(631b) Pt/Ni Single-Atom Alloys Encapsulated in SiO2 Nanotubes: X-Ray Absorption Spectroscopy Studies

Atomically dispersed Pt on Ni can increase the catalytic stability by enhancing Ni reducibility and minimizing carbon accumulation during methane reforming. In this work, Pt/Ni single-atom alloys (SAAs) located inside the SiO₂ nanotubes are synthesized by reverse microemulsion. The SiO₂-encapsualted nanoparticles with an average size of 20 nm are composed of Ni and CeO₂ species, and Pt species are atomically distributed mainly on the Ni nanoparticles. The single-atom Pt formations on Ni are confirmed by aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM) images and extended X-ray absorption fine structure (EXAFS) analysis. When Pt/Ni atomic ratio is less than 0.008 (Pt⁸/Ni), singly distributed Pt atoms are presented on Ni, and Pt⁸/Ni shows the most well dispersed Pt/Ni SAAs. X-ray absorption near edge structure (XANES) of Pt⁸/Ni indicates that Pt has a metallic property without having any cationic states on Ni surface. The EXAFS of Pt⁸/Ni SAA shows Pt–Ni contribution in the first shell with the coordination number of 5, implying that Pt atoms are isolated on the outermost layer of Ni species. On the other hand, when Pt/Ni atomic ratio exceeds over 0.008, Pt nanoclusters form, and large Pt nanoparticles can be observed at the high Pt/Ni atomic ratio of 0.03.

The reaction data shows that the Pt⁸/Ni SAA exhibits long stability during 20 h of dry reforming of methane, whereas other catalysts deactivate due to the carbon formation. The great performance of Pt⁸/Ni SAA can be explained by efficient H₂ spillover from Pt to Ni species due to the high Pt–Ni interaction. The lower performance observed on the wet impregnated catalysts is associated with the Pt–O contribution observed in the first shell due to the Pt–CeO₂ interaction. The Pt⁸/Ni SAA can also maintain its structure after the reaction, verified by EXAFS analysis.