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X-ray spectroscopy reveals role of nickel in PtNi bimetallic catalyst for green hydrogen production

X-ray spectroscopy was used to investigate the role of nickel in platinum-nickel electrocatalysts for glycerol conversion to chemicals and green hydrogen. The results showed that nickel modifies the electronic structure of platinum, providing fundamental insights for developing selective glycerol oxidation catalysts.

Glycerol electrooxidation reaction (GEOR) to coproduce green hydrogen (H2) and valuable chemicals at low potential constitutes a promising strategy to phase out fossil fuels in the energy and chemical sectors [1]. Platinum-nickel (PtNi) bimetallic electrocatalysts are potential candidates for GEOR; however, the role of Ni remains unknown. Although some studies suggest that it directly participates in the reaction via a bifunctional mechanism, others suggest that it simply modulates the electronic structure of Pt to weaken the Pt*-CO and Pt*-C bonds, thus increasing their reactivity with neighbouring *OH species. Such ambiguities have hindered the development of more active catalysts.

This work investigated the role of Ni in PtNi electrocatalysts. By increasing the Ni precursor content, three different PtNi bimetallic nanoparticles were formed, denoted as PtNi1, PtNi2, and PtNi3, each with a distinctive catalytic performance for GEOR. Scanning/transmission electron microscopy (S/TEM) revealed the incorporation of Ni atoms into the Pt lattice (Figure 78a). Energy-dispersive X-ray spectroscopy (EDX) mapping demonstrated that by increasing the Ni content, the obtained PtNi nanoparticles change from a Pt-rich surface in PtNi1 to a uniformly distributed Pt−Ni atomic structure in PtNi2, to a partially Ni-rich surface in PtNi3.

X-ray absorption spectroscopy (XAS) was employed at beamline BM28 to examine the local atomistic and electronic structure of Pt and Ni, enabling the construction of a precise model of the PtNi bimetallic nanoparticles (Figure 78b). The X-ray absorption near-edge structure (XANES) spectra at the Pt L3- and Ni K-edges showed that both Pt and Ni were partially oxidised. From the extended X-ray absorption fine structure (EXAFS) spectra, it was clear that the Pt−Pt pair coordination number followed a descending trend from PtNi1 to PtNi3, indicating less Pt- rich local areas, while the Pt−Ni pair coordination peak was the highest for PtNi2, which suggests that the atoms are most homogeneously coordinated, consistent with the STEM studies.

PtNi2 exhibited the highest GEOR activity normalised to the mass and surface area (Figure 79a). From the

Fig. 78: a) TEM, atomic-resolution aberration-corrected high-angle annular dark-field-STEM images and

EDX mapping of PtNi catalysts. b) Pt L3-edge and Ni K-edge XANES/EXAFS spectra of three as-prepared PtNi samples, ex-situ, compared with reference materials.