Surface Chemistry of TiO₂ Photocatalysts

Citation

Szczepankiewicz, Steven Henry (2001) Surface Chemistry of TiO₂ Photocatalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HABJ-3T48. https://resolver.caltech.edu/CaltechETD:etd-05232006-094537

Abstract

The surface chemistry of TiO₂ photocatalysts have been characterized by infrared spectroscopy, alcalimetric titrations, and photocatalytic reactivity. The fundamental processes governing electron transfer at the surface were investigated by inhibiting the photocatalytic reactions.

Free and trapped charge carriers in polycrystalline TiO₂ following bandgap irradiation are characterized by diffuse reflectance IR spectroscopy (DRIFTS). Irradiation of anatase in vacuo or in the presence of CD₃OD leads to decreased overall reflectance, to an absorbance at 3717 cm⁻¹, and to the decline of a 3647 cm⁻¹ band. x persists for days in vacuum or dry O₂ and is only bleached by the action of [O₂ + H₂O(g) + hv]. It is assigned to a Ti(III)O-H vibration ensuing electron trapping at the acidic Ti(IV)OH centers that absorb at 3647 cm⁻¹. Irradiation under O₂ yields a band at 3683 cm⁻¹ (Y) that corresponds to bound OH radicals. These last indefinitely in vacuo, but decay in a few hours under O₂. Since none of the above phenomena occur on defective TiO₂-x, materials obtained by thermal annealing under vacuum, photochemical damage to actual catalysts involves concurrent surface reorganization. The presence of the 3717 cm⁻¹ band is used to confirm this reorganization.

The decreased reflectivity is due to a spectrum-wide absorption signal proportional to λ^1.7 (λ = wavelength/µm), which indicates the presence of free conduction band electrons coupled with acoustic phonons in the lattice. Free electrons appear to decay according to saturation kinetics. The fitted parameters indicate a limited number of reversible trapping states. The concentration of these states appears to be diminished by sequential UV treatments. A broad IR absorption peak centered at 3380 cm⁻¹ is attributed to an electronic transition from an occupied surface electron trap 0.42 eV below the conduction band. The free carrier decay lifetime is lengthened as the samples are dehydrated.

Electric fields generated by photoexcited charge carriers in TiO₂ (anatase) produce Stark effect intensity and wavelength shifts for surface TiO-H stretching vibrations. Shallow electron-trapping states, observed as broad absorption bands above 3000 cm⁻¹, produce an apparently homogeneous electric field. Intensity changes and corresponding wavelength shifts for v(TiO-H) are proportional to the magnitude and polarity of the electric field. O₂ is shown to reversibly abstract electrons from shallow trapping states. These results suggest that shallow electron traps are not associated with localized structures, but rather are delocalized across the TiO₂ surface.

The effect of poisoning the photocatayst is monitored during the oxidation of a phosphorous-containing organic substrate. The photoassisted oxidation of dimethyl-methyl-phosphonate (DMMP) over UV-irradiated TiO₂ is investigated as a simulant for warfare nerve agent detoxification. Adsorption uptake measurements are performed by MS analysis of a fine DMMP aerosol up and downstream of an adsorbant TiO₂ film. Photodesorption processes, gas phase intermediates and mineral products are quantified in situ by headspace GC-MS analysis of a static system. Non-volatile intermediates and products are analyzed by HPLC analysis of both aqueous and organic extractions from the Ti02 film. Adsorbed intermediates are characterized and quantified in situ by DRIFTS of TiO₂ powders. Specific site binding of DMMIP and catalyst poisoning are observed in the DRIFT spectra. A proposed mechanism suggests rapid detoxification of DMMP as a simulant, but extensive poisoning of the catalyst by the end products.

In a comparative investigation of titania surface chemistry, TiO₂ Q particles are synthesized and characterized by transmission electron microscopy, potentiometric titration, infrared analysis, and photocatalytic reactivity. TEM images show small individual particulates of anatase about 2 nm in diameter. Potentiometric titrations confirm the presence of a highly protonated gel layer on the surface of these particles. The infrared spectra indicate that this layer is amorphous. An autocatalytic enhancement of the photocatalysts during photooxidation of methyl orange suggests that reaction intermediates cause the collapse of the hydrated gel layer into a more active surface.

Two supplementary investigations are presented which confirm the previous results. MAS NMR data is presented which shows a paramagnetic influence from surface trapped electrons. The data also suggests localized trapping at a characteristic type of TiOH surface moiety. Theoretical calculations also confirm the band assignments presented in the previous chapters by reproducing the data with the predicted structural assignments.

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