Mechanism of the Decomposition of Hydrazine Monohydrate on Pd/Al2O3 Studied by in Situ IR Spectroscopy

Abstract

Pd-containing catalysts (1% Pd/Al₂O₃ and 5% Pd/Al₂O₃) supported on aluminum oxide were studied in the decomposition reaction of hydrazine monohydrate. According to in situ IR-spectroscopic data, hydrazine monohydrate was adsorbed in a linear form on the coordinatively unsaturated sites of the catalyst surface. As the temperature was increased, the adsorbed hydrazine monohydrate lost a water molecule with a change in the geometry of the molecular complex. The adsorption of hydrazine on a support and its diffusion onto palladium clusters is a more advantageous process than direct adsorption on active sites. This circumstance shows that the hydrazine adsorbed on the support can be an intermediate in the process of its decomposition. The test catalysts had a maximum activity at a temperature of about 100°C. At temperatures in a range of 100−120°C, the ratio between hydrogen and nitrogen concentrations in the reaction products was 2, which corresponds to 100% selectivity for hydrogen. The selectivity decreased significantly with the reaction temperature. The high selectivity for hydrogen at low temperatures was explained by the fact that N₂H₄ was chemisorbed through the formation of hydrogen–metal bonds. The hydrogen–metal bond strength in such a complex is higher than the nitrogen–metal bond strength; hence, the N−H bond breaking barrier is lower than the N−N bond breaking barrier, and this fact led to the breaking of an N–H bond and the preservation of an N–N bond. At elevated temperatures, some of the formed hydrogen atoms recombined, and the other reacted with the surface complexes of hydrazine to form the intermediate NH₃−NH₃, in which N–N bond breaking led to the appearance of ammonia molecules in the gas phase.