IMAGINE A ÒSELF-CLEANINGÓ WORLD
Paul Chin
You may say that I'm a dreamer, but I'm not the only one
I hope someday you'll join us, and the world will live as
one
- John Lennon, Imagine
From the yesterday
decades of yellow submarines to
bell bottoms to New Kids on the Block, culture has evolved with humans, and
technology is no different. Gone
will be the need to clean windows eight days a week. Glass
companies have manufactured Òself-cleaningÓ windows, billed under name brands
such as PPG SuncleanTM and Pilkington ActivTM, containing
a thin film of titanium dioxide (TiO2), a photocatalyst which
utilizes ultraviolet light on a good day sunshine for (a) photocatalytic oxidation (PCO) of adsorbed
organic materials, and (b) photo-induced superhydrophilicity (PSH) to assist
with water washing of the surface by allowing water to come together instead of beading up when it rains. Using
these two TiO2 photoreactive properties may minimize the need to
send washers outside of big skyscrapers to clean window exteriors, or to remove
the bird ÒmishapsÓ off Hank SteinbrennerÕs luxury box window every time the
Boston Red Sox score in the new Yankee Stadium.
Though this revolutionary technology is truly amazing, researchers canÕt
just let it be. A field analysis method is needed to characterize the
TiO2 film at initial and continuing times, and various environmental
conditions. Furthermore, the
creation of these Òself-cleaningÓ surfaces logically leads to consideration of testing potential
organic material deposits found in outdoor air.
Work completed by the Ollis
research group at Raleigh, NC, focused on using the organic dyes Acid Blue 9
and Reactive Black 5 for visual and quantitative PCO data on Pilkington ActivTM
glass1. The visual
results showed a return of aesthetic clarity on the dye-coated ActivTM
glass after 1000 min of near-UV illumination (irradiance: 1.4 mW/cm2). Using UV-Vis spectroscopy, our lab
quantitatively observed a monotonic decrease in peak absorbance as a function
of time. A two-step reaction
mechanism was developed to help(!) describe
the kinetics of dye degradation. The
kinetic model indicated that the colored dye forms a colored intermediate
before its colorless final product. This model can be applied to
characterize the change in TiO2 kinetics at initial and continuing
times. Additionally, here comes
the sun, solar experiments conducted under
variable UV light intensity, relative humidity, and temperature exhibited
similar dye decolorization rates compared to work completed in an indoor
photoreactor. Our model fit the
solar data well even though the UV irradiance varies with time.
Though dyes are a good
characterization material, they do not simulate organic pollutants encountered
in the atmosphere. Take a walk
down Penny Lane or enter any
industrialized U.S. city, and what you will find besides crime, poverty,
obnoxious sport fans, and general helter skelter, is the deposition of soot on exterior surfaces
(e.g., buildings, statues).
Work completed in our lab
systematically studied various TiO2 and soot thickness to determine
their effect on the PCO rate of soot2 using a quartz crystal
microbalance (QCM). Results of
UV-Vis spectroscopy revealed that our lab could
linearly and reproducibly deposit soot using an analytical linear rotor
rotating past a hurricane lamp.
By changing the amount of soot deposited on top of the TiO2-coated
QCM crystals, experiments revealed a range of behaviors from complete soot
mineralization (< 0.7 mm soot
thickness), to partial oxidation (1 mm
thickness), to minimal soot oxidation (2 mm
thickness) caused by soot screening of the incident UV light. Alternatively, varying the TiO2
thickness did not demonstrate significant changes in the soot destruction rate because oxidation of soot in direct contact with TiO2
is the dominant reaction compared to lateral and remote soot oxidation, and the
soot / TiO2 contact area is independent of TiO2
thickness. A series / parallel
reaction mechanism was successfully constructed and utilized to describe the
reaction kinetics of our soot PCO for t > 1,000 min. If we get
back to previous literature3,4
for PCO of soot on thick (c.a. 1 mm)
TiO2 films, we can work it out
that our model5 adequately describes their data. Finally
our lab illustrated TiO2 PCO of carbon black, often studied as a
model soot, though further tests were needed to elucidate the kinetic reaction
mechanism.
While continual development of
these novel Òself-cleaningÓ surfaces will transform our hard dayÕs night (and possibly put WindexTM out of
business), additional research is required to perfect the TiO2 thin
films. Further experimentation of
organic deposits is needed to comprehend fully the applicability of these
Òself-cleaningÓ surfaces under realistic conditions. After a long and winding road, one can imagine how this technology will take us across
the universe.
The End.
1
Chin P., Ollis D.F., Catalysis Today 2007,
123 (1), 177-188.
2 Chin
P., Grant C.S., Ollis D.F., ÒQuantitative Photocatalyzed Soot Oxidation on
Titanium DioxideÓ, submitted to Journal of Catalysis 2008.
3 Mills
A., Wang J., Crow M., Chemosphere 2006,
64 (6), 1032-1035.
4 Lee N.C., Choi W.Y., Journal of Physical Chemistry B 2002, 106 (45), 11818-11822.
5
Chin P., Roberts G.W., Ollis D.F., Industrial & Engineering Chemistry
Research 2007, 46 (23),
7598-7604.