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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2014, 136, 11034-11042.

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