Zeolites photocatalysis

Zinc containing zeolites exhibit potential applications in optics, electronics, sensors and photocatalysis [1]. In catalytic applications, the addition of Zn produces an increase in both the catalytic activity and the selectivity towards aromatization of small alkanes in catalytic reforming reactions [2,3]. Zinc species promote dehydrogenation reactions avoiding the preferential removal of hydrogen by transfer to hydroearbon... 

The direct observation of the (Cu-NO) species by IR spectroscopy (774) and of its behavior under U V irradiation by EPR measurements (775, 777) suggests that a local electron transfer from the excited state of the Cu ion (3d 4s ) to the anti-77-bonding orbital of NO and simutaneously an electron transfer from the jr-bonding orbital of another NO to the vacant orbital of the Cu" ion (Sd 4s ) lead to the decomposition of two NO molecules on the Cu" site, selectively forming N2 and O2 under UV irradiation, even at 275 K. The data show that new and unique photocatalysis can be achieved within the small cavities of the zeolite. 

For the last two decades, attention has been focused on redressing the ozone depletion in the earth s protective layer. It is believed that chlorine radicals dissociated from chlorofluorocarbons (CFCs), upon irradiation of sun s UV in the stratosphere, promotes the ozone depletion. Hence, in addition to development of CFC alternatives there is an urgent need for the safe disposal of CFCs. Several processes such as pyrolysis, incineration, photocatalysis, oxidative destruction over metal oxide or zeolite catalysts and destruction at very high temperatures ( by plasma technique ) are reported in the literature for the disposal of CFCs[ 1-5]. But all these processes yield harmful products like CO, HF/F2 etc. Catalytic conversion of chlorinated organics in presence of hydrogen seems to be a better technique as it yields either hydrofluorocarbons(HFCs) or hydrochlorofluorocarbons(HCFCs) whose ozone depletion potential is either zero or very low and yet most of these products act as CFC alternatives. 

Investigations are being performed to gain further understanding of the photocatalysis of a-methylstyrene by TiOg added zeolite for example in using other types of zeolites (with different structure and acidity). 

We leave to specialists the task to describe the desired new approaches in the preparation of zeolites and mesoporous materials. Much work remains to be done for the scale-up of fabrication of MCM and similar materials. The manufacture of specially structured catalysts for very compact devices, like on-board fuel cells and automobile reforming units, is certainly able to bring much information valuable for the fabrication of many specific catalysts in environmental protection, especially for reverse-flow reactors. The demands of photocatalysis also impose constraints. 

Cadmium selenide is of interest for photosensitized electron transfer reactions used for solar energy conversion and photocatalysis. Small ensembles of CdSe have been synthesized within the cages of zeolite Y by ion exchange with Cd(II) and subsequent treatment with H2Se. [235] The presence of CdSe clusters with Cd-Se bond lengths of 2.60 A was inferred from EXAFS data measured at both the Cd and Se absorption edges. 

It should be noted that the photon energy and flux emitted from these discharges are not sufficient to explain the enhancement in destruction by photocatalysis. While Ti02 is found to be effective in more than doubling the CFC-12 destruction efficiency, the NaX zeolite catalyst which has no effect in enhancing the CFC-12 destruction (Fig. 6.2) reduces the NOx formation by more than a factor of two compared with the use of a plasma discharge alone. Ideally in plasma catalysis, we wish to achieve both enhancement in destruction and minimisation of unwanted by-products. 

Keywords Water treatment Photocatalysis Desalinisation Capacitive deionization Water filtration Heavy metal Zeolite Alginate Charcoal Activated carbon Chlorination UV Ozonation Nanofiltration... 

Transition metal oxide containing zeolites are used in selective oxidation reactions, ammoxidation, aromatization, photocatalysis and the selective catalytic reduction of NO. Often, isolated, zeolite-boimd oxidic species are identified as the most active sites in these reactions. Therefore, the preparation procedures are usually aimed at creating very small intrazeolitic oxide clusters or even isolated metal oxo-spedes. 

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