Modified nano-crystalline ferrites for high-temperature WGS membrane reactor applications

Khan, Ataullah; Chen, Ping; Boolchand, P.; Smirniotis, Panagiotis G.

doi:10.1016/j.jcat.2007.10.018

Cited by 126 publications

(99 citation statements)

References 25 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

487

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

487

463

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“…The substitution of Fe sites in spinel structure promotes the Fe 3+ ↔ Fe 2+ redox couple and accordingly the redox properties of ferrites. From previous studies [16][17][18][19][20], the dopants (Mn, Co, Ni, Cu, Zn) modified the catalytic activity of ferrites. Recently, Mn substitution in spinel ferrite has been found greatly improving the catalytic activity in HCHO oxidation [21], as evidenced by the obvious decrease of about 100 • C in both the temperatures of 90% HCHO conversion and 90% CO 2 generation.…”

Section: Introductionmentioning

confidence: 97%

The variation of cationic microstructure in Mn-doped spinel ferrite during calcination and its effect on formaldehyde catalytic oxidation

Liang

Liu

et al. 2016

Journal of Hazardous Materials

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“…In the last few years, spinel ferrites have been extensively pursued, due to their high thermal resistance, strong magnetism and catalytic activity [25]. The substitution of transition metals (Co, Ni, Cu, Zn) in spinel ferrites has been found modifying its redox activity of ferrites [26][27][28][29][30]. The transition metal dopants in the spinel structure generally promote the formation of mixed or inverse spinel structures [31].…”

Section: Introductionmentioning

confidence: 99%

“…The transition metal dopants in the spinel structure generally promote the formation of mixed or inverse spinel structures [31]. The physico-chemical properties of ferrites are strongly dependent on the sites, nature and amount of the transition metals on the octahedral or tetrahedral sites in the spinel structure [27]. In most instances, these substitutions improve the thermal, textural stability and redox properties.…”

Section: Introductionmentioning

confidence: 99%