Over 90 % of all chemical manufacture uses a solid catalyst. Related work thus responds to major societal demand. This study is of water-gas shift on platinum for hydrogen production. The close-packed Pt(111) surface catalyses this process.
Many chemical reactions involve bond-dissociation. This is also true for reactions at solid surfaces, in which the dissociation step is often limiting but facilitated in comparison to gas phase reaction channels. Since bond-breaking is poorly described by Hartree-Fock and DFT methods, this work adopts Quantum Monte Carlo (QMC) methodology. QMC is a stochastic approach to solving the Schrödinger equation recently came of age for heterogeneous systems involving solids.
The present work considers co-adsorption of water and carbon monoxide on Pt(111). The water is partially dissociated while its oxygen atom binds to CO losing a hydrogen atom. This concerted step is rate-limiting. The resulting adsorbed formate species then decomposes to readily eliminated carbon dioxide and the clean-fuel product is H$_2$.
The Transition-State geometry can be optimized using molecular Quantum Monte Carlo force constants, on the basis of our earlier work using the CASINO software.
Our embedded active site approach is used. This allows a high-level configuration interaction (CI) wave-function to be used, expanded in plane-waves and embedded in the metal lattice exposing its close-packed face. The resulting periodic function is used to guide the Quantum Monte Carlo calculation.
Results are given here on mechanism and QMC activation barrier for water addition to CO pre-adsorbed on Pt(111) of 17 +/- 0.2 c.f. apparent measured value of 17.05 kcal/mol. They are encouraging for investigating similar or complex catalytic systems.
The description of the chemical bond between a solid surface and an atom or a molecule is the fundamental basis for understanding a broad range of scientific problems in heterogeneous catalysis, semiconductor device fabrication, and fuel cells. Widespread understandings are based on the molecular orbital theory and focused on the degree of filling of antibonding surface-adsorbate states that weaken bonding on surfaces. The unoccupied antibonding surface-adsorbate states are often tacitly assumed to be irrelevant. Here, we show that most antibonding states become unoccupied because the electron…
Enhanced Hydrogen Evolution Catalysis of Pentlandite due to the Increases in Coordination Number and Sulfur Vacancy during Cubic-Hexagonal Phase Transition
Hydrogen production as a clean, sustainable replacement for fossil fuels is gathering pace. Doubling the capacity of Paris-CDG airport has been halted, even with the upcoming Olympic Games, until hydrogen powered planes can be used. It is thus timely to work on catalytic selective hydrogen production and optimise catalyst structure. Over 90 % of all chemical manufacture uses a solid catalyst. This work describes adsorption of carbon-monoxide (CO) on platinum thin films, supported by cheap Al(111). CO reacts with water to produce hydrogen (water-gas shift). Quantum Monte Carlo methods are the o…
Increasing anthropogenic carbon dioxide (CO$_2$) emissions have led to rising global temperatures and climate change. Using earth-abundant metal-oxide catalysts such as Cu$_2$O for reducing CO$_2$ through RWGS reaction seems lucrative. In this work, we have used Cu$_2$O nanostructures and identified its activity, stability, and selectivity for reducing CO$_2$ to carbon monoxide (CO) which can be further hydrogenated to higher hydrocarbons using Fisher Tropsch synthesis. We have observed that the rate of CO$_2$ conversion increases by 4 times and significantly drops at 300 C where the catalyst …
Surface Van Hove singularity (SVHS), defined as the surface states near the Fermi level (EF) in low-dimensional systems, triggers exciting physical phenomena distinct from bulk. We herein explore theoretically the potential role of SVHS in catalysis taking CO oxidation reaction as prototype over graphene/Ca2N (Gra/Ca2N) heterojunction and Pt2HgSe3 (001) surface. It is demonstrated that both systems with SVHS could serve as an electron bath to promote O2 adsorption and subsequent CO oxidation with low energy barriers of 0.2 ~ 0.6 eV for Gra/Ca2N and Pt2HgSe3 (001) surface. Importantly, the cata…
We present a density-functional theory based kinetic Monte Carlo study of CO oxidation at the (111) facet of RuO$_2$. We compare the detailed insight into elementary processes, steady-state surface coverages and catalytic activity to equivalent published simulation data for the frequently studied RuO$_2$(110) facet. Qualitative differences are identified in virtually every aspect ranging from binding energetics over lateral interactions to the interplay of elementary processes at the different active sites. Nevertheless, particularly at technologically relevant elevated temperatures, near-ambi…
