The adsorption properties of O2 molecules on anionic, cationic, and neutral Aun clusters (n=1–6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2–0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au3 and Au5 clusters can adsorb one O2 molecule.

Cerium oxide cluster cations (Ce_mO_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 (CH₄, C₂H₄, and C₂H₆) 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_mO_2m ⁺ clusters strongly suggests the following reactions: (1) Ce_mO_2m ⁺ + C₂H₄ → Ce_mO_2m−1 ⁺ + C₂H₄O (m = 2–6); (2) Ce_mO_2m ⁺ + CO → Ce_mO_2m−1 ⁺ + CO₂ (m = 4–6); and (3) Ce_mO_2m ⁺ + CH₄/C₂H₆ → Ce_mO_2mH⁺ + CH₃/C₂H₅ (m = 2–4). Density functional theory (DFT) calculations are performed to study reaction mechanisms of Ce₂O₄ ⁺ + X (X = CO, CH₄, C₂H₄, and C₂H₆). The calculated results are in good agreement with the experimental observations. The structural and bonding properties of Ce_mO_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 CeO₂ moiety, which can be considered as the active site in the cluster. To further understand the nature of the active sites in Ce_mO_2m ⁺ clusters, the fast flow reaction experiments are also carried out on zirconium oxide clusters Zr_mO_n ⁺ , because both Zr ([Kr]4d²5s² ) and Ce ([Xe]4f¹5d¹6s² ) 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_mO_2m ⁺ + C₂H₄ → Zr_mO_2m−1 ⁺ + C₂H₄O (m = 1–4) reported in the literature, hydrogen abstraction reactions Zr_mO_2m ⁺ + CH₄/C₂H₆ → Zr_mO_2mH⁺ + CH₃/C₂H₅ are also identified. The rate constants of CO oxidation as well as hydrogen abstraction by Ce_mO_2m ⁺ and Zr_mO_2m ⁺ are very different. The reactivity and selectivity of Ce_mO_2m ⁺versus Zr_mO_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_mO_2m ⁺ clusters is unique and may be considered in the use and design of cerium oxide based catalysts.

We introduce chemical structures and reactivity of oxygen-centred radicals (O⁻˙) over transition metal oxide (TMO) clusters based on mass spectrometric and density functional theory studies. Two main issues will be discussed: (1) the compositions of TMO clusters that have the bonding characteristics of (or contain) the O⁻˙ radicals; and (2) the dependences (cluster structures, sizes, charge states, metal types, etc.) of the chemical reactivity and selectivity for the O⁻˙ radicals over TMO clusters. One of the goals of cluster chemistry is to understand the elementary reactions involved with complex heterogeneous catalysis. The study of the O⁻˙ containing TMO clusters permits rather detailed descriptions for how mono-nuclear oxygen-centred radicals may exist and react with small molecules over TMO based catalysts.