La, V, and Fe promotion of Rh/SiO2 for CO hydrogenation: Detailed analysis of kinetics and mechanism

It has been reported widely that Rh-based catalysts can exhibit high selectivity to C2+ oxygenates during CO hydrogenation, with promoters playing an important role in this behavior. In this study, the effects of the addition of La, V, and/or Fe promoters on the kinetics of the formation of various products were determined and the mechanistic pathways were delineated using a Langmuir-Hinshelwood approach. Non-promoted and promoted Rh supported on SiO2 were prepared using the incipient wetness impregnation method and were reduced in H-2 at 500 degrees C before reaction. The kinetic study was carried out using a fixed-bed differential reactor. It was found that, in general, increasing H-2 pressure resulted in increased activities, while increasing CO partial pressure had an opposite effect. However, the specific influence of H-2 or CO partial pressure on the activity and selectivities differed greatly with different promoters. There was a more significant change in the activity of the La-V doubly promoted Rh catalyst with H-2 or CO partial pressure than for other catalysts, which may be due to a synergistic effect between La and V. The Fe singly promoted catalyst showed different trends in both rate and selectivity from other catalysts, suggesting a different promoting mechanism than La or V. Based on the fact that hydrogen-assisted CO dissociation has been reported to best describe the mechanism for Rh catalysts, Langmuir-Hinshelwood rate expressions for the formation of methane and of ethanol were derived and compared to the experimentally derived power-law parameters. It was found that the addition of different promoters appeared to result in different rate-limiting steps. (C) 2009 Elsevier Inc. All rights reserved.