Surface-Bound Metals

Catalysis of hydrolytic reactions may also occur by surface-bound metals. Although there has been a greater focus on the study of metal catalysis by dissolved metal ions, there is increasing evidence to suggest that catalysis by surface-bound metals may be of greater importance in environmental systems such as groundwater aquifers. Stone (1989) postulates 3 mechanisms for catalysis at the mineral-water 

Effect of various metal oxides on the loss of phenyl picolinate (PHP) from solution via hydrolysis. All suspensions contained 10 g/L oxide, 1 x10 M acetate buffer (pH 5.0) and 5x10 M NaCI. From Torrents and Stone (1991). (Reprinted by permission from the American Chemical Society). 

The investigators proposed that the rate enhancement observed in Ti02 and FeOOH suspensions was due to chelation of the carbonyl oxygen and pyridineal nitrogen of PHP by surface-bound metals (Mechanism 2.110). 

Ti02 and FeOOH suspensions also suppressed the dependence of PHP hydrolysis kinetics on pH. In particle-free solution, the hydrolysis of PHP is pH dependent, exhibiting base-catalysis. In the presence of the metal oxides, however, hydrolysis was found to be pH independent, suggesting that H2O could compete with the stronger nucleophile, OH , for the surface activated carbonyl group. 

Favorable electrostatic interactions at the mineral-water interface can also result in the catalyzed hydrolysis of substrates (Mechanism 3). For example, hydrolysis rates of monophenyl terephthalate (MPT ) in aluminum oxide suspensions were an order of magnitude greater than rates measured in homogeneous solutions (2.112) (Stone, 1989). 

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Amine-terminated, full-generation PAMAM and PPI dendrimers, as well as carboxylate-terminated half-generation PAMAM dendrimers, can directly bind metal ions to their surfaces via coordination to the amine or acid functionality. A partial hst of metal ions that have been bound to these dendrimers in this way includes Na+, K+, Cs+, Rb+, Fe +, Fe +, Gd +, Cu+, Cu +, Ag+, Mn +, Pd, Zn, Co, Rh+,Ru +,andPt + [18,19,27,36,54,82-96]. Tuxro et al.have also shown that the metal ion complexes, such as tris(2,2 -bipyridine)ruthenium (Rulbpylj), can be attached to PAMAM dendrimer surfaces by electrostatic attraction [97]. A wide variety of other famihes of dendrimers have also been prepared that bind metal ions to their periphery. These have recently been reviewed [3]. Such surface-bound metal ions can be used to probe dendrimer structure using optical spectroscopy, mass spectrometry, and electron paramagnetic resonance (EPR) [86-88,90,97-99]. 

Figure 4.3 Different configurations of surface hydroxyl groups produce different, atomically dispersed surface bound metal species.

Figure 13.20 (a) Phenyl picoli-nate may coordinate to a surface bound metal by forming a five-membered bidentate complex. (b) The isomeric phenyl isonicotinate may form only a (weak) monoden-tate complex. 

In summary, the theoretical results representing the rhenium carbonyls on MgO are in very good agreement with the symmetry indicated by the vibrational spectra and with the coordination numbers and bond distances indicated by EXAFS spectroscopy. Thus, the rhenium carbonyls are regarded as prototype supported metal complexes. They reaffirm the strong analogy between surface-bound metal complexes and molecular metal complexes. The MgO surface is clearly identified as a polydentate ligand. 

Au nanocrystals were prepared as following 50 mg calcined mesoporous silica was immersed in 100 mL aqueous solution of HAuCl4-3H20 (3 mg / 100 mL). The solvent was evaporated in a rotary evaporator, and subsequently, 100 mL CH2CI2 was added to induce the outer surface bound metal precursors to move into the channels of mesoporous silica. CH2CI2 was also removed by evaporation. The obtained solids were dried at room temperature and reduced in a constant H2 flow at 773 K for 3 h. Silica frameworks were dissolved using a 10 wt% solution of HF to obtain unsupported Au nanoparticles. 

Recent studies on supported metal clusters show that they may offer a new generation of heterogeneous catalysts (72-76). Surface-bound metal-bimetal clusters, however, especially under reaction conditions, are difficult to characterize (7i, 14, 17), but some information has been accumulated that provides valuable understanding of surface-bound clusters which, on activation, may be converted to active catalysts. These may exhibit distinctive activities and selectivities that differ from those offered by conventionally prepared metal catalysts (12-14, 18). The remainder of this article is devoted to the organometallic surface chemistry of the interaction between metal clusters... 

Preparation and Characterization of Surface-Bound Metal Species Deduced by Physicochemical Studies... 

Organometallic precursor Activation support Surface-bound metal species Characterization methods"... 

Some reactions such as alkene isomerization, alkene hydrogenation, and H2 -I- D2 exchange can be used as sensitive chemical probes of the coordination environment of metal atoms associated with surface-bound metal clusters. Other catalytic reactions such as CO -I- H2 and alkane hydroge-nolysis, which are sensitive to metal ensemble sizes, are applied as a further structural probe. Several attempts have been made to stabilize cluster frameworks in such a way that catalytic activity is maintained. One of the more promising approaches involves the introduction of a capping group into the... 

Clusters, alloys and poisoning. An overview Molecular organometallic chemistry on surfaces reactivity of metal carbonyls on metal oxides Mechanisms of skeletal rearrangements of hydrocarbons on metals elementary steps Surface-bound metal hydrocarbyls. Organometallic connections between heterogeneous and homogeneous catalysis... 

The sampling of water and soil for speciation analysis is not significantly different than for total metal content determinations. The distribution of metals ranges from truly dissolved to particulate-bound species. This includes inorganic complexes or complexes with organic molecules, surface-bound metals (organic and inorganic... 

Figure 3 Schematic illustration of the surface-bound metal chelates, (a) Tri-dentate IDA and (b) pentadentate TED, both with a coordination number of 6.

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