Reactivity of Atomic Oxygen Radical Anions Bound to Titania and Zirconia Nanoparticles in the Gas Phase: Low-Temperature Oxidation of Carbon Monoxide

Reactivity of Atomic Oxygen Radical Anions in Metal Oxide Clusters

Atomic oxygen radical anion (O⋅-) represents an important type of reactive centre that exists in both chemical and biological systems. Gas-phase atomic clusters can be studied under isolated and well controlled conditions. Studies of O⋅--containing clusters in the gas-phase provide a unique strategy to interpret the chemistry of O⋅- radicals at a strictly molecular level. This review summarizes the research progresses made since 2013 for the reactivity of O⋅- radicals in the atomically precise metal oxide clusters including negatively charged, nanosized, and neutral heteronuclear metal clusters benefitting from the development of advanced experimental techniques. New electronic and geometric factors to control the reactivity and product selectivity of O⋅- radicals under dark and photo-irradiation conditions have been revealed. The detailed mechanisms of O⋅- generation have been discussed for the reaction systems of nanosized and heteroatom-doped metal oxide clusters. The catalytic reactions mediated by the O⋅- radicals in metal clusters have also been successfully established and the microscopic mechanisms about the dynamic generation and depletion of O⋅- radicals have been clearly understood. The studies of O⋅- containing metal oxide clusters in the gas-phase provided new insights into the chemistry of reactive oxygen species in related condensed-phase systems.

Gas-phase reactions driven by polarized metal-metal bonding in atomic clusters

Multimetallic catalysts exhibit great potential in the activation and catalytic transformation of small molecules. The polarized metal-metal bonds have been gradually recognized to account for the reactivity of multimetallic catalysts due to the synergistic effect of different metal centers. Gas-phase reactions on atomic clusters that compositionally resemble the active sites on related condensed-phase catalysts provide a widely accepted strategy to clarify the nature of polarized metal-metal bonds and the mechanistic details of elementary steps involved in the catalysis driven by this unique chemical bonding. This perspective review concerns the progress in the fundamental understanding of industrially and environmentally important reactions that are closely related to the polarized metal-metal bonds in clusters at a strictly molecular level. The following topics have been summarized and discussed: (1) catalytic CO oxidation with O₂, H₂O, and NO as oxidants (2) and the activation of other inert molecules (e.g., CH₄, CO₂, and N₂) mediated with clusters featuring polarized metal-metal bonding. It turns out that the findings in the gas phase parallel the catalytic behaviors of condensed-phase catalysts and the knowledge can prove to be essential in inspiring future design of promising catalysts.

Photooxidation of Isoprene by Titanium Oxide Cluster Anions with Dimensions up to a Nanosize

Titania (TiO₂) nanoparticles are active photocatalysts, and isoprene (C₅H₈) is a biogenic volatile organic compound that contributes crucially to global particulate matter generation. Herein, the direct photooxidation of isoprene by titanium oxide cluster anions with dimensions up to a nanosize by both ultraviolet (UV) and visible (Vis) light excitations has been successfully identified through mass spectrometric experiments combined with quantum chemistry calculations. The potential role of "dry" titania in atmospheric isoprene oxidation has been revealed, and a clear picture of the photooxidation mechanism on titanium oxide nanoparticles has been provided explicitly at the molecular level. The adsorption of isoprene on the atomic oxygen radicals (O^•-) of titanium oxide clusters leads to the formation of the crucial interfacial state (IS) within the band gap of titanium oxides. This IS is demonstrated to be the significant factor in delivering the electron from the π orbital of C₅H₈ to the Ti_3d orbital in the photooxidation process (C₅H₈ + Ti⁴⁺-O^•- → C₅H₈O + Ti³⁺) and creating photoactivity in the Vis region. It is revealed that after the photogeneration of the O^•- radicals by UV excitation on the TiO₂ particle surface, the subsequent reactions can be induced by Vis excitation through the IS. This multicolor strategy in both the UV and Vis regions can enhance the efficiency of solar energy harvesting and improve the product yield of the photocatalysis on TiO₂ nanoparticles. New insights have been provided into both the atmospheric chemistry of isoprene and the photochemistry of TiO₂ nanoparticles.

A low-temperature oxyl transfer to carbon monoxide from the ZnII–oxyl site in a zeolite catalyst

We demonstrated that the Zn^II–oxyl bond specifically formed by the zeolite lattice ligation has the capability of transferring the oxyl to CO even at 150 K with the generation of a single Zn^I˙ species.

