Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO 4 ] .+ /C 2 H 4 Couple by Gas‐Phase Vibrational Spectroscopy

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 O^2- site next to the Ni²⁺ center. The CH₃ ⋅ attaches as CH₃ ⁺ to O^2- 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.

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.

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.

Characterization of the non-covalent docking motif in the isolated reactant complex of a double proton-coupled electron transfer reaction with cryogenic ion spectroscopy

The solution kinetics of a proton-coupled electron transfer reaction involving two-electron oxidation of a Ru compound with concomitant transfer of two protons to a quinone derivative have been interpreted to indicate the formation of a long-lived intermediate between the reactants. We characterize the ionic reactants, products, and an entrance channel reaction complex in the gas phase using high-resolution mass spectrometry augmented by cryogenic ion IR photodissociation spectroscopy. Collisional activation of this trapped entrance channel complex does not drive the reaction to products but rather yields dissociation back to reactants. Electronic structure calculations indicate that there are four low-lying isomeric forms of the non-covalently bound complex. Comparison of their predicted vibrational spectra with the observed band pattern indicates that the C=O groups of the ortho-quinone attach to protons on two different -NH2 groups of the reactant scaffold, exhibiting strong O-H-N contact motifs. Since collisional activation does not lead to the products observed in the liquid phase, these results indicate that the reaction most likely proceeds through reorientation of the H-atom donor ligand about the metal center.