X-ray-absorption fine structure in embedded atoms

Rehr, J. J.; Booth, C. H.; Bridges, F.; Zabinsky, S. I.

doi:10.1103/physrevb.49.12347

Cited by 180 publications

(173 citation statements)

References 11 publications

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“…It was first shown by Holland et al [28] that µ 0 (k) = µ free (k)(1 + χ AX (k)), with µ free being the free atomic background and χ AX (k) being a fine structure attributed to AXAFS. As was pointed out by Rehr et al [29], the AXAFS contribution will be visible as a peak in the Fourier transform at an unphysically short distance, often less than 1.5 Å. Ramaker et al [21] have recently discussed the origin and the parameters determining the AXAFS. The AXAFS is caused by the scattering of the photoelectron off the deep valence electrons in the periphery of the absorbing atom.…”

Section: Axafsmentioning

confidence: 98%

“…In the last few years, several research groups [28,29] have reported on the oscillatory structure detected in the atomic background µ 0 that is removed from the raw absorption data. It was first shown by Holland et al [28] that µ 0 (k) = µ free (k)(1 + χ AX (k)), with µ free being the free atomic background and χ AX (k) being a fine structure attributed to AXAFS.…”

Section: Axafsmentioning

confidence: 99%

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The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

Koningsberger

Graaf

Mojet

et al. 2000

Applied Catalysis A: General

108

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The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La +3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed to the effect of a higher Si/Al ratio and non-framework Al in the K-USY.EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consisting of 14-20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding peak of the Pt/Na-Y data. A detailed analysis of the L 2 and the L 3 X-ray absorption near edge structure revealed a shape resonance due to the Pt-H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy (E res ) between the AS and the Fermi-level (E F ) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles.Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H + , Na + , La 3+ and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst.While the TOF is dependent on the metal-support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size.

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Section: Axafsmentioning

confidence: 98%

Section: Axafsmentioning

confidence: 99%

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

Koningsberger

Graaf

Mojet

et al. 2000

Applied Catalysis A: General

108

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“…The pseudo-radial distribution function FT(v(k)*k 3 ) was calculated by Fourier transforming the k 3 -weighted experimental v(k) function, multiplied by a Bessel window, into reciprocal space. EXAFS data analysis was performed using theoretical backscattering phases and amplitudes calculated with the ab-initio multiple-scattering code FEFF7 [38]. Single scattering and multiple scattering paths in the a-MoO 3 model structure were calculated up to 5.0 Å with a lower limit of 2.0% in amplitude with respect to the strongest backscattering path.…”

Section: Sample Characterizationmentioning

confidence: 99%

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

Huang

Bensch

Sigle³

et al. 2007

J Mater Sci

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MoO 3 was dispersed onto mesoporous SBA-15 by using ammonium heptamolybdate as MoO 3 source. The formation of MoO 3 was carried out by heating the loaded material to 500°C for 3 h in air. Below 13 wt% Mo loading, no reflections of MoO 3 occur in the X-ray powder patterns and even for high MoO 3 contents, the intensities of the reflections are much lower than expected for fully crystalline material. A detailed XAFS analysis reveals that at low Mo contents, the metastable hexagonal modification of MoO 3 is formed despite the high calcination temperature of 500°C. It is highly likely that the nanosize of the particles and the interaction between MoO 3 and SBA-15 stabilize the metastable modification of the material. Nitrogen physisorption experiments show the typical type-IV isotherms indicating that the mesoporosity of the materials is preserved despite the large amount of MoO 3 .Transmission electron micrographs demonstrate the presence of MoO 3 inside the SBA-15 support. The Raman spectra display a remarkable size-dependent intensity loss and several features give evidences for a bond formation between nano-sized MoO 3 particles and the silica support. Moreover, the spectroscopic details suggest the formation of (MoO 3 ) n oligomers.

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“…[8,15] The results mentioned above for bulk metal systems, organometallic systems, and supported metal catalysts have shown that AXAFS is a viable tool for probing the influence of the local surroundings on the electronic properties of a central atom. Although the presence of an AXAFS contribution in the X-ray absorption data of oxide systems has been established, [20,27] to the best of our knowledge, no systematic study of the influence of the chemical surroundings, including the effect of the support, on the AXAFS of small metal oxide clusters has yet been carried out.In the present work, three case studies have been used to determine the influence of changes in the first and next nearest neighbour shells on the AXAFS of vanadium bulk reference compounds and alumina-supported vanadium oxide clusters. In particular, the effects of oxidation/reduction treatments and vanadium oxide loading on the Fourier transform AXAFS peak (both intensity and centroid position) of alumina-supported vanadium oxide clusters have been determined.…”

mentioning

confidence: 99%

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

Keller

Weckhuysen

Koningsberger

2007

Chemistry A European J

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IntroductionMany parameters affect the catalytic properties of transition-metal oxide catalysts, for example, metal oxide loading, pre-treatment conditions, molecular structure, electronic structure, and the nature of the supporting oxide. [1][2][3][4][5][6][7] Various researchers have investigated the activities and selectivities of metal oxide catalysts. However, little is known about the parameters that are truly responsible for the catalytic action of transition-metal oxides.To study the effect of the support on the catalytic performance, one has to find a tool that is capable of probing the influence of the local environment (e.g., the support or promoters) on the properties of the catalytically active species, that is, a metal particle or metal oxide cluster. Metal-A C H T U N G T R E N N U N G (oxide)-support interactions can be ascribed to a structural (particle/cluster size and shape) effect, but can also have an electronic origin. For supported metal particles, the effect of the support on the catalytic properties has been determined by several techniques, for example, XPS, FT-IR analysis of adsorbed CO, and newly developed delta XANES and atomic XAFS (AXAFS) techniques. [8][9][10][11][12][13][14][15] In particular, the application of AXAFS has, in our opinion, led to a better understanding of metal-support interaction effects. [8,[15][16][17] AXAFS is sensitive to changes in the potential around the X-ray absorbing atom. The potential field around an atom is mainly determined by the first and second coordination spheres. O Grady et al. have shown for Pt/Ru alloys that the AXAFS intensity changes systematically with the composition of the alloy. [18] Thus, when the number of Ru atoms around a Pt atom increases, the intensity of the FT AXAFS peak increases and the peak centroid shifts to lower values of R. Comparison of these effects with the effects observed for a Pt electrode as a function of the applied voltage suggests that the first coordination shell of the Pt in the Pt/Ru alloy influences the electronic properties (inter-A C H T U N G T R E N N U N G atomic potential) of the Pt atom. [18,19] The effect of changes in the coordination around the absorber atom has been further illustrated for Pt by van Dorssen et al., who compared the AXAFS of a Pt foil with that of bulk Na 2 Pt(OH) 6 . The AXAFS intensity was found to be significantly higher in the Abstract: The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and aluminasupported vanadium oxide clusters has been investigated. Using Na 3 VO 4 and V 2 O 5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been dete...

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X-ray-absorption fine structure in embedded atoms

Cited by 180 publications

References 11 publications

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

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