Evaluation of the Oxygen Mobility in CePO4-Supported Catalysts: Mechanistic Implications on the Water-Gas Shift Reaction

Abstract

The hexagonal and monoclinic phases of CePO4 have been demonstrated to be excellent catalytic supports for Pt-based water-gas shift (WGS) catalysts. Consequently, the elucidation of the WGS reaction mechanism in these materials constitutes a fundamental aspect in order to explain their catalytic behavior. Because the observed WGS reaction path is closely related to the absence or presence of oxygen vacancies in the support, the study of the oxygen mobility in these solids constitutes a key factor for the understanding of the structure of the materials and its influence on the reaction mechanism. In this study, the oxygen mobility in CePO4 supports and the corresponding Pt catalysts has been evaluated by means of isotopic exchange experiments using 18O2 and C18O2 as probe molecules. Results demonstrate that the evaluated solids present a low exchange activity when 18O2 is used, indicating the absence of oxygen vacancies in these solids, thus suggesting a poor influence of the WGS redox mechanism. On the contrary, a high oxygen exchange activity is observed using C18O2, demonstrating that the exchange in these materials takes place through the formation of carbonate-like intermediates, thus suggesting the associative mechanism of the WGS reaction as the preferred path in these solids. Operando diffuse reflectance infrared spectroscopy experiments under WGS reaction conditions confirm these results, proving that the WGS reaction in the studied materials takes place through a formate-mediated associative mechanism. Copyright © 2020 American Chemical Society.