Chapter 9: Photocatalytic CO₂ Conversion to Fuels by Novel Green Photocatalytic Materials

In this chapter the necessity of solar energy storage is explained first, followed by a calculation of the photocatalytic rates necessary for practical application of artificial photosynthesis systems to achieve effective solar energy storage. Next, we introduce TiO₂ as an active CO₂ and H₂O conversion photocatalyst, including the current understanding of CO₂ sorption modes, and mechanistic details of the conversion of H₂O and CO₂ to hydrocarbons. Subsequently, various modifications of TiO₂ are described that enhance photocatalytic performance, with the focus on earth-abundant elements, including copper, iron, and nickel, in the form of metal or metal oxide nanoparticles and composites. Next, the creation of isolated Ti sites in mesoporous silica framework materials is discussed, followed by a description of the mechanism of CO₂ conversion to hydrocarbons in these materials. Several attempts to create visible light-sensitive MMCT binuclear sites are also addressed. For effective CO₂ reduction, water oxidation activity is essential. We will discuss some issues related to methodology to determine water oxidation efficacy, and discuss the mechanism of activity of IrO_x and CoO_x sites when present in mesoporous silica-based materials. Some attention is also paid to the prevention of the reverse reaction (hydrocarbons and oxygen to form CO₂ and water), which would significantly decrease conversion efficiency. We conclude the overview of TiO₂-based systems by discussing composites with graphene and metal organic framework materials. Finally, the currently achieved activity data are reflected upon, and implications of for avenues of future research activity are identified.