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Physisorption of CO on ZnO(101¯0)under ambient conditions could cause the reaction between CO and lattice oxygen of ZnO to form CO2, while the collective interaction of weakly adsorbed molecules also modulates chemisorption and resulted in a new adsorbate structure for CO2on ZnO.

Under an industry-related high-temperature oxidation atmosphere, the structure and chemical states of metal nanocatalysts meeting sustainable development challenges change dramatically, deteriorating the activity and/or lowering the yield. Theoretically revealing the mechanisms of oxygen-induced structure evolution and establishing a framework to distinguish them are vital to improving the operando stability and rational design of metal nanocatalysts. Here, we studied the oxygen-induced disintegration and Ostwald ripening of Ni, Cu, Pt, Pd, and Ag nanoparticles on TiO2(110) using first-principles-based thermodynamic and kinetic simulations. It was found that oxygen promotes Ostwald ripening via the formation of Ag/Ag–O and Pd intermediates on the support and volatile gaseous PtO2complexes, induces disintegration of Ni nanoparticles to Ni–O complexes, and leads to the formation of copper oxide. These differences in the deactivation pathways can be attributed to the dependence of the ripening activation energies and disintegration free energies on the interaction between metal atoms/complexes and TiO2(110). Revealed knowledge and corresponding models provide valuable insights into the general mechanisms governing the structural evolution of supported nanocatalysts under reaction conditions.

X-ray absorption spectroscopy (XAS) is a powerful technique for simultaneously characterizing the oxidation states and local structures of working catalysts and battery materials, among others. However, deciphering the apparent oxidation state through XAS remains challenging because of the high sensitivity of spectra to multiple factors. Here, comprehensive first-principles calculations of X-ray absorption near-edge structure (XANES) spectra of a series of Fe-based catalysts and battery electrodes, including FeO, Fe2O3, Fe(OH)2, FeOOH, FeAl2O4, and MFeO2(M = Li, Na, or K), were performed to shed light on the issue by dissecting the dependence of XANES line shapes on detailed electronic and geometric structures. We revealed that, in comparison with the composition and factors usually extracted from XAS measurements (i.e., the oxidation state and local structure), nonlocal structures (e.g., crystal structure) that cannot be straightforwardly obtained from XAS experiments are more dominant factors of the XANES line shapes of the main edge, main peak, and postedge. As demonstrated through Fe compounds with the same or similar nonlocal structures, their line shapes are alike and shift in response to a change in the chemical environment. On the other hand, we found that the local coordination (octahedra vs tetrahedra) and the oxidation state are more dominant on the intensity of the pre-edge and the energy splitting between the pre-edge and the main edge, respectively. We demonstrated that clarifying these control factors and origins not only provides valuable insights into reliable assignment of spectra and understanding semiempirical rules but also unravels the structural information that is not directly accessible in XAS measurements.

Preferential oxidation of CO in excess H2(PROX) has been extensively explored for selective removal of CO with minimum H2consumption to prevent CO poisoning of the Pt-based anode in a proton-exchange-membrane fuel cell (PEMFC). Unmodified platinum group metal catalysts are widely used for this reaction, yet they still show unsatisfying activity for CO oxidation at low temperatures, because their stronger adsorption of CO would poison the active site. Cu-based catalysts are alternatives, but they suffer from structural instability. Therefore, designing a more efficient catalyst for PROX is highly required. In this work, a bimetallic Cu/Rh catalyst is designed that facilitates significantly weakening the CO poisoning effect due to its comparable adsorption strengths of O2and CO. As compared to the dissociative mechanism, CO oxidation via the OCOO-mediated associative mechanism on this catalyst is found to be more favorable, and the barriers of the steps in the catalytic cycle are modest, suggesting a high low-temperature activity for CO oxidation. Moreover, it is found that a Cu/Rh catalyst exhibits lower selectivity for H2oxidation than that for CO oxidation. Additionally, the systematic studies of the surface segregation of Cu/Rh induced by the adsorption of species in PROX show that a Cu/Rh catalyst exhibits a good structural stability under the typical PROX conditions. These results demonstrate that the designed bimetallic Cu/Rh catalyst is promising for the PROX reaction at low temperatures.

Atomically dispersed metal catalysts maximize atom efficiency and display unique catalytic properties compared with regular metal nanoparticles. However, achieving high reactivity while preserving high stability at appreciable loadings remains challenging. Here we solve the challenge by synergizing metal–support interactions and spatial confinement, which enables the fabrication of highly loaded atomic nickel (3.1 wt%) along with dense atomic copper grippers (8.1 wt%) on a graphitic carbon nitride support. For the semi-hydrogenation of acetylene in excess ethylene, the fabricated catalyst shows extraordinary catalytic performance in terms of activity, selectivity and stability—far superior to supported atomic nickel alone in the absence of a synergizing effect. Comprehensive characterization and theoretical calculations reveal that the active nickel site confined in two stable hydroxylated copper grippers dynamically changes by breaking the interfacial nickel–support bonds on reactant adsorption and making these bonds on product desorption. Such a dynamic effect confers high catalytic performance, providing an avenue to rationally design efficient, stable and highly loaded, yet atomically dispersed, catalysts.

