On the adsorption of acetonitrile on pure and sulfated tetragonal zirconia (t-ZrO2)

The ambient temperature adsorption of acetonitrile, in its perdeuterated form (CD3CN), was used for the surface characterization of both pure and sulfate-doped t-ZrO2. In most pure t-ZrO2 specimens, the absence of terminal and bi-bridged surface hydroxyls hinders completely the base-catalyzedCD3CNreactions which yield on other oxides either acetamide-like species or carbanionic surface species. In the case of sulfated t-ZrO2, even if some terminal hydroxyls are present, the mentioned surface reactions are hindered by the presence of acidic (sulfate) functionalities. On pure t-ZrO2, the predominant adsorbed species are represented by CD3CN molecules Lewis-coordinated to coordinatively unsaturated (cus) surface cationic sites. These sites are made available either by surface dehydration or by ligand-displacement of surfacecoordinated (undissociated) water molecules. Lewis-coordinated CD3CN species dominate also on sulfated t-ZrO2, and the relevant adsorbing sites are made available both through the two mechanisms active on pure t-ZrO2 and through an additional ligand-displacement effect concerning polydentate surface sulfates. On sulfated t-ZrO2 systems, there exists an abundant protonic (Brønsted) acidity, a fraction of which can be revealed by CD3CN adsorption through the formation of a relatively strong H-bonded species. The CN vibrational mode for these H-bonded species is distinct from that of weakly adsorbed H-bonded and/or physisorbed species.