Promotion Mechanisms of Iron Oxide-Based High Temperature Water-Gas Shift Catalysts by Chromium and Copper

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 Cu-0 nanoparticles (similar to 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 similar to 1/3 of the surface of the Cu nanoparticles was covered by a FeOx 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.