Structure Of Bimetallic Nanoparticles Studied By X-ray Absorption Fine Structure Spectroscopy

Bimetallic nanoparticles (NPs) have more flexible composition and structurecompared with single metallic NPs, and show many unusual physical and chemicalproperties. Besides, it is easy to regulate and control their composition, size,architectural structure and to obtain nanomaterial with controllable structure andproperty. In the past two decade, bimetallic NPs had extensive use in heterogeneticcatalysis. Bimetallic NPs as catalyst usually show higher selectivity, activity andstability in comparison with single metallic NPs.Currently, platinum-based bimetallic NPs are important catalyst for fuel cell andare mainly used for catalyzing the reaction on anode and cathode. Besides,rhodium-based bimetallic NPs show potential in catalytic synthesis of ethanol fromsyngas. However, it is not sufficient to study these important catalytic processes,especially, it is difficult to obtain clear structure-property correlation and deeplyunderstand the mechanism for promoting effects of doped metal. It mainly resultsfrom that the structures of bimetallic NPs may become complicated by manyinfluencing factors, such as the size of NPs, amount of the second metal, synthesismethods, etc. In addition, their structures are sensitive to the environment and can beinfluenced by atmosphere and temperature. Therefore, it is a challenge in energyfield that how to characterize the structure under in-situ reaction condition andobtain more comprehensive and detailed structural information.X-ray absorption fine structure (XAFS) technique based on synchrotronradiation has element selectivity and can detect the local geometric and electronicstructure for both metal elements respectively, and can be used for in situmeasurement and provide complementary information, including the extent ofoxidation, coordination number, bond distance, atom alloying ratio. In this paper, thestructure of bimetallic NPs was studied by XAFS technique on Beamline BL14W1at the Shanghai Synchrotron Radiation Facility (SSRF), and in-situ XAFS techniqueat different temperature was developed. Combining with TEM and synchrotron radiation XRD techniques et al., the structure of Pt-based and Rh-based bimetallicNPs was studied by systematic XAFS analysis, main results of this paper are shownas following.(1) The experimental equipment of in-situ XAFS used for characterizing thestructure of catalysts was installed and tested. This equipment can be used forcollecting high quality XAFS data in both transmission and fluorescence mode, andcan be employed for in-situ measurements under different atmospheres (ordinarypressure) at different temperatures (from RT to350oC). This work provides thesupport of equipment and method for the study of subsequent Pt-Ni and Rh-Mn NPs,in addition, it accumulates valuable experience for developing in-situ XAFStechniques on Beamline BL14W1.(2) The structure of carbon supported Pt-Ni bimetallic NPs (PtnNi/C, n=3,2,1)with nominal Pt/Ni atomic ratios was studied by XAFS. The results show that thestructure of Pt-Ni NPs can be influenced by the amount of Ni. The NPs ofas-prepared Pt3Ni/C have a quasi core-shell structure consisting of a core of metallicPt surrounded by many small NiOxclusters. In contrast, both Pt2Ni/C and Pt1Ni/Care demonstrated to have an alloy structure with partial oxidation on the surface, thePt-Pt bond length shows obvious contract as a result of the existence of the Pt-Nibond. Further in-situ XAFS experimental data show that the PtnNi/C became theexpected bimetallic alloy under the atmosphere of H2at393K. The possiblecatalytic activity of PtnNi/C as anode material in direct methanol fuel cells (DMFCs)for methanol oxidation reaction (MOR) was also discussed and a reasonable explainfor the promoting behavior of Ni for MOR was proposed.(3) The structure of carbon supported Pt-Co and Pt-Pb bimetallic NPs wasstudied. The results show that the NPs of PtmCon/C (n=3,1,1, m=1,1,3) samplesare alloy structure, the order of extent of alloy is Pt1Co3/C> Pt1Co1/C> Pt3Co1/C,and the alloying ratio of Pt atom (N(Pt-Co)/N(Pt-M)) increases with decreasing Pt-Ptbond distance. The Pt1Co3/C sample has significant Pt-Co interaction without Ptoxidation, it indicates that there is significant electron transfer from Ni to Pt. Incontrast, Pt1Co1/C and Pt3Co1/C sample have more serious Co segregate. These NPs can be attributed to partial solid solution alloy, it is consistent with the case that thedifferent of the metal atom radius between Pt and Co is about10%(<15%). However,no significant Pt-Pb interaction was observed in PtnPb/C (n=3,2,1) samples, it isconsistent with the case that the different of the metal atom radius between Pt and Pbis about27%(>25%). The Pb atoms in PtnPb/C exist in the form of PbOx. Theexistence of PbOximproves the dispersion of Pt metal and increases the oxidation ofPt, and results shorter Pt-Pt bond distance. These work provide important structuralinformation to deeply understand that the second metal how to modulate the localgeometric and electronic structure of Pt metal, and further understand the promotingcatalytic behavior of the second metal.(4) The structure of Rh-Mn NPs in multi-walled carbon nanotubes(Rh-Mn@CNTs) under different atmospheres and temperatures was studied. Theresults show that the Rh-Mn NPs in Rh-Mn@CNTs sample before reduction werecomposed of Rh2O3clusters and mixed Mn oxide species. After reduction in a10%H2-90%He atmosphere, the mixed Mn oxides were converted into nearly pure MnO.In contrast, the Rh2O3clusters were easily decomposed to metallic Rh clusters evenunder a He atmosphere at250°C. The Rh clusters remained in the metal state underthe next reduction atmosphere, but their dispersion in the Rh-Mn NPs increased withincreasing temperature. No significant Mn-Rh or Mn-O-Rh interaction in thereduced NPs was observed from the XAFS analysis. This work provides thereference information for understanding the structure of Rh-Mn NPs under reactioncondition and the promoting catalytic behavior of Mn for the synthesis of ethanolfrom syngas.In conclusion, the in-situ XAFS method for bimetallic NPs was successfullyachieved, its reliability and application performance was also examined and certifiedby using Pt-Ni NPs et al. as study objects, this paper provides research experiencefor the development of Beamline BL14W1at SSRF, and the new beamlines forenergy and material related to XAFS technique in SSRF phase Ⅱbeamline project.