Chemistry of Light-Based Technologies: Photocatalysis

Catherine Bridges Connor Hayes

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

1 metric tons of CO2 released into the atmosphere. 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

ox red

catalyst e- e-

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

1 Catherine Bridges Connor 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

+ ½ H2 H

e- - - Electron hv e e Trap

Excitation Relaxation (luminescence) EBG = hv H2O e- + Electron OH+H e- e- Relay 2OH H2O2 H2O2 H2O + ½ O2

2 Catherine Bridges Connor 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

11 methanol, can also be split photocatalytically to produce H2 with even greater success. Another

12 common application of photocatalysis is the reduction of CO2 to CO. Not only will this process decrease the amount of CO2 in the atmosphere, but CO can also easily be converted into liquid

2 fuels using H2 and electrocatalytic processes. 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

H O 2 CO CO2

H2 NO - NOx 3

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C. Statement of Need and Outline of Approach

Materials and Methods

Results

Discussion

Conclusion

References

4 1 “Overview data for the United States.” U.S. Energy Information Administration. 30 May 2013. (accessed 2015-01-31)

2 Kubiak, C.; Benson, E.; Sathrum, A.; Smieja, J. Electrocatalytic and homogenous approaches to conversion of CO2 to liquid fuels. Chem. Soc. Rev. 2008, 38, 89-99.

3 Schultz, D.; Yoon, T. Solar Synthesis: Prospects in Visible Light Photocatalysis. Science 2014, 343, 985-993.

4 Otanicar, T.; Golden, J. Comparative Environmental and Economic Analysis of Conventional and Nanofluid Solar Hot Water Technologies. Environ. Sc. Technol. 2009, 43, 6082- 6087.

5 “Concentrating Solar Power Basics.” National Renewable Energy Laboratory. 25 July

2014. (accessed 2015-01-31)

6 Mickey, C. Solar Photovoltaic Cells. J. Chem. Educ. 1981, 58, 418-423.

7 Blatti, J.; Burkart, M. Releasing Stored Solar Energy within Pond Scum: Biodiesel form Algal Lipids. J. Chem. Educ. 2012, 891, 239-242.

8 Banerjee, S.; Pillai, S.; Falaras, P.; O’Shea, K.; Byrne, J.; Dionysiou, D. New Insights into the Mechanism of Visible Light Photocatalysis. J. Phys. Chem. Lett. 2014, 5, 2543-2554.

9 Wallington, T.; Anderson, J.; Mueller, S.; Morris, E.; Winkler, S.; Ginder, J. Corn Ethanol Production, Food Exports, and Indirect Land Use Change. Environ. Sci. Technol. 2012, 46, 6379-6384.

10 Luo, S.; Mejia, E.; Friedrich, A.; Pazidis, A.; Junge, H.; Surkus, A.; Jackstell, R.; Denurra, S.; Gladiali, S.; Lochbrunner, S.; Beller, M. Photocatalytic Water Reduction with Copper- Based Photosensitizers: A Noble-Metal-Free System. Angew. Chem. Int. Ed. 2013, 52, 419-423.

11 Guo, Q., Yang, W., Xu, C., Chen, M., Yang, X., Molecular Hydrogen Formation from

Photocatalysis of Methanol on Anatase-TiO2. J. Am. Chem. Soc. 2014, 136, 602-605.

12 Kim, W.; Yuan, G.; McClure, B.; Frei, H. Light Induced Cardon Dioxide Reduction by Water at Binuclear ZrOCoII Unit Coupled to Ir Oxide Nanocluster Catalyst. J. Am. Chem. Soc. 2014, 136, 11034-11042.

13 Mendes, A.; Angelo, J.; Andrade, L.; Madeira, L. An overview of photocatalysis phenomena applied to NOx abatement. J. Environ. Mngmt. 2013, 129, 522-539.