Synchrontron Radiation Study On The Structure And Performance Of Bimetallic Catalysts

Catalysis is the pillar of modern chemistry and a crucial field in modern chemical industry,about 85%of chemical production needs to be completed by catalytic process.The catalytic process involves the breaking of old chemical bonds and the formation of new chemical bonds.As the core catalyst of catalytic reaction,chemical bonds are bound to form with reactants in the catalytic process,producing a variety of intermediate species in the reaction,resulting in changes in the surface structure,composition,size and morphology,as well as changes in the atomic valence and geometric configuration of active sites.These basic problems are not only related to the efficient use of catalyst,but also related to the further design and optimization of catalyst performance.Therefore,only in the real condition of catalytic reaction,monitoring its geometry and electronic structure characteristics,obtaining the real structure,understanding its change rule,and linking the real structure and catalytic performance of catalyst,can we correctly understand the catalytic behavior,guide the optimization of catalyst structure and improve its catalytic performance.In this paper,bimetallic catalysts including Pt and Co were designed and synthesized,and the CO oxidation were used as probe reaction.The dynamic changes of structure,compositions and chenmical states of bimetallic catalysts were studied by synchrotron radiation X-ray absorption fine structure spectroscopy(XAFS),diffuse reflection infrared Fourier transform spectroscopy(DRIFTS)and other characterization methods.It is of great significance to obtain the real structure of the catalyst under working condition to clarify the realationship between the structure and performance of bimetallic catalyst,which can help us understand the catalytic reaction behavior of the catalyst and provide new ideas for the optimization design of new catalyst.The specific research contents of this paper are as follows:1.The structure dynamics and active site changes of Platinum-cobalt-hydroxide nanocatalysts under CO oxidationThe structure,compositions and chemical states of metal catalysts are prone to dynamic changes in response to reaction conditions,while the structure change of bimetallic catalyst is more complex.In this work,Pt-Co(OH)2/SiO2 bimetallic catalysts with different particle sizes(1.3 and 1.9 nm)were prepared by urea deposition precipitation method.They have much higher catalytic CO oxidation performance than Pt/SiO2 with similar size and Pt loading.The Pt-Co(OH)2/SiO2 catalyst with a particle size of 1.3 nm can completely transform CO into CO2 at 345 K.The apparent activation energy Eα(14 kJ mol-1)and TOF(0.068 s-1)are much better than Pt/SiO2(Eα=64 kJ mol-1,TOF=0.003 s-1).A combination of in situ XAFS and DRIFTS has been used to monitor the temperature-dependent structural dynamics in bimetallic Pt-Co(OH)2 nanocatalysts during CO oxidation.The results show that Pt-Co alloy is formed in Pt-Co(OH)2 nanocatalysy after H2 reduction at 473 K.Alloying with electron-donating Co promotes the catalytic activity of metallic Pt for CO oxidation at low temperature.At elevated temperatures under the reaction atmosphere,O2 drives the segregation of the Pt-Co alloy into cobalt oxide and platinum metal that due to the greater affinity of O2 and Co metal atom.The CoOx species former by the segregated Co interacts with the metal Pt to forms an interface;the segregation will not occur in the CO atmosphere without O2.At 473 K or above,Pt-Co alloy is completely segregated,and the Pt-CoOx interface is the active site of the CO oxidation reaction,corresponding to Eα=53 kJ mol-1.Reduction at high temperature could recover the formation of the Pt-Co alloy with the same alloying extent,which proves that the alloy/dealloying process is dynamic reversible.2.Enhanced catalytic activity of PROX by metal-oxide interface of Pt-CoOx sub-nanocluster catalystPrecisely constructing active metal-oxide interfaces and disclosing their atomic interfacial structure under realistic reaction conditions are of great importance for optimizing catalytic performance and establishing structure-activity relations.Here,we report that Atomic Layer Deposition technology is used to precisely control the atomic deposition,and a two-dimensional planar sub-nano Pt cluster with quasi atomic layer thickness is obtained,the cluster size is 0.94 nm which is decorated with CoOx species to construct the Pt-CoOx interface.Planar subnano-clusters possess high metal-atom efficiency,distinct electronic properties and catalytic functions.In the reation of preferential oxidation of CO in the H2(PROX),the Pt-CoOx/SiO2 catalyst can achieve 100%CO conversion and 100%selectivity in a wide temperature range of 25-140℃and with TOF as high as 0.39 s-1,which is obviously better than the catalyst with three-dimensional nano particle or single atom Pt structure.In situ XAFS further revealed the real structure of the sub-nanocluster catalyst in the PROX reaction,that Pt cluster and CO1Ox species formed Pt-CoOx interface through the O atom after high temperature reduction,and the sub-nanocluster remained stable in the catalytic reaction.The results of DRIFTS and XPS show that the chage transfer from Pt to CoOx in the Pt-CoOx cluster reduces the charge feedback of Pt to CO molecules,thus weakening the adsorption strength of CO on Pt and effectively enhanceing its catalytiac performance.3.Structure of subnano Pt-Co bimetallic particle and and catalytic properties for the hydrolysis of aminoboraneBimetallic catalysts have unique properites compared with single element catalysts due to the changes of surface geometry and electronic structure.In this work,a series of subnano PtCo bimetallic particles with different composition and particle size were prepared by adjusting Co/Pt atom ratio,reduction temperature and reduction time.TEM,XAFS,DRIFTS were used to characterize the morphology and structure of the bimetallic catalyst.It was found that the higher reduction temperature,the smaller particle size;the longer reduction time,the smaller particle size;when the Co/Pt atomic ratio was 0.6,the smallest metal particles could be obtained under the same reduction condition.The particle structure in H2 reducing atmosphere and air atmosphere was characterized by XAFS.It was found that Pt-Co alloy structure was formed by reduction in H2 atmosphere,and the proportion of alloy structure was large.At this time,the the higher reduction temperature,the larger particle size.In the air atmosphere,Pt-Co alloy structure segregation and the proportion of alloy structure decreased,the higher the reduction temperature,the smaller particle size of the sample.The structure interaction degree of Pt-Co alloy formed in H2 reduction has an important influence on the final particle size in air atmosphere.In the reaction of the hydrolization of aminoborane to produce hydrogen,the CoPt/Al2O3 sample has a significant improvement in performance compared with Pt/SiO2.The effect of Co/Pt ratio on the catalytic performance is larger than that of particle size.The interaction of PtCo promotes the improvement of catalytic performance.