A TPD and SFG study of propionic acid adsorbed on Ni(110) surface

Propionic acid adsorbed on Ni(110) surface was studied by temperature programmed desorption (TPD) and infrared-visible sum frequency generation (SFG) spectroscopy. The TPD results indicated that propionic acid molecularly adsorbed on the Ni(110) surface at 113 K and that the multilayers of propionic acid started to desorb at 213 K. The molecularly adsorbed propionic acid on the first layer decomposed on further heating to form propionate species which decomposed at 390 K to H₂(g) and CO₂(g), leaving carbon on the surface. The SFG spectra of the propionic acid adsorbed in various thickness at 113 K gave vibrational peaks at 2885 cm^-1, 2956 cm^-1, and 2990 cm^-1, assigned to the symmetric CH stretching mode of the CH₂ group, symmetric CH stretching mode of the CH₃ group, and antisymmetric CH stretching mode of the CH₂ group, respectively, of propionic acid. The multilayered films gave peaks under both the (pp) and (sp) polarization combinations of visible and infrared pulses but the signals disappeared for the (sp) polarization combination when the coverage was lower than one monolayer (ML). The absence of the SFG peaks for the (sp) polarization combination on the low coverage surface was ascribed to the difference in molecular orientation and molecular SFG tensor. The SFG spectrum of the surface which was dosed at 300 K gave vibrational peaks at 2885 cm^-1, 2948 cm^-1 and 2990 cm^-1 only for the (pp) polarization combination. These peaks were assigned to the symmetric CH stretching mode of the CH₂ group, symmetric CH stretching mode of the CH₃ group, and the antisymmetric CH stretching mode of the CH₂ group of propionate, the product of dissociative adsorption.