Photocatalysis for Indoor Air Cleaning : In situ FTIR and DFT Study of Acetaldehyde Photo-Oxidation on TiO2 and Sulfate-Modified TiO2

Abstract: Acetaldehyde is among the group of VOC’s causing the so-called "sick building" syndrome. WHO has estimated that this problem, related to the indoor air-quality, affects one third of all buildings worldwide. TiO2 is an attractive material for the photocatalytic removal of VOC’s in the indoor environment. Deactivation of the photocatalyst is, however, commonly deteriorating its performance over usage time, and ways to mitigate this problem must be invented. Thus modification of TiO2 surface properties is an area of considerable interest. It is possible to enhance the surface acidity of TiO2 through sulfation which effectively lowers the affinity of acidic intermediates. In addition, surface modification of photocatalysts may be used to control adsorption and wetting properties, as well as providing scavengers for photo-excited electrons and holes to suppress unwanted recombination and increase the quantum yield.In this work the interaction of gaseous acetaldehyde with TiO2 and SO4-modified anatase TiO2 is investigated. Films were prepared by doctor-blading of commercial TiO2 nanoparticle suspensions and characterized by means of XRD and TEM. The adsorption and photo-oxidation of acetaldehyde were studied with in situ FTIR spectroscopy and DFT calculations.On TiO2 nanoparticles, which exhibit predominantly (101) facets, in situ FTIR shows that acetaldehyde adsorption is accompanied by the appearance of a hitherto non-assigned absorption band at 1643 cm−1, shown to be due to acetaldehyde dimers. The results are supported by DFT calculations performed at the M06/6-31++G'' level. Vibrational frequencies calculated within a partially relaxed cluster model for molecular acetaldehyde and its dimer, and for the corresponding adsorbed species on the anatase (101) surface, were in good agreement with the experimental results. Inclusion of dimer formation is shown to give an improved description of the reaction kinetics on TiO2.Based on mode-resolved in situ FTIR, kinetic models were constructed, which describe the observed photo-oxidation surface products on both TiO2 and SO4-modified TiO2. The surface concentration of main surface products and corresponding reaction rates were determined. It was observed that formate is the major reaction product, whose further oxidation limits the complete oxidation to gaseous species. Formate thus acts as an inhibitor, blocking adsorption sites, and is responsible for photocatalyst deactivation. The oxidation reaction is characterized by two reaction pathways, one fast and one slow, associated with two types of surface reaction sites. On the SO4-modified TiO2 fewer intermediates are accumulated, and it resists deactivation much better compared with the pure TiO2 photocatalyst, which is attributed to the acidic character of the modified surface. The results presented here are of interest for applications to photocatalytic air purification, and surfaces with controlled wettability.

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