Specific reaction rates (cm-2-s-1) of the catalytic decomposition of ammonia, the isotopic exchange between ammonia and deuterium, and the inhibition of the decomposition of ammonia by hydrogen (with ammonia to hydrogen partial pressure ratios varying from 1:1 to 1:4) on a polycrystalline platinum wire have been measured in a continuous stirred tank microreactor at pressures between 5 x 10-7 and 0.6 Torr and temperatures between 400 and 1200 K. At relatively low temperatures and/or high pressures, nitrogen adatoms are the dominant surface species, and the recombinative desorption of nitrogen controls the rate of decomposition of ammonia. At relatively high temperatures and/or low pressures, the surface coverage of all species is low, and a competition between the desorption of molecular ammonia and the cleavage of an N-H bond of molecularly adsorbed ammonia controls the rate of reaction. The kinetics of decomposition of ammonia as well as the results for the NH3 + D2 exchange reaction are described quantitatively by a mechanistic model employing independently measured adsorption-desorption parameters of NH3 and H2, and desorption parameters of N2. The model was extended to incorporate the nitrogen coverage-dependence of the rate coefficient of hydrogen desorption to describe the inhibition of the decomposition by hydrogen. The hydrogenation of NH2(a) to produce molecularly adsorbed ammonia is predicted to be the dominant factor in the inhibition of the decomposition of ammonia.
The kinetics of adsorption and desorption of deuterium have been studied on Pt(110)-(1x2) surfaces on which various fractional coverages of nitrogen adatoms were deposited via the decomposition of ammonia at 400 K. Nitrogen selectively blocks the high temperature β2-state of deuterium prior to poisoning the low temperature β1-state. No evidence of a 'long-range' electronic perturbation of the surface by the nitrogen adatoms was found. The adsorption kinetics of deuterium on both clean and nitrogen-precovered Pt(110)-(1x2) surfaces were Langmuirian. Nitrogen modifies the preexponential factor and the activation energy of desorption of deuterium on Pt(110)-(1x2) by essentially rescaling the effective coverage of the deuterium. The results are consistent with findings from previous studies of the inhibition of the decomposition of ammonia by hydrogen on polycrystalline platinum.
Steady-state specific reaction rates have also been measured for the catalytic decomposition of ammonia on a Ru(001) surface at pressures of 10-6 and 2 x 10-6 Torr and temperatures between approximately 500 and 1250 K. Qualitatively, the kinetics are similar to those observed for ammonia decomposition on the polycrystalline platinum surface. Based on thermal desorption measurements during the steady-state decomposition of ammonia at 2 x 10-6 Torr, nitrogen adatoms are the dominant surface species, and the recombinative desorption of nitrogen is the major (and probably the only) elementary reaction that produces molecular nitrogen. The mechanistic model developed previously describes accurately the pressure and temperature dependence of both the decomposition kinetics and the measured steady-state coverage of nitrogen adatoms.
The isotopic exchange reaction between 15NH3 and deuter m at steady-state has been studied on Ru(001) for a partial pressure ratio of ammonia to deuterium of 4:1 with a total pressure of 2.5 x 10-6 Torr at temperatures between 380 and 720 K. All three exchange products were observed, and a dissociative exchange mechanism was found to describe quantitatively the experimental data. This mechanistic model is discussed in terms of a potential energy diagram that describes the catalytic decomposition (or synthesis) of ammonia on Ru(001). The energy levels of and the activation barriers separating the chemisorbed intermediates in the ammonia decomposition and synthesis reactions, namely, NH3, NH2+H, NH+2H, N+3H are determined, and the dissociative chemisorption of molecular nitrogen on Ru(001) is found to be activated with an activation energy estimated to be approximately 5 kcal-mol-1 in the limit of zero surface coverage of nitrogen adatoms. Direct comparsion between the estimated barrier for the dissociative adsorption of nitrogen on polycrystalline platinum and Ru(001) surfaces indicates clearly that ruthenium is a superior catalyst to platinum for the synthesis of ammonia.
