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

A graded nano-alloy of Au sub(100-x)Pt sub(x) (x= 7, 15, 23, 32, 40, 51, 62, 73 and 86) nanoparticles (NPs) formed by co-reduction of HAuCl sub(4) and H sub(2)PtCl sub(6) and the details are presented in this work. Au sub(100-x)Pt sub(x) NPs were characterized using surface plasmon resonance (SPR) absorption spectroscopy and transmission electron microscopy (TEM). The NPs were dispersed in Vulcan carbon (Au sub(100-x)Pt sub(x)/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 as-formed 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 Au sub(100-x)Pt sub(x) NPs is kinetically controlled rather than being controlled by the thermodynamic stability. The results demonstrate the formation of graded alloys of Au sub(100-x)Pt sub(x) 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= 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 intermediates.