(781f) New Insights Into the Water-Gas Shift Reaction Over Bulk CuO*Cr2O3*Fe2O3 Mixed Oxide Catalysts: A Combined Operando Raman-IR-XAS-XPS Investigation

The bulk chromia-iron mixed oxide catalyst is the primary catalyst for the high temperature (310-450^oC) water-gas shift (WGS) reaction for the production of hydrogen and carbon dioxide from steam and carbon monoxide.The commercial lifetime of pure magnetite catalysts is limited because of thermal sintering and chromium oxide addition, 8-12% Cr₂O₃, has been found to stabilize the surface area and extend the catalyst life to 2-5 years. Additionally, a few percent CuO is added to enhance the activity. Despite numerous characterization studies, the role of the chromia and copper promoters is still not completely understood. The absence of fundamental in situ and operando spectroscopic studies of the bulk CuO*Cr₂O₃*Fe₂O₃ WGS shift catalyst during the WGS reaction in the catalysis literature has hindered the development of molecular level insights about the catalytic active sites, surface reaction intermediates and the reaction mechanism. In order to address the state of the iron oxide catalyst under reaction conditions, the role of the chromia and copper promoters, and the nature of the catalytic active site, in situ and operandoRaman spectroscopy, IR spectroscopy, X-ray Absorption Spectroscopy (XAS), and Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) studies under reaction conditions were undertaken in the present investigation.

     The operando Raman spectroscopy studies during the high temperature WGS confirm that crystalline Fe₃O₄ is the active bulk phase in Cr- and Cu-doped samples and surface dioxo (O=)₂CrO₂ species are reduced. The corresponding operando IR spectroscopy measurements revealed that no surface reaction intermediates are present, even when the reaction is performed at the temperature limit of ~225^oC. Operando XANES measurements (Cr K-edge) confirm the reduction of Cr⁺⁶ (dioxo species) to Cr⁺³ during WGS reaction conditions. The EXAFS Fe K-edge data confirm that the Cr-doped samples stabilize as Fe₃O₄ under steady-state WGS reaction conditions, while unpromoted Fe₂O₃ is active as an amorphous phase consisting mostly of a core of metallic Fe with an Fe₃O₄ shell. Recent in situ and operando AP-XPS data has revealed, for the first time, that some Cr⁺⁶ species remain on the catalyst surface under reaction conditions while CuO reduces to surface Cu⁰ metal and sinters. The new fundamental insights are allowing for establishment of the WGS reaction mechanism and the nature of the catalytic active site during relevant WGS reaction conditions.