UV Photocatalysis

Introduction

A photocatalytic reaction proceeds in the presence of light--typically visible or UV--and catalyst--often a semi-conductive transition metal oxide such as titanium dioxide (TiO₂). This experiment will give an introduction into wastewater treatment via photocatalysis by measuring the degradation kinetics of methylene blue as a function of catalyst loading, hydrogen peroxide concentration, and experimental setup.

Background

Review your textbook on chemical reaction engineering, particularly those chapters which cover experimental determination of rate laws and heterogeneous catalysis . Most often, a pseudo first-order kinetic model is chosen to determine rate laws in this experiment.

Theory

Photocatalysis is such a large field that there are tens of thousands of papers to read; it's recommended that you give yourself a morning--and maybe and afternoon--to do so. If you're unwilling or unable to read everything, we've categorized a few favorites below to help you along.

Review Articles

These articles provide a good overview and context for the field but intentionally lack the specificity of research articles.

• Parameters affecting the photocatalytic degradation of dyes using TiO₂-based photocatalysts: A review.
• Photocatalytic degradation for environmental applications - a review (good coverage of environmental aspects).
• Titanium dioxide photocatalysis (good physical chemistry aspects).
• Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress, and problems.
• Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment: A review (a broad review which includes much nano-TiO₂).
• Treatment of hazardous organic and inorganc compounds through aqueous-phase photocatalysis: A review.
• Photophysical, photochemical and photocatalytic aspects of metal nanoparticles (good review from a quality journal).
• Photocatalysis on TiO₂ surfaces: Principles, mechanisms, and selected results (classic review with over a thousand citations. You can find the crystal structure of various titanium oxides here).

Research Articles: Slurry Reactors

These articles were chosen for their investigation of dye degradation in slurry reactors.

Research Articles: H₂O₂/TiO₂ Systems

These articles were chosen for their use of H2O2 and TiO2 for dye degradation. Note that some are written in a manner similar to our lab reports but occasionally employ poor writing styles.

Standard Operating Procedure

Caution: methylene blue is a potent textile dye. Be sure to wear proper PPE to avoid staining your clothes and work carefully to avoid spills.

Preparation

1. Familiarize yourself with the UV/VIS spectrophotometer and the data acquisition software. Note that there are two types of measurements:
   1. Absorption spectrum. Use this to determine the wavelength of maximum absorbance.
   2. Point measurement. Use this to measure absorbance at a single, pre-set wavelength.
2. Create a calibration curve.
   1. Prepare about 100 mL of a solution of known dye concentration; typically 10 ppm is sufficient.
   2. Acquire the absorption spectrum of this solution. If you see a plateau instead of peaks then the solution is too concentrated and needs to be diluted.
   3. Note the wavelength of maximum absorbance, \(\lambda_{\textrm{max}}\).
   4. Measure absorbance at \(\lambda_{\textrm{max}}\) for several diluted solutions, including deionized (DI) water. Minimize waste by using measurement pipets and plastic cuvettes for dilution series.
3. Measure the volume of the reactor-flask system.
   1. Select a small or medium-sized glass spinner flask.
   2. Place the flask on the stir plate and connect the pump lines.
   3. Fill the flask with enough DI water to submerge the appropriate pump line.
   4. Run the peristaltic pump until continuous circulation is achieved between the flask and UV reactor, adding water as necessary to keep the appropriate pump line submerged. Flow through the UV reactor should be from bottom to top.
   5. The total volume of water used in Steps 3 and 5 is the volume of water you should use to prepare solutions for subsequent runs.
   6. Drain the system. Use a large glass spinner flask or other appropriate glass flask to store your waste. Do not dump waste down the drain.

Basic Operation

1. In the spinner flask, prepare an appropriate volume of solution of known (measured) dye concentration (~10 ppm) with TiO₂ (~1-10 g / 500 mL) in DI water. Use the stir plate to break up clumps of TiO₂.
2. Add H₂O₂ (1-5 mL) to "kick-start" the reaction .
3. Turn on the pump, stirrer, and UV reactor.
4. At regular intervals, collect a sample, filter out the TiO₂ using syringe filters, and measure absorbance and \(\lambda_{\textrm{max}}\).
   1. Absorbance measurements are meaningless unless the sample is adequately filtered.
   2. Do not filter the samples near your face! It's easy to rupture the syringe filter or connections during the filtration process if you go too fast.
   3. About 20 mL of solution can be filtered before the filter should be replaced.

Shutdown

1. Turn off the UV reactor.
2. Drain the UV reactor and consolidate waste into a single, large spinner flask.
   1. If more waste was produced than can fit in a single flask, then fill a single flask and place excess waste in another glass container; the latter need not be a spinner flask.
3. Add about 5 mL H₂O₂, then turn on pump, stirrer, and UV reactor.
4. Before you leave the lab, inform the TA or Instructor of which flasks are waste.