Abstract
Photomineralization of methane in air (10.0–1000 ppm (mass/volume) of C) at100%relative humidity (dioxygen as oxygen donor) was systematically studied at318±3 K in an annular laboratory-scale reactor by photocatalytic membranes immobilizing titanium dioxide as a function of substrate concentration, absorbed power per unit length of membrane, reactor geometry, and concentration of a proprietary vanadium alkoxide as photopromoter. Kinetics of both substrate disappearance, to yield intermediates, and total organic carbon (TOC) disappearance, to yield carbon dioxide, were followed. At a fixed value of irradiance (0.30 W⋅cm-1), the mineralization experiments in gaseous phase were repeated as a function of flow rate (4–400 m3⋅h−1). Moreover, at a standard flow rate of 300 m3⋅h−1, the ratio between the overall reaction volume and the length of the membrane was varied, substantially by varying the volume of reservoir, from and to which circulation of gaseous stream took place. Photomineralization of methane in aqueous solutions was also studied, in the same annular reactor and in the same conditions, but in a concentration range of 0.8–2.0 ppm of C, and by using stoichiometric hydrogen peroxide as an oxygen donor. A kinetic model was employed, from which, by a set of differential equations, four final optimised parameters,k1andK1,k2andK2, were calculated, which is able to fit the whole kinetic profile adequately. The influence of irradiance onk1andk2, as well as of flow rate onK1andK2, is rationalized. The influence of reactor geometry onkvalues is discussed in view of standardization procedures of photocatalytic experiments. Modeling of quantum yields, as a function of substrate concentration and irradiance, as well as of concentration of photopromoter, was carried out very satisfactorily. Kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading to mineralization, were considered, and it is paralleled by a second competition kinetics involving superoxide radical anion.
Highlights
Air and water pollution is notoriously a serious problem facing all anthropic activities throughout the world
For experiments in which the photopromoter was added, the same procedure described in the preceding paragraph was adopted, and the Φ∞ values obtained at a high value of absorbed irradiance, typically Φ∞ values at 4 W·cm−1, much smaller than the maximum allowable Φ∞ values corresponding to irradiances tending to zero, were plotted as a function of concentration of photopromoter in the photocatalytic membrane
Photomineralization of methane in air at 100% relative humidity, and in a concentration range corresponding to 10.0–1000 ppm of carbon, was studied at 318 ± 3 K, in a laboratory-scale reactor, by using dioxygen of air as an oxygen donor, and by photocatalytic membranes immobilizing 30 ± 3% of titanium dioxide, as a function of absorbed power per unit length of membrane, expressed in W·cm−1
Summary
Air and water pollution is notoriously a serious problem facing all anthropic activities throughout the world. The major problem in photocatalysis, from the point of view of industrial applicability, is given by the necessity of immobilizing the semiconductor photocatalyst, by firmly anchoring it to a suitably reacting structure, and of reaching the highest performance as possible in photocatalytic reactors, in continuous mode [11] These two aspects (kind of immobilization, as linked to kinetic modeling of the whole course of the photomineralization process, and quantum yields) need to be considered as priority options for engineering applications. On the contrary, in a wide range of concentrations, and over all the kinetic concentration profile of TOC, leading to full mineralization, has been approached systematically in previous papers of this series [12,13,14, 18], mostly for processes carried out in aqueous solutions This has been done from the standpoint of quantum yields and energy efficiencies, a very important aspect, which is often neglected in the literature pertinent to photocatalysis. The dependence of quantum yields on the concentration of substrate and on photon flow will be considered and analyzed, with the goal of attaining, by all these variables expressed in the most convenient ways, a complete rationalization of the photocatalytic process, both in liquid and gaseous phases, from the engineering point of view, as well from the perspective of establishing a reliable standardization method of the photocatalytic performance of materials
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