Reaction of water vapor with α-Al2O3(0001) and α-Fe2O3(0001) surfaces: synchrotron X-ray photoemission studies and thermodynamic calculations

Liu, Ping N.; Kendelewicz, T.; Brown, Gordon E.; Nelson, Eric J.; Chambers, Scott A.

doi:10.1016/s0039-6028(98)00661-x

Cited by 255 publications

(229 citation statements)

References 60 publications

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“…The oxygen-terminated FeO(111) surface is chemically inert and a physisorption of water monomers is observed, followed by the formation a hydrogen bonded bilayer and finally by condensation of ice. On the iron-terminated Fe 3 O 4 (111) surface a dissociative chemisorption of water occurs, which was also observed on (0001) and (012) surfaces of α-Fe 2 O 3 55,56 . On Fe 3 O 4 (111) the chemisorption is not related to surface defects but occurs on regular surface areas.…”

Section: Active Sites For Water Chemisorptionmentioning

confidence: 64%

Structure and reactivity of iron oxide surfaces

Shaikhutdinov¹,

Joseph²,

Kuhrs³

et al. 1999

Faraday Disc.

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Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111) substrates and used for studying structure-reactivity correlations. The film morphologies and their atomic surface structures were characterized by scanning tunneling microscopy and low energy electron diffraction including multiple scattering calculations. The adsorption of water, ethylbenzene, and styrene was investigated by temperature programmed desorption and photoelectron spectroscopy. A dissociative chemisorption of water and a molecular chemisorption of ethylbenzene and styrene is observed on all oxides that expose metal cations in their topmost layers, whereas purely oxygen terminated FeO(111) monolayer films are chemically inert and only physisorption occurs. Regarding the technical styrene synthesis reaction which is performed over iron oxide based catalysts, we find a decreasing chemisorption strength of the reaction product molecule styrene if compared to ethylbenzene when going from Fe 3 O 4 (111) over α-Fe 2 O 3 (0001) to KFe x O y (111). Extrapolation of the adsorbate coverages to the technical styrene synthesis reaction conditions using the Langmuir isotherm for coadsorption suggests an increasing catalytic activity along the same direction. This result agrees with previous kinetic experiments performed at elevated gas pressures over the model systems studied here and over polycrystalline iron oxide catalyst samples. It indicates that the iron oxide surface chemistry does not change across the pressure-gap and that the model systems simulate technical styrene synthesis catalysts in a realistic way.

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Section: Active Sites For Water Chemisorptionmentioning

confidence: 64%

Structure and reactivity of iron oxide surfaces

Shaikhutdinov¹,

Joseph²,

Kuhrs³

et al. 1999

Faraday Disc.

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“…OH species on the (0001) face of hematite were found to be difficult to remove (in contrast to surface OH groups on other hematite planes) and were thermally stable up to at least 1073 K in O 2 atmosphere as evidenced by infrared spectroscopy [303], and OH on Fe terminated α-Fe 2 O 3 (0001) could not be removed by excessive heating at 900K without reducing the near-surface region to Fe(II) [301]. Also the isostructural α-Al 2 O 3 (0001) surface has been found to get hydroxilated easily [63,297,298,[304][305][306][307][308]. This was confirmed by theory [296].…”

Section: Discussionmentioning

confidence: 92%

“…Equ. ( This hydroxylation was observed on single-crystalline surfaces of several transition metal-oxides [59,[60][61][62][63]. Polar surface planes can be stabilized by hydroxylation, because it reduces excess surface charge and, therefore, the electrostatic surface dipole moment (see Section 2.8).…”

Section: Adsorption Processmentioning

confidence: 95%

Surface chemistry and catalysis on well-defined epitaxial iron-oxide layers

Weiß

Ranke

2002

Progress in Surface Science

579

610

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Metal-oxide based catalysts are used for many important synthesis reactions in the chemical industry. A better understanding of the catalyst operation can be achieved by studying elemantary reaction steps on well-defined model catalyst systems. For the dehydrogenation of ethylbenzene to styrene in the presence of steam both unpromoted and potassium promoted iron-oxide catalysts are active. Here we review the work done over unpromoted single-crystalline FeO(111), Fe 3 O 4 (111) and α-Fe 2 O 3 (0001) films grown epitaxially on Pt(111) substrates. Their geometric and electronic surface structures were characterized by STM, LEED, electron microscopy and electron spectroscopic techniques. In an integrative approach, the interaction of water, ethylbenzene and styrene with these films was investigated mainly by thermal desorption and photoelectron emission spectroscopy. The adsorptiondesorption energetics and kinetics depend on the oxide surface terminations and are correlated to the electronic structures and acid-base properties of the corresponding oxide phases, which reveal insight into the nature of the active sites and into the catalytic function of semiconducting oxides in general. Catalytic studies, using a batch reactor arrangement at high gas pressures and post reaction surface analysis, showed that only α-Fe 2 O 3 (0001) containing surface defects is catalytically active, whereas Fe 3 O 4 (111) is always inactive. This can be related to the elementary adsorption and desorption properties observed in ultrahigh vacuum, which indicates that the surface chemical properties of the iron-oxide films do not change significantly across the "pressure-gap". A model is proposed according to which the active site involves a regular acidic surface sites and a defect site next to it. The results on metal-oxide surface chemistry also have implications for other fields, such as environmental science, biophysics and chemical sensors.-2 -"2kyEi9FYSQjBVrZxIPQ.FHIAC_WRa02_review.doc", Datum: 19.02.03

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“…Interestingly though, the relaxations determined from the structure optimization were significantly smaller than those predicted by theory (see Figure 57). The half-metal termination has also been reported based on an XPD study [349] conducted on an α- [345], but as yet there is no atomic-scale imaging of this preparation technique to assess the defect density or whether multiple terminations coexist (the relatively poor LEED IV R-factor of 0.34 suggests this may be the case [345]). An alternative approach could be that of Bedzyk and co-workers [356] who report obtaining a sharp (1×1) LEED pattern when a natural single crystal was Ar + sputtered and then annealed at 723 K in a stream of atomic oxygen.…”

Section: Maghemite Surfacesmentioning

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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Reaction of water vapor with α-Al2O3(0001) and α-Fe2O3(0001) surfaces: synchrotron X-ray photoemission studies and thermodynamic calculations

Cited by 255 publications

References 60 publications

Structure and reactivity of iron oxide surfaces

Structure and reactivity of iron oxide surfaces

Surface chemistry and catalysis on well-defined epitaxial iron-oxide layers

Iron oxide surfaces

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