Photocatalysis intermediates

Turchi, C. S., Ollis, D. F., 1989, Mixed reactant photocatalysis Intermediates and mutual rate inhibition. / Catal, 119 483 96. 

Water Treatment. Several components must be treated simultaneously in a multicomponent mixture as available in wastewaters to prove the technology of heterogeneous photocatalysis. The formation and subsequent elimination of intermediates in the photooxidative process must be monitored, identifying all intermediates and final products. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

In 1991 and 1993, Ziessel123,129 reported mechanistic studies for the photocatalysis of the Ir(m) complexes, and proposed the mechanism shown in Scheme 41 to explain the results. A condensed version of the work was arranged by Vlcek.132 Many intermediates were isolated or detected spectroscopically using NMR, infrared spectroscopy, or UV-Vis, and XRD was employed to determine the detailed... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

In early works, the reactivity associated widi TiO2 photocatalysis was assigned to free OH radicals, but more recently die oxidative species has been identified as bound HO radicals and photogenerated holes. Reaction of the substrate with these species would require either die direct adsorption of organic compounds to the semiconductor interface or Fickian diffusion of the substrate to the semiconductor surface under depletion conditions. However, these two pathways imply different mechanisms, and in some cases different reaction products or intermediates. 

The majority of studies on the TiOz photocatalysis of organophosphorus compounds rely primarily on the disappearance of the initial substrate coupled with the monitoring of the mineralization products. Because of the complex mixtures formed in the TiOz photocatalysis of these compounds, there is limited information on the reaction pathways and mechanisms involved in the degradation processes. A recent report by Konstantinou et al. identified a number of intermediate products in the degradation of dichlofenthion and bromophos methyl by TiOz... 

Sonication is a tool for improvement of chemical processes such as photocatalytic reaction. The improvements of reaction rates, yields and selectivity, the generation of reactive intermediate species and so on were reviewed.36) Some examples have been also shown in this chapter. The development of a new reaction pass by the combined effect of photocatalysis and sonolysis is expected in the near future. The contribution to Green Chemistry is one of typical examples. 

The photocatalytic cleavage of water in the Dion-Jacobson series is presented first followed by studies on Ruddlesden-Popper series and finally on the intermediate series. Table 16.3 lists the oxides employed for photocatalysis studies. 

Finally, recent catalytic studies in which silicon-metal intermediates have been implicated include the photocatalysis of hydrosilation (Si-Fe, 452 Si-Co, 448 Si-Rh, 441) and the double silylation of alkynes with disilanes (Si-Pd, 457). 

Pervaporation - photocatalysis In the described systems the membrane usually permeates water and rejects the reactants, enhancing their residence time in the photoreactor. However, it is known that some intermediate products of the photo-catalytic degradation of organic compounds can negatively affect the reaction rate, therefore, in some cases it is useful to eliminate these by-products in order to improve the thermodynamic and/or the kinetics of the reaction. 

To this purpose, in a study on the photocatalytic degradation of 4-chlorophenol, Camera-Roda and Santarelli [89] proposed an integrated system in which photocatalysis is coupled with pervaporation as process intensification for water detoxification. Pervaporation represents a useful separation process in the case of the removal of VOCs and in this study it is used to remove continuously and at higher rate the organic intermediates that are formed in the first steps of the photocatalytic degradation of the weakly permeable 4-CP. 

One of the main objectives in the use of a membrane process coupled to a photocatalytic reaction is the possibility of recovering and reusing the catalyst. Moreover, when the process is used for the degradation of organic pollutants, the membrane must be able to reject the compounds and their intermediate products, while if the photocatalysis is applied to a synthesis, often the membrane have to separate the product(s) from the environment reaction. Therefore, in a PMR the choice of a suitable membrane is essential to obtain an efficient system. 

A comparison of these new processes with Ti02 photocatalysis for the degradation of aniline [148] showed that the former are faster. Although all these methods are considered to proceed via OH radicals, some different intermediates were detected in the electrochemical and photocatalytic experiments. p-Benzoquinone and NH4+ appeared in all solutions tested. A recent study [149] reports 87% and 99% TOC removals after 4 hr of... 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

Anpo, M. Photocatalysis on small particle Ti02 catalysts. Reaction intermediates and reaction mechanisms, Res. Chem. Intermed. 1989, 11, 67. 

Vinodgopal, K. Stafford, U. Gray, K. A. Kamat, P. V. Electrochemically assisted photocatalysis. II. The role of oxygen and reaction intermediates in the degradation of 4-chlorophenol on immobilized Ti02 particles, J. Phys. Chem 1994, 98, 6797. 

Minero, C. Aliberti, C. Pelizzetti, E. Terzian, R. Serpone, N. Kinetic studies in heterogeneous photocatalysis. 6. AMI simulated sunlight photodegradation over titania in aqueous media A first case of fluorinated aromatics and identification of intermediates, Langmuir 1991, 7, 928. 

Another interesting combination is heterogeneous photocatalysis with ultrasonic irradiation, because this process hinders the inactivation of the catalyst by reaction intermediates, which usually block the catalyst. Ultrasound also reduces mass transfer limitations occurring in the case of immobilized catalysts (see [8] for a detailed description of this combined process). 

The photocatalytic degradation of TCE was studied intensively, both in the liquid phase and in the gas phase. Unlike in the aqueous phase, where the quantum efficiency is no more than a few percents (Alberici and Jardim, 1997 Pruden and Ollis, 1983), the quantum efficiency in the gas phase can be higher than 100% (Upadhya and Ollis, 1998). This difference was attributed to the existence of two mechanisms. That the mechanism in the liquid phase was different than that in the gas phase could be deduced also by the fact that the intermediate products dichloroacetaldehyde (DCA) and dichloroacetic acid (DCAA) were identified only during liquid-phase photocatalysis (Pruden and Ollis, 1983). 

Chapter 2 considers the removal of inorganic water contaminants using photocatalysis. Metal cations react via one-electron steps first leading to unstable chemical intermediates, and later to stable species. Three possible mechanisms are identified (a) direct reduction via photo-generated conduction band electrons, (b) indirect reduction by intermediates generated from electron donors, and (c) oxidative removal by electron holes or hydroxyl radicals. The provided examples show the significance of these mechanisms for the removal of water contaminants such as chromium, mercury, lead, uranium, and arsenic. 

Re diimine complexes act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. Examples include the tricarbonyl complexes yac-[Re(Q -diimine)(CO)3L]" [n = 0, L = halide n = 1, L = NCMe, P(OR)3 a-diimine = 1,4-disubstituted 1,4-diazabuta-l,3-dienes or bpy and related chelating N-heterocycles], for example, fac-[Re(dmb)(CO)3(NCMe)]+, 5 [Re(dmb)(CO)3]2" and fac-[Re(bpy)(CO)3 P(OPfl)3 ]+. Electron-transfer from an amine electron donor (e g. triethanolamine or triethylamine) to the excited state complex is usually considered as the initiation of the photocatalysis, and metallocarboxylates and metallo-carboxyUc acids have been proposed as intermediates in the formation of CO. The electrocatalytic process is triggered by a 1-electron or a 2-electron cathodically induced chloride (X) or L ligand dissociation to form the catalytic species. ... 

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