Fischer–Tropsch synthesis, the conversion of CO and H2to long-chain hydrocarbons, is performed at relatively low temperatures and high pressures over the most commonly encountered iron-, ruthenium-, or cobalt-based catalysts. Identification of the morphologies and structure evolution of FTS catalysts under reaction conditions are essential for understanding the structure–reactivity relationship. In this work, we performed a comprehensive ab initiothermodynamics study to provide an understanding of the morphology evolution of cobalt (Co) catalyst with hexagonal close-packed (HCP) and face-centered cubic (FCC) crystal structures under a CO atmosphere. CO adsorption on numerous surfaces of HCP Co and FCC Co at different coverages were investigated. On both HCP Co and FCC Co, lateral interaction is attractive at lower coverage and becomes repulsive at higher coverage. Compared to FCC Co, though in average CO adsorption on HCP Co is stronger at lower coverage, they become similar at higher coverage due to the overwhelming lateral repulsion. We established the phase diagrams and the morphology evolution of FCC Co and HCP Co as a function of CO chemical potentials. The most probable exposed facets in FCC Co and HCP Co Wulff shapes were revealed under different CO atmospheres. At a relatively low CO chemical potential, many open facets could be exposed in Co equilibrium morphology, including {101̅2}, {101̅1}, {101̅0}, and {112̅0} facets for HCP Co and {311}, {110}, and {100} facets for FCC Co. In contrast, at a relatively high CO chemical potential, FCC Co has an octahedron shape composed entirely by close-packed {111} facets, while HCP Co is hexagonal prism shaped composed mainly by close-packed {0001} and {101̅0} facets. The morphology evolution of HCP and FCC phases would have great impact on the inherited catalytic performance of Co nanoparticles, thus will selectively regulate their chemical reactivity.

Crystal structures and surface energies of transition metals are of fundamental importance to the activity, selectivity, and stability in heterogeneous catalysis, but the interplay between crystal structures and surface energies as well as its dependence on composition remains elusive. In the present work, we performed comprehensive density functional theory calculations of Co, Ni, Ru, Rh, Pd, Os, and Ir in both hexagonal close-packed (hcp) and face-centered cubic (fcc) phases and considered numerous surfaces to derive the equilibrium morphology and the surface energy. Irrespective of the transition metals considered, the fcc phase exposes mainly {111} and {100} facets, whereas the hcp phase exposes mainly {0001}, {101̅0}, and {101̅1} facets. For Co, Ru, and Os preferring the hcp bulk at ambient conditions, the corresponding surface energies are found higher to be than those in the fcc bulk, whereas for Ni, Rh, Pd, and Ir preferring the fcc bulk phase at ambient conditions, the opposite trend is found. A negative linear relationship of the surface energy difference between the fcc and hcp phases with respect to the corresponding bulk energy difference is established; a phase with a higher bulk energy has a lower surface energy as compensation. The compensation effect on the surface energy and the bulk energy provides a driving force for the size-induced phase transition of the nanoparticles. The results are used to rationalize the available experiments, and the insights revealed might be used to design better catalysts.

CO methanation is an industrially important reaction for the removal of trace amounts of CO from the hydrogen feed for ammonia production and in proton exchange membrane fuel cells. Although the H-assisted CO dissociation mechanism has been extensively elucidated, discrepancies exist in determining through which C1-oxygenate intermediates the C–O bonds are broken. Using density functional theory calculations and microkinetic studies, we show that theoretical studies can reach agreement in C–O bond scission viathe CHO intermediate on Co(0001) at a low coverage regime, and this mainly controls the CO methanation rate. This mechanism is independent of the functionals considered and the presence of graphitic carbon, and likely also pertains to other Co surface structures, including some open facets and step sites. The work provides fundamental insights into the mechanistic discrepancies relating to CO activation and methanation on hexagonal close-packed Co catalysts, which can potentially be used to design improved CO hydrogenation catalysts.

The efficient electrochemical hydrogen evolution reaction (HER) plays a key role in accelerating sustainable H2production from water electrolysis, but its large-scale applications are hindered by the high cost of the state-of-the-art Pt catalyst. In this work, submonolayer Pt was controllably deposited on an intermetallic Pd3Pb nanoplate (AL-Pt/Pd3Pb). The atomic efficiency and electronic structure of the active surface Pt layer were largely optimized, greatly enhancing the acidic HER. AL-Pt/Pd3Pb exhibits an outstanding HER activity with an overpotential of only 13.8 mV at 10 mA/cm2and a high mass activity of 7834 A/gPd+Ptat −0.05 V, both largely surpassing those of commercial Pt/C (30 mV, 1486 A/gPt). In addition, AL-Pt/Pd3Pb shows excellent stability and robustness. Theoretical calculations show that the improved activity is mainly derived from the charge transfer from Pd3Pb to Pt, resulting in a strong electrostatic interaction that can stabilize the transition state and lower the barrier.

Non-oxidative direct methane conversion provides a potentially economic and environmental friendly route for the use of natural gas and shale gas, but this process suffers the disadvantages of low activity and selectivity and harsh operating conditions. Using density functional calculations, we develop the relations in heats of adsorption of CHx(x= 0–4) species and catalytic performance of conventional Fe, Ru, and Co-based catalysts and identify the key factors that affect the activity and selectivity as methane adsorption and the relative strength of CH2and CH adsorption. Based on the analysis, we design the single Ru sites embedded in rutile TiO2(110) catalyst, which tunes the adsorption strength of CHxcompared with the traditional Ru-based catalyst, particularly weakening CH adsorption relative to CH2adsorption, thus leading to increased activity, improved selectivity toward ethylene, and strong resistance toward coking. This work highlights the impact of surface coordination environment, achieving fundamental insight that can be used to design and develop improved catalysts for direct methane conversion and other important reactions of technological interest.

