Alloy formation in supported Pd-Pt nanoparticles prepared by flame spray pyrolysis J.-D. Grunwaldt 1 , R. Strobel 1,2 , S. Hannemann 1 , S.E. Pratsinis 2 and A. Baiker 1 Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, 8093 Zürich, Switzerland 2 Particle Technology Laboratory, ETH Zentrum, CH-8092 Zürich, Switzerland Palladium-based catalysts are known to be effective for methane combustion [1,2]. The mechanism for the PdO reduction and re-oxidation has been studied extensively during the past years. However, the use of monometallic palladium has some drawbacks, e.g. deactivation due to sintering at higher temperatures and hysteresis effects. Therefore the addition of noble metals such as Pt, Rh and Ru have been proposed. Preparation routes like co-precipation, impregnation, selective deposition by redox methods or the deposition of heterobinuclear complexes are often used to afford bimetallic noble metal particles [3]. In the present study, flame spay pyrolysis as a one-step preparation method was used to prepare supported Pt-Pd/Al 2 O 3 catalysts [4]. Comparing the monometallic Pd-based catalysts with the corresponding alumina supported Pt-Pd particles uncovered that the addition of small amounts of platinum made the palladium more resistant against sintering and lowered the deactivation observed during methane combustion. In order to prove alloy formation in such kind of catalysts, EXAFS was applied. For this purpose the catalysts were reduced at 250°C in flowing 5%H 2 /He. The experiments were carried out at beamline X1, using Si(311) and Si(111) double crystals for monochromatization. The catalyst samples were loaded in a capillary and treated in 5%H 2 /He [5,6]. Three ionization chambers were used to measure the incident and outcoming X-ray intensities, located before and after the in situ cell as well as after a Pd or Pt foil for energy calibration. The ionization chamber gases (N 2 , Ar, Kr) were adjusted in a way that about 10% were absorbed in the first ionization chamber and 40% in the second and third ionization chamber. EXAFS spectra were taken in the step scanning mode around the Pd K and Pt L 3 -edge between 24000 eV and 25800 eV and 11400 eV and 12700 eV, respectively. During dynamic changes (temperature, gas change) faster spectra in the region 243000 – 24600 eV were taken in the continuous scanning mode (QEXAFS). The raw data were background corrected and normalised using the WINXAS 3.0 software [7]. The EXAFS data were fitted in R-space using scattering amplitudes and phase shifts of the Pd-Pd and Pd-Pt shell calculated by the FEFF code [8]. The Pd K- and Pt L 3 -XANES region of the as-prepared materials showed that both platinum and palladium are mainly in oxidized state. These results are also supported by the EXAFS data. For example, in Figure 1, the Fourier transformed EXAFS spectrum of 1.5%Pd-1%Pt/Al 2 O 3 is depicted. The strong backscattering peak around 1.8 Å (not corrected for phase shift) indicates the backscattering by oxygen neighbours. Moreover, no contribution at 2.8-3.5 Å is found, which would be characteristic for PdO particles. Hence, palladium is finely dispersed in the oxide matrix. However, after reduction at 250 °C backscattering by palladium and/or platinum occurs as also the comparison to a Pd foil underlines. Both palladium and platinum were reduced around 100 °C. Figure 1: Comparison of Fourier transformed EXAFS data of (1) 1.5%Pd-1%Pt/Al 2 O 3 (as-prepared), (2) 1.5%Pd-1%Pt/Al 2 O 3 (reduced at 250°C), (3) 2%Pd- 0.5%Pt/Al 2 O 3 (reduced), (4) 2.5%Pd/Al 2 O 3 (reduced) and (5) Pd foil (divided by 3). 253