Facile N≡N Bond Cleavage by Anionic Trimetallic Clusters V_3-x Ta_x C₄^- (x=0-3): A DFT Study

Activation of N₂ on anionic trimetallic V_3-x Ta_x C₄^- (x=0-3) clusters was theoretically studied employing density functional theory. For all studied clusters, initial adsorption of N₂ (end-on) on one of the metal atoms (denoted as Site 1) is transferred to an of end-on: side-on: side-on coordination on three metal atoms, prior to N₂ dissociation. The whole reaction is exothermic and has no global energy barriers, indicating that the dissociation of N₂ is facile under mild conditions. The reaction process can be divided into two processes: N₂ transfer (TRF) and N-N dissociation (DIS). For V-series clusters, which has a V atom on Site 1, the rate-determining step is DIS, while for Ta-series clusters with a Ta on Site 1, TRF may be the rate-determining step or has energy barriers similar to those of DIS. The overall energy barriers for heteronuclear V₂ TaC₄^- and VTa₂ C₄^- clusters are lower than those for homonuclear V₃ C₄^- and Ta₃ C₄^- , showing that the doping effect is beneficial for the activation and dissociation of N₂ . In particular, V-Ta₂ C₄^- has low energy barriers in both TRF and DIS, and it has the highest N₂ adsorption energy and a high reaction heat release. Therefore, a trimetallic heteronuclear V-series cluster, V-Ta₂ C₄^- , is suggested to have high reactivity to N₂ activation, and may serve as a prototype for designing related catalysts at a molecular level.