O-atom transport catalysis by neutral manganese oxide clusters in the gas phase: Reactions with CO, C2H4, NO2, and O2

Abstract

Reactions of CO, C2H4, NO2, and O2 with neutral Mn(m)O(n) clusters in a fast flow reactor are investigated both experimentally and theoretically. Single photon ionization at 118 nm is used to detect neutral cluster distributions through time of flight mass spectrometry. Mn(m)O(n) clusters are generated through laser ablation of a manganese target in the presence of 5% O2/He carrier gas. A strong size dependent reactivity of Mn(m)O(n) clusters is characterized. Reactions Mn2O5/Mn3O7 + CO → Mn2O4/Mn3O6 + CO2 are found for CO oxidation by Mn(m)O(n) clusters, while only association products Mn2O(3-5)C2H4 and Mn3O(5-7)C2H4 are observed for reactions of C2H4 with small Mn(m)O(n) clusters. Reactions of Mn(m)O(n) clusters with NO2 and O2 are also investigated, and the small Mn2O(n) clusters are easily oxidized by NO2. This activation suggests that a catalytic cycle can be generated for the Mn2O5 cluster: Mn2O5 + CO + NO2 → Mn2O4 + CO2 + NO2 → Mn2O5 + CO2 + NO. Density functional theory (DFT) calculations are performed to explore the potential energy surfaces for the reactions Mn2O(4,5)/Mn3O7 + CO → Mn2O(3,4)/Mn3O6 + CO2, Mn2O5 + C2H4 → Mn2O4 + CH3CHO, and Mn2O4 + NO2 → Mn2O5 + NO. Barrierless and thermodynamically favorable pathways are obtained for Mn2O5∕Mn3O7 + CO and Mn2O4 + NO2 reactions. A catalytic cycle for CO oxidation by NO2 over a manganese oxide surface is proposed based on our experimental and theoretical investigations. The various atom related reaction mechanisms explored by DFT are in good agreement with the experimental results. Condensed phase manganese oxide is suggested to be a good catalyst for low temperature CO oxidation by NO2, especially for an oxygen rich sample.