Surface composition and catalytic evolution of [Au.sub.x][Pd.sub.1-x] nanoparticles under CO/[O.sub.2] reaction in Torr pressure regime and at 200°C

• Published in: Catalysis letters 2011-05, Vol.141 (5), p.633
• Main Authors: Alayoglu, Selim, Tao, Franklin, Altoe, Virginia, Specht, Colin, Zhu, Zhongwei, Aksoy, Funda, Butcher, Derek R, Renzas, Russ J, Liu, Zhi, Somorjai, Gabor A
• Format: Article
• Language: English
• Subjects: Alloys; Analysis; Nanoparticles; Palladium; Palladium catalysts; X-ray spectroscopy
• ISSN: 1011-372X; 1572-879X
• DOI: 10.1007/s10562-011-0565-7

[Au.sub.x][Pd.sub.1_x] (x = 0, 0.25, 0.5, 0.75, 1) nanoparticle (NP) catalysts (8-11 nm) were synthesized by a one-pot reaction strategy using colloidal chemistry. XPS depth profiles with variable X-ray energies and scanning transmission electron microscopy (STEM) analyses show that the as-synthesized [Au.sub.x][Pd.sub.1-x] (x = 0.25 and 0.5) bimetallic NPs have gradient alloy structures with Au-rich cores and Pd-rich shells. The evolution of composition and structure in the surface region corresponding to a mean free path of 0.6-0.8 nm (i.e., 2-3 layers to the bulk from the particle surface) was studied with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) under CO/[O.sub.2] reaction in the Torr pressure regime. Under the reaction conditions of 80 mTorr CO and 200 mTorr [O.sub.2] at 200°C, the surface region of [Au.sub.0.75][Pd.sub.0.25] NP is Au-rich (~70% by Au). All [Au.sub.x][Pd.sub.1-x] (x = 0.25, 0.5, 0.75) NP catalysts have higher turnover rates for the model CO/[O.sub.2] reaction than pure Pd and pure Au NPs. The Pd-rich [Au.sub.0.25][Pd.sub.0.75] NPs show the highest turnover rates and the Pd-rich [Au.sub.0.5][Pd.sub.0.5] NPs the lowest turnover rates at 200°C. Interestingly, the Au-rich [Au.sub.0.75][Pd.sub.0.25] NPs exhibit steady-state turnover rates which are intermediate to those of the Pd-rich bimetallic nanoparticles.