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.