Abstract
We investigate the water gas shift reaction (WGSR) on copper using three levels of exchange-correlation (XC) functionals of increasing complexity: the Perdew–Burke–Ernzerhof (PBE) functional, the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional, and the exact exchange plus the random phase approximation (RPA) correlation functional. We show that DFT predictions for the kinetics of the WGSR strongly depend on the choice of XC functionals. It is important to have accurate CO adsorption energy. Due to PBE’s overestimation of CO’s adsorption energy, it predicts small free surface and a negative (incorrect) reaction order for CO. HSE and RPA largely avoid such overestimation and predict large free surface and positive reaction orders for CO. The key finding in this work is that the prediction for the WGSR mechanism also depends on the choice of XC functionals. PBE and HSE predict the carboxyl mechanism, while RPA predicts that both redox and carboxyl mechanisms are important. These results suggest that caution should be paid when using approximate XC functionals to model heterogeneous catalysis (such as the WGSR investigated here) in which several mechanisms compete. In addition, we also observed one problem for RPA: It underestimates the overall WGSR energy and predicts a negative reaction order for CO2. In addition, we examine formate’s role in the WGSR over copper. Both DFT and previous experiments suggest that formate does not participate much in the reaction and cannot cause a negative reaction for CO2 for the WGSR over copper.
Published Version
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