TiO2 Photocatalysis for Organics
Alan Van Matre
Table of Contents
Current uses of TiO2 Photocatalysis .............4
Application of TiO2 Photocatalysis ...............6
Figure I ........................................................8
Figure II .......................................................9
The definition of photocatalysis is basically the acceleration of a photoreaction by the presence of a catalyst. A more in depth approach would include that the catalyst may accelerate the photoreaction by interaction with the substrate in its ground or excited state and/or with a primary photoproduct, depending upon the mechanism of the photoreaction .
Catalysis by definition, implicates a catalytic entity that participates and accelerates the chemical transformation of a substrate, itself remaining unaltered at the end of each catalytic cycle . In photocatalysis, no energy is stored; there is merely an acceleration of a slow event by a photon- assisted process.
It would probably be a good time to introduce a few of the many ways in which photocatalysis works in basic easy to understand terms. In figure one; M is the metal containing catalyst or catalyst precursor, O is the organic reactant, P is the product, C’ is the photoassistor also pseudocatalyst, R is the primary photoproduct, and hv is the irradiation via ultraviolet or visible light. The simplest most basic equation is that the irradiated subject is changed to an excited state thus increasing the ease of bond making and braking which ultimately renders the organic reactant to a desired product. As previously mentioned figure one includes schemes 5-9. Scheme 5 illustrates the process, commonly termed photosensitization, in which the interaction between electronically excited M and ground-state substrate activates the latter and regenerates M. In scheme 6 the reaction of M* (the exited M) produces a ground state species, C’, which assists the transformation of substrate to product and then reverts to M. Scheme 7 describes the case in which M catalyzes the reaction of an electronically excited organic substrate via formation of an excited complex, M-O*. Scheme 8 involves a metal-catalyzed reaction of a primary photoproduct, R. Lastly, Scheme 9 illustrates a transformation that results from irradiation of a ground state M-O complex .
Machines or specially designed equipment are needed in this field because inconveniences and detrimental factors in direct solar photolysis are the lack of sunlight absorption by the substrates, attenuation of the sunlight, and the relatively shallow penetration depth of sunlight in natural aquatic bodies.
There are many types of catalyst, some act on very few substrates while some act on many substrates. The best way to cleanse a wastewater would be to use a photocatalysis process that can be effective on a multitude of contaminants or in other words a heterogeneous environment of contaminants. Metal oxides work well in this case. It is true that many oxides work well, WO3, and ZnO but in scientific studies it has been proven that TiO2 has an advantage over the others.
The reasons that TiO2 does so well and is desired as an agent
in remediation of wastewater is based on several factors. 1. The process
occurs under ambient conditions. 2. The formation of photocyclized intermediate
products, unlike direct photolysis techniques, is avoided. 3. Oxidation
of the substrates to CO2 is complete. 4. The photocatalyst is
inexpensive and has a high turnover. 5. TiO2 can be supported
on suitable reactor substrates. 6. The process offers great potential as
an industrial technology to detoxify wastewaters .
Current Uses of TiO2 Photocatalysis
Researchers have used photocatalytic oxidation (PCO) to break down and destroy many types of organic pollutants. It has been used to purify drinking water, destroy bacteria and viruses, remove metals from waste streams, and breakdown organics into simpler components of water and CO2.
After photocatalysis was realized to be a great oxidation mechanism, researchers began testing it on many different compounds, and in many different processes. To date, this technology has been used to detoxify drinking water, decontaminate industrial wastewater, and purify air streams.
Photocatalytic Treatment of Water
Some of the first experiments showed that chlorinated aliphatic hydrocarbons were dechlorinated and mineralized . This means that the compounds were broken down into water and CO2. Before long researchers realized that this advanced oxidation technique could be used on many compounds, including some aromatics that are resistant to normal oxidation reactions . According to Purifics, an industrial water treatment company specializing in photocatalysis, many different chemicals have proven to be detoxified or removed from water . These chemicals include:
Organic Families Toxic Compounds
amine compounds cyanide
Photocatalysis has not only been proven to remove pollutants from water, but also nuisance color, taste and odor compounds . Tests have also proven TiO2 to effectively remove bacteria, and viruses from water supplies. A study by Ireland et. al. showed that TiO2 oxidation effectively removed Escheria coli (E. coli) from drinking water .
Photocatalytic Treatment of Air
Treatment of polluted air streams is often more efficient that treating liquid waste streams. Gas phase kinetics allow reactions to occur much faster than in the liquid phase. This fact has lead some people to utilize air stripping of pollutants from liquid phase for treatment in the gas phase. In the process of treating air streams, TiO2 must be suspended on some sort of surface to allow the gas to pass over it and react. This is usually some sort of matrix with a high surface area, which the UV light is shown upon.
An air treatment system for ethylene removal has been developed at University
of Wisconsin-Madison . This system will be placed in produce sections
of grocery stores to remove the naturally occurring ethylene that causes
fruits and vegetables to spoil. The UV light has also shown to reduce bacteria,
molds and odors .
Application of TiO2 Photocatalysis
A common application for TiO2 photocatalysis is the mineralization of trichlorotmehtane (CHCl3). Trichloromethane is a suspected carcinogenic chloroform produced from dissolved organic matter during conventional water chlorination procedures. This purification process is shown to be very effective in an experiment performed by David F. Ollis, a chemical engineer at North Carolina University. According to his results published in Environmental Science and Technology, "The simultaneous presence of illumination and TiO2 produced the chloride ion and caused the disappearance of chloroform" . The basic general equation for chloroform breakdown is given below:
H2O + CHCl3 + (1/2)O2 => CO2 + 3HCl
The oxygen needed for the experiment is aerated throughout the contaminated
water. The statement given states that a chloride ion is produced. Figure
II shows that the ions combine with hydrogen to form a more desirable compound
HCl . Ollis’ experiment shows great promise and provides ample information
to show its success. Figure II also shows the differences in using only
one of the procedures at a time compared to using both procedures at the
same time. The results are staggering showing a combined effort associated
with a drastic drop in tricholormethane. Overall the experiment is well
documented and explained. One problem addressed when using the insoluble
catalyst TiO2 is the need to recover and reuse this material.
After much research, we would suggest experimenting with immobilizing the
catalyst perhaps placing TiO2 in glass. Immobilizing the catalyst
can cause a wide range of problems from lower efficiency to difficulties
in mass transport, but that is beyond the scope of the paper.
2. Matthews, R. W. in Phtotocatalytic Purification and Treatment of Water and Air, Ollis, D.F., Al-Ekabi, H., Eds. Elsevier: Amsterdam, 1993.
3. Purifics web page. Process Description. http://www.purifics.com/Prodctin.htm.
4. Field Testing of A Solar Assisted System for Detoxification of Water. http://es.epa.gov/ncerqa_abstracts.
5. Ireland, J.C. et. al. Inactivation of Escheria coli by Titanium Dioxide Photocatalytic Oxidation. Applied and Environmental Microbiology. May 1993.
6. Biotechnology Education. UW Clean-Air Technology Headed for the Produce