Methane Activation and Catalytic Ethylene Formation on Free Au2+

Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars' atmosphere

We report gas-phase reactions of free iron-oxide clusters, FenOm+, and their Ar adducts with methane in the context of chemical processes in Mars' atmosphere. Methane activation was observed to produce FenOmCH2+/FenOmCD2+ and FenOmC+, where the reactivity exhibited size and composition dependence. For example, the rate coefficients of methane activation for Fe3O+ and Fe4O+ were estimated to be 1 × 10-13 and 3 × 10-13 cm3 s-1, respectively. Based on these reaction rate coefficients, the presence of iron-oxide clusters/particles with a density as low as 107 cm-3 in Mars' atmosphere would explain the rapid loss of methane observed recently by the Curiosity rover.

Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

Ligand effect on Ru-centered species toward methane activation

Ligands have been known to profoundly affect the chemical transformations of methane, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we demonstrate that the conversion of methane can be regulated by Ru centered cations with a series of ligands (C, CH, CNH, CHCNH). Gas-phase experiments complemented by theoretical dynamic analysis were performed to explore the essences and principles governing the ligand effect. In contrast to the inert Ru+, [RuC]+, and [RuCNH]+ toward CH4, the dehydrogenation dominates the reaction of ligand-regulated systems [RuCH]+/CH4 and [RuCHCNH]+/CH4. In active cases, CH acts as active sites, and regulates the activation of CH4 assisted by the "seemingly inert" CNH ligand.

Dicoordinate Au(I)-Ethylene Complexes as Hydroamination Catalysts

A series of gold(I)-ethylene π-complexes containing a family of bulky phosphine ligands has been prepared. The use of these sterically congested ligands is crucial to stabilize the gold(I)-ethylene bond and prevent decomposition, boosting up their catalytic performance in the highly underexplored hydroamination of ethylene. The precatalysts bearing the most sterically demanding phosphines showed the best results reaching full conversion to the hydroaminated products under notably mild conditions (1 bar of ethylene pressure at 60 °C). Kinetic analysis together with density functional theory calculations revealed that the assistance of a second molecule of the nucleophile as a proton shuttle is preferred even when using an extremely congested cavity-shaped Au(I) complex. In addition, we have measured a strong primary kinetic isotopic effect that is consistent with the involvement of X-H bond-breaking events in the protodeauration turnover-limiting step.

Gas-Phase Synthesis of Metal Olefins: Plasma-Assisted Methane Dehydrogenation and C═C Bond Formation

Methane dehydrogenation and C-C coupling under mild conditions are very important but challenging in chemistry. Utilizing a customized time of flight mass spectrometer combined with a magnetron sputtering (MagS) cluster source, here, we have conducted a study on the reactions of methane with small silver and copper clusters simply by introducing methane in argon as the working gas for sputtering. Interestingly, a series of [M(C_nH_2n)]⁺ (M = Cu and Ag; n = 2-12) clusters were observed, indicating high-efficiency methane dehydrogenation in such a plasma-assisted chamber system. Density functional theory calculations find the lowest energy structures of the [M(C_nH_2n)]⁺ series pertaining to olefins indicative of both C-H bond activation of methane and C-C bond coupling. We analyzed the interactions involved in the [Cu(C_nH_2n)]⁺ and [Ag(C_nH_2n)]⁺ (n = 1-6) clusters and demonstrated the reaction coordinates for the "Cu⁺ + CH₄" and "Ag⁺ + CH₄." It is illustrated that the presence of a second methane molecule enables us to reduce the necessary energy of dehydrogenation, which concurs with the experimental observation of an absence of the metal carbine products Cu⁺CH₂ and Ag⁺CH₂, which are short-lived. Also, it is elucidated that the higher-lying excitation states of Cu⁺ and Ag⁺ ions enable more favorable dehydrogenation process and C═C bond formation, shedding light on the plasma assistance of the essence.

