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.