Role of promoters on Rh/SiO 2 in CO hydrogenation: A comparison using DRIFTS

Abstract

La, V, Zn, Cu, Fe, Li and Ag promoted Rh/SiO 2 catalysts were investigated for the synthesis of ethanol during CO hydrogenation at 230 °C and 1.8 atm. As is well known, the activity and selectivity depend heavily on the choice of promoter. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to probe the effects of La, V, Zn and Cu on CO adsorption and hydrogenation. From the IR study, it was found that the behavior of CO adsorbed on the differently promoted catalysts was very different. While La enhanced total CO adsorption, the addition of V, Zn and Cu suppressed CO adsorption to different extents. The doubly promoted Rh-La/V/SiO 2 showed only moderate CO adsorption. Results from DRIFTS suggest that the higher catalytic activity (compared to the non-promoted catalyst) observed for the La singly promoted Rh/SiO 2 catalyst may primarily be caused by an increase in the concentration of the adsorbed CO species in the presence of H 2, possibly due to the formation of new active sites at the LaO x -Rh interface. The higher catalytic activity of the V singly promoted Rh/SiO 2 catalyst could be ascribed to an increased desorption rate/reactivity of the adsorbed CO species. The La and V doubly promoted catalyst showed both new adsorbed CO species and increased desorption rate/reactivity of the adsorbed species during CO hydrogenation due to a synergistic promoting effect of La and V. The addition of Zn or Cu promoters significantly reduced the desorption rate/reactivity of the adsorbed CO species on Rh/SiO 2, leading apparently to the much reduced activities for CO hydrogenation observed.