Local Symmetry-breaking Active Centers For CO₂ Hydrogenation

The ever-increasing utilization of fossil fuels leads to the rise of CO2 emission,resulting in severe environmental problems such as greenhouse effect.A pivotal route to reduce carbon emission is to elevate the ratio of renewable energy.Unfortunately,electricity generated by renewable energy such as wind and solar energy is intermittent,which is difficult to be merged into the electricity grid system.To this end,we propose an anthropogenic carbon loop for intertwined goals of economic growth,environmental conservation,and energy security.Specially,intermittent renewable energy is used to drive water electrolysis for H2 production.Over efficient catalysts,the produced H2 reacted with CO2 which derives from the exhausts of power,chemical,cement,and fermentation plants,resulting in the formation of liquid fuels.The use of liquid fuels in turn emits CO2.Compared with battery and high-pressure gases,liquid fuels such as methanol not only possess higher energy density,but also benefit from convenient storage and transportation.The activation of CO2 is difficult,because CO2 has stable C=O bonds with a bond length as short as 1.16 A.However,it is not proper to activate CO2 by directly increasing the reaction temperature.This is because CO2 hydrogenation into liquid fuels is an exothermic process where elevating the temperature decreases both the balanced conversion of CO2 and the balanced selectivity for fossil fuels.Therefore,the key scientific issue for CO2 hydrogenation lies in how to design catalysts which enable efficient activation of CO2.Herein,we propose a catalyst design principle of constructing symmetry-breaking active centers for activating non-polar CO2 molecules.From the perspective of electronic properties,there is prominent charge density gradient in a symmetry-breaking center,resulting in perturbing electronic structures of non-polar CO2 and polarizing the adsorbed species.From the perspective of adsorption configuration,a symmetry-breaking center gives a local torque which can facilely bend the linear CO2 molecule.Based on this design principle,we focused on the construction and mechanism of local symmetry-breaking centers,in terms of single-atom catalysts,for CO2 hydrogenation.(1)We prepared local symmetry-breaking centers of Pt-O-H,and revealed how the active centers influenced the reaction path and selectivity.We loaded Pt single atoms in MIL-101 to construct Pt-O ensemble.Such Pt-O ensemble catalyzed the dissociation of H2,resulting in the formation of Pt-O-H ensemble.Based on mechanistic studies,we found that H atoms in Pt-O-H hydrogenated CO2 to form HCOO*,followed by subsequent hydrogenation of HCOO*into formic acid and methanol.In comparison,H atoms in Pt-H added on CO2 to generate COOH*,followed by subsequent conversion of COOH*into CO and methane.(2)We loaded Pt single atoms on MoS2 to form isolated and neighboring Pt-S-H active centers,as well as revealed the synergy between neighboring symmetry-breaking centers.Based on mechanistic studies,we found that isolated active centers enabled direct conversion of CO2 into methanol without formic acid as the intermediate.In comparison,neighboring active centers worked in synergy to alter the reaction path of CO2 hydrogenation,where CO2 was hydrogenated into formic acid and methanol in sequence.