(672a) Model Studies of Supported Catalyst Preparation - Pd Deposition On Iron Oxide Films From the Liquid Phase

In this contribution we present results of surface science studies related to the preparation of supported metal catalysts on well-defined oxide surfaces applying procedures used in real catalysis. While in most UHV model studies of supported metal catalysts the metal is deposited onto a clean surface by physical vapor deposition, the standard methods in real catalyst preparation, e.g. impregnation or deposition-precipitation, are wet chemical routes with the important processes occurring at the solid-liquid interface, followed by calcination and reduction to transform the catalyst precursor into the active compound. The aim of the present work is to apply those preparation procedures to well-defined oxide surfaces using thin oxide films as substrates that allow standard surface science characterization techniques to be applied. We present Scanning Tunneling Microscopy (STM), X-ray Photoemission Spectroscopy (XPS), Temperature Programmed Desorption (TPD) and Infrared Reflection Absorption Spectroscopy (IRAS) results for Pd deposition from PdCl₂ containing solutions of different pH onto Fe₃O₄(111) films.

STM showed that uniformly distributed Pd nanoparticles can be prepared by using the deposition-precipitation method. XPS was used to follow the evolution of Pd species at each preparation stage and to study the influence of Pd precursor solution pH on the speciation of Pd complexes on the surface. The Pd particles were further characterized by CO chemisorption with TPD and IRAS. Interestingly, different chemisorption properties of Pd/Fe₃O₄ were found after CO or H₂ reduction at 500 K. CO-reduced Pd samples showed normal CO adsorption behavior, while a striking suppression of CO adsorption was brought about by H₂ reduction. XPS results indicated a change in the electronic structure of Pd after H₂ reduction which can be recovered by reoxidation and CO reduction. The results of these experiments will be compared with the respective results for Pd clusters prepared by physical vapor deposition on Fe₃O₄(111).