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

Abstract

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 Rh₇ 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 CH_x (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 Rh₇/TiO₂ catalyst; as a result, the MRh/TiO₂ catalyst can suppress CH₄ production and facilitate C₂ oxygenate formation.