Gold Atoms Supported on Gas-Phase Cerium Oxide Cluster Ions: Stable Stoichiometry and Reactivity with CO

Hydrogen Storage Mechanism by Gas-Phase Vanadium Cluster Cations Revealed by Thermal Desorption Spectrometry

The adsorption of hydrogen on gas-phase vanadium cluster cations, Vn+ (n = 3-14), at 300 K and desorption of hydrogen from hydride clusters, VnHm+, upon heating were observed experimentally by combined thermal desorption spectrometry and mass spectrometry analyses. The ratio m/n was approximately 1.3 for all n values at 300 K, which was reduced to approximately zero at 1000 K. For n = 4, stable cluster geometries of V4Hm+ (m = 0, 2, 4, and 6) were investigated by DFT calculations, revealing that V4 adopted a trigonal pyramidal structure and the H atoms adsorbed mainly on the μ2 bridge sites. The adsorption reaction pathway of one H2 molecule on V4+ was also investigated. The experimentally estimated desorption energies of the H2 molecules were consistent with their calculated binding energies. Among the observed hydride clusters, V6H8+ was found to be significantly thermally durable, probably because of its close-packed octahedral V6 core structure, with H atoms occupying all hollow sites.

Zooming in on the initial steps of catalytic NO reduction using metal clusters

The study of reactions relevant to heterogeneous catalysis on the surface of well-defined metal clusters with full control over the number of consituent atoms and elemental composition can lead to a detailed insight into the interactions between metal and reactants. We here review experimental and theoretical studies involving the adsorption of NO molecules on mostly rhodium-based clusters under near-thermal conditions in a molecular beam. We show how IR spectrosopic characterization can give information on the binding nature of NO to the clusters for at least the first three NO molecules. The complementary technique of thermal desorption spectrometry reveals at what temperatures multiple NO molecules on the cluster surface desorb or combine to form rhodium oxides followed by N₂ elimination. Variation of the cluster elemental composition can be a powerful method to identify how the propensity of the critical first step of NO dissociation can be increased. The testing of such concepts with atomic detail can be of great help in guiding the choices in rational catalyst design.

The study of reactions relevant to heterogeneous catalysis on metal clusters with full control over the number of constituent atoms and elemental composition can lead to a detailed insight into the interactions governing catalytic functionality.

Lanthanide Oxides: From Diatomics to High-Spin, Strongly Correlated Homo- and Heterometallic Clusters

Small lanthanide (Ln) oxide clusters present both experimental and theoretical challenges because of their partially filled, core-like 4f ⁿ orbitals, a feature that results in a plethora of close-lying and fundamentally similar electronic states. These clusters provide a bottom-up approach toward understanding the electronic structure of defective or doped bulk material but also can offer a challenge to the theorists to find a method robust enough to capture electronic structure patterns that emerge from within the 4f ⁿ (0 < n < 14) series. In this Feature Article, we explore the electronic structures of small lanthanide oxide clusters that deviate from bulk stoichiometry using anion photoelectron spectroscopy and supporting density functional theory calculations. We will describe the evolution of electronic structure with oxidation and how Ln_xO_y^- cluster reactivities can be correlated with specific Ln-local orbital occupancies. These strongly correlated systems offer additional insights into how interactions between electrons and electronically complex neutrals can lead to detachment transitions that lie outside of the sudden one-electron detachment approximation generally assumed in anion photoelectron spectroscopy. With a better understanding of how we can control nominally forbidden transitions to sample an array of spin states, we suggest that more in-depth studies on the magnetic states of these systems can be explored. Extending these studies to other Ln-based materials with hidden magnetic phases, along with sequentially ligated single molecule magnets, could advance current understanding of these systems.

Catalytic Co-Conversion of CH4 and CO2 Mediated by Rhodium-Titanium Oxide Anions RhTiO2. Angew Chem Int Ed Engl 2021;60:13788-13792. [PMID: 33890352 PMCID: PMC8251526 DOI: 10.1002/anie.202103808]

Catalytic co‐conversion of methane with carbon dioxide to produce syngas (2 H₂+2 CO) involves complicated elementary steps and almost all the elementary reactions are performed at the same high temperature conditions in practical thermocatalysis. Here, we demonstrate by mass spectrometric experiments that RhTiO₂⁻ promotes the co‐conversion of CH₄ and CO₂ to free 2 H₂+CO and an adsorbed CO (CO_ads) at room temperature; the only elementary step that requires the input of external energy is desorption of CO_ads from the RhTiO₂CO⁻ to reform RhTiO₂⁻. This study not only identifies a promising active species for dry (CO₂) reforming of methane to syngas, but also emphasizes the importance of temperature control over elementary steps in practical catalysis, which may significantly alleviate the carbon deposition originating from the pyrolysis of methane.

Rhodium chemistry: A gas phase cluster study

Due to the extraordinary catalytic activity in redox reactions, the noble metal, rhodium, has substantial industrial and laboratory applications in the production of value-added chemicals, synthesis of biomedicine, removal of automotive exhaust gas, and so on. The main drawback of rhodium catalysts is its high-cost, so it is of great importance to maximize the atomic efficiency of the precious metal by recognizing the structure-activity relationship of catalytically active sites and clarifying the root cause of the exceptional performance. This Perspective concerns the significant progress on the fundamental understanding of rhodium chemistry at a strictly molecular level by the joint experimental and computational study of the reactivity of isolated Rh-based gas phase clusters that can serve as ideal models for the active sites of condensed-phase catalysts. The substrates cover the important organic and inorganic molecules including CH₄, CO, NO, N₂, and H₂. The electronic origin for the reactivity evolution of bare Rh_x ^q clusters as a function of size is revealed. The doping effect and support effect as well as the synergistic effect among heteroatoms on the reactivity and product selectivity of Rh-containing species are discussed. The ingenious employment of diverse experimental techniques to assist the Rh₁- and Rh₂-doped clusters in catalyzing the challenging endothermic reactions is also emphasized. It turns out that the chemical behavior of Rh identified from the gas phase cluster study parallels the performance of condensed-phase rhodium catalysts. The mechanistic aspects derived from Rh-based cluster systems may provide new clues for the design of better performing rhodium catalysts including the single Rh atom catalysts.

