Acetonitrile adsorption as an IR spectroscopic probe for surface acidity/basicity of pure and modified zirconias

Abstract

The ambient temperature adsorption/desorption of acetonitrile (ACN), in its deuterated form, was used for the surface characterisation of several ZrO₂-based systems. On pure m-ZrO₂, ACN uptake originates four types of chemisorptive interaction: σ-coordination to coordinatively unsaturated (cus) surface cations (Lewis acid sites), formation of weak H-bonding with bridging surface hydroxyls, and two surface reactions. The latter are a partial hydrolytic reaction, leading to (at least) three acetamide-like surface anionic species, and the extraction by basic surface sites of one H (D) atom, leading to monomeric and/or polymeric surface carbanionic species. The proportion among the four types of interaction is mainly determined by the samples activation conditions. Comparison of m-ZrO₂ data with some preliminary data of ACN adsorption on t-ZrO₂ suggest that the two surface reactions of ACN depend on the presence at the ZrO₂ surface of terminal (single-coordinated) hydroxyl groups, whose IR vibrational frequency is at ∼ 3775 cm⁻¹. This hypothesis has been confirmed by ACN adsorption/desorption on some surface-modified m-ZrO₂ systems: (i) m-ZrO₂ pre-treated with chloroform, which selectively interacts with terminal hydroxyls; (ii) m-ZrO₂ pre-treated with a weak acid (CO₂), which interacts weakly with surface basic sites; (iii) m-ZrO₂ with the surface partly covered by strong acid groups (sulfates). The latter system (sulfated m-ZrO₂) presents, when highly hydrated, a medium-weak Brønsted acidity, otherwise absent on all other m-ZrO₂ systems considered.