Hydrogen storage and stability properties of Pd–Pt solid-solution nanoparticles revealed via atomic and electronic structure

Bimetallic Pd1−xPtx solid-solution nanoparticles (NPs) display charging/discharging of hydrogen gas, which has relevance for fuel cell technologies; however, the constituent elements are immiscible in the bulk phase. We examined these material systems using high-energy synchrotron X-ray diffraction, X-ray absorption fine structure and hard X-ray photoelectron spectroscopy techniques. Recent studies have demonstrated the hydrogen storage properties and catalytic activities of Pd-Pt alloys; however, comprehensive details of their structural and electronic functionality at the atomic scale have yet to be reported. Three-dimensional atomic-scale structure results obtained from the pair distribution function (PDF) and reverse Monte Carlo (RMC) methods suggest the formation of a highly disordered structure with a high cavity-volume-fraction for low-Pt content NPs. The NP conduction band features, as extracted from X-ray absorption near-edge spectra at the Pd and Pt LIII-edge, suggest that the Pd conduction band is filled by Pt valence electrons. This behaviour is consistent with observations of the hydrogen storage capacity of these NPs. The broadening of the valence band width and the down-shift of the d-band centre away from the Fermi level upon Pt substitution also provided evidence for enhanced stability of the hydride (ΔH) features of the Pd1−xPtx solid-solution NPs with a Pt content of 8-21 atomic percent.


Transmission Electron Microscopy (TEM) analysis
The size of the prepared samples was determined from transmission electron microscopy (TEM) images, which were obtained using a Hitachi HT7700 transmission electron microscope operated at 100 kV accelerate voltage. The samples dispersed with ethonal were drop-cast onto a carbon-coated copper grid and allowed to dry under ambient conditions. The mean diameter and distributions were estimated by averaging over 200 particles. The average particle sizes of the Pd1-xPtx solid-solution NPs for compositions where x = 0, 0.08, 0.15, 0.21, and 0.5 are 6.1  0.8, 6.7  0.9, 7.4  0.9, 8.1  1.0, and 11.2  1.7 nm, respectively 1 .

Inductively coupled plasma mass spectrometry (ICP-MS) analysis
The atomic composition of Pt in the Pd1-xPtx solid-solution NPs were estimated to be 8, 15, 21 and 50% with errors smaller than 1% by inductively coupled plasma mass spectrometry (ICP-MS) 1 .

S2. High-energy X-ray diffraction data analysis
X-ray diffraction measurements were performed using a two-axis diffractometer installed at the BL04B2 beamline 2 of the third-generation synchrotron radiation facility SPring-8, Hyogo, Japan. The incident X-ray beam was 61.46 keV, with a wavelength of 0.02019 nm; it was generated using an Si(220) monochromator. The Pd1-xPtx solid-solution and core/shell NPs were loaded into a capillary column and measured at room temperature.
Fine powders of bulk 99.9% Pd and Pt were used as the reference materials. The XRD data were corrected for background, polarization and absorption. As shown in Figs. 1 and S1, the corrected XRD data were then normalized to the structure factor S(Q) and Fourier transformed to produce the pair distribution function g(r) data using the SPring-8 BL04B2 PDF analysis software.

S3. Pair distribution function (PDF) analysis
The atomic scale structure of a Pd1-xPtx solid-solution NPs can be described quantitatively in terms of pair distribution function (PDF), g(r), which indicates the average probability of finding another atom within a specified volume at a distance from an origin atom as a function of the radial distance r. 3 This is Fourier transform of the total structure factor S(Q), defined by following equation: where  is the atomic number density and r is the radial distance. The diffraction wave vector, Q, is defined by Q = 4 sin/, where  is half the scattering angle and  is the wavelength of the incident X-rays.
Instead of the g(r), another widely used correlation function is the reduced pair distribution function, G(r), defined by The main advantage of G(r) function is the one directly obtained from the Fourier transform of total structure factor S(Q) without knowing average number density  of the materials. 4 The other useful correlation function is radial distribution function (RDF) is given by = 4π 2 ( ).
At the large values of r, the RDF tends to the smooth parabolic function 4r 2 . On the other hand, the RDF should be equal to zero at small values of r. The area under the respective peak in the RDF equal to the number of neighbors to the origin atom (i.e. the average coordination number). The total correlation function, T(r), is also used for data plotting. This is defined by ( ) = 4 ( ).

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The T(r) also known as reduced RDF, which is easily obtained by dividing the RDF by r and improve the precision of peaks for better estimation of average coordination number.
Crystal structural information of Pd1-xPtx solid-solution NPs were obtained by structural refinement of reduced PDF G(r) data using PDFgui software package 5 .

S4. Reverse Monte Carlo (RMC) simulation
The 3D structural models of Pd1-xPtx solid-solution NPs were generated by RMC modeling method using RMC_POT software 6 furnished for the case of non-periodic boundary conditions. RMC model of Pd0.79Pt0.21 solid-solution NPs was constructed by 13815 (Pd; 10914, Pt; 2901) total atoms in a spherical configuration closely resembling that of a spherical nanoparticles with 8.1 nm in diameter and 0.0496461 Å −3 in number density. As shown in Figure 3, the calculated total structure factor S(Q) and experimental data shows good agreement for Pd0.79Pt0.21,Pd0.85Pt0.15, and Pd0.92Pt0.08 solid-solution NPs.

S5. X-ray absorption fine structure (XAFS) analysis
The XAFS experiment was performed at the Pt LIII-edge and Pd K-edge in transmission mode at room temperature. Figure S4 show the X-ray absorption near-edge structure (XANES) spectra at Pd K-edge for Pd-Pt solid solution NPs and Pd metal foil. The extended X-ray absorption fine structure (EXAFS) spectra at the Pt LIII-edge and Pd Kedge were Fourier-transformed to real space (R-space) in the k range 3.0 to 15.0 Å -1 . Figure S6 show the Fourier transformations of Pt LIII-edge k 3 -weighted EXAFS spectra (phase shift uncorrected) of Pd1-xPtx solid solution NPs and Pt metal foil. The Pd K-edge EXAFS fit was performed in R-space using Artemis program in the IFEFFIT 7 . Here the single scattering path between the Pd core atom and its nearest neighbor was fit with k 3weight. The results of Pd-K edge EXAFS refinement of Pd1-xPtx solid-solution NPs are reported in the Table S1.

S6. X-ray absorption near-edge structure (XANES) of Pd LIII-edge
As shown in Figure S5a,