Degradation of phenol and benzoic acid in the presence of a TiO2-based heterogeneous photocatalyst

This paper reports the results of a study on the titanium dioxide Wackherr’s ‘‘Oxyde de titane standard’’, which shows very interesting photocatalytic activity. Produced for cosmetic purposes as a white pigment, its features make it very interesting in the field of heterogeneous photocatalysis. The results obtained with this photocatalyst are compared with the behaviour of the well-studied and widely used TiO2 Degussa P25 under the same conditions. In particular, the TiO2 Wackherr induces relevantly faster phenol degradation than P25 when high photocatalyst loading is used (up to 2.00 g l-1), as phenol degradation rate in the presence of TiO2 Wackherr continues to increase with increasing photocatalyst loading. This is most likely due to the lower radiation scattering in the UV region of TiO2 Wackherr when compared with Degussa P25, which is linked with the higher particle size of TiO2 Wackherr. The initial phenol degradation rate by TiO2 Wackherr as a function of phenol concentration has a maximum for [phenol] ≈ 3x10-4 M, and decreases for higher concentration values. The addition of fluoride ions to TiO2 Wackherr at pH 3.7 increases phenol degradation rate, as already found for Degussa P25. The increase is more relevant for higher phenol concentration values and makes the maximum as a function of phenol concentration to disappear. Comparable results are also obtained when benzoic acid is used as a substrate, with some differences from phenol that can be accounted for by benzoic acid more strongly interacting with the surface. The use of TiO2 Wackherr in heterogeneous photocatalysis can be desirable when high photocatalyst loading is required.