Infrared studies of reactions on oxide surfaces. 5. Lewis acid sites on dehydroxylated silica

Abstract

When silica is subjected to a vacuum degassing at about 600 °C two new infrared bands are observed at 908 and 888 cm""1 and the intensity of these bands increases with higher degassing temperature reaching a maximum at about 1200 °C. These bands are associated with the formation of a new active site on to which pyridine and trimethylamine can reversibly coordinate without dissociation of the site or of the adsorbate. It is suggested that a surface silicon atom of strained siloxane bridge is electron deficient thereby acting as a Lewis acid center. The nature and unusual chemical reactivity of this site are discussed in the following

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…As already reported in Section 2, 3.1.4 and 3.1.7, the S2R concentration is rather small (in the initial works 137,535,536 it was reported to be around 0.1 S2R/nm 2 , although more recent data after severe activation steps suggests that it can reach up to 0.5 S2R/nm 2 ). Interestingly, the formation of S2R surface defects is quite general as they have also been identified by their IR fingerprint in crystalline all-silica silicalite.…”

“…Two bands at 888-891 and 908-910 cm -1 had been observed to be present in silicas activated at high temperature (fortunately, they lie in a narrow transparent window in the silica spectrum, between Si-O-Si bending and stretching modes) and to disappear upon adsorption of Lewis bases or Lewis acids 137 (see below, Section 3.1.7). Michalske and Bunker 138 hypothesized that they were specifically attributable to S2R (which they called « edgeshared defects »), and they were able to substantiate this claim by comparing with the vibrational spectra of molecular disiloxanes 139 (Figure 10).…”

Simulating liquid and amorphous silicon dioxide using real-space pseudopotentials

“…33,44,45 In order to understand the vibrational spectrum of the 2m ring, we calculated the vibrational density of states with the 24-atom system. Figure 12 shows our calculations in comparison to the CPMD simulation (dashed line) and experiment (circles).…”

“…In IR experiment on dehydroxylated a-SiO 2 surface, two unique peaks at 888 and 908 cm −1 have been reported. 44 These two peaks are regarded to be a strained defect, i.e., the edge-sharing structure on the surface. We note that the IR experiments focused on the surface that was thermally treated.…”

CrO /SiO2 catalysts prepared by metal vapour synthesis: Physical–chemical characterisation and functional testing in oxidative dehydrogenation of propane

“…Strained Si-O-Si bridges, reactive toward ammonia, are known to be formed also on bare silica, but after outgassing treatment at significantly higher temperatures [37][38][39]. The formation of such particular structure is then apparently promoted by the involvement of the silica support in the fixation of chromium species.…”