Rational electronic control of carbon dioxide reduction over cobalt oxide

The selective reduction of carbon dioxide (CO2) into fuels and chemicals is expected to play a key role in achieving a carbon–neutral sustainable energy economy. Activation of CO2 is the most important step in this process. As the electron transfer to CO2 is the rate-limiting step for CO2 activation, it is significant to modulate the electronic structure of CO2 reduction catalysts to enhance their activities. However, the intrinsic relationships between electronic properties of catalysts and their catalytic activities remains somewhat unclear, which limits the rational design of highly-efficient CO2 reduction catalysts. Herein, a catalyst-insulator–metal system consisting of Al as an electron-donor material was designed to modulate the electronic structure of cobalt oxide (Co3O4) catalyst. The electrons in Al can be efficiently tunneled to Co3O4 across an ultrathin self-formed Al2O3 insulating layer. Experimental and theoretical results undoubtedly confirm that high electron density on Co3O4 favours the adsorption and activation of CO2, while lowers the energy barrier for the formation of COOH* and especially lowers the desorption energy barrier of CO* intermediates, thereby significantly accelerating the reaction kinetics of CO2-to-CO photoreduction. The turnover frequency per Co atom of Co3O4/Al2O3-Al is much higher than that of Co3O4. The apparent quantum yield (AQY) of Co3O4/Al2O3-Al catalyst highly reaches 3.8% at 420 nm, which is superior to those of most reported catalysts. Besides, the increase in electron density on Co3O4 can effectively inhibit the competing hydrogen evolution reaction. The selectivity of CO increases from 57.9% for Co3O4 to 82.4% for Co3O4/Al2O3-Al. Significantly, the catalytic efficiency can be rationally tuned through controlling the content and particle size of Al. This work stablishes the links between the electronic structure of catalysts and their catalytic activities toward CO2 reduction reaction. Moreover, this Al2O3-Al structure reported here, may be used as an unprecedented platform for the electronic effect study of other heterogeneous catalysts.