Structural and catalytic analysis of gold-palladium composite nanoalloys

The present thesis describes studies on gold-palladium nanoalloys which have been synthesized and stabilized in spherical polyelectrolyte brushes (SPB) as composite particles (AuPd@SPB). Different compositions with atomistic ratios of 25:75, 50:50, and 75:25 (Au:Pd) have been synthesized as well as neat Au@SPB and Pd@SPB nanoparticles. Detailed structural analyses have been performed by high resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The AuPd@SPB nanoalloys appear as well-defined facetted metallic crystals in a size range of 1 – 3 nm. No phase segregation or core-shell structure has been observed neither by HR-TEM nor by EXAFS. By HR-TEM analysis some steps appear at the edge of the facetted nanoalloys atall Au:Pd ratios that have been synthesized. A semi-quantitative analysis has been applied to calculate an index for the frequency of surface defects for these nanoalloys. It is found that the AuPd@SPB particles with an atomistic ratio of 75:25 (Au:Pd) contain the highest number of surface defects,as derived fromthis semi-quantitative analysis. The atomistic arrangement of gold and palladium in the nanoalloys has beenanalyzedby EXAFS measurements.It turned out that there are slight enrichments of palladium at the surface of the particles which is attended by an increasing gradient of gold towards the center of the nanoalloys. This effect is significant for the alloy compositions of 25 molar % and 50 molar % of gold. For the alloy system with 75 molar % of gold this effect is negligible. In this case the atomistic arrangement is equal to a homogeneous randomly mixed bimetallic nanoalloy. Furthermore palladium atoms of the species Pd are observed among metallic palladium in the bimetallic nanoalloys and the palladium nanoparticles by EXAFS spectroscopy. In addition, the EXAFS analysis gives evidence of shortening of interatomic distances in metallic nanoparticles. To gain further insight about the impact of the stabilizing system on the metal nanoparticles,gold nanoparticles with different sizes (1.8 – 2.7 nm) have been synthesized inside SPBand analyzed by EXAFS. In this case the lattice parameter depends linear on the inverse particles size which accords V well with the literature for metal nanoparticles stabilized by different supporting systems. However, it turned out that the slope of this linear correlationdependson the type of the stabilizer system. Thus, an influence of the stabilizer system on the metal nanoparticles is found. Thenanoalloys have been applied as catalysts for the reduction of 4-nitrophenol by sodium borohydride which has been chosen as model reaction to study the catalytic behavior of metal nanoparticles. The AuPd@SPB nanoalloys show an enhanced catalytic performance compared to the neat metal nanoparticles, e.g., Au@SPB, Pd@SPB, and Pt@SPB. A maximum in the catalytic activity is observed for a nanoalloy system with the atomistic ratio of 75:25 (Au:Pd). The catalytic behavior of this alloy composition has been studied in terms of the Langmuir-Hinshelwood model which has been developed in previous studies on SPB stabilized metal nanoparticles. [1] Furthermore a delay time appears for the applied model reaction in the case of the AuPd@SPB nanoalloys which accords well with previous results of Au@SPB nanoparticles where this induction period is related to a surface restructuring at the catalyst. [1] Moreover, to study the impact of the SPB stabilizing system on catalysis, support-free gold nanoparticles which have been generated by laser ablation were applied as catalyst for the previous described model reaction. The catalytic behavior of these gold nanoparticles is analyzed in terms of the Langmuir-Hinshelwood model and compared to SPB stabilized metal nanoparticles. A central result of this investigation is that the polymeric stabilizer does not influence the catalytic activity of the metal nanoparticles but the induction period.