Mechanism of the Hydrazine Monohydrate Decomposition by Means of IR Spectroscopy <i>In Situ</i>

Abstract

Pd-containing catalysts (1%Pd/Al2O3 and 5%Pd/Al2O3) deposited on aluminum oxide were studied in the decomposition reaction of hydrazine monohydrate. According to in situ IR spectroscopy, it was found that hydrazine monohydrate is adsorbed on the coordination unsaturated centers of the catalyst surface in a linear form. When the temperature rises, the adsorbed hydrazine monohydrate loses a water molecule, which is accompanied by a change in the geometry of the molecular complex. Adsorption of hydrazine on a support and its diffusion onto palladium clusters is a more advantageous process than direct adsorption on active centers. This circumstance shows that the hydrazine adsorbed on the support can be an intermediate of its decomposition process. The studied catalysts have a maximum activity in the temperature range of 100–120°C, while the ratio of hydrogen and nitrogen concentrations in the reaction products was equal to 2, which corresponds to 100% selectivity for hydrogen. As the reaction temperature increases, the selectivity decreases significantly. The explanation of the high selectivity for hydrogen at low temperatures is due to the fact that the adsorption of N2H4 is carried out 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 barrier for breaking the N–H bond is lower than the barrier for breaking N–N bond, which leads to breaking N–H bond and preserving the N–N bond. At elevated temperatures, some of the hydrogen atoms formed recombine, the other reacts with the surface complexes of hydrazine to form the intermediate NH3–NH3, the breaking of the bond in which leads to the formation of ammonia molecules in the gas phase.