Transition metal oxide clusters with character of oxygen-centered radical: a DFT study

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.

Data pertaining to the catalytic capabilities of transition metal oxides for fuel cell applications

The burning of fossil fuels produces pollutants and has a negative effect on the environment; however, it is still the primary source of energy for much of the globe today. This is why there has been a surge in interest in studying how to generate energy in a more environmentally friendly and long-term fashion. The widespread use of fuel cell technology-which efficiently converts electrochemical energy to electrical energy while producing almost no carbon emissions-is a prime illustration of this effort. The oxygen reduction reaction (ORR), which is utilized for catalysis inside fuel cell membranes, is slow, and platinum (Pt) is expensive and unstable, which limits the efficiency and broad application of fuel cell technology. This work investigates nanomaterials made of titanium, cobalt, and tungsten oxides as potential inexpensive and active electrocatalysts. Nanomaterials made of cobalt, tungsten, and titanium oxides have become increasingly popular as potential materials with catalytic capabilities that are both inexpensive and effective, especially when compared to conventional platinum catalysts. When used as fuel cell catalysts, the bimetallic compositions of these transition metals and oxygen have been the subject of surprisingly little theoretical and experimental investigation. Crystallographic surfaces of CoWO4 (011), CoWO4 (100), CoWO4 (111), Co3WO8 (001), Co3WO8 (101), Co3WO8 (011), TiWO4 (100), TiWO4 (101), and TiWO4 (110) are the principal focus of this investigation into their catalytic capacities. The electronic characteristics of the structures were studied using Density Functional Theory (DFT) with CASTEP and DMol3, and oxygen adsorption on the different surface configurations was done using the Adsorption Locator module.

Experimental Reactivity of (MoO3)NO- (N = 1-21) Cluster Anions with C1-C4 Alkanes: A Simple Model to Predict the Reactivity with Methane

Metal oxide clusters with atomic oxygen radical anions are important model systems to study the mechanisms of activating and transforming very stable alkane molecules under ambient conditions. It is extremely challenging to characterize the activation and conversion of methane, the most stable alkane molecule, by metal oxide cluster anions due to the low reactivity of the anionic species. In this study, using a ship-lock type reactor that could be run at relatively high pressure conditions to provide a high number of collisions in ion-molecule reactions, the rate constants of the reactions between (MoO3)NO- (N = 1-21) cluster anions and the light alkanes (C1-C4) were measured under thermal collision conditions. The relationships among the reaction rates of different alkanes were obtained to establish a model to predict the low rate constants with methane from the high rate constants with C2-C4 alkanes. The model was tested by using available experimental results in literature. This study provides a new method to estimate the relatively low reactivity of atomic oxygen radical anions with methane on metal oxide clusters.

Exposing the Oxygen-Centered Radical Character of the Tetraoxido Ruthenium(VIII) Cation [RuO4 ]. Chemphyschem 2023;24:e202300390. [PMID: 37589334 DOI: 10.1002/cphc.202300390]

The tetraoxido ruthenium(VIII) radical cation, [RuO4 ]+ , should be a strong oxidizing agent, but has been difficult to produce and investigate so far. In our X-ray absorption spectroscopy study, in combination with quantum-chemical calculations, we show that [RuO4 ]+ , produced via oxidation of ruthenium cations by ozone in the gas phase, forms the oxygen-centered radical ground state. The oxygen-centered radical character of [RuO4 ]+ is identified by the chemical shift at the ruthenium M3 edge, indicative of ruthenium(VIII), and by the presence of a characteristic low-energy transition at the oxygen K edge, involving an oxygen-centered singly-occupied molecular orbital, which is suppressed when the oxygen-centered radical is quenched by hydrogenation of [RuO4 ]+ to the closed-shell [RuO4 H]+ ion. Hydrogen-atom abstraction from methane is calculated to be only slightly less exothermic for [RuO4 ]+ than for [OsO4 ]+ .

Size Effects in Gas-phase C-H Activation

The gas-phase clusters reaction permits addressing fundamental aspects of the challenges related to C-H activation. The size effect plays a key role in the activation processes as it may substantially affect both the reactivity and selectivity. In this paper, we reviewed the size effect related to the hydrocarbon oxidation by early transition metal oxides and main group metal oxides, methane activation mediated by late transition metals. Based on mass-spectrometry experiments in conjunction with quantum chemical calculations, mechanistic discussions were reviewed to present how and why the size greatly regulates the reactivity and product distribution.