Oxidation of isoprene by titanium oxide cluster cations in the gas phase

The heterogeneous oxidation of isoprene (C5H8) by metal-oxide particles, such as the typical mineral aerosols TiO2, plays an important role in the isoprene atmospheric chemistry. However, the underlying mechanism of C5H8 oxidation remains elusive owing to the complexities of aerosol surfaces and reaction channels. Herein, we report the gas-phase reactions of TixOy+ (x = 1-7, y = 1-14) cations with isoprene by using mass spectrometry and density functional theory (DFT) calculations. Five types of reaction channels were observed: association, hydrogen atom transfer (HAT), C-C bond cleavage, combined oxygen atom transfer (OAT) and HAT and combined OAT and C-C bond cleavage. It is noteworthy that formaldehyde is known as the major oxidation product of isoprene/hydroxyl radicals in the atmosphere. In addition, CO has not been observed in the reactions of isoprene with gas-phase ions. Therefore, the reaction mechanisms of CH2O and CO generation observed in Ti2O5+/C5H8 and Ti4O8+/C5H8 systems were further investigated by DFT calculations, and the calculated results are in agreement with the experimental observations. In these two reactions, both Ti and O atoms can be the adsorption sites for C5H8. The reaction channels and mechanistic information gained in these gas-phase model reactions may offer fundamental insights relevant to the corresponding oxidation processes over titanium oxide aerosols in the atmosphere.

Ligand-Mediated Reactivity in CO Oxidation of Niobium-Nickel Monoxide Carbonyl Complexes: The Crucial Roles of the Multiple Adsorption of CO Molecules

The heteronuclear metal oxide complexes are of great significance in heterogeneous catalytic oxidation of CO. However, previous studies are mainly focused on the composition of metal oxide, charge state, the support and the active oxygen species, with little attention paid to adsorbed CO ligands. Herein, the ligand-mediated reactivity in CO oxidation of niobium-nickel monoxide carbonyl complexes has been successfully identified. The NbNiO(CO) _n^- ( n = 5-6) anions are determined to be O-bridged complexes. In contrast, the NbNiO(CO) _n^- ( n = 7-8) anions are characterized to be η²-CO₂-tagged complexes. The crucial roles of the multiply adsorbed CO molecules that can facilitate not only the competitive binding with bridging oxygen atom to the transition metal centers but also the electron accumulation of transition metal atoms have been discovered. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over heteronuclear metal oxide under CO-rich feed condition.

Size-Dependent Reactivity of Nano-Sized Neutral Manganese Oxide Clusters toward Ethylene

Neutral manganese oxide clusters with the general composition Mn_2 N O_3 N+x (N=2-22; x=-1, 0, 1) with dimensions up to a nanosize were prepared by laser ablation and reacted with C₂ H₄ in a fast flow reactor. The size-dependent reactivity of C₂ H₄ adsorption on these clusters was experimentally identified and the adsorption reactivity decreases generally with an increase of the cluster size. Density functional theory calculations were performed to study the geometrical and electronic structures of the Mn_2 N O_3 N (N=1-6) clusters. The calculated results indicated that the coordination number and the charge distribution of the metal centers are responsible for the experimentally observed size-dependent reactivity. The highly charged Mn atoms with low coordination are preferential to adsorb C₂ H₄ . In contrast, the neutral manganese oxide clusters are completely inert toward the saturated hydrocarbon molecule C₂ H₆ . This work provides new perspectives to design related materials in the separation of hydrocarbon molecules.

A nine-atom rhodium-aluminum oxide cluster oxidizes five carbon monoxide molecules

Noble metals can promote the direct participation of lattice oxygen of very stable oxide materials such as aluminum oxide, to oxidize reactant molecules, while the fundamental mechanism of noble metal catalysis is elusive. Here we report that a single atom of rhodium, a powerful noble metal catalyst, can promote the transfer of five oxygen atoms to oxidize carbon monoxide from a nine-atom rhodium-aluminum oxide cluster. This is a sharp improvement in the field of cluster science where the transfer of at most two oxygen atoms from a doped cluster is more commonly observed. Rhodium functions not only as the preferred trapping site to anchor and oxidize carbon monoxide by the oxygen atoms in direct connection with rhodium but also the primarily oxidative centre to accumulate the large amounts of electrons and the polarity of rhodium is ultimately transformed from positive to negative.

