Abstract
Experimental detection of the Pt 5d densities of states in the valence band is conducted on a series of Pt-Ni alloys by high energy resolution valence-to-core X-ray emission spectroscopy (VTC-XES) at the Pt L3-edge. VTC-XES measurements reveal that the Pt d-band centroid shifts away from the Fermi level upon dilution, accompanied by concentration-dependent Pt d-band width. The competition between the strain effect and ligand effect is observed experimentally for the first time. It is found that the d-band widths in Pt3Ni and PtNi are broader than that of Pt metal due to compressive strain which overcompensates the effect of dilution, while it is narrower in PtNi3 where the ligand effect dominates. VTC-XES is demonstrated to be a powerful tool to study the Pt d-band contribution to the valence band of Pt-based bimetallic. The implication for the enhanced activity of Pt-Ni catalysts in oxygen reduction reaction is discussed.
Highlights
Pt-based alloys have been studied as one of the most promising catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs)[1,2,3]
The X-ray emission process with the excitation energy scanned across the Pt L 3-edge is illustrated in Fig. 1 in three energy regions: (a) RIXS, ω resulting from an intermediate state and final state differentiated by the excitation of a valence electron into the conduction band via energy loss (ΔE2 = Ω—ω), by the incident X-ray, Ω; the process is enhanced by the proximity of the Pt 2p3/2 to 5d5/2,3/2 dipole transition; (b) VTC above threshold (E0, the point of inflection of the rising absorption edge) and near resonance; and (c) VTC above resonance
The energy of the emitted X-ray is very close to the excitation energy which will appear in the spectrum as an elastically scattered (ES) peak, as shown in the right column of Fig. 2
Summary
Pt-based alloys have been studied as one of the most promising catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs)[1,2,3]. As a result of the evolution of the bright and tunable incident X-ray and high energy resolution detection, the valence band of heavy metal elements can be experimentally determined from VTC-XES.
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