Methanol Chemisorption and Reaction on the (111) Crystallographic Plane of NiAl

Abstract

Thermal chemistry of CH3OH on the (111) crystallographic plane of NiAl has been investigated with temperature-programmed desorption (TPD), synchrotron-based photoelectron spectroscopy, high-resolution electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). It is inferred from EELS results that CH3OH, at least in part, adsorbs molecularly at 120 K and completely converts to methoxy, CH3O(ad), by 200 K. Photoelectron spectroscopy indicates that the majority of methoxy binds strongly to the Al component, inducing a 1 eV shift of the Al 2p core level to higher binding energy, relative to the same Al core level of clean NiAl(111). Comparison of EELS results for methoxy on NiAl(110) and NiAl(100) to NiAl(111) opens up the possibility that methoxy adopts at least two different binding sites on the (111) plane of NiAl. One of these sites has a strong contribution from Ni, and a second site has a lesser contribution from Ni (i.e., “Al-rich” site), and methoxy decomposition on this latter site leads to no oxygenated gaseous product. Thermal decomposition of methoxy results in the production of gaseous H2, CO, CH4, and CH3 in the temperature range 200−650 K (aluminum oxide forms by 650 K). It is thought that CO and probably CH4 result from the decomposition of methoxy on the “Al site” with strong Ni character.