Structural Design Of Pd-Based Electrocatalysts For CO₂ Reduction

Electrochemical CO₂ reduction is a promising pathway to reduce CO₂concentration in the atmosphere,and simultaneously convert it into value-added chemicals.Efficient electrocatalysts are highly desired in this field.Pd-based electrocatalysts have been widely investigated,but they still need further improvement,regarding the high cost,easy CO poisoning,unsatisfied catalytic activity and stability,and debatable reaction mechanism.In this dissertation,the design of Pd-based electrocatalysts and corresponding catalytic performance of CO₂reduction were investigated.Strategies were employed to optimize its electronic structure and adsorption configuration by designing Pd-based bimetallic catalysts,regulating alloy composition,constructing the Pd@Ag core-shell structure,and modifying Pd nanocrystalline surfaces.Furthermore,thermodynamic and kinetic processes of CO₂ reduction reaction on Pd-based catalysts were investigated.The main content and results of this paper are briefly described as follows:1.Pd_1-xAg_x alloy was constructed to study the electronic effect on electrocatalytic CO₂reduction.According to the experimental results and DFT calculations,the change of Pd/Ag ratio in Pd_1-xAg_x significantly altered the electronic structure,which further affected the catalytic selectivity.Among Pd_1-xAg_x/C samples,Pd_0.75Ag_0.25/C showed95.3%Faraday efficiency of CO at-0.6 V?vs.RHE?,and achieved excellent stability during 20 hours.Through kinetic analysis,it was discovered that varied Pd/Ag ratio in Pd_1-xAg_x/C was corresponding to the transformation of rate-determining steps during CO₂ reduction,which was further confirmed by DFT calculation.Moreover,the DFT calculation revealed that Pd_0.75Ag_0.25 could effectively reduce the adsorption strength of*CO,but adsorption strength of*COOH was nearly unaffected.Therefore,the inherent scaling relationship of the reaction intermediates could be broken,resulting in excellent catalytic performance on Pd_0.75Ag_0.25/C.2.Pd octahedron nanocrystals were used as seed crystals,and Ag was grown by non-classical epitaxial growth mode to form cubic Pd@Ag core-shell nanostructures.The growth pathway of the nanocrystals was also investigated.The results indicated that the introduced appropriate amount of surfactants could effectively overcome the obstacles of lattice mismatch between Pd and Ag.Therefore,Ag atoms could grow on Pd octahedral nanocrystals and evolve into a cubic core-shell structure exposing Ag?100?facet.Pd@Ag nanocrystals exhibited better catalytic activity and selectivity than Pd nanocrystals and Ag/C in the electrocatalytic CO₂ reduction,which could be attributed to crystal facet effect,defects and strain effects,and electron transfer between the inner layer Pd and the outer layer Ag in Pd@Ag nanostructures.3.A facile electrochemistry method was used to modify Pd octahedron surface with Bi atoms,which significantly changed the catalytic selectivity of CO₂ reduction.Experimental characterization indicated that Bi atoms were distributed on Pd octahedron surfaces.Moreover,there were strong electronic interactions between Bi and Pd,indicating that Bi atoms combined with the outer Pd to form Pd-Bi nanostructure.In the process of catalyzing CO₂ reduction,Pd-Bi nanostructures,which had similar morphological structure with Pd octahedral nanocrystals,displayed obvious distinctions in catalytic activity and selectivity.DFT calculations revealed that Pd nanocrystal surfaces modified Bi atoms successfully weakened*CO adsorption.As a result,Pd-Bi nanostructures exhibited higher catalytic activity toward CO₂ reduction.Especially,the introduction of Bi atoms could effectively reduce formation energy of the HCOO*intermediates,which facilitated the conversion of CO₂ to HCOOH.In summary,this work developed efficient Pd-based electrocatalysts for CO₂reduction via electronic regulation on Pd-Ag alloys,Pd@Ag,and Bi modification on Pd octahedron nanocrystal surfaces.Meanwhile,DFT calculation gave insightful interpretation to the structure-activity relationship of the electrocatalyst.This work is expected to broaden the road for rational design of excellent catalysts toward efficient CO₂ reduction.