The particle migration and coalescence (PMC) kinetics of a supported metal are the main deactivation mechanisms restricting the successful industrialization of nanoparticles, but the theoretical insights regarding these kinetics are lacking. One key issue is the lack of a physical model to predict the effects of metal-support interaction (MSI) on PMC kinetics. In this paper, we report a theoretical study of PMC kinetics and their dependence on MSI. A new particle diffusion model is proposed based on the surface premelting hypothesis that considers the contact angle of a hemispherical particle on the support. Enhanced MSI suppresses PMC by increasing the radius of curvature and the interfacial adhesion energy, even though the accompanying reduction in the geometry factor partially promotes PMC kinetics. The increased surface energy increases the chemical potential of the atoms in the particle, which is conducive to PMC; an increased surface energy also results in enhanced MSI, which suppresses PMC. The competition between these two contradictory effects leads to a critical contact angle where the surface energy has no influence on the diffusion and resulting PMC kinetics. The proposed diffusion theory mode lincluding the effects of the support and the corresponding kinetic simulations, shed light onto the support-dependence of PMC kinetics and provide a foundation for further optimization and design of supported particles with better stability.

The nature of the active sites and their structure sensitivity are the keys to rational design of efficient catalysts but have been debated for almost one century in heterogeneous catalysis. Though the Brønsted–Evans–Polanyi (BEP) relationship along with linear scaling relation has long been used to study the reactivity, explicit geometry, and composition properties are absent in this relationship, a fact that prevents its exploration in structure sensitivity of supported catalysts. In this work, based on interpretable multitask symbolic regression and a comprehensive first-principles data set, we discovered a structure descriptor, the topological under-coordinated number mediated by number of valence electrons and the lattice constant, to successfully address the structure sensitivity of metal catalysts. The database used for training, testing, and transferability investigation includes bond-breaking barriers of 20 distinct chemical bonds over 10 transition metals, two metal crystallographic phases, and 17 different facets. The resulting 2D descriptor composing the structure term and the reaction energy term shows great accuracy to predict the reaction barriers and generalizability over the data set with diverse chemical bonds in symmetry, bond order, and steric hindrance. The theory is physical and concise, providing a constructive strategy not only to understand the structure sensitivity but also to decipher the entangled geometric and electronic effects of metal catalysts. The insights revealed are valuable for the rational design of the site-specific metal catalysts.

The development of alternative alloy catalysts with high activity, surpassing platinum group metals, for the oxygen reduction reaction (ORR) is urgently needed in the field of electrocatalysis. The Ag-based single-atom alloy (AgSAA) cluster has been proposed as a promising catalyst for the ORR; however, enhancing its activity under operational conditions remains challenging due to limited insights into its actual active site. Here, we demonstrate that the operando formation of the MOx(OH)ycomplex serves as the key active site for catalyzing the ORR over AgSAA cluster catalysts, as revealed through comprehensive neural network potential molecular dynamics simulations combined with first-principles calculations. The volcano plot of the ORR over the MOx(OH)ycomplex addresses the gaps inherent in traditional metallic alloy models for pure AgSAA cluster catalysts in ORR catalysis. The appropriate orbital hybridization between OH and the dopant metal in the MOx(OH)ycomplexes indicated that the Ag54Co1, Ag54Pd1, and Ag54Au1clusters are optimal AgSAA catalysts for the ORR. Our work underscores the significance of theoretical modeling considering the reaction atmosphere in uncovering the true active site for the ORR, which can be extended to other reaction systems for rational catalyst design.

Identifying the intrinsic activity of the distinct sites which coexist in oxide-supported metal particles is vital but challenging for rational design of catalysts. We treat the challenge here by density functional theory calculations to differentiate unbiasedly the intrinsic reactivity of a variety of sites observed under reaction conditions for methanol steam reforming on Cu/ZnO catalyst. Metallic Cu and CuZn alloy are found to be less active but highly selective toward formaldehyde because water dissociation is demanding, which limits the formation of hydroxyl and subsequent coupling necessary to yield CO2. Cu/ZnO interface is highly active and selective for H2/CO2because of its superior activity for water and methanol activation. Distinct hydrogen affinity at Cu/ZnO interface also leads to more favorable CO2production via H2COO, in contrast to via HCOOH at (bi)metallic sites. The distinct reactivity of various structural motifs exposed and the importance of the metal/oxide for selectivity revealed is valuable for optimal design of catalysts.

The dynamic and kinetic evolution of supported metal particles in the presence of reactants is decisive in shaping the nature of the catalytic active sites and the deactivation process. Ostwald ripening of FeO/Pt(111) supported Au particles in the presence of carbon monoxide is addressed here by first-principles kinetics. It is found that CO stabilizes the ripening monomer (Au atom) by forming favorable Au carbonyls with lower total activation energy, and corresponding phase diagram at wide range of temperature and CO pressures is constructed. Evolution of particle number, dispersion and particle size distribution of supported Au particles are explored. Great influence of CO promotion on ripening kinetics is revealed and explored in details, and mbar range of CO can lower the onset temperature of ripening by a few hundred kelvins. The present work reveals the crucial role of the metal-reactant complexes formed under reaction conditions on ripening of metal catalysts.

