overall WGS reaction on Mg(3)O(3)/Cu(111) show that water dissociation may not be the key step to control the WGS reaction on Mg(3)O(3)/Cu(111) and the removal of H from Mg(3)O(3) can be problematic. The strong interaction between H and O from Mg(3)O(3) blocks the O sites for further water dissociation and therefore the WGS reaction. Our study observes a very different behavior of oxide clusters in such small size from the bigger ones supported on Cu(111) and provides new insight into the rational design of the WGS catalysts.

Water-gas shift reaction on oxide∕Cu(111): Rational catalyst screening from density functional theory

Developing improved catalysts based on a fundamental understanding of reaction mechanism has become one of the grand challenges in catalysis. A theoretical understanding and screening the metal-oxide composite catalysts for the water-gas shift (WGS) reaction is presented here. Density functional theory was employed to identify the key step for the WGS reaction on the Au, Cu-oxide catalysts, where the calculated reaction energy for water dissociation correlates well with the experimental measured WGS activity. Accordingly, the calculated reaction energy for water dissociation was used as the scaling descriptor to screen the inverse model catalysts, oxide∕Cu(111), for the better WGS activity. Our calculations predict that the WGS activity increases in a sequence: Cu(111), ZnO∕Cu(111) < TiO(2)∕Cu(111), ZrO(2)∕Cu(111) < MoO(3)∕Cu(111). Our results imply that the high performances of Au, Cu-oxide nanocatalysts in the WGS reaction rely heavily on the direct participation of both oxide and metal sites. The degree that the oxide is reduced by Cu plays an important role in determining the WGS activity of oxide∕Cu catalysts. The reducible oxide can be transformed from the fully oxidized form to the reduced form due to the interaction with Cu and, therefore, the transfer of electron density from Cu, which helps in releasing the bottleneck water dissociation and, therefore, facilitating the WGS reaction on copper.