INTERACTION OF AMMONIA WITH HYDROGEN ON AL(111)

The interaction of NH3 with clean and hydrogenated Al(111) surfaces is studied using temperature programmed desorption and infrared reflecti on absorption spectroscopy. On clean Al(111), NH3 adsorbs molecularly and desorbs by 200 K. In contrast, on the H-covered surface, a new NH3 desorption state (beta) is seen at 225 K. The beta-NH3 coverage track s that of H, reaching a saturation coverage of about 0.58 NH3/Al surfa ce atom on the fully H-covered surface. Infrared spectroscopy shows th at H causes appreciable change in NH3 adsorption geometry. NH3 prefere ntially adsorbs via a Lewis acid-base interaction at monohydride sites with the Al-N bond inclined a say from the surface normal, whereas on the H-Free surface this bond is essentially normal to the surface. (C ) 1997 Elsevier Science B.V.