A Synergistic effect on the atomic cluster M4 supported on MN4-graphene (M = Fe, Ni) for the hydrogen evolution reaction.

The development of stable and efficient non-noble metal catalysts for the hydrogen evolution reaction (HER) can greatly promote the utilization of hydrogen energy. Herein, we investigated four potential model catalysts of the atomic cluster M4 supported on MN4-graphene substrates (M = Fe, Ni) from first-principles, i.e., Fe4@FeN4-Gr, Fe4@NiN4-Gr, Ni4@FeN4-Gr and Ni4@NiN4-Gr, respectively. Using density functional theory (DFT) calculations, the synergistic effect enhances the stability and HER activity of these supported M4@MN4-Gr. It is found that the Gibbs free energy of hydrogen adsorption (ΔGH*) of Ni4@FeN4-Gr is only -0.168 eV with the best exchange current. We further explored the pH effect on the HER performance and determined the ideal pH range of these potential model catalysts. Four model catalysts can follow the Volmer-Tafel pathway if considering the implicit solvation effect. These results provide an effective guidance for the rational design of electro-catalysts.

Methane activation on single-atom Ir-doped metal nanoparticles from first principles.

The breaking of the C-H bond of CH4 is of great importance, and one of the most efficient strategies in heterogeneous catalysis is to alter the electronic structure of a surface by doping it with different metal elements or controlling the stoichiometry. We present an in-depth study on methane activation on pure metal and single-atom Ir-doped alloy nanoparticles, which are constructed based on (100), (110), (111) surfaces using density functional theory (DFT) calculations. DFT results show that the dissociation barriers of CH4 on the Ir-doped alloy surfaces are about 0.3-0.4 eV, much lower than those on the pure metal surfaces (i.e., 0.6-0.8 eV). DFT-based transition state theory further reveals the rates of the first C-H activation on single-atom Ir-doped alloy nanoparticles at the early stages. Importantly, a strong temperature dependence is mainly contributed by the proportion of the exposed (110) surface. The Ir-doped Pt nanoparticle is found to be an efficient catalyst for methane activation in potential industrial applications. These important results are helpful for further designing new metal catalysts for methane activation at the atomic/molecular level.

Dissociative chemisorption of O2 on Agn and Agn-1Ir (n = 3-26) clusters: a first-principle study.

Understanding the interactions between O2 and small metal clusters is of great importance in exploring heterogeneous catalysis particularly involving an oxidation reaction. We herein present the dissociative chemisorption of O2 on Agn and Agn-1Ir clusters (n = 3-26) by using density functional theory calculations. Combining a particle swarm optimization algorithm and a minima hopping method, we have optimized and obtained stable geometric structures of Agn and Agn-1Ir clusters without and with O2 adsorption. Some important physical parameters, including bond length, adsorption energy, dissociation barriers and bader charge, have been systematically calculated for appraising the stability and reactivity of Agn and Agn-1Ir clusters. It is found that the dopant Ir atom can largely enhance the stability and promote the O2 dissociation, especially on small Agn-1Ir clusters (n = 3-10). It is mainly attributed to the dopant Ir atom being completely exposed outside the Ag atoms. For O2 adsorption and dissociation on large Agn-1Ir clusters (n = 11-26), the dissociation barriers are much higher due to the dopant Ir emerging into the core of Agn-1Ir clusters, which is very similar to those on large Agn (n = 11-26). Microkinetic simulation results provide direct evidence for high reaction temperature and pressure effects on improving O2 dissociation on Agn and Agn-1Ir clusters especially for small clusters (n < 10). It is found that the Ag5Ir cluster is the most suitable nanocluster for promoting O2 dissociation at the given reaction temperatures and pressures. Our theoretical work is helpful for the rational design of doped silver nanocluster catalysts in future experiments.

Abnormal subsurface hydrogen diffusion behaviors in heterogeneous hydrogenation reactions.

Hydrogen adsorption and diffusion behaviors on noble metal model catalyst surfaces and into the subsurfaces are of paramount significance in the exploration of novel heterogenous catalytic hydrogenation reactions. We present an in-depth study of hydrogen adsorption on and diffusion into the subsurfaces of three typical 5d noble metals from three-dimensional electronically adiabatic potential energy surfaces (PESs) by interpolating plenty of ab initio density functional theory (DFT) configuration-energy points. The surfaces and subsurfaces regions of the relaxed Ir(100) and (111), Pt(100) and (111), and Au(100) and (111) surfaces, are, respectively, taken into account. For hydrogen adsorption on the (100) surfaces, the lowest adsorption energy site is the Bridge site, instead of the traditional Hollow site. Hydrogen prefers to follow the indirect pathway with a lower diffusion barrier, in the competition with the direct pathway with much higher diffusion barrier. For hydrogen diffusion on the (111) surfaces, hydrogen follows the pathway from Top site to fcc site on the surface and prefers an up-down direct pathway into the subsurface. Importantly, the nudged elastic band (NEB) based on the PESs can reproduce those results calculated from the NEB(DFT) very well. The developed highly-accurate and efficient approach based on the PESs helps us to further investigate the more complex reactant diffusion dynamics at surfaces.

