Cerium oxide cluster cations (Ce(m)O(n)(+), m = 2-16; n = 2m, 2m +/- 1 and 2m +/- 2) are prepared by laser ablation and reacted with carbon monoxide (CO) and small hydrocarbon molecules (CH(4), C(2)H(4), and C(2)H(6)) in a fast flow reactor. A time of flight mass spectrometer is used to detect the cluster distribution before and after the reactions. The observation of oxygen reduction and hydrogen pickup of Ce(m)O(2m)(+) clusters strongly suggests the following reactions: (1) Ce(m)O(2m)(+) + C(2)H(4) --> Ce(m)O(2m-1)(+) + C(2)H(4)O (m = 2-6); (2) Ce(m)O(2m)(+) + CO --> Ce(m)O(2m-1)(+) + CO(2) (m = 4-6); and (3) Ce(m)O(2m)(+) + CH(4)/C(2)H(6) --> Ce(m)O(2m)H(+) + CH(3)/C(2)H(5) (m = 2-4). Density functional theory (DFT) calculations are performed to study reaction mechanisms of Ce(2)O(4)(+) + X (X = CO, CH(4), C(2)H(4), and C(2)H(6)). The calculated results are in good agreement with the experimental observations. The structural and bonding properties of Ce(m)O(2m)(+) (m = 2-5) clusters are also investigated by the DFT calculations. The unpaired electron in each of the clusters is mainly distributed over one Ce atom (4f and 5p orbitals) and two O atoms (2p orbital) in a CeO(2) moiety, which can be considered as the active site in the cluster. To further understand the nature of the active sites in Ce(m)O(2m)(+) clusters, the fast flow reaction experiments are also carried out on zirconium oxide clusters Zr(m)O(n)(+), because both Zr ([Kr]4d(2)5s(2)) and Ce ([Xe]4f(1)5d(1)6s(2)) have the same number of valence electrons while the latter has one more f and one less d electrons. In addition to the oxygen transfer reactions such as Zr(m)O(2m)(+) + C(2)H(4) --> Zr(m)O(2m-1)(+) + C(2)H(4)O (m = 1-4) reported in the literature, hydrogen abstraction reactions Zr(m)O(2m)(+) + CH(4)/C(2)H(6) --> Zr(m)O(2m)H(+) + CH(3)/C(2)H(5) are also identified. The rate constants of CO oxidation as well as hydrogen abstraction by Ce(m)O(2m)(+) and Zr(m)O(2m)(+) are very different. The reactivity and selectivity of Ce(m)O(2m)(+) versus Zr(m)O(2m)(+) can be well rationalized based on the DFT calculations. The oxygen transfer and hydrogen abstraction reactions studied in this work are of widespread importance. The nature of the active sites of Ce(m)O(2m)(+) clusters is unique and may be considered in the use and design of cerium oxide based catalysts.

Reactions of small neutral iron oxide clusters (FeO(1-3) and Fe(2)O(4,5)) with carbon monoxide (CO) are investigated by experiments and first-principle calculations. The iron oxide clusters are generated by reaction of laser-ablation-generated iron plasma with O(2) in a supersonic expansion and are reacted with carbon monoxide in a fast flow reactor. Detection of the neutral clusters is through ionization with vacuum UV laser (118 nm) radiation and time-of-flight mass spectrometry. The FeO(2) and FeO(3) neutral clusters are reactive toward CO, whereas Fe(2)O(4), Fe(2)O(5), and possibly FeO are not reactive. A higher reactivity for FeO(2) [sigma(FeO(2) + CO) > 3 x 10(-17) cm(2)] than for FeO(3) [sigma(FeO(3) + CO) approximately 1 x 10(-17) cm(2)] is observed. Density functional theory (DFT) calculations are carried out to interpret the experimental observations and to generate the reaction mechanisms. The reaction pathways with negative or very small overall barriers are identified for CO oxidation by FeO(2) and FeO(3). The lower reactivity of FeO(3) with respect to FeO(2) may be related to a spin inversion process present in the reaction of FeO(3) with CO. Significant reaction barriers are calculated for the reactions of FeO and Fe(2)O(4-5) with CO. The DFT results are in good agreement with experimental observations. Molecular-level reaction mechanisms for CO oxidation by O(2), facilitated by condensed phase iron oxides as catalysts, are suggested.