Hydrogen peroxide (H2O2) is widely used in almost all industrial areas, particularly in the chemical industry and environmental protection. The only degradation product of its use is water, and thus it has played a large role in environmentally friendly methods in the chemical industry. Hydrogen peroxide is produced on an industrial scale by the anthraquinone oxidation (AO) process. However, this process can hardly be considered a green method. It involves the sequential hydrogenation and oxidation of an alkylanthraquinone precursor dissolved in a mixture of organic solvents followed by liquid-liquid extraction to recover H2O2. The AO process is a multistep method that requires significant energy input and generates waste, which has a negative effect on its sustainability and production costs. The transport, storage, and handling of bulk H2O2 involve hazards and escalating expenses. Thus, novel, cleaner methods for the production of H2O2 are being explored. The direct synthesis of H2O2 from O2 and H2 using a variety of catalysts, and the factors influencing the formation and decomposition of H2O2 are examined in detail in this Review.

The molecularly dispersed TiO 2 /SiO 2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of titanium isopropoxide. Experimental monolayer dispersion of surface titanium oxide species on SiO 2 was reached at ∼4 Ti atoms/nm 2 with a two-step impregnation procedure. The surface structures of the molecularly dispersed TiO 2 /SiO 2 under various environments were extensively investigated by in-situ spectroscopic techniques (e.g., Raman, UV-vis-NIR DRS, and XANES) as well as XPS. The combined characterization techniques revealed the consumption of surface Si-OH groups and the formation of Ti-O-Si bridging bonds. In the dehydrated state, the surface Ti atoms in the 1% TiO 2 /SiO 2 sample (0.24 Ti atoms/nm 2 ) are predominantly found to be isolated TiO 4 units, whereas at maximum surface coverage (∼4 Ti atoms/nm 2 ), two-dimensional polymerized TiO 5 units are dominant on the silica surface. The in-situ spectroscopic studies demonstrated that the coordination and ligands of the surface Ti cations change upon hydration/dehydration as well as during methanol oxidation. Methanol oxidation showed that the molecularly dispersed surface titanium oxide species exhibit completely different catalytic behavior (predominantly redox products) compared to bulk titanium oxide (predominantly dehydration products). Furthermore, the TOF of the surface titanium oxide species is strongly dependent on their local structures and varies by 1 order of magnitude (isolated TiO 4 . polymerized TiO 5 ). These new results provide fundamental insights about molecular structure-reactivity/selectivity relationships of the molecularly dispersed TiO 2 /SiO 2 supported oxides.

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