Effect of Ce Additive on CO2 Hydrogenation over Rh/SiO2 Catalysts.

Abstract

The effects of Ce additive on CO2 hydrogenation over Rh/SiO2 catalysts were studied. Ce addition drastically increased CO2 conversion. The main product was CO over unpromoted Rh/SiO2 catalyst, but methane over Ce promoted Rh/SiO2 catalyst. Moreover, Ce additive promoted ethanol production. The mechanism of the promotion effect of Ce was investigated by XRD, TEM, EDX and FT-IR. The mean metal particle size of Rh-Ce/SiO2 estimated by XRD and TEM analysis was smaller than that for Rh/SiO2 and Ce/SiO2. In-situ FT-IR during reaction and CO2 adsorption on unpromoted catalyst showed CO2 molecules were adsorbed as CO species as found in CO adsorption. However, a new peak attributed to carboxylate species was observed at around 1570cm-1 by CO2 adsorption as well as during reaction over the Ce promoted Rh/SiO2 catalyst. This result indicates that the Ce additive enhanced CO2 adsorption on the catalyst, leading to increased activity. The new adsorption band at around 1700cm-1 may be attributed to a CO molecule bonded through both the carbon and oxygen atoms due to higher metal dispersion. These findings suggest that the Ce additive enhanced CO dissociation and CO insertion over Rh/SiO2 catalysts, resulting in promotion of CH4 and ethanol formation.