Reactions of hydrazoic acid on TiO ₂ nanoparticles: An experimental and computational study

This article reports the results of a computational and experimental study on the reaction of hydrazoic acid, HN ₃, adsorbed on 15-20 nm TiO ₂ particle films. Experimentally, FTIR spectra of HN ₃(a) have been measured by varying HN ₃ dosage, UV irradiation time and surface annealing temperature. Three sharp peaks, related to v _a(NNN) of HN ₃(a) and N ₃(a) with different configurations in the 2000-2200 cm _-1 region, and a broad band absorption, related to associated and isolated HN(a) and HO(a) adsorptions in the 3000-3800 cm ^-1 region, have been detected. Computationally, molecular structures, vibrational frequencies and adsorption energies of possible adsorbates including HN ₃ and its derivatives, N ₃, N ₂, NH, and H, have been predicted by first-principles calculations based on the density functional theory (DFT) and the pseudopotential method. On the basis of the experimental and computational results, the peak appeared at 2075 cm ^-1, which increases at a faster rate with HN ₃ exposure time, is attributed to a stable adsorbate, N3-Ti(a), with the predicted adsorption energy, E _ads = 13 kcal/mol. The peak at 2118 cm ^-1, which survives at the highest surface temperature in the heating experiment, is attributable to the most stable adsorbate, Ti-N ₂N(H)-O(a) with E _ads = 36 kcal/mol. The peak at 2170 cm ^-1, which vanishes most readily in all of the aforementioned experiments, is related to less stable molecular adsorbates, end-on HN ₃-Ti(a) with E _ads = 5 kcal/mol and side-on HN(N ₂)-Ti(a) with E _ads = 8 kcal/mol. A potential energy diagram for the formation of various absorbates with their transition states has been established for the HN ₃/TiO ₂ system. On the basis of the predicted desorption energies, the four most stable products of the HN ₃ reaction on TiO ₂ are H-O(a), 118 kcal/mol; HN-O(a), 85 kcal/ mol; Ti-N ₂N(H)-O(a), 36 kcal/mol; and N ₃-O(a), 19 kcal/mol.