This book introduces the theory and applications of nanometer photocatalysis, and it briefly presents the concept of photocatalysts, photocatalytic reaction mechanisms and kinetics, and photocatalytic reactor design. In addition, the use of photocatalysis in the control of flue-gas pollutants is discussed in detail. The book also describes how a photocatalytic reactor is designed and implemented to evaluate the photocatalytic oxidation capacity of different photocatalysts on elemental mercury in a simulated flue gas. After that, the effect of photocatalysts on the SO2, NOx and Hg removal in the flue gas is studied. Photocatalytic cleaning technology can be applied not only in gas pollutant cleaning at power plants, but also in wastewater purification. Readers gain a comprehensive understanding of possible mercury emission control methods and the industrial applications of these technologies.