Catalyst photocatalytic process

Besides improvements in catalyst characteristics [28], the low productivity of a photocatalytic process can also be improved by reactor design. In photocatalytic research on a laboratory scale, the most widely applied reactors are the top illumination or annular reactors containing a suspended catalyst [29]. This type of... 

In the photocatalytic oxidation cycle, on the other hand, the pigment plays a part as a catalyst. The process of photocatalytic oxidation caused by Ti02 pigments is fully understood now (1, 2, 3, 4). 

Metal oxides are an important elass of heterogeneous catalysts. They find direct application in a variety of reactions, from acid-base to redox reactions, in photocatalytic processes, and as catalysts for environmental protection. In addition, they are widely used as supports for other active components (metal particles or other metal oxides), although often they act not only as a support, but actively participate in the reaction mechanism." ... 

The photonic excitation of the catalyst represents the initial step of the activation of the photocatalytic process and the hydroxyl radicals are the primary oxidant in these systems, although the recombination of the ed) hv ), that produces thermal energy, can occur, with a reduction of photocatalytic activity. 

Among the various catalysts employed in the photocatalytic processes, the most popular and used is titanium dioxide, Ti02, thanks to its strong catalytic activity, high chemical stability in aqueous media and in a large range of pH (0-14), low cost due to the abundance of Ti (0.44% of the Earth s crust) and long lifetime of electron-hole pairs. 

The pH plays an important role in the efficiency of the photocatalytic process. In particular, the occurrence of aggregation phenomena involving the suspended Ti02 catalyst particles together with their precipitation has been observed at acidic values. The last aspect determines a reduction of the catalytic active sites and therefore a decrease of the catalyst activity. 

The early work of photoelectrochemical hydrogen production using Ti02 as catalyst, was reported by Fujishima and Honda [61]. Subsequently, the interest for the photocatalytic processes has grown significantly, although the number of the reported photocatalysts used for water splitting is still limited. 

Despite the great potentialities of the photocatalytic processes, their application at the industrial level is limited by different factors. Besides the problems regarding the high reactivity of the system and the low selectivity of the classical catalysts on which, as previously described, many studies have been realized in recent years, one of the main problems is the recovery of the catalyst from the reaction environment. 

The recovery of the photocatalyst from the reaction environment represents one of the main problems of the photocatalytic process that limits its industrial application. Although this process step can be obviated by the use of immobilized catalyst, the suspended system has more attractive features [76]. Therefore, the separation of the photocatalyst from the treated solution and its recycle is one of the challenges in further development of this technology. 

Photoinduced catalysis means the photogeneration of a catalyst that subsequently promotes a catalyzed reaction. Photons are required to generate the catalyst only. Thus, the efficiency of such processes depends only on the activity of the catalyst produced photochemically and, in homogeneous photocatalysis, the turnover number (TON) is the useful tool. The TON is usually expressed as the number of moles of product formed per mole of catalyst and, for photoassisted catalysis, TON <1, whereas for photogenerated catalysis TON >1 and even 1 [135], Therefore, high turnovers of photochemically produced catalysts are one of the main criteria concerning efficient photocatalytic processes. Quantum yields (ratio of moles of product formed to the number of photons absorbed) >1 may occur. The same is true for photoinduced chain reactions. 

Catalyzed photolysis refers to catalysis of a photochemical reaction, for which there is a physical pathway for decay of the system back to its ground state (Figure 6.17). When the photocatalytic process occurs through photoexcitation of the catalyst, the physical decay may occur through recombination and/or thermal photoionization of the excited states, which ultimately leads to regeneration of the original state of the catalyst. Note that catalyzed photolysis is not catalytic in photons, contrary to photogenerated catalysis. 

This chapter presents a quantitative method to determine the photoadsorption capacity of a polycrystalline semiconductor oxide irradiated in liquid-solid system. The determination is performed imder reaction conditions so that it is really indicative of the photoadsorption capacity. The method uses the experimental results obtained in typical batch photoreactivity runs on this ground it has been applied to the following photocatalytic processes carried out in aqueous suspensions (i) oxidation of phenol in the presence of a commercial Ti02 catalyst (Degussa P25) and... 

The photocatalytic Cr(Vl) reduction is more feasible at low pH because the net reaction consumes protons (Equations (18) and (19)), but use of neutral or alkaline conditions can be more convenient because Cr(lll) can be precipitated as the hydroxide and immobilized, avoiding expensive separation steps after the photocatalytic process, an adequate acid or strong basic treatment easily separates Cr(lll) from the catalyst (Lin et al., 1993). 

The overall reaction rate of photocatalytic processes is usually slow compared to conventional chemical reaction rates due to low concentration level of the pollutants, and therefore, there is a need to provide large amounts of active catalyst inside the reactor. Although the effective surface area of the porous anatase catalyst coating is high, there can only be a thin coating (about 1 pm thick) applied to a surface. Thus, the amount of active catalyst in the reactor is limited, and even if individual degradation... 

Gabriele Centi and Sighnda Perathoner (University of Messina, Italy) examine the use of solid catalysts for the removal of contaminants from water supplies. This includes photocatalytic processes as well as oxidation and reduction reactions. There are a wide range of catalysts used in these various processes. In addition to their activity, deactivation is often a critical concern. The authors show that there are significant challenges remaining in this area. 

In the case of photogenerated catalysis, two different but equivalent models are worth considering the Langmuir-Hinshelwood photocatalytic process and the Eley-Rideal photocatalytic process. The former is described by Mechanism II, in which the reaction occurs at a photochemically active surface when light is absorbed by the catalyst and leads to the generation of surface electrons (e ) and holes (h" ). 

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