Low temperature CO oxidation over Rh supported on N-doped carbon

Abstract

Some isolated transition metals supported on nitrogen-doped carbon (M–N–C) are effective catalysts for reactions involving O₂, including low temperature CO oxidation. In this work, screening of various M–N–C materials using quantum chemical calculations showed that group 9 transition metals (Co, Rh, and Ir) in nitrogen-doped carbon have similar binding affinities for CO and O₂ and were able to form a stable CO–O₂ intermediate, which are criteria for a low-temperature CO oxidation catalyst. A Rh–N–C catalyst was therefore synthesized and evaluated for CO oxidation. The steady-state reaction at low temperature (<403 K) over Rh–N–C had positive reaction orders in both CO and O₂ with a very small apparent activation energy. Results from kinetic experiments and quantum chemical calculations are consistent with a reaction path involving weak adsorption of CO onto Rh ions with turnover coming from CO-assisted activation of weakly adsorbed O₂. The reaction mechanism does not involve a redox cycle with Rh and appears to be general in nature for low temperature CO oxidation. These findings may be conceptually useful for the design of other catalysts for reactions involving dioxygen activation.