DFT and kinetic evidences of the preferential CO oxidation pattern of manganese dioxide catalysts in hydrogen stream (PROX)

Abstract

The oxidation functionality of Mn(IV) sites has been assessed by density functional theory (DFT) analysis of adsorption and activation energies of CO, H2 and O2 on a model Mn4O8 cluster. DFT calculations indicate that Mn(IV) atoms prompt an easy CO conversion to CO2 via a reaction path involving both catalyst and gas-phase oxygen species, while much greater energy barriers hinder H2 oxidation. Accordingly, a MnCeOx catalyst (Mnat/Ceat, 5) with large exposure of Mn(IV) sites shows a remarkable CO oxidation performance at T ≥ 293 K and no H2 oxidation activity below 393 K. Empiric kinetics disclose that the catalyst-oxygen abstraction step determines both CO and H2 oxidation rate, although different activation energies favor the preferential oxidation (PROX) pattern of the studied catalyst (353–423 K). Conversion-selectivity of 100%, high stability during 72 h reaction time and moderate inhibiting effects of water and CO2 feeding reveal the potential of MnO2 materials as efficient, low-cost and robust PROX catalysts.