(446f) Photocatalytic Thermodynamic Efficiency Factors (PTEF) During Enhanced Phenol Degradation by Iron Ions

Heterogeneous photocatalysis belongs to the Advanced Oxidation Processes (AOP), a technique based on the formation of non-selective highly?reactive radicals as initiators of the oxidative degradation. The use of an oxidant, a semiconductor material and UV promotes oxidation reactions through the cyclic formation and consumption of hydroxyl radicals, which attack the organic compounds.1 An organic molecule can potentially be degraded to carbon dioxide, water and mineral acids (1).

Phenol has been extensively used as a model compound to both understand the photocatalytic reaction mechanisms and to test the performance of various photocatalytic reactors. Kinetic models for the photocatalytic oxidation of phenol and other phenolic compounds have been mainly based on initial rates of reaction. Such kinetic models fail to account for the formation of reaction intermediates species. More recently a series-parallel kinetic model based on some measurable aromatic and aliphatic acid intermediates was developed. These models involve in the different steps, variable quantities of OH? free radicals (1).

Regarding energy efficiencies for ranking photo conversion reactors, their importance have been emphasized in a number of studies. In spite of this, determination of photocatalytic reactor efficiency has remained a challenge due to the many variables involved, such as reaction rates, reaction mechanism, OH? free radicals involved in various reaction steps, kinetic constants, adsorption parameters, irradiation field, light absorbed by the photocatalyst, amount and type of photocatalyst, type of organic or inorganic species to be converted.

To address this issue, the photochemical thermodynamic efficiency factor (PTEF) was proposed as the ratio of the energy used to produce the OH radical used in the photocatalytic reaction to the energy absorbed by the photocatalyst (2). In the very first approach, the efficiency factors were calculated using the information from the first minutes of photoconversion and calculating the initial reaction rate. Values in the range of 1-2 % were obtained.

The Photocatalytic Thermodynamic Efficiency Factor (PTEF) is a parameter that can be used in photocatalytic reactors to establish photon energy utilization as the ratio of the energy used to generate OH? free radicals and the energy absorbed by the TiO2 photocatalyst. The PTEF evaluation requires the assessment of the total rate of OH? free radicals at any given time during the photoconversion of organic species. A key parameter in this assessment is the availability of the complete spectrum of measurable chemical species including various intermediates. Quantification of different chemical species and their evolution with irradiation time allow via stoichiometric relationships to calculate the OH? radicals consumed in the photocatalytic reactor. PTEFs and quantum yields (QY) were reported recently for phenol photocatalytic conversion in water media (free of iron ions) displaying 71% and 19% maximum QYs and PTEFs respectively (3).

In the present study, the QY and PTEF are reviewed further, considering the photo conversion of phenol in water media enhanced by iron ions. It is shown using the more realistic RN2 Model including both phenolic species and carboxylic acids that the maximum QYs and PTEFs reach up to 85% and 23% levels respectively.

These encouraging efficiency factors demonstrate the favorable prospects of photocatalysis and Photo-CREC-Water reactors operated under optimum photocatalyst loading conditions (0.14 g/L), with only a small fraction of the total absorbed photons potentially lost in photon recombination. Hydroxyl Radicals, Phenol, Heterogeneous Photocatalysis, Reactor Efficiency, Iron

References

(1) de Lasa, H.; Serrano, B.; Salaices, M. Photocatalytic Reaction Engineering, Springer Publishers, 2005. (2) Serrano, B.; de Lasa H. Photocatalytic Degradation of Water Organic Pollutants. Kinetic Modeling and Energy Efficiency, Ind. Eng. Chem.. Res., 1997, 36, 4705?4711. (3) Serrano, B.; Ortiz-Gomez, A.; Moreira-del-Rio, J.; de Lasa, H., Energy Efficiency in Photocatalytic Reactors for the Full Span of Reaction Times, Ind. Eng. Chem. Res. 2009, Ind. Eng. Chem. Res., 2009, 48 (22), pp 9864?9876