ABSTRACT

Carbon dioxide (CO2) is the chief greenhouse gas that results from human activities, which causes global warming and climate change. Based on the atmospheric CO2 measurement data at Mauna Loa Observatory, Hawaii, the concentration of CO2 in the atmosphere is increasing each decade at an accelerating rate (ESRL 2014). The upper safety limit for atmospheric CO2 is 350 ppm; however, atmospheric CO2 levels have remained higher than 350 ppm since early 1988, now reaching levels of 399 ppm. There are three possible strategies for reducing atmospheric CO2:

• Reduction of the produced amount of CO2 from power generation stations and industrial sites

• Usage of CO2 as a feedstock of nearly zero cost for conversion to fuels and chemicals • Storage of it using carbon sequestration and storage (CCS) technologies (Centi and

Parathoner 2009; Mikkelsen et al. 2010; Richter et al. 2013; Hu et al. 2013)

The conversion of CO2 to fuels, chemicals, and materials has attracted much attention in recent years (Ma et al. 2009; Morris et al. 2009; Jensen et al. 2011; Windle and Perutz 2012; Liu et al. 2012; Inglis et al. 2012; Tran et al. 2012; Dhakshinamoorthy et al. 2012; Mori et al. 2012; Fan et al. 2013; Izumi 2013; Handoko et al. 2013; Tahir and Amin 2013; Stechel and Miller 2013; Mao et al. 2013; Kondratenko et al. 2013; Das and Daud 2014). However, CO2 is a kinetically and thermodynamically stable compound, which is difcult to oxidize or reduce to other fuels and/or chemicals at low temperatures. Reduction of CO2 needs high energy input, cofeeding of a high-energy reactant such as H2, and an excellent catalyst capable of driving its selective conversion to targeted chemicals. CO2 can be theoretically reduced to hydrocarbons and/or alcohols at high temperature and high

9.1 Introduction .......................................................................................................................... 173 9.2 Characteristics of CO2 Molecule .......................................................................................... 174 9.3 Mechanism of Photocatalytic Reduction of CO2 .................................................................. 174 9.4 Photocatalysts for Reduction of CO2 .................................................................................... 177

9.4.1 TiO2 and Related Titanium-Containing Solids......................................................... 177 9.4.2 Other Metal Oxide Photocatalysts ............................................................................ 179 9.4.3 Nonoxide Semiconductor Photocatalysts ................................................................. 179

9.5 Operation Conditions ............................................................................................................ 181 9.5.1 Pressure Effect .......................................................................................................... 181 9.5.2 Reductant Effect ....................................................................................................... 181 9.5.3 Temperature Effect ................................................................................................... 181 9.5.4 Reactor-Type Effect .................................................................................................. 182

9.6 Conclusions and Outlook ...................................................................................................... 182 References ...................................................................................................................................... 183

pressure. However, the process is quite complex, and a large amount of energy is essential. It would be attractive to capture CO2 from the atmosphere or the exhaust of power stations or factories, and convert it to fuel and chemicals by using photocatalysts and a sustainable source of energy such as sunlight. In combination with photocatalytic H2 production from water, the solar-driven CO2 reduction to fuels and chemicals could be a very promising process for utilization of CO2. Here, the mechanism of photocatalytic reduction of CO2, the catalytic activity of the developed catalysts for the CO2 reduction, and its advantages and challenges are introduced and discussed.