First-Principles-Based Kinetic Monte Carlo Simulation of the Structure Sensitivity of the Water–Gas Shift Reaction on Platinum Surfaces

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Abstract

Precious metals, such as platinum, have recently been explored as catalysts for the water–gas shift (WGS) reaction. Previous studies have elucidated the underlying reaction mechanism using mean-field models; however, the contribution of different site types, namely, steps and terraces, on the overall reaction rate is an open question, and it remains unclear how structure sensitive the WGS reaction is. The present work addresses these questions using a multiscale modeling approach that integrates density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulation. We calculate the reaction barriers and pre-exponential factors of the elementary steps of the WGS mechanism at steps and terraces using DFT. These elementary steps include adsorption and desorption, water and hydroxyl decomposition, and the formation of carboxyl and formate intermediates followed by the formation of CO2. We subsequently incorporate the calculated values into a KMC framework and calculate the turnover frequency for Pt(111), Pt(211), and Pt(322) under a wide range of conditions. Our results indicate that for industrially relevant conditions, the WGS reaction is practically structure-insensitive with both the steps and the terraces contributing to the overall activity, whereas for low CO:H2O ratios, the steps are much more active than the terraces. Investigation of the surface coverages and analysis of the reaction statistics reveals that this change in structure sensitivity stems from changes in the most abundant surface species as well as the primary pathway through which the chemistry proceeds. Our study provides the first evidence that the active site may be condition specific and may entail multiple individual sites under certain conditions.

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