Effect of Hydrogen Sulfide and Methanethiol Adsorption on Acidic Properties of Metal Oxides: An Infrared Study

Adsorption of H2S and CH3SH on SiO2, Al2O3, TiO2, and ZrO2 and the resulting changes of surface acidity were studied by means of IR spectroscopy using CO (at 77 K) and 2,6-dimethylpyridine (DMP) (at 300 K) as probe molecules. Both H2S and CH3SH form H-bond with surface OH groups of all the adsorbents. For silica it is the only way of adsorption, while coordination as well as dissociative adsorption leading to formation of OH groups and molecular water occur on the three other oxides. Two types of induced Brønsted acidity were established, according to the mechanism of adsorption of the sulfur-containing compound. In the presence of large amount of sulfur-containing molecules, a reversible increase of the OH group acidity occurs, as revealed by the frequency shifts of the bands of H-bonded CO molecules and of perturbed OH groups on increasing H2S coverage, or by the increased intensity of protonated DMP in the presence of H2S. This effect is explained by the interaction of molecular H2S with oxygen atom of the OH group. For Al2O3, TiO2, and ZrO2, dissociative adsorption of H2S and CH3SH results in the appearance of new OH groups that account for an irreversible, at least at 300 K, increase of Brønsted acidity revealed by higher intensity of corresponding bands of H-bonded CO and of protonated 2,6-dimethylpyridine. These new OH groups correspond to the most acidic of those that normally exist at the surface of the considered metal oxides. In no case the S−H groups of adsorbed molecules or surface SH groups formed by dissociation could account for the observed Brønsted acidity increase.