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.fail edit

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.

Each team should always create its own calibration curve. During the Winter quarter, most teams should focus on obtaining quality data using simple initial rate and pseudo first-order kinetics. As the quarter progresses and turns into the Spring quarter, "better" conditions--as determined by the initial teams--can be used to analyze the reaction using Langmuir-Hinshelwood kinetics.

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. Note that nanometer-sized TiO₂ particles are used very differently from the micron-sized particles we use in the lab, and that only a small number of studies use them in a slurry as we do. We found that nanometer-sized particles were a poor choice for a teaching lab due to safety concerns.

• Photocatalytic degradation pathway of methylene blue in water.
• TiO₂-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations.
• Photocatalytic degradation of various types of dyes in water by UV-irradiated titania.
• Kinetics of photocatalytic degradation of reactive dyes in a TiO₂ slurry reactor.
• Photocatalytic degradation of various dyes by combustion synthesized nano anatase TiO₂.
• Variation of Langmuir absorption constant determined for TiO₂-photocatalyzed degradation of acetophenone under different light intensity.
• Adsorption of methylene blue and acid blue 40 on titania from aqueous solution.
• Photodestruction and COD removal of toxic dye erioglaucine by TiO₂-UV process: influence of operation parameters.
• A general treatment and classification of the solute absorption isotherm.

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.

• Photocatalytic degradation of disperse blue 1 using UV/TiO₂/H₂O₂ process. Note that this is not a great journal, the writing isn't that great, and the H₂O₂ concentration was never stated. The mechanism is good compared to the next two references but still not complete; look in one of the previous references for the full mechanism.
• Treatment of Remazol brilliant blue dye effluent by advanced photo oxidation process in TiO₂/UV and H₂O₂/UV reactors.
• H₂O₂/TiO₂ photocatalytic oxidation of metol. Identification of intermediates and reaction pathways.

Standard Operating Procedure

Safety

• Lab coats and protective eyewear should be worn at all times.
• Dispose of broken glassware in the appropriately labeled receptacles; used filters can be disposed of in the trash.
• Do not dispose of waste down the drain! Use a flask to collect all waste then notify the TA or instructor at the end of the lab period.
• Methylene blue is a potent textile dye. When connecting tubing or transferring liquid, take caution not to splash.
• Relevant chemical information has been listed in Table 1.

Table 1. Chemicals and their formulas, hazards, and suppliers for UV photocatalysis.

NameMSDS	Formula	Hazard	Supplier	SKU
Methylene blue	C16H18N3SCl		unk	unk
Hydrogen peroxide (5%)	H2O2		any	any
Titanium dioxide	TiO2		unk	unk

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.