(583ej) Efficiencies of the Photocatalytic Reactors During the Remediation of Water, Air and Hydrogen Production

The main objective of this study is to report reactor efficiencies of three different batch photocatalytic reactors during water and air decontamination and hydrogen production respectively.

Water remediation: Water decontamination studies were carried out in the Photo CREC – Water – II photoreactor, a 7 litre bench scale slurry reactor using six TiO₂-based_. Phenol was used as a model compound in all cases. A reaction scheme based on the detected compounds was proposed. This Unified Kinetic Model describes the behavior of the reacting system including phenol and its intermediates.

The reactor efficiency was calculated for the complete experimental reaction span, which is based on the consumption of the free radicals OH and the absorbed irradiation by the TiO₂ catalyst, using the quantum efficiency (QY) and Photochemical and Thermodynamic Efficiency Factor (PTEF). The total reaction rate of OH radicals was expressed in terms of the detectable compound concentrations, which required knowing the stoichiometric equations for each reaction step. The calculated quantum efficiencies reached about 50 % while a value of 13% was obtained for the PTEF. This indicates that the designed photo reactors prototypes are appropriate for scaling up.

Air remediation: Air treatment studies were carried out on the Photo CREC Dual a 57 liters reactor with the catalyst attached to a mesh located in the annular section. Air is put in recirculation; contact of pollutants in the air with the catalyst causes the chemical reaction. Inside the annular region of this reactor, a specially designed sensor with optical fiber was placed to study the irradiation field, which affects the chemical reaction.

The model compounds were acetone and acetaldehyde, the enthalpy of formation of OH radicals was calculated in air. The fraction of energy from irradiation consumed to produce the OH radical is 0.0131 (or 1.31 %), appreciably smaller than that of the water, 0.27 (or 27 %) indicating that hydroxyl radicals are easily formed in air phase, leading to quantum efficiencies (QY) above 100 %, and PTEF values of less than 100% being in agreement with thermodynamics. This means that in air, a photon is better used for remediation than it is in water. Oxidation reactions are in chain in air systems and it is known that other free radicals are involved during remediation. These high efficiency values indicate that heterogeneous photocatalysis is a very promising technology for air purification.

Hydrogen production: Experiments were carried out in the reactor photo CREC Water II adapted for the production of hydrogen. A hermetic new tank was implemented in order to accurately measure hydrogen concentrations. During the photocatalytic experiments nitrogen or argon are used as carrier gases and ethanol as hole scavenger. Given our ample experience in water and air treatment it was possible to extend the research to the hydrogen production.

TiO₂ catalyst was impregnated with different loads of platinum, which functions as electron deposit that reduces the recombination of electrical charges, and then the electron reacts with the proton resulting from the water dissociation, producing the free radical hydrogen, which then forms a hydrogen molecule. It is necessary to carry out these reactions in absence of air so that oxygen do not compete with electrons, this might reduce reactor efficiency.

The synthesized catalysts were characterized with BET, XRD, SEM/EDX, etc. CO₂, CO, H₂, CH₄ were detected with GC-detector TCD-FID during hydrogen production with photocatalysis.

For pH = 10 and 2% v/v ethanol and argon conditions, no hydrogen was produced with or without TiO₂ catalyst. On the other hand, for TiO₂ – 1% Pt, production of hydrogen was 0.131 micromoles of H₂ / cm³. In this case, the quantum efficiency profile showed a maximum value of 6 %, achieved at one hour of experiment.

 In the case of  pH = 5, hydrogen production was not observed with or without TiO₂ catalyst. However, when TiO₂-1% Pt was used hydrogen concentration profiles are lineal, and the maximum production was 0.27 micromol of H₂/cm³ . In this case,  quantum efficiency of 8 % was achieved and maintained constant during all the experiment. It was found that 0.14 g of catalyst /Liter was the best value for the absorption of light and chemical reaction.

The obtained values for the reactor efficiencies allow us to argue that the designed photo CREC reactors are useful for establishing criteria to scale up the reactors. With this study we aim to solve environmental problems through the remediation of water and air and the supply of energy through hydrogen production, as an alternative source of energy. Heterogeneous photocatalysis is a very valuable technology under construction and the challenge is to design and ensemble industrial pieces of equipment able to effectively use solar radiation.