Rational Design of Promoted Iron Oxide-Based Catalysts for the High Temperature-Water Gas Shift (HT-WGS) Reaction

Israel Wachs, Chemical * Biomolecular Engineering, Lehigh University, Bethlehem, USA
Minghui Zhu, Chemical * Biomolecular Engineering, Lehigh University, Bethlehem, Usa
Ozgen Yalçın, Chemical Engineering/chemical & Biomolecular Engineering, Middle Eastern Technical University/lehigh University, Ankara/bethlehem, Turkey/usa
Isik Onal, Chemical Engineering, Middle Eastern Technical University, Ankara, Turkey


Industrial H2 is mostly produced by methane steam reforming (CH4 + H2O ↔ CO + 3 H2) followed by the water-gas shift (WGS) reaction (H2O + CO ↔ H2 + CO2) to further increase the concentration of H2. The WGS reaction is conducted in two stages with the CuO-Cr2O3-Fe2O3 catalyst at high temperatures (HT) and the Cu-ZnO-Al2O3 catalyst at low temperatures (LT) because this exothermic reaction is equilibrium limited. The fundamental structures and promotion mechanisms of Cu and Cr for the CuO-Cr2O3-Fe2O3 catalyst, however, are still not well understood at the molecular level due to the absence of surface characterization studies under reaction conditions. This lack of understanding has prevented the rational design of new Cr-free iron oxide-based HT-WGS catalysts that don’t have the toxic hexavalent Cr(VI), which is the objective in development of the next generation of HT-WGS catalysts.

The Cr- and Cu-promoted iron oxide catalysts were investigated as a function of reaction conditions. In situ Raman and HS-LEIS measurements revealed that the initial calcined catalyst consists of a bulk hematite Fe2-xMxO3 solid solution, M = Cr+3 and Cu+2, that is surface enriched with dioxo (O=)2Cr+6(-O-Fe)2 sites. In situ Raman, HS-LEIS and XANES analysis showed that during the reducing HT-WGS reaction conditions the bulk hematite Fe2-xMxO3 phase transforms to the bulk Fe3-xCr+3xO4 (magnetite) solid solution, the surface dioxo (O=)2Cr+6(-O-Fe)2 sites reduce to Cr+3 and dissolve into the magnetite phase, and the Cu+2 cations reduce to metallic Cu0 nanoparticles (~3nm) residing on the magnetite phase. In addition, ~1/3 of the surface of the Cu nanoparticles becomes covered by an overlayer of FeOx due to strong metal support interaction (SMSI) effect between the reduced iron oxide and metallic copper.

These new insights were used to guide the rational design of Cr-free catalysts.