Mechanism of the Periodic Unsteady-State Water–Gas Shift Reaction on Highly Dispersed Cu-Loaded CeO<sub>2</sub> Catalysts
Ningqiang Zhang, Shinta Miyazaki, Yucheng Qian, Yuan Jing, Takashi Toyao, Ken‐ichi Shimizu
Abstract
Kinetic analyses of Ce 4+ ↔ Ce 3+ redox and CO 2 /H 2 formation for the unsteady-state water–gas shift (WGS) reaction under periodic CO ↔ H 2 O feeds to Cu/CeO 2 catalysts are carried out by in situ/operando ultraviolet–vis and infrared studies at 350 °C. Under CO, the Ce 4+ –OH species are reduced to produce H 2, CO 2, and Ce 3+ –□ (oxygen vacancy). Under the subsequent feed of H 2 O, Ce 3+ –□ is reoxidized by H 2 O to yield H 2 and Ce 4+ –OH species. The rates of Ce 4+ reduction/Ce 3+ reoxidation are close to those of CO 2 /H 2 formation for various Cu/CeO 2 catalysts with different Cu loadings, providing quantitative evidence of the redox-based mechanism of the unsteady-state WGS reaction. Ce 3+ –□ reoxidation by H 2 O has a lower apparent barrier than the Ce 4+ –O reduction step. The H 2 O-promoted desorption of the adsorbed carbonates is responsible for CO 2 formation under H 2 O. The characterization results suggest that the number of interfacial sites between the CeO 2 and Cu species increases with decreasing Cu loading. Turnover frequencies per surface Cu site for the Ce 4+ ↔ Ce 3+ redox reaction and CO 2 /H 2 formation increase with the number of interface sites. An associative redox mechanism based on the redox reaction between the oxidized state (Cu 2+ –OH adjacent to Ce 4+ and Ce 4+ –OH) and the reduced state (Cu + –□–Ce 3+ ) is proposed as the main catalytic cycle of the unsteady-state WGS reaction.