Metal oxide plays an important role on stability and catalytic performance of supported metal nanoparticles, but mechanistic understanding of structure sensitivity and optimization of the oxide supports remains elusive in heterogeneous catalysis. Taking Ostwald ripening of platinum nanoparticles supported on titanium dioxide (TiO2) as an example, we reveal here a great structure sensitivity of oxide facets and crystal phases on sintering of supported metal nanoparticles through first-principles kinetic simulation. Total activation energies of the Pt ripening on various pristine TiO2surfaces of both anatase and rutile phases are calculated by density functional theory, and Ostwald ripening under isothermal condition and temperature programmed condition are simulated numerically. Calculated total activation energies are found inversely proportional to the corresponding oxide surface energies, and vary considerably from 1.76 to 3.56 eV. The Pt ripening rate on the pristine TiO2surfaces follows the order of r(001) ≈ a(001) ≫ a(100) ≈ r(101) > r(100) > a(101) ≈ r(110). For TiO2support exposing different facets, not only their intrinsic ripening rate but also their relative surface area determines the overall ripening kinetics and formation of transit bimodal particle size distribution. For pristine anatase TiO2exposing a(001) and a(101) facets, ripening starts on a(101) facets only after ripening on a(001) facets finishes due to their order of magnitude difference in ripening rate, resulting a step-wise increase of average particle size with ripening time. For pristine rutile TiO2exposing r(101) and r(110) facets, ripening could proceed simultaneously on both facets due to their modest difference in ripening rate, and the average particle size increases monotonically with ripening time. Compared to rutile TiO2, anatase TiO2supports are less resistant to the metal nanoparticles ripening since a(001) facets with high ripening rate is likely to be exposed. The present work is compared to available experiments and the theoretical framework established could be expanded to various metal and oxide systems.

Identification of the structure sensitivity of nitrogen molecule (N2) activation and ammonia synthesis on metal surfaces is important for the mechanistic understanding and rational design of more efficient catalysts. In the present work, density functional theory calculations together with microkinetic simulations were performed to study the influence of cobalt crystal structures including hexagonal close-packed (HCP) and face-centered cubic (FCC) on nitrogen molecule dissociation and ammonia synthesis. Molecular and dissociative adsorption energies of N2as well as dissociation barriers are calculated for a total of ten cobalt surfaces. It is found that molecular adsorption energies on Co surfaces vary modestly on the order of 0.25 eV, whereas dissociative adsorption energies and the corresponding barriers vary considerably in magnitude by about 0.80 eV. First-principles microkinetic simulations show that HCP Co displays higher activity than FCC cobalt for nitrogen molecule dissociation and ammonia synthesis due to the higher intrinsic activity and density of active sites of HCP cobalt. Nitrogen molecule dissociation is the rate-determining step of ammonia synthesis due to the weak interaction between nitrogen and cobalt. The crystal phase sensitivity of nitrogen molecule dissociation on cobalt is compared with the dissociation of an isoelectronic molecule, carbon monoxide on cobalt, ruthenium, and nickel. This work provides valuable insights into nitrogen molecule dissociation and ammonia synthesis on cobalt catalysts with different crystal phases, and highlights the interplay between activated molecules and catalyst composition on the crystal phase sensitivity.

Supported transition metal (TM) particles on oxides severely deactivate because of sintering. Investigation of the dependence of Ostwald ripening kinetics on the composition and size of the metal particles is essential for understanding the sintering mechanism. On the basis of the first-principles kinetics simulation, we study here the ripening kinetics of TiO2(110)-supported late TMs (including Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au) in a wide range of particle size. Density functional theory calculations show that the total activation energies of ripening are decided by the corresponding formation energy of the metal monomer on TiO2(110) and vary in the range of 3 eV following the order of Ag < Cu < Pd < Au < Ni < Rh < Pt < Ru < Ir. Isothermal and temperature ramping kinetic simulations are performed, and the corresponding half-life time and onset temperature of ripening are extracted, respectively. The results show that the half-life time of ripening exponentially increases with the total activation energy of the metal from Ag to Ir. The onset temperature of ripening increases more than hundreds of kelvin, which is consistent with variation in the melting points of the bulk counterpart. The ripening rate is found to dramatically increase with the decrease of the particle size, and the corresponding size effect increases pronouncedly with the total activation energy from Ag to Ir. This work provides valuable insights into the ripening kinetics of oxide-supported metal particles and is helpful in designing stable nanocatalysts.

A theoretical microkinetic model is developed to describe the decomposition of methanol into formaldehyde, carbon monoxide, and hydrogen on eight transition metal surfaces using density functional theory (DFT) calculations and transition state theory (TST). The chemical kinetics are then analyzed using a recently developed technique, the sum over histories representation (SOHR), that clearly reveals the chemical pathways followed by the system. The model itself consists of 10 reversible hydrogen abstraction reactions as well as the adsorption–desorption of methanol, formaldehyde, hydrogen, and carbon monoxide. The computed rate coefficients are fit to generalized Arrhenius expressions that are applicable to a wide range of conditions. While the lateral interactions are not explicitly computed, the effects of surface coverage are included using a site-blocking model for the kinetics. The SOHR method allows the chemical pathways followed by the surface species to be determined and weighted by unique probabilities. Locating the most probable chemical pathways is very useful in understanding the selectivity of product formation and can be used to determine the “optimal” reaction conditions.

