Metal-Organic Frameworks Towards Photo-and Electro Catalytic Carbon Dioxide Reduction

The fastly growing environmental concerns and energy demand are considered to be the factors that restrict the development of society.An ideal method to address these questions is to convert carbon dioxide into fuels by sunlight.Because CO2 is a stable sinus gas,a large amount of energy is required for carbon dioxide reduction.In addition,the selectivity for main products is low.Therefore,the development of efficient electrocatalysts and the construction of efficient systems are crucial for the realization of carbon dioxide reduction and solar energy storage.Metal-organic framework(MOF)thin film(Re-MOF/FTO)was deposited onto FTO electrode by liquid-phase epitaxy(LPE)method.The Re-MOF/FTO(active area is 1 cm2)showed a Faraday efficiency(FE)of 93.0%for reduction of CO2 to CO at a potential of-1.6 V vs.NHE in the organic phase system.During the 2 h of eletrocatalytic reaction in this system,40.50 ?mol carbon monoxide was produced.The FE is higher than that of non-MOF system and homogeneous system.Electrochemical study and DFT calculation(This part of the work is by Thomas Heine)indicated that the charge transport is along the[001]direction via a redox hopping mechanism,which is beneficial for efficient CO2 reduction.To construct a highly efficient and selective system for electrochemical reduction of CO2,a 2D MOF nanosheets Zn-TCPP(Co)-MOF was developed and then deposited on carbon paper(CP)to prepare the working electrode[Zn-TCPP(Co)-MOF/CP](active area is 1 cm2).The Zn-TCPP(Co)-MOF/CP showed a stable current density of-8.4 mA/cm-2 and FE of 86.2%for reduction of CO2 to CO at a potential of-0.7 V vs.RHE in a aqueous system.During the 5 h of eletrocatalytic reaction in this system,661.00 ?mol carbon monoxide was produced.The overall system was constructed for the eletrocatalytic reduction of CO2 coupled to the eletrocatalytic oxidation of water,with Zn-TCPP(Co)-MOF/CP as a bifunctional electrode.The overall system showed a stable current density of-8.1 mA/cm-2 for 3 h and FE of 83.4%for reduction of CO2 to CO at a potential of 2.7 V,generating 379.20 ?mol of carbon monoxide,72.30 ?mol of hydrogen and 217.53 ?mol of oxygen.The Zn-TCPP(Co)-MOF as catalyst also showed photocatalytic CO2 reduction activity under the three-component system.Additionally,the surfactant-assisted method was used to prepare the uniformed 2D MOF nanosheets Zn-MOF-n.Two efficient systems were constructed for the photocatalytic reduction of CO2,with Zn-MOF-n as photosensitizer,and the CO2L or ZIF-67 as catalyst.During the 6 h of photocatalytic reaction in these systems,0.69 ?mol and 21.20 ?mol carbon monoxide was produced,selectivity for CO were 81.5%and 91.0%.These systems showed higher photocatalytic activity and selectivity owing to the better charge transport ability,and the more efficient separation and longer lifetime of photogenerated charge carriers with the Zn-MOF-n.A three-component system was constructed for the photocatalytic reduction of CO2,with CoSn(OH)6 as catalyst,and the Ru(bpy)3(PF6)2 as photosensitizer.During the 90 min of photocatalytic reaction in this system,19.30 ?mol carbon monoxide was produced.The photocatalytic activity and carbon monoxide selectivity were further improved by adding weak Bronsted acids,as proton sources,to the system.The system shows a higher stability when using Zn-MOF-n as photosensitizer.