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Title: Dynamics of CrO 3 –Fe 2 O 3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

Abstract

A series of supported CrO 3/Fe 2O 3 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=) 2Cr 6+O 2 species and a bulk Fe 2O 3 phase containing some Cr 3+ 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 Fe 2O 3 bulk phase becomes Fe 3O 4 ,and surface dioxo (O=) 2Cr 6+O 2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeO x sites, but some minor reduced surface chromia sites are also retained. The Fe 3–-xCr xO 4 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 C 16O 2/H 2 → C 18O 2/H 2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites frommore » 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.« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1354339
Report Number(s):
BNL-112855-2016-JA
Journal ID: ISSN 2155-5435
DOE Contract Number:
SC00112704
Resource Type:
Journal Article
Resource Relation:
Journal Name: ACS Catalysis; Journal Volume: 6; Journal Issue: 7
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; in situ; isotope exchange; metal oxide; operando; spectroscopy; surface; water−gas shift

Citation Formats

Keturakis, Christopher J., Zhu, Minghui, Gibson, Emma K., Daturi, Marco, Tao, Franklin, Frenkel, Anatoly I., and Wachs, Israel E. Dynamics of CrO 3 –Fe 2 O 3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity. United States: N. p., 2016. Web. doi:10.1021/acscatal.6b01281.
Keturakis, Christopher J., Zhu, Minghui, Gibson, Emma K., Daturi, Marco, Tao, Franklin, Frenkel, Anatoly I., & Wachs, Israel E. Dynamics of CrO 3 –Fe 2 O 3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity. United States. doi:10.1021/acscatal.6b01281.
Keturakis, Christopher J., Zhu, Minghui, Gibson, Emma K., Daturi, Marco, Tao, Franklin, Frenkel, Anatoly I., and Wachs, Israel E. 2016. "Dynamics of CrO 3 –Fe 2 O 3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity". United States. doi:10.1021/acscatal.6b01281.
@article{osti_1354339,
title = {Dynamics of CrO 3 –Fe 2 O 3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity},
author = {Keturakis, Christopher J. and Zhu, Minghui and Gibson, Emma K. and Daturi, Marco and Tao, Franklin and Frenkel, Anatoly I. and Wachs, Israel E.},
abstractNote = {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 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 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.},
doi = {10.1021/acscatal.6b01281},
journal = {ACS Catalysis},
number = 7,
volume = 6,
place = {United States},
year = 2016,
month = 6
}
  • The goal of this study was to identify the most suitable chromium-free iron-based catalysts for the HTS (high temperature shift) reaction of a fuel processor using LPG. Hexavalent chromium (Cr6+) in the commercial HTS catalyst has been regarded as hazardous material. We selected Ni and Co as the substitution for chromium in the Fe-based HTS catalyst and investigated the HTS activities of these Crfree catalysts at LPG reformate condition. Cr-free Fe-based catalysts which contain Ni, Zn, or Co instead of Cr were prepared by coprecipitation method and the performance of the catalysts in HTS was evaluated under gas mixture conditionsmore » (42% H2, 10% CO, 37% H2O, 8% CO2, and 3% CH4; R (reduction factor): about 1.2) similar to the gases from steam reforming of LPG (100% conversion at steam/carbon ratio = 3), which is higher than R (under 1) of typically studied LNG reformate condition. Among the prepared Cr-free Febased catalysts, the 5 wt%-Co/Fe/20 wt%-Ni and 5 wt%-Zn/Fe/20 wt%-Ni catalysts showed good catalytic activity under this reaction condition simulating LPG reformate gas.« less
  • The Pt-ceria synergy may be described as the dehydrogenation of formate formed on the surface of the partially reducible oxide (PRO), ceria, by Pt across the interface, with H{sub 2}O participating in the transition state. However, due to the rising costs of rare earth oxides like ceria, replacement by a less expensive partially reducible oxide, like manganese oxide, is desirable. In this contribution, a comparison between Pt/ceria and Pt/manganese oxide catalysts possessing comparable Pt dispersions reveals that there are significant differences and certain similarities in the nature of the two Pt/PRO catalysts. With ceria, partial reduction involves reduction of themore » oxide surface shell, with Ce{sup 3+} at the surface and Ce{sup 4+} in the bulk. In the case of manganese oxide, partial reduction results in a mixture of Mn{sup 3+} and Mn{sup 2+}, with Mn{sup 2+} located at the surface. With Pt/CeO{sub X}, a high density of defect-associated bridging OH groups react with CO to yield a high density of the formate intermediate. With Pt/MnO{sub X}, the fraction of reactive OH groups is low and much lower formate band intensities result upon CO adsorption; moreover, there is a greater fraction of OH groups that are essentially unreactive. Thus, much lower CO conversion rates are observed with Pt/MnO{sub X} during low temperature water-gas shift. As with ceria, increasing the Pt loading facilitates partial reduction of MnO{sub X} to lower temperature, indicating metal-oxide interactions should be taken into account.« less
  • Combined in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies have been conducted to follow gold structural changes of low-content (<1%Au) gold-ceria catalysts in water-gas shift (WGS) reaction tests at 100 and 200 C; and after heating the used catalysts in oxygen gas at 150 C. Gold in the fresh (400 C-calcined) material was atomically dispersed in cerium oxide. Under WGS reaction conditions, reduction of the oxidized gold species was observed, accompanied by gradual gold aggregation. The Au-Au coordination number is zero for the fresh material, but increases with the reaction temperature, to 6.5more » {+-} 2.4 (after use at 100 C) and to 8.7 {+-} 1.5 (after 200 C) in a gas mixture containing 5% CO- 3% H2O in helium. The second important parameter is the reaction gas composition which determines the extent of Au-O reduction. The lower the reduction potential of the reaction gas mixture, the more oxidized the gold is in the used catalyst, and the higher its activity. The maximum activity of Au-CeO2 was that of the fully dispersed Au-O-Ce fresh material. Loss of surface oxygen took place during reaction, as measured by H2-TPR of the used samples, and it was commensurate with the activity loss. Attempts to reoxidize and redisperse the gold by heating in oxygen gas at 150 C were not effective. However, we report here that complete recovery of the surface oxygen amount and redispersion of gold in ceria was possible after a 400 C- oxygen treatment of both the 100 C- and 200 C- used catalyst samples, with concomitant recovery of the initial catalyst activity. These tests were conducted by consecutive H2-TPR/steady-state catalyst activity measurements in the same microreactor.« less
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