High CO2and H2selectivity are important issues for methanol steam reforming (MSR) to provide H2as a clean energy carrier. The structure sensitivity and the factors governing the activity and selectivity for MSR on Cu catalysts are systematically investigated using density functional theory calculations and microkinetic simulation. Potential energy surfaces including water dissociation, methanol dehydrogenation, formaldehyde desorption and coupling with oxygen-containing intermediates, and formation of hydrogen and carbon dioxide are calculated over Cu(111), (100), (221), (211), and (110) surfaces, respectively. It is found that Cu(110) facet is the most active and selective toward carbon dioxide; Cu(221) is also active but highly selective toward formaldehyde, whereas Cu(111) is nearly inactive. Degree of rate control analysis shows that the activity is controlled mainly by methanol dehydrogenation to formaldehyde, whereas the degree of selectivity control shows that the selectivity toward formaldehyde or carbon dioxide depends sensitively on competition between formaldehyde desorption and coupling with surface oxygen. For Cu(110), abundance of both methoxy and oxygen as well as available vacant sites key for its high activity and selectivity toward carbon dioxide, whereas lack of oxygen on Cu(221) makes the corresponding surface highly selective for formaldehyde. The present work highlights the great influence of Cu surface orientations on the activity and selectivity for MSR.

The influence of the magnetism of transition metal oxide, nickel(II) oxide (NiO), on its surface reactivity and the dependence of surface reactivity on surface orientation and reactant magnetism were studied by density functional theory plus U calculations. We considered five different antiferromagnetically ordered structures and one ferromagnetically ordered structure, NiO(001) and Ni(011) surfaces, paramagnetic molecule NO, and nonparamagnetic molecule CO. The calculations showed that the dependence of surface energies on magnetism was modest, ranging from 49 to 54 meV/Å2for NiO(001) and from 162 to 172 meV/Å2for NiO(011). On NiO(001), both molecules preferred the top site of the Ni cation exclusively for all NiO magnetic structures considered, and calculated adsorption energies ranged from –0.33 to –0.37 eV for CO and from –0.42 to –0.46 eV for NO. On NiO(011), both molecules preferred the bridge site of two Ni cations irrespective of the NiO magnetism. It was found that rather than the long-range magnetism of bulk NiO, the local magnetic order of two coordinated Ni cations binding to the adsorbed molecule had a pronounced influence on adsorption. The calculated NO adsorption energy at the (↑↓) bridge sites ranged from –0.99 to –1.05 eV, and become stronger at the (↑↑) bridge sites with values of –1.21 to –1.30 eV. For CO, although the calculated adsorption energies at the (↑↓) bridge sites (–0.73 to –0.75 eV) were very close to those at the (↑↑) bridge sites (–0.71 to –0.72 eV), their electron hybridizations were very different. The present work highlights the importance of the local magnetic order of transition metal oxides on molecular adsorption at multi-fold sites.

Supported single-atom catalysts have attracted increasing interest due to their high atomic efficiencies and simple structures used to establish the structure–activity relations in catalysis. In this contribution, we present a density functional theory study of ZnO supported single transition metal (TM: Mn, Fe, Co, and Ni) atoms for the water gas shift reaction (WGSR). We find that these single TM1atoms prefer to substitute in the surface Zn lattice and exhibit promising activity for stabilizing the intermediates (i.e.CO, OH, and COOH) involved in WGSR. Meanwhile, the surface lattice O coordinated with the TM1atoms is responsible for the hydrogen abstraction process. The formation of COOH viathe association between CO and OH is the rate-limiting step in the catalytic cycle. Microkinetic modeling analysis is used to determine the activity trend, and a volcano-like plot between the calculated rates and the binding energies of COOH is obtained, suggesting that the COOH binding energy might be a good activity descriptor for catalyst screening. Among these single atoms, the single Ni1atom exhibits the highest activity and is promising for WGSR.

Structure sensitivity on chain growth and selectivity in cobalt catalyzed Fischer–Tropsch synthesis (FTS) were studied by density functional theory (DFT) calculations. It is found that at a lower CO coverage, chain growth tends to proceed viaa CO insertion mechanism on close-packed Co (0001) and stepped Co, with CH4as the main product. However, a carbide mechanism is preferable on more open Co (1011) accompanied with higher selectivity to C2hydrocarbons than CH4. The origin is identified from the structure sensitive adsorption of the key intermediates, specifically the least “saturated” C/CH species, which exhibit a relatively strong dependence on the structure evolution. With increasing CO coverage, the CO insertion mechanism becomes more favorable, and both FTS activity and C2hydrocarbon selectivity increase on Co (0001). This work highlights the intrinsic structure and coverage effects, achieving fundamental insight that can potentially be used to design and develop improved catalysts for FTS and other important reactions in syngas conversion.

