Metal porphyrins (M = Ti, Fe, Co, Ni, Cu, or Zn) as potential catalysts for the oxidation of CO by N2O: insight from DFT calculations

Abstract

The oxidation of CO by N₂O over M–porphyrin (M = Ti, Fe, Co, Ni, Cu, and Zn) catalysts has been investigated via density functional theory calculations. The whole reaction process is divided into two steps: the catalytic decomposition of N₂O that breaks the N–O bond resulting in O–M active species, and the carbon atoms of the CO molecule reaction with O–M to form CO₂. For the rate-controlled step of the reaction, the porphyrins of different metal centers appear in different positions. The barrier height of N₂O decomposition on Ti–porphyrin is 3.8 kcal mol⁻¹, and the barrier height of CO oxidation is 21.9 kcal mol⁻¹. The rate-controlled step appears in the process of oxidation of CO. However, for Fe–porphyrin, the barrier height of N₂O decomposition is 24.2 kcal mol⁻¹, and the barrier height of CO oxidation is 11.4 kcal mol⁻¹. The rate-controlled step appears in the process of N₂O decomposition. For the catalytic decomposition of N₂O, the Ti–porphyrin has a low activation energy barrier, which may be due to the smaller gap between the highest occupied molecular orbital (HOMO) of the metal porphyrin and the lowest unoccupied molecular orbital (LUMO) of N₂O for Ti–porphyrin compared to Fe–porphyrin.