Ensemble Effects in Nanostructured TiO₂ Used in the Gas-Phase Photooxidation of Trichloroethylene

Abstract

Nanostructured TiO2 with different crystal sizes (i.e., 3, 5, and 6 nm) but identical aggregate size (i.e., 100 nm) and shape (sphere) were successfully made using a modified sol−gel method. TiO2 catalysts with crystal size of 5 nm and aggregate size of 100 nm were also prepared. This batch of catalysts has similar phase structure, surface area, band-gap energy, and degree of surface hydroxylation, and differs mainly in their crystallinity. Synchrotron XRD, XANES and EXAFS, Raman spectroscopy and transmission electron microscopy (TEM) were used to characterize the structural properties of the nanometer sized TiO2, whereas ultraviolet, X-ray photoelectron and electron paramagnetic resonance spectroscopy provided important information on the electronic properties of these nanoparticles. The catalytic performance of the nanostructured TiO2 for gas-phase photocatalytic oxidation of trichloroethylene exhibited strong dependency on both crystal size and crystallinity.

Enhanced Catalytic Activity of Sub‐nanometer Titania Clusters Confined inside Double‐Wall Carbon Nanotubes

“…Earlier studies have correlated the pre-edge peak positions and their intensities of titania with the coordination symmetry around titanium. [26,27] The pre-edge features of TD samples indicate that five-and six-coordinated titanium are likely to coexist, as it was reported for unsupported small titania nanoparticles. [26,27] Interestingly, the 4970.5 eV peak of TD-in shifts to a lower energy (4970.0 eV) upon in situ heat-treatment at 300 8C while that of TD-out does not change.…”

“…[26,27] The pre-edge features of TD samples indicate that five-and six-coordinated titanium are likely to coexist, as it was reported for unsupported small titania nanoparticles. [26,27] Interestingly, the 4970.5 eV peak of TD-in shifts to a lower energy (4970.0 eV) upon in situ heat-treatment at 300 8C while that of TD-out does not change. A further increase of the heat-treatment temperature does not cause additional changes in the pre-edge region for both samples.…”

“…[25][26][27] Therefore, we investigated the titanium by XANES in order to gain further insight into the structure of titania confined in DWNTs. Figure 3 a-b show that the Ti XANES features of the TD samples resemble those of anatase, indicating that the oxidation state is Ti 4+ .…”

Techniques for the Study of the Electronic Properties.

“…A general feature shared by all nanostructured oxide materials with size typically below 10-15 nm is that they display broader CRs with respect to well-crystallized references. 19,21,24,25,26,31,33,39 This can be observed in the inset of Figure 6.1; the derivative spectra give evidence of a larger overlap between CRs. The reason for this can be understood in terms of the f-rule.…”

Techniques for the Study of the Structural Properties

“…A point to mention is that in some case, like ZrO 2 oxides, coexistence of two crystalline (or one amorphous) phases can lead to destructive interference effects and misleading interpretation of results. 54 For titanium oxides, the studies usually identify the presence of fivefoldcoordinated species in the nanostructured anatase and rutile surfaces, with a shortening of the Ti-O first shell bond distance 37,38 which in some cases is identified as a result of presence of Ti hydroxyl groups at the surface 37 . Note however that the inclusion of the third cumulant for studying mesoporous (amorphous by XRD but with a surface local structure similar to nanostructured anatase) titania suggests a larger Ti-O average first coordination distance with respect to the rutile/anatase phase 50 .…”