Photocatalytic parameters

Nevertheless, by selecting or modifying some photocatalytic parameters, such us the semiconductor surface or the wavelength, it is possible to control and modify the types of products and their distributions. 

Another point to be considered is the value of specific surface area of the photocatalysts. Many authors express the photocatalytic parameters in terms of per area and denote them as area-related parameters. Notably in photocatalysis, the area-related parameters do not characterize the catalyst as they do in heterogeneous dark catalysis. In fact, the properties of a photocatalyst are not directly proportional to its area, because the light may not equally reach every part of a particle. In the most frequent cases the surface area accessed by the light cannot be determined, and thus it is recommended to use the total surface area in terms of the BET(N2) area. This means that generally a lower limit of the quantity under examination is obtained, because the surface determined by N2 adsorption will be larger than that reached by the radiation. 

However, in recent years, some studies have been performed on the application of photocatalysis in reaction of synthesis, such as selective reduction. These studies demonstrated that high selectivity could be obtained in photoreduction processes in comparison to conventional methods by appropriately selecting or modifying some photocatalytic parameters such as the semiconductor surface or the excitation wavelength. 

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). 

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. 

Sivakumar M, Atsuya T, Kyuichi Y, Toru T, Teruyuki K, Yasuo I (2010) Dependence of sonochemical parameters on the platinization of rutile titania - an observation of a pronounced increase in photocatalytic efficiencies. Ultrason Sonochem 17(3) 621-627... 

Some other studies showed that the combination of the three polymorphs with reduced crystallite size and high surface area can lead to the best photocatalysts for 4-chlorophenol degradation [37], or that particles in the dimension range 25-40 nm give the best performances [38]. Therefore, many elements contribute to the final photocatalytic activity and sometimes the increased contribution of one parameter can compensate for the decrease of another one. For example, better photocatalytic activity can be obtained even if the surface area decreases, with a concomitant increase in the crystallinity of the sample, which finally results in a higher number of electron-hole pairs formed on the surface by UV illumination and in their increased lifetime (slower recombination) [39]. Better crystallinity can be obtained with the use of ionic liquids during the synthesis [39], with a consequent increase of activity. 

Akpan, U.G. and Hameed, B.H. (2009) Parameters affecting the photocatalytic degradation of dyes using Ti02-based photocatalysts a review. Journal of Hazardous materials, 170, 520-529. 

Sobana, N. and Swaminafhan, M. (2007) The effect of operational parameters on the photocatalytic degradation of Acid Red 18 by ZnO. Separation and Purification Technology, 56, 101-107. 

Daneshvar, N Salari, D Khataee AR. Photocatalytic degradation of azo dye acid red 14 in water investigation of the effect of operational parameters. Journal of Photochemistry and Photobiology A Chemistry, 2003 157, 111-116. 

Lee K, Nam WS, Han GY (2004) Photocatalytic water splitting in alkaline aqueous solution using redox mediator 1 parameter study. Int J Hydrogen Energy 29 1343-1347... 

Figure 2 Model calculation according to Ref. 37 of the photocatalytic rate as a function of parameters F = 02 and Z = 03 (see text), given 4) = 0.001 and ai = 0.01. Note that the rate shows a maximum which position depends on the photon flux. At low Z, the rate shape is comparable to those observed in Fig. 1. At high Z, a Langmuirian shape is produced.

These selectivities remain constant within the duration of the experiments (3 h). Because of its high selectivity in cyclohexanone, this reaction was chosen to study the influence of various parameters on the photocatalytic activity (see section 2). 

The influence of various physical parameters was studied in the case of the oxidation of cyclohexane selected as a model reaction. Since the main product is cyclohexanone, its rate of formation was chosen as representative of the photocatalytic activity of the system. 

We report here results on the photocatalytic oxidation by oxygen of neat--liquid toluene, under UV-irradiation, by using unloaded and iron-loaded titania catalysts. Several experimental conditions have been selected to investigate parameters which can influence the chemical yields and the distribution of pro ducts particular attention has been devoted to investigate the evolution of chemical yields and selectivity by extending the irradiation times up to 12 h. 

The addition of Fe ions to the solution also increased the rate of photo-catalytic degradation. The TOC reduction was 80% in the presence of Fe ions and 10% in the absence of Fe ions. pH is the key parameter in determining the photocatalytic degradation of anilines as well as other compounds previously studied. The high photocatalytic degradation near the pHpzc has also been observed for other compounds studied with Ti and Zn. The major mechanism is the attack of photogenerated OH radicals on the aniline molecule under alkaline conditions. The addition of Fe ions to the solution can enhance the formation of OH radicals and lead to higher photocatalytic rates. 

Polynuclear clusters fill the gap between mononuclear and extended solid transition metal vibronic systems. The applications of the theory of vibronic interaction allow to describe physical and chemical properties of these systems, sometimes directly linked to their application. The Jahn-Teller distortion found for the rhenium clusters defines the architecture of hybrid inorganic-organic materials and, as a result, their electric and magnetic properties. The application of the vibronic theory to the decatungstate cluster elucidates the details of its reactivity in the photocatalytic reaction. The modern DFT methods give a key to the calculations of key parameters of the vibronic theory. In future, we will assist at the combination of these methods with phenomenological approaches leading to the description of vibronic effects in physical and chemical properties of polynuclear clusters from first principles. 

Some parameters that influence the photocatalytic process are ... 

For the development of a continuous photocatalytic reactor, applicable at the industrial level, it is important to consider some parameters such as the catalyst configuration, the specific illuminated surface area, the UV source, the mass-transfer rate and the scale-up possibilities [69]. 

Parameters Influencing the Photocatalytic Membrane Reactors (PMRs) Performance... 

In the development of a photocatalytic membrane reactor it is important to take into account some parameters that influence the performance of the system and its applicability to the industrial level. 

By means of preliminary batch tests some important parameters that influence the photocatalytic oxidation of benzene to phenol were investigated. In particular, the obtained results showed an increase of phenol production depending on the pH of the aqueous Ti02 suspensions, the catalyst concentration and the radiation intensity. 

Photocatalytic reactions with one reaction partner only we may call photo-sorption processes at the surface of a solid. The investigation of photosorption processes is the first step towards an understanding of photocatalytic reactions. Because fewer parameters are involved than in photocatalytic reactions, a large number of investigations on photosorption have been carried out. For the same reason, photosorption will be discussed to some extent in this context. 

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