Au–Pt graded nano-alloy formation and its manifestation in small organics oxidation reaction

A graded nano-alloy of Au100_xPtx (x ¼ 7, 15, 23, 32, 40, 51, 62, 73 and 86) nanoparticles (NPs) formed by co-reduction of HAuCl4 and H2PtCl6 and the details are presented in this work. Au100_xPtx NPs were characterized using surface plasmon resonance (SPR) absorption spectroscopy and transmission electron microscopy (TEM). The NPs were dispersed in Vulcan carbon (Au100_xPtx/C) and annealed at 250, 400, 600 and 800 degree C. The as-formed and annealed materials were characterized using TEM, high resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (XRD), cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS). The CV studies indicate excess Pt on the surface, which is corroborated by XPS and HR-TEM results. The XRD data show that Vegard's law is obeyed by the asformed material and the materials annealed at 250 and 400 degree C, indicating that these materials are not nano-alloys. The studies clearly indicate that the formation of Au100_xPtx NPs is kinetically controlled rather than being controlled by the thermodynamic stability. The results demonstrate the formation of graded alloys of Au100_xPtx NPs. Pt excess in the graded nano-alloy is reflected favourably in the electrochemical oxidation of small organics. In the methanol oxidation reaction (MOR), the peak current value per mg of Pt increases as a function of x, reaches a maximum value at x ¼ 23 and the ratio of forward current to reverse current for MOR reached an unprecedented value of 6.7, which shows the catalyst’s stability against poisoning by carbonaceous intermediate