Pd-Mn Silica Supported Catalysts: 1. Formation of the Bimetallic Particles

The synthesis of Pd-Mn silica-supported catalysts from bis-acetylacetonates precursors is described. EXAFS and X-ray diffraction lead to a clear understanding of the mode of fixation of the two complexes on the silica surface. Whereas Pd(acac)2forms large crystals, weakly interacting with the support, the Mn complex undergoes a partial decomposition, leading to a molecular layer of Mn(acac)x. The decomposition of these precursors followed by thermal methods confirms a marked difference in the mode of interaction between the silanol groups of the support and the two acetylacetonate complexes. After activation of these precursors, TEM and analytical microscopy provide a picture of the distribution of palladium and manganese on the support. Manganese, for the most part, is spread in the form of a thin layer of oxide over the silica surface. The remaining part is incorporated in 3–6-nm bimetallic Pd-Mn particles, as shown by EXAFS experiments performed at the palladium K-edge. The comparison of EXAFS and EDX results indicates that the alloy particles are partly surrounded by manganese oxide.