The Reactive Sites of Methane Activation: A Comparison of IrC3+ with PtC3.

The activation reactions of methane mediated by metal carbide ions MC3+ (M = Ir and Pt) were comparatively studied at room temperature using the techniques of mass spectrometry in conjunction with theoretical calculations. MC3+ (M = Ir and Pt) ions reacted with CH4 at room temperature forming MC2H2+/C2H2 and MC4H2+/H2 as the major products for both systems. Besides that, PtC3+ could abstract a hydrogen atom from CH4 to generate PtC3H+/CH3, while IrC3+ could not. Quantum chemical calculations showed that the MC3+ (M = Ir and Pt) ions have a linear M-C-C-C structure. The first C-H activation took place on the Ir atom for IrC3+. The terminal carbon atom was the reactive site for the first C-H bond activation of PtC3+, which was beneficial to generate PtC3H+/CH3. The orbitals of the different metal influence the selection of the reactive sites for methane activation, which results in the different reaction channels. This study investigates the molecular-level mechanisms of the reactive sites of methane activation.

C/C Exchange in Activation/Coupling Reaction of Acetylene and Methane Mediated by Os+: A Comparison with Ir+, Pt+, and Au.

The activation and coupling reactions of methane and acetylene mediated by M+ (M = Os, Ir, Pt, and Au) have been comparatively studied at room temperature by the techniques of mass spectrometry in conjunction with theoretical calculations. Studies have shown that Os+ and Ir+ can mediate the activation/coupling reaction of CH4 and C2H2, while Pt+ and Au+ cannot, which could be explained by the number of empty valence orbitals in the metal atom. In addition, there are different competition channels for the reaction mediated by Os+ and Ir+: an expected dehydrogenation and an unexpected C/C exchange. We find that if the rare C/C exchange reaction takes place, there are symmetric carbon atoms in the reaction intermediate and the C/C exchange reaction is favored kinetically. The C/C exchange reaction must be considered, which will affect the yield of the products in the primary reaction. This study shows the molecular-level mechanisms which include the C/C exchange reaction in the activation and coupling reaction of organic compounds mediated by different metals.

Reactions of Transition-Metal Carbyne Cations with Ethylene in the Gas Phase.

The reactions of iridium- and osmium-carbyne hydride cations [HIrCH]+ and [HOsCH]+ with ethylene have been studied using mass spectrometry with isotopic-labeling in the gas phase. The carbyne reactivity is compared with that of the rhodium, cobalt, and iron analogues [TMCH2]+ (TM = Fe, Co, and Rh), which were determined to have the carbene structures. Besides the cycloaddition/dehydrogenation reaction in forming the [TMC3H4]+ + H2 (TM = Ir and Os) products, a second reaction pathway producing the [TMC2H2]+ ion and CH4 via triple hydrogen atom transfer reactions to the carbyne carbon is observed to be the major channel. The latter channel is not observed in the rhodium, cobalt, and iron carbene cation reactions. Quantum-chemical calculations indicate that the distinct reactivity is not due to different initial structures of the reactants. Both reaction channels are predicted to be thermodynamically exothermic and kinetically facile for the carbyne cations, and the reactions proceed with the initial formation of a carbene intermediate via hydride-carbyne coupling. The latter channel is also exothermic but kinetically unfavorable for the rhodium, cobalt, and iron carbene cations.

Surface construction of loose Co(OH)2 shell derived from ZIF-67 nanocube for efficient oxygen evolution.

The rational design of nanostructure is very important for improving the number and effective utilization of active sites of the electrocatalysts. Here, a core-shell nanostructure composed of ZIF-67 core and Co(OH)2 shell (ZIF-67@Co(OH)2) has been obtained by subjecting ZIF-67 nanocube to the optimal high temperature etching process. After refluxing and etching in ethanol/water mixed solution, the loose Co(OH)2 shell can be constructed based on the surface of etched ZIF nanocube, which provides the obviously abundant active cobalt sites and better contact for oxygen evolution reaction (OER). Compared to the whole hollow Co(OH)2 nanocube, the solid ZIF-67 core in ZIF-67@Co(OH)2 can be favorable for charge transfer and provide the stable structure. The synergistic effect between Co(OH)2 shell and ZIF-67 core under suitable etching regulation can realize the optimized electrocatalysis for OER. The performance measurements show that Co(OH)2-1 after refluxing 1 h demonstrates the excellent activity requiring 354 mV overpotential at the current density of 10 mA cm-2 and the good stability. The enhanced mechanism may be due to the formation of loose Co(OH)2 as shell with fully exposed active sites, as well as the synergistic effect between ZIF-67 core and Co(OH)2 shell. Therefore, the surface construction of active composites based on ZIF precursor may be a new strategy for efficient electrocatalysis for water splitting.