Synthesis Of Palladium And Silver Nanoclusters With Atomic Precision And Their Catalytic Properties

Metal nanoparticle catalysts are an important type of heterogeneous catalysts,which are widely used in catalytic oxidation,hydrogenation and other reactions However,nanoparticle catalysts have many disadvantages,such as large particle size and uneven distribution;quantum size effect is weakened and easily deactivated;the composition and structure are complex,and the active site is difficult to determine The uniform size of metal nanoclusters and the precise atomic structure are conducive to in-depth study of the relationship between structure and properties.On the other hand,compared with single-component clusters,the physicochemical properties of bimetallic clusters will change greatly.This is because the introduction of exogenous atoms changes the electronic structure of single-component clusters.The significant synergistic effect formed between the two metal atoms makes bimetallic clusters have great potential for catalysis.The electron transfer between different metal atoms in bimetallic clusters and the strong metal-support interaction(SMSI)can significantly regulate the catalytic properties and provide new ideas for optimizing the catalyst design.In the work of this paper,PdCl2 was selected as the precursor to prepare Pd3Cl(PPh2)2(PPh3)3(abbreviated as Pd3)clusters with three palladium atoms arranged in triangles and the catalytic behavior of nitrobenzene and benzaldehyde in the synthesis of benzylaniline catalyzed by Pd3 clusters were systematically studied.AgNO3,AuClPPh3,PdCl2 and K2PtCl4 were used as precursors to synthesize M@Ag24(M Ag,Au,Pd,Pt)nanoclusters and used as a model to catalyze CO oxidation.The main contents are as follows1.An atomic-precise Pd3 cluster catalyst was utilized.It showed better performance than the conventional Pd nanoparticle catalyst in catalyzing the one-pot synthesis of benzalaniline from nitrobenzene and benzaldehyde.Our catalyst exhibited efficient conversion and selectivity toward the partially hydrogenated product benzalaniline.However,conventional Pd nanoparticles afforded a completely hydrogenated product benzylaniline.For comparison,the benzalaniline was more easily desorbed from the surface of the Pd3 cluster catalyst,thereby inhibiting further hydrogenation of the benzalaniline,and improving the selectivity of the Pd3 cluster catalyst to benzalaniline.Density functional calculations further confirmed that the interaction between benzalaniline and Pd3 clusters was relatively weak,so it was easier to desorb from Pd3 clusters,thus avoiding its further hydrogenation to generate benzylaniline.2.M@Ag24(M=Ag,Au,Pd,Pt)nanoclusters with a fixed number of atoms were prepared and supported on the manganese-cerium mixed oxide carrier MnxCe1-xO2 The M@Ag24/MnxCe1-xO2(M=Ag,Au,Pd,Pt)catalytic CO oxidation law is:The catalytic activity increased first and then decreased with the increase of pretreatment temperature.When activated at 200? in an O2 atmosphere,the order of catalytic activity of the four clusters is:Pt@Ag24>Au@Ag24>Pd@Ag24>Ag@Ag24.In-situ infrared spectroscopy of CO adsorption on the four cluster catalysts showed that the central atom M(M=Ag,Au,Pd,Pt)did not migrate to the cluster surface at this activation temperature,but the electrons from the central atom M(M=Au,Pd,Pt)transferred to the silver atoms in the Ag24 shell,thus resulted in different levels of electron density on the silver atoms in the shells of the four clusters Pd@Ag24>Au@Ag24?Pt@Ag24>Ag@Ag24.Raman spectroscopy test found that the carrier MnxCe1-xO2 itself had no Raman signal.Compared with commercial CeO2,the Raman signals of M@Ag24/MnxCe1-xO2(M=Ag,Au,Pd,Pt)had different blue shifts Pt@Ag24>Au@Ag24>Pd@Ag24>Ag@Ag24,which showed that these four clusters had different enhancements to the oxidizing ability of the catalyst.In summary,M@Ag24/MnxCe1-xO2(M=Ag,Au,Pd,Pt)existed the interaction of different metal atoms in the clusters and the SMSI effect between the clusters and the carrier,and caused a difference in its catalytic activity.After Pt@Ag24/MnxCe1-xO2 was pretreated at 200? in an O2 atmosphere,it had the largest electron density and the number of oxygen vacancies in the Ag24 shell,so it had the best catalytic activity.