Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives.

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Highly selective NH3 synthesis from N2 on electron-rich Bi0 in a pressurized electrolyzer.

Electrochemical conversion of N2 into ammonia presents a sustainable pathway to produce hydrogen storage carrier but yet requires further advancement in electrocatalyst design and electrolyzer integration. This technology suffers from low selectivity and yield owing to the extremely strong N≡N bond and the exceptionally low solubility of N2 in aqueous systems. A high NH3 synthesis performance is restricted by the high activation energy of N≡N bond and the supply insufficiency of N2 to active sites. This paper describes the introduction of electron-rich Bi0 sites into Ag catalysts with a high-pressure electrolyzer that enables a dramatically enhanced Faradaic efficiency of 44.0% and yield of 28.43 µg cm-2 h-1 at 4.0 MPa. Combined with density functional theory results, in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy demonstrates that N2 reduction reaction follows an associative mechanism, in which a high coverage of N-N bond and -NH2 intermediates suggest electron-rich Bi0 boosts sound activation of N2 molecules and low hydrogenation barrier. The proposed strategy of engineering electrochemical catalysts and devices provides powerful guidelines for achieving industrial-level green ammonia production.

Modulation of *CHxO Adsorption to Facilitate Electrocatalytic Reduction of CO2 to CH4 over Cu-Based Catalysts.

Copper (Cu) can efficiently catalyze the electrochemical CO2 reduction reaction (CO2RR) to produce value-added fuels and chemicals, among which methane (CH4) has drawn attention due to its high mass energy density. However, the linear scaling relationship between the adsorption energies of *CO and *CHxO on Cu restricts the selectivity toward CH4. Alloying a secondary metal in Cu provides a new freedom to break the linear scaling relationship, thus regulating the product distribution. This paper describes a controllable electrodeposition approach to alloying Cu with oxophilic metal (M) to steer the reaction pathway toward CH4. The optimized La5Cu95 electrocatalyst exhibits a CH4 Faradaic efficiency of 64.5%, with the partial current density of 193.5 mA cm-2. The introduction of oxophilic La could lower the energy barrier for *CO hydrogenation to *CHxO by strengthening the M-O bond, which would also promote the breakage of the C-O bond in *CH3O for the formation of CH4. This work provides a new avenue for the design of Cu-based electrocatalysts to achieve high selectivity in CO2RR through the modulation of the adsorption behaviors of key intermediates.

High-Throughput Screening of Electrocatalysts for Nitrogen Reduction Reactions Accelerated by Interpretable Intrinsic Descriptor.

Developing easily accessible descriptors is crucial but challenging to rationally design single-atom catalysts (SACs). This paper describes a simple and interpretable activity descriptor, which is easily obtained from the atomic databases. The defined descriptor proves to accelerate high-throughput screening of more than 700 graphene-based SACs without computations, universal for 3-5d transition metals and C/N/P/B/O-based coordination environments. Meanwhile, the analytical formula of this descriptor reveals the structure-activity relationship at the molecular orbital level. Using electrochemical nitrogen reduction as an example, this descriptor's guidance role has been experimentally validated by 13 previous reports as well as our synthesized 4 SACs. Orderly combining machine learning with physical insights, this work provides a new generalized strategy for low-cost high-throughput screening while comprehensive understanding the structure-mechanism-activity relationship.

Modeling CoCu Nanoparticles Using Neural Network-Accelerated Monte Carlo Simulations.

The correct description of catalytic reactions happening on bimetallic particles is not feasible without proper accounting of the segregation process. In this study, we tried to shed light on the structure of large CoCu particles, for which quite controversial results were published before. However, density functional theory (DFT) is challenging to be directly used for the systematic study of nanometer-sized particles. Therefore, we constructed a neural network-based potential and further applied it to the Monte Carlo simulations for the description of the segregation phenomenon. The resulting approach shows high efficiency and can be used in systems with thousands of atoms. The accuracy and transferability of the model to other sizes and compositions make this methodology useful for solving segregation problems.

Data-Driven Interpretable Descriptors for the Structure-Activity Relationship of Surface Lattice Oxygen on Doped Vanadium Oxides.

Understanding the structure-activity relationship of surface lattice oxygen is critical but challenging to design efficient redox catalysts. This paper describes data-driven redox activity descriptors on doped vanadium oxides combining density functional theory and interpretable machine learning. We corroborate that the p-band center is the most crucial feature for the activity. Besides, some features from the coordination environment, including unoccupied d-band center, s- and d-band fillings, also play important roles in tuning the oxygen activity. Further analysis reveals that data-driven descriptors could decode more information about electron transfer during the redox process. Based on the descriptors, we report that atomic Re- and W-doping could inhibit over-oxidation in the chemical looping oxidative dehydrogenation of propane, which is verified by subsequent experiments and calculations. This work sheds light on the structure-activity relationship of lattice oxygen for the rational design of redox catalysts.

