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catalyst preparation; surface science; palladium; iron oxide; thin film; nanoparticle; model catalyst,; wet chemical; PVD
Abstract:
In this work, results of surface science studies related to the preparation of supported metal catalysts on well-defined oxide surfaces applying procedures used in real-world catalysis are presented. Specifically, a thin Fe₃O₄(111) film grown on Pt(111) was chosen as support and its surface was exposed to solutions of PdCl₂ in order to deposit a Pd precursor. Scanning Tunneling Microscopy (STM) combined with X-ray Photoelectron Spectroscopy (XPS) have been applied to track both morphological and chemical changes that come along with the transformation of the adsorbed Pd precursor into nanoparticles in the early stages of catalyst preparation. The chemical properties of the prepared Pd model catalyst have been investigated by Temperature Programmed Desorption (TPD) and Infrared Reflection Absorption Spectroscopy (IRAS).
For the supported Pd model catalyst prepared by impregnation of Fe₃O₄(111) films with acidic (pH 1.3) PdCl₂ solution, STM and XPS results provide evidence for homogeneous nucleation of Pd particles out of a monolayer of adsorbed precursor, and an enhancement of the particle-support interaction with increasing annealing temperature. Chlorine, which remains on the model catalyst surface after vacuum annealing, could be removed by oxidation/reduction. However such a treatment leads to particle sintering. A Pd sample prepared through deposition-precipitation using a pH 10 PdCl₂ solution shows a different particle evolution behavior as compared to the pH 1.3 sample because of the different precursor speciation and chemical decomposition mechanism. A comparison of Pd/Fe₃O₄ samples created via Pd evaporation under UHV conditions and that resulting from the solution deposition of Pd-hydroxo complexes reveals that changes in the interfacial functionalization of the support surface govern the differences in Pd nucleation behavior observed. The effect of solution pH on Pd loading and particle size during preparation has been studied. Homogeneously distributed Pd particles with narrow size distribution are formed on Fe₃O₄(111) from low and high pH PdCl₂ solutions. Post-preparation treatment by oxidation and CO reduction leads to CO chemisorptive features characteristic of Pd nanoparticles. H₂ reduction of a Pd/Fe₃O₄ sample, however, induces the strong metal support interaction (SMSI) with Pd-Fe alloy formation.
