Dissociative Adsorption of N₂ onto Size-Selected Ti⁺ and TiO⁺ (n ≤ 16) toward Nitrogen Fixation

The reactivity of titanium cluster ions and their oxides with molecular dinitrogen was examined using a tandem-type mass spectrometer at a low collision energy of 0.5 eV. The clusters can adsorb dinitrogen and release a titanium atom to consume the obtained excess energy. This indicates that N₂ adsorption energy is large enough to break the titanium cluster. While the smaller clusters have relatively low reactivity, the measured reaction cross section increases with the cluster size and reaches nearly one-third of the Langevin cross section at the larger clusters. Density functional theory calculations indicate that the reaction proceeds exothermically and that molecular dinitrogen adsorbs on the clusters dissociatively. It is suggested that the energy levels of the highest occupied molecular orbitals of the titanium clusters are decisively important for N₂ activation.