Surface Modification And Restruction Of Noble Metal Nanoparticles And Its Application In Electrocatalysis

In recent several decades,the rapid development of nanotechnology has promoted its wide application in energy,catalysis,and environment.As a new type of catalytic material,metal nanocatalysts have attracted more and more attention due to their unique size and crystal effects.In the catalytic system,the surface structure of the nanometal determines the activity,selectivity and stability of its catalytic reaction.The traditional colloid synthesis method can easily control the surface morphology and crystal orientation of the nanocrystals by selecting different ligands,but the presence of the ligand also has a great influence on the catalytic reaction.It has been widely accept that the effect of ligands on the surface of nanoparticles on the catalytic reaction can be divided into electronic effects and steric effects.However,it is very difficult to distinguish these two effects.If we can find a way to distinguish these two effects and accordingly regulate them,we can use the rich effect to control the catalytic activity.Therefore,For this reason,the research content of this thesis will follow the following several directions:In chapter 2,we proposed two electrochemical strategies:"oxidizing Pt" and "hydrogen evolution",which successfully removed the ligands on the surface of Pt nanoparticles.To the oxidizing Pt method,up potential>1.3 V and cycling number(n)>20 are preferred to remove the ligands.The water-soluble ligands(such as PVP,CTAB,NaOAc)can be removed by just one cycle after thoroughly washed by water to remove the unattached ligands.However,the oil-soluble ligands(such as OAm,TPP,thiol)need more cycles(n>20),which maybe due to the strong coordination between these ligands with Pt NPs.To the hydrogen break method,the generated hydrogen during hydrogen evolution reaction(HER,potential<-0.1 V)can break the interaction between the ligands and Pt NPs,resulting in the cleaned surface of Pt.The reverse adsorption of ligands and their removal demonstrate the reliability of these two methods.Moreover,these methods can be extended to remove the ligands from other noble metals,such as Pd and Au NPs.The successful removal of ligands is so important in getting the reliable data in many areas which are not limited to electrocatalysis,electrochemical sensing,in-situ techniques including SERS,FTIR.In Chapter 3,we systematically studied the effect of ligand coverage on the surface of Au nanoparticles on catalytic reactivity.We used two methods:potential cycling and annealing to carefully remove ligands on the surface of Au nanoparticles and successfully regulate the coverage of ligands on the surface of Au nanoparticles.Potential cycling enables a mild removal of ligands without changing the size of AuNPs and thus provides ideal models to identify the intrinsic influence of surface coverage of ligands on oxygen reduction reaction(ORR).The ORR activity and selectivity is quantitatively evaluated by plotting half potential and transferred electron number versus the surface coverage of ligand.Thermal annealing at different temperatures produces different sized AuNPs with various surface ligand coverage.Compared to the size effect,we find that the surface coverage of ligands plays a dominant role on ORR activity of AuNPs.Most importantly,capping AuNPs with oleylamine and sodium citrate and the reverse poisoning clean AuNPs with thiol,butylamine and CTAB further reveal the difference of ORR activity/selectivity of the capped AuNPs is directly related to the surface coverage of the ligands regardless the diverse chemical nature of these ligands.This work highlights that the surface coverage of ligands should be considered to be an important factor accounting for metal-organic interfaces to catalytic reactions.In chapter 4,we successfully separate the electronic and steric effects of ligands by adjusting the coverage of pyridine on the Pt surface.X-ray photoelectron spectroscopy,in situ CO infrared spectroscopy,and CO electrochemical stripping experiments demonstrated that the electron density of the Pt surface increases with increasing pyridine coverage.We found that as the pyridine coverage increases,the specific activity of the oxygen reduction reaction(ORR)gradually increases,while the mass activity of ORR increases first and then decreases.By using density functional theory(DFT)calculation,we found that the increase of ORR specific activity is positively correlated with the electron density at the Pt surface,and the change of ORR mass activity is the combined result of the electron density on the Pt surface and the steric effect of pyridine.On the contrary,with the increase of pyridine coverage,the specific activity and mass activity of the methanol oxidation reaction(MOR)gradually decreased.Combining with the theoretical and experimental results,we found that both of the increase of electron density and the increase of steric hindrance are not conducive to methanol oxidation.Utilizing the electronic and steric effects of pyridine,a bifunctional methanol fuel cell cathode catalyst can be obtained.We used other ligands to further modify the Pt nanoparticles.The experimental results confirmed the above conclusion,that is,by changing the coverage of the ligand,we can successfully separate the electronic and steric effects of the ligands.In chapter 5,we prepared a highly open dendritic PtCu alloy nanoparticle by using hexadecylamine as a ligand.The ligand on the nanoparticle surface can be removed by electrochemical strategy.The composition of the PtCu nanodendrites can be easily tuned by changing the molar ratio of the precursors.The PtCu nanodendrites exhibit efficient catalytic activity toward the methanol oxidation reaction(MOR).Particularly,the Pt1Cu1 nanodendrites exert 4.6×increase in the specific activity and 3.8× increase in the mass activity compared to the commercial Pt/C catalyst.The mechanism of the enhancement was comprehensively studied.The enhanced catalytic activities can be ascribed to the high index surface of the branched structure and the electronic effect between the alloy metals.Specifically,the addition of Cu downshifts the binding energy of Pt,increasing the CO-tolerance ability of PtCu nanodendrites and,hence,improves their MOR activities.Moreover,the PtCu nanodendrites display better stability and durability for MOR compared to Pt/C.The approach can be adapted to synthesize desired Pt-based nanodendrites for various catalytic reactions.