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

Abstract

L 2 and L 3 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 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.

Theoretical Study on Pt Particle Adsorbate Bonding: Influence of Support Ionicity and Implications for Catalysis

“…A detailed analysis of the kinetic data for neopentane hydrogenolysis as reported by us 10 recently showed that the H coverage directly affected the neopentane-Pt bond strength through the Frumkin isotherm and thereby changed the activation energy for the reaction. Preliminary results for the hydrogenation of benzene and other alkenes catalyzed by Pt on supports with different ionicities show that the change in H coverage may be the most responsible for the observed change in reactivity.…”

“…In addition, the inhibition by hydrogen of the hydrogenolysis of alkanes is much stronger for basic supports: the order in H 2 ranges from -2.5 (basic support) to -1.5 (acidic). [10][11][12][13] Although the effect of the support on the catalytic properties of the supported metal particles has been well established, at least for reactions involving H, the nature of this metal-support interaction has been the subject of much debate. Explanations have involved the formation of metal-proton adducts on Brønsted acidic supports, 5,14 electron transfer between support and particle, 4,15,16 the polarization of the metal particle by nearby cations, 17 and a change in the ionization potential of the metal, due to a shift in the d-band density of states.…”

Support-induced compensation effects in H/D exchange of cyclopentane

“…Since the reactants are always the same molecules, the changes in the chemisorption strengths must be caused by changes in the electronic properties of the supported Pt particles. This is discussed in more detail in an other paper [39].…”

Synthesis of highly dispersed palladium alumina supported particles: Influence of the particle surface density on physico-chemical properties

“…These interactions play a key role in the final properties of the metallic active sites, controlling morphology, electronic properties and finally overall catalytic performances [19][20][21].…”