A Facile N≡N Bond Cleavage by the Trinuclear Metal Center in Vanadium Carbide Cluster Anions V3C4. J Am Chem Soc 2020;142:10747-10754. [PMID: 32450693 DOI: 10.1021/jacs.0c02021]

Cleavage of the triple N≡N bond by metal clusters is of fundamental interest and practical importance in nitrogen fixation. Previous studies of N≡N bond cleavage by gas-phase metal clusters emphasized the importance of the dinuclear metal centers. Herein, the dissociative adsorption of N₂ and subsequent C-N coupling on trinuclear carbide cluster anions V₃C₄^- under thermal collision conditions have been characterized by employing mass spectrometry (collision induced dissociation), cryogenic photoelectron imaging spectroscopy, and quantum chemistry calculations. A theoretical analysis identified a crucial adsorption intermediate with N₂ bonded with the V₃ metal core in the end-on/side-on/side-on (ESS) mode, which most likely enables the facile cleavage of the N≡N bond. Such a vital N₂ coordination in the ESS mode is a result of symmetry-matched interactions between the occupied orbitals of the metal core and both of the two empty π* orbitals of N₂. Furthermore, carbon ligands also play a considerable role in enhancing the reactivity of the metal core toward N₂. This study strongly suggests a new mechanism of N≡N bond cleavage by gas-phase metal clusters.

Thermodynamics and Kinetics of Gas-Phase CO Oxidation on the Scandium Monoxide Carbonyl Complexes

The CO chemisorption onto the ScO⁺ cation was investigated using infrared photodissociation spectroscopy combined with density functional theory calculations. The spectra were recorded in the CO stretching vibrational region for the OSc(CO)_n⁺ (n = 4-6) complex series. Comparisons of the experimental spectra with the simulated ones have established the geometries and present strong evidence that all of the CO ligands are chemisorbed, which could not be readily oxidized by scandium monoxide core into CO₂. Complementary calculations demonstrate that, regardless of the thermodynamic feasibility, the CO oxidation on the scandium monoxide carbonyl complexes is kinetically unfavorable due to the significant barriers involved in the CO oxidation process. Nevertheless, the consecutive CO adsorption has a positive influence on the Sc-O bond activation.

Direct Conversion of Methane with Carbon Dioxide Mediated by RhVO3 - Cluster Anions

Direct conversion of methane with carbon dioxide to value-added chemicals is attractive but extremely challenging because of the thermodynamic stability and kinetic inertness of both molecules. Herein, the first dinuclear cluster species, RhVO₃ ^- , has been designed to mediate the co-conversion of CH₄ and CO₂ to oxygenated products, CH₃ OH and CH₂ O, in the temperature range of 393-600 K. The resulting cluster ions RhVO₃ CO^- after CH₃ OH formation can further desorb the [CO] unit to regenerate the RhVO₃ ^- cluster, leading to the successful establishment of a catalytic cycle for methanol production from CH₄ and CO₂ (CH₄ +CO₂ →CH₃ OH+CO). The exceptional activity of Rh-V dinuclear oxide cluster (RhVO₃ ^- ) identified herein provides a new mechanism for co-conversion of two very stable molecules CH₄ and CO₂ .

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.

Selective Generation of Free Hydrogen Atoms in the Reaction of Methane with Diatomic Gold Boride Cations

The thermal reaction of diatomic gold boride cation AuB+ with methane has been studied by using state-of-the-art mass spectrometry in conjunction with density functional theory calculations. The AuB+ ion can activate a methane molecule to produce exclusively the free hydrogen atom, an important intermediate in hydrocarbon transformation. This result is different from the reactivity of AuC+ and CuB+ counterparts with methane in previous studies. The AuC+ cation mainly transforms methane into ethylene. The CuB+ reaction system principally generates the free hydrogen atoms, but it also gives rise a portion of ethylene-like product H2B−CH2. The B atom of AuB+ is the active site to activate methane. The strong relativistic effect on gold plays an important role for the product selectivity. The mechanistic insights obtained from this study provide guidance for rational design of active sites with high product selectivity toward methane activation.

Coupling of Methane and Carbon Dioxide Mediated by Diatomic Copper Boride Cations

The use of CH₄ and CO₂ to produce value-added chemicals via direct C-C coupling is a challenging chemistry problem because of the inertness of these two molecules. Herein, mass spectrometric experiments and high-level quantum-chemical calculations have identified the first diatomic species (CuB⁺ ) that can couple CH₄ with CO₂ under thermal collision conditions to produce ketene (H₂ C=C=O), an important intermediate in synthetic chemistry. The order to feed the reactants (CH₄ and CO₂ ) is important and CH₄ should be firstly fed to produce the C₂ product. Molecular-level mechanisms including control of product selectivity have been revealed for coupling of CH₄ with CO₂ under mild conditions.

Thermal activation of methane by vanadium boride cluster cations VBn+ (n = 3–6)

A new type of active species, transition metal boride cluster cations [VB_n⁺ (n = 3–6)], has been experimentally identified to dehydrogenate methane under thermal collision conditions. The B₃ unit in VB₃⁺ cluster is polarized by the V⁺ cation to activate CH₄.