Experimental and Theoretical Study of the Reactions between Cerium Oxide Cluster Anions and Carbon Monoxide: Size-Dependent Reactivity of CenO2n+1–(n= 1–21)

Reactions of cerium oxide cluster anions with carbon monoxide are investigated by time-of-flight mass spectrometry and density functional theory computations aided with molecular dynamics simulations. Interesting size-dependent reactivity of the Ce n O 2 n +1 – cluster series with n = 1–21 is observed: (1) the small n = 1–3 clusters have no or very low reactivity toward CO, (2) the large n = 4–21 clusters can oxidize CO to produce CO 2, and (3) the n = 4 (Ce 4 O 9 – ), 6 (Ce 6 O 13 – ), 7 (Ce 7 O 15 – ), and 12 (Ce 12 O 25 – ) clusters have relatively higher reactivity than their neighboring systems Ce 3 O 7 –, Ce 5 O 11 –, Ce 8 O 17 –, etc. Theoretical study indicates that the Ce n O 2 n +1 – clusters contain oxygen-centered radicals (O –• ) and the nature of the spin density distributions within the clusters controls the experimentally observed size-dependent reactivity. The experiment and theory in this study suggest that the metal oxide clusters as large as Ce 21 O 43 – can contain the reactive O –• centers, at which the size may be large enough to mimic related active sites in condensed phase catalysts. Oxidation of CO by O 2 at low temperature is of widespread importance and reactive oxygen species including O –• are usually involved. The nature of the O –• radicals is demonstrated to be able to further address the goodness of nanocrystalline CeO 2 in the low-termperautre CO oxidation.