Paper SS2-MoA10
Formation, Thermal Stability and In-situ Gas Phase Catalytic Properties of Supported Au, Fe, and Au_xFe_1-x Nanoparticles

Monday, November 9, 2009, 5:00 pm, Room N

Metal-loaded diblock-copolymer micelles have been used to synthesize size-selected self-assembled Au, Fe, and Au-Fe nanoparticles (NPs). In-situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the formation and thermal stability of Au_xFe_1-x (x = 1, 0.8, 0.5, 0.2, 0) NPs supported on TiO₂(110) [1]. Upon step-wise annealing from 300°C to 1060°C, a remarkable thermal stability of the Au-Fe NPs was observed, maintaining their original hexagonal spatial arrangement on the TiO₂ surface up to 900°C. A majority phase of a gold-iron alloy (solid solution) was achieved for our Au_0.5Fe_0.5 NPs in the temperature range of 700°C - 800°C, and for Au_0.2Fe_0.8 NPs at 800°C, while a phase mixture of bcc Fe and Au-Fe alloy was observed for the Au_0.8Fe_0.2 system at 800°C - 900°C. For all samples the segregation of Au atoms towards the NP surface was detected upon high temperature annealing (800°C) in vacuum. Nearly complete Au desorption was observed by XPS at 900°C for Au_0.2Fe_0.8 NPs, at 1000°C for Au_0.5Fe_0.5 NPs, and at 1060°C for Au_0.8Fe_0.2 NPs. The enhanced thermal stability of Au in the Au_0.8Fe_0.2 NPs is believed to be related to the formation of core(Fe)/shell(Au) structures. Furthermore, contrary to the case of pure Fe or Fe-rich NPs where nearly complete Fe desorption or Fe diffusion into TiO₂ was observed at 1000°C, an Fe signal was detected at this temperature for the Au-rich samples (Au_0.8Fe_0.2 and Au_0.5Fe_0.5).

The reactivity of our pure Au, pure Fe, bimetallic Au-Fe and a sample with a mixture of pure Au and pure Fe NPs will be compared via in-situ (UHV) temperature programmed desorption measurements. CO oxidation has been used as probe reaction.

[1] A. Naitabdi, L.K. Ono, F. Behafarid, B. Roldan Cuenya, J. Phys. Chem. C 113 (2009) 1433.