Dehydrogenation of diborane on small Nb n + clusters.

The reactivity of Nb n + (1 ≤ n ≤ 21) clusters with B 2 H 6 is studied by using a self-developed multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). The Nb n + clusters were generated by a magnetron sputtering source and reacted with the B 2 H 6 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 Nb n B 1-4 + and Nb n B 1-4 H 1,2,4 + were detected, indicative of the removal of H 2 and likely BH x moieties. Interestingly, these Nb n B 1-4 + and Nb n B 1-4 H 1,2,4 + products are limited to 3 ≤ n ≤ 6, suggesting that the small Nb n + clusters are relatively more reactive than the larger Nb n > 6 + clusters under the same conditions. By varying the B 2 H 6 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 Nb 1 + cations accounts for the weakened dehydrogenation efficiency. Nb 2 + forms partial dehydrogenation products at a faster rate. In contrast, the Nb n> 6 + clusters are subject to more flexible vibrational relaxation which disperse the energy gain of B 2 H 6 -adsorption and thus are unable to overcome the energy barriers for subsequent hydrogen atom transfer and H 2 release.