arXiv subjects

Philip E Hoggan

Publications and source records attributed to Philip E Hoggan.

6 recordsLinked to original sources

arXiv2026

Short time-to-solution Quantum Monte Carlo for catalysed hydrogen synthesis. Tools give CO hydrolysis activation barriers to 1kJ/mol on Pt(111)

Hydrogen synthesis is a clean, sustainable alternative to fossil fuel \cite{gals}. It has come of age: prototyping various aspects of hydrogen power are hot topics. In 9 out of 10 reactions, a solid catalyst is used. Here hydrogen production (via water-gas shift) is studied. Adsorbed reactants are optimidsed on model Pt(111). Focus is on partial O-H bond dissociation, when CO is co-adsorbed with water on this plane. hydrogen is the product. Many chemical reactions involve bond-dissociation. This process is often the key to rate-limiting reaction steps at solid surfaces. Bond-breaking is poorly described by Hartree-Fock and DFT methods, our embedded active site approach is used. We showcase Quantum Monte Carlo (QMC) methodology using the ground-state Slater Determinant of a simple four primitive-cell layer model, oriented to expose Pt (111), to initialise the QMC. This stochastic approach solves the Schr{ö}dinger equation. It recently came of age for heterogeneous systems involving solids. During hydrolysis of carbon monoxide, initial O-H bond stretch is rate-limiting. Its dissociation energy is offset by surface Pt-H bond formation. The reactive formate (H-O-C=O) species formed by initial hydrolysis of CO, also interacting with a vicinal Pt. The products are hydrogen (CO$_2$ by-product is mineralised. A H-atom dissociates from the formate, another is desorbed from Pt(111). This yields pure hydrogen. Single-determinant work with a novel averaging procedure is compared to a high-level configuration interaction (CI) wave-function. Activation barriers are given to 0.86kJ/mol (c.f. 0.7 of the CI benchmark). Active sites embedded in metal lattice (111) faces. These trial wave-functions guide QMC.

cond-mat.mtrl-sci↗

arXiv2020

Quantum Monte Carlo method for metal catalysis: case study of hydrogen production on Pt(111)

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.

cond-mat.mtrl-sci↗

arXiv2019

Selective hydrogen production at platinum surfaces investigated by the Quantum Monte Carlo approach to chemical reactions

The present letter describes an atomic scale investigation of a chemical reaction for selective hydrogen production. This clean fuel is a sustainable energy source. Because electron transfer is the key to such reactions, accurate methods based on quantum theory are used. The reaction between water and carbon monoxide has been used industrially with metal catalysts, usually Platinum. There is a considerable economic and environmental challenge underpinning this application of a fundamental process limited by bond dissociation. That is the process often limiting reaction rates for industrial catalysis. Most mainstream quantum approaches fail to greater or lesser degree in the description of bond dissociation. The present work presents a promising alternative: the initial analysis of a considerable mass of statistical data generated by the atomic-scale Quantum Monte Carlo method to very stringent statistical accuracy for essential information on the hydrogen production via the water-gas shift reaction with platinum catalyst. This is encouraging for establishing less well-known benchmark values of industrial reaction barriers on Pt(111).

physics.chem-ph↗

arXiv2018

Analytical evaluation of relativistic molecular integrals. II. Computational aspect for relativistic molecular auxiliary functions

The Slater-type orbital basis with non-integer principal quantum numbers is a physically and mathematically motivated choice for molecular electronic structure calculations in both non-relativistic and relativistic theory. The non-analyticity of these orbitals at $r=0$, however, requires analytical relations for multi-center integrals to be derived. This is nearly insurmountable. Previous papers by present authors eliminated this difficulty. Highly accurate results can be achieved by the procedure described in these papers, which place no restrictions on quantum numbers in all ranges of orbital parameters. The purpose of this work is to investigate computational aspects of the formulae given in the previous paper. It is to present a method which helps to increase computational efficiency. In terms of the processing time, evaluation of integrals over Slater-type orbitals with non-integer principal quantum numbers are competitive with those over Slater-type orbitals with integer principal quantum numbers.

quant-ph↗

arXiv2017

Quantum Monte Carlo calculation of the bridge reaction barrier to hydrogen dissociation on Pt(111)

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 of pseudo-potentials in these calculations and we show that those of Pt are less problematic than those for Cu, particularly for QMC work. In this letter, we determine the activation barrier to hydrogen dissociation on Pt(111) using the bridging geometry. This is found to agree to better than chemical accuracy with recent Specific Reaction Parameter (SRP-DFT) work, which is itself within chemical accuracy of measurement (Kroes et al Cplett 2017). The results suggest the dissociation barrier for hydrogen on Pt(111) is close to 5.4 (QMC) and 6.2 (SRP-DFT) kcal/mol with a QMC standard error of 1.08 kcal/mol. This is encouraging for establishing less well-known benchmark values of industrial reaction barriers on Pt(111).

cond-mat.mtrl-sci↗

arXiv2015

Quantum Monte Carlo activation barrier for hydrogen dissociation on copper to unprecedented accuracy

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 dissociation reaction pathway for stretched and distant equilibrium H$_2$ are given by using Density Functional Theory (DFT) calculations in a plane wave basis. The PBE wave-functions at these bond-lengths serve as trial input for Quantum Monte Carlo (QMC) simulations of the ground states to obtain highly accurate correlated results for the associated activation barriers indicating the catalytic effect on this dissociation. This correlation varies as bonds dissociate, requiring its accurate evaluation. Finite size effects and fixed-node error are possible limitations to accuracy of this type of QMC study. We are able to limit fixed node error, using certain trial wave-functions. The finite size effect is considerable, although comparing two adsorbed geometries cancels about 90% with respect to clean surfaces. The pseudo-potential used to represent the atomic core of copper must also be determined carefully: we leave 11 active electrons but include the 3d shell in the pseudo-potential.

cond-mat.mtrl-sci↗