Application of XANES Spectra to Supported Catalysts

Abstract

XANES spectroscopy is applied to the characterization of surface species of supported catalysts. In case of vanadium oxide supported on silica (V2O5/SiO2) at low loadings, comparison of the vanadium K-edge XANES spectrum with the spectra of the authentic samples allowed the determination that the surface species is a VO4 tetrahedron where three oxygen atoms of the four are linked to silica support. The coordination environment changed easily by adsorption of water molecules, indicating the direct attachment of water molecules to vanadium atoms. A platinum particle of Pt-sulfated ZrO2, known as a catalyst promoting super acidic reactions, consists of a core of platinum metal particle and surface layers of platinum oxide. The analysis of XANES spectrum of the catalyst sample by using those of Pt metal and α-PtO2 goves the fraction of α-PtO2. Fe ions in Fe, Mn-sulfated ZrO2 catalyst, knowing as a catalyst promoting super acidic reactions, were found to be present inside the ZrO2 as a solid solution and the ions does not directly participate in the super acidic reaction. Mn ions are the site of adsorption of reactant gas molecules. The in situ XANES spectra at Fe K-edge did not change throughout the reaction and those at Mn K-edge were affected by introduction of reaction gas. Europium amide complex in zeolite is a mixture of trivalent and divalent Eu species. Eu divalent species is an active species for abase-catalyzed reaction. The fraction of Eu divalent species could be determined by danalysis of XANES spectra of Eu L3-edge. The analysis applied to Pt XANES spectra can be applied to the XANES spectra of Rh/TiO2 at Rh K-edge. In the case of a Rh/TiO2 photocatalyst, the reactivity of the catalyst varies with the loading amount of Rh. XANES analysis of the Rh/TiO2 catalysts showed that surface rhodium species are a mixture of Rh metal and highly dispersed Rh2O3 and the fraction of Rh2O3 varies with the loading amount. We found that Rh atoms are present as an oxide form at low loadings even after heating the catalyst in the presence of hydrogen at 673 K.