Oxidation of composition-selected cerium oxide cluster cations by O2

Geometrical Structures of Gas-Phase Cerium Oxide Cluster Cations after Reaction with Nitric Oxide Studied by Ion Mobility Mass Spectrometry

Cerium oxide cluster cations were reacted with nitric oxide molecules and then measured by ion mobility mass spectrometry (IMMS). Ce_nO_2n+1N⁺ species appeared as products of the reaction Ce_nO_2n⁺ + NO → Ce_nO_2n+1N⁺, and their collision cross sections (CCSs) with helium were obtained by IMMS. The experimental CCSs of Ce_nO_2n+1N⁺ were 2-6 Ų larger than those of Ce_nO_2n⁺ for n = 4-10. Geometrical structures of Ce₄O₉N⁺ and Ce₅O₁₁N⁺ were assigned by comparing experimental CCSs with theoretically calculated CCSs of candidate structures. The suggested structures showed that the adsorbed NO molecule is oxidized by the Ce_nO_2n⁺ cluster into a nitrite (NO₂^-) or nitrate (NO₃^-). The Ce_nO_2n+1N⁺ species are regarded as intermediates of the NO oxidation reaction Ce_nO_2n⁺ + NO → Ce_nO_2n-1⁺ + NO₂, and therefore, the present results are helpful for understanding redox reactions involving gas-phase Ce_nO_2n⁺ cluster ions.

Reaction mechanism between small-sized Ce clusters and water molecules II: an ab initio investigation on Cen (n = 1-3) + mH2O (m = 2-6)

Possible reactions between the products of the three independent reactions involving a small Ce cluster and a single water molecule, Cen + H2O (n = 1-3), and an additional H2O molecule are systematically investigated. The ground-state isomers of the final products and the reaction pathways involving multiple water molecules are predicted. We find that under either ambient or UV-irradiation conditions, all the reactions can entail low energy barriers. In addition, the final products of the reaction between Cen and more than two H2O molecules are also predicted through an extensive structural search. The calculated reaction energies suggest that although small-sized Ce clusters can react with more than two water molecules, the reactions with one or two water molecules are dominant. The electronic structures of all the ground-state isomers and the corresponding oxidation states of Ce atoms in these isomers are computed and determined via the natural bond orbital (NBO) method. The results indicate that a single Ce atom and a Ce2 cluster can react with a maximum of four and six water molecules, respectively, while a Ce3 cluster can react with more than six water molecules. This comprehensive study offers an improved understanding of the mechanism underlying the reactions between a single Ce atom or a small Ce cluster and two or more H2O molecules. Knowledge obtained from this study can be helpful for the development of high-performance Ce-doped or Ce-based catalysts.

Reaction mechanism between small-sized Ce clusters and water molecules: an ab initio investigation on Cen + H2O

Reactions of small-sized cerium clusters Cen (n = 1-3) with a single water molecule are systematically investigated theoretically. The ground state structures of the Cen/H2O complex and the reaction pathways between Cen + H2O are predicted. Our results show the size-dependent reactivity of small-sized Ce clusters. The calculated reaction energies and reaction barriers indicate that the reactivity between Cen and water becomes higher with increasing cluster size. The predicted reaction pathways show that the single Ce atom and the Ce2 and Ce3 clusters can all easily react with H2O and dissociate the water molecule. Under UV-irradiation, the reaction of a Ce atom with a single H2O molecule may even release an H2 molecule. The reaction of either Ce2 or Ce3 with a single H2O molecule can fully dissociate the H2O into H and O atoms while it is bonded with the Ce cluster. The electronic configuration and oxidation states of the Ce atoms in the products and the higher occupied molecular orbitals are analyzed by using the natural bond orbital (NBO) analysis method, from which the high reactivity between the reaction products of Cen + H2O and an additional H2O molecule is predicted. Our results offer deeper molecular insights into the chemical reactivity of Ce, which could be helpful for developing more efficient Ce-doped or Ce-based catalysts.

Photodissociation of Cerium Oxide Nanocluster Cations

Cerium oxide cluster cations, CexOy(+), are produced via laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. The mass spectrum displays a strongly preferred oxide stoichiometry for each cluster with a specific number of metal atoms x, with x ≤ y. Specifically, the most prominent clusters correspond to the formula CeO(CeO2)n(+). The cluster cations are mass selected and photodissociated with a Nd:YAG laser at either 532 or 355 nm. The prominent clusters dissociate to produce smaller species also having a similar CeO(CeO2)n(+) formula, always with apparent leaving groups of (CeO2). The production of CeO(CeO2)n(+) from the dissociation of many cluster sizes establishes the relative stability of these clusters. Furthermore, the consistent loss of neutral CeO2 shows that the smallest neutral clusters adopt the same oxidation state (IV) as the most common form of bulk cerium oxide. Clusters with higher oxygen content than the CeO(CeO2)n(+) masses are present with much lower abundance. These species dissociate by the loss of O2, leaving surviving clusters with the CeO(CeO2)n(+) formula. Density functional theory calculations on these clusters suggest structures composed of stable CeO(CeO2)n(+) cores with excess oxygen bound to the surface as a superoxide unit (O2(-)).