KMC study on the promotion of the water–gas shift reaction by CO-induced clustering on Cu(111)

Abstract

The Water–Gas Shift Reaction (WGSR) is a critical process for hydrogen production and purification. However, typically employed low-index metal surface models have limitations in capturing the catalytic active sites under actual reaction conditions. Inspired by the recently surface science studies that CO can induce the formation of copper clusters on the Cu(111) surface, we investigated the dynamic process of copper cluster formation induced by CO on the Cu(111) surface and its impact on the WGSR by employing Density Functional Theory (DFT) combined with kinetic Monte Carlo (kMC) simulation. The kMC results indicated that ejection barriers that detachment of a copper atom from the step edges of adjacent terraces, defect density on Cu(111), and CO adsorption free energy significantly influence cluster formation and morphology, with lower ejection barrier and higher defect density as well as more negative of CO adsorption free energy favors the formation of high-density small copper cluster like Cu₇ or smaller observed by experiment. Models for Cu₃ and Cu₇ clusters were established to assess their effect on WGSR by mean-field microkinetic simulation modeling (MF-MKM) as well as kMC. Our results revealed that the adsorption energy on the clusters is higher than on the Cu(111) surface, due to the decrease in the coordination number of copper atoms. The activation energy for water dissociation on the clusters is lower than on Cu(111). Microkinetic analysis indicated that the activity order for WGSR is Cu₇/Cu(111) > Cu₃/Cu(111) > Cu(111), and the reliability of the MF-MKM method was confirmed by comparing the WGSR rates with those predicted by kMC. The activity order is attributed to the reduced activation energy for water dissociation and the ability of the clusters to recombine H₂ on the terrace. This work elucidates the rules of cluster formation induced by CO and its potential impact on the reactivity of the WGSR.