4.8 Article

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

Journal

ACS CATALYSIS
Volume 6, Issue 7, Pages 4786-4798

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acscatal.6b01281

Keywords

water-gas shift; operando; in situ; spectroscopy; metal oxide; isotope exchange; surface

Funding

  1. NSF IREE [0609018]
  2. NSF [1511689]
  3. U.S. DOE [DE-FG02-03ER15476, DE-FG02-05ER15688, DE-AC02-98CH10886]
  4. Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy [DE-SC0014561]
  5. Directorate For Engineering
  6. Div Of Chem, Bioeng, Env, & Transp Sys [1511689] Funding Source: National Science Foundation

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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=)(2)Cr6+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 Fe2O3,and surface dioxo (O=)(2)Cr6+O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium iron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with (CO2)-O-16/H-2 -> (CO2)-O-18/H-2 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.

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