Water dissociating on rigid Ni(100): A quantum dynamics study on a full-dimensional potential energy surface.

We constructed a nine-dimensional (9D) potential energy surface (PES) for the dissociative chemisorption of H2O on a rigid Ni(100) surface using the neural network method based on roughly 110 000 energies obtained from extensive density functional theory (DFT) calculations. The resulting PES is accurate and smooth, based on the small fitting errors and the good agreement between the fitted PES and the direct DFT calculations. Time dependent wave packet calculations also showed that the PES is very well converged with respect to the fitting procedure. The dissociation probabilities of H2O initially in the ground rovibrational state from 9D quantum dynamics calculations are quite different from the site-specific results from the seven-dimensional (7D) calculations, indicating the importance of full-dimensional quantum dynamics to quantitatively characterize this gas-surface reaction. It is found that the validity of the site-averaging approximation with exact potential holds well, where the site-averaging dissociation probability over 15 fixed impact sites obtained from 7D quantum dynamics calculations can accurately approximate the 9D dissociation probability for H2O in the ground rovibrational state.

Hydrogen diffusion into the subsurfaces of model metal catalysts from first principles.

Diffusion pathways of atomic hydrogen on model catalyst surfaces and into subsurfaces are of great significance in the exploration of novel catalytic hydrogenation in heterogeneous catalysis. We present in detail the diffusion pathways of hydrogen on seven different open and closed model catalyst surfaces from first principles calculations. Seven transition metal catalysts with thirteen different crystal surfaces, i.e., Co(001), Ni(100) and Ni(111), Pd(100) and (111), Pt(100) and (111), Cu(100) and (111), Ag(100) and (111) and Au(100) and (111), are taken into account. Thirteen corresponding potential energy surfaces (PESs) are constructed for modelling hydrogen diffusion on these model catalyst surfaces and into the subsurfaces by interpolating ab initio density functional theory energy points (∼2000 for each surface). The minimum energy diffusion pathways for hydrogen on the surfaces and into the subsurfaces are globally searched for based on PESs using a mesh method, and are in excellent agreement with those calculated from the nudged elastic band method. Furthermore, the important substrate relaxation effect can decrease the diffusion barriers for hydrogen into catalyst subsurfaces. The high reactivity of subsurface reactants mainly comes from the residual energy of subsurface hydrogen emerging from the subsurface onto the surface.

Methane dissociation on Ni(111): A seven-dimensional to nine-dimensional quantum dynamics study.

As one benchmark system of CH4 dissociation on the Ni(111) surface, it is of great significance to explore the role of each degree of freedom (DOF) of reactant CH4 in its first C-H bond dissociation from quantum dynamics simulations. Here, the influence of the CH stretching DOF of methyl limited in C3v symmetry is quantitatively investigated as well as the important role of azimuth. We calculated the sticking probabilities, S0, of ground state (GS) CH4 dissociation on a rigid Ni(111) surface by performing some seven-dimensional to nine-dimensional (9D) quantum dynamics simulations based on one highly accurate and fifteen-dimensional (15D) ab initio potential energy surface which we recently developed. Our direct quantum dynamics results show that S0 of GS CH4 on four given surface impact sites are weakly enhanced by adding the CH stretching DOF of methyl but strongly weakened by the DOF of azimuth. Furthermore, using a 9D quantum dynamics model, we improve the post-treatment model for treating the influence of surface impact sites through a linear relationship between the effective potential barriers and the distances relative to that on the transition state site. These developed high-dimensional quantum dynamics models and improved post-treatments can be usefully extended for studying some complex polyatomic gas-surface reactions by other theoretical groups.

Communication: Methane dissociation on Ni(111) surface: Importance of azimuth and surface impact site.

Understanding the role of reactant ro-vibrational degrees of freedom (DOFs) in reaction dynamics of polyatomic molecular dissociation on metal surfaces is of great importance to explore the complex chemical reaction mechanism. Here, we present an expensive quantum dynamics study of the dissociative chemisorption of CH4 on a rigid Ni(111) surface by developing an accurate nine-dimensional quantum dynamical model including the DOF of azimuth. Based on a highly accurate fifteen-dimensional potential energy surface built from first principles, our simulations elucidate that the dissociation probability of CH4 has the strong dependence on azimuth and surface impact site. Some improvements are suggested to obtain the accurate dissociation probability from quantum dynamics simulations.

CH4 dissociation on Ni(111): a quantum dynamics study of lattice thermal motion.

Lattice thermal motion is of great importance because it has a significant effect on molecule activation on metal surfaces. Here, we present an in-depth quantum dynamics study of lattice thermal motion for methane dissociation on some static distorted Ni(111) surfaces based on an accurate, fourteen-dimensional potential energy surface fitted to ∼10(5)ab initio energy points. Our study reproduces the tendency that the sticking probability of ground state methane increases (decreases) as the lattice atom moves upward (downward), and thus represents the first validation of the applicability of the energy-shifting scheme to polyatomic molecular gas-surface reactions. Furthermore, we improve on the linear model proposed by Jackson's group and introduce a new model that is applicable to a broad range of surface temperatures.

Methane dissociation on Ni(111): A fifteen-dimensional potential energy surface using neural network method.

In the present work, we develop a highly accurate, fifteen-dimensional potential energy surface (PES) of CH4 interacting on a rigid flat Ni(111) surface with the methodology of neural network (NN) fit to a database consisted of about 194 208 ab initio density functional theory (DFT) energy points. Some careful tests of the accuracy of the fitting PES are given through the descriptions of the fitting quality, vibrational spectrum of CH4 in vacuum, transition state (TS) geometries as well as the activation barriers. Using a 25-60-60-1 NN structure, we obtain one of the best PESs with the least root mean square errors: 10.11 meV for the entrance region and 17.00 meV for the interaction and product regions. Our PES can reproduce the DFT results very well in particular for the important TS structures. Furthermore, we present the sticking probability S0 of ground state CH4 at the experimental surface temperature using some sudden approximations by Jackson's group. An in-depth explanation is given for the underestimated sticking probability.

Effect of Bystander Hydrogen Atoms on Hydrogen Desorption on Single-Atom Alloy Surfaces: Insights from Simulated Temperature-Programmed Desorption Spectra.

Single-atom alloy (SAA) catalysts exhibit unique and excellent catalytic properties in heterogeneous hydrogenation/dehydrogenation reactions. A thorough understanding of the microscopic surface processes is essential to improve the catalytic performance. Here, from a new perspective of the temperature-programmed desorption (TPD) spectra of hydrogen (H) on two common SAA surfaces, Pt@Cu(111) and Pd@Cu(111), we reveal and confirm the key influence of H atoms attached to Pt/Pd dopants, i.e., the H atom bystander, on the desorption process of surface H atoms. It is found that only after considering the effect of the H atom bystander can the simulated TPD spectra well reproduce the experimentally observed higher desorption temperature on Pt@Cu(111) than on Pd@Cu(111) and the leftward shift of the TPD peak with increasing H atom coverage; otherwise, the features are inconsistent with experiments. Our work provides direct evidence for the effect of bystander H atoms from a simulation perspective.

Stacking Fault-Enriched MoNi4/MoO2 Enables High-Performance Hydrogen Evolution.

Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of hydrogen economy a reality. Although platinum (Pt)-based catalysts show good performance toward hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, a non-Pt, high-performance electrocatalyst for HER achieved by engineering high fractions of stacking fault (SF) defects for MoNi4/MoO2 nanosheets (d-MoNi) through a combined chemical and thermal reduction strategy is shown. The d-MoNi catalyst offers ultralow overpotentials of 78 and 121 mV for HER at current densities of 500 and 1000 mA cm-2 in 1 M KOH, respectively. The defect-rich d-MoNi exhibits four times higher turnover frequency than the benchmark 20% Pt/C, together with its excellent durability (> 100 h), making it one of the best-performing non-Pt catalysts for HER. The experimental and theoretical results reveal that the abundant SFs in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the associated combination of *H into hydrogen and molecular hydrogen desorption, enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging electrochemical energy conversion and storage applications.

Promotion Effect of Well-Defined Deposited Water Layer on Carbon Monoxide Oxidation Catalyzed by Single-Atom Alloys.

Single-atom alloys (SAAs) exhibit excellent catalytic performance and unique electronic structures, emerging as promising catalysts for potential industrial reactions. While most of them have been widely employed under reducing conditions, few are applied in oxidation reactions. Herein, using density functional theory calculations and microkinetic simulations, we demonstrate that a well-defined one water layer can improve CO oxidation on model SAAs, with reaction rates increased by orders of magnitude. It is found that the formation of hydrogen bonds and the transfer of charges effectively enhance the adsorption and activation of oxygen molecules at the H2O/SAA interfaces, which not only increases the surface coverage of O2 species but also reduces the energy barrier of CO oxidation. The proposed strategy in this work would extend the application range of SAA catalysts to oxidation reactions.

Monometallic interphasic synergy via nano-hetero-interfacing for hydrogen evolution in alkaline electrolytes.

