Environ. Sci. Technol. 2000, 34, 3435-3442
The Intrinsic Catalytic Activity in Photoreactors LEV DAVYDOV, SOTIRIS E. PRATSINIS, AND PANAGIOTIS G. SMIRNIOTIS* Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171
A methodology for determining intrinsic activity in aqueous heterogeneous photocatalytic reactors was introduced. The most common commercial titania powders (Degussa P25, Aldrich anatase, Aldrich anatase 325) with high BET surface area were evaluated for the UV-induced heterogeneous degradation of phenol to test the proposed methodology. It was demonstrated that kinetic data obtained from commonly used photocatalytic reactors (such as liquidphase reactor) do not correspond to the intrinsic activity of catalysts due to strong nonuniformity of the internal radiation field. In contrast to the vast majority of the previous studies, which report scattered data for different reactors, the present work offers an objective means to characterize photocatalysts. The data obtained from a variable reaction zone were found to approach intrinsic in a differential volume (annular reaction zone of 0.15 cm thickness and 6 cm length). A correction factor (η) representing the ratio of the radical generation rate in the laboratory to that in the reference-ideal photocatalytic reactors was introduced. This allowed for the obtaining of the intrinsic activity (kint) of the photocatalyst from apparent kinetic measurements (kapp). The proposed methodology was tested for an annular reactor for the above reaction (first order) and showed good agreement with the data. The intrinsic reaction rate constants kint were found to be independent of the reactor size for cylindrical geometry for the three light sources employed (100, 200, and 450 W) and variable catalyst concentration. The calculated turnover frequencies (TOF) allowed for a realistic comparison of the intrinsic activity per unit surface area of the individual sites. The sites of Aldrich anatase powders were found to be more active (2fold) than those of Degussa P25.
Introduction Several variables have been introduced to describe photocatalytic activity. Primary quantum yield represents the number of radicals produced in the primary process (most frequently hydroxyl radicals) on the surface of a photocatalyst per single photon absorbed (1). It is probably the most accurate way to characterize photocatalytic activity. This variable is, however, difficult to obtain in simple photocatalytic experiments since only indirect methods are currently available to quantify primary processes. One of them is the use of a probe molecule capable of admitting only one hydroxyl radical (such as methanol) after which the oxidation ceases completely (2). Overall quantum yield or quantum * Corresponding author phone: (513)556-1474; fax: (513)556-3473; e-mail:
[email protected]. 10.1021/es990198r CCC: $19.00 Published on Web 06/30/2000
2000 American Chemical Society
efficiency represents the number of molecules degraded per photon sent into the system or absorbed by the system (1). It is probably the most common value to be used by different researchers for characterization of photocatalysts, and it was shown (3) to vary significantly with the reaction zone length and thickness. Apparent reaction rate constants are also widely used for evaluation of photocatalysts for the degradation of particular compounds since most phototocatalytic reactions are pseudo-first-order for dilute organics. They, however, differ significantly as a function of the radiative power of the lamp and concentration of the catalyst employed in the same reactor (4), which restrains their use as a universal characteristic. Several researchers attempted to relate photocatalytic reaction rate with radiation intensity. Okamoto et al. attempted (5) to incorporate the formation rate of active hydroxyl radicals into the reaction rate equation by
-
dC ) Φ•OHIn f(c,PO2) dt
(1)
where Φ•OH is the quantum yield for the formation of hydroxyl radicals, I is the rate of radiation absorption, PO2 is the oxygen partial pressure, and n is a constant. A number of attempts have also been made to define a common means of comparison of reaction rates for different photocatalysts. For instance, an expression was proposed (6) for the intrinsic rate of hydrogen production in the photocatalytic cleavage of water. It represented the ratio of the hydrogen production rate to the optical transparency (ratio of the transmitted radiation to the incident radiation) of the slurry. Relative photonic efficiency (7) was introduced as yet another means to compare the photocatalytic behavior of different semiconductors. Its value for a standardized system (catalyst, reactor, and actinometer) was proposed to be unity. The quantum yield of each particular photocatalytic system then becomes a product of its relative photonic efficiency with the apparent quantum yield. Rigorous kinetic approach coupled with photon balance was also used (8) to model the photocatalytic degradation of trichloroethylene in water in a reactor with a parabolic reflector. Local volumetric rate of energy absorption was used to characterize the radiation field inside the photoreactor, and major species’ balances accounted for the photochemical transformation. The present study attempts the determination of intrinsic reaction rate constants for photocatalysts from their apparent activity and the radiation field in the photoreactor. The performance of various TiO2 powders in the photocatalytic degradation of phenolic compounds (shown to obey firstorder rate law (3, 4)) in an annular reactor is used to test our proposal. The present method is based on measurements of the incident photon fluxes at different reactor locations and uncoupling the nonuniformity of the radiation field from apparent activity. The reaction rate constants obtained from our model are then compared to the experimental ones obtained for select catalysts in a differential photocatalytic volume (3).
Experimental Section Commercial titania powders were evaluated for the photocatalytic degradation of phenol. The titania powders utilized in the present study were Aldrich anatase, Aldrich anatase 325, and Degussa P25; their characteristics are shown in Table 1. The choice of the catalysts was based on their different photocatalytic properties (Degussa P25 the highest, Aldrich anatase 325 the lowest). The photocatalytic degradation of VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Photocatalytic Powders of Titania Employed in the Present Study BET surface primary particle anatase area, m2/g size, nm content, wt % 75a 10a 9.75a
Degussa P25 Aldrich Anatase Anatase 325 mesh
21c 156c
