Preparation Of Pd-based Bimetallic Catalysts Via One-pot Method And Their Electrocatalytic Properties Toward Oxidation Of C2 Alcohols

In the 21st century,energy shortage and environment pollution are still the two most important problems to be solved.Fossil energy is not renewable and will eventually be exhausted with the continuous exploitation by human beings.Therefore,the development and application of new energy is urgent!Fuel Cell is a new type of energy conversion device that can efficiently and environmentally-friendly convert chemical energy stored in fuel directly into electricity without burning it.In present,the commercial fuel cells under development are mainly proton exchange membrane fuel cells（such as methanol proton exchange membrane fuel cells）,but their cost,working performance,energy conversion efficiency still need to be further improved and perfected.In recent years,with the rapid development of anion exchange membrane,alkaline direct alcohol fuel cell（ADAFC）,as an important member of anion exchange membrane fuel cell,has been favored by researchers.The results show that some precious metal and transition metal catalysts exhibit better electrocatalytic activity and higher anti-poisoning ability toward alcohol oxidation in alkaline system than that in acidic system.Based on the above,the development and research of anode catalyst and carrier for alkaline direct alcohol fuel cell is the focus and hotspot in the field of fuel cells.As the anode catalyst of alkaline direct alcohol fuel cell,Pd is the best alternative to Pt in terms of cost and performance.In order to further improve the electrocatalytic performance of Pd-based catalysts,increasing the inherent activity of the original catalytic active site and increasing the number of catalytic active sites are the two most commonly used and effective strategies.（1）Specific methods to improve the inherent activity of the original catalytic active site:generating Pd-based alloys,introducing doping elements,introducing other components to produce interface effects.With the above methods,the electronic structure of Pd nanoparticles can be regulated or recombined,and the electrocatalytic performance of Pd-based catalysts can be improved.（2）Specific methods to increase the number of catalytic active sites:synthesizing low-dimensional Pd-based catalysts with specific nanostructures or selecting catalyst carriers with high specific surface area,high conductivity,high stability and porous structure.Suitable catalyst carrier can not only improve the dispersion and stability of Pd nanoparticles,but also produce synergistic effect with metal nanoparticles,change the electronic structure of metal nanoparticles,optimize the adsorption performance of reaction intermediates on its surface,and then improve the electrocatalytic properties of the Pd-based catalyst.Based on the above research background,five kinds of Pd-based bimetallic nanocatalysts（Pd Au/MXene,PdCu/N-MXene,PdCu/PEDOT,Pd Ni/PEDOT,Pd Sn/PEDOT）were synthesized by one-pot method in this thesis.The electrocatalytic properties（electrocatalytic activity,anti-poisoning ability,durability,etc.）of the above prepared catalysts towardC2-alcohol（ethanol and ethylene glycol）oxidation in alkaline system were studied.The specific research contents and results are as follows:（1）With MXene（Ti₃C₂T_x）as the carrier,PdCl₄^2-and Au Cl₄^-were simultaneously reduced and deposited on the surface of MXene by one-pot method;The Pd_0.5Au_1.5/MXene catalyst with the best catalytic performance was optimized by changing the experimental conditions.Then,the electrocatalytic oxidation of ethanol,ethylene glycol and methanol were systematically studied with Pd_0.5Au_1.5/MXene catalyst.The experimental results show that Pd Au alloy is formed and its morphology is about 88 nm nanospheres,and the Pd Au alloy nanoparticles were uniformly composited with MXene.The obtained Pd_0.5Au_1.5/MXene catalyst showed better electrocatalytic properties and anti-poisoning ability toward methanol,ethanol and ethylene glycol oxidation in alkaline medium than commercial Pd/C catalyst.The reasons were analyzed as follows:The formation of Pd Au alloy changes the electronic structure of Pd nanoparticles,optimizes the adsorption energy of the reaction intermediates,and improves the electrocatalytic performance of the catalyst.In addition,the interaction between Pd_0.5Au_1.5nanoparticles and the electronegative functional groups on the surface of MXene produces strong synergistic effect,which is conducive to the improvement of catalytic properties of the catalyst.