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

Kinetic analyses of Ce4+ <-> Ce3+ redoxand CO2/H-2 formation for the unsteady-statewater-gas shift (WGS) reaction under periodic CO <-> H2O feeds to Cu/CeO2 catalysts are carried out byin situ/operando ultraviolet-vis and infrared studies at 350 degrees C. Under CO, the Ce4+-OH species are reducedto produce H-2, CO2, and Ce3+-(oxygen vacancy). Under the subsequent feed of H2O, Ce3+- is reoxidized by H2O to yieldH(2) and Ce4+-OH species. The rates ofCe(4+) reduction/Ce3+ reoxidation are close tothose of CO2/H-2 formation for various Cu/CeO2 catalysts with different Cu loadings, providing quantitativeevidence of the redox-based mechanism of the unsteady-state WGS reaction.Ce3+- reoxidation by H2O hasa lower apparent barrier than the Ce4+-O reductionstep. The H2O-promoted desorption of the adsorbed carbonatesis responsible for CO2 formation under H2O.The characterization results suggest that the number of interfacialsites between the CeO2 and Cu species increases with decreasingCu loading. Turnover frequencies per surface Cu site for the Ce4+ <-> Ce3+ redox reaction and CO2/H-2 formation increase with the number of interface sites.An associative redox mechanism based on the redox reaction betweenthe oxidized state (Cu2+-OH adjacent to Ce4+ and Ce4+-OH) and the reduced state (Cu+--Ce3+) is proposed as the maincatalytic cycle of the unsteady-state WGS reaction.