Advancing Inorganic Coordination Chemistry by Spectroscopy of Isolated Molecules: Methods and Applications

A unique feature of the work carried out in the Collaborative Research Center 3MET continues to be its emphasis on innovative, advanced experimental methods which hyphenate mass-selection with further analytical tools such as laser spectroscopy for the study of isolated molecular ions. This allows to probe the intrinsic properties of the species of interest free of perturbing solvent or matrix effects. This review explains these methods and uses examples from past and ongoing 3MET studies of specific classes of multicenter metal complexes to illustrate how coordination chemistry can be advanced by applying them. As a corollary, we will show how the challenges involved in providing well-defined, for example monoisomeric, samples of the molecular ions have helped to further improve the methods themselves thus also making them applicable to many other areas of chemistry.

A dicoordinate gold(I)-ethylene complex

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold. Besides, the bonding situation of this species has been investigated by means of state-of-the-art Density Functional Theory (DFT) calculations indicating that π-backdonation plays a minor role compared with tricoordinate analogues.

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold.

Room-Temperature Methane Activation Mediated by Free Tantalum Cluster Cations: Size-by-Size Reactivity

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spin-density-functional-theory calculations. Ta_n⁺ clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1-4) dehydrogenate CH₄ and concurrently eliminate H₂, while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta⁺ cation, which dehydrogenates consecutively up to four CH₄ molecules and is predicted theoretically to promote formation of a [Ta(CH₂-CH₂-CH₂)(CH₂)]⁺ product, exhibiting C-C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

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.

Gold(I) ethylene complexes supported by electron-rich scorpionates

Ethylene complexes of gold(i) have been stabilized by electron-rich, κ2-bound tris(pyrazolyl)borate ligands. Large up-field shifts of olefinic carbon NMR resonances and relatively long C[double bond, length as m-dash]C distances of gold bound ethylene are indicative of significant Au(i) → ethylene π-backbonding relative to the analog supported by a weakly donating ligand, consistent with the computational data.

Consecutive methane activation mediated by single metal boride cluster anions NbB4. Phys Chem Chem Phys 2021;23:12592-12599. [PMID: 34047332 DOI: 10.1039/d1cp01418h]

Cleavage of all C-H bonds in two methane molecules by gas-phase cluster ions at room temperature is a challenging task. Herein, mass spectrometry and quantum chemical calculations have been used to identify one single metal boride cluster anions NbB4- that can activate eight C-H bonds in two methane molecules and release one H2 molecule each time under thermal collision conditions. In these consecutive reactions, the loaded Nb atoms and the support B4 units play different roles but act synergistically to activate CH4, which is responsible for the interesting reactivity of NbB4-. Moreover, there are some mechanistic differences in these two reactions, including the mechanisms for the first C-H bond activation steps, dihydrogen desorption sites, and major electron donors. This study shows that non-noble metal boride species are reactive enough to facilitate thermal C-H bond cleavages, and boron-based materials may be one kind of potential support material facilitating surface hydrogen transport.

The Optical Spectrum of Au2. Angew Chem Int Ed Engl 2020;59:21403-21408. [PMID: 32888257 PMCID: PMC7756737 DOI: 10.1002/anie.202011337]

The electronic structure of the Au2 + cation is essential for understanding its catalytic activity. We present the optical spectrum of mass-selected Au2 + measured via photodissociation spectroscopy. Two vibrationally resolved band systems are observed in the 290-450 nm range (at ca. 440 and ca. 325 nm), which both exhibit rather irregular structure indicative of strong vibronic and spin-orbit coupling. The experimental spectra are compared to high-level quantum-chemical calculations at the CASSCF-MRCI level including spin-orbit coupling. The results demonstrate that the understanding of the electronic structure of this simple, seemingly H2 + -like diatomic molecular ion strictly requires multireference and relativistic treatment including spin-orbit effects. The calculations reveal that multiple electronic states contribute to each respective band system. It is shown that popular DFT methods completely fail to describe the complex vibronic pattern of this fundamental diatomic cation.