Facile fabrication of FeN nanoparticles/nitrogen-doped graphene core-shell hybrid and its use as a platform for NADH detection in human blood serum

Herein, we present a novel strategy for the synthesis of an iron nitride nanoparticles-encapsulated nitrogen-doped graphene (FeN NPs/NG) core-shell hierarchical nanostructure to boost the electrochemical performance in a highly sensitive, selective, reproducible, and stable sensing platform for nicotinamide adenine dinucleotide (NADH). This core-shell hierarchical nanostructure provides an excellent conductive network for effective charge transfer and avoids the agglomeration and restacking of NG sheets, which provides better access to the electrode material for NADH oxidation. The FeN NPs/NG core-shell hierarchical nanostructure demonstrates direct and mediatorless responses to NADH oxidation at a low potential. This material displays a high sensitivity of 0.028μA/μMcm(2), a wide linear range from 0.4 to 718μM, and a detection limit of 25nM with a fast response time of less than 3s. The interferences from common interferents, such as glucose, uric acid, dopamine, and ascorbic acid, are negligible. The fabricated sensor was further tested for the determination of NADH in human blood serum. The resulting high sensitivity, excellent selectivity, outstanding stability, and good reproducibility make the proposed FeN NPs/NG core-shell hierarchical nanostructure as a promising candidate for biomedical applications.

Methane activation by gold-doped titanium oxide cluster anions with closed-shell electronic structures

The reactivity of closed-shell gas phase cluster anions AuTi₃O₇^- and AuTi₃O₈^- with methane under thermal collision conditions was studied by mass spectrometric experiments and quantum chemical calculations. Methane activation was observed with the formation of AuCH₃ in both cases, while the formation of formaldehyde was also identified in the reaction system of AuTi₃O₈^-. The cooperative effect of the separated Au⁺ and O^2- ions on the clusters induces the cleavage of the first C-H bond of methane. Further activation of the second C-H bond by a peroxide ion O₂^2- leads to the formation of formaldehyde. This study shows that closed-shell species on metal oxides can be reactive enough to facilitate thermal H-CH₃ bond cleavage and the subsequent conversion.

Reactions of metal cluster anions with inorganic and organic molecules in the gas phase

Progress on the activation and transformation of important inorganic and organic molecules by negatively charged bare metal clusters as well as ligated systems with oxygen, carbon, and nitrogen, among others.

The reactivity of stoichiometric tungsten oxide clusters towards carbon monoxide: the effects of cluster sizes and charge states

Density functional theory (DFT) calculations are employed to investigate the reactivity of tungsten oxide clusters towards carbon monoxide. Extensive structural searches show that all the ground-state structures of (WO3)n(+) (n = 1-4) contain an oxygen radical center with a lengthened W-O bond which is highly active in the oxidation of carbon monoxide. Energy profiles are calculated to determine the reaction mechanisms and evaluate the effect of cluster sizes. The monomer WO3(+) has the highest reactivity among the stoichiometric clusters of different sizes (WO3)n(+) (n = 1-4). The reaction mechanisms for CO with mono-nuclear stoichiometric tungsten oxide clusters with different charges (WO3(-/0/+)) are also studied to clarify the influence of charge states. Our calculated results show that the ability to oxidize CO gets weaker from WO3(+) to WO3(-) as the negative charge accumulates progressively.

Catalytic oxidation of CO by N2O on neutral Y2MO5 (M = Y, Al) clusters: a density functional theory study

The full catalytic cycle of CO oxidation by N₂O on neutral Y₂MO₅ (M = Y, Al) clusters has been studied in the current work.

CO Oxidation Promoted by Gold Atoms Loosely Attached in AuFeO3(-) Cluster Anions

Time-of-flight mass spectrometry experiment shows that upon the interactions with carbon monoxide, the mass-selected AuFeO3(-) oxide cluster anions can evaporate neutral gold atoms in a hexapole collision cell and oxidize CO into CO2 in an ion trap reactor. The computational studies identify that the gold atom is loosely attached in the AuFeO3(-) cluster, and the different reaction channels can be attributed to different cluster velocities. The structure of the AuFeO3(-) cluster is very flexible, and the approach of CO induces significant geometrical and electronic structure changes of AuFeO3(-), which facilitates the exposure of the positively charged gold atom to trap and oxidize CO. The CO oxidation by the AuFeO3(-) cluster follows the Au-assisted Mars-van Krevelen mechanism, in which the direct participation of the surface lattice oxygen (O(2-)) is proposed.

Highly regioselective hydride transfer, oxidative dehydrogenation, and hydrogen-atom abstraction in the thermal gas-phase chemistry of [Zn(OH)]+/C3H8

The metal center of [Zn(OH)]⁺ serves as active site in the regiospecific hydride transfer to generate [i-C₃H₇]⁺ as major product in the reaction with C₃H₈. In the [Zn(OH)]⁺–C₃H₈ system, a high regioselectivity features the remarkable chemoselectivity.