Moderate Surface Segregation Promotes Selective Ethanol Production in CO2 Hydrogenation Reaction over CoCu Catalysts.

Cobalt-copper (CoCu) catalysts have industrial potential in CO/CO2 hydrogenation reactions, and CoCu alloy has been elucidated as a major active phase during reactions. However, due to elemental surface segregation and dealloying phenomena, the actual surface morphology of CoCu alloy is still unclear. Combining theory and experiment, the dual effect of surface segregation and varied CO coverage over the CoCu(111) surface on the reactivity in CO2 hydrogenation reactions is explored. The relationship between C-O bond scission and further hydrogenation of intermediate *CH2 O was discovered to be a key step to promote ethanol production. The theoretical investigation suggests that moderate Co segregation provides a suitable surface Co ensemble with lateral interactions of co-adsorbed *CO, leading to promoted selectivity to ethanol, in agreement with theory-inspired experiments.

Nature of the Active Sites of Copper Zinc Catalysts for Carbon Dioxide Electroreduction.

The electrochemical CO2 reduction (CO2 ER) to multi-carbon chemical feedstocks over Cu-based catalysts is of considerable attraction but suffers with the ambiguous nature of active sites, which hinder the rational design of catalysts and large-scale industrialization. This paper describes a large-scale simulation to obtain realistic CuZn nanoparticle models and the atom-level structure of active sites for C2+ products on CuZn catalysts in CO2 ER, combining neural network based global optimization and density functional theory calculations. Upon analyzing over 2000 surface sites through high throughput tests based on NN potential, two kinds of active sites are identified, balanced Cu-Zn sites and Zn-heavy Cu-Zn sites, both facilitating C-C coupling, which are verified by subsequent calculational and experimental investigations. This work provides a paradigm for the design of high-performance Cu-based catalysts and may offer a general strategy to identify accurately the atomic structures of active sites in complex catalytic systems.

Synergistic Mechanism of Platinum-GaOx Catalysts for Propane Dehydrogenation.

The synergy between metals and metal oxides can effectively improve the heterogeneous catalytic process. This paper describes the intrinsic effect of Pt modification over GaOx (Pt-GaOx ) on propane dehydrogenation. The presence of Pt promotes H2 dissociation and surface coverage of hydrogen species, which is beneficial for the activation of C-H in propane. With excessive Pt, Gaδ+ can be further reduced to form Pt-Ga alloy with less surface hydrogen species. Consequently, the relative propylene formation rate between Pt-GaOx and the summed contribution of individual Pt and GaOx increases linearly with the content of hydrogen species. Optimally, the relative propylene formation rate of Pt-GaOx with 0.03 wt % Pt exceeds 25 % of the summed contribution of individual components.

Theoretical insights into single-atom catalysts.

Single-atom catalysts (SACs) with atomically dispersed metals have emerged as a new class of heterogeneous catalysts and have attracted considerable interest because they offer 100% metal atom utilization and show excellent catalytic behavior compared with traditionally supported nano-particles. However, it is challenging to explore the active sites and catalytic mechanisms of SACs through common characterization methods due to the isolated single atoms. Therefore, employing theoretical calculations to determine the nature of SACs' active sites and the reaction mechanisms is particularly meaningful. This paper describes the nature of SACs by summarizing the diverse applications and properties of SACs, which starts from computational simulation on a couple of important applications of SACs. Then the distinctive and fundamental properties of SACs are discussed. At last, the challenges and future perspectives of computational calculations for SACs are outlined.

Controllable Cu0 -Cu+ Sites for Electrocatalytic Reduction of Carbon Dioxide.

Cu-based electrocatalysts facilitate CO2 electrochemical reduction (CO2 ER) to produce multi-carbon products. However, the roles of Cu0 and Cu+ and the mechanistic understanding remain elusive. This paper describes the controllable construction of Cu0 -Cu+ sites derived from the well-dispersed cupric oxide particles supported on copper phyllosilicate lamella to enhance CO2 ER performance. 20 % Cu/CuSiO3 shows the superior CO2 ER performance with 51.8 % C2 H4 Faraday efficiency at -1.1 V vs reversible hydrogen electrode during the 6 hour test. In situ attenuated total reflection infrared spectra and density functional theory (DFT) calculations were employed to elucidate the reaction mechanism. The enhancement in CO2 ER activity is mainly attributed to the synergism of Cu0 -Cu+ pairs: Cu0 activates CO2 and facilitates the following electron transfers; Cu+ strengthens *CO adsorption to further boost C-C coupling. We provide a strategy to rationally design Cu-based catalysts with viable valence states to boost CO2 ER.

Controllable Distribution of Oxygen Vacancies in Grain Boundaries of p-Si/TiO2 Heterojunction Photocathodes for Solar Water Splitting.

