CHARACTERIZATION OF EUROPEAN STANDARD CATALYSTS BY RADIAL ELECTRONIC DENSITY DISTRIBUTION AND EXAFS SPECTROSCOPY TECHNIQUES

E.M.Moroz, S.V.Bogdanov, D.A.Zyuzin, V.V.Kriventsov

Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Science, Novosibirsk 630090, Russia

Keywords: catalysts, phase composition, electronic density, interatomic distances

Euro-Pt-1 (Pt/SiO₂) and EO-Cat (V-W/TiO₂) catalysts were studied by the XRD, radial electronic density distribution (RED), and EXAFS spectroscopy techniques.

The supported metallic catalyst Euro-Pt-1contains 6.3 wt% Pt. Diffraction pattern of the sample exhibits only very diffused maxima of metallic platinum and SiO₂ phases. A half-width of Pt (1.1.1) maximum corresponds to crystallites ~18Å in size.

The EO-Cat catalyst, according to element analysis, contains following oxide phases: TiO₂ (78 wt%), WO₃ (9 wt%), SiO₂ (6.5 wt%), V₂O₅ (3.15 wt%). Diffraction pattern of the sample exhibits maxima of TiO₂ phase (anatase). A half-width of these maxima corresponds to crystallites ~320Å in size. Anatase is observed without peak shifts.

Phase composition of the catalysts was precisely characterized with the X-ray RED and EXAFS spectroscopy techniques. In order to elucidate qualitative phase composition of catalysts we have brought into correlation coordination peak positions on RED curves with the interatomic distances of certain phases. To determine amounts of phases we have compared experimental areas of coordination peaks with those calculated for certain phases.

Differential RED curve of Euro-Pt-1 catalyst contains coordination peaks corresponding to interatomic distances of the f.c.c. Pt⁰ structure and oxide PtO phase. Correlation between Pt⁰ and PtO phases calculated from the peak areas is 1 : 2.

Two platinum particle models were tested against the experimental EXAFS spectra: the traditional model (1), which is characterized by the distance Pt-Pt normally distributed around the same mean, both in the particle bulk and on its surface, and the model (2), which has two different mean Pt-Pt distances for the particle bulk and its surface. Model 1 gives underestimated coordination numbers for the platinum particle structure, which do not fit the data of electron spectroscopy and RED. Model 2 provides coordination numbers corresponding to the dispersity and the proportion of a Pt⁰ phase in the sample. Pt-Pt distances on the platinum surface particles appear to be shortened.

The modeling of the EO-Cat phase composition by RED curves showed the catalyst do not contain WO₃ phase but W⁺⁶ ions place in the anatase structure forming solid solution.