Mechanistic Details of the Heterogeneous Decomposition of Ammonia on Platinum

Article abstract of DOI:10.1021/j100261a017

Absolute reaction rates have been measured for the catalytic decomposition of NH3 and ND3 and for the NH3 + D2 exchange reaction over a polycrystalline platinum wire at pressures between 5 x 10^-7 and 0.5 torr and temperatures between 400 and 1200 K in a continuous flow microreactor.At relatively low pressures and/or high temperatures, a primary isotope effect was observed for the decomposition of ND3, indicating that a surface reaction involving N-H bond cleavage is the rate-limiting step.Under these conditions, the order of the decomposition reaction is unity with respect to ammonia pressure with an apparent activation energy of 4.2 kcal/mol.A coverages increase, corresponding to relatively high pressures and/or low temperatures, the order of the decomposition reaction is zero with respect to ammonia, and the reaction rate becomes controlled by nitrogen desorption.In this case the apparent activation energy of the decomposition reaction is 22 kcal/mol.The kinetics of the NH3 + D2 exchange reaction have been used, together with data concerning the adsorption-desorption parameters of NH3, H2, and N2 as well as the reaction intermediates NH and NH2, to develop a mechanistic model which descibes the reaction rate over a wide range of experimental conditions and which includes the energetics of each intermediate step in the decomposition reaction.This model is discussed in terms of a potential energy diagram for ammonia decomposition on platinum.