P 60 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
ON THE LOCAL STRUCTURE OF CATALYTIC Au/Pd NANOPARTICLES STABILIZED ON SPHERICAL POLYELECTROLYTE BRUSHES
W. Szczerba1∗, J. Kaiser2, H. Riesemeier1, U. Reinholz1, M. Radtke1, L. Yu2, M. Ballauff2, and A.F. Th¨unemann1
1BAM Federal Institute for Materials Research and Testing, Unter den Eichen 97, 12205 Berlin, Germany
2Helmholtz-Centre Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
Keywords: EXAFS, gold, palladium, nanoparticle, nanoalloy
∗e-mail : wojciech.szczerba@bam.de
Nowadays, metal nanoallys are attracting more interests than metal nanoparticles. Their physical and chemical properties can be variegated by the type of composition, size and structure. The main application of such alloys is in catalysis, because of their synergistic effect, which means that the alloy particles show much higher catalytic activity than that of the pure single metal particle.
Recently, we have demonstrated that spherical polyelectrolyte brushes (SPB) can work efficiently as carrier system for the immobilization of Au- Pd nanoalloys. HR-TEM demonstrates that well- defined alloy nanoparticles can be obtained with size in the range of 1 – 2 nm. Figure 1 shows the schematic structure of Spherical Polyelectrolyte Brushes and TEM images of Au/Pd alloys.
In order to get precise information about the structure and composition of alloy particles, ex-
tended X-ray absorption fine structure (EXAFS) was the method of choice for us. Noble metal alloys containing Au/Pd have been measured for various compositions at the Au L3-edge and Pd K -edge at BESSY II in Berlin.
In order to interpret the data models employ- ing the FEFF code have been established. It has been found that the Au EXAFS is purely metal- lic, whereas the Pd EXAFS exhibits a strong non- metallic Pd signal, and a significantly smaller metal- lic contribution. The models suggest that the par- ticles consist of a Au/Pd alloy with an increasing Au gradient toward the center. The particle surface is most probably covered by a monolayer of non- metallic palladium compounds, e.g. Pd-O, Pd-Cl, which does not give any contrast in TEM, but con- tributes significantly to the Pd XAFS signal.
Figure 1 : Au/Pd nanoalloys immobilised in spherical polyelectrolyte bruch.
Figure 2 : FT of the Au EXAFS of the Au25Pd75 nanoparticle and the first shell fit of a Au/Pd alloy model.
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