ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012) P 23
XRD STUDY OF UNIFORMITY AND INTERDIFFUSION IN PdCo AND PdAg NANOALLOYS
Zbigniew Kaszkur∗
Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warszawa, Poland
∗e-mail : zbig@ichf.edu.pl
Nanoalloys of PdCo and PdAg may have a very interesting physical and chemical properties. PdCo can be ferromagnetic as Pd nanocrystals alone may become ferromagnetic and Co doping stabilizes this property, leading to a potential applications in mag- netic storage media and electrical spin injection [1].
On the other hand both nanoalloys are known as a good catalyst e.g. in formic acid oxidation to be ap- plied in formic acid fuel cells [2, 3]. PdAg is effective as a catalyst in hydrogenation and hydrodechlorina- tion reactions [4]. Palladium contents within the al- loy offers a simple way to test uniformity of the stud- ied material. Exposition of the sample to hydrogen at RT leads to transition of Pd and its diluted al- loys to a hydride phase with the lattice constant increase dependent on concentration. Any observa- tion of a broad, structured XRD reflection may indi- cate presence of microvolumes of alloy with different metal-companion concentration (Figure 1). A gas- phase controlled temperature-programmed experi- ment may reveal information on sintering kinetics as well as kinetics of segregation.
For all practical applications phenomena of sur- face segregation and phase thermal stability is of fundamental importance. From the first observa- tion of the Kirkendall effect [5] it is believed that the random walk metal transport phenomena pro- ceed through vacancy mechanism. For nanocrystalls the vacancy disappearance rate can be higher due to surface effect and this may affect the diffusion rate. Surface segregation in alloys is a special case of
interdiffusion moving up the concentration gradient- thus violating the first Fick law. Kinetics of a sur- face segregation in solids could be estimated on the basis of Auger Electron Spectroscopy data [e.g. 6 for Ag in Cu]. The in situ XRD offers a unique op- portunity to observe its time evolution e.g. when reversal of segregation is induced by chemisorption.
We present several experimental results shed- ding some light on the uniformity of the studied alloys as well on the interdiffusion rate for several 20 wt.% Pd3Co supported on carbon (samples cour- tesy of prof. KuanWen Wang, Taiwan) and Pd7Ag3
supported on silica and metalic sponge sample.
References
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Figure 1 : XRD in situ patterns of a sample of Pd3Co/C. Intensity vs. scattering angle (CuKα).
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