Precursor interactions, support metal

The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters... 

The rhenium interacts strongly with the oxygen atoms of the support and also with platinum platinum interacts less strongly with the support than rhenium. One is tempted to generalize that when one of the metals in a supported bimetallic cluster is noble and the other oxophihc, the oxophUic metal interacts more strongly with the support than the noble metal if the bimetalhc frame of the precursor is maintained nearly intact, then this metal-support interaction helps keep the noble metal highly dispersed. 

Lopez et al. [27] prepared Pd/SiC>2 catalysts under both acidic (pH = 3) and basic (pH = 9) conditions in the sol-gel step and reported that an acid medium promotes the formation of small metal crystallites. This finding is consistent with the formation of a micro-porous silica gel network at a low pH. By comparing samples prepared by the sol-gel method and impregnation, these authors found in the former a stronger metal-support interaction which they ascribed to the square planar palladium complex used as a precursor. Finally, their results showed that the method of preparation as well as the conditions used in each method impact on how these catalysts deactivate in the hydrogenation of phenylacetylene. 

Some interesting conclusions can be drawn from the TPR experiments (Figure 1). First, the reduction feature of RU/AI2O3 catalyst differs from that observed for Ru/AC. Ruthenium precursors supported on carbon are reduced at lower temperatures. This fact is indicative of different metal-support interactions. Furthermore, in all the AC supported catalysts a second H2 consumption peak appears at temperatures close to 673 K. This peak is accompanied by the production of CH4, which can be originated by the partial gasification of the carbon species of the support near the metal particle [10]. Also, this peak near 673 K could indicate the presence of some Ru" species stabilized by interaction with the carbonaceous support, which would become reduced at this temperature. Moreover, the addition of MgO to the Ru/C catalyst shifts the reduction of ruthenium to higher temperatures. Thus, we can deduce that in the Ru-Mg/AC catalysts the ruthenium particles are in close interaction with the MgO. 

Anchoring of metal complexes through interaction with surface hydroxyl groups of inorganic supports continues to be of interest. Studies with catalysts prepared with allyl, carbonyl, chloride, and ethoxy ligands have been reported. Kuznetsov and co-workers conclude that the precursors of metathesis-active centres of surface metal complexes, prepared by anchoring allyl and ethoxy compounds of Mo, W, and Re to silica, are co-ordinatively unsaturated metal ions with oxidation number +4. Metathesis activity of the surface species depends on the ligand environment of the metal ion. 

The catalysts derived from HT precursors provide information on the role of the type and amount of active metal, activation procedure and composition of the inert matrix, without any interaction due to side phase segregation or lack of homogeneity. Unlike that claimed previously by Swamy and coworkers [13, 14], the Co/Al (75.0/25.0, as atomic ratio %) mixed oxide shows a very low activity, due to the severe reaction conditions and, mainly, the presence of water, in agreement with previous data for transition metals supported on cubic mixed oxides derived from HT precursors [15]. The addition of a small amount of Rh [i.e., the Rh/Co/Al (0.5/75.0/24.5, as atomic ratio %) catalyst] improves the catalytic performances significantly, giving higher conversions than those of the analogous Rh/Mg/Al catalysts. Even so the level of conversion is still not satisfactory. 

TPR analysis allows to study reducibility, nature and number or distribution of different reducible species present in the catalytic precursor sample [5-7], Thus, metal-support interaction, state in which the supported metals can be found in the catalyst, and mutual effect on their reducibility can be studied [8,9]. 

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