D.C. Koningsberger

XAFS Characterization of the Binuclear Iron Complex in Overexchanged Fe/ZSM5 -Structure and Reactivity

We have characterized, with emphasis on XAFS spectroscopy, one of the most promising DeNOx catalysts, i.e., Fe/ZSM5 prepared through the FeCl3sublimation technique. XAFS is a very useful tool for this purpose since it is element specific and can be used in situ, namely, in the presence of the reactants and at reaction temperature. In this communication it will be pointed out that the as-synthesized Fe/ZSM5 catalyst contains stable binuclear iron oxo/hydroxo-complexes. The reaction of these complexes with the probe molecule CO clearly shows that only one of the oxygen shells around iron changes while the iron contribution is not affected, the iron complex remaining binuclear.

In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

L2 and L3 X-ray absorption near edge spectra (XANES) on supported Pt particles, with and without chemisorbed hydrogen, are shown to reflect the type of hydrogen-binding site on the Pt surface. FEFF8 ab initio multiple scattering calculations are used to determine XANES spectral fingerprints for the atop vs threefold H binding sites on Pt. Comparison of the experimental XANES data with the theoretical fingerprints, and further theoretical results, show that the acid/base properties of the support have a profound influence on the hydrogen coverage, and therefore on the mode of hydrogen adsorption on the Pt surface. As the electron richness of the support oxygen atoms increases (i.e., with increasing alkalinity of the support), the H coverage increases and the hydrogen-binding site of the strongly adsorbed hydrogen changes from atop to threefold. This site change is primarily responsible for the observed changes in previously reported kinetic data, which show an increase in negative order (roughly from −1. 5t o−2.5) in hydrogen partial pressure for neopentane hydrogenolysis with increasing support alkalinity. This change in negative order directly reflects the greater number of vacant Pt sites that must be available to allow adsorption of the neopentane. A compensation relation is found in the kinetic data of Pt on different supports resulting directly from this change in hydrogen coverage. This implies that the experimentally determined kinetic parameters are apparent values. These apparent values are correlated to the intrinsic kinetic parameters via the thermodynamic properties of the sorption of the reactants, described by the Temkin equation. The TOF of neopentane hydrogenolysis over several catalysts, measured in previous work, decreases with the increasing alkalinity of the support. This can now be directly explained as the result of the change in hydrogen coverage using a Frumkin isotherm, implying that the neopentane adsorption becomes weaker with increased hydrogen coverage. These conclusions, that hydrogen drives the catalysis, are further supported by density functional calculations on small Pt 4 clusters, which show that the acid/base properties of the support have a much larger direct influence on Pt–H bonding than on Pt–CH n bonding.  2003 Elsevier Science (USA). All rights reserved.

Interpretation of the Al K- and LII/III-edges of aluminium oxides: differences between tetrahedral and octahedral Al explained by different local symmetries

The Al K- and LII/III-edge XANES of aluminium oxide are interpreted using empirical molecular orbital theory (EHMO) and ab initio self-consistent field real space multiple scattering calculations (FEFF8). Most features in the XANES at the K- and LII/III-edges are interpreted as shape resonances; although some fine structure, visible at both edges, arises from multiple scattering over the medium range (~15 A). The change in local symmetry between octahedral and tetrahedral Al explains the observed differences in the electronic structure. First, Al p–d hybridization is allowed only in tetrahedral symmetry, resulting in a lower absorption edge in tetrahedral Al than in the octahedral. Second, only in octahedral Al do the oxygen orbitals near the valence band maximum (the HOMOs) have the right symmetry to mix with the Al p orbitals just above the band gap (the LUMOs). This gives a more screened core hole in the octahedral case. Calculations on distorted octahedral Al sites reveal both p–d and s–d hybridizations; however, the latter is less prominent. The diffuse d orbitals, which hybridize with the p or s orbitals in tetrahedral or distorted octahedral symmetry, are primarily responsible for the fine structure in the near-edge region (0–15 eV) that is determined by medium-range scattering (up to ~15 A). The observed difference in the magnitude of this fine structure at the K- and LII/III-edges is caused by the different degrees of d orbital hybridization with the s and p orbitals.

A new model for the metal-support interaction Evidence for a shift in the energy of the valence orbitals

The catalytic and spectroscopic properties of Pt supported on LTL zeolite are greatly affected by the acidity/alkalinity of the support. The turnover frequency (TOF) for neopentane hydrogenolysis and isomerization decreases from acidic to neutral to alkaline. In addition, in the infrared spectra, there is a decrease in the linear to bridging ratio of adsorbed CO which parallels the catalytic activity, indicating that the changes in TOF are due to a modification of the electronic properties of the Pt particles resulting from the metal‐support interaction. The local structure of the Pt particles has also been determined by EXAFS spectroscopy. The Pt atoms are in contact with the oxygen ions of the support. None of the Al, Si or K ions are within bonding distance of the Pt. In addition, analysis of the L III and LII near-edge spectra suggest that, contrary to the generally accepted model, the number of electrons in the valence band is unchanged by the support interaction. Furthermore, at the LIII edge in the presence of chemisorbed hydrogen, a Pt‐H antibonding orbital is observed near the Fermi level. Isolation of this shape resonance indicates that the energy difference between the antibonding orbital and the Fermi level increases with increasing acidity of the support and correlates with the TOF. Based on the analysis of the Pt‐H shape resonance, a new model for the metal‐support interaction is proposed where the binding energy of the Pt valence orbitals increase as the charge of the support oxide ion becomes more positive, i.e., becomes more acidic. The catalytic and spectroscopic properties are discussed in the context of this new model.

