Importance Of Au Nanostructures In CO₂ Electroreduction

Efficient and selective conversion of CO2 into fuels is a promoting way to solve greenhouse effect and recycle chemical energy as well.Electrochemical conversion of CO2 into fuels under ambient temperature and pressure is a promising approach,however,is limited by the high overpotential,slow kinetics and the accompanied vice reaction of hydrogen evolution,which result in low conversion rate and poor selectivity.Using proper metal nanocatalysts can effectively improve the performance of CO2 electrochemical reduction.As reported,Au nanocatalysts exhibit high activity and good selectivity for the electrochemical reduction of CO2 to CO.Nanostructures of the gold nanocatalysts determine the adsorption and desorption of reactants,intermediates and products,which strongly affects the CO2 conversion.Therefore,in this thesis,Au nanocatalysts with different morphologies and sizes have been prepared for exploring the structure effects on electrochemical reduction of CO2,including:1.Effect of Au nanocatalysts structure on CO2 electrochemical reduction reactionGold colloids and trioctahedron with 50 nm diameter were synthesized and utilized for CO2 electrochemical reduction.Electrochemical studies show that both the nanocatalysts exhibit conversion of CO2 into CO with high efficiency and selectivity.The maximum Faradaic conversion efficiency for CO formation on the Au trioctahedron reaches 72.42%(-0.5 V),which is 1.7 times as high as that of the Au colloids(43.32%at-0.7 V).The XRD and HRTEM characterizations show that the ratio of high-index facets especially(221)facets on Au trioctahedron is much higher than the Au standard card,while the ratio of low-index facets especially(111)facets on Au colloids is higher than the Au standard card.Density functional theory calculations show that the(221)facet has a lower potential barrier than(111)facet for CO2 conversion,which is benefiting for the formation and stabilization of COOH*intermediates.Therefore,the(221)facet shows higher catalytic activity toward CO2 electrochemical reduction reaction because of the easier binding reaction and higher binding energy of CO2 and COOH*on(221)facet as compared to the ones on(111)facet.2.Effect of Au nanocatalysts size on CO2 electrochemical reduction reactionGold trioctahedra with sizes of 50nm,75 nm,and 100 nm were synthesized in order to systematically study the size effect on CO2 electrochemical reduction reaction.As revealed by the electrochemical results,high efficient and selective conversion of CO2 to CO can be achieved on the above gold nanocatalysts.It is found that with the increase of the gold nanocatalysts size,the ratio of edge sites to facet sites on(221)facet decreases,and the Faradaic efficiency for CO decreases from 72.42%,60.73%to 47.43%,respectively;while the overpotential increases from 390 mV,590 mV to 690 mV.The IR result shows that with the size decrease of the Au trioctahedron,the vibration frequency of the surface adsorbed CO shows red-shift,indicating the stronger CO binding energy on smaller catalysts.Density functional theory calculations show that the edge sites of(221)facet have the better electrocatalytic activity toward CO2 electrochemical reduction than the facet sites owning to the lower coordination environment of the edge atoms with more dangling bonds around.Thus,the electrocatalytic activity of the gold trioctahedra is determined by both the number of surface active sites and the intrinsic activity of each reaction sites.The gold nanocatalysts with smaller size have a higher ratio of edge sites to plane sites,and thus,higher catalytic performance for CO2 electrochemical conversion.In summary,controlling the morphology of the gold nanocatalysts is an effective means to regulate the properties of the active sites,which is beneficial for increasing the selectivity and reaction rate of the gold nanocatalysts.In addition,controlling the gold nanocatalysts size can regulate the ratio of active sites,which further improves electrocatalytic performance.Therefore,precise design of the gold nanocatalysts structure is a promising means to control the surface properties of gold nanocatalysts with high conversion efficiency and selectivity for CO2 electrochemical reduction reaction.