Adsorption and Thermal Decomposition of Methanol on the (100) Surface of NiAl: A Comparison to NiAl(110)

Abstract

The surface reactivity of the (100) plane of stoichiometric NiAl toward methanol has been investigated with temperature-programmed desorption (TPD), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and high-resolution electron energy loss spectroscopy (EELS). The atomic composition of the outermost surface of NiAl(100) is predominately Al, in contrast to the (110) plane of NiAl which is capped by an outermost layer that contains both Al and Ni. In an effort to understand the effects of the atomic composition of the surface on reactivity, results for CH3OH/NiAl(100) are compared to prior results for CH3OH/NiAl(110). Methanol chemisorption on both planes of NiAl, at 120 K, is primarily associative (some dissociation cannot be ruled out). Chemisorbed methanol transforms into surface methoxy in the temperature interval of 120 and 200 K on both NiAl(100) and NiAl(110). On the (110) surface, subsequent C-H and C-O bond breaking steps at higher temperatures lead to the evolution of gaseous H2, 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 generates a much smaller amount of CH4 and the reaction channel leading to gaseous CO is completely suppressed. The presence of Ni atoms in the top layer of the (110) surface is suspected to be responsible for both CH4 and CO production on NiAl(110).