Dehydrogenation of diborane on small Nb⁺ clusters

The reactivity of Nb⁺ (1 ≤ n ≤ 21) clusters with B₂H₆ is studied by using a self-developed multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). The Nb⁺ clusters were generated by a magnetron sputtering source and reacted with the B₂H₆ gas under fully thermalized conditions in the downstream flow tube where the reaction time was accurately controlled and adjustable. The complete and partial dehydrogenation products NbB_1-4⁺ and NbB_1-4H_1,2,4⁺ were detected, indicative of the removal of H₂ and likely BH moieties. Interestingly, these NbB_1-4⁺ and NbB_1-4H_1,2,4⁺ products are limited to 3 ≤ n ≤ 6, suggesting that the small Nb⁺ clusters are relatively more reactive than the larger Nb⁺ clusters under the same conditions. By varying the B₂H₆ gas concentrations and the reactant doses introduced into the flow tube, and by changing the reaction time, we performed a detailed analysis of the reaction dynamics in combination with the DFT-calculated thermodynamics. It is demonstrated that the lack of cooperative active sites on the Nb₁⁺ cations accounts for the weakened dehydrogenation efficiency. Nb₂⁺ forms partial dehydrogenation products at a faster rate. In contrast, the Nb⁺ clusters are subject to more flexible vibrational relaxation which disperse the energy gain of B₂H₆-adsorption and thus are unable to overcome the energy barriers for subsequent hydrogen atom transfer and H₂ release.