Exploring the dependence of the structure–activity relationship of catalysts is important for improving the activity and selectivity in heterogeneous catalysis. Among other factors, the influence of the crystal phases, face-centered cubic (FCC) and hexagonal close-packed (HCP), of Ni catalysts on CO dissociation is studied by density functional theory (DFT). Surface energies of numerous FCC and HCP facets are calculated to construct the corresponding morphologies, and the exposed facets (six facets for FCC Ni and five facets for HCP Ni) are used to investigate the CO dissociation. For FCC Ni, (311) is the most active facet with the least barrier of 1.58 eV, followed by (100) and (211) with barriers of 1.63 and 1.75 eV, respectively. For HCP Ni, (101̅2) is the most active facet with the least barrier of 1.73 eV, followed by (101̅1) with a barrier of 1.86 eV. On both FCC and HCP Ni, CO dissociation shows a dramatic structural sensitivity irrespective of direct or H-assisted pathway. Compared to the direct dissociation, the H-assisted dissociation is kinetically favorable on both FCC and HCP Ni. With increase of dissociation barrier, the preferred dissociation pathway changes from COH intermediate to CHO intermediate. FCC Ni can expose abundant facets with low barrier. The result is compared with more active cobalt catalysts showing an opposite dependence on the crystal phases. The revealed insights regarding the crystal phase and the composition of catalysts upon activation of the diatomic molecules provide a new perspective for rational design of catalysts to expose more active sites for a higher specific activity.

Studying catalytic hydrogenation reactions on oxide surfaces at the atomic scale has been challenging because of the typical occurrence of these processes at ambient or elevated pressures, rendering them less accessible to atomic-scale techniques. Here, we report an atomic-scale study on H2dissociation and the hydrogenation of CO and CO2on ZnO using ambient pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. We directly visualized the heterolytic dissociation of H2on ZnO(101̅0) under ambient pressure and found that dissociation reaction does not require the assistance of surface defects. The presence of CO or CO2on ZnO at 300 K does not impede the availability of surface sites for H2dissociation; instead, CO can even enhance the stability of coadsorbed hydride species, thereby facilitating their dissociative adsorption. Our results show that hydride is the active species for hydrogenation, while hydroxyl cannot hydrogenate CO/CO2on ZnO. Both AP studies and DFT calculations showed that the hydrogenation of CO2on ZnO is thermodynamically and kinetically more favorable compared to that of CO hydrogenation. Our results point toward a two-step mechanism for CO hydrogenation, involving initial oxidation to CO2at step sites on ZnO followed by reaction with hydride to form formate. These findings provide molecular insights into the hydrogenation of CO/CO2on ZnO and deepen our understanding of syngas conversion and oxide catalysis in general.

Controllable synthesis of metallic nanocrystals (NCs) with tunable phase, uniform shape, and size is of multidisciplinary interests but has still remained challenging. Herein, a robust phase control strategy is developed, in which seeds with a given phase are added to guide the epitaxial growth of the target metal to inherit the seeds’ phase. Through this strategy, M@Ru (M = Pt, Pd) NCs in the face-centered cubic (fcc) phase, a metastable phase for Ru under ambient conditions, were synthesized with the hydrothermal method. The Pt@Ru NCs showed not only the pure fccphase but also high morphology selectivity to tetrahedrons surrounded by {111} facets. As revealed by density function theory (DFT) calculations, the preferentially epitaxial growth of Ru atom layers on the nonclosest-packed facets of hetero fccmetal seeds led to the formation of fccRu shells. Furthermore, the fccPt@Ru tetrahedrons/C showed electrocatalytic activity enhancement with more than an order of magnitude toward hydrogen oxidation reaction (HOR) in acidic electrolyte compared with hydrothermally synthesized Ru/C. Electrochemical measurement combined with DFT calculations revealed that the optimum HOR activity should be achieved on well-crystallized fccRu catalysts exposing maximum {111} facets.

Photocatalysis of a single methanol molecule on the TiO2(110) surface was investigated using a high-resolution scanning tunneling microscopy (STM) technique. Three different types of elementary methanol photocatalytic processes, methanol photodissociation, photoinduced migration of formaldehyde, and formaldehyde photodesorption, were clearly observed. Detailed chemical structures of the intermediates were obtained through careful comparisons between experimental STM images and theoretical simulations based on density functional theory (DFT) calculations. This work demonstrates that elementary photocatalytic processes of a single methanol molecule on the surface can be followed step by step using advanced STM imaging techniques. Such a study can provide unprecedented insights into the surface photocatalytic processes and will greatly help us to understand photocatalysis at the most fundamental level.

We present a model of the surface kinetics of the dehydrogenation reaction of methanol on the Pd(111), Pt(111), and Ni(111) metal surfaces. The mechanism consists of 10 reversible dehydrogenation reactions that lead to the final products of CO and H2. The rate coefficients for each step are calculated using ab initiotransition state theory that employs a new approach to obtain the symmetry factors. The potential energies and frequencies of the reagents and transition states are computed using plane wave DFT with the PW91 exchange correlation functional. The mechanism is investigated for low coverages using a global sensitivity analysis that monitors the response of a target function of the kinetics to the value of the rate coefficients. On Pd(111) and Ni(111), the reaction COH → CO + H is found to be rate limiting, and overall rates are highly dependent upon the decomposition time of the COH intermediate. Reactions at branches in the reaction network are also particularly important in the kinetics. A stochastic atom-following approach to pathway analysis is used to elucidate both the pathway probabilities in the kinetics and the dependence of the pathways on the values of the key rate coefficients of the mechanisms. On Pd(111) and Ni(111) there exists significant competition between the pathway containing the slow step and faster pathways that bypass the slow step. A discussion is given of the dependence of the model target’s probability density function on the chemical pathways.

