Metal-support interactions silica supported metals

S. H. Overbury, L. Ortiz-Soto, H. G. Zhu, B. Lee, M. D. Amiridis, and S. Dai, Comparison of Au catalysts supported on mesoporous titania and silica Investigation of Au particle size effects and metal-support interactions, Catal. Lett. 95(3-4), 99-106 (2004). 

The focus of these studies has been on identifying mild activation conditions to prevent nanoparticle agglomeration. Infrared spectroscopy indicated that titania plays an active role in dendrimer adsorption and decomposition in contrast, adsorption of DENs on silica is dominated by metal-support interactions. Relatively mild (150° C) activation conditions were identified and optimized for Pt and Au catalysts. Comparable conditions yield clean nanoparticles that are active CO oxidation catalysts. Supported Pt catalysts are also active in toluene hydrogenation test reactions. 

Smith and co-workers investigated the effect of metal support interaction on the CF formation on a series of Co-silica catalysts.The metal support interaction was manipulated by addition of either BaO, La203 or Zr02 to silica. The rate of catalyst deactivation was found to increase with the increase in the metal support interaction. Competition between CF formation and encapsulating carbon formation controlled the catalyst deactivation rate. In case of the catalysts with high metal support interaction, the encapsulating carbon formation was dominant and hence led to a rapid deactivation of the catalyst. 

The influence of the support on the activity of catalysts for the CO/H2 reaction, especially in the so-called Strong Metal-Support Interaction systems needs to be considered. Silica is normally considered a relatively... 

Salama, T. M., Hattori,H., Kita,H., Ebitani, K., and Tanaka, T., X-ray adsorption spectroscopic and electron paramagnetic resonance studies on the strong metal-support interaction of platinum supported on titania dispersed on silica, J. Chem. Soc. Faraday Trans. 89(12), 2067 (1993). 

The role of iron clusters in Fischer-Tropsch catalysis has been the focus of considerable studies. Cagnoli et al. have recently studied the role of Fe clusters on silica and alumina supports for methanation.22 Chemisorption, catalysis and Mossbauer spectroscopy experiments were used to study the effect of dispersion and the role of various supports. Although several oxidation states of iron were observed, the focus of this research was on Fe clusters which were found to be on the order of 12 A crystal size. The authors proposed that metal support interactions were greater for silica than alumina supports and that selectivity differences between these catalysts were due to differences in surface properties of silica vs. alumina. Differences in selectivity for Fe/SiC>2 catalysts at different H2/CO ratios were attributed to differences in coadsorption of H2 and CO. Selectivity differences are difficult to explain in such systems even when only one metal is present. 

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. 

In summary, our study shows the power and the necessity of computational methods in unravelling the structural chemistry of silica-supported metal clusters. It has been shown that the stmcture of such clusters is highly sensitive to the processes by which the cluster-support interaction occurs and not just dependent on the strength of the binding interaction. Further studies are aimed at also investigating the electronic structure of supported clusters and the effects of ligands on cluster structure. 

The entries in Table 1 for Cu(acac)2 have been assigned in the literature and we used these assignments to interpret the effect of the interaction with a hydrophilic surface such as silica. The 2[y-(C-H)] vibration is observed at 1552 cm in the IR spectrum of polycrystalline Cu(acac)2 and in the IR spectrum of a silica-supported metal complex that has been partially decomposed by heating so as to remove only one (acac) ligand so that the stoichiometry of the resulting supported metal complex was Cu(acac). Thermal decomposition of the supported metal complex in a TGA confirmed that the (acac) ligands were removed... 

Metal-support interactions can be defined as being weak, medium or strong.28 Non-reducible metal oxides such as silica, alumina, and magnesia as well as carbon or graphite are considered to exert only a weak influence on the metal and, thus, exhibit only a weak metal-support interaction (WMSI). Zeolites (Chapters 10 and 13) exert a medium metal-support interaction (MMS1)28 while metals supported on reducible oxides when reduced at high temperatures exhibit a strong metal-support interaction (SMSI).27-32... 

Palladium-silica catalysts prepared from tetra-ammine palladous nitrate (to avoid chlorine introduction) showed a marked reduction effect , viz, the specific activity for benzene hydrogenation decreased with increased reduction temperature, i.e., 573 or 723Various explanations were considered, including a metal-support interaction. After reduction at 873 K, X-ray diffraction provided clear evidence of chemical reaction and at lower temperatures silicon insertion into palladium might still occur, which could either disrupt the palladium ensembles required for benzene adsorption or modify the properties of single palladium atoms, if these are the active sites. 

Metal-support interaction was observed in two nickel-silica catalysts, but they were prepared either by precipitating a complex carbonate from nickel nitrate solution containing Si02 as a slurry or co-precipitating the carbonate from a solution of nickel nitrate and sodium silicate. The similar spectra from the unreduced catalysts resembled in shape and binding energy a NiSi03 standard and were quite distinct from NiO. The interaction was... 

Magnetic measurements, which included magnetization-temperature behavior and particle size determination, were also made on this series of catalysts as a function of reduction treatment. These results, in conjunction with those obtained from kinetic studies, provided a physical picture of the different mechanisms for the niobia and phosphate supports. The same picture is consistent with the less interacting nature of niobia-silica, which should prove useful as a model system for the study of metal-support interactions in general. 

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