The rational design of hydrogen evolution reaction (HER) electrocatalysts which are competitive with platinum is an outstanding challenge to make power-to-gas technologies economically viable. Here, we introduce the delafossites PdCrO$_2$, PdCoO$_2$ and PtCoO$_2$ as a new family of electrocatalysts for the HER in acidic media. We show that in PdCoO$_2$ the inherently strained Pd metal sublattice acts as a pseudomorphic template for the growth of a strained (by +2.3%) Pd rich capping layer under reductive conditions. The surface modification continuously improves the electrocatalytic activity b…
Accurate modeling of chemically reactive systems has traditionally relied on either expensive ab initio approaches or flexible bond-order force fields such as ReaxFF that require considerable time, effort, and expertise to parameterize. Here, we introduce FLARE++, a Bayesian active learning method for training reactive many-body force fields on the fly during molecular dynamics (MD) simulations. During the automated training loop, the predictive uncertainties of a sparse Gaussian process (SGP) force field are evaluated at each timestep of an MD simulation to determine whether additional ab ini…
We describe a first-principles statistical mechanics approach enabling us to simulate the steady-state situation of heterogeneous catalysis. In a first step density-functional theory together with transition-state theory is employed to obtain the energetics of all relevant elementary processes. Subsequently the statistical mechanics problem is solved by the kinetic Monte Carlo method, which fully accounts for the correlations, fluctuations, and spatial distributions of the chemicals at the surface of the catalyst under steady-state conditions. Applying this approach to the catalytic oxidation …
The search for highly efficient and low-cost catalysts based on earth abundant elements is one of the main driving forces in organic and inorganic chemistry. Current strategies for the heterogeneous catalyst design focus on increasing the density of active regions and the optimization of Gibbs free energy of chemical reaction via doping. In this work, we investigate the chemical properties of moire metal for hydrogen evolution reaction. We show that the local shift of metallic atoms introduces a variation in surface hydrogen bonding strength and leads to the spatially dependent Gibbs free ener…
Topological quantum catalysts (TQCs), where metallic surface states from nontrivial band topology serve as the mechanism to favor heterogeneous catalysis processes, have been well demonstrated in three dimensional (3D) examples but have been rarely discussed in 2D scale. Here, we develop a design scheme to realize 2D TQCs with showing traversing nodal line at the Brillouin zone boundary, large Fermi arc on the edge, and nearly zero Gibbs free energy (ΔGH*) for hydrogen evolution reaction (HER). We demonstrate the 2D Cu2C2N4 sheet is a such example. The material manifests an open nodal line tra…
Here we present an overview of recent fundamental studies on the nature of the interaction between individual metal atoms and metal clusters and the conjugated surfaces of graphene and carbon nanotube with a particular focus on the electronic structure and chemical bonding at the metal-graphene interface. We discuss the relevance of organometallic complexes of graphitic materials to the development of a fundamental understanding of these interactions and their application in atomtronics as atomic interconnects, high mobility organometallic transistor devices, high-frequency electronic devices,…
An understanding of hydrogen diffusion on metal surfaces is important, not just for its role in heterogeneous catalysis and hydrogen fuel cell technology, but also because it provides model systems where tunneling can be studied under well-defined conditions. Here we report helium spin-echo measurements of the atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K. Quantum effects are evident at temperatures as high as 200 K, while below 120 K we observe a tunneling-dominated temperature independent jump rate of 1.9$\times$10$^9$ s$^{-1}$, many orders of magnitude faster …
Density functional theory (DFT) can be quite advantageous in advancing the field of catalysis because of the microscopic insights it provides, and thus can guide experimental searches of novel catalysts. Several recent works have demonstrated that low-dimensional materials can be very efficient catalysts. Graphene quantum dots (GQDs) have gained much attention in past years due to their unique properties like low toxicity, chemical inertness, biocompatibility, crystallinity, etc. These properties of GQDs which are due to quantum confinement and edge effects facilitate their applications in var…
