Semiconducting oxide photocatalysts for reduction of CO2 to methanol

The explosive growth in anthropogenic energy consumption, coupled with the consequent environmental pollution, have been acknowledged as two impending challenges confronting humanity. Photocatalytic CO2 reduction to produce value-added hydrocarbon fuels, by using abundant solar energy and redundant atmospheric CO2, is an innovative way to satisfy global energy requirements whilst simultaneously reducing atmospheric CO2 levels. Although this notion is several decades old, it has unfortunately been lingering in a state of infancy due to inherently poor CO2-to-fuel conversion efficiencies, and the generation of low-value products (e.g., CO, HCHO). These pitfalls hamper this process from any potential commercial breakthrough and are primarily fuelled by the lack of progress in developing high-performance photocatalytic materials. Fortunately, the advent of nanotechnology has recently introduced many promising novel materials for this purpose. Here, we review photocatalysts with proven potential for converting CO2 into methanol, a high-value, energy-dense hydrocarbon fuel that is easily transported using existing pipeline infrastructure. Methanol possesses multifarious applications in the automobile, industrial and petrochemical sector. In addition, the development of direct methanol fuel cells (DMFCs) has introduced the tantalizing prospect of using methanol as a medium for storing solar energy that is easily converted to electricity via DMFCs. As such, methanol is an ideal fuel, with numerous advantages over its counterparts. This article reviews several photocatalysts that have been reported for this environmentally sustainable process of converting CO2 into methanol by photocatalysis. Specifically, the performance enhancement effected by adding dopant atoms, forming heterostructured composites and nanostructures, is investigated in terms of four key areas: (1) enhanced visible light sensitivity, (2) improved adsorption of reactants on the catalytic surface, (3) lowered electron-hole recombination and (4) increased CO2 reduction kinetics. The trends deduced therein are invaluable for researchers developing novel photocatalytic materials, which will utilize sunlight to convert CO2 into methanol with enhanced efficiency, thus ushering in the era of a green methanol-based economy.