Influence of charge state and active site structure of tetrahedral copper and silver clusters on the methane activation

IR Spectroscopic Characterization of Methane Adsorption on Copper Clusters Cun+ (n = 2-4)

The interaction of CH₄ with cationic copper clusters has been studied with infrared-multiple photon dissociation (IRMPD) spectroscopy. Cu_n⁺ (n = 2-4) formed by laser ablation were reacted with CH₄. The formed complexes were irradiated with the IR light of the free-electron laser for intracavity experiments (FELICE), and the fragments were mass-analyzed with a reflectron time-of-flight mass spectrometer. The structures of the Cu_n⁺-CH₄ complexes are assigned on the basis of comparison between the resulting IRMPD spectra to spectra of different isomers calculated with density functional theory (DFT). For all sizes n, the structure found is one with molecularly adsorbed CH₄. Only slight deformations of the CH₄ molecule have been identified upon adsorption on the clusters, which results in redshifts of the spectroscopic bands. This deformation can be explained by charge transfer from the cluster to the adsorbed methane molecule.

Enhancement of the Catalytic Activities of Heteronuclear Bimetallic Cations for the C-H Bond Activation of Cyclohexane

Heterometallic cations NiCu⁺ and CoNi⁺ can easily induce triple dehydrogenation of cyclohexane with high yield, and monometallic cations Ni⁺ and Co⁺ only give rise to double dehydrogenation with low yield. Reaction mechanisms of the six C-H bond activations for cyclohexane are systematically investigated by comparing the difference between bimetallic cations and monometallic ones. Fragment molecular orbital analysis clearly indicates that charge transfer (CT) occurs from the occupied interacting orbital of the metallic cation to the σ*-antibonding orbital of the first, third, and fifth activated C-H bonds in transition states. The synergistic effects of heteronuclear bimetallic cations result in the destabilization of the occupied interacting orbital in bimetallic cations, which raise the reactivity of bimetallic cations and enhance the CT between catalysts and substrates. Contrary to the absence of the third dehydrogenation product in the mononuclear metallic cation catalytic reaction, a significant amount of the third dehydrogenation product is observed in the presence of heteronuclear cations (NiCu⁺ and CoNi⁺). π back-bonding between Ni of heteronuclear metallic cations and the substrate cyclohexadiene plays an essential role in lowering the energies of transition states, which accelerate the third dehydrogenation. The reasons why heteronuclear bimetallic cations are more reactive than monometallic ones are discussed in detail.