Preparation Of Atomically Dispersed Pd–Based Catalysts By Nitric Acid Vapor Process And Catalytic Performance

In recent years,the application of nanotechnology in the field of catalysis has become more and more extensive,especially the preparation of supported metal nanoparticles catalysts,which has been favored by many researchers.However,as the catalytic reaction occurs on the surface of the metal nanoparticle catalyst,usually only the metal atoms on the surface participate in the reaction,while the metal atoms in the bulk phase do not play a role,which greatly reduces the utilization rate of the metal atoms.Therefore,in order to improve the utilization of metals,the emergence and research of atomically dispersed single-atom catalysts in recent years have brought new opportunities and challenges for the development of nano-metal catalysts.The concept of single atomic catalyst(SACs)is simply that the catalyst prepared by uniformly dispersing the supporting metals on the surface of the support at the atomic level.In order to suppress the agglomeration of metal atoms at high temperature and other conditions,the atomic-dispersed metal atoms can be anchored by the functional groups and defects on the surface of the support.Single atom can significantly improve the utilization rate of metal atoms,increase the specific surface area,reduce the cost of catalyst preparation,and realize the rational utilization of metal resources.However,with the decreasing particle size of the supported metal,its surface free energy also increases and its kinetic stability decreases.Therefore,in the process of catalytic preparation and catalytic reaction,the single-atom catalyst is very easy to aggregate into particles or larger clusters.Therefore,the efficient preparation of single-atom catalysts still faces challenges.In this paper,a new synthesis method is proposed based on the current"top down"preparation strategy of single-atom catalyst.Firstly,use simple deposition precipitation method preparation of Pd nanoparticles catalyst particles,then using nitric acid steam for Pd nanoparticles particles dispersed again to obtain single atom Pd catalyst and study for benzene acetylene selective hydrogenation reaction and Suzuki coupling reaction to explore the catalytic performance of single-atom catalyst prepared by the nitric acid vapor etching method.The main research includes the following aspects:(1)In this paper,a graphene-supported Pd nanoparticle composite(Pd_n/G)was prepared by loading Pd nanoparticle particles on the surface of graphene using the traditional deposition and precipitation method with graphene as the support.The support of the catalyst was graphene G,and the metal precursor was palladium nitrate(Pd(NO₃)₂).The Pd_n/G catalyst was treated with nitric acid vapor,and the Pd nanoparticles were redispersed on the surface of the graphene support.The atomically dispersed single-atom Pd catalyst(Pd₁/G)supported on graphene was obtained.(2)The catalytic activity and stability of graphene supported palladium single-atom catalyst for selective hydrogenation of phenylacetylene were studied in this paper.The effects of Pd_n/G and Pd₁/G prepared by deposition and precipitation on the catalytic activity and stability of selective hydrogenation of phenylacetylene were investigated.The results showed that the selective hydrogenation activity of Pd₁/G was much higher than that of Pd_n/G under the same reaction conditions.Attributed to nitric acid steam etching,the Pd nanoparticles on graphene particles dispersed again,and metal particle size significantly decreases,at the same time the dispersion of the metal improves.The Pd particles disperses on the graphene highly and uniformly,thus raising the the activity and selectivity and stability of the catalyst of phenyl acetylene selective hydrogenation reaction,(3)The catalytic performance of Pd₁/G catalyst in Suzuki coupling reaction was also studied in this paper.In our experiment,the best reaction conditions were explored,and the differences of catalytic performance of Pd_n/G and Pd₁/G catalysts were compared under the same reaction conditions.It was found that the catalytic performance of Pd_n/G and Pd₁/G was the best when the volume ratio of alcohol to water was 5:3 and the mass of catalyst was 15 mg.In addition,in the Suzuki coupling reaction,the catalytic activities of Pd_n/G and Pd₁/G gradually increased with the reaction time increasing,and finally reaches the optimum performance at this temperature.