259827 New Insights Into the Water-Gas Shift Reaction Over Bulk Cr2O3*Fe2O3 Mixed Oxide Catalysts: A Combined Operando Raman-IR-XAS-MS Investigation
The bulk chromia-iron mixed oxide catalyst is the primary catalyst for the high temperature (310-450oC) 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. Despite numerous characterization studies, the role of the chromia promoter is still not completely understood. The absence of fundamental in situ and operando spectroscopic studies of the bulk 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 promoter, and the nature of the catalytic active site, in situ Ambient Pressure-XPS (AP-XPS) and operando Raman, IR, and XAS spectroscopic studies under reaction conditions were undertaken in the present investigation.
The operando Raman spectroscopy studies during the WGS confirm that crystalline Fe3O4 is the active bulk phase in Cr-doped samples and that crystalline Cr2O3 NPs are not present. The corresponding operando IR spectroscopy measurements revealed that dioxo surface (O=)2CrO2 species are also present under oxidizing conditions and become reduced during the WGS reaction. The IR measurements also revealed that no surface reaction intermediates are present, even when the reaction is performed at the WGS reaction temperature limit of ~225oC. Operando XANES measurements of the Cr K-edge confirm the reduction of Cr+6 (dioxo species) to Cr+3 during steady-state 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 iron oxide shell consisting of an indistinguishable FeOx state (+2 or +3). Recent in situ AP-XPS data has revealed, for the first time, that some Cr+6species are not reduced and remain on the catalyst surface during reaction conditions.
The new fundamental insights are allowing for the establishment of molecular level models, based on direct observations during relevant WGS reaction conditions, for the WGS reaction mechanism and the nature of the catalytic active site for the high temperature chromia-iron WGS catalysts.