Ab Initio Study of CO Hydrogenation to Oxygenates on Reduced Rh Terraces and Stepped Surfaces

Previously reported syngas conversion experiments on silica-supported Rh nanoparticles show that CO conversion and oxygenate selectivity vary as a function of nanoparticle size. Theoretical studies in the literature have examined the effect of steps on CO dissociation, but structure sensitivity for C-1 and C-2 oxygenates has not been systematically investigated. In this study, density functional theory-based reaction energetics and kinetics for C-H, C-C, C-O, and O-H bond formation on flat Rh(111) and stepped Rh(211) surfaces are reported and compared. Multiple paths for methanol and ethanol formation are considered to ascertain the lowest energy pathways. Nearly an identical methanol formation route via CO -> CHO -> CH2O -> CH3O -> CH3OH is found to be favored on both Rh terrace and (211) sites. CO insertion into CH2 is deduced to be the precursor for C-2 oxygenate formation irrespective of site structure. Ethanol formation pathways, however, are determined to be markedly different on flat and stepped Rh surfaces in terms of barriers and intermediates. Our results show that reaction pathways are typically preferred on Rh step sites irrespective of the bond-breaking and -making (C-H, C-C, and C-O) reactions considered.