Researchers from Soochow University in China and the University of Toronto have developed a new photocatalyst for the hydrogenation of CO₂ to methanol with 50% selectivity under simulated solar irradiation. A paper on their work appears in the journal Joule.

The solar methanol production of the defect-laden indium oxide, In₂O_3-x(OH)_y, with a rod-like nanocrystal superstructure, can be stabilized at a rate of 0.06 mmol g_cat^-1 h^-1 at atmospheric pressure. This is 120 times higher than that of the best-known photocatalysts. The research team suggests that this discovery bodes well for the development of a low-pressure solar methanol process using CO₂ and renewable H₂ feedstocks.

The development of methanol synthesis from the hydrogenation of CO₂ is important for achieving a greener chemical industry. Methanol is consumed at a scale of 65 million tons/year (2013) and is considered to be a future energy carrier due to its high volume-specific energy density. It is also a convenient medium for the safe storage of H₂ and an important renewable feedstock for the chemical industry, as around 30% of known chemicals can be derived from methanol. Furthermore, the desire of the transportation industry to replace diesel by dimethyl ether or its higher homolog oxy-methylene ether (OME-1), as a cleaner burning fuel has re-focused interest on its production from the catalytic dehydration of methanol made from CO₂.

… It remains a big challenge to achieve effective methanol production from CO₂ hydrogenation with a reasonably high rate and selectivity at atmospheric pressure. It would be of considerable scientific and technological interest if this reaction could be photocatalytic because, together with renewable solar energy, methanol can be produced without generating greenhouse gas emissions, which would make it an important feedstock for the chemical industry as well as an energy carrier for transportation and fuel cells.

… Here, we report that defect-laden indium oxide, In₂O_3-x(OH)_y, with a rod-like nano-crystal superstructure, can effectively catalyze the hydrogenation of CO₂ to methanol under illumination at atmospheric pressure at a champion rate of around 0.06 mmol g_cat^-1 h^-1 for long-term reaction, impressive 50% selectivity, and notable long-term operational stability. Success in this endeavor is enabled by recognition of the photocatalytic activity of surface frustrated Lewis pairs (SFLP), comprising Lewis acidic coordinately unsaturated In surface sites proximal to Lewis basic In hydroxide surface sites in non-stoichiometric metal oxides, exemplified by archetypal nano-crystal forms of In₂O_3-x(OH)_y. SFLP have been shown to increase their Lewis acidity and Lewis basicity respectively in the excited state compared with the ground state, thereby facilitating the photochemical RWGS reaction, with a decrease in activation energy by 20 kJ mol^-1 under illumination.

—Wang et al.

The researchers emphasized that the source of hydrogen remains key to the environmental viability of photocatalysis. They envision a process whereby the _H2 would be sourced via electrolysis driven by renewable electricity.

Resources

• Wang et al. (2018) “Photocatalytic Hydrogenation of Carbon Dioxide with High Selectivity to Methanol at Atmospheric Pressure,” Joule doi: 10.1016/j.joule.2018.03.007