Catherine BridgesConnor Hayes
Chemistry of Light-Based Technologies: Photocatalysis
Introduction
A. General Purpose/ Uses of Light-Based Technologies
The world is consuming fossil fuels at an alarming rate. In 2012, the United States alone
exhausted almost 900 million tons of coal and 26,000 billion cubic feet of natural gas over the
course of the year, with an additional 19,000 barrels of petroleum burned daily.1 However, this
amount of carbon fuel spending came at a high cost, resulting in an additional 5,200 million
metric tons of CO2 released into the atmosphere.1 Such an increase in carbon dioxide levels and
other greenhouse gases are the leading cause of global warming, which poses a serious threat to
the delicate ecosystems on Earth.2 Renewable energy sources, such as solar power, provide a
viable alternative to fossil fuels and lack the production of harmful pollutants. Shown in Scheme
1, radiant energy has already been proven effective within the community in a variety of ways.
Scheme 1. Five Major Modes of Solar Energy
e-e-
ox red
catalyst
Research and development of photocatalysis is being pursued intensively for its promise to
convert light into a chemical driving force.3 Solar energy is also harnessed to heat water both
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Catherine BridgesConnor Hayes
residentially and industrially, eliminating the energy utilized by water heaters and producing
electricity in power plants via steam-driven turbines.4,5 Alternatively, electricity for power grids
can be directly generated from solar cells capitalizing on the photovoltaic effect.6 Applications
of solar power extend far beyond the household, as photosynthetic microalgae and other plants
are being exploited to synthesize biodiesel via the transesterification of their fatty acids.7 Solar
energy is quickly becoming the most useful alternative energy available.
B. General Types of Photocatalysis Applications
A major mode of solar energy being probed by scientists is photocatalysis. Photocatalysis is the
process of capturing light to perform a redox reaction using an activated substrate, known as a
photocatalyst, which remains unchanged in the overall reaction.8 The basic mechanism of a
semiconducting photocatalyst involves the absorption of a photon to excite an electron from the
conducting band to the valence band followed by possible recombination or redox chemistry at
the active site (Scheme 2).8 Natural photocatalysts, such as chlorophyll in corn, have been used
Scheme 2. Mechanism of a Semiconducting Photocatalyst
e-
H2O
OH+H+
H+½ H2
hvExcitationEBG = hv
2OH
hv
H2O2e-
Relaxation(luminescence)
e-
e-
e-
e-
H2O2 H2O + ½ O2
ElectronTrap
ElectronRelay
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Catherine BridgesConnor Hayes
in the production of biofuels like ethanol.9 Molecular hydrogen evolution, however, is a process
benefitting from man-made photocatalysts. Solar-activated copper complexes have been shown
to efficiently split water to produce this clean, carbon-free energy source.10 Liquid fuels, namely
methanol, can also be split photocatalytically to produce H2 with even greater success.11 Another
common application of photocatalysis is the reduction of CO2 to CO.12 Not only will this process
decrease the amount of CO2 in the atmosphere, but CO can also easily be converted into liquid
fuels using H2 and electrocatalytic processes.2 Photocatalysis offers a solution for neutralizing
existing air pollutants as well. Released by the combustion of fossil fuels, NO and NO2 are
responsible for many negative environmental side effects including acid rains, global warming,
and human disease.13 However, through photocatalytic oxidation, these harmful nitrogenous
oxides can be converted into a less destructive nitrate compound.12 These major modes of
photocatalysis are illustrated below (Scheme 3).
Scheme 3. Five Major Uses of Photocatalysis
Photocatalyst
H2O
H2
CO2CO
NOxNO3
-
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Catherine BridgesConnor Hayes
C. Statement of Need and Outline of Approach
Materials and Methods
Results
Discussion
Conclusion
References
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