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

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 (TiCl4) and titanium isopropoxide (TTIP) as metal precursors. The deposition rate of titania films from TiCl4 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, ν6b (638 cm-1), ν9a (1200 cm-1), and ν2 (3103 cm-1), provide strong spectroscopic evidence for Brønsted acid sites on the surface.