**4. Conclusions**

Selected spirooxindole natural products.

56 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

increase in Zn content in sunlight.

with no significant loss of catalytic activity.

(MnZnFe2O4 [120]).

**Mixed-metal or core-shell ferrites.** Ferrites containing 2 metal ions, additionally to iron, are much more widespread in the nano-catalysis; their nanoparticle size can vary in a broad range, from ultrasmall particles (5–8 nm) up to 100 nm or more (in case of supported NPs). Both nanosized and out-of-nano-sized mixed-metal ferrite NPs can be synthesized by a variety of methods, in particular classic sol-gel and co-precipitation methods or microwave heating

*Cobalt-based ferrite nanoparticles*. For cobalt-containing ferrite NPs, as well as for zinc ferrite above, one of important applications is the methanol decomposition to CO and hydrogen. Thus, Cu1-*x*Co*x*Fe2O4 (0<*x*<1, 8–40 nm in size) was applied as a nanodimensional powder for this purpose [121]. The stabilization of the cubic structure with the substitution of copper ions by cobalt in mixed Cu-Co ferrites was observed. Cobalt containing ferrites exhibited higher and more stable catalytic activity and selectivity in *methanol decomposition* to CO and hydrogen in comparison with the CuFe2O4 one. Photocatalytic properties of the cobalt zinc ferrite Co1 *<sup>x</sup>*Zn*x*Fe2O4 (0<*x*<1) nanoparticles (10.5−14.8 nm in size), prepared by a hydrothermal method, were studied on the example of *degradation of methyl blue* in aqueous solution [122]. It was elucidated that the oxidation-reduction potential of methyl blue aqueous solution in presence of the ferrite nano-particles at pH=7 under natural sunlight irradiation was negative and increased with increase in Zn content. The degradation rate of methyl blue also decreases as

*Nickel-based ferrite nanoparticles*. Similar to cobalt ferrites, several nickel-containing mixed or core-shell ferrites have been reported as nanocatalysts but in more narrow size range (18–50 nm). Thus, a magnetically separable catalyst consisting of ferric hydrogen sulfate (FHS) supported on silica‐coated nickel ferrite nanoparticles (50 nm) was prepared [123]. This catalyst was shown to be an efficient heterogeneous catalyst for the *synthesis of 1,8*‐*dioxodecahydroacri‐ dines* (reaction *21*) under solvent‐free conditions. The catalyst can be recycled several times

*Other mixed-metal ferrite nanoparticles*. Ferrite nanoparticles, containing other metals and applied in the catalysis, are represented more chaotically in the available literature. Thus, the spinel ferrites Cu1-*x*Cd*x*[Fe1-*x*Al*x*Cr1-*x*Mn*x*]O4, where 0<*x<*1, having unknown particle size, were Iron-based nanoparticles, utilized in catalytic reactions described in this chapter, possess different sizes, from ultrasmall (2 nm) to 100 nm. They are obtained mainly by wet-chemical sol-gel or co-precipitation methods, sometimes combined with simple calcination at high temperatures, sonochemical technique, mechanical high-energy ball milling, or spark plasma sintering, among other methods. Microwave heating or hydrothermal route are also frequently used. Due to magnetic properties, these nano catalysts can be easily recovered from reaction systems and reused up to several runs almost without loss of catalytic activity.

Catalytic processes with application of iron-based nanocomposites are in a wide range. Notable attention is paid to methanol decomposition to CO and methane or to CO and hydrogen. Other catalyzed organic reactions consist of oxidation of various alkenes, aldol, alkylation and dehydrogenation reactions, synthesis of various organic compounds such as, for example, quinoxaline derivatives [126], *β*,*γ*-unsaturated ketones, arylidene barbituric acid derivatives, α-aminonitriles, nopol, 1,4-dihydropyridines, and 1,8‐dioxodecahydro-acridines. Degradation/decomposition processes are also reported, for instance decomposition of H2O2 or photocatalytic degradation of methylene blue. Some of catalyzed reactions might have great practical applications, for instance transesterification of soybean oil to biodiesel. In addition, small iron-based particles could also be considered [127] as substituents of noble metals in a variety of catalytic transformations. Several iron nanomaterials could have biological appli‐ cations, such as peroxidase-like catalytic activity of Fe3O4 ultrasmall NPs [128].

We note that the total number of nano-iron composites applications for catalytic purposes is still not high, so it could be a perfect research niche for further applications of these nanoma‐ terials in a variety of organic processes.
