Characteristics of Rh-SiO2 Catalyst Prepared Using Microemulsion. An Investigation from the Bahavior of Adsorbed CO Species.

Abstract

Spectroscopic investigations were conducted to elucidate the difference in surface properties between two kinds of Rh-SiO2 catalysts prepared by using microemulsion and by the conventional impregnation method, by means of FT-IR measurements using CO as a probe molecule. The gem-dicarbonyl species adsorbed on isolated Rh1 sites were observed in the IR spectra of the Rh-SiO2 catalyst prepared by using microemulsion even at a temperature as high as 300°C, while it was not observed at this temperature in the spectra of the Rh-SiO2 catalysts prepared by the impregnation method. This result suggested that the disruption of Rh clusters caused by CO gas, leading to the formation of isolated Rh1 sites, still occurred prominently on the Rh-SiO2 catalysts prepared by using microemulsion even at such a high temperature that agglomeration occurred on the catalyst prepared by the impregnation method. It is quite likely that the disruption occurred prominently because the Rh particles of the catalyst existed in an oxidized state. Therefore, the oxidation state of Rh particles was investigated by XPS measurements, which showed that there were more Rh atoms in an oxidized state in the Rh-SiO2 catalyst prepared by using microemulsion. It is concluded, therefore, that the Rh-SiO2 catalyst prepared by using microemulsion had significant features attributed to a strong interaction between Rh particles and SiO2 support.