Thermal Methane Conversion to Formaldehyde Promoted by Single Platinum Atoms in PtAl2O4−Cluster Anions

Analogy of C-Pt and C-O Chemical Bonding in the Diatomic CPt and CO

The conjugate-pair molecules of CO and CPt provide a prototype of the autogenic isolobal relationship between the O and Pt atoms that can rationalize the structure and reactivity trends of platinum carbides. Herein, the photoelectron detachment at 532 nm has been recorded for the gas-phase CPt- by using the photoelectron velocity-map imaging spectroscopy. The vibrationally resolved ground-state transition reveals a wealth of information concerning the electronic ground states of CPt0/-1. Although the triple bonding characters in both CPt and CO have fortified the autogenic isolobal relationship between O and Pt, further detailed bonding comparisons have revealed the subtle nuance in the triple bonding. The triple bonds of CO comprise one σ donor bond from the O atom to the C atom and two degenerate π electron-sharing bonds. In contrast, the triple bonds of CPt involve three dative bonds, including one σ donor bond from the C atom to the Pt atom and two degenerate π back-donation bonds from the Pt atom to the C atom.

Dinitrogen Activation in the Gas Phase: Spectroscopic Characterization of C-N Coupling in the V3 C+ +N2 Reaction

We report on cluster-mediated C-N bond formation in the gas phase using N₂ as a nitrogen source. The V₃ C⁺ +N₂ reaction is studied by a combination of ion-trap mass spectrometry with infrared photodissociation (IRPD) spectroscopy and complemented by electronic structure calculations. The proposed reaction mechanism is spectroscopically validated by identifying the structures of the reactant and product ions. V₃ C⁺ exhibits a pyramidal structure of C₁ -symmetry. N₂ activation is initiated by adsorption in an end-on fashion at a vanadium site, followed by spontaneous cleavage of the N≡N triple bond and subsequent C-N coupling. The IRPD spectrum of the metal nitride product [NV₃ (C=N)]⁺ exhibits characteristic C=N double bond (1530 cm^-1 ) and V-N single bond (770, 541 and 522 cm^-1 ) stretching bands.

Gas-Phase Mechanism of O.- /Ni2+ -Mediated Methane Conversion to Formaldehyde

The gas-phase reaction of NiAl2 O4 + with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2 O4 H+ and NiAl2 O3 H2 + , are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl2 O4 + contains Ni in the +II oxidation state and features a terminal Al-O.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH3 ⋅) or formaldehyde (CH2 O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.- site, followed by either CH3 ⋅ loss or CH3 ⋅ migration to an O2- site next to the Ni2+ center. The CH3 ⋅ attaches as CH3 + to O2- and its unpaired electron is transferred to the Ni-center reducing it to Ni+ . The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.

Methane Activation by (MoO3 )5 O- Cluster Anions: The Importance of Orbital Orientation

The reactivity of the molybdenum oxide cluster anion (MoO₃ )₅ O^- , bearing an unpaired electron at a bridging oxygen atom (O_b ^.- ), towards methane under thermal collision conditions has been studied by mass spectrometry and density functional theory calculations. This reaction follows the mechanism of hydrogen atom transfer (HAT) and is facilitated by the O_b ^.- radical center. The reactivity of (MoO₃ )₅ O^- can be traced back to the appropriate orientation of the lowest unoccupied molecular orbitals (LUMO) that is essentially the 2p orbital of the O_b ^.- atom. This study not only makes up the blank of thermal methane activation by the O_b ^.- radical on negatively charged clusters but also yields new insights into methane activation by the atomic oxygen radical anions.

Conversion of carbon dioxide to a novel molecule NCNBO- mediated by NbBN2- anions at room temperature

The activation of carbon dioxide (CO₂) mediated by NbBN₂^- cluster anions under the conditions of thermal collision has been investigated by time-of-flight mass spectrometry combined with density functional theory calculations. Two CO double bonds in the CO₂ molecule are completely broken and two C-N bonds are further generated to form the novel molecule NCNBO^-. To the best of our knowledge, this new molecule is synthesized and reported for the first time. In addition, one oxygen atom transfer channel produces another product, NbBN₂O^-. Both of the Nb and B atoms in NbBN₂^- donate electrons to reduce CO₂, and the carbon atom originating from CO₂ serves as an electron reservoir. The reaction of NbB^- with N₂ was also investigated theoretically, and the formation of NbBN₂^- from this reaction is thermodynamically and kinetically quite favorable, indicating that NCNBO^- might be produced from the coupling of N₂ and CO₂ mediated by NbB^- anions.

