Coordination number dependence of Pt catalysts for catalytic decomposition of aqueous hydroxylammonium nitrate (HAN)

To uncover the relation between active metals structure and the adsorption, activation as well as its decomposition performance of hydroxylamine nitrate (HAN) that has been considered to be a promising low-toxicity monopropellant for spacecrafts, the performance of xPt/Al₂O₃ with various Pt loadings (x = 0.1, 0.5, 1 and 2, indicates the mass percentage of Pt) was evaluated in the aqueous HAN (10 wt%) decomposition. It was found that the both the NH₃OH⁺ conversion and the turnover frequency (TOF) enhanced as Pt loadings increased while NO₃^- was hardly converted at the temperature of lower than 120 ℃ for xPt/Al₂O₃, and 2Pt/Al₂O₃ exhibited the best activity with almost 100 % NH₃OH⁺ conversion, 20 % NH₄⁺ selectivity and 80 % N₂O selectivity at around 120 ℃, respectively. This originated from the increased Pt coordinated number of active sites with sufficiently continuous Pt sites caused by its enhanced particle size with loadings, which was beneficial to the cracking of N-O, O-H and N-H bonds together thus the activation and conversion of NH₃OH⁺ to NH₄⁺ and N₂O.