Dynamics of CrO₃-Fe₂O₃ Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

A series of supported CrO₃/Fe₂O₃ catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O=)₂Cr⁶⁺O₂ species and a bulk Fe₂O₃ phase containing some Cr³⁺ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe₂O₃ bulk phase becomes Fe₃O₄,and surface dioxo (O=)₂Cr⁶⁺O₂ species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium-iron oxide catalyst surface is dominated by FeO_x sites, but some minor reduced surface chromia sites are also retained. The Fe_{3 - x}Cr_xO₄ solid solution stabilizes the iron oxide phase from reducing to metallic Fe⁰ and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C¹⁶O₂/H₂ → C¹⁸O₂/H₂ isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O∗ sites from the surface region of the chromium-iron oxide catalysts are involved in the RWGS reaction. The number of redox O∗ sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O∗ sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect.