Photocatalytic approach

Electrocatalysis at metal electrodes in aqueous (1.2) and non-aqueous ( ) solvents, phthalocyanine ( ) and ruthenium ( ) coated carbon, n-type semiconductors (6.7.8),and photocathodes (9,10) have been explored in an effort to develop effective catalysts for the synthesis of reduced products from carbon dioxide. The electrocatalytic and photocatalytic approaches have high faradaic efficiency of carbon dioxide reduction (1,6). but very low current densities. Hence the rate of product formation is low. Increasing current densities to provide meaningful amounts of product, substantially reduces carbon dioxide reduction in favor of hydrogen evolution. This reduction in current efficiency is a difficult problem to surmount in light of the probable electrostatic repulsion of carbon dioxide, or the aqueous bicarbonate ion, from a negatively charged cathode (11,12). 

In addition, Luo s group has used a visible-light photocatalytic approach for the C-H imidation of a number of heteroarenes with bromosaccharin 279 as the nitrogen source (Scheme 13.40) [78]. This reaction proceeds via a radical chain pathway with termination by electron transfer- proton transfer (ET - PT). 

In the last two decades we have witnessed in photocatalysis, as a science, a continuous shift from phenomenological approaches to studies at the molecular level. With accumulation of information obtained in such studies, the accents in the work aimed at development of new photocatalysts and new photocatalytic reactions and technologies, are expected to more and more shift from empirical search to intentional design. 

We recently demonstrated that photocatalyzed destruction rates of low quantum efficiency contaminant compoimds in air can be promoted substantially by addition of a high quantum efficiency contaminant, trichloroethylene (TCE), in a single pass fixed bed illuminated catalyst, using a residence time of several milliseconds [1-3]. Perchloroethylene (PCE) and trichloropropene (TCP) were also shown to promote contaminant conversion [2]. These results establish a novel potential process approach to cost-effective photocatalytic air treatment for contaminant removal. 

Photocatalytic oxidation is a novel approach for the selective synthesis of aldehyde and acid from alcohol because the synthesis reaction can take place at mild conditions. These reactions are characterized by the transfer of light-induced charge carriers (i.e., photogenerated electron and hole pairs) to the electron donors and acceptors adsorbed on the semiconductor catalyst surface (1-4). Infrared (IR) spectroscopy is a useful technique for determining the dynamic behavior of adsorbed species and photogenerated electrons (5-7). 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

The extension of this approach to artificial leaves based on titanates, niobates, tantalates, metal nitrides and phosphides, metal sulfides, and other transition metal oxides appears possible and useful in order to enhance the photocatalytic efficiency. In addition, the construction of multicomponent systems such as Ti02-CdS or MoS2-CdSe for overall water splitting could also lead to further improvements. This... 

Zhou, Y., Krumeich, F., Heel, A., and Patzke, G.R. (2010) One-step hydrothermal coating approach to photocatalytically active oxide composites. Dalton Transactions, 39 (26), 6043-6048. 

Yu, J. and Zhang, J. (2010) A simple template-free approach to Ti02 hollow spheres with enhanced photocatalytic activity. Dalton Transactions, 39 (25), 5860-5867. 

I. Bouzaida, C. Ferronato, J.M. Chovelon, M.E. Rammah and J.M. Herrmann, Heterogeneous photocatalytic degradation of the anthraquinone dye, Acid Blue 25 (AB25) a kinetic approach. J. Photochem. Photobiol.A Chem., 168 (2004) 23-30. 

Fig. 8.13 Z-scheme approach to photocatalytic water splitting using a DSSC based tandem cell.

Photocatalytic oxidation over illuminated titanium dioxide has been demonstrated to be effective at removing low concentrations of a variety of hazardous aromatic contaminants from air at ambient temperatures. At low contaminant concentration levels and modest humidity levels, complete or nearly complete oxidation of aromatic contaminants can be obtained in photocatalytic systems. Although aromatic contaminants are less reactive than many other potential air pollutants, and apparent catalyst deactivation may occur in simations where recalcitrant reaction intermediates build up on the catalyst surface, several approaches have already been developed to counter these potential problems. The introduction of a chlorine source, either in the form of a reactive chloro-olefin cofeed or an HCl-pretreated catalyst, has been demonstrated to promote the photocatalytic oxidation of... 

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