Electrocatalytic synergy is a functional yet underrated concept in electrocatalysis. Often, it materializes as intermetallic interaction between different metals. We demonstrate interphasic synergy in monometallic structures is as much effective. An interphasic synergy between Ni(OH)2 and Ni-N/Ni-C phases is reported for alkaline hydrogen evolution reaction that lowers the energy barriers for hydrogen adsorption-desorption and facilitates that of hydroxyl intermediates. This makes ready-to-serve Ni active sites and allocates a large amount of Ni d-states at Fermi level to promote charge redistribution from Ni(OH)2 to Ni-N/Ni-C and the co-adsorption of Hads and OHads intermediates on Ni-N/Ni-C moieties. As a result, a Ni(OH)2@Ni-N/Ni-C hetero-hierarchical nanostructure is developed, lowering the overpotentials to deliver -10 and -100 mA cm-2 in alkaline media by 102 and 113 mV, respectively, compared to monophasic Ni(OH)2 catalyst. This study unveils the interphasic synergy as an effective strategy to design monometallic electrocatalysts for water splitting and other energy applications.

Rational Design of Mixed Matrix Membranes Modulated by Trisilver Complex for Efficient Propylene/Propane Separation.

The application of membrane-based separation processes for propylene/propane (C3 H6 /C3 H8 ) is extremely promising and attractive as it is poised to reduce the high operation cost of the established low temperature distillation process, but major challenges remain in achieving high gas selectivity/permeability and long-term membrane stability. Herein, a C3 H6 facilitated transport membrane using trisilver pyrazolate (Ag3 pz3 ) as a carrier filler is reported, which is uniformly dispersed in a polymer of intrinsic microporosity (PIM-1) matrix at the molecular level (≈15 nm), verified by several analytical techniques, including 3D-reconstructed focused ion beam scanning electron microscropy (FIB-SEM) tomography. The π-acidic Ag3 pz3 combines preferentially with π-basic C3 H6 , which is confirmed by density functional theory calculations showing that the silver ions in Ag3 pz3 form a reversible π complex with C3 H6 , endowing the membranes with superior C3 H6 affinity. The resulting membranes exhibit superior stability, C3 H6 /C3 H8 selectivity as high as ≈200 and excellent C3 H6 permeability of 306 Barrer, surpassing the upper bound selectivity/permeability performance line of polymeric membranes. This work provides a conceptually new approach of using coordinatively unsaturated 0D complexes as fillers in mixed matrix membranes, which can accomplish olefin/alkane separation with high performance.

Electronic Structure Engineering of Single-Atom Ru Sites via Co-N4 Sites for Bifunctional pH-Universal Water Splitting.

The development of bifunctional water-splitting electrocatalysts that are efficient and stable over a wide range of pH is of great significance but challenging. Here, an atomically dispersed Ru/Co dual-sites catalyst is reported anchored on N-doped carbon (Ru/Co-N-C) for outstanding oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in both acidic and alkaline electrolytes. The Ru/Co-N-C catalyst requires the overpotential of only 13 and 23 mV for HER, 232 and 247 mV for OER to deliver a current density of 10 mA cmgeo -2 in 0.5 m H2 SO4 and 1 m KOH, respectively, outperforming benchmark catalysts Pt/C and RuO2 . Theoretical calculations reveal that the introduction of Co-N4 sites into Ru/Co-N-C efficiently modify the electronic structure of Ru by enlarging Ru-O covalency and increasing Ru electron density, which in turn optimize the bonding strength between oxygen/hydrogen intermediate species with Ru sites, thereby enhancing OER and HER performance. Furthermore, the incorporation of Co-N4 sites induces electron redistribution around Ru-N4, thus enhancing corrosion-resistance of Ru/Co-N-C during acid and alkaline electrolysis. The Ru/Co-N-C has been applied in a proton exchange membrane water electrolyzer and steady operation is demonstrated at a high current density of 450 mA cmgeo -2 for 330 h.

Hot-Atom Mechanism in Syngas Methanation on Precovered Pd(100) Surfaces.

The excess energy of subsurface hydrogen species may facilitate overcoming reaction barriers and remarkably alters the reaction pathways. We present an in-depth study on the different reactivity of surface and subsurface hydrogen species in syngas methanation on the O/C-covered Pd(100) by using density functional theory calculations and microkinetic simulations. It is shown that the apparent energy barriers to form H2O and CH4 are reduced by 0.87 and 0.61 eV for the case in which the hot subsurface hydrogen species are involved in the whole hydrogenation process. The activity of O-covered Pd(100) is better than that of the C-covered surface, and the reactivity of subsurface hydrogen species is much higher than that of surface hydrogen species under ambient conditions. Increasing CO partial pressure strongly enhances the reactivity of subsurface hydrogen species in syngas methanation on the O-covered Pd(100). These important results are helpful for understanding the hot-atom mechanism through subsurface heterogeneous catalysis.