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

The surface structure and elemental composition of a series of calcine d 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 f or supported bimetallic catalysts involving up to three separate, laye red phases on spherical support particles are used for the first time. These apply an average takeoff angle for photoelectrons from spherica l particles whose radii are large compared to the electron attenuation length. Calcined monometallic Co/Nb2O5 and Rh/Nb2O5, and four calcine d bimetallic Co-Rh/Nb2O5 catalysts with similar Co loadings (approxima te 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 he re for the first time. The catalysts were prepared by incipient wetnes s 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-contain ing 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 surfac e-enriched. The measured XPS peak intensities were compared to the val ues predicted from several different structural models of the oxide pa rticles. These models assumed large spherical Nb2O5 particles (diamete r approximate to 60 nm to match the BET area) with the Co and Rh oxide s covering fractions of this support's surface in several particle arr angements. 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) is lands 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 sub monolayer 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 th e reduction temperature of the Rh2O3 was not strongly influenced by th e presence of Co3O4. Thus, the TPR results are consistent with this bi layer island model, with Rh2O3 on top of Co3O4, The Co+2 species was n ot obvious in TPR due to its low concentration. (C) 1995 Academic Pres s, Inc.