Two-dimensional materials supported by single atom catalysis (SACs) are foreseen to replace platinum for large-scale industrial scalability of sustainable hydrogen generation. Here, a series of metal (Al, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn) and non-metal (B, C, N, O, F, Si, P, S, Cl) single atoms embedded on various active sites of g-C$_3$N$_4$ are screened by DFT calculations and six machine learning (ML) algorithms (support vector regression, gradient boosting regression, random forest regression, AdaBoost regression, multilayer perceptron regression, ridge regression). Our results based on f…
In this study, we demonstrate sustainable and green nanotechnology for room-temperature synthesis of H$_x$WO$_3$ ($0<x<0.5$) via a novel reaction pathway induced by mechanical energy. A simple mixture of monoclinic WO$_3$ powder and polyolefin (polypropylene) is used to obtain H$_x$WO$_3$ nanoparticles that show high crystallinity even through high-energy ball milling synthesis. The composite of H$_x$WO$_3$ nanoparticles and nanocarbon by-products exhibit unique optoelectronic properties along with outstanding enhancement of photocatalytic performance in the decomposition of azo-dye water poll…
We investigate the binding of single and quadruple hydrogen molecules on a positively charged Ca ion. By comparing with benchmark quantum Monte Carlo (QMC) calculations we demonstrate wide variability in other more approximate electronic structure methods including common density functionals. Single determinant QMC calculations find no binding at short range by approximately 0.1 eV for the quadruple hydrogen molecule case, for a fixed hydrogen bond length of 0.77 Angstrom. Density functional calculations using common functionals such a LDA and B3LYP differ substantially from the QMC binding cu…
The possible importance of oxide formation for the catalytic activity of transition metals in heterogenous oxidation catalysis has evoked a lively discussion over the recent years. On the more noble transition metals (like Pd, Pt or Ag) the low stability of the common bulk oxides suggests primarily sub-nanometer thin oxide films, so-called surface oxides, as potential candidates that may be stabilized under gas phase conditions representative of technological oxidation catalysis. We address this issue for the Pd(100) model catalyst surface with first-principles kinetic Monte Carlo (kMC) simula…
We describe a simple scheme to perform phonon calculations with quantum Monte Carlo (QMC) methods, and demonstrate it on metallic hydrogen. Because of the energy and length scales of metallic hydrogen, and the statistical noise inherent to QMC methods, the conventional manner of calculating force constants is prohibitively expensive. We show that our alternate approach is nearly 100 times more efficient in resolving the force constants needed to calculate the phonon spectrum in the harmonic approximation. This requires only the calculation of atomic forces, as in the conventional approach, and…
Many chemical reactions involve bond-dissociation. This is also true for reactions at solid surfaces, in which the dissociation step is often limiting but facilitated in comparison to gas phase reaction channels. This work considers hydrogen dissociation. Reliable molecular beam results are available for this reaction at some copper surfaces. Heterogeneous catalysis by copper is simulated. It was investigated in our previous work since it is in many ways a prototype metal presenting a close-packed surface here. These hydrogen molecules are adsorbed at Cu(111) and fixed geometries on the dissoc…
Structural defects in materials such as vacancies, grain boundaries, and dislocations may trap hydrogen and a local accumulation of hydrogen at these defects can lead to the degradation of the materials properties. An important aspect in obtaining insight into hydrogen induced embrittlement on the atomistic level is to understand the diffusion of hydrogen in these materials. In our study we employ kinetic Monte Carlo (kMC) simulations to investigate hydrogen diffusion in bcc iron within different microstructures. All input data to the kMC model, such as available sites, solution energies, and …
Hydrogen single-particle dynamics in solid LiH at T=20 K has been studied through the incoherent inelastic neutron scattering technique. A careful analysis of the scattering data has allowed for the determination of a reliable hydrogen-projected density of phonon states and, from this, of three relevant physical quantities: mean squared displacement, mean kinetic energy, and Einstein frequency. In order to interpret these experimental findings, a fully-quantum microscopic calculation has been carried out using the variational Monte Carlo method. The agreement achieved between neutron scatterin…
