In turn, Iron cobaltite (FeCo
2O
4) has been also studied in environmental remediation and energy storage, thanks to its electrical properties and electrochemical performance
[45]. In the energy storage field, Mohamed et al. demonstrated that iron cobaltite nanorods show a better capacity and lower overpotential as the cathode of lithium−O
2 batteries than other metal cobaltites (Mn, Ni, and Zn)
[46] because the FeCo
2O
4 surface has the highest number of electropositive Co
3+ active sites that improve the oxygen adsorption and Fe
2+ in the tetrahedral site that favors the release of electrons to reduce oxygen. Yadav et al.
[47] demonstrated that iron cobaltites are also efficient for supercapacitive and photocatalytic applications due to the valence states of the Fe
3+/Fe
2+ and Co
3+/Co
2+ species, which are considered active catalytic sites. These nanoflake-like iron cobaltites present a capacitance as high as 1230 F g
−1 (5 mV s
−1) with a good rate capability and superior cycling stability and also show a good photocatalytic performance achieving up to 94.19% degradation of crystal violet dye under sunlight illumination. However, despite this, very little work has been carried out with reference to their evaluation as a Fenton-like catalyst
[48][49]. Zhang et al.
[48] synthesized nitrogen-containing carbon/FeCo
2O
4 composites and analyzed their performances as Fenton catalysts for the degradation of methylene blue obtaining almost 100% removal in 10 min without pH adjustment, which was attributed to the uniform distribution of bimetals and nitrogen doping, which ensured the exposure of sites with high catalytic activity. Zhao et al.
[49] analyzed the behavior of FeCo
2O
4/g–C
3N
4 as a photo-Fenton catalyst in the degradation of rhodamine B (RhB), obtaining 98% degradation in 45 min, which was attributed to a synergetic interaction between photocatalytic and Fenton-like reactions and the effective separation of the photogenerated charges. Therefore, it is proposed that the use of iron cobaltite as a pristine catalyst (without doping or support) in the Fenton reaction may be promising for future applications.
In general, spinel ferrites turn out to be attractive materials for catalytic activities in Fenton processes, mainly because they manage to improve the generation reaction of Fe
2+ and achieve a synergistic effect between metal ions with valences similar to those of iron involved in the process (M
2+, M
3+, M: Metal).
Perovskites are other types of materials with promising catalytic activity that can be synthesized by the abovementioned methods (Chemical coprecipitation, sol–gel, and hydro/solvothermal)
[50][51][52][53]. Perovskites can be defined as a type of mixed oxide with different formulations, binary (ABO), ternary (AA’BO or ABB’O), and quaternary (AA’BB’O), where A and B are cations sites occupied by alkali metals, alkaline-earth metals or rare-earth metals and transition metals, respectively
[54].
Some perovskites have been studied in different Fenton-type reactions, for example, Carrasco-Díaz et al.
[55] removed paracetamol by Fenton reaction using LaCu
1−xM
xO
3 (M = Mn, Ti) as the catalyst and determined that the most active catalyst was the one that contained the highest amount of Cu
2+ at the surface. Moreover, they found that the titanium and manganese species seem not to be responsible for the improvement of activity with respect to the sample LaCuO
3. Li et al.
[56] synthetized a Ca
1−xFeO
3−δ perovskite and determined that the A-site cation can distort the FeO
6 octahedra in the perovskite and regulate the oxygen vacancies (OV) concentration; in this way, an A-site cation deficient of Ca
0.9FeO
3-δ results in an improved H
2O
2 activation for the degradation of tetracycline by a Fenton-like process. Similarly, Xie et al.
[57] found that the copper incorporation in LaCoO
3 perovskite improved the electro-Fenton activity due to the enhancement of redox activity and oxygen vacancies, but in this case, by the substitution of B-site elements, which synergistically promoted the activation of hydrogen peroxide to a hydroxyl radical (
•OH). On the other hand, Rusevova et al.
[58] degraded phenol via heterogeneous Fenton-like reactions using iron-containing LaFeO
3, and BiFeO
3 perovskites, and made a comparison with data reported in the literature using, as a catalyst, nano-sized Fe(II, III) oxide particles, determining that the perovskites synthetized
in th
is work had a higher catalytic activity. Zhao et al.
[59] determined that BiFeO
3 supported in carbon aerogel (BFO/CA) with a three-dimensional (3D) structure improves the catalytic activity and stability of BiFeO
3, resulting in an interesting strategy for the development of advanced catalysts for its possible application in Fenton processes.
In summary, perovskites are materials similar to spinel ferrites, and the contribution of two different metal species (formation of OVs) can be of interest to the Fenton process.