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The ambient temperature adsorption=desorption of acetonitrile (ACN), in its deuterated form, was used for the surface characterisation of several ZrO 2 -based systems. On pure m-ZrO 2 , ACN uptake originates four types of chemisorptive interaction: s-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-ZrO 2 data with some preliminary data of ACN adsorption on t-ZrO 2 suggest that the two surface reactions of ACN depend on the presence at the ZrO 2 surface of terminal (single-coordinated) hydroxyl groups, whose IR vibrational frequency is at $ 3775 cm À1 . This hypothesis has been confirmed by ACN adsorption=desorption on some surface-modified m-ZrO 2 systems: (i) m-ZrO 2 pre-treated with chloroform, which selectively interacts with terminal hydroxyls; (ii) m-ZrO 2 pre-treated with a weak acid (CO 2 ), which interacts weakly with surface basic sites; (iii) m-ZrO 2 with the surface partly covered by strong acid groups (sulfates). The latter system (sulfated m-ZrO 2 ) presents, when highly hydrated, a medium-weak Brnsted acidity, otherwise absent on all other m-ZrO 2 systems considered.

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