Enhanced Stabilization and Deposition of Pt Nanocrystals on Carbon by Dumbbell-like Polyethyleniminated Poly(oxypropylene)diamine

Pseudo-dendritic polyethyleniminated poly(oxypropylene)diamine (D400(EI)₂₀) was used as a stabilizer and promoter to prepare Pt nanoparticles in aqueous solution, which was then deposited on carbon surface followed by calcination. After being deposited on carbon surface, no Pt⁰ could be detected in the solution phase. In all steps, the increasing molar ratio of the amino groups of D400(EI)₂₀ to H₂PtCl₆ ([N]/[Pt]) drastically reduced the size and the polydispersity and kept a constant low value after [N]/[Pt] = 20. Under a [N]/[Pt] ratio of 20, the particle sizes obtained from transmission electron microscopy (TEM) were very small in solution (2.7−2.4 nm) and remained the same after being deposited on carbon surface (2.7−2.4 nm), and were only slightly increased to 3.6−3.0 nm after calcination. The stabilizing ability of D400(EI)₂₀ to Pt on carbon surface before and after calcination can be interpreted by the existence of binding energy between Pt and amine nitrogen. The X-ray diffraction (XRD) pattern together with the TEM image reveals that the obtained Pt nanoparticles exist in single-crystal form. The results of photoelectron spectroscopy (XPS) evidence that the metallic Pt(0) rather than the oxidized Pt is the predominant species in the Pt/C catalysts. The electrochemical active surface (EAS) area of the Pt/C catalyst is only slightly higher than that of the E-TEK Pt/C catalyst, but the utilization factor (93.4%) is remarkably higher than the latter (62.8%). The increasing time of thermal treatment increases the crystallinity of Pt(0) on carbon, accompanied by the increasing EAS areas, which corresponds to its enhanced electrocatalytic performance to methanol oxidation.