**Author details**

*Redox*

higher temperature.

higher concentration of Pt2+ and lower Pt0

on the active sites (mainly metal oxides) via nucleophilic attacks on C–Cl bond. The results obtained showed decreasing catalytic activity in the following order: MnKIT-6 > CrKIT-6 > CuKIT-6 > FeKIT-6 > Sn/KIT-6, in correlation with redox properties of the catalysts. Therefore, the best catalytic combustion activity of MnKIT-6 catalysts benefited from their large specific surface area, good Mn dispersion, better reducibility, and large amount of chemisorbed oxygen species. However, as the content of Mn species increasing, due to increased agglomeration of Mn, more MnOx clusters gave rise to pore plugging and cover adsorption sites, leading to weaker catalytic activity. Mesoporous silica KIT-6 with 3D interconnected mesopores offered a confined environment, a better dispersion of the oxide phase, and a faster diffusion of the reactants and products. The Mo/KIT-6, Fe/KIT-6, and Mo–Fe/KIT-6 were tested as potential alternatives to noble metals in the conversion of MCP [57]. Only the isolated tetrahedral Fe ions and highly dispersed small FeOx nanoclusters were responsible for the endocyclic C-C bond rupture at substituted C-C. The synergy between Fe and Mo was not observed at low temperatures and for total conversion but only for ring-opening reactions at

In particular, the good performance of the Ti(IV)-doped SBA-15-supported catalysts in CH4 oxidation was due to the combination of Ti(IV) structurally incorporated into the silica lattice and present as surface-dispersed TiO2 particles. The negative effect of the Ti(IV) over the HMS-supported catalysts was related to the high acidity induced by the more homogeneous incorporation of Ti(IV) into the silica structure. The loss of catalyst activity during CH4 oxidation reaction was not necessarily related to the sintering of the active sites but rather to the variation of the oxygen storage capacity and oxygen mobility strictly related to the support properties. For PdTi-SBA-15 catalysts, combustion of methane occurs through a redox or Mars van Krevelen mechanism was accepted [58]. Accordingly, during this reaction, PdO was locally reduced to Pd by methane, producing H2O and CO2, and then Pd was reoxidized by oxygen. Although the active species was considered PdO, it was widely recognized that metallic Pd plays the important role in decomposing and activating the methane molecule. Similar conclusions were obtained also for PtTiKIT-6 catalysts [16]. Therefore, the efficiency of all process was related to the redox properties of the catalyst and to the oxygen mobility. The maintenance of the activity, in spite of the significant palladium oxide sintering, was attributed to the good interaction between surface titania and silica. Such interaction favored the formation of Ti–O–Si linkages with an increase of the oxidizing potential of the Ti(IV) cations. In the presence of a support metal interaction, the increase of the PdO particle size [58], respectively PtO [16] after the long reaction is not detrimental. Another effect of metal–support interaction was observed for Ce-TiKIT-6 samples. In this case, two factors were influencing the Pt activity: dispersion and size of the platinum species and their concentration on the surface. Two opposite effects determined a decreasing in the CH4 conversion. The

A good correlation was obtained between catalytic activity, and reducibility of the

It can be concluded that OMSs containing transition metals showed extremely promising redox properties in the oxidation of organic compounds with larger molecules thanks to the mesoporous support. The applications and performances of the redox couple diversity resulted by association of transition metals on an increasing

catalysts was also obtained for samples with Ti and Ce supported on KIT-6.

concentration on the catalyst surface.

**30**

**5. Conclusions**

Viorica Parvulescu Institute of Physical Chemistry "Ilie Murgulescu" of the Romanian Academy, Bucharest, Romania

\*Address all correspondence to: vpirvulescu@icf.ro

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
