Abstract
Structural changes of a commercial supported PdAu/SiO 2 catalyst for VAM production aged under laboratory and industrial conditions were studied by XRD, TEM/EDXS, XPS, ICP-OES and FTIR spectroscopy. The fresh catalyst contains alloy particles with a wide range of Pd/Au ratios formed by agglomerated crystallites of different sizes. The particles do not show any noticeable sintering during deactivation up to 6 month use but undergo serious restructuring which is connected to the migration of Pd to the particle surface during early stages on stream followed by formation of water-soluble Pd, most probably Pd acetate, which is leached out of the catalyst upon extended time on stream. Considering earlier studies which identified single Pd atoms isolated from each other by surrounding Au atoms as active sites [M. Chen, D. Kumar, C.-W. Yi, D.W. Goodman, Science 310 (2005) 291], it is concluded that the partial enrichment of Pd on the exposed noble metal alloy surface is a major reason for loss of activity in the present type of catalyst. Additionally it is demonstrated that simultaneous calculation of crystallite sizes and alloy compositions from the width and position of the XRD reflections, as usually performed, can in principal, give information on dynamic changes in the noble metal phase but it is not a meaningful approach for calculations both of crystallite size and composition in such inhomogeneous systems.
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