Environmental and economic constraints necessitate further improvement of the activity and selectivity of dispersed Pd, Rh, and Pt metals for use in NOxreduction. We present here a density functional theory plus Ustudy of NO reduction with CO catalyzed by a single Pd1atom embedded in CeO2(111) (noted as Pd1/CeO2(111)). The complete catalytic cycle for NO + CO → CO2+ 1/2N2, including competitive adsorption of reactants, generation of the oxygen vacancy by CO, formation of N2O2* intermediates, scission of N–O bonds, and formation of N2, is mapped out. The calculations indicate that Pd1/CeO2(111) is active and selective toward N2, in agreement with available experimental literature. The key intermediate N2O2* toward N2is identified, and the rate-limiting step is the first deoxygenation step of N2O2* with a barrier of 1.43 eV. The Pd1–OVpair embedded in CeO2(111) is proposed to be the active site, responsible not only for the formation of N2O2* by the reaction of two NO molecules but also for the subsequent two deoxygenation steps to make N2. Detailed electronic structure analysis indicates that the formation of the Pd1–OVpair and the synergetic effect between Pd 4d electron and reducibility of CeO2are responsible for the catalytic activity of single Pd atom embedded in ceria.

Disintegration of supported nanoparticles (NPs) in the presence of reactants can lead to catalyst deactivation or be exploited to redisperse sintered catalysts. To better understand the stability of TiO2(110)-supported three-way catalysts Rh, Pd, and Pt NPs during NOxand CO reduction, we present an ab initio thermodynamics study of the feasibility for these NPs to disintegrate into adatom-reactant complexes across a large parameter space of temperatures, pressures, and sizes. The tendencies for disintegration and redispersion between supported Rh, Pd, and Pt NPs are established. Compared to both Pd and Pt, Rh NPs are found to be more susceptible to either NO- or CO-induced disintegration, due to the large and exothermic formation energy of the Rh adatom complexes. Moreover, NO is a more efficient reactant for particle redispersion than CO. These findings provide valuable insights for how to either prevent reactant-induced NP disintegration or facilitate reactant-induced redispersion of sintered catalysts.

The stoichiometric single- and bilayer ZnO(0001) have been prepared by reactive deposition of Zn on Au(111) and studied in detail with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Both single- and bilayer ZnO(0001) adopt a planar, graphite-like structure similar to freestanding ZnO(0001) due to the weak van der Waals interactions dominating their adhesion with the Au(111) substrate. At higher temperature, the single-layer ZnO(0001) converts gradually to bilayer ZnO(0001) due to the twice stronger interaction between two ZnO layers than the interfacial adhesion of ZnO with Au substrate. It is found that Cu atoms on the surface of bilayer ZnO(0001) are mobile with a diffusion barrier of 0.31 eV and likely to agglomerate and form nanosized particles at low coverages; while Cu atoms tend to penetrate a single layer of ZnO(0001) with a barrier of 0.10 eV, resulting in a Cu free surface.

The interaction of an Fe atom, an Fe dimer, a one-dimensional Fe nanowire, and an FeO nanowire with a single-walled armchair carbon nanotube (CNT) (8, 8) is investigated using density functional theory calculations. The results show that for all iron species the bonding with the outside wall of the CNT is stronger than that with the inside wall. Analysis of the electron distribution of the CNT shows that the curvature of the CNT induces a significant electron disparity at the inside and outside regions and more electrons are distributed on the outer surface. The properties of the frontier orbitals of the CNT are studied, and the results show that the highest occupied molecular orbital and lowest unoccupied molecular orbital are mainly located outside the tube, which may account for the in- and out-dependent interactions of Fe species with the CNT surface and hence different chemical reactivities of CNT-loaded metals.

The catalytic role of the PtFe cation ensemble presented at the perimeters of the FeO film supported on Pt(111) for low‐temperature CO oxidation and the promotion of water on activity were studied by using DFT calculations. We found that the perimeter sites along the edge of the FeO islands on Pt provided a favorable ensemble that consisted of coordinatively unsaturated ferrous species and nearby Pt atoms for O2and H2O activation free from CO poison. A dissociative oxygen atom at the PtFe cation ensemble reacts easily with CO adsorbed on nearby Pt. The OH group from water dissociation not only facilitates activation of the oxygen molecule, more importantly it opens a facile reaction channel for CO oxidation through the formation of the carboxyl intermediate. The presence of the OH group on the FeO film strengthens interfacial interactions between FeO and Pt(111), which would make the FeO film more resistant to further oxidation. The importance of the PtFe cation ensemble and the role of water as a cocatalyst for low‐temperature CO oxidation is highlighted.

Identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure–reactivity relationship and rationale of the design of new catalysts, but remains challenging. Among others, the boundaries at metal nanoparticles and supported oxides were found to be important and attributed to the active sites in various catalytic reactions. To reveal the nature of the active sites at the boundaries, the catalytic role of the inverse 3d transition-metal oxide nanoislands on Pt(111) for low-temperature CO oxidation was studied by density functional theory calculations. A characteristic Pt–cation ensemble at the oxide/metal boundaries as the active sites is identified. In Pt–cation ensembles, coordinate-unsaturated (CUS) cations exposed at the edges of oxide nanoislands are highly active for O2adsorption and dissociation, and less-reactive Pt binds modestly with dissociated O responsible for the facile CO oxidation. Inverse VIIIB-oxide/Pt boundaries exhibit high activities for low-temperature CO oxidation, and the corresponding activity decreases gradually from Fe to Co to Ni. The results rationalize a wide range of the experimental findings. To take advantage of the high oxidizing activity of low-valent VIIIB cations, FeO/Pt and CoO/Pt catalysts are appropriate for the reactions under oxygen-poor conditions, whereas NiO/Pt for the reactions under oxygen-rich conditions. The dependence of the activity and valence state of the Pt–cation ensemble at the oxide/metal boundaries is discussed, and the insight of the metal–cation ensemble as the active sites is highlighted.

