Amorphous titanium dioxide, activity

Synthesis of mesoporous TiO/M composites and their photocatalytic activity in hydrogen evolution. Hydrolysis of Ti(TV) tetrabutoxide with subsequent sol-gel transformation in the presence of dibenzo-18-crown-6 ether as a template yields amorphous titanium dioxide (Fig. 1). Calcination of the parental amorphous sample at 500 °C causes crystallization of Ti02 and formation of a porous material with high specific surface area and a narrow pore size distribution with the average pore diameter 5.0 nm (Table 1, sample No. 2). Combination of the hydrothermal treatment (HTT) of Ti02 samples at 100 - 175 °C with their subsequent calcination... 

Crabtree et al. reported the immobilisation of the dimeric mixed-valence species [(tpy)(H20)Mn (0)2Mn (tpy)(H20)] (5) onto titanium dioxide nanoparticles with different degrees of crystallinity. The complex was directly adsorbed onto the titanium dioxide surface for ciystalline titanium dioxide, the manganese(iii)-manganese(iv) dimer transformed to manganese(iv)-manganese(iv) dimer or tetramer-like structure, whereas it remained stable when anchored to titanium-dioxide nanoparticles with low crystallinity. The o>ygen-evolution measurements performed in the presence of CAN showed that the dimeric mixed-valence complex adsorbed on amorphous titanium dioxide support was not catalytically active, whereas the oxidised/dissociated complex formed in the crystalline titanium dioxide nanoparticles showed catalytic activity however, no detailed explanation was provided for the differences in catalytic activity. ... 

Nanoporous materials like zeolites and related materials, mesoporous molecular sieves, clays, pillared clays, the majority of silica, alumina, active carbons, titanium dioxides, magnesium oxides, carbon nanotubes and metal-organic frameworks are the most widely studied and applied adsorbents. In the case of crystalline and ordered nanoporous materials such as zeolites and related materials, and mesoporous molecular sieves, their categorization as nanoporous materials are not debated. However, in the case of amorphous porous materials, they possess bigger pores together with pores sized less than 100 nm. Nevertheless, in the majority of cases, the nanoporous component is the most important part of the porosity. 

Among the supports that have been used in the preparation of supported transition metal nanoparticles are carbon, silica, alumina, titanium dioxide, and polymeric supports [57], and the most frequently used support is alumina [56], These supports normally produce an effect on the catalytic activity of the metallic nanoparticles supported on the amorphous material [60], In Chapter 3, different methods for the preparation of metallic catalysts supported on amorphous solids were described [61-71],... 

Concurrently with the discovery and development in this country of the catalytic conversion of paraffins to aromatics (131) three different groups in the U.S.S.R. discovered this reaction independently of each other. Moldavskil and co-workers (238,239) showed that paraffins with six or more carbon atoms form aromatics by closure of a six-membered ring. For example, n-octane gives xylene and some ethylbenzene over amorphous chromia at about 470°C. Olefins also undergo this reaction. In subsequent publications, the group headed by Moldavskil demonstrated that molybdenum sulfide, titanium dioxide, and other oxides as well as activated carbon also may be used for dehydrocyclization (237,239). 

Titanium dioxide and titanium incorporated into aiumino-siiicate photocataiysts have been studied for the photodegradation of dichioromethane into carbon dioxide and HCi. Different forms of titanium dioxide have been produced such as rutiie and anatase as weii as an amorphous form. Titanium piiiared ciays have been found to be more active than titanium ciays. Carbon feit was aiso used as a support for titanium species and this materiai is the most active we have studied. By enriching solutions of dichloromethane with oxygen or by pre-irradiating the titanium catalyst, faster rates of reaction and larger conversions are obtained. 

Whilst Schemes 3-5 above are quite well documented, more recent studies on both polyalkenes and polyamides clearly indicate that titanium dioxide and zinc oxide pigments owe their photocatalytic activity to catalysis of the photodecomposition of hydroperoxides in amorphous regions of the polymers. 

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