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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Sham, Tsun-Kong

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 5d5/2 and 5d3/2 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.

Summary for Lay Audience

Platinum is an excellent catalyst in many chemical reactions. However, because of the high price and limited production, it is desirable to reduce the usage of platinum while maintaining high performance of the catalysts. To achieve this goal, one needs to target the key factor for catalytic activity, which is the valence band of platinum, and apply controllable modifications.

In this thesis, the chemical reaction in a hydrogen fuel cell is used to illustrate the connection between the electronic structure of platinum and its catalytic activity. Bimetallic systems combining platinum and a cheap transition metal, as a promising solution, have been studied in detail to reveal the change of platinum electronic structure. In specific, platinum-nickel bulk alloys, platinum-nickel nanoparticles, and platinum-copper nanoparticles are synthesized and characterized by advanced synchrotron X-ray techniques. These synchrotron X-ray techniques have great advantages over traditional lab techniques in studying the fundamental properties of materials including electronic structure, chemical environment, and crystal structure.

As a result, it is found that platinum attracts electrons from first-row transition metals upon alloying, while the valence band of platinum shrinks and shifts to lower energy. On the other hand, the shape of the unoccupied electron states above the valence band is found to follow a linear relationship between the two types of platinum 5d sates. This linear relationship is established by combining the results from different bimetallic systems and thus is considered generally valid. Other than the physical implication, this finding also provides guidance for future experiments and data analysis.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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