Using anion photoelectron spectroscopy of cluster models to gain insights into mechanisms of catalyst-mediated H2 production from water

Metal oxide cluster models of catalyst materials offer a powerful platform for probing the molecular-scale features and interactions that govern catalysis. This perspective gives an overview of studies implementing the combination of anion photoelectron (PE) spectroscopy and density functional theory calculations toward exploring cluster models of metal oxides and metal-oxide supported Pt that catalytically drive the hydrogen evolution reaction (HER) or the water-gas shift reaction. The utility in the combination of these experimental and computational techniques lies in our ability to unambiguously determine electronic and molecular structures, which can then connect to results of reactivity studies. In particular, we focus on the activity of oxygen vacancies modeled by suboxide clusters, the critical mechanistic step of forming proximal metal hydride and hydroxide groups as a prerequisite for H2 production, and the structural features that lead to trapped dihydroxide groups. The pronounced asymmetric oxidation found in heterometallic group 6 oxides and near-neighbor group 5/group 6 results in higher activity toward water, while group 7/group 6 oxides form very specific stoichiometries that suggest facile regeneration. Studies on the trans-periodic combination of cerium oxide and platinum as a model for ceria supported Pt atoms and nanoparticles reveal striking negative charge accumulation by Pt, which, combined with the ionic conductivity of ceria, suggests a mechanism for the exceptionally high activity of this system towards the water-gas shift reaction.

Understanding structure-activity relation in VxOy clusters of varied stoichiometry and sizes through conceptual density functional approach

Computations have been performed on V_xO_y clusters (with x = 1-8, y = 1-21) to explore their structure, stability, and reactivity based on local and global reactivity descriptors defined within the formalism of density functional theory (DFT). The vertical and adiabatic ionization energies and electron affinities are in accordance with Franck-Condon principle and suggest that the V_xO_y clusters are more likely to be electron acceptors than donors. The structure and reactivity of V_xO_y clusters delicately depend on their oxygen content and environment. Distinct active sites have been identified for each cluster species on the basis of coordination, symmetry, and charge distribution. The propensity of all the reactive sites towards an approaching electrophile and/or nucleophile has been studied using local reactivity descriptor. In oxygen-poor clusters, the vanadium atoms are more prone to nucleophilic attack. With an increase in oxygen concentration, the coordination number of vanadium increases and reaches four-fold, the site for nucleophilic attack shifts to terminal oxygens. We conclude that of all the stoichiometries, the stable V_xO_y clusters have the (VO₃)_a(V₂O₅)_b formula unit_. The localization of positive charge density in cubic cage structure of V₈O₂₀ successfully traps halide ions (F^-, Cl^-, and Br^-). In view of increasing use of metal oxide clusters in heterogeneous catalysis, the understanding of structure-activity relationship in vanadium oxides' clusters provided in the current study is highly desirable.

Efficient Liberation of Ammonia from Thermal Reaction of ScNH+ Cations and Water

Ammonia synthesis by using water as a hydrogen source is a challenging task. Laser-ablation-generated ScNH⁺ cations have been mass-selected using a quadrupole mass filter and reacted with H₂O in a linear ion trap reactor under thermal collision conditions. Through mass spectrometry in conjunction with density functional theory calculations, we found that ammonia is released as the product in the reaction of ScNH⁺ with H₂O, and this reaction is with high efficiency and selectivity, and the rate constant for the reaction is (1.14 ± 0.23) × 10^-10 cm³ molecule^-1 s^-1, corresponding to the reaction efficiency of 15%. Metal imido complexes (*MNH) are one of the important intermediates in the currently reported NH₃ synthetic reactions. The gas-phase ScNH⁺ cation can be a simplified model of *MNH over catalysts of NH₃ synthesis, and the facile proton transfer mechanism obtained in this model system may offer fundamental mechanistic insights into how to design catalysts for ammonia production by using water as the hydrogen source under ambient conditions.

Oxygen radical character in group 11 oxygen fluorides

Transition metal complexes bearing terminal oxido ligands are quite common, yet group 11 terminal oxo complexes remain elusive. Here we show that excited coinage metal atoms M (M = Au, Ag, Cu) react with OF₂ to form hypofluorites FOMF and group 11 oxygen metal fluorides OMF₂, OAuF and OAgF. These compounds have been characterized by IR matrix-isolation spectroscopy in conjunction with state-of-the-art quantum-chemical calculations. The oxygen fluorides are formed by photolysis of the initially prepared hypofluorites. The linear molecules OAgF and OAuF have a ³Σ ⁻ ground state with a biradical character. Two unpaired electrons are located mainly at the oxygen ligand in antibonding O−M π* orbitals. For the ²B₂ ground state of the OM^IIIF₂ compounds only an O−M single bond arises and a significant spin-density contribution was found at the oxygen atom as well.

