Determination of dispersion of precious metals on CeO2-containing supports

Precious metal (PM) dispersions on CeO2-containing supports were examined. When dispersions were evaluated by the H2-O2 titration method, the CeO2-containing supports hide the true redox of PM because of the large oxygen storage-reduction capacity of the oxide, resulting in higher apparent dispersions than the exact values. When Pt dispersions on the CeO2 support were evaluated by the ordinary CO pulse method, CO was adsorbed on the CeO2 support as carbonate species accompanying the oxidation with lattice oxygen. Therefore, the dispersions estimated from the CO uptake were also higher than the exact value. Infrared spectra also clearly evidenced the presence of carbonate species after CO adsorption. To prevent the CO adsorption on CeO2 as the carbonate species, we treated Pt/CeO2 with CO2 before CO adsorption. The strong bands arising from carbonate species on CeO2 were observed in the infrared spectra. When CO was adsorbed on CO2-terated Pt/CeO2, CO was selectively adsorbed on the surface atoms of Pt particles. By the O2-CO2-H2-CO pulse method, the amount of CO adsorbed on surface atoms of the Pt particles could be separated, and a precise dispersion was determined.