Active sites of stoichiometric cerium oxide cations (CemO2m+) probed by reactions with carbon monoxide and small hydrocarbon molecules

Abstract

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.