While transition metal complexes bearing terminal oxido ligands are common, those of group 11 elements have yet to be experimentally observed. Here, Riedel and colleagues synthesise molecular oxygen fluorides of copper, silver and gold, and show that the oxo ligands possess radical character.

Effect of hydrogenation on the structure and magnetic properties of an iron oxide cluster

Hydrogenation of an iron oxide particle influences the geometrical topology and total magnetic moment and invokes different superexchange mechanisms.

Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O^•). Whenever the radical is delocalized, e.g., in [MgO]_n^•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga₂O₃) to [MgO]₂^•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al₂O₂]^•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH₃ moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]₂^•+ by Al₂O₃ enables HAT and PCET to compete. Similarly, [ZnO]^•+ activates methane by PCET generating many products. Adding a CH₃CN ligand to form [(CH₃CN)ZnO]^•+ leads to a single HAT product. The CH₃CN dipole acts as an OEF that switches off PCET. [MC]⁺ cations (M = Au, Cu) act by different mechanisms, dictated by the M⁺-C bond covalence. For example, Cu⁺, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu⁺.

Thermal Methane Activation by the Metal-Free Cluster Cation [Si2 O4 ]

The thermal reaction of methane with the metal-free cluster cation [Si₂ O₄ ]^.+ has been examined by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. In addition to generating a methyl radical via hydrogen-atom abstraction, [Si₂ O₄ ]^.+ can selectively oxidize methane to formaldehyde. The mechanisms of these rather efficient reactions have been elucidated by high-level quantum-chemical calculations.

CO oxidation by the atomic oxygen on silver clusters: structurally dependent mechanisms generating free or chemically bonded CO2

Oxidation of CO by the atomic oxygen on Ag_nO⁻ (n = 1–8) forms free or chemically bonded CO₂.

Compositions and structures of niobium oxide cluster ions, NbmOn±, (m = 2–12), revealed by ion mobility mass spectrometry

The compositions and structures of niobium oxide cluster ions were studied and compared with vanadium oxide.

Thermal Methane Activation by [Si2 O5 ].+ and [Si2 O5 H2 ].+ : Reactivity Enhancement by Hydrogenation

The thermal reactions of methane with the oxygen-rich cluster cations [Si₂ O₅ ]^⋅+ and [Si₂ O₅ H₂ ]^⋅+ have been examined using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry in conjunction with state-of-the-art quantum chemical calculations. In contrast to the inertness of [Si₂ O₅ ]^.+ towards methane, the hydrogenated cluster [Si₂ O₅ H₂ ]^.+ brings about hydrogen-atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed.

Adsorption of a single gold or silver atom on vanadium oxide clusters

The single Au atom can be adsorbed on both V and O sites of vanadium oxide clusters with quite large binding energies, illustrating the stabilization of noble atoms in single-atom catalysts.

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.

Adsorption of O2 on anionic silver clusters: spins and electron binding energies dominate in the range up to nano sizes

Exploring the reactivity of metal clusters is an important task in cluster science, while only a few previous studies involve the reactions of nano-sized ones.

High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides

The reactions between methane and niobium oxide cluster cations were studied and compared to those employing vanadium oxides. Hydrogen atom abstraction (HAA) reactions were identified over stoichiometric (Nb2O5)N(+) clusters for N as large as 14 with a time-of-flight mass spectrometer. The reactivity of (Nb2O5)N(+) clusters decreases as the N increases, and it is higher than that of (V 2O5)N(+) for N ≥ 4. Theoretical studies were conducted on (Nb2O5)N(+) (N = 2-6) by density functional calculations. HAA reactions on these clusters are all favorable thermodynamically and kinetically. The difference of the reactivity with respect to the cluster size and metal type (Nb vs V) was attributed to thermodynamics, kinetics, the electron capture ability, and the distribution of the unpaired spin density. Nanosized Nb oxide clusters show higher HAA reactivity than V oxides, indicating that niobia may serve as promising catalysts for practical methane conversion.

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.