Ongoing developments in automatic generation of microkinetic models for heterogeneous catalysis using RMG

Abstract

Kinetic parameters for surface reactions can be predicted using a combination of Density Functional Theory (DFT) calculations, scaling relations, and machine learning algorithms; however, construction of microkinetic models still requires a knowledge of all the possible, or at least reasonable, reaction pathways.The recently developed Reaction Mechanism Generator (RMG) for heterogeneous catalysis, now included in RMG version 3.0, is built upon well-established, open-source software that can provide detailed reaction mechanisms from user-supplied initial conditions without making a priori assumptions of a rate-limiting step or dominant pathway. As a result of this work, RMG is now able to estimate adsorbate thermochemistry and construct detailed microkinetic models on a range of hypothetical metal surfaces using linear scaling relationships, predict new surface reactions for polymerization, and use coverage dependent modifiers. Linear scaling relationships are a simple, computationally efficient way to estimate adsorption energies by scaling the energy of a surface species on one metal to any other metal. By conducting simulations with sensitivity analyses, users can not only determine the rate limiting step on each surface by plotting a ``volcano surface'' for the degree of rate control of each reaction as a function of elemental binding energies, but also screen novel catalysts for desirable properties. The catalytic partial oxidation of methane was investigated to demonstrate the utility of this new tool and explored catalysts to maximize the yield of synthesis gas. Sensitivity analyses show that while the dissociative adsorption of O2 has the highest degree of rate control, the interactions between individual reactions and reactor conditions are complex, which result in a dynamic rate-limiting step across differing metals. The dimerization and oligomerization of ethylene to 1-butene was studied to demonstrate new surface reactions in a semi-batch reactor on Pt(111) and a metal with carbon and oxygen binding energies of -5.5~eV and -2.5~eV, respectively. Sensitivity analyses showed the most important characteristic of the metal catalyst was for the adsorption of ethylene to favor dissociative adsorption retaining the carbon-carbon double bond. Comparison of the catalytic partial oxidation of methane on Pt(111) with and without the new surface families shows an immense improvement in the predictive capability of the model, especially the temperature profile. Coverage dependence was demonstrated by showing the catalytic partial oxidation of methane with and without the coverage modifiers on Pt(111) to remove any errors from linear scaling. The model using coverage dependencies shows better agreement to the experiment results, and the addition of such modifiers improves the predictive accuracy of the RMG models. Portions of this abstract have been reprinted with permission from Automated Mechanism Generation Using Linear Scaling Relationships and Sensitivity Analyses Applied to Catalytic Partial Oxidation of Methane,Emily J. Mazeau, Priyanka Satpute, Katrín Blöndal, C. Franklin Goldsmith, and Richard H. West, ACS Catalysis, 2021, 11, (12), 7114-7125. Copyright 2021 American Chemical Society.--Author's abstract