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2.
Dogmas and misconceptions in heterogeneous photocatalysis. Some enlightened reflections 总被引:1,自引:0,他引:1
In a recent article, Ollis analyzed heretofore reported photocatalyst kinetics of surface photochemical reactions that take place in heterogeneous systems and that rely heavily on the Langmuir-Hinshelwood (LH) kinetic model to interpret the experimental observations. This model assumes a fast adsorption/desorption equilibrium step and a subsequent slow surface step. His interesting analysis of the experimental results reported in 2000 by Emeline and co-workers, Xu and Langford, and Martyanov and Savinov prompted our reexamination of the LH kinetic model along with several other dogmas that continue to propagate in the heterogeneous photocatalytic landscape. This short article discusses some of these issues and reexamines certain misinterpretations. Specifically, we reexamine (1) the a priori assumed validity of the LH kinetic model in heterogeneous photocatalysis, (2) the recombination of photogenerated free charge carriers on the solid (metal oxide) photocatalyst by the band-to-band recombination pathway, and (3) the mistaken assertion that the kinetics of a heterogeneous photoreaction are either only first-order dependent or half-order dependent on photon flow (i.e., light irradiance). 相似文献
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A simplified and energy-saving integrated device consisting of a microwave applicator and an ultrasonic homogenizer has been fabricated to generate liquid plasma in a medium possessing high dielectric factors, for example water. The microwave waveguide and the ultrasonic transducer were interconnected through a tungsten/titanium alloy stick acting both as the microwave antenna and as the horn of the ultrasonic homogenizer. Both microwaves and ultrasonic waves are simultaneously transmitted to the aqueous media through the tungsten tip of the antenna. The microwave discharge liquid plasma was easily generated in solution during ultrasonic cavitation. The simple device was evaluated by carrying out the degradation of the perfluorooctanoic acid (PFOA), a system highly recalcitrant to degradation by conventional advanced oxidation processes (AOPs). PFOA is 59% degraded in an aqueous medium after only 90 s of irradiation by the plasma. Intermediates were identified by electrospray mass spectral techniques in the negative ion mode. 相似文献
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Sonication of aqueous solutions causes cavitation in the liquid which results in the formation of H· and ·OH radical species that can be used to reduce or oxidize certain chemical compounds. This article focuses on the effect of ultrasounds in homogeneous reactions to examine the type of chemistry that ensues. It also deals with a rather novel method of using ultrasounds in combination with photochemical methods of inducing chemical reactions; this sonophotochemical technique remains to be explored and exploited. Finally, we examine and explore the potential utilization of ultrasounds to convert environmental hazardous substances into more benign substrates, or better still to mineralize organics into carbon dioxide. 相似文献
5.
Emeline AV Abramkin DA Zonov IS Sheremetyeva NV Rudakova AV Ryabchuk VK Serpone N 《Langmuir : the ACS journal of surfaces and colloids》2012,28(19):7368-7373
The present study explored photoinduced radical processes caused by interaction of CH(4) and NH(3) with a photoexcited surface of a complex metal oxide: magnesium-aluminum spinel (MgAl(2)O(4); MAS). UV irradiation of MAS in vacuo yielded V-type color centers as evidenced by the 360 nm band in difference diffuse reflectance spectra. Interaction of these H-bearing molecules with photogenerated surface-active hole states (O(S)(-)?) yielded radical species which on recombination produced more complex molecules (including heteroatomic species) relative to the initial molecules. For the MAS/CH(4) system, photoinduced dissociative adsorption of CH(4) on surface-active hole centers produced ?CH(3) radicals that recombined to yield CH(3)CH(3). For MAS/NH(3), a similar dissociative adsorption process led to formation of ?NH(2) radicals with formation of NH(2)NH(2) as an intermediate product; continued UV irradiation ultimately yielded N(2). For the mixed MAS/CH(4)/NH(3) system, however, interaction of adsorbed NH(3) and CH(4) on the UV-activated surface of MAS yielded ?NH(2) and ?CH(3) radicals, respectively, which produced CH(3)-NH(2) followed by loss of the remaining hydrogens to form a surface-adsorbed cyanide, CN(S), species. Recombination of photochemically produced radicals released sufficient energy to re-excite the solid spinel, generating new surface-active sites and a flash luminescence (emission decay time at 520 nm, τ ~ 6 s for the MAS/NH(3) case) referred to as the PhICL effect. 相似文献
6.
Application of ultrasound to a system that contains at least one liquid phase produces microscopic bubbles in the liquid which
undergo periodic expansions and contractions. Some of these microbubbles eventually destabilize and collapse violently, generating
temperatures in the thousands of degrees Kelvin and pressures in the hundreds of atmospheres. This phenomenon, known as cavitational
implosion, favors the production of free solvent radicals that react amongst themselves and with other substrates in the system.
In addition, ultrasound accelerates reactions that involve single electron transfers but seems to have no effect on reactions
that proceed via ionic mechanisms for reasons that remain unclear. In practical terms, ultrasound allows the synthesis of
novel compounds as well as the improved preparations of standard compounds. Sonication is more than just more efficient stirring. The high temperatures produced on cavitation, both in the cavity and at the interface, could lead to molecular combustion
of the substrate and of the solvent to form radical species which could then initiate reactions. 相似文献
7.
Irradiation of acidic (pH 2) solutions of RuL32+ 2Cl? (L = diisopropyl 2,2′-bipyridine-4,4′ -dicarboxylate) in the presence of TiO2 at 100°C leads to the loss of one bipyridyl ligand and the chemical fixation of RuL2 at the surface of teh TiO2 particles through formation of Ru-O-Ti bonds. These surface complexes are very stable and shift the absorption onset of TiO2 beyond 600 mm. Efficient sensitization H2-generation is achieved with this system beginning in the wave length domain between 590 and 665 nm. Preliminary water cleavage experiments with bifunctional TiO2/Pt/RuO2 redox catalyst are reported. 相似文献
8.
Nick Serpone Satoshi Horikoshi Alexei V. Emeline 《Journal of Photochemistry and Photobiology, C: Photochemistry Reviews》2010,11(2-3):114-131
This review article focuses, albeit non-exhaustively, on the influence of microwave radiation on photoassisted processes often referred to as Advanced Oxidation Processes. In particular, we describe and illustrate the possible advantages of microwaves in TiO2-assisted photodegradations and photomineralizations of various organic pollutants such as herbicides and endocrine disruptors, among others. Described are also various reactor configurations involving UV/visible radiation and microwaves, with the former being supplied either by traditional Hg lamps or alternatively by electrodeless lamps activated by microwaves. To place the use of microwaves on processes occurring in aqueous TiO2 dispersions in perspective regarding environmental applications, we first introduce the various sources of pollutants and subsequently describe in brief the various advanced oxidation processes such as UV/peroxidation, UV/ozonation and the photo-Fenton process(es) in addition to direct photolysis either by sunlight or by artificial light sources. 相似文献
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Several studies have used microwaves as a heat source for carrying out various types of reactions employing circulation reaction vessels. The microwave flow chemical synthesis methodology is most appropriate in the use of microwaves in chemical syntheses. It can attenuate the problem of microwave heating (non‐uniform heating and penetration depth) and maximize the benefits (rapid heating and first temperature adjustments). In this brief review, we examine and explain some of the relevant features of microwave heating with applicative examples of the usage of microwave flow chemistry equipment in carrying out organic syntheses, enzymatic reactions, and (not least) nanoparticle syntheses. 相似文献