We present Monte Carlo simulations on Eley-Rideal abstraction reactions of atomic hydrogen chemisorbed on graphite. The results are obtained via a hybrid approach where energy barriers derived from density functional theory calculations are used as input to Monte Carlo simulations. By comparing with experimental data, we discriminate between contributions from different Eley-Rideal mechanisms. A combination of two different mechanisms yields good quantitative and qualitative agreement between the experimentally derived and the simulated Eley-Rideal abstraction cross sections and surface config…
The ab-initio phase diagram of dense hydrogen is very sensitive to errors in the treatment of electronic correlation. Recently, it has been shown that the choice of the density functional has a large effect on the predicted location of both the liquid-liquid phase transition and the solid insulator-to-metal transition in dense hydrogen. To identify the most accurate functional for dense hydrogen applications, we systematically benchmark some of the most commonly used functionals using Quantum Monte Carlo. By considering several measures of functional accuracy, we conclude that the van der Waal…
We discuss electronic properties and their evolution for the linear chain of $H_2$ molecules in the presence of a uniform external force $f$ acting along the chain. The system is described by an extended Hubbard model within a fully microscopic approach. Explicitly, the microscopic parameters describing the intra- and inter-site Coulomb interactions are determined together with the hopping integrals by optimizing the system ground state energy and the single-particle wave functions in the correlated state. The many-body wave function is taken in the Jastrow form and the Variational Monte-Carlo…
Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energeti…
The water gas shift reaction (WGS) is important and widely applied in the production of H2. Cu modified perovskites are promising catalysts for WGS reactions in hydrogen generation. However, the structure-dependent stability and reaction pathways of such materials remain unclear. Herein, we report catalytically active Cu modified SrTiO3 (nominally SrTi1-xCuxO3) prepared by a modified polymeric precursor method. Microstructural analysis revealed a partially segregated CuO phase in the as-prepared materials. Operando X-ray diffraction and absorption spectroscopy showed the reduction of CuO into …
Topological states of matter and their corresponding properties are classical research topics in condensed matter physics. Quite recently, the application of materials that feature these states has been extended to the field of electrochemical catalysis and become an emerging research topic that is receiving increasing interest. In particular, several recent experimental studies performed on topological semimetals have already revealed high catalytic performance towards hydrogen evolution reaction (HER), strongly promoting acceptance of the fresh concept of the topological catalyst. This emerg…
Titanium dioxide, TiO$_2$, has multiple applications in catalysis, energy conversion and memristive devices because of its electronic structure. Most of these applications utilize the naturally existing phases: rutile, anatase and brookite. Despite the simple form of TiO$_2$ and its wide uses, there is long-standing disagreement between theory and experiment on the energetic ordering of these phases that has never been resolved. We present the first analysis of phase stability at zero temperature using the highly accurate many-body fixed node diffusion Quantum Monte Carlo (QMC) method. We also…
We use the diffusion quantum Monte Carlo to revisit the enthalpy-pressure phase diagram of the various products from the different proposed decompositions of H$_2$S at pressures above 150~GPa. Our results entails a revision of the ground-state enthalpy-pressure phase diagram. Specifically, we find that the C2/c HS$_2$ structure is persistent up to 440~GPa before undergoing a phase transition into the C2/m phase. Contrary to density functional theory, our calculations suggest that the C2/m phase of HS is more stable than the I4$_1$/amd HS structure over the whole pressure range from 150 to 400 …
Water-metal interfaces are ubiquitous and play a key role in many chemical processes, from catalysis to corrosion. Whereas water adlayers on atomically flat transition metal surfaces have been investigated in depth, little is known about the chemistry of water on stepped surfaces, commonly occurring in realistic situations. Using first-principles simulations we study the adsorption of water on a stepped platinum surface. We find that water adsorbs preferentially at the step edge, forming linear clusters or chains, stabilized by the cooperative effect of chemical bonds with the substrate and hy…
