Surface Acidity and Properties of TiO2/SiO2 Catalysts Prepared by Atomic Layer Deposition: UV-visible Diffuse Reflectance, DRIFTS, and Visible Raman Spectroscopy Studies
NORTHWESTERN UNIV EVANSTON IL DEPT OF CHEMISTRY
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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 deg C temperature range, which is slightly higher than that from TTIP at 150 deg 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 Br nsted acid sites. Additionally, three new pronounced bands in the Raman spectra, 6b 638 cm-1, 9a 1200 cm-1, and 2 3103 cm-1, provide strong spectroscopic evidence for Br nsted acid sites on the surface.
- Organic Chemistry
- Physical Chemistry
- Atomic and Molecular Physics and Spectroscopy