Insight into the promoting role of Rh doped on Pt(111) in methanol electro-oxidation

The bi-functional mechanism has been widely accepted for describing the binary catalysts in methanol electrooxidation, in which Pt sites can catalyze the methanol dehydrogenation and the second elements (e.g. Ru, Sn) can activate water to oxidants. However, in terms of binary PtRh, the surface Rh sites being identified not to be oxidized as easily as Ru, may participate in the dehydrogenation reaction at low potentials rather than just activate water. In this work, we investigate the roles of doped Rh on Pt(111) in methanol electrooxidation by density functional theory (DFT) calculations for the first time. We find that the methanol dehydrogenation activity is enhanced on Rh/Pt(111) because Rh can eliminate significantly the repulsive interaction between carbon fragment and H at the transition state. The formation potential of OH* on Rh/Pt(111) is 0.36 V (vs SHE) which is lower than 0.64 V (vs SHE) on Pt(111). Meantime, the coupling barrier between CO* and OH* is also decreased from 0.47 eV to 0.24 eV. By considering coverage, the individual Rh atom on surface is identified to be more effective than the neighbouring atoms for methanol electrooxidation, and thus a moderate Rh coverage is suggested. This work provides some insight into the binary catalysts for methanol electrooxidation.