(658b) Operando Raman and IR Spectroscopy During the Water-Gas Shift Reaction Over Bulk Cr2O3*Fe2O3 Mixed Oxide Catalysts

The high temperature water-gas shift (WGS) reaction is commercially conducted over bulk Cr₂O₃*Fe₂O₃ (~10% CrO₃) catalysts. Much is known about the bulk properties of this catalytic system: the active catalyst consists of the Fe₃O₄ phase and Cr⁺³ cations are dissolved in the Fe₃O₄ lattice. A separate phase of Cr₂O₃ nanoparticles (NPs) has never been found and the only thing known about the surface of this catalyst system is that it is enriched with chromium. The objective of this investigation is to learn more about the bulk and surface aspects of this important catalyst system during WGS reaction conditions, which has never been reported, by employing Raman and IR operando spectroscopy investigations. The operando Raman spectroscopy studies during WGS confirm that Fe₃O₄ is the active phase and that crystalline Cr₂O₃ NPs are not present. The corresponding IR spectroscopy measurements revealed that dioxo surface (O=)₂CrO₂ species are also present under oxidizing conditions that become reduced during WGS reaction. The surface CrOx species enhance the reaction rate and are intimately involved in the formation of surface formate intermediates. These studies strongly suggest that it is the surface CrOx species that are the catalytic active sites for the high temperature WGS reaction and that the Fe₃O₄ phase just serves as a support for the chromium oxide sites.