A wide variety of methods have been applied to prepare nanostructured Ti02 employing wet chemical, physical deposition or hybrid methods. They generally yield products with different structures (anatase or rutile), crystallinity, porosity and contaminants. As a consequence the surface properties of the products depend on the preparation technique. Unfortunately, a significant fraction of reported studies involve the T1O2 powder denoted P25 from Degussa AG. which is prepared by vapour-phase oxidation of TiCU. This process yields a fairly narrow size distribution ((d}=20-25 nm). However the structure is a mixture of anatase (ca. 75 at.%) and rutile particles and contains chlorine as a main contaminant. Thus the reactivity of P25 cannot easily be related to the surface properties of pure phase TiOi. Chlorine free TiC>2 prepared by the sulphate route (usually FeTiOj) contain sulphate ions, which affects its surface acidity. High phase purity and contaminate free Ti02 can be prepared by the alkoxide route (usually Ti(OC3H7)4 or Ti(OC4H<j)4). However, both the sulphate and alkoxide routes require a final calcination step to remove water and carbon contaminants, respectively. Since residues and contaminants from the synthesis can affect particle growth and phase transformations (which is particle size dependent—see above), calcination introduces variations in the final product which may be difficult to control. Large progress has been made in various film fabrication methods to prepare TiC>2 films with unique properties. In particular physical and chemical vapour phase deposition techniques (PVD and CVD, respectively) allow for high purity, contaminant free crystalline structures without further pretreatment steps (calcination). These latter methods introduce an additional degree of freedom which affects the final TiC>2 product, namely the substrate. Interestingly, it has been shown that Ti02 with different growth orientations of surface faces can be obtained compared with conventional wet chemical methods. In particular, it has been reported that TiC>2 films prepared by CVD on glass substrates yield structures with preferential (112) oriented crystallites which have beneficial photocatalytic activity.13 Similarly, surface modified Al/Ti on glass by anodization and subsequent alternating sol-gel dip coating yield preferential (004) oriented crystallites with up to 6 times higher photooxidation rate of acetaldehyde gas.14 The origin of the beneficial activity of these novel TiC>2 films are not known and has been tentatively explained by formation of hollow nanostructures with unusual high exposed surface area (films with both (112) 3 and (004) 14 crystallite orientation), or a high number density of active sites (films with (112) faces l5). Despite the enormous efforts to synthesize and improve the performance of Ti02 photocatalysts during the past say 30 years, these recent results suggest that we still have plenty of room for improvements and still lack fundamental insight what makes a good TiC>2 photocatalyst.
In this paper we take an approach which is a compromise between traditional photocatalysis work and the surface science approach. We take the advantage of the progress of nanofabrication methods to prepare well-defined TiC>2 that expose different (distributions of) crystallite orientations. We explore Ti02 prepared by a broad range of physical and wet chemical methods. We review our work employing in situ vibrational spectroscopy to explore at the molecular level the interaction of probe molecules with TiC>2. We also include new data which has not been published before. By scrutinizing the adsorption properties and the photochemical reaction pathways of the probe molecules by in situ molecular spectroscopy and relating them to the detailed atomistic structure of the Ti02 obtained by high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray diffraction (XRD), we are able to correlate and identify rate determining steps and thus provide independent data (other than kinetic data) that supports a mechanistic interpretation of the observed difference between TiCh having different structures, crystallinity and contaminants. We exemplify this with the case study of photocatalytic oxidation of propane, where acetone and formate are found to be a key intermediates that limits the oxidation rate.
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