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The heterogeneous decomposition of hydrazine.

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posted on 19.11.2015, 08:47 by Courtnay Frederick. Sayer
The heterogeneous decomposition of liquid hydrazine on supported platinum group metal catalysts has been found to follow one, or both of the reactions. On rhodium, palladium and platinum catalysts reaction (2) occurs while on the ruthenium catalyst both reactions occur. Reaction (2) occurs following the dissociative chemisorption of hydrazine molecules in the form of amide radicals followed by a Langmuir-Rideal reaction between adsorbed radicals and a solution phase hydrazine molecule to form the products. Reaction (1) occurs following the associative adsorption of hydrazine molecules to form an activated complex which then breaks down to yield the products. These mechanisms are consistent with the dual plane theory postulated by Cosser and Tompkins to explain the decomposition of hydrazine on tungsten films. The decomposition of hydrazine on the supported iridium catalyst was observed to follow reaction (1) but the kinetics were found to be complex. For concentrations of hydrazine up to 3 mole dm-3 the reaction was observed to be first order with an activation energy of 35 kJ mole-1. Increasing the concentration of hydrazine results in a change in kinetics to a reaction having an activation energy of 65 kJ mole-1 and although the rate is a linear function of hydrazine concentration the reaction is not true first order. The hydrazine concentration at which the change in kinetics occurred was found to increase with increasing oxide content of the catalyst. It has been postulated that at low concentrations of hydrazine the associative adsorption of hydrazine molecules occurs on the sites of higher oxidation state as these are energetically favoured. Only when these sites have been saturated does adsorption and decomposition of hydrazine occur on the sites of lower oxidation state.


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University of Leicester

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