Abstract
The adsorption and decomposition of NH3 and ND3 on W{100} have been studied in detail using a molecular beam technique. Sticking probabilities and desorption rates of gases resulting from the decomposition reaction have been followed at crystal temperatures between 300 and 910 K. Isothermal desorption of D2(H2) was followed as a function of ND3(NH3) beam time on an initially clean surface. Variations in the shape of the resulting isothermal desorption curves as a function of surface temperature are explained via the changes in activation energies for desorption of D2 and decomposition of adsorbed ND3 as a function of N coverage. A significant kinetic isotope effect is found in H2 and D2 isothermal desorption during the decomposition of mixed NH3 and ND3 beams only at high N(a) coverages. A kinetic model and energetic scheme are presented for the ammonia decomposition and synthesis reactions on W{100} which are consistent with all of the experimental data, and show that the synthesis reaction is blocked by a large activation barrier to the initial hydrogenation of adsorbed nitrogen atoms.
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