Electronic and local structures of Pt-based bimetallic alloy and core-shell systems

Jiatang Chen, The University of Western Ontario

Abstract

This thesis investigates the electronic structure of Pt for catalysis applications. The importance of the Pt 5d band is discussed in terms of the bonding capability of Pt. The oxygen reduction reaction in proton exchange membrane fuel cells is chosen as the catalytic reaction model to illustrate the effect of Pt 5d states on Pt-O interaction. Pt-based bimetallic systems are introduced as a solution for the high price and limited resources of Pt. Despite lower usage of Pt, the tuning capability to optimize the Pt 5d band in bimetallic catalysts is supposed to provide superior catalytic activity. Advanced synchrotron X-ray techniques including normal X-ray absorption fine structure (XAFS), X-ray ptychography, and high energy resolution fluorescence detected (HERFD) X-ray absorption/emission spectroscopy (XAS/XES) are combined with laboratory characterization techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) to study the behavior of Pt upon alloying or forming core-shell structures with 3d transition metals.

Three Pt-based bimetallic systems are studied, including Pt-Ni bulk alloys, Pt-Ni nanoparticles (NPs), and Pt-Cu NPs. Pt-Ni bulk alloys are synthesized as model compounds to study the many-body effect, charge redistribution, and local structure of Pt upon alloying. It is found that Pt gains 5d electrons, resulting in more symmetric Pt 4f XPS peaks when diluted in Ni, while Ni loses 4p and 4d electrons, resulting in an increase of K-edge XAS whiteline (WL) intensity, more symmetric Ni 2p XPS peaks, and stronger shake-up satellites. The downshifting of the Pt valence states and upshifting of Ni valence states are also observed with ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) calculations. Pt-Ni NPs, as the Pt-Ni systems in nanoscale, are used to track the evolution process including the alloying and de-alloying of Pt during the synthesis of the bimetallic systems. It is found that Pt goes through five distinct stages, i.e. core frame, tight network, outer frame, thin skin, and particulate shell. The electronic and local structures at each stage are tracked with XAS. Pt-Cu NPs are in the form of core-shell or alloy NPs, with a very low amount of Pt either on the surface or in the bulk. For the 8-nm Cu@Pt core-shell NPs, several monolayers of Pt are deposited on the Cu core, exhibiting good controllability by the polyol reduction method.

The advantages of HERFD-XAS/XES are demonstrated in studying the Pt 5d band of Pt-Ni and Pt-Cu bimetallic systems. In the valence-to-core (VTC) XES experiments, the widths of the VTC emission lines and energy transfers show the shrinking and downshifting of the Pt valence band upon alloying with Ni. For HERFD-XAS, significantly narrowed WLs, enhanced near-edge XAS features, and easily removable background have enabled detailed analysis of the WL peaks with high accuracy. Combining the HERFD-XAS results for Pt-Ni and Pt-Cu bimetallic systems, Pt foil, and Pt NPs, a general linear relationship between the WL areas of Pt L₃- and L₂-edges is established. Physically, this linear relationship indicates that the unoccupied Pt 5d_5/2 and 5d_3/2 states also have a linear relationship. Experimentally, this finding suggests that measuring the Pt L₃-edge alone will provide enough information to study the unoccupied Pt 5d states of Pt-based metallic systems.