Water–Gas Shift Reaction on Titania-Supported Single-Metal-Atom Catalysts: The Role of Cation (Ti) and Oxygen Vacancy
Shuang Zhu, Hui Wang, Kaiwei Wan, Lingju Guo, Tao He, Xinghua Shi
Abstract
Oxide-supported single-metal-atom catalysts have shown extraordinary catalytic properties for a water–gas shift (WGS) reaction. Herein, a series of single metal atoms (M = Cu, Ag, Au, Ni, Pd, and Pt) embedded in cation (Ti) or oxygen vacancy on the surface of rutile TiO2(110) are systematically studied. We found that different types of single atoms or different embedded styles of single atoms will have significantly different effects on the reaction mechanism of the WGS reaction. On the embedded cation (Ti) catalyst (M@Ti1–xO2), the dominant pathway of the WGS reaction follows the redox mechanism, but the carboxyl pathway is preferred on the embedded oxygen vacancy catalyst (M@TiO2–x). For the redox pathways on M@Ti1–xO2, we found that the adsorption strength of CO is related to its activation. For the association mechanism pathway on M@TiO2–x, it is found that the Gibbs free energies of different single atoms are similar. However, unlike M@Ti1–xO2, the active site of the WGS reaction is no longer directly above the single atom but on the Ti atom adjacent to the single atom. Finally, a Brønsted–Evans–Polanyi (BEP) relationship between the Gibbs free energy of the transition state and adsorption free energy of the reactant gas molecule (CO) is provided, which can be used as a relative quantity to describe the single-atom catalyst (SAC) reaction. Our results could provide some useful information on the design of new single-atom catalyst systems.