The thermal chemistry of ammonia on Ni(110)

The thermal chemistry of NH, on Ni(110) surfaces has been investigated using temperature-programmed desorption (TPD). The thermal activation of ammonia-saturated Ni(110) leads primarily to molecular desorption, but some decomposition is identified by the appearance of hydrogen and nitrogen TPD peaks ca 400 and 800 K, respectively. Previous reports have shown that ammonia dehydrogenation is readily induced by mild electron bombardment, but this study found dehydrogenation to persist despite the absence of any electron current at the surface, indicating that both thermal- and electron-induced dissociation processes are possible in the NH3/Ni(110) system. Our experimental data also identified the onset of the thermally-activated ammonia dehydrogenation at about 300 K. Evidence is given to suggest that the surface intermediate formed subsequent to this dehydrogenation is NH2,ads. Further activation of the NH2,ads moiety by the Ni(110) surface results in its dehydrogenation to NHads and N-ads around 380 K, but also to some NH2,ads +H-ads recombination to NH3(g) ca 360 K. After considering this recombination step, it was calculated that as much as 23% of adsorbed ammonia dissociates by thermal activation, a much greater value than previously reported. (C) 1999 Elsevier Science B.V. All rights reserved.