INTERACTIONS OF ALCOHOLS WITH A NIO(100) SURFACE STUDIED BY HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY AND TEMPERATURE-PROGRAMMED DESORPTION SPECTROSCOPY

Interactions of alcohols with well-defined NiO(100) films prepared on Mo(100) have been studied using combined high-resolution electron energy loss spectroscopy (HREELS) /temperature-programmed desorption spectroscopy (TPD). The results show that alcohols adsorb associatively on NiO(100) at 90 K and reversibly desorb upon heating under ultrahigh-vacuum conditions. Both methanol and ethanol are bonded to the cation sites of NiO(100) via their oxygen atom with the methyl and ethyl group, respectively, directed away from the surface. At low coverages, there is a repulsive interaction between adsorbed alcohols, resulting in a decrease in the apparent activation energy of desorption. Below monolayer coverages,the change in the relative mode intensities in the HREELS spectra with respect to the methanol coverage correlates excellently with two desorption states evident in the TPD studies, indicating reorientation of a portion of the adsorbed methanol at high coverages. In the case of ethanol adsorbed on NiO(100), however, only a single desorption state above 200 K is observed, consistent with the HREELS data which show that the relative mode intensities remain essentially unchanged within the monolayer coverage range. The absence of the nu(OH) loss feature in the region of OH vibrations below monolayer coverages indicates the formation of strong hydrogen bonds between the hydroxyl hydrogen and the lattice oxygen. This feature, along with consideration of the surface symmetry of NiO(100), leads to the conclusion that the hydroxyl proton of the molecules likely fluctuates about the neighboring anion sites, acting as a mobile proton. This result is important to our understanding of the nature of the precursor to the dissociated state in an acid-base reaction of the following type: AH + B reversible A- + HB+.