Silicon is a promising photocathode material in photoelectrochemical water splitting for hydrogen production, but it is primarily limited by photocorrosion in aqueous electrolytes. As an extensively used protective material, crystalline TiO2 could protect Si photoelectrode against corrosion. However, a large number of grain boundaries (GBs) in polycrystalline TiO2 would induce excessive recombination centers, impeding the carrier transport. This paper describes the introduction of oxygen vacancies (Ovac ) with controllable spatial distribution for GBs to promote carrier transport. Two kinds of Ovac distribution, Ovac along GBs and Ovac inside grains, are compared, where the latter one is demonstrated to facilitate carrier transport owing to the formation of tunneling paths across GBs. Consequently, a simple p-Si/TiO2 /Pt heterojunction photocathode with controllable Ovac distribution in TiO2 shows a +400 mV onset potential shift and yields an applied bias photon-to-current efficiency of 5.9 %, which is the best efficiency reported among silicon photocathodes except for silicon homojunction.

Coupling of Cu(100) and (110) Facets Promotes Carbon Dioxide Conversion to Hydrocarbons and Alcohols.

Copper can efficiently electro-catalyze carbon dioxide reduction to C2+ products (C2 H4 , C2 H5 OH, n-propanol). However, the correlation between the activity and active sites remains ambiguous, impeding further improvements in their performance. The facet effect of copper crystals to promote CO adsorption and C-C coupling and consequently yield a superior selectivity for C2+ products is described. We achieve a high Faradaic efficiency (FE) of 87 % and a large partial current density of 217 mA cm-2 toward C2+ products on Cu(OH)2 -D at only -0.54 V versus the reversible hydrogen electrode in a flow-cell electrolyzer. With further coupled to a Si solar cell, record-high solar conversion efficiencies of 4.47 % and 6.4 % are achieved for C2 H4 and C2+ products, respectively. This study provides an in-depth understanding of the selective formation of C2+ products on Cu and paves the way for the practical application of electrocatalytic or solar-driven CO2 reduction.

Strong Electronic Oxide-Support Interaction over In2O3/ZrO2 for Highly Selective CO2 Hydrogenation to Methanol.

Metal oxides are widely employed in heterogeneous catalysis, but it remains challenging to determine their exact structure and understand the reaction mechanisms at the molecular level due to their structural complexity, in particular for binary oxides. This paper describes the observation of the strong electronic interaction between In2O3 and monoclinic ZrO2 (m-ZrO2) by quasi-in-situ XPS experiments combined with theoretical studies, which leads to support-dependent methanol selectivity. In2O3/m-ZrO2 exhibits methanol selectivity up to 84.6% with a CO2 conversion of 12.1%. Moreover, at a wide range of temperatures, the methanol yield of In2O3/m-ZrO2 is much higher than that of In2O3/t-ZrO2 (t-: tetragonal), which is due to the high dispersion of the In-O-In structure over m-ZrO2 as determined by in situ Raman spectra. The electron transfer from m-ZrO2 to In2O3 is confirmed by XPS and DFT calculations and improves the electron density of In2O3, which promotes H2 dissociation and hydrogenation of formate intermediates to methanol. The concept of the electronic interaction between an oxide and a support provides guidelines to develop hydrogenation catalysts.

Grain-Boundary-Rich Copper for Efficient Solar-Driven Electrochemical CO2 Reduction to Ethylene and Ethanol.

The grain boundary in copper-based electrocatalysts has been demonstrated to improve the selectivity of solar-driven electrochemical CO2 reduction toward multicarbon products. However, the approach to form grain boundaries in copper is still limited. This paper describes a controllable grain growth of copper electrodeposition via poly(vinylpyrrolidone) used as an additive. A grain-boundary-rich metallic copper could be obtained to convert CO2 into ethylene and ethanol with a high selectivity of 70% over a wide potential range. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy unveils that the existence of grain boundaries enhances the adsorption of the key intermediate (*CO) on the copper surface to boost the further CO2 reduction. When coupling with a commercially available Si solar cell, the device achieves a remarkable solar-to-C2-products conversion efficiency of 3.88% at a large current density of 52 mA·cm-2. This low-cost and efficient device is promising for large-scale application of solar-driven CO2 reduction.

FeO6 Octahedral Distortion Activates Lattice Oxygen in Perovskite Ferrite for Methane Partial Oxidation Coupled with CO2 Splitting.

