*3.3. Catalytic Wet Peroxide Oxidation (H2O2/Catalyst/Air)*

Catalytic wet peroxide oxidation (CWPO) processes are based on the degradation of contaminants by the combined action of a solid catalyst, hydrogen peroxide and air in aqueous solution. In a certain extent, they are similar to Fenton-like processes, because they use H2O2 as oxidant; however CWPO processes are carried out in the presence of a flow of air or under pressurized air. They can work with high oxidation efficiency in a wide range of pH without leaching or production of sludge.

Apart from Fenton-type catalysts, including zeolitic materials or composite metal oxides, a reduced number of perovskites has also been tested for CWPO applications. It is important to note that very little theoretical and experimental information is available and there are only a few examples of CWPO using perovskites as heterogeneous catalysts.

The first application of a perovskite in CWPO was carried out by Ovejero et al. [85], who compared the activity of LaTi0.45Cu0.55O3 with that of other catalysts containing Fe or Cu in the degradation of phenol. The reactions were carried out at 100 ◦C in a system pressurized with air at 1 MPa. The perovskite led to a complete elimination of phenol and a TOC removal of 92% in 45 min. The leaching of copper was 22%; however it was significantly lower than the leaching measured for other catalysts (between 64 and 74%). Considering the fact of the high stability of copper ions in perovskite structures, it is probable that most of the leached copper proceeded from La2CuO4 oxide, phase detected by XRD together with the perovskite phase. LaTi0.45Cu0.55O3 was reused in a second cycle, the TOC removal decreasing only to 90%.

Three years later, the same authors extended the study of the CWPO degradation of phenol to other perovskites, in order to elucidate the influence of different reaction conditions (temperature, H2O2 peroxide concentration, catalyst concentration and air pressure) on the performance [86]. Three perovskites of LaTi1–xCuxO3 composition, with different substitution degree, were tested in the reaction. TOC removal values comprised between 88 and 94% were achieved at 100 ◦C under air pressure (1 MPa) and stoichiometric amount of H2O2 after 2 h. The temperature exerted a significant effect on the activity and only a 15% of TOC removal was reached when reaction was developed at 40 ◦C. The catalysts could be easily regenerated by calcination in air, leading to similar activity in the second run.

Less drastic operation conditions that the reported above were applied by Faye et al. [87] for the degradation of the same contaminant, phenol, by the action of several LaFeO3 perovskites, synthesized by self-combustion method by varying the glycine/NO3 − molar ratio. Thus, authors used a flow of air at atmospheric pressure and mild temperatures, 25 or 40 ◦C. Depending on synthesis conditions, strong differences in the structural, textural and reducibility characteristics were observed. The perovskite having the highest surface area exhibited the highest TOC abatement (76%) and very low iron leaching (0.27 wt%) after 4 h of reaction at 40 ◦C. The perovskites were better catalysts than Fe2O3, for which a TOC removal of only 10% was reached after 4 h.