C2oxygenate (acetaldehyde, ethanol, etc.) formation from syngas (CO + H2) is an important industrial process for the production of clean liquid energy fuels and valuable chemical feedstocks that are catalyzed industrially by Rh modified with Mn and Fe, etc. In an effort to identify catalysts based on less expensive metals and higher C2oxygenate selectivity, density functional theory (DFT) calculations were performed to tune the relative activity of the selectivity-determining steps, i.e., CO insertion in CHx(x= 1, 2, 3) versus CHxhydrogenation by changing composition and structure of material. We find that the Rh-decorated Cu alloy catalyst has significantly lower CO insertion barriers compared to pristine Rh(111) and vicinal Rh(553) surfaces, whereas the variation of CHxhydrogenation barriers on the three surfaces is modest. A semiquantitative kinetic analysis based on DFT calculations shows that the C2oxygenate selectivity on RhCu(111) is substantially improved, with the production rate of C2oxygenates slightly higher than CH4under experimental conditions, compared with Rh(111) and Rh(553) that are highly selective to CH4. Our calculations suggest that the improved C2oxygenate selectivity on the RhCu alloy is primarily due to the fact that CO insertion is rather sensitive, whereas hydrogenation is insensitive to the ensemble effect. Furthermore, the Rh-decorated Cu alloy has stronger resistance toward coking and lower constituent cost compared to pure Rh catalysts and is thus a promising candidate for an improved C2oxygenate synthesis catalyst.

AZ91D alloy composites reinforced by CNTs/SiCp were fabricated using stir casting process. The mechanical properties of the composites were tested, observed and analyzed the microstructure, the fractographs were observed and analyzed via scanning electron microscope. The results showed that CNTs/SiCp could not only refine the grains of the composites, but also bear the load of resistance to deformation. Compared with the matrix alloy, the tensile strength, the elastic modulus, the micro-hardness and the elongation rate of the composites had been enhanced significantly. But the mechanical properties would be fell down with the more addition of CNTs/SiCp.

In elastic deformation range, this paper reports a finite element study of effect of length and diameter of the fiber on intrfacial stress of AZ91D metal matrix composites reinorced by short fiber. The nuit cell model is developde in ansys. It was found no infuence the fiber length has on the interfacial regions. There is considerable increase in intefacial von-Mises stress near the fiber end with increase in fiber diameter, and the region of stress transfer become wider. So, the risk of crack and debonding increase.

HCO insertion into CHxexhibits superior or similar activity to CO insertion and carbene coupling according to DFT calculations, which thus reveal a new reaction channel for chain growth in syngas conversion. The picture shows schematically the lower reaction barrier for HCO versus CO insertion and the optimized transition state for insertion of HCO into CH2.

Based on the classical electrostatic theory, the distributions of potential and electrical field at the apex of the carbon nanotubes (CNTs), both in normal-gate type triode structure and under-gate type triode structure, were simulated and calculated respectively. The gate electrode's position and gate aperture's effect on CNTs' field emission characteristics were analyzed. The results indicate that under-gate structure, compared with normal-gate structure, has better field emission performance and lower threshold voltage. Both the gate aperture and the distance between gate electrode and CNTs' apex have crucial effect on field enhancement factors of normal-gate structure and under-gate structure.

The electroless deposition is an excellent method for the preparation of the CNTs with Ni-P layers. Pristine CNTs were first oxygen-functionalized by treating them with a mixture of concentrated (HNO3 :H2SO4=1:3) acids. The SWNTs were obtained in the suspension of purification solution, In this work, the heat-treatment at 400°С transformed the amorphous Ni-P layer into the nanocrystalline Ni-P (crystalline Ni and Ni3P intermetallic compound) layer. The Ni-P-SWNTs before and after heat-treatment were studied using XRD and SAED. It is observed that the lattice parameters of Ni-P layers has difference from the bulk’s, indicting that the lattice change has taken place.

Methanol steam reforming (MSR) is an important industrial process for hydrogen production, and fundamental understanding of the reaction mechanism is crucial to improve the catalytic activity and selectivity. In the present work, we present a comparative mechanistic study of the MSR reaction on two key model systems, Cu(111) and Pd(111), with distinct selectivity using density functional theory calculations. We find that, on Cu(111), methanol dehydrogenation to formaldehyde is favorable first through the O−H bond scission, and the final products are dominated by carbon dioxide and hydrogen. On Pd(111), formaldehyde is also found to be an important intermediate; however, it comes through the C−H bond breaking first, and the final products are mainly CO and hydrogen. We find that the distinct selectivity on the Cu(111) and Pd(111) surfaces originates from the different reactivities of HCHO on the two surfaces. On Cu(111), HCHO tends to react with the hydroxyl to form hydroxymethoxy followed by its decomposition to CO2. In contrast, direct dehydrogenation of HCHO to CO is favorable on Pd(111). Finally, we find that there is a good linear correlation between the transition-state energies and the final-state energies for the elementary reactions involved in the MSR reaction, which may be useful for computational design and optimization of the catalysts.