ADSORPTION AND THERMAL-DECOMPOSITION OF METHANOL ON THE (100) SURFACEOF NIAL - A COMPARISON TO NIAL(110)

The surface reactivity of the (100) plane of stoichiometric NiAl towar d methanol has been investigated with temperature-programmed desorptio n (TPD), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS ), and high-resolution electron energy loss spectroscopy (EELS). The a tomic composition of the outermost surface of NiAl(100) is predominate ly Al, in contrast to the (110) plane of NiAl which is capped by an ou termost layer that contains both Al and Ni. In an effort to understand the effects of the atomic composition of the surface on reactivity, r esults for CH3OH/NiAl(100) are compared to prior results for CH3OH/NiA l(110). Methanol chemisorption on both planes of NiAl, at 120 K, is pr imarily associative (some dissociation cannot be ruled out). Chemisorb ed methanol transforms into surface methoxy in the temperature interva l of 120 and 200 K on both NiAl(100) and NiAl(110). On the (110) surfa ce, subsequent C-H and C-O bond breaking steps at higher temperatures lead to the evolution of gaseous H-2, CO, CH4, CH3 radicals, and small amounts of C2H4, in addition to the deposition of surface oxygen and carbon. The peak temperature of CH4 desorption from NiAl(110) is near 350 K and the maximum rate of CH3 radical desorption is located at 570 K. In contrast, methoxy decomposition on the NiAl(100) surface genera tes a much smaller amount of CH4 and the reaction channel leading to g aseous CO is completely suppressed. The presence of Ni atoms in the to p layer of the (110) surface is suspected to be responsible for both C H4 and CO production on NiAl(110). (C) 1995 Academic Press, Inc.