Table of Contents
page
Introduction
2
Current uses of TiO2 Photocatalysis
4
Application of TiO2 Photocatalysis
6
Figure I
8
Figure II
9
References
10
Photocatalysis
Introduction
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
[1].
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 [1]. 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
2
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 [1].
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 [1].
3
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 [2]. 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 [2]. According to Purifics, an industrial water treatment company
specializing in photocatalysis, many different chemicals have proven to be detoxified or
removed from water [3]. These chemicals include:
Organic Families
Toxic Compounds
alkanes
alkenes
alkynes
ethers
PCB’s
PAH’s
dioxins
furans
4
aldehydes
ketones
alcohols
amine compounds
amide compounds
esters
pesticides
herbicides
phenols
cyanide
Treatment of water can be accomplished by adding a powdered form of TiO2 to the water,
or it can be immobilized on a substrate. If TiO2 is in solution then some sort of recovery
system is necessary in order to reuse the catalyst.
Photocatalysis has not only been proven to remove pollutants from water, but also nuisance
color, taste and odor compounds [4]. 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 [5].
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 [6]. This system will be placed in produce sections of grocery stores to
5
remove the naturally occurring ethylene that causes fruits and vegetables to spoil. The UV
light has also shown to reduce bacteria, molds and odors [6].
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[2]. 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” [2]. 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 [2]. 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
6
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.
7
8
9
REFERENCES
1. Kutal, C., Serpone, N., Photosensitive Metal Organic Systems: Mechanistic Principles
and Applications. American Chemical Society, Washington D.C. 1993.
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 Section.
http://www.biotech.wisc.edu/Education/biotechnews/air.html.
10
11