（2）Firstly,nitrogen doped MXene（N-Ti₃C₂T_x）was prepared by hydrothermal method.Then,the Pd_xCu_y/N-Ti₃C₂T_xcatalyst was prepared by in-situ co-reduction of PdCl₄^2-andCu²⁺adsorbed on the surface of N-Ti₃C₂T_xvia one-pot method.By adjusting the experimental conditions,the optimized catalyst Pd₁Cu₁/N-Ti₃C₂T_xwas obtained,and its electrocatalytic properties toward ethanol oxidation in alkaline medium was systematically studied.The experimental results show that Pd₁Cu₁alloy nanoparticles are uniformly deposited on the surface of N-Ti₃C₂T_xnanosheets,and the doping of N atoms increases the layer spacing of MXene nanosheets,providing a large number of attachment sites for the deposition of Pd₁Cu₁nanoparticles,thus improving the effective active area and stability of Pd₁Cu₁nanoparticles.Compared with commercial Pd/C and Pd/N-Ti₃C₂T_x,the obtained Pd₁Cu₁/N-Ti₃C₂T_xcatalysts showed better electrocatalytic performance for the oxidation of ethanol in alkaline system.The reasons are as follows:the formation of PdCu alloy reorganizes and optimizes the electronic structure of Pd,and then endow Pd with higher electrocatalytic activity.The high specific surface area of N-Ti₃C₂T_ximproves the dispersion and active area of Pd₁Cu₁nanoparticles,and further enhances the catalytic reaction efficiency of ethanol.In addition,the synergistic effect between N-Ti₃C₂T_xand Pd₁Cu₁nanoparticles was enhanced by the abundant functional groups on the surface of N-Ti₃C₂T_x,which further enhanced the electrocatalytic properties of the catalyst.（3）By one-pot method,the Pd/PEDOT composite was prepared by mixing excess PdCl₄^2-with EDOT through synchronous reduction-oxidation polymerization processes.Then Cu²⁺/Ni²⁺and Na BH₄were added successively to prepare Pd_xCu_y/PEDOT and Pd_xNi_y/PEDOT catalysts.After a series of optimization experiments,the best samples Pd₁Cu₃/PEDOT and Pd₇Ni₃/PEDOT were selected,and the electrocatalytic properties of Pd₁Cu_3/PEDOT and Pd₇Ni₃/PEDOT in alkaline system were systematically studied.The experimental results show that PEDOT is amorphous granule,fluffy and porous,and shows good dispersibility.Pd₁Cu₃and Pd₇Ni₃nanoparticles are uniformly dispersed on the surface of PEDOT particles or wrapped inside.Both Pd₁Cu₃/PEDOT and Pd₇Ni₃/PEDOT showed better electrocatalytic performance than commercial Pd/C and Pd/PEDOT catalysts toward the oxidation of ethylene glycol in alkaline system.The reason is that Cu/Ni doping changes the electronic structure of Pd and enhances its catalytic activity.Amorphous PEDOT particles provide a large number of adhesion sites for Pd₁Cu₃and Pd₇Ni₃nanoparticles,which improves the dispersibility and effective active area of the metal particles.The interaction between electronegative O and S atoms in PEDOT and Pd-based nanoparticles can optimize the electronic structure of Pd particles,and further improve the electrocatalytic activity and anti-poisoning ability of Pd-based catalysts.（4）by one-pot method,the Pd/PEDOT composite was prepared by mixing excess PdCl₄^2-with EDOT via synchronous reduction-oxidation polymerization.Then Sn²⁺and Na BH₄were added to prepare Pd_xSn_y/PEDOT catalyst.The best catalyst sample of Pd₄Sn₆/PEDOT was optimized by changing the experimental conditions,and the electrocatalytic properties of Pd₄Sn₆/PEDOT towardC2 alcohols in alkaline medium were systematically studied.The experimental results show that Pd₄Sn₆nanoparticles are uniformly dispersed on the surface or inside of PEDOT particles,and Pd₄Sn₆exists in alloy state.For the oxidation of C2 alcohols in alkaline medium,Pd₄Sn₆/PEDOT showed better electrocatalytic performance than commercial Pd/C,Pd/PEDOT and Pd₄Sn₆catalysts.The reasons are explained as follows:the strong interaction between Pd₄Sn₆alloy,PEDOT and metal nanoparticles optimize the electronic structure of Pd,and then enhances the electrocatalytic activity of Pd-based catalyst.The amorphous and porous structure of PEDOT particles not only improve the dispersibility and effective active area of Pd₄Sn₆nanoparticles,but also provide a large number of diffusion channels for the reaction intermediates,improving the mass transfer efficiency and thus enhancing the catalytic oxidation efficiency of alcohols.In summary,the introduction of metals Au,Cu,Ni and Sn can significantly enhance the electrocatalytic activity of Pd catalyst,improve its mass activity,reduce the amount of Pd,and thus reduce the cost of catalyst.The carrier MXene or PEDOT can not only provide a large number of attachment sites for Pd-based nanoparticles to improve their dispersion,but also interact with metal nanoparticles to change the electronic structure of metal particles,thereby optimizing their surface adsorption energy and improving the catalytic performance of Pd-based catalysts.Toward the oxidation of C2 alcohols in alkaline system,the electrocatalytic properties of the above catalysts are ranked as follows:Pd₄Sn₆/PEDOT>Pd₇Ni₃/PEDOT>Pd₁Cu₃/PEDOT>Pd₁Cu₁/N-Ti₃C₂T_x>Pd_0.5Au_1.5/MXene.It can be concluded that PEDOT is more suitable than MXene Ti₃C₂T_xas the support for Pd-based bimetallic catalysts toward the electrocatalytic oxidation of C2 alcohols in alkaline medium.