Mechanistic and Adsorption Studies of Relevance to Photocatalysts on Titanium Grafted Mesoporous Silicalites

Abstract

Ti-SBA-15 and Ti-MCM-41 were synthesized and evaluated as possible photocatalysts for the reduction of CO 2 , and for the photo-Kolbe decomposition of acetic acid. UV-Raman was used to study the adsorption of carbon dioxide, water, formic acid, and acetic acid over Ti-MCM-41 by monitoring the UV enhanced resonance peak of the totally symmetric stretching band of the grafted Ti species at 1,085 cm -1 . Acetic and formic acid dissociate on Ti-SBA-15 and Ti-MCM-41 to form acetate and formate, respectively. The conjugate bases subsequently interact strongly with Ti sites. Water interacts with the Ti sites, while no change in the amplitude of the 1,085 cm -1 band is observed in the presence of CO 2 . Photocatalysis experiments indicate that these mesoporous silicalites are active in the photo-Kolbe decomposition of acetic acid. CO 2 is formed by reaction of a hole with the acetate carboxylate groups. The methyl radical co-products react with a surface proton and an electron to form methane. No products resulting from the dimerization of methyl radicals are observed, presumably because of the highly dispersed active sites.

Tuning the Acid/Base and Structural Properties of Titanate-Loaded Mesoporous Silica by Grafting of Zinc Oxide

“…As the pretreatment temperature in this study was higher, these surface sites may have been removed. In a recent study Danon et al 15 studied the interaction of TiO x /SBA-15 with CO 2 and H 2 O by means of Raman spectroscopy. In agreement with the results of our study, they observed that there is hardly any interaction between CO 2 and TiO x /SBA-15.…”

“…It has also been suggested that due to the generally fast electron− hole recombination kinetics, an interfacial electron transfer seems only kinetically competitive, when the reactant is already preadsorbed. 15,24 Therefore, it is desirable not only to characterize but also to increase the amount of surface-bound CO 2 and H 2 O. 7,18 While the synthesis of ZnO inside micro-or mesoporous silica has previously been reported, the acid/base properties of the obtained species are barely characterized.…”

“…A study by Danon et al monitored the UV resonance Raman vibrational band of the tetrahedral TiO 4 species upon adsorption of CO 2 or H 2 O, confirming the adsorption of H 2 O at this site, but there was no indication for the adsorption of CO 2 . Neither the studies by Anpo et al nor that by Danon et al proved the adsorption of CO 2 on isolated TiO x species by means of a direct or quantitative technique such as temperature-programmed desorption (TPD), thermogravimetry, or microcalorimetry. The adsorption of CO 2 usually requires the presence of basic sites, and CO 2 is a frequently used probe molecule for the detection of basic sites .…”

New insight into the structure of dispersed titania by combining normal-mode analysis with experiment

“…Previous studies have addressed the structure of silicasupported titania both experimentally [8][9][10][11][12][13][14] and theoretically [14,8,15]. From these studies it can be concluded that, at low content, Ti atoms are predominantly present in tetrahedral coordination, whereas with higher Ti content increasing crosslinking (Ti O Ti) and changes in the coordination gain importance.…”

“…For comparison with the calculations, UV Raman, UV-vis, and FT-IR spectroscopy have been used to characterize the structure of silica SBA-15-supported titania (Ti density: 0.7 Ti/nm 2 ). Raman experiments have been performed at an UV excitation wavelength to allow for selective resonance enhancement [9,17].…”