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Abstract

The Cr and Cu promotion mechanisms of high temperature water–gas shift (HT-WGS) iron oxide catalysts, synthesized by coprecipitation, 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 Cr⁶⁺ for the initial catalyst and reduced to Cr³⁺ during the HT-WGS reactions, with the Cr³⁺ 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 Cu²⁺ cations dissolved in the Fe₂O₃ bulk lattice and reduced to metallic Cu⁰ nanoparticles (∼3 nm) 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.