Characterization of surface structure and oxidation/reduction behavior of Pd-Pt/Al₂O₃ model catalysts

Structural and morphological characterizations of bimetallic Pd-Pt/Al2O3 model catalysts are performed using X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, and CO chemisorption. Further, the catalysts were studied under oxidizing and reducing conditions using both X-ray absorption spectroscopy and low-energy ion scattering spectroscopy. For the as-prepared catalysts, the existence of alloyed bimetallic Pd-Pt particles and of (tetragonal) PdO was found for the samples calcined at 800 °C. PdO is present in the form of crystals at the surface of the Pd-Pt particles or as isolated PdO crystals on the support oxide. Bimetallic Pd-Pt nanoparticles were only formed on the Pd-Pt catalysts after calcination at 800 °C. The results show that the Pd-Pt nanoparticles undergo reversible changes in surface structure composition and chemical state in response to oxidizing or reducing conditions. Under oxidizing conditions Pd segregates to the shell and oxidizes forming PdO, while under reducing conditions regions with metallic Pd and Pd-Pt alloys were observed at the surface. No bimetallic Pd-Pt nanoparticles were observed for the sample initially calcined at 500 °C, but instead isolated monometallic particles, where small Pt particles are easily oxidized under O2 treatment. In the monometallic catalysts, the Pd is found to be completely oxidized already after calcination and to consist of metallic Pd after reductive treatment.