Particle and Phase Thicknesses from XPS Analysis of Supported Bimetallic Catalysts: Calcined Co-Rh/Nb 2O 5

Abstract

The surface structure and elemental composition of a series of calcined Co-Rh/Nb 2O 5 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/Nb 2O 5 and Rh/Nb 2O 5, and four calcined bimetallic Co-Rh/Nb 2O 5 catalysts with similar Co loadings (≍ 1.9 wt%) and variable Rh loadings (0.3 to 2.3 wt%) were examined. Reference spectra for pure CoNb 2O 6 (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, Co 3O 4 and Co +2, on all calcined Co-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 Nb 2O 5 particles (diameter ≍ 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 Nb 2O 5 support surface, whereas the Co 3O 4 forms thick (>2.5 nm) islands covered by Rh2O3 on approximate to 3.6% of the support surface. The thickness of the Rh 2O 3 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 Co 3O 4 in this model. The reduction temperature of the Co 3O 4 strongly decreased as the Rh/Co bulk atomic ratio increased, while the reduction temperature of the Rh 2O 3 was not strongly influenced by the presence of Co 3O 4. Thus, the TPR results are consistent with this bilayer island model, with Rh 2O 3 on top of Co 3O 4, The Co +2 species was not obvious in TPR due to its low concentration.