Photoassisted Selective Steam and Dry Reforming of Methane to Syngas Catalyzed by Rhodium-Vanadium Bimetallic Oxide Cluster Anions at Room Temperature

Photoassisted steam reforming and dry (CO₂ ) reforming of methane (SRM and DRM) at room temperature with high syngas selectivity have been achieved in the gas-phase catalysis for the first time. The catalysts used are bimetallic rhodium-vanadium oxide cluster anions of Rh₂ VO_1-3 ^- . Both the oxidation of methane and reduction of H₂ O/CO₂ can take place efficiently in the dark while the pivotal step to govern syngas selectivity is photo-excitation of the reaction intermediates Rh₂ VO_2,3 CH₂ ^- to specific electronically excited states that can selectively produce CO and H₂ . Electronic excitation over Rh₂ VO_2,3 CH₂ ^- to control the syngas selectivity is further confirmed from the comparison with the thermal excitation of Rh₂ VO_2,3 CH₂ ^- , which leads to diversity of products. The atomic-level mechanism obtained from the well-controlled cluster reactions provides insight into the process of selective syngas production from the photocatalytic SRM and DRM reactions over supported metal oxide catalysts.

Reaction mechanism of the Me3AuPMe3-H2 plasma-enhanced ALD process

The reaction mechanism of the recently reported Me₃AuPMe₃-H₂ plasma gold ALD process was investigated using in situ characterization techniques in a pump-type ALD system. In situ RAIRS and in vacuo XPS measurements confirm that the CH₃ and PMe₃ ligands remain on the gold surface after chemisorption of the precursor, causing self-limiting adsorption. Remaining surface groups are removed by the H₂ plasma in the form of CH₄ and likely as PH_xMe_y groups, allowing chemisorption of new precursor molecules during the next exposure. The decomposition behaviour of the Me₃AuPMe₃ precursor on a Au surface is also presented and linked to the stability of the precursor ligands that govern the self-limiting growth during ALD. Desorption of the CH₃ ligands occurs at all substrate temperatures during evacuation to high vacuum, occurring faster at higher temperatures. The PMe₃ ligand is found to be less stable on a gold surface at higher substrate temperatures and is accompanied by an increase in precusor decomposition on a gold surface, indicating that the temperature dependent stability of the precursor ligands is an important factor to ensure self-limiting precursor adsorption during ALD. Remarkably, precursor decomposition does not occur on a SiO₂ surface, in situ transmission absorption infrared experiments indicate that nucleation on a SiO₂ surface occurs on Si-OH groups. Finally, we comment on the use of different co-reactants during PE-ALD of Au and we report on different PE-ALD growth with the reported O₂ plasma and H₂O process in pump-type versus flow-type ALD systems.

Methane activation by heteronuclear diatomic AuRh+ cation: comparison with homonuclear Au2+ and Rh2. Phys Chem Chem Phys 2020;22:6231-6238. [PMID: 32129335 DOI: 10.1039/c9cp05699h]

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH₄ with Au₂⁺, AuRh⁺ and Rh₂⁺ cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au₂⁺ has the lowest reactivity, and it can activate methane but only produce H-Au₂-CH₃⁺ without H₂ release; Rh₂⁺ has the highest reactivity, and it can produce both carbene-type Rh₂-CH₂⁺ and carbyne-type H-Rh₂-CH⁺ with H₂ release; AuRh⁺ also has high reactivity to produce only AuRh-CH₂⁺ with H₂, avoiding the excessive dehydrogenation of CH₄. Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals.

Catalytic Non-Oxidative Coupling of Methane on Ta8O2

Mass-selected Ta₈O₂⁺ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H₂, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C-C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

Theoretical study of methane adsorption and C─H bond activation over Fe-embedded graphene: Effect of external electric field

The effect of an external electric field (EF) on the methane adsorption and its activation on iron-embedded graphene (Fe-GPs) are investigated by using the M06-L density functional method. The EF is applied in the perpendicular direction to the graphene in the range of -0.015 to +0.015 a.u. with the interval of 0.005 a.u. The effects of EF on the adsorption, transition state and product complexes of the methane activation reaction are revealed. The binding energies of methane on Fe site in Fe-GPs are increased from -12.9 to -15.3, -18.1 and -21.5 kcal/mol for the negative EF of -0.005, -0.010 and -0.015, respectively. By applying positive EF, the activation barriers for methane activation are reduced in range of 3-8 kcal/mol (around 12-31%) and the reaction energies are more exothermic. The positive EF kinetically favors the reaction compared to the system without EF. The adsorption and activation of methane on Fe-GPs can be easily tuned by adjusting the external electric field for various applications. © 2019 Wiley Periodicals, Inc.

