Photocatalytic degradation of toluene over doped and coupled (Ti,M)O2 (M = Sn or Zr) nanocrystalline oxides: Influence of the heteroatom distribution on deactivation

In this work, we have studied the photocatalytic oxidation (PCO) of toluene over Sn- and Zr-doped TiO2, and coupled TiO2/SnO2 and TiO2/ZrO2 catalysts. The TiO2 sample doped with Sn (8% of metal ions) is composed by the anatase and rutile phases of TiO2, while the Zr-doped sample (same dopant content) contains only the anatase phase. The coupled photocatalysts are formed, in addition to the phases present in their doped counterparts, by a segregated MO2 phase (M: Sn or Zr). For the photocatalytic degradation of toluene, higher rates in the stationary state are obtained with the coupled catalysts with respect to the doped ones and the TiO2 references (both synthetic and Degussa P25). In the case of the coupled photocatalysts, these higher rates are due to the absence of the deactivation that does occur for the rest of samples. Fresh and used photocatalysts have been studied by FTIR and EPR spectroscopies and by solid/liquid extraction in methanol, followed by GC/MS analysis. The obtained results lead us to conclude that, while structural and electronic modifications, due to the guest cations, are responsible for the high activity of doped samples observed in previous studies for a reaction not causing catalyst deactivation (methylcyclohexane PCO), other factors are crucial for the PCO of toluene. For this reaction, there is a relationship between surface water, adsorbed intermediates and resistance to deactivation, and thus the modifications in the amount and arrangement of surface water molecules caused by the second oxide may be the cause of the high degradation rate obtained with the coupled TiO2/SnO2 and TiO2/ZrO2 photocatalysts.