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Structure-Photocatalytic Relationships of Well-Defined TiO2 Nanodomains

Charles A. Roberts and Israel E. Wachs. Chemical Engineering, Lehigh University, 111 Research Dr., B323 Iacocca Hall, Bethlehem, PA 18015

The goal of this study to examine the photocatalysis of well-defined TiO2 nanodomains supported on SiO2 and to determine their structure-photocatalytic relationships. Understanding how catalytic structure relates to photocatalytic properties (defect formation, adsorption, electron excitation) in these catalysts of known structure will lead to more rapid development in the discovery of improved photocatalysts for specific reactions. Thus, 1-60% TiO2/SiO2 catalysts were synthesized by incipient wetness impregnation of Ti-isopropoxide into the SiO2 support (Cab-O-Sil), drying and calcination at 500 oC. The molecular and electronic structures of the TiO2 nanodomains were determined with in situ Raman and UV-vis spectroscopy. The nature of the TiO2 nandomain was found to change in the following manner as a function of the titania loading: isolated site (1% TiO2/SiO2) < polymeric chain (12% TiO2/SiO2) < 2D sheets (20-40% TiO2/SiO2) < 3D nanoclusters (60% TiO2/SiO2). Electron paramagnetic resonance (EPR) was conducted to examine the nature of defects created on the surface by reduction in flowing 5% H2/He. The effect of UV illumination on the defects was also studied since the EPR cell was equipped with a polychromatic lamp. The structure of oxygen adsorbed on the surface was examined by low temp (120 K) adsorption in flowing 10% O2/He. The same reduction and adsorption experiments were also performed using in situ UV-Raman spectroscopy (266 nm excitation). In order to study propylene oxidation adsorbates under reaction conditions, a novel experimental design was required. An operando DRIFTS experimental apparatus was designed using a specially modified Harrick Praying Mantis cell for simultaneous spectrum collection , UV irradiation, and real time product analysis via an online mass spectrometer. Finally studies were conducted using time resolved fluorescence spectroscopy to determine if the type of TiO2 nanodomain present affects the decay rate of excited electron states.

The primary results included a striking similarity in the behavior of an F' defect center signal in the in situ EPR and a band at 1599 cm-1 in the UV-Raman for the 12 and 60% TiO2/SiO2 catalysts, suggesting a possible induced Raman signal that could be used as an indirect method for tracking photocatalytic behavior. The EPR studies also showed that the F' defect center thermally most stable on the 12% TiO2/SiO2 catalyst. It is known from the structural characterization studies that the 12% TiO2/SiO2 catalysts consists entirely of polymeric surface chain nanostructures. Thus, the polymeric surface titania chain is preferential for the formation of F' center defects and, thus, preferential for photocatalytic activity. The DRIFT study revealed that the adsorption of propylene was preferential to acetone intermediates as evidenced by vibrations at 1688 and 1421 cm-1 under reaction gas flow and UV irradiation. Furthermore, the studies showed that catalysts containing crystalline nanodomains more readily formed the acetone surface species. Catalysts promoted with WOx species were also examined and showed no difference in the adsorption spectra.