Metal-Substrate Interaction

From catalysis it is well-known that the metal-substrate interaction influences the reactivity of supported nanoparticles. For instance, for noble metal particles on oxidic supports, the hydrogenation and hydrogenolysis activity is much greater if the support has a higher acidity (high concentration of acidic —OH groups at the surface) than for neutral or alkaline oxidic supports. The influence of the presence of a support on the catalytic activity of metal nanoparticles has been ascribed to [70, 75-79]  

The changes in metal nano particle reactivity due to a support are often explained in terms of electronic eflfects. Experimentally a shift in electronic levels, especially 

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The variable activity of RNase toward different RNA preparations has been tracked down in part to the variable metal content of the substrates [see Wojnar and Roth (4-76), and earlier references quoted]. Takahashi et al. (477) have reported that Mg2+, Ca2+, and Mn2+ have little or no effect on step 1 or step 2 activity when these are assayed with low molecular weight substrates. However, Ca2+ and Mg2+ do interact with RNA and they inhibit the RNase-catalyzed reaction at pH 7 because of this interaction with substrate (478). Eichhorn et al. (479) found activation by Mg2+ and various transition metals at pH 5. In any event it is clear that in general each metal can be expected to show different effects as a function of pH, ionic strength, specific buffer effects, etc. A substantial correlation of much of the data has been made by Alger (480) who studied RNA and C > p substrates over wide ranges of metal concentration. Activation appeared to involve predominantly metal-substrate interactions while inhibition occurred with direct enzyme-metal interaction. 

In metal peroxide chemistry, the heterolytic or homolytic nature of catalytic oxidation seems to be strongly dependent on the heterolytic or homolytic dissociation mode of the peroxide intermediate, for which the triangular coordination mode of the peroxide moiety of the metal appears to be a key feature. Heterolytic oxidations require attainable coordination sites on the metal, involve strained metallacyclic reaction intermediates, and are highly selective. In contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate interactions and are less selective. In the important field of palladium oxidation chemistry, hydroperoxo... 

The possibility of using the ordering effect to induce unusual properties in the supported complexes was suggested by the authors, who included among the peculiar features the enhanced metal-substrate interactions, the reorientational of substrate molecules within the solvent cage and the possibility of directing the approach of molecules to catalytically active centers. 

Vanadium. Vanadyl complexes of the three ligands 14, 24, and 28 were prepared from vanadyl sulfate and the metal-free ligands. They have the expected spectroscopic properties, and this is confirmed by the X-ray crystal structure of the vanadyl complex of 24, which is shown in Fig. 23(a) and has metal-donor bond distances in the expected range. In terms of reactivity, these complexes are rather unspectacular. No haloperoxidase reactivity was observed, and oxidation of alkanes and alkenes is rather sluggish (138). This is not unexpected since with the pentadentate ligands there is no free site at the vanadium(IV) center for metal-substrate interaction. The same is true for the vanadium(V) oxo-peroxo complexes with tetradentate coordination of the pentadentate ligand. 

Ideally, the substrate should be sufficiently inert to guarantee that the metal-substrate interaction will not affect the properties of the metal particle. Often, however, a relatively strong interaction can occur between substrate and metal species, in particular for small clusters. In a sense, the oxide surface can be considered to be a very special case of a ligand environment stabilizing the metal cluster. The metal electronic states are, however, always perturbed by interaction with the substrate, just as for proper ligands. 

For Au NPs supported on the MgO(lOO) surface TEM has shown that even very small clusters are well faceted and fee ordered (36). This is explained by the strong adhesion and small lattice mismatch ( 3%) between Au and MgO, which favors epitaxial Wulf-Kaischew-Iike morphologies. For larger lattice mismatch (e.g. Pd/MgO) NPs may be significantly strained and contain dislocations. To model these effects theoretically requires carefiil treatment of the metal-substrate interaction (37). In contrast for weakly interacting supports, such as graphite, strained decahedral and icosahedral clusters have been observed by TEM (33). 

We are gratefiil for the financial support by the Deutsche Forschungsgemein-schaft (DFG) within the scope of our Collaborative Research Center (SFB 558) Metal-Substrate Interactions in Heterogeneous Catalysis. The authors thank all staff members of our department who contributed to this work, especially Lamy Khodeir, as well as Melanie Kurtz, Natalia Bauer, Hagen... 

Several examples of metal-catalyzed enantioselective Friedel-Crafts arylations to conjugated acceptor systems are known. Similar to the 1,2-addition reaction, most of the substrates are bidentate in nature to ensure a tight-binding, rigid metal-substrate interaction (Figure 8.11). 

For Cr, Ni, Cu and Pd depositions on a-alumina (Ealet, 1993), the strength of the metal-substrate interactions was found to be a function of the surface reduction the higher the surface reduction, the stronger the Al-Pd and Al-Cu interactions, and the weaker the Al-Cr interaction. This suggests that for metal-substrate interfaces such that %m — Zai is large and xo — Xm is low, the interactions are more sensitive to the surface reduction. The understanding of these effects is lacking. 

With respect to the work-function changes induced by H2O adsorption there is always a decrease of 0.6 to 1.3 eV observed indicating that the lower half of the bilayer -pointing to the metal substrate -interacts with the oxygen part. For higher exposures multilayer ice is condensed which does not contribute very much to a further work-function decrease as the H2O dipoles are apparently randomly oriented. 

Figure 3. Different metal growth types depending on the metal ad-soibate-metal substrate interaction and the lattice misfit (a) Adsorbate-adsorbate interaction, is stronger than adsorbate—...

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