Mechanism of the Periodic Unsteady-State Water–Gas Shift Reaction on Highly Dispersed Cu-Loaded CeO2 Catalysts

Abstract

Kinetic analyses of Ce4+ ↔ Ce3+ redox and CO2/H2 formation for the unsteady-state water–gas shift (WGS) reaction under periodic CO ↔ H2O feeds to Cu/CeO2 catalysts are carried out by in situ/operando ultraviolet–vis and infrared studies at 350 °C. Under CO, the Ce4+–OH species are reduced to produce H2, CO2, and Ce3+–□ (oxygen vacancy). Under the subsequent feed of H2O, Ce3+–□ is reoxidized by H2O to yield H2 and Ce4+–OH species. The rates of Ce4+ reduction/Ce3+ reoxidation are close to those of CO2/H2 formation for various Cu/CeO2 catalysts with different Cu loadings, providing quantitative evidence of the redox-based mechanism of the unsteady-state WGS reaction. Ce3+–□ reoxidation by H2O has a lower apparent barrier than the Ce4+–O reduction step. The H2O-promoted desorption of the adsorbed carbonates is responsible for CO2 formation under H2O. The characterization results suggest that the number of interfacial sites between the CeO2 and Cu species increases with decreasing Cu loading. Turnover frequencies per surface Cu site for the Ce4+ ↔ Ce3+ redox reaction and CO2/H2 formation increase with the number of interface sites. An associative redox mechanism based on the redox reaction between the oxidized state (Cu2+–OH adjacent to Ce4+ and Ce4+–OH) and the reduced state (Cu+–□–Ce3+) is proposed as the main catalytic cycle of the unsteady-state WGS reaction.