We present a quantum Monte Carlo study of the hydrogen-benzene system where binding is very weak. We demonstrate that the binding is well described at both variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) levels by a Jastrow correlated single determinant geminal wave function with an optimized compact basis set that includes diffuse orbitals. Agreement between VMC and fixed-node DMC binding energies is found to be within 0.18 mHa, suggesting the calculations are well-converged with respect to the basis. Essentially the same binding is also found in independent DMC calculations usi…
We compute the electrical conductivity for liquid hydrogen at high pressure using quantum Monte Carlo. The method uses Coupled Electron-Ion Monte Carlo to generate configurations of liquid hydrogen. For each configuration correlated sampling of electrons is performed in order to calculate a set of lowest many-body eigenstates and current-current correlation functions of the system, which are summed over in the many-body Kubo formula to give AC electrical conductivity directly. The extrapolated DC conductivity at 3000 K for several densities shows a liquid semiconductor to liquid-metal transiti…
A theoretical study is reported of the molecular-to-atomic transition in solid hydrogen at high pressure. We use the diffusion quantum Monte Carlo method to calculate the static lattice energies of the competing phases and a density-functional-theory-based vibrational self-consistent field method to calculate anharmonic vibrational properties. We find a small but significant contribution to the vibrational energy from anharmonicity. A transition from the molecular Cmca-12 direct to the atomic I4_1/amd phase is found at 374 GPa. The vibrational contribution lowers the transition pressure by 91 …
The hybrid nanostructures of nitrogen doped carbon materials and nonprecious transition metals are among the most promising electrocatalysts to replace noble metal catalysts for renewable energy applications. However, the fundamental principles governing the catalytic activity of such hybrid materials remain elusive. Herein, we systematically explore the electrocatalytic properties of transition metals, transition metal oxides and carbides substrates covered by nitrogen-doped graphitic sheets for hydrogen evolution reaction (HER). Our first-principles calculations show that the graphitic sheet…
Systematic Fine-Tuning of MACE Interatomic Potentials for Catalysis
University of South Carolina（南卡罗来纳大学）；National Laboratory of the Rockies（落基山国家实验室）；Rensselaer Polytechnic Institute（伦斯勒理工学院）；University of Cambridge（剑桥大学）；Max Planck Institute for Polymer Research（马克斯·普朗克聚合物研究所）；University of Toronto（多伦多大学）
A metal oxide support and a catalytically active metal are the two main ingredients for complex catalysts used in heterogeneous catalysis. The gas environment can change the catalyst during the reaction, modifying its structural and electronic properties. Here, we use monochromated electron energy loss spectroscopy (EELS) to reveal hydrogen-pressure-dependent changes of the electronic structure at the Pd/rutile-TiO$_2$ interface in an environmental transmission electron microscope (ETEM). Hydrogen-induced changes are observed in rutile-TiO$_2$ within $2$~nm from the interface at $10$~Pa of hyd…
Grand Canonical Monte Carlo (GCMC) simulations are performed to study hydrogen physisorption in different nano carbon porous materials made up of different substructures including carbon nanotubes (CNT), graphene sheets and C60. Hydrogen weight percentage (wt%) at different temperatures with pressure ranging from 1 to 20MPa are predicted. Fugacity and quantum effects on hydrogen adsorption are investigated. Different structural dimensions including the sizes of the substructures and spacing between the substructures are used to study the geometrical effects on hydrogen storage capacity in carb…
The quantitative correlation of the catalytic activity with microscopic structure of heterogeneous catalysts is a major challenge for the field of catalysis science. It requests synergistic capabilities to tailor the structure with atomic scale precision and to control the catalytic reaction to proceed through well-defined pathways. Here we leverage on the controlled growth of MoS2 atomically thin films to demonstrate that the catalytic activity of MoS2 for the hydrogen evolution reaction decreases by a factor of ~4.47 for the addition of every one more layer. Similar layer dependence is also …
We have used diffusion Monte Carlo (DMC) simulations to calculate the energy barrier for H$_2$ dissociation on the Mg(0001) surface. The calculations employ pseudopotentials and systematically improvable B-spline basis sets to expand the single particle orbitals used to construct the trial wavefunctions. Extensive tests on system size, time step, and other sources of errors, performed on periodically repeated systems of up to 550 atoms, show that all these errors together can be reduced to $\sim 0.03$ eV. The DMC dissociation barrier is calculated to be $1.18 \pm 0.03$ eV, and is compared to t…
