Photoelectrocatalysis in water treatment

researcher: Yasmina Bennani

promotor: prof. Luuk Rietveld

 

Research objectives
Parameters that influence photoelectrocatalytic (PEC) efficiency of TiO2 film electrodes prepared by paint-thermal decomposition method were studied in this research. Building upon the knowledge gained in the previous research [1], the objective of the study was to determine the optimal experimental conditions of selected process variables like layer thickness and energy flux and to determine first-order kinetic rate constants from the experimental data. Together with the mentioned optimization variables, this should provide the necessary basis for the design of electrodes which can be used in a large-scale photoelectrocatalytic reactors.

Project outline
Introduction
For a given system, the rate of the photocatalytic reaction at the surface of the catalyst is determined by the concentrations of both the photoholes in the anode and the scavengers at the cathode surface. The former is determined by the intensity of the incident radiation, layer thickness and the latter by the bulk concentration of the scavenger.In fully optimizing the photoelectrocatalytic processes that occur at TiO2 film electrodes, the improvement of photocatalytic activity is the research focus in this study. When enhancing degradation kinetics of organic compounds for practical applications [2], it should be taken into account that working conditions are case-specific and need to be carefully optimized.

Approach
TiO2 film electrodes were manufactured according to the paint-thermal decomposition method [3]. To experimentally determine the optimal conditions for the selected process parameters, initial phenol concentration, TiO2 layer thickness and solar light intensity, were varied. A photoluminescence technique was used in order to evaluate the photocatalytic activity of the TiO2 films with different thicknesses, while the other parameters were monitored by measuring the phenol degradation in an aqueous solution.

Results

Figure 1. Maximum PL intensity of 2-OHTA at 425 nm as a function of the number of TiO2 layers, at applied potential of 1 V.

Figure 2. Electrical energy per order using different solar light (UV300-400, W/m2) intensities for different degradation percentages of phenol

 

  • The film thickness is a key parameter in controlling the electron transfer as well as the light-harvesting efficiency. The observations suggested that there might be an optimal number of layers of TiO2 film for an effective PEC reaction, being 6 layers.
  • Proper dispersion and size of the TiO2 particles in the layers is the key to achieving optimum optical performance.
  • Increase in the initial concentration of phenol increased the probability of phenol molecules to react with •OH. However, at higher initial phenol concentrations, the available surface area for •OH formation becomes the rate-limiting factor, as there is an excess of phenol molecules at the surface.
  • Elimination of mass transfer limitations and ensuring that any other possible influencing parameters (cathode/anode ratio, limiting current) are constant is essential for optimization of other PEC system parameters (energy demand, layer thickness). 
  • Required organic pollutant removal efficiency of the system is determined by the   balance between reaction rate kinetics (retention time in the reactor) and energy consumption of the system.

Scientific relevance
The study about optimization of experimental conditions should predict the photodegradation behaviour under different conditions and use of energy, leading to a better understanding of the process and a better design of reactors for PEC.

Social relevance
Photoelectrocatalysis is a relatively cheap green technology. It does not generate secondary waste nor involve the use of additional chemicals, and offers improved beneficial uses of produced water. It can generate and store energy, use renewable energy source (solar light), remove organics, produce clean water and recover valuable materials from produced water with little or no negative impact on the environment.

Literature

  • Y. Bennani, A. S. El-Kalliny, P. Appel and L.C. Rietveld,  “Enhanced solar light photoelectrocatalytic activity in water by anatase-to-rutile TiO2 transformation”, Journal of Advanced Oxidation Technology, vol. 17, no. 2, pp. 285-296, 2014.
  • K. Mogyorosi, A. Farkas, I. Dekany, TiO2-based photocatalytic degradation of 2-chlorophenol adsorbed on hydrophobic clay, Environ. Sci. Technol. 36, 3618–3624, 2002.
  • H. Beer, Improvements in or relating to electrodes for electrolysis, patent GB1, 147, 442, 1969.