Water–Gas Shift Reaction over Cu_xO/Cu(111) (x < 2) from a DFT-MKM-kMC Study

Cu-based catalysts benefiting from their low cost and high catalytic activity are widely used in the low-temperature water–gas shift reaction (WGSR) industry. However, there is still a lack of understanding of surface oxides (Cu_xO/Cu(111)) and their influence on the catalytic activity. Herein, we focus on these issues, systematically study the relative stability of copper surface oxides over Cu(111) by ab initio atomistic thermodynamics, and then identify their surface population by Boltzmann statistical mechanics. It was found that p4, p4-OCu_3, and p4-(OCu₃)₂ take up a certain proportion of Cu(111) under ideal conditions. The catalytic activity for WGSR was investigated through a combined approach consisting of density functional theory and multisite mean-field microkinetic modeling (MF-MKM) as well as kinetic Monte Carlo (kMC) simulation on these surfaces. The simulation results illustrate that with the ratio of Cu⁺/(Cu⁰ + Cu⁺) increasing, the catalytic activity exhibits a “volcano-type” relationship, in agreement with the experimental observation. Furthermore, the weakly oxidized phase, p4-OCu₃, in which the ratio of Cu⁺/(Cu⁰ + Cu⁺) on the surface equals 0.273, has the best catalytic activity in this paper. That is because its suitable geometric structure enhances the adsorption of H₂O, thus leading to high activity. It is possible that Cu_xO–Cu⁰ can serve as the active site in Cu(111)-catalyzed-WGSR, in which Cu_xO is used to activate H₂O while Cu⁰ is used to form H₂, and the synergistic effect between them is vital to catalyze WGSR. Besides, doping with Pt or Zn can improve the catalytic performance of p4-OCu₃ by enhancing the CO adsorption or lowering the activation energy of the H₂ combination. It is hoped that our results show that the appropriate Cu⁺/(Cu⁰ + Cu⁺) ratio is the WGSR active site and may extend to other systems like Cu(100)-catalyzed WGSR and even Cu/CeO₂.