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 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.

Preparation of Zirconium Oxide Powder Using Zirconium Carboxylate Precursors

“…These display a sharp band at about 2,339 cm −1 that is attributed to the adsorbed CO 2 species on the surface that resulted from the thermal decomposition of the carboxylate ligand. This assignment is in good agreement with previous studies on the adsorption of CO 2 onto ZrO 2 and modified ZrO 2 [28,29]. The peak observed at about 1,424 cm −1 for the zirconium propionate calcined at 470 ∘ C (Figure 3(b)) probably corresponds to the presence of some carbonate species adsorbed on the surface of t-ZrO 2 .…”

The Effect of Al-Alkyls on the Phillips Catalyst for Ethylene Polymerization: The Case of Diethylaluminum Ethoxide (DEALE)

“…This is clearly evident by looking to the spectra of CD 3 CN adsorbed on SiO 2 +DEALE (Figure 4c). Two bands are observed in the ̃(CN) region, at 2276 and 2267 cm -1 , which are assigned to CD 3 CN adsorbed on the OH groups and to liquid-like acetonitrile, respectively [48,50,51,57]. Both bands decrease rapidly in intensity upon degassing up to disappear, indicating that the interaction of d-acetonitrile with the silica surface is quite weak and fully reversible.…”

“…To obtain a more complete description of the Cr(II)/SiO 2 +DEALE system, the accessibility of the Cr sites was successively probed by the adsorption of CD 3 CN at room temperature. This molecule has been largely employed to probe both acid and basic sites on metal oxides and zeolites [48][49][50][51][52][53][54][55][56][57].…”

Mechanism of acrylonitrile decomposition over Cu-ZSM-5

“…A weak band at 3376 cm −1 is regarded as additional evidence for adsorbed NH 3 [13,14]. The band at 2306 cm −1 has been also ascribed to AN adsorbed on Lewis acid sites [15], but in our study this band increased at first and then decreased with increasing temperature, which suggests that it overlaps with the band of other products, perhaps -NCO [16]. The disappearance of this band at 400 • C coincided with the peak for HNCO formation during TPSR measurement.…”