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.

Selective C-O Coupling Hidden in the Thermal Reaction of [Al2 CuO5 ]+ with Methane

The thermal gas-phase reaction of [Al₂ CuO₅ ]⁺ with methane has been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. The generation of atomic [Cu]⁺ from the [Al₂ CuO₅ ]⁺ /CH₄ couple corresponds to the only reaction channel. Labeling experiments and computational studies strongly suggest that methane activation is indeed involved in the production of [Cu]⁺ , and generation of CH₂ O prevails. Mechanistic aspects and the associated doping effects are discussed.

Fe2 O+ Cation Mediated Propane Oxidation by Dioxygen in the Gas Phase

The mass-selected Fe₂ O⁺ cation mediated propane oxidation by O₂ was investigated by mass spectrometry and density functional theory calculations. In the reaction of Fe₂ O⁺ with C₃ H₈ , H₂ was liberated by C-H bond activation to give Fe₂ OC₃ H₆⁺ . Interestingly, when a mixture of C₃ H₈ /O₂ was introduced into the reactor, an intense signal that corresponded to the Fe₂ O₂⁺ cation was present; the experiments indicated that O₂ was activated in its reaction with Fe₂ O(C₃ H₆ )⁺ to give Fe₂ O₂⁺ and C₃ H₆ O (acetone or propanal). A Langmuir-Hinshelwood-like mechanism was adopted in the propane oxidation reaction by O₂ on gas-phase Fe₂ O⁺ cations. In comparison with the absence of Fe₂ O₂⁺ in the reaction of Fe₂ O⁺ with O₂ , the ligand effect of C₃ H₆ on Fe₂ OC₃ H₆⁺ is important in the oxygen activation reaction. The theoretical results are consistent with the experimental observations. The propane oxidation by O₂ in the presence of Fe₂ O⁺ might be applied as a model for alkane and O₂ activations over iron oxide catalysts, and the mechanisms and kinetic data are useful for understanding corresponding heterogeneous reactions.

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.

Electronic Origin of the Competitive Mechanisms in the Thermal Activation of Methane by the Heteronuclear Cluster Oxide [Al2 ZnO4 ]

The thermal gas-phase reactions of [Al₂ ZnO₄ ]^.+ with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. Two competitive mechanisms, that is, hydrogen-atom transfer (HAT) and proton-coupled electron transfer (PCET) are operative. Interestingly, while the HAT process is influenced by the polarity of the transition structure, both the ionic nature of the metal-oxygen bond and the structural rigidity of the cluster oxide affect the PCET pathway. As compared to the previously reported homonuclear [Al₂ O₃ ]^.+ and [ZnO]^.+ , the heteronuclear oxide [Al₂ ZnO₄ ]^.+ exhibits a much higher chemoselectivity towards methane. The electronic origins of the doping effect have been explored.

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.

Thermal Dihydrogen Activation by a Closed-Shell AuCeO2(+) Cluster

Laser-ablation-generated AuCeO2(+) and CeO2(+) oxide clusters were mass-selected using a quadrupole mass filter and reacted with H2 in an ion trap reactor at ambient conditions. The reactions were characterized by mass spectrometry and density functional theory calculations. The gold-cerium bimetallic oxide cluster AuCeO2(+) is more reactive in H2 activation than the pure cerium oxide cluster CeO2(+). The gold atom is the active adsorption site and facilitates the heterolytic cleavage of H2 in collaboration with the separated O(2-) ion of the CeO2 support. To the best of our knowledge, this is the first example of thermal H2 activation by a closed-shell atomic cluster, which provides molecular-level insights into the single gold atom catalysis over metal oxide supports.