Steady-state decomposition of ammonia on the ruthenium(001) surface

Abstract

Steady-state specific reaction rates for the catalytic decomposition of ammonia at pressures of 1 x 10/sup -6/ and 2 x 10/sup -6/ Torr have been measured on the Ru(001) surface at temperatures between approximately 500 and 1250 K. For temperatures above 750 K, the reaction rate approaches first order in ammonia pressure, and the apparent activation energy is 5.0 +/- 0.3 kcal x mol/sup -1/. A kinetic isotope effect is observed at these high temperatures, and the activation energy for the decomposition of deuteriated ammonia is 6.6 +/- 0.3 kcal x mol/sup -1/. This apparent activation energy is associated with the difference in activation energies between that of the cleavage of a nitrogen-hydrogen (nitrogen-deuterium) bond in chemisorbed ammonia and that of the desorption of molecularly chemisorbed ammonia. At lower temperatures, the reaction rate is independent of ammonia pressure, and the apparent activation energy is 43 +/- 3 kcal x mol/sup -1/, which is associated with the recombinative desorption of nitrogen. From transient thermal desorption mass spectrometric measurements, the activation energy of the recombinative desorption of nitrogen on Ru(001) is 44.0 +/- 0.5 kcal x mol/sup -1/, and the preexponential factor of the desorption rate coefficient is (1.3 +/- 0.6) xmore » 10/sup -3/ cm/sup 2/ x s/sup -1/. On the basis of thermal desorption measurements during the steady-state decomposition of ammonia at 2 x 10/sup -6/ Torr, nitrogen adatoms are shown to be the dominant surface species under these experimental conditions. A mechanistic model that has been introduced previously describes accurately the pressure and temperature dependence of both the measured decomposition kinetics and the steady-state coverage of nitrogen adatoms.« less