Development of chromium-free iron-based catalysts for high-temperature water-gas shift reaction

Natesakhawat, Sittichai; Wang, Xueqin; Zhang, Lingzhi; Ozkan, Umit S.

doi:10.1016/j.molcata.2006.07.013

Cited by 146 publications

(78 citation statements)

References 37 publications

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“…In recent years however, motivation to understand the mechanisms of the WGS is quickly regaining momentum because of the vastly increased demand for H 2 , and because cost-efficient water-gas shift catalysts without (toxic) chromium are required on environmental grounds. This has led to repeated attempts to study how and why the addition of different metals to Fe 3 O 4 can promote the reaction [443][444][445][446][447]. Understanding WGS on any particular Fe 3 O 4 surface at the atomic scale requires a detailed knowledge of CO and H 2 O adsorption, and how the surface responds to these reactants at the reaction temperature.…”

Section: Oxygen (O 2 )mentioning

confidence: 99%

Iron oxide surfaces

Parkinson

2016

Surface Science Reports

501

463

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The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), hematite (α-Fe 2 O 3 ), and wüstite (Fe 1-x O) is reviewed. The paper starts with a summary of applications where iron oxide surfaces play a major role, including corrosion, catalysis, spintronics, magnetic nanoparticles (MNPs), biomedicine, photoelectrochemical water splitting and groundwater remediation. The bulk structure and properties are then briefly presented; each compound is based on a close-packed anion lattice, with a different distribution and oxidation state of the Fe cations in interstitial sites. The bulk defect chemistry is dominated by cation vacancies and interstitials (not oxygen vacancies) and this provides the context to understand iron oxide surfaces, which represent the front line in reduction and oxidation processes. The atomic-scale structure of the low-index surfaces of iron oxides is the major focus of this review. Fe 3 O 4 is the most studied iron oxide in surface science, primarily because its stability range corresponds nicely to the ultra-high vacuum environment, and because it is an electrical conductor, which makes it straightforward to study with the most commonly used surface science methods such as photoemission spectroscopies (XPS, UPS) and scanning tunneling microscopy (STM). The impact of the surfaces on the measurement of bulk properties such as magnetism, the Verwey transition and the (predicted) half-metallicity is discussed.The best understood iron oxide surface at present is probably Fe 3 O 4 (100); the structure is known with a high degree of precision and the major defects and properties are well characterised. A major factor in this is that a termination at the Fe oct -O plane can be reproducibly prepared by a variety of methods, as long as the surface is annealed in 10 Such straightforward preparation of a monophase termination is generally not the case for other iron oxide surfaces. All available evidence suggests the oft-studied (√2×√2)R45° reconstruction results from a rearrangement of the cation lattice in the outermost unit cell in which two octahedral cations are replaced by one tetrahedral interstitial, a motif conceptually similar to well-known Koch-Cohen defects in Fe (0001) is the most studied hematite surface, but 2 difficulties preparing stoichiometric surfaces under UHV conditions have hampered a definitive determination of the structure. There is evidence for at least three terminations: a bulk-like termination at the oxygen plane, a termination with half of the cation layer, and a termination with ferryl groups. When the surface is reduced the so-called "bi-phase" structure is formed, which eventually transforms to a Fe 3 O 4 (111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe 1-x O and α-Fe 2 O 3 (0001) islands was recently challenged and a new structure based on a thin film of Fe 3 O 4 (111) on α-Fe 2 O 3 (0001) was proposed. The merits of the compet...

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Section: Oxygen (O 2 )mentioning

confidence: 99%

Iron oxide surfaces

Parkinson

2016

Surface Science Reports

501

463

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“…1 For example, potential promotional roles of Co, Bu, Ag, Ba, Ce for Fe-Cr-O catalysts were examined. [2][3][4][5][6][7][8][9][10] Fe-Cu-Al-O catalysts 2,3,9 reported recently exhibit a comparable catalytic performance to Fe-Cr-O and other Fe-based catalysts. 8 Cu is considered as a promoter for the high temperature WGS catalysts, Fe-Cu-Al-O.…”

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confidence: 99%

The role of copper in catalytic performance of a Fe–Cu–Al–O catalyst for water gas shift reaction

Wang

Zhang

et al. 2013

Chem. Commun.

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A Fe-Cu-Al-O water gas shift catalyst with a Fe : Cu atomic ratio of 4 : 1 upon pretreatment at 350 8C in H 2 exhibits a conversion higher than a physical mixture of Fe-Al-O and Cu-Al-O by B40% over a temperature range of 300 8C-450 8C. In situ ambient pressure X-ray photoelectron spectroscopy studies suggest that the surface region of Fe-Cu-Al-O was restructured into a double-layer structure consisting of a surface layer of

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“…After dehydration, the iron-chromium oxide catalyst was first allowed to equilibrate under C 16 O2/H2 reverse WGS reaction conditions (10ml/min C 16 O2, 10ml/min H2) at 330ºC for 1 hour, then the catalyst was flushed by inert gas (20ml/min He) for 10 min followed by switch to isotopic reverse WGS reaction conditions (10ml/min C 18 O2, 10ml/min H2). The C 16 O2/C 18 O2 isotope exchange was monitored with the online mass spectrometer by recording the evolution of H2 16 19,44,46,48). All mass spec signals were normalized to the same maximum and minimum intensity to observe their transient behavior.…”

Section: O2/c 18 O2 Isotope Exchange During Rwgsmentioning

confidence: 99%

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

et al. 2016

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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=)2Cr +6 O2 species and a bulk Fe2O3 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 reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 and surface dioxo (O=)2Cr +6 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 o and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C 16 O2/H2 → C 18 O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* from the surface region of the chromium-iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites were quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly an 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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Development of chromium-free iron-based catalysts for high-temperature water-gas shift reaction

Cited by 146 publications

References 37 publications

Iron oxide surfaces

Iron oxide surfaces

The role of copper in catalytic performance of a Fe–Cu–Al–O catalyst for water gas shift reaction

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

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Development of chromium-free iron-based catalysts for high-temperature water-gas shift reaction

Cited by 146 publications

References 37 publications

Iron oxide surfaces

Iron oxide surfaces

The role of copper in catalytic performance of a Fe–Cu–Al–O catalyst for water gas shift reaction

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

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Product

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