A synthesis of cutting-edge research for converting CO₂ into renewable fuel

Converting CO₂ to chemical fuels—essentially reducing CO₂—is a maximizing course of action: it offers a plentiful renewable energy source and remediates global warming caused by CO₂ emissions. Researchers around the world are exploring photoreduction of CO₂, which utilizes abundant light in the conversion. In The Journal of Chemical Physics a group of theoretical chemists surveys photocatalytic CO₂ reduction studies and explicates key challenges.

Co-author and theoretical chemist Peijun Hu says that scientists have demonstrated reducing CO₂ to chemical fuels, but because of the high stability of CO₂ molecules, this conversion requires a lot of energy. Moreover, reducing CO₂ requires that it be adsorbed on the surface of the catalyst, and sometimes adsorption becomes the rate-determining step.

The adsorption and activation of CO₂ molecules are complex processes, as these involve many geometric possibilities and participation of photogenerated electrons. Due to the complexity, unproductive pathways leading to non-fuel products can easily ensue.

The solvent is also critical, and lead author and chemical theorist Chao Peng points out that the solvent effect can change adsorption structures of CO₂ and reaction energies. Moreover, in aqueous solution, photoreduction of protons to hydrogen competes with CO₂ reduction, adding to the challenge, Hu says.

Of course, photocatalysts, which absorb light, are central for CO₂ photoreduction. Research has made strides on photocatalysts, including on the modification of photocatalysts to improve the charge separation, the light absorption efficiency and the reactions. In one focus, scientists have described the effects of oxygen vacancies on the adsorptive TiO₂ surface. These vacancies can, in fact, enhance the reduction.

Fundamental questions remain, but progress is occurring for CO₂ fuel conversion. Hu adds that collaboration between experimentalists and theorists will be very helpful for this effort.

Source: “Perspective: Photocatalytic reduction of CO₂ to solar fuels over semiconductors,” by Chao Peng, Glenn Reid, Haifeng Wang, and Peijun Hu, The Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4985624 .