A study of metal-support interactions on lanthana- and ceria-supported noble metals

The focus of this work has been the connection between structure and particle size for precious metals supported on rare-earth oxides. First, the adsorption properties of small Pd particles were studied as a function of their size via {dollar}sp{lcub}15{rcub}{dollar}NO, CO and H{dollar}sb2{dollar} adsorption. CO and H{dollar}sb2{dollar} adsorption on Pd were found to be independent of particle size. In contrast, NO adsorption was found to be strongly structure sensitive; the main desorption product following NO adsorption on small particles was N{dollar}sb2,{dollar} at temperatures higher than 600K, while large particles and Pd single crystals did not dissociate NO.; The role of ceria structure on the catalytic properties of supported metals was probed using adsorption and computer simulations, and the size of ceria crystallites was shown to dramatically influence the oxygen donation capability of ceria, with small crystallites being the most active. TPD studies of CO on precious metals supported on ceria showed CO{dollar}sb2{dollar} desorption only when the ceria support consisted of small crystallites. Steady-state CO oxidation rates were also significantly different when the ceria support consisted of small crystallites. Simulated annealing calculations confirmed that ceria reducibility is a strong function of crystallite size, with very small ceria clusters exhibiting large fluctuations in the energy required for oxygen removal as a function of cluster size, resembling the "magic numbers" observed on metals.; Finally, experimental evidence was found for the surface of polycrystalline lanthana films to be reducible in CO, especially in the presence of noble metals. Reduced lanthana can adsorb and decompose NO in a temperature range similar to that observed for Pt and Rh. Substantial amounts of the NO on reduced lanthana react to form N{dollar}sb2{dollar} and N{dollar}sb2{dollar}O. However, as for ceria, high-temperature annealing results in deactivation of lanthana.