PARTICLE AND PHASE THICKNESSES FROM XPS ANALYSIS OF SUPPORTED BIMETALLIC CATALYSTS - CALCINED CO-RH/NB2O5

摘要

The surface structure and elemental composition of a series of calcined Co-Rh/Nb2O5 bimetallic catalysts have been investigated using X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). New formulae for the quantitative analysis of XPS intensities for supported bimetallic catalysts involving up to three separate, layered phases on spherical support particles are used for the first time. These apply an average takeoff angle for photoelectrons from spherical particles whose radii are large compared to the electron attenuation length. Calcined monometallic Co/Nb2O5 and Rh/Nb2O5, and four calcined bimetallic Co-Rh/Nb2O5 catalysts with similar Co loadings (approximate to 1.9 wt%) and variable Rh loadings (0.3 to 2.3 wt%) were examined. Reference spectra for pure CoNb2O6 (columbite) are also presented here for the first time. The catalysts were prepared by incipient wetness impregnation and calcined at 673 K to generate the oxide precursors. The XPS lineshapes and the Co (2p) spinorbit splitting indicated the presence of two Co species, Co3O4 and Co+2, on all calcined Go-containing catalysts. The measured XPS Co/Nb and Rh/Nb atomic ratios for the catalysts were factors of 1.5-2 and 2-4.5, respectively, greater than the bulk atomic ratios, showing that both Co and Rh oxides were surface-enriched. The measured XPS peak intensities were compared to the values predicted from several different structural models of the oxide particles. These models assumed large spherical Nb2O5 particles (diameter approximate to 60 nm to match the BET area) with the Co and Rh oxides covering fractions of this support's surface in several particle arrangements. The best agreement, according to the minimum least squares criteria, was found for a model in which the Co+2 phase is adsorbed on the Nb2O5 support surface, whereas the Co3O4 forms thick (>2.5 nm) islands covered by Rh2O3 on approximate to 3.6% of the support surface. The thickness of the Rh2O3 overlayer increased to 2.6 nm as the Rh/Co bulk atomic ratio increased to 0.72. The Co+2 phase was present at submonolayer concentrations. Between 78 and 90 mole% of the total Co was present as Co3O4 in this model. The reduction temperature of the Co3O4 strongly decreased as the Rh/Co bulk atomic ratio increased, while the reduction temperature of the Rh2O3 was not strongly influenced by the presence of Co3O4. Thus, the TPR results are consistent with this bilayer island model, with Rh2O3 on top of Co3O4, The Co+2 species was not obvious in TPR due to its low concentration. (C) 1995 Academic Press, Inc. [References: 88]