Catalytic selectivity of Rh/TiO2 catalyst in syngas conversion to ethanol: probing into the mechanism and functions of TiO2 support and promoter

The catalytic selectivity, the functions of a TiO2 support and promoter, and the mechanism of ethanol synthesis from syngas on a Rh/TiO2 model catalyst have been fully identified. Our results show that all species preferentially interact with Rh-7 clusters of a Rh/TiO2 catalyst, rather than the support and cluster-support interface. CO -> CHO -> CH2O -> CH3O is an optimal pathway. CH3 formed via the CH3O -> CH3 + O route is the most favored CHx (x = 1-3) monomer, and this route is more favorable than methanol formation by CH3O hydrogenation; CO insertion into CH3 can then form CH3CO, followed by successive hydrogenation to ethanol. Methane is formed by CH3 hydrogenation. The Rh/TiO2 catalyst exhibits better catalytic activity and selectivity toward CH3 than CH3OH formation. Starting from the CH3 species, CH4 formation is more favorable than CH3CO formation; thus, ethanol productivity and selectivity on a Rh/TiO2 catalyst with a support is determined only by CH4 formation, which is similar to that on a pure Rh catalyst without a support. Introducing an Fe promoter into the Rh/TiO2 catalyst effectively suppresses methane production, and promotes CH3CO formation. Therefore, compared to a pure Rh catalyst without a support, the TiO2 support serves only to promote the activity and selectivity of CH3 formation, and provide more CH3 species for ethanol formation; methane formation is independent of the Rh catalyst support, and depends only on the promoter. In order to achieve high ethanol productivity and selectivity, an effective Rh-based catalyst must contain a suitable combination of supports and promoters, in which the promoter, M, should have characteristics that draw the d-band center of the MRh/TiO2 catalyst closer to the Fermi level compared to the Rh-7/TiO2 catalyst; as a result, the MRh/TiO2 catalyst can suppress CH4 production and facilitate C-2 oxygenate formation.