Adsorption and thermal decomposition of NH on NiAl 001/and 3 NiAl 111/

The adsorption and thermal decomposition of ammonia NH3.on NiAl 001.and NiAl 111.was investigated by means of high-resolution electron energy loss spectroscopy EELS.and temperature programmed desorption TPD.. At 80 K, NH3 adsorbs molecularly forming NH3 multilayers on both surfaces. These multilayers desorb by heating the samples to about 100 K. On NiAl 001.the further thermal decomposition of the remaining monolayer-NH3 takes place for considerably lower temperatures than on NiAl 111.. For the 001.surface, heating to 150 K leads to a partial decomposition of NH3 to NH. After annealing at 380 K, the NH3 and NH species are dissociated completely and atomic nitrogen is adsorbed on the NiAl 001. surface. In the case of NiAl 111., the decomposition of NH3 to NH occurs at about 260 K. In contrast to NiAl 001., evidence is found for a recombinative thermal desorption of N2, H2, and NH3 at 134, 270, and 275 K, respectively. At 420 K, NiAl 111.is covered with NH which decomposes to atomic nitrogen upon annealing at 750 K. We assume the different atomic composition of the outermost layer of NiAl 001. and NiAl 111. to be responsible for the observed differences. q1998 Elsevier Science B.V.