This article first indicates the various types of temperature-induced oxygen isotopic exchange (OIE) that can occur between labeled gaseous O2 and solid ox

The photocatalytic removal rates r of dichloroacetic acid (DCAA), 4-chlorobenzoic acid (4-CBA) and 4-chlorophenol (4-CP) in water were compared for TiO2 samples used either as a powder or as a coating on a fiber glass tissue, SiO₂ being the binder. From SEM-EDX measurements it was deduced that SiO₂ prevails over TiO₂ in the coating top layers and ¹⁸O₂-Ti₁₆O₂ isotopic exchange showed that the accessibility of O2 to TiO2 was markedly reduced when TiO₂ was thus coated. The unfavorable effect of the restricted TiO₂ accessibility on r was drastic for DCAA, much less pronounced for 4-CBA, and still smaller for 4-CP. It is inferred that DCAA can be attacked only when it directly interacts with TiO₂, whereas 4-CP can also react within the near-TiO₂ surface water layers. The 4-CBA intermediate behavior is in line with the structural similarities of 4-CBA with DCAA and 4-CP.

Titanium dioxide nanotubes (TNT) have mainly been used in dye sensitized solar cells, essentially because of a higher transport rate of electrons from the adsorbed photo-excited dye to the Ti electrode onto which TNT instead of TiO₂ nanoparticles (TNP) are attached. The dimension ranges and the two main synthesis methods of TNT are briefly indicated here. Not surprisingly, the particular and regular texture of TNT was also expected to improve the photocatalytic efficacy for pollutant removal in air and water with respect to TNP. In this short review, the validity of this expectation is checked using the regrettably small number of literature comparisons between TNT and commercialized TNP referring to films of similar thickness and layers or slurries containing an equal TiO₂ mass. Although the irradiated geometrical area differed for each study, it was identical for each comparison considered here. For the removal of toluene (methylbenzene) or acetaldehyde (ethanal) in air, the average ratio of the efficacy of TNT over that of TiO₂ P25 was about 1.5, and for the removal of dyes in water, it was around 1. This lack of major improvement with TNT compared to TNP could partially be due to TNT texture disorders as seems to be suggested by the better average performance of anodic oxidation-prepared TNT. It could also come from the fact that the properties influencing the efficacy are more numerous, their interrelations more complex and their effects more important for pollutant removal than for dye sensitized solar cells and photoelectrocatalysis where the electron transport rate is the crucial parameter.

This paper outlines the basic events that occur when a semiconductor, in contact with aerated liquid water containing low concentrations of pollutants, is photoexcited. First, the factors favouring this recombination of photoproduced charges are recalled, and the difficulties in decreasing the recombination are underlined. Second, the formation and identification of the species resulting from charge transfer with O₂, H₂O and pollutants are presented. Third, the reactions involving these species are considered, and methods for assessing their respective importance are critically reviewed. Fourth, on the basis of the view that the photocatalyst surface is covered by tightly bound water layers which hinder the adsorption of many organic pollutants, the hypothesis according to which primary photocatalytic events can take place within the near-surface solution layers is discussed.

TiO₂ photocatalysis and ultrasound are advanced oxidation processes for water treatment. Our study aimed at showing, via the purposely chosen title compounds, that these techniques can be complementary. For C₆H₅COCF₃ (PTMK), the photocatalytic removal rate was higher than the ultrasonic (515 kHz) removal rate in the presence of TiO₂ in the dark, whereas it was the opposite for octan-1-ol under the conditions employed. Simultaneous UV and ultrasound irradiation of the TiO₂ suspension led, for PTMK, to a removal rate about equal to the sum of the removal rates observed for separate irradiations, and decreased by a factor of approximately 20% for octan-1-ol as if the photocatalytic degradation was suppressed by the dominant distribution of octan-1-ol to the cavitation bubbles. This distribution was substantiated by the large detrimental effect of octan-1-ol on the PTMK ultrasonic removal rate. The concurrent use of both techniques allowed a faster removal of both pollutants in binary mixtures. The amount of CF₃COOH from PTMK was approximately eight times lower in sonicated, than in UV-irradiated, TiO₂ suspensions. Several intermediate products showed the occurrence of chemical transformations occurring in and/or on the cavitation bubbles. COD decline and CO₂ formation were initially higher for ultrasonic than for photocatalytic treatment. However, complete mineralization (except for CF₃COOH) was achieved more rapidly by photocatalysis and even more rapidly by simultaneous use of both techniques.