Characterization of Rh-Co/SiO 2 catalysts for CO 2 hydrogenation with TEM, XPS and FT-IR

Abstract

Rh-Co/SiO 2 catalysts, which showed remarkable methanol formation in CO 2 hydrogenation, were characterized by various methods such as TEM, EDX, XPS, and in situ FT-IR. The added amount of Co was varied from 0 to 2 in atomic ratio to Rh. The mean size of metal particles determined by TEM observation decreased with the amount of Co added. In addition, the metal particle size distribution of Rh-Co/SiO 2 catalysts observed by TEM was narrower than that of Rh/SiO 2. EDX analysis of metal particle showed that Rh-Co alloy was formed on the Co-promoted catalysts. The results of XPS indicated that Co species existing on metal surfaces were more than 80 at.%. A good correlation was obtained between methanol selectivity and the surface Rh composition of Rh-Co/SiO 2 catalysts determined by XPS analysis. The selectivity to methanol increased with the surface composition of Rh. Adsorbed CO species on the Rh-Co alloy (cobalt rhodium carbonyl) were observed on Co-promoted catalysts in the spectra of in situ FT-IR during CO 2 hydrogenation reaction. These results indicated that methanol formation was promoted on the interface between Rh and Co. The electron-donating effect from Co to Rh was observed in situ FT-IR observation of CO 2 adsorption on Rh-Co/SiO 2. Moreover, the temperature at which adsorbed CO species reacted with H 2 over Co-promoted Rh/SiO 2 catalysts was higher than that over unpromoted one. Judging from these findings of in situ FT-IR and CO 2 hydrogenation reactivity, it was concluded that adsorbed CO derived from CO 2 was stabilized by Co additive.