Infrared Spectroscopy of Gold Carbene Cation (AuCH2+): Covalent or Dative Bonding?

The present work explores the structure of the gold carbene cation, AuCH₂⁺, using infrared multiple photon dissociation action spectroscopy and density functional theory (DFT). Unlike several other 5d transition-metal cations (M⁺ = Ta⁺, W⁺, Os⁺, Ir⁺, and Pt⁺) that react with methane by dehydrogenation to form MCH₂⁺ species, gold cations are unreactive with methane at thermal energies. Instead, the metal carbene is formed by reacting atomic gold cations formed in a laser ablation source with ethylene oxide (cC₂H₄O) pulsed into a reaction channel downstream. The resulting [Au,C,2H]⁺ product photofragmented by loss of H₂ as induced by radiation provided by the free-electron laser for intracavity experiments in the 300-1800 cm^-1 range. Comparison of the experimental spectrum, obtained by monitoring the appearance of AuC⁺, and DFT calculated spectra leads to the identification of the ground-state carbene, AuCH₂⁺ (¹A₁), as the species formed, as previously postulated theoretically. Unlike the covalent double bonds formed by the lighter, open-shell 5d transition metals, the closed-shell Au⁺ (¹S, 5d¹⁰) atom binds to methylene by donation of a pair of electrons from CH₂(¹A₁) into the empty 6s orbital of gold coupled with π back-bonding, i.e., dative bonding, as explored computationally. Contributions to the AuC⁺ appearance spectrum from larger complexes are also considered, and H₂CAu⁺(c-C₂H₄O) seems likely to contribute one band observed.

Selective Activation of the C-H Bond in Methane by Single Platinum Atomic Anions

Mass spectrometric analysis of the anionic products of interaction between platinum atomic anions, Pt^- , and methane, CH₄ and CD₄ , in a collision cell shows the preferred generation of [PtCH₄ ]^- and [PtCD₄ ]^- complexes and a low tendency toward dehydrogenation. [PtCH₄ ]^- is shown to be H-Pt-CH₃ ^- by a synergy between anion photoelectron spectroscopy and quantum chemical calculations, implying the rupture of a single C-H bond. The calculated reaction pathway accounts for the observed selective activation of methane by Pt^- . This study presents the first example of methane activation by a single atomic anion.

Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations

The stability patterns of single silver, platinum, and palladium atom doped gold cluster cations, MAuN−1+ (M = Ag, Pt, Pd; N = 3–6), are investigated by a combination of photofragmentation experiments and density functional theory calculations. The mass spectra of the photofragmented clusters reveal an odd-even pattern in the abundances of AgAuN−1+, with local maxima for clusters containing an even number of valence electrons, similarly to pure AuN+. The odd-even pattern, however, disappears upon Pt and Pd doping. Computed dissociation energies agree well with the experimental findings for the different doped clusters. The effect of Ag, Pt, and Pd doping is discussed on the basis of an analysis of the density of states of the N = 3–5 clusters. Whereas Ag delocalizes its 5s valence electron in all sizes, this process is size-specific for Pt and Pd.

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.

Au2+ cannot catalyze conversion of methane to ethene at low temperature

The previously reported conversion of methane to ethene catalyzed by Au₂⁺ at thermal energies is investigated through a combination of experiment and theory. The conversion is found not to occur, in-line with well-established thermodynamics.

Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping

The reactivity of small metallic clusters, nanoparticles composed of a countable number of atoms (typically up to ∼100 atoms), has attracted much attention due to the fascinating properties these objects possess toward a variety of molecules. Cluster reactivity often is significantly different from the homologous bulk, with gold as prototypical example. Bulk gold is the noblest of all metals, whereas small gold clusters react with carbon monoxide, molecular oxygen, and hydrocarbons, among others. Furthermore, cluster reactivity is strongly size and composition dependent, allowing a wide range of tuning possibilities. The study of cluster reactivity usually follows two routes of investigation. In the first, research aims for fundamental understanding of mechanisms, mainly driven by curiosity. One consequence of the inherent small size of a cluster is that atoms can arrange themselves very differently from the crystallographic structure of the homologous bulk. In addition, quantum confinement effects dominate the electronic structure of a cluster with atom-like electronic shells instead of the electronic bands in bulk. These features result in a very rich and size-dependent interaction of a cluster with small molecules, governed by a fine interplay between the geometry and the electronic structure of the system. An alternative research approach uses the investigation of chemical reactions of isolated small clusters in the gas phase as model systems for the reactions taking place in more complex systems. This offers several advantages compared to more conventional methods and techniques used to study such complex systems. First, clusters can be produced under well-defined conditions, with control over size, composition, and charge state. Second, clusters in the gas phase solely interact with the molecule(s) chosen by the researcher, since contaminations are limited by the high vacuum conditions of the experiments. Third, due to the small number of atoms involved, detailed quantum chemical calculations can be performed on the systems under investigation. Thus, even though gas phase clusters differ significantly in size and in environmental conditions from those encountered, for example, in industrial catalysis, they can be used to unravel the complicated nature of a metal-molecule chemical bonding process. In this Account, both routes of investigation are discussed. The nature of the interaction between small gas phase clusters with diverse molecules is described, stressing the broader relevance of these studies. Particular emphasis is given to the effect of heteroatom doping. By adding a different element to a cluster, its geometric and electronic structure is modified, thereby altering its reactivity. Specifically, the effect of varying size and composition of doped gold, platinum, and aluminum clusters on their reactivity toward diverse molecules, relevant for catalytic applications, is discussed. Most studies presented here combine experiments based on mass spectrometric techniques with density functional theory calculations, allowing a deep understanding of the reaction mechanisms at a molecular level.

Methane Activation by Gas Phase Atomic Clusters

The increasing supply of natural gas has created a strong demand for developing efficient catalytic processes to upgrade methane, the most stable alkane molecule, into value-added chemicals. Currently, methane conversion in laboratory and industry is mostly performed under high-temperature conditions. A lot of effort has been devoted to exploring chemical entities that are able to activate the C-H bond of methane at lower temperatures, preferably room temperature. Gas phase atomic clusters with limited numbers of atoms are ideal models of active sites on heterogeneous catalysts. The cluster systems are being actively studied to activate methane under room-temperature conditions. State-of-the-art mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations have been combined in our laboratory to reveal the molecular-level mechanisms of methane activation by atomic clusters. In this Account, we summarize our recent progress on thermal methane activation by metal oxide clusters doped with noble-metal atoms (Au, Pt, and Rh) as well as by oxygen-free species including carbides and borides of base metals (V, Ta, Mo, and Fe). In contrast to the generations of CH₃^• free radicals in many of the previously reported cluster reactions with methane, the generations of stable products such as formaldehyde, acetylene, and syngas as well as closed-shell species AuCH₃ and B₃CH₃ have been identified for the cluster reaction systems herein. Besides the well recognized mechanisms of methane activation by the O^-• radicals through hydrogen atom abstraction and by metal atoms through oxidative addition, the new mechanisms of synergistic methane activation by Lewis acid-base pairs (such as Au^δ+-O^δ- and B^δ+-B^δ-) and by dinuclear metal centers (such as Ta-Ta) have been recently revealed. In the reactions between methane and oxide clusters doped with noble-metal atoms, the oxide cluster "supports" can accept the H atoms and the CH _x species delivered through the noble-metal atoms and then transform methane into stable oxygenated compounds. The product selectivity (such as formaldehyde versus syngas) can be controlled by different noble-metal atoms (such as Pt versus Rh). The electronic structures of base metal centers can be engineered through carburization so that the low-spin states can be accessible to reduce the C-H bond of methane. Such active base metal centers in low-spin states resemble related noble-metal atoms in methane activation. The boron clusters (such as B₃ in VB₃⁺) can be polarized by the metal cations to form the Lewis acid-base pair B^δ+-B^δ- to cleave the C-H bond of methane very easily. These molecular-level mechanisms may well be operative in related heterogeneous catalysis and can be a fundamental basis to design efficient catalysts for activation and conversion of methane under mild conditions.

