(629f) Promotion Mechanisms of Iron Oxide-Based High Temperature-Water Gas Shift (HT-WGS) Catalysts By Chromium and Copper

The Cr and Cu promotion mechanisms of high temperature water-gas shift (HT-WGS) iron oxide catalysts, synthesized by co-precipitation, were investigated as a function of reaction conditions. XRD and in situ Raman characterization showed that the initial calcined catalysts consisted of the Fe₂O₃ (hematite) bulk phase and transformed to the Fe₃O₄ (magnetite) phase during the HT-WGS reactions. In situ NAP-XPS and HS-LEIS surface analysis revealed that Cr was surface enriched as Cr⁺⁶ for the initial catalyst and reduced to Cr⁺³ during the HT-WGS reactions, with the Cr⁺³ dissolving into the bulk iron oxide lattice forming a solid solution. In situ NAP-XPS, XANES and HS-LEIS characterization indicated that Cu was initially present as Cu⁺² cations dissolved in the Fe₂O₃ bulk lattice and reduced to metallic Cu⁰ nanoparticles (~3nm) on the external surface of the iron oxide support during the HT-WGS reactions. In situ HS-LEIS surface analysis also suggests that ~1/3 of the surface of the Cu nanoparticles was covered by a FeO_x overlayer. The CO-TPR probe demonstrated that Cr does not chemically promote the iron oxide catalyst and that only Cu is a chemical promoter for the iron oxide HT-WGS catalysts. The Cu promoter introduces new catalytic active sites that enhance the reaction rates of the WGS reactions.