Reactions of CO, C 2 H 4 , NO 2 , and O 2 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% O 2 /He carrier gas. A strong size dependent reactivity of Mn m O n clusters is characterized. Reactions Mn 2 O 5 /Mn 3 O 7

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... + CO → Mn 2 O 4 /Mn 3 O 6 + CO 2 are found for CO oxidation by Mn m O n clusters, while only association products Mn 2 O 3-5 C 2 H 4 and Mn 3 O 5-7 C 2 H 4 are observed for reactions of C 2 H 4 with small Mn m O n clusters. Reactions of Mn m O n clusters with NO 2 and O 2 are also investigated, and the small Mn 2 O n clusters are easily oxidized by NO 2 . This activation suggests that a catalytic cycle can be generated for the Mn 2 O 5 cluster: Mn 2 O 5 + CO + NO 2 → Mn 2 O 4 + CO 2 + NO 2 → Mn 2 O 5 + CO 2 + NO. Density functional theory (DFT) calculations are performed to explore the potential energy surfaces for the reactions Mn 2 O 4,+ NO 2 → Mn 2 O 5 + NO. Barrierless and thermodynamically favorable pathways are obtained for Mn 2 O 5 /Mn 3 O 7 + CO and Mn 2 O 4 + NO 2 reactions. A catalytic cycle for CO oxidation by NO 2 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 NO 2 , especially for an oxygen rich sample. Yin, Wang, and Bernstein J. Chem. Phys. 139, 084307 (2013) FIG. 10. Possible catalytic cycle for the CO oxidation by NO 2 over manganese oxide catalysts at the molecular level.