Dehydrogenation of diborane on small Nb clusters.

The reactivity of Nb (1 ≤ ≤ 21) clusters with BH 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 BH 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 and NbBH were detected, indicative of the removal of H and likely BH moieties. Interestingly, these NbB and NbBH products are limited to 3 ≤ ≤ 6, suggesting that the small Nb clusters are relatively more reactive than the larger Nb clusters under the same conditions. By varying the BH 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 cations accounts for the weakened dehydrogenation efficiency. Nb 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 BH-adsorption and thus are unable to overcome the energy barriers for subsequent hydrogen atom transfer and H release.