Inhibition by hydrogen of the heterogeneous decomposition of ammonia on platinum

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Absolute reaction rates have been measured in a continuous flow microreactor for the inhibition by hydrogen of the heterogeneous decomposition of ammonia over a polycrystalline platinum wire at pressures between 10 T and 0.6 torr, with ammonia to hydrogen partial pressure ratios varying from 1:1 to 1:4 and temperatures between 400 and 1200 K. Inhibition of the decomposition is observed at relatively high total pressures (0.2-0.6 torr) for all temperatures studied. The reaction orders with respect to hydrogen and ammonia are -1.5 and 1.0, respectively, with an apparent activation energy of 38 kcal/mol. For relatively low total pressures (10 T-5 x 10 T torr), inhibition is only observed at temperatures below 650 K, where the activation energy varies from 33 to 55 kcal/mol and the reaction order with respect to hydrogen approaches -1.5 at low temperatures. The decomposition becomes uninhibited at high temperatures and the activation energy is 4.5 kcal/mol. The kinetics of this reaction as well as previous results for the NH3 + D2 exchange reaction are described quantitatively by a mechanistic model employing independently measured adsorption-desorption parameters of NH3, N2 and H2, where the rate coefficient for hydrogen desorption is a function of the fractional surface coverage ofmore » nitrogen adatoms. The hydrogenation of NH2(a) to produce molecularly adsorbed ammonia is predicted to be the dominant factor in the inhibition of the ammonia decomposition. 37 references, 5 figures, 1 table.« less