The photocatalytic conversion of CO 2 occurs through a series of processes in which electrons are transferred. This can result in various products, including carbon monoxide (CO), methanol (CH 3 OH), methane (CH 4 ), and other hydrocarbons. Eight electrons must be transferred on the way from CO 2 to CH 4 — more than for other C 1 products. Methane is the thermodynamically favorable end product, but the competing reaction to form CO only requires two electrons and is much faster, so it is kinetically favored. Effective and selective methanization is thus particularly challenging.
A team led by Hefeng Cheng (Shandong University, Jinan, China) and co-workers has now developed a practical approach to efficiently convert CO 2 to CH 4 using solar energy. The key to their success is a novel catalyst with single gold atoms. Because gold atoms aggregate in conventional preparative methods, the team developed a new strategy that uses a complex exchange to produce the catalyst.
Because of their unique electronic structures, single-atom catalysts behave differently from conventional metal nanoparticles. Also, when fixed to a suitable support, nearly all single atoms are available as active catalytic centers. In this new catalyst, single gold atoms are anchored to an ultrathin zinc–indium sulfide nanolayer and are each coordinated to only two sulfur atoms. Under sunlight, the catalyst was demonstrated to be very active with a CH 4 selectivity of 77 %.
A photosensitizer (a ruthenium complex) absorbs light, becomes excited, and accepts an electron that is made available by an electron donor (triethanolamine). It then passes the electron on to the catalyst. The single gold atoms on the surface of the support act as “electron pumps”. They capture the electrons significantly more effectively than gold nanoparticles and transfer them to CO 2 molecules and intermediates.
Detailed characterization and computations reveal that the catalyst activates the CO 2 molecules to a much greater degree than gold nanoparticles, more strongly adsorbs the excited *CO intermediates, lowers the energy barrier for binding hydrogen ions, and stabilizes the *CH 3 intermediate. This allows CH 4 to be the favored product and minimizes the release of CO.
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Dr. Hefeng Cheng is a Professor at the State Key Laboratory of Crystal Materials of Shandong University. His main research focus is defect chemistry engineering in semiconductor photocatalysis for solar energy harvesting and conversion, including water splitting, CO 2 reduction and so on.
Angewandte Chemie International Edition
Experimental study
Not applicable
Low-Coordination Single Au Atoms on Ultrathin ZnIn2S4 Nanosheets for Selective Photocatalytic CO2 Reduction towards CH4
21-Aug-2022
The authors declare no conflict of interest.