In situ characterization of adsorbates in solid-liquid interfaces by reflection-absorption infrared spectroscopy

A cell has been designed for the in situ characterization of catalytic intermediates adsorbed at liquid-solid interfaces by single-reflection infrared spectroscopy. The design of our cell is based on previous setups used for the study of electrochemical systems, and consists of a cylindrical Teflon body terminated by a calcium fluoride prism cut in a trapezoidal shape with two faces beveled to 60degrees to let the infrared beam in and out of the cell on one end and by a polished platinum disk (the solid sample) mounted on a retractable rod on the other. The solid sample is cleaned by repeated electrochemical oxidation-reduction cycles using KClO4, HClO4, or H2SO4 electrolytes, and a thin liquid film is then trapped between the two surfaces for the liquid-solid adsorption studies. The performance of this cell was first tested by studies on the adsorption of carbon monoxide from 1.0 M H2SO4 solutions. Significant signals were detected for the adsorbed CO, comparable to those seen under vacuum or under high pressures of CO gas, and also to data reported previously. A more detailed characterization of our experimental setup was carried out with cinchona alkaloids. Discrimination between adsorbed and dissolved species was accomplished by dividing spectra obtained with p- and s-polarized light, and was corroborated by a number of tests, including a variation in the liquid film thickness and the nature of the adsorbing surface. The importance of the in situ characterization was assessed by comparing spectra for quinoline deposited under vacuum versus adsorbed from a carbon tetrachloride solution.