Hydrogenation is an effective way to tune material property1-5. Traditional techniques for doping hydrogen atoms into solid materials are very costly due to the need for noble metal catalysis and high-temperature/pressure annealing treatment or even high energy proton implantation in vacuum condition5-8. Acid solution contains plenty of freely-wandering protons, but it is difficult to act as a proton source for doping, since the protons always cause corrosions by destroying solid lattices before residing into them. Here we achieve a facile way to hydrogenate monoclinic vanadium dioxide (VO2) w…
Quantum materials such as Topological Insulators (TI) have been promising due to diverse applications of their robust surface/edge states in the bulk (3D) and two-dimensional (2D) regime. Such conducting surface states in 3D systems, host "\textit{electron bath}" which are known to facilitate catalysis. However, the analogous effects in 2D scenarios wherein, conducting helical edge states giving rise to Fermionic accumulation has been scarcely addressed. Using density functional theory based \textit{first-principles} calculations, we demonstrate that, the conducting edge states in 2D TI such a…
The design and development of inexpensive highly efficient electrocatalysts for hydrogen production, underpins several emerging clean-energy technologies. In this work, for the first time, molybdenum disulfide (MoS2) nanodots have been synthesized by ionic liquid assisted grinding exfoliation of bulk platelets and isolated by sequential centrifugation. The nanodots have a thickness of up to 7 layers (4 nm) and an average lateral size smaller than 20 nm. Detailed structural characterization established that the nanodots retained the crystalline quality and low oxidation states of the bulk mater…
We survey the phase diagram of high-pressure molecular hydrogen with path integral molecular dynamics using a machine-learned interatomic potential trained with Quantum Monte Carlo forces and energies. Besides the HCP and C2/c-24 phases, we find two new stable phases both with molecular centers in the Fmmm-4 structure, separated by a molecular orientation transition with temperature. The high temperature isotropic Fmmm-4 phase has a reentrant melting line with a maximum at higher temperature (1450K at 150GPa) than previously estimated and crosses the liquid-liquid transition line around 1200K …
Diffusion Monte Carlo (DMC) is being recognized as a higher-accuracy, albeit more computationally expensive, alternative to Density Functional Theory (DFT) for energy predictions of catalytic systems. A major computational bottleneck in the use of DMC for catalysis is the need to perform finite-size extrapolations by simulating increasingly large periodic cells (supercells) to eliminate many-body finite-size effects and obtain energies in the thermodynamic limit. Here, we show that this computational cost can be significantly reduced by leveraging the cancellation of many-body finite-size erro…
We analyze in detail the electronic properties of high pressure hydrogen around the liquid-liquid phase transition based on Coupled Electron-Ion Monte Carlo calculations. Computing the off-diagonal single particle density matrix and the momentum distribution we discuss localization properties of the electrons. The abrupt changes of these distributions indicate a metal to insulator transition occurring together with the structural transition from the atomic to molecular fluid. We further discuss the electron-proton and electron-electron pair correlation functions, which also change abruptly at …
In many-body nonlinear systems with sufficient anharmonicity, a special kind of lattice vibrations, namely, Localized Anharmonic Vibrations (LAV) can be excited either thermally or by external triggering, in which the amplitude of atomic oscillations greatly exceeds that of harmonic oscillations (phonons) that determine the system temperature. Coherency and persistence of LAV may have drastic effect on chemical and nuclear reaction rates due to time-periodic modulation of reaction sites. One example is a strong acceleration of chemical reaction rates driven by thermally-activated "jumps" over …
The yardstick of new first-principles approaches to key points on reaction paths at metal surfaces is chemical accuracy compared to reliable experiment. By this we mean that such values as the activation barrier are required to within 1 kcal/mol. Quantum Monte Carlo (QMC) is a promising (albeit lengthy) first-principles method for this and we are now beyond the dawn of QMC benchmarks for these systems, since hydrogen dissociation on Cu(111) has been studied with quite adequate accuracy in two improving QMC studies Hoggan ArXiv 2015, K. Doblhoff-Deir et al JCTC (2017). Pt and Cu require the use…
