Effects of preparation procedure on the surface properties of rhodium supported on γ-alumina

Abstract

The decomposition of rhodium(III) nitrate, chloropentamminerhodium(III) chloride and rhodium(III) sulphite in atmospheres of hydrogen, nitrogen and nitrogen (80%)+ oxygen (20%) have been investigated by thermogravimetric analysis with the salts either pure or impregnated onto a γ-alumina support. The decomposition of rhodium(III) sulphite failed to give dispersions of rhodium metal on alumina but the reduction of the other salts to Rh⁰ was complete in nitrogen or hydrogen at temperatures between 400 and 690 K.

Hydrogen and CO uptake data have shown that rhodium(III) nitrate led to a more finely divided dispersion of metal than did chloropentamminerhodium(III) chloride. Infrared spectra of CO absorbed on the Rh/Al₂O₃ dispersions were consistent with this conclusion. The effects of sintering and oxygen adsorption on the infrared spectra are reported. Linear and bridge-bonded CO were formed on exposed metal planes or two-dimensional rafts at Rh⁰ sites which were also active for the chemisorption of oxygen. Gem-dicarbonyl species were formed at monatomically dispersed rhodium sites or edge sites in rafts, both of which had cationic character through interaction with the alumina support. These sites were not active for the adsorption of oxygen.