Modulating lattice oxygen in metal oxides that conducts partial oxidation of methane in balancing C-H activation and syngas selectivity remains challenging. This paper describes the discovery of distorting FeO6 octahedra in La1-xCexFeO3 (x = 0, 0.25 0.5, 0.75, 1) orthorhombic perovskites for the promotion of lattice oxygen activation. By combined electrical conductivity relaxation measurements and density functional theory calculations studies, this paper describes the enhancement of FeO6 octahedral distortion in La1-xCexFeO3 promoting their bulk oxygen mobility and surface oxygen exchange capability. Consequently, La0.5Ce0.5FeO3 with the highest FeO6 distortion achieves exceptional syngas productivity of ∼3 and 8 times higher than LaFeO3 and CeFeO3, respectively, in CH4 partial oxidation step with simultaneous high CO2 conversion (92%) in the CO2-splitting step at 850 °C. The results exemplify the feasibility to tailor the active lattice oxygen of perovskite by modulating the distortion of BO6 in ABO3, which ultimately influences their reaction performance in chemical looping processes.

Enriched Surface Oxygen Vacancies of Photoanodes by Photoetching with Enhanced Charge Separation.

A facile photoetching approach is described that alleviates the negative effects from bulk defects by confining the oxygen vacancy (Ovac ) at the surface of BiVO4 photoanode, by 10-minute photoetching. This strategy could induce enriched Ovac at the surface of BiVO4 , which avoids the formation of excessive bulk defects. A mechanism is proposed to explain the enhanced charge separation at the BiVO4  /electrolyte interface, which is supported by density functional theory (DFT) calculations. The optimized BiVO4 with enriched surface Ovac presents the highest photocurrent among undoped BiVO4 photoanodes. Upon loading FeOOH/NiOOH cocatalysts, photoetched BiVO4 photoanode reaches a considerable water oxidation photocurrent of 3.0 mA cm-2 at 0.6 V vs. reversible hydrogen electrode. An unbiased solar-to-hydrogen conversion efficiency of 3.5 % is realized by this BiVO4 photoanode and a Si photocathode under 1 sun illumination.

Coverage-Dependent Behaviors of Vanadium Oxides for Chemical Looping Oxidative Dehydrogenation.

Chemical looping provides an energy- and cost-effective route for alkane utilization. However, there is considerable CO2 co-production caused by kinetically mismatched O2- bulk diffusion and surface reaction in current chemical looping oxidative dehydrogenation systems, rendering a decreased olefin productivity. Sub-monolayer or monolayer vanadia nanostructures are successfully constructed to suppress CO2 production in oxidative dehydrogenation of propane by evading the interference of O2- bulk diffusion (monolayer versus multi-layers). The highly dispersed vanadia nanostructures on titanium dioxide support showed over 90 % propylene selectivity at 500 °C, exhibiting turnover frequency of 1.9×10-2  s-1 , which is over 20 times greater than that of conventional crystalline V2 O5 . Combining in situ spectroscopic characterizations and DFT calculations, we reveal the loading-reaction barrier relationship through the vanadia/titanium interfacial interaction.

Enhanced CO2 Electroreduction on Neighboring Zn/Co Monomers by Electronic Effect.

It is of great significance to reveal the detailed mechanism of neighboring effects between monomers, as they could not only affect the intermediate bonding but also change the reaction pathway. This paper describes the electronic effect between neighboring Zn/Co monomers effectively promoting CO2 electroreduction to CO. Zn and Co atoms coordinated on N doped carbon (ZnCoNC) show a CO faradaic efficiency of 93.2 % at -0.5 V versus RHE during a 30-hours test. Extended X-ray absorption fine structure measurements (EXAFS) indicated no direct metal-metal bonding and X-ray absorption near-edge structure (XANES) showed the electronic effect between Zn/Co monomers. In situ attenuated total reflection-infrared spectroscopy (ATR-IR) and density functional theory (DFT) calculations further revealed that the electronic effect between Zn/Co enhanced the *COOH intermediate bonding on Zn sites and thus promoted CO production. This work could act as a promising way to reveal the mechanism of neighboring monomers and to influence catalysis.

Ultrathin Pd-Au Shells with Controllable Alloying Degree on Pd Nanocubes toward Carbon Dioxide Reduction.

Electrocatalytic reduction of carbon dioxide (CO2ER) to reusable carbon resources is a significant step to balance the carbon cycle. This Communication describes a seed-mediated growth method to synthesize ultrathin Pd-Au alloy nanoshells with controllable alloying degree on Pd nanocubes. Specifically, Pd@Pd3Au7 nanocrystals (NCs) show superior CO2ER performance, with a 94% CO faraday efficiency (FE) at -0.5 V vs reversible hydrogen electrode and approaching 100% CO FE from -0.6 to -0.9 V. The enhancement primarily originates from ensemble and ligand effects, i.e., appropriately proportional Pd-Au sites and electronic back-donation from Au to Pd. In situ attenuated total reflection infrared spectra and density functional theory calculations clarify the reaction mechanism. This work may offer a general strategy for the synthesis of bimetallic NCs to explore the structure-activity relationship in catalytic reactions.