Dynamics of CrO3–Fe2O3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

详细信息    查看全文

• 作者：Christopher J. Keturakis ; Minghui Zhu ; Emma K. Gibson ; Marco Daturi ; Franklin Tao ; Anatoly I. Frenkel ; Israel E. Wachs
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：July 1, 2016
• 年：2016
• 卷：6
• 期：7
• 页码：4786-4798
• 全文大小：767K
• 年卷期：0
• ISSN：2155-5435

文摘

A series of supported CrO₃/Fe₂O₃ catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O═)₂Cr⁶⁺O₂ species and a bulk Fe₂O₃ phase containing some Cr³⁺ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe₂O₃ bulk phase becomes Fe₃O₄ ,and surface dioxo (O═)₂Cr⁶⁺O₂ species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeO_<i>xi> sites, but some minor reduced surface chromia sites are also retained. The Fe_3–<i>-i>xCr_<i>xi>O₄ solid solution stabilizes the iron oxide phase from reducing to metallic Fe⁰ and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C¹⁶O₂/H₂ → C¹⁸O₂/H₂ isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromium–iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect.