DFT Study of the Role of the Promoter in V-Promoted Rh Catalysts for CO Hydrogenation

Unpromoted rhodium catalysts for CO hydrogenation generally produce methane, however promoters like V have shown to dramatically increase selectivity towards oxygenates.  Density functional theory (DFT) calculations are performed with the focus on the determination of new pathways at the interface between the promoter (V) and the active metal (Rh).  The goal is to understand the physical relationship between the promoter and the catalyst under realistic syngas conversion conditions of high CO surface coverage, incorporation of which has shown to significantly shift the thermodynamics.  Promotion is modeled both as an isolated metal atom on Rh(211) step edge, and as an oxide cluster on Rh(111) surface.  This work provides a systematic insight into the observed catalytic performance of the promoted catalyst viewed as a combined effect of surface structure, adsorbate coverage and identity of the promoter.  Promoter effects on C‐O cleavage, C‐H and/or C‐C bond formation reactions are analyzed via changes to the Rh d-band filling.  Chemical promotion through an H-assisted CO dissociation mechanism via formyl (HCO) as an intermediate species was found to be a primary pathway for CO activation on V promoted catalyst, due to the high CO dissociation barriers of 1.61 and 1.93 eV for Rh(111)/VO₂ and RhV(211) surface, respectively.  The observed decrease in adsorbate binding energy on the undercoordinated Rh step edge could be attributed to both surface coverage and more predominant V promotion (DE = -0.58 eV), the combined effect resulting in a highly reactive surface that facilitates hydrogenation of CO species (E_act = 0.76 eV) as well as a particularly low HCO insertion barriers to CH_x of 0.52, 0.32 and 0 eV for x = 1, 2, 3, respectively.  H-assisted CO dissociation is thermodynamically favored on Rh(111)/VO₂ with a 0.49 eV energy barrier. The oxide promoted surface provides low dissociation barriers assumed to be due to the undercoordination of vanadium atom of the oxide, achieved upon the oxygen removal via H₂O molecule.  HCO insertion to CH_x, assumed to be independent of interface, is exothermic with 0.76, 1.01 and 1.41 eV barriers for x = 1, 2, 3, respectively.  V is found to promote H₂O formation due to a repulsive behavior caused by V behaving as a mild Lewis acidic when embedded in Rh surface and OH intermediate (increased O electronegativity).