CO desorption and oxidation on CeO 2-supported Rh: Evidence for two types of Rh sites

Abstract

Previous co-adsorption studies of CO and oxygen on Rh single-crystal surfaces have shown that formation of CO 2 is the dominant pathway during temperature-programmed desorption and that the reaction limits the competing pathway of CO and O 2 desorption. The present work finds that this selectivity is altered on CeO 2-supported Rh islands. In addition to facile CO oxidation, a new CO adsorption state is found that is associated with the Rh but is observed only in the presence of both the CeO 2 support and pre-adsorbed oxygen. This state is characterized by its low desorption temperature (140–250 K), well below the CO 2 formation temperatures of 290 K ( β 2 ) and 360 K ( β 1 ), and by its IRAS frequency of 2100 cm −1, higher than observed for either CO adsorbed on the supported Rh islands (2057–2066 cm −1) or CO co-adsorbed with O on Rh islands (2074 cm −1). The data suggest that oxygen and subsequent CO exposure of CeO 2 supported Rh creates atomically dispersed gem-dicarbonyl, Rh(CO) 2, from the edge of Rh islands. As temperature increases, part of the dicarbonyl desorbs at low temperature, and the remainder is converted to CO 2. The low-temperature desorption pathway has a selectivity as high as 46%, with the remaining CO oxidized to CO 2.