Heterogeneous catalysis photocatalysts

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

The potential of electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy is also not fully explored in photocatalyt-ic studies which is primarily used for the rapid screening and high throughput evaluation of photocatalysts. The use of in-situ analytical techniques used for heterogeneous catalysis is extensively reviewed elsewhere [213-218]. 

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

Turnover numbers and turnover rates are relatively easily measured in homogeneous (photo)catalysis however, their determination has eluded several attempts in heterogeneous (photo)catalysis because of the required knowledge of the number of (photo)catalytically active sites on a (solid) photocatalyst. These turnovers appear to depend on how the (photo)catalytic process is described (Serpone et al, 2000). 

At present it is not possible to suggest more precise estimates than those made in this article, of the actual role of particular photocatalytic reactions in the atmosphere. To improve present knowledge, it is necessary to study in laboratories the quantitative characteristics of heterogeneous photocatalysis and thermal catalysis over natural aerosols, and under conditions that would be more close to those in the atmosphere. The most important characteristic to be measured is the quantum yield of photocatalytic reactions of atmospheric components on atmospheric aerosols containing Fe203, Ti02, and ZnO, since these are the most plausible candidates for the role of photocatalysts due to their appropriate photochemical properties and rather high concentration in the troposphere. 

Metallic oxides having different shapes and sizes have received considerable attention because of their theoretical, technological applications in various organic reactions. Catalysts, for example, are mostly nanoscale particles, and catalysis is a nanoscale phenomenon. In the case of various reactions, separation of the catalyst from the reaction mixture is the main problem and loss of product occurs. Nano-particles prepared on the resin can easily be applied as a heterogeneous catalyst for efficient recovery and recycling of the photocatalyst from liquid-phase reactions. 

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. 

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