Photocatalytic oxidations

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

It is well known that photocatalytic oxidation of or nic pollutants follows Langmuir-Hinshelwood kinetics[6]. Therefore, this kind of reaction can be represented as follows. 

Photocatalytic oxidation tests were performed in a photoreactor [2]. The flat, rectangular stainless steel reactor has dimensions of 578 mm x 113 mm and inlet/outlet ports at the two ends. The aerogel catalyst placed in a recess located at the center of the reactor was uniformly irradiated by... 

Fig. 3. Photocatalytic oxidation test of titania-silica aerogels, (a) TCE (air 400 seem, TCE 113 ppm), (b) isopropanol (air 200 seem, isopropanol 268 ppm).

Photocatalytic oxidation of TCE in a Ti02-coated activated carbon fluidized bed reactor... 

Adsorption and photocatalytic oxidation of TCE with Ti02-coated activated carbon was also carried out in a cylindrical continuous flow fluidized bed reactor with 65cm height and 68cm inside diameter (Figure 1). 

Frank SN, Bard AJ (1977) Heterogeneous photocatalytic oxidation of cyanide and sulfite in aqueous solutions at semiconductor powders. J Phys Chem 81 1484—1488... 

Photocatalytic Oxidation of Air Contaminants by Chlorine (Cl) or Hydroxyl (OH) Radicals or Holes (h+) Mechanistic Correlations... 

Photocatalytic oxidation of ethanol on Pt/ri02 and Nafion coated Ti02 catalysts were studied using in situ infrared IR techniques. Infrared studies show that the reaction produced acetaldehyde, acetic acid, acetate, formic acid, formate, and CO2/H2O. Modification of the Ti02 catalyst by Pt and Nafion slowed down the oxidation reaction through site blocking. Incorporation of Pt was found to favor formation of formate (HCOO ), indicating Pt decreases the rate of oxidation of formate more than that of its formation. 

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 adsorbed acetaldehyde (CHsCHOad), adsorbed acetate (CHsCOO ad) and adsorbed formate (HCOO ad) relative IR intensity during the first 20 min of photocatalytic oxidation are presented in Figure 52.3 for the Pt/Ti02 catalyst and Ti02 catalyst (Aeroxide P25S Degussa). Results of Ti02 are included for... 

The observations of CHsCOO , CHsCHOad, and HCOO confirm that the photocatalytic oxidation of ethanol follow a reaction pathway consistent with our previous study (8), as shown in the following scheme. 

Figure 52.4. CO2 Formation during the photocatalytic oxidation of ethanol on the Pt/ri02, Ti02, and 1 1 Nafion-Ti02, catalyst surface at 303 K.

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. 

Use of combination of cavitation and advanced oxidation processes such as ozonation, chemical oxidation using hydrogen peroxide and photocatalytic oxidation and use of combination of ultrasound and microwave irradiations. 

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 synergistic effects of combining photocatalytic oxidation with cavitation can be possibly attributed to ... 

Tang WZ, An H (1995) UV/Ti02 photocatalytic oxidation of commercial dyes in aqueous solutions. Chemosphere 31 4157-4170... 

Photocatalytic oxidation can be an effective way of removing pollutants in the gas phase, such as NOx, SOx, and volatile organic compounds (VOCs). 

Cao, G., Li, Y., Zhang, Q., and Wang, H. (2010) Synthesis and characterization ofLa203/Ti02 F and the visible light photocatalytic oxidation of 4-chlorophenol. Journal of Hazardous materials, 178 (1-3), 440-449. 

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