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. Nonpromoted and promoted Rh supported on SiO2 were prepared using the incipient wetness impregnation method and were reduced in H2 at 500 °C before reaction. The kinetic study was carried out using a fixed-bed differential reactor. It was found that, in general, increasing H2 pressure resulted in increased activities, while increasing CO partial pressure had an opposite effect. However, the specific influence of H2 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 H2 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.