Surface Acidity and Properties of TiO₂/SiO₂ Catalysts Prepared by Atomic Layer Deposition: UV−visible Diffuse Reflectance, DRIFTS, and Visible Raman Spectroscopy Studies

Abstract

Highly uniform submonolayer to multilayer thin films of titanium dioxide supported on high surface area silica gel have been synthesized by atomic layer deposition (ALD) using titanium tetrachloride (TiCl 4) and titanium isopropoxide (TTIP) as metal precursors. The deposition rate of titania films from TiCl 4 was found to be stable in the 150-300°C temperature range, which is slightly higher than that from TTIP at 150°C. UV-visible diffuse reflectance spectroscopy (DRS) shows that the coordination geometry of Ti cations depends on the number of ALD cycles and the precursor but is essentially independent of deposition temperature. Using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and visible Raman spectroscopy with pyridine as a probe molecule, we found all of the titania films studied to exhibit Lewis acidity but only films containing chloride or carbonyl impurities possessed Brønsted acid sites. Additionally, three new pronounced bands in the Raman spectra, ν 6b (638 cm-1), ν 9a (1200 cm-1), and ν 2 (3103 cm-1), provide strong spectroscopic evidence for Brønsted acid sites on the surface.

Preparation and Foliar Application of Oligochitosan - Nanosilica on the Enhancement of Soybean Seed Yield

“…In the range of 200 -800 nm, the nanosilica had no absorption (Fig. 4d), the same result was also obtained by Lu et al (2009) [28]. While the UVVis spectrum of OC-nanosilica exhibited a shift of the 262 nm peak of oligochitosan to 271 nm and a shoulder in the range between 300 and 350 nm.…”

Controlled deposition of titanium oxide overcoats by non-hydrolytic sol gel for improved catalyst selectivity and stability

“…A third band at 1490 cm À1 can be attributed to both Lewis and Brønsted acid sites but the absence of bands at 1545 or 1640 cm À1 reveals the absence of significant Brønsted acidity on all materials [63,64]. Since Brønsted acidity in silicasupported TiO 2 materials has been recently attributed to the presence of chlorides and alkoxides impurities [60,65], those data further confirmed the efficient elimination of any precursor residue from the metal oxide framework during post-synthetic treatment. The maximum intensity for the peak attributed to Lewis acid sites was observed for 6 TiO 2 layers where hydrogen bonding was observed as well with a band at 1590 cm…”

“…While SBA-15 does not have significant acidity, acidity can be generated by the formation of mixed oxide at the titania-silica interface [60]. We observed controlled acid site formation during overcoating.…”

Flower-like tungsten oxide particles: Synthesis, characterization and dimethyl methylphosphonate sensing properties

“…Acidic metal oxides have abundant Lewis and Brønsted sites, where adsorption and/or catalysis may take place [15,16]. As the amount of water on the surface of metal oxides can alter the number of those acid sites, a TG analysis was applied to examine the state of water on the surface of WO 3 .…”