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.

Selective C-H Bond Cleavage in Methane by Small Gold Clusters

Methane represents the major constituent of natural gas. It is primarily used only as a source of energy by means of combustion, but could also serve as an abundant hydrocarbon feedstock for high quality chemicals. One of the major challenges in catalysis research nowadays is therefore the development of materials that selectively cleave one of the four C-H bonds of methane and thus make it amenable for further chemical conversion into valuable compounds. By employing infrared spectroscopy and first-principles calculations it is uncovered herein that the interaction of methane with small gold cluster cations leads to selective C-H bond dissociation and the formation of hydrido methyl complexes, H-Au_x⁺ -CH₃ . The distinctive selectivity offered by these gold clusters originates from a fine interplay between the closed-shell nature of the d states and relativistic effects in gold. Such fine balance in fundamental interactions could prove to be a tunable feature in the rational design of a catalyst.

Methane Activation by Tantalum Carbide Cluster Anions Ta2C4. J Phys Chem Lett 2017;8:605-610. [PMID: 28088857 DOI: 10.1021/acs.jpclett.6b02568]

Methane activation by transition metals is of fundamental interest and practical importance, as this process is extensively involved in the natural gas conversion to fuels and value-added chemicals. While single-metal centers have been well recognized as active sites for methane activation, the active center composed of two or more metal atoms is rarely addressed and the detailed reaction mechanism remains unclear. Here, by using state-of-the-art time-of-flight mass spectrometry, cryogenic anion photoelectron imaging spectroscopy, and quantum-chemical calculations, the cooperation of the two Ta atoms in a dinuclear carbide cluster Ta₂C₄^- for methane activation has been identified. The C-H bond activation takes place predominantly around one Ta atom in the initial stage of the reaction and the second Ta atom accepts the delivered H atom from the C-H bond cleavage. The well-resolved vibrational spectra of the cryogenically cooled anions agree well with theoretical simulations, allowing the clear characterization of the structure of Ta₂C₄^- cluster. The reactivity comparison between Ta₂C₄^- cluster and the carbon-less analogues (Ta₂C₃^- and Ta₂C₂^-) demonstrated that the cooperative effect of the two metal atoms can be well tuned by the carbon ligands in terms of methane activation and transformation.

Small gold clusters catalyzing the conversion of glycerol to epichlorohydrin

The conversion of glycerol to epichlorohydrin (GTE) is of great interest because the product is widely used in plastics, rubbers and adhesives, and also contributes to the disposal of the reactant glycerol, a major by-product in biodiesel production.

Jones oxidation of glycerol catalysed by small gold clusters

We present here a joint theoretical and experimental study on the oxidation reactivity of glycerol catalysed by chemically pure small Au clusters in the absence and presence of H₂O₂.

Ménage-à-trois: single-atom catalysis, mass spectrometry, and computational chemistry

Genuine, single-atom catalysis can be realized in the gas phase and probed by mass spectrometry combined with computational chemistry.

Thermal Methane Conversion to Syngas Mediated by Rh1-Doped Aluminum Oxide Cluster Cations RhAl3O4<sup/>

Laser ablation generated RhAl₃O₄⁺ heteronuclear metal oxide cluster cations have been mass-selected using a quadrupole mass filter and reacted with CH₄ or CD₄ in a linear ion trap reactor under thermal collision conditions. The reactions have been characterized by state-of-the-art mass spectrometry and quantum chemistry calculations. The RhAl₃O₄⁺ cluster can activate four C-H bonds of a methane molecule and convert methane to syngas, an important intermediate product in methane conversion to value-added chemicals. The Rh atom is the active site for activation of the C-H bonds of methane. The high electron-withdrawing capability of Rh atom is the driving force to promote the conversion of methane to syngas. The polarity of Rh oxidation state is changed from positive to negative after the reaction. This study has provided the first example of methane conversion to syngas by heteronuclear metal oxide clusters under thermal collision conditions. Furthermore, the molecular level origin has been revealed for the condensed-phase experimental observation that trace amounts of Rh can promote the participation of lattice oxygen of chemically very inert support (Al₂O₃) to oxidize methane to carbon monoxide.