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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 −1.5 to −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 Pt4 clusters, which show that the acid/base properties of the support have a much larger direct influence on Pt&z.sbnd;H bonding than on Pt&z.sbnd;CHn bonding. [Copyright &y& Elsevier]

The exchange of hydrogen for deuterium (H/D exchange) in cyclopentane was measured for Pt catalysts with supports of various acidity. With an earlier developed Monte-Carlo model the contributions of the various possible intermediates in the H/D exchange can directly be measured. It was shown that the activity and the selectivity in the H/D exchange over the supported Pt catalysts strongly depend on the support acid/base properties. The activities of the various catalysts show a compensation effect. The compensation effect can directly be correlated to the contribution of the various exchange mechanisms, which proceed via different intermediates. The contribution of each intermediate depends on the electronic properties of the Pt particles, which in turn depend on the support acidity. This shows that the compensation effect is caused by support-induced changes in the adsorption modes of cyclopentane. [Copyright &y& Elsevier]

In situ XAFS spectroscopy was applied in order to determine the catalytically active sites in Fe/ZSM5, prepared by FeCl3 sublimation. The number of (inactive) spectators in this catalyst was minimized by using a specially dedicated calcination procedure. The catalytic activity of Fe/ZSM5 during the collection of the XAFS data was monitored via chemiluminescence analysis of the gas outlet. Binuclear Fe complexes with a Fe&z.sbnd;O&z.sbnd;Fe core were found to be catalytically active species in this material. During heat treatment in He to 350 °C the Fe complexes undergo auto-reduction, ascribed to the removal of oxygen from Fe&z.sbnd;O&z.sbnd;Fe bridges (closest Fe&z.sbnd;O-coordination sphere) and the formation of Fe&z.sbnd;□&z.sbnd;Fe vacancies. Treatment with isobutane results in a further slight average reduction of iron, accompanied by an additional removal of oxygen from the Fe&z.sbnd;O&z.sbnd;Fe bridges. Fe in the binuclear complexes is readily oxidized when NO, NO+O2, or a typical HC-SCR mixture (NO, i-C4H10, O2) is fed to the Fe/ZSM5 catalyst. Reoxidation is accompanied by filling of the Fe&z.sbnd;□&z.sbnd;Fe vacancies. Under HC-SCR working conditions the average oxidation state of iron is 3+. [Copyright &y& Elsevier]

The structure of the iron species in mildly calcined over-exchanged Fe/ZSM5, prepared by CVD of FeCl3, was studied during heat treatments in He or O2/He (50:50) by coupling in situ Fe K edge HR-XANES and EXAFS. The majority of iron appears to be present as Fe-binuclear complexes. EXAFS shows that the closest Fe&z.sbnd;O shell in the complexes can be described with a [HO&z.sbnd;Fe&z.sbnd;O&z.sbnd;Fe&z.sbnd;OH]2+ core. Heating to a moderate temperature (up to ∼150 °C) results in the desorption of water from the Fe-coordination sphere, in He as well as in a O2/He (50:50) mixture. The composition of the gas phase strongly influences the changes occurring to the binuclear complexes in the 150–350 °C temperature range. Above 250 °C in He a significant fraction of iron undergoes autoreduction. This is accompanied by the removal of approximately one oxygen atom from the closest Fe&z.sbnd;O shell, ascribable to the Fe&z.sbnd;O&z.sbnd;Fe bridging oxygen atom. The presence of oxygen in the gas phase (PO2=0.5 bar), on the contrary, suppresses the auto-reduction capability of the binuclear complexes by inhibiting the removal of the Fe&z.sbnd;O&z.sbnd;Fe bridging oxygen. [Copyright &y& Elsevier]

The exchange of hydrogen for deuterium (H/D exchange) in cyclopentane (CP) is a promising test reaction for studying support effects in heterogeneously metal-catalyzed reactions, such as differences in stability and activity in, e.g., benzene or tetralin hydrogenation reactions. In order to employ this test reaction, a full understanding of the H/D exchange mechanism is essential. However, clear insight in the observed kinetics and selectivities is lacking in the literature. In this paper, a kinetic model that adequately describes the observed activity and orders in CP and D2 is developed. It is shown that the selectivity is determined in a series of reactions after the rate-determining step. To understand the observed selectivities a Monte Carlo model is developed which accurately simulates the observed exchange patterns and reveals the relative contributions of four competitive intermediates. One intermediate (a π-bonded η2-cyclopentene) can rotate in a nonactivated mechanism via an allylic intermediate. The model reveals the number of rotations each intermediate experiences and can be as high as ∼ 20, even if only 5 D atoms are observed in the product. This number of rotations is a better measure of the H/D exchange than the conventional “multiplicity.” The H/D exchange of CP together with these insights can now be applied to investigate in detail the influence of the support on the catalytic properties of supported metal particles. [Copyright &y& Elsevier]

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57Fe Mo¨ssbauer spectroscopy, 27Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Brønsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination. [Copyright &y& Elsevier]