Silica-bound clusters

For the silica-bound clusters (see Fig. /), hydrogen-terminated fragments of silica were used to model the interaction between the clusters and the silica surface. Due to the amorphous character of the pore walls of MCM41 silica, various fully-relaxed, fragments represented possible internal pore surface sites. The use of hydrogen-terminated fragments of silica to represent extended silica supports, as well as being... 

The DFT optimised cluster FSl is found to have the most energetically favourable free-space structure compared to other clusters investigated and is closest in morphology to silica-bound cluster Bl. Cluster B1 is also found to be the best representative silica-bound cluster with respect to the experimental data found thus far and is compared favourably with the experimental EXAFS data for both coordinations and bond-lengths in ref. 54. Conversely cluster Bl is not found to be the lowest energy cluster structure compared with other possible clusters bound to a silica four-ring in the same manner. 

Ruthenium and osmium analogs of the /r-oxo bound cluster are known. For example, (/r-H)Ru3(CO)io(/r-OSiEt3) and (/u.-H)Os3(CO)io(/r-OSiEt3) have been prepared, and the (/n-oxo)-bound triruthenium and the triosmium clusters can be selectively prepared on the surface of silica and alumina/ ... 

There is a correlation between the amount of adsorbed methane and WAW (Figure 1.44d). Lower amounts of adsorbed methane is observed for samples at minimal (/ =0.005 g/g) and maximal (1 g/g) hydration of silica. The former includes too little water to form effective nanoporosity (between the surface of adjacent silica nanoparticles, clusters, and domains of unfrozen water and ice nanocrystallites), and the latter includes too much water, which can form a nearly continuous surface SAW film (Figure 1.44a), and bound water is less clustered. In other words, strong clustering of bound water (Figure 1.46 Table 1.13) is a necessary condition for the maximal adsorption of methane onto nanosilica. 

Considerable attention has also been paid in recent years to hybrid polymer-silica nanocomposites prepared mostly via a sol-gel process [219,243-254] where 10-100 nm size 3D silica domains (clusters) were covalently bound to the polymer. Polymers deprived of groups reactive in the sol-gel process but prone to hydrogen bonding with silanols of silica nanoparticles have also been successfully incorporated into nanophase-separated hybrid materials [246,248-252]. Some polymer-silica nanocomposites, in particular silica core-polymer shell nanoparticles [255-258], were prepared using 3D fumed silica nanoparticles. 

Fig. 10 Relative total energies of DFT-optimised RU12CU4C112 clusters in free-space and bound to a cluster representation of the silica substrate.

Judging from IR, EXAFS, and UV-Vis spectra (63), larger nuclearity Os clusters such as Ose,(CO),8 and H20sjoC(CO)24 are not fragmented on alumina and MgO even on thermal activation at 523 K in a CO -I- H2 atmosphere. The stable Osg and Os clusters are bound to one or two oxygen atoms shared with the silica or alumina support (Fig. 18), and they retain their metal framework even on hydroxyl-containing surfaces and at elevated temperatures. 

The theoretical results have also indicated that when metal atoms are bound to specific defects their chemical activity may change, in particular can increase. This is likely to be true also for small metal clusters. This has not been fully appreciated so far. In fact, even inert supports, like silica, alumina, or magnesia, can interact strongly with the supported metal if this is bound at a defect site and can have a direct role in the chemistry of the supported species. Some preliminary calculations on supported clusters, however, suggest that the effect of the defect on the cluster electronic structure is restricted to very small, really nanometric clusters of about ten atoms size [224]. Should the size of active catalysts in real applications go down to this size, the specific interaction with the substrate could no longer be ignored in the interpretation of the catalytic activity. 

---