Diffuse reflectance infrared spectra of stearic acid self-assembled on fine silver particles

Abstract

The diffuse reflectance infrared Fourier transform (DRIFT) spectrum of stearic acid (STA) that has been self-assembled on 2 μm-sized silver particles is recorded. Comparing with the reflection-absorption infrared (RAIR) spectrum taken for the same molecule on a vacuum-evaporated silver film, the usual surface selection rule that applies to flat metal surfaces is confirmed to be applicable even to the surface of fine metal particles. It can be readily concluded that the STA molecule is chemisorbed on silver as carboxylate with its two oxygen atoms bound symmetrically to the surface. From the DRIFT spectral pattern, there is also evidence that the adsorbed stearate species should consist of fully extended trans-zigzag carbon chains in a crystalline state. Furthermore, we have demonstrated that the temperature dependence of the DRIFT spectral pattern is comparable to that of the RAIR spectral pattern. The stability of the adsorbates as well as their possible order–disorder phase transition could be examined through the temperature dependence of the DRIFT peak positions and intensities; the DRIFT spectral pattern of STA on Ag was quite reversible below 390 K, but it became irreversible once the temperature was raised above 390 K.