IR-visible sum-frequency generation study of methanol adsorption and reaction on Ni(100)

Abstract

Infrared-visible sum-frequency generation (SFG) has been used to study the adsorption and reaction of methanol on Ni (100) in ultra-high vacuum. SFG spectra at 0.2 cm −1 resolution are reported for the CO and CH and OH vibrational stretching regions. The SFG signals arising from the adsorbate vibrational resonances are strongly influenced by a nonresonant background from the metal surface. The interference between the resonant and nonresonant components leads to complex lineshapes. Most significantly, there is a distinct change in the lineshape of the CH stretching vibrations as methanol reacts to form methoxy. The complex lineshapes can be deconvoluted into simple Lorentzian resonant terms and a wavelength independent, nonresonant term. The angles at which methanol and methoxy are bound to the surface are determined from the relative intensities of the methyl group CH resonances. The carbon monoxide stretching vibration, which is observed upon the dissociation of methoxy, is strongly perturbed by the coadsorbed hydrogen. Following thermal desorption of the hydrogen, the CO vibrational frequency shifts to 2030 cm −1, characteristic of an atop binding site.