Understanding atomic x-ray absorption fine structure in x-ray absorption spectra

The origin and important parameters determining the intensity of atomic x-ray absorption fine structure (AXAFS) are described both in chemical and physical terms. A full mathematical derivation is presented and new criteria are given for removal of the background to extract the total (EXAFS and AXAFS) from the experimental absorption cross-section. The embedded-atom potential, the interstitial potential and the distribution of the absorber-atom electron density are all found to be important in determining the AXAFS intensity. Application is made to spherical Pt metal clusters, where it is shown that the AXAFS intensity of the central atom is much larger than that of the surface atoms. However, the average AXAFS intensity per platinum atom is found not to depend significantly on cluster size. On the other hand, variation of the metal cluster support does considerably change the intensity as well as the imaginary part of the AXAFS. Hence, AXAFS can be a very useful probe of the effects of metal-support interactions in supported noble-metal catalysts.

The Direct Influence of the Support on the Electronic Structure of the Active Sites in Supported Metal Catalysts : Evidence from Pt-H Anti-Bonding Shape Resonance and Pt-CO FTIR Data

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by support properties such as the presence of protons, type of charge compensating cations, Si/Al ratio and/or presence of extra-framework Al. The metal–support interaction is relatively independent of the metal (Pd or Pt) or the type of support (microporous zeolites such as LTL and Y or macroporous supports such as SiO2). As the alkalinity of the support increases (i.e., with increasing electronic charge on the support oxygen ions), the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a systematic shift from linear to bridge bonded CO as indicated by the IR spectra. This is a strong indication of a change in the electronic structure of the catalytically active Pt surface atoms. Analysis of the Pt–H anti-bonding shape resonance present in the Pt X-ray absorption spectra of the L3 edge indicates that the difference in energy between the Pt–H anti-bonding orbital and the Fermi level decreases as the alkalinity of the support increases. The results from the IR and Pt–H shape resonance data directly show that the support influences the position in energy of the metal valence orbitals. The ionisation potential of the catalytically active Pt surface atoms decreases with increasing support alkalinity, i.e., with increasing electron charge on the support oxygen ions. This shift leads to a weakening of the Pt–H bond.

Apparatus for In Situ X-Ray Absorption Fine Structure Studies on Catalytic Systems in the Energy Range 1000 eV < E < 3500 eV

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal ...

The Selective Oxidation of n-Butane to Maleic Anhydride : Comparison of Bulk and Supported V-P-O Catalysts

V–P–O catalysts supported on the surface of silica and titania particles were studied and compared with bulk V–P–O. The catalytic performance was tested in the η‐butane oxidation reaction to maleic anhydride, and the structure of the equilibrated catalysts was characterised with X‐ray absorption spectroscopy (EXAFS) and (low‐temperature) ESR spectroscopy. Our results show considerable differences in catalytic performance between VPO/TiO2 on the one hand, and VPO/SiO2 and VPO/bulk on the other hand, the yield to maleic anhydride being comparable for VPO/bulk and VPO/SiO2. The differences in catalytic behaviour are attributed to differences in the local structure around vanadium (EXAFS). Furthermore, different spin exchange interactions between vanadium atoms in the three samples have been observed (ESR). The combination of characterisation methods suggests that the structure of the supported V–P–O phase is amorphous and differs considerably from that of bulk crystalline vanadylpyrophosphate. We therefore propose that the oxidation of η‐butane to maleic anhydride takes place over an amorphous surface V–P–O phase. This finding has high relevance for our understanding of the catalytic activity of bulk crystalline V–P–O catalysts as well.

Separation of double-electron and atomic XAFS contributions in x-ray absorption spectra of Pt foil and Na2Pt(OH)6

Both double-electron excitation (DEE) and atomic XAFS (AXAFS) contributions are shown to be present in L23 x-ray absorption spectra of Pt foil and Na2Pt(OH)6. In the Fourier transform of the spectra, the DEE feature appears at lower R values than the AXAFS feature. A background subtraction procedure is developed that leaves the DEE contribution in the background and the AXAFS in the oscillatory part of the spectrum. The separation of the DEE and the AXAFS contributions is possible only when using a cubic spline function routine in which the smoothing parameter can be continuously adjusted.

Flexible aluminium coordination of zeolites as function of temperature and water content, an in-situ method to determine aluminium coordinations

The aluminium coordinations in zeolites H-Beta and H-Y have been quantitatively investigated as a function of temperature in the presence and absence of water. In-situ Al K edge X-ray Absorption Spectroscopy shows that a framework tetrahedrally coordinated aluminium is stable in inert to at least 725 K. However, in the presence of water, already at room temperature, part of the framework tetrahedral aluminium is converted to an octahedral coordination. This octahedral aluminium is not stable in inert at 375 K, where it quantitatively reverts to the tetrahedral framework coordination.