Unsupported stabilization

The presence of shielding compounds interferes with subsequent processes, as the formation of metal-support interactions is able to stabilize supported particles. Moreover, the shielding effect of the colloid protectors prevents the contact of metal particles with the reacting molecules, thus avoiding the use of unsupported colloidal particles as a catalytic system [11]. 

In addition to Au and noble metals, Ni-Zn nanoclusters with an amorphous structure were successfully deposited on Ti02 nanoclusters. The state of Ni was metallic. The catalytic activity of Ni-Zn/Ti02 in olefin hydrogenation was ca. 10 times higher than unsupported Ni nanoclusters. Selective deposition onto Ti02 and the addition of Zn seemed to play an important role to stabilize Ni nanoclusters and to decrease the size of Ni nanoclusters, respectively. Also, clearly Zn promoted the hydrogenation activity of Ni and inhibit the growth of the size, but did not substantially affect Ni nature itself... 

The great majority of platinum(I) complexes are binuclear with monofunctional or bifunctional bridging groups. However, there is also a series of unsupported dimers with the general structure shown in (12). These are generally stabilized by phosphine, carbonyl, and isocyanide ligands.17 Dimeric hydride complexes can have terminal or bridging hydrides and these are discussed above in Section 6.5.2.1.4. 

In the case of methyloxirane, however, on Pt and Pd catalysts the extent of the rupture of the sterically hindered bond is indicative of the electrophilic character of the catalyst. Unsupported or silica-supported ion-exchanged catalysts cleave the sterically less hindered bond, whereas on the impregnated catalysts, the rupture of the more hindered C-O bond is dominant.290 It is likely that Pt or Pd surface metal ions are responsible for the rupture of the sterically more hindered bond and residual chlorine from the catalyst preparation can stabilize these ions in the hydrogen atmosphere. 

The EXAFS data recorded after exposure to air of the unsupported Co-Mo catalysts with different cobalt content allow one to examine the effect of cobalt. In spite of a great uncertainty in the coordination numbers, the promoted catalysts seem to have a somewhat smaller domain size than the unpromoted catalyst as indicated both by the smaller second shell coordination numbers and by the larger effect of air exposure (i.e., reduced sulfur coordination number in first shell). This influence of cobalt on the domain size may be related to the possibility that cobalt atoms located at edges of M0S2 stabilize the domains towards growth in the basal plane direction. Recent results on C0-M0/AI2O3 catalysts indicate that Co may also have a similar stabilizing effect in supported catalysts (36). 

Dispersion of POMs onto inert solid supports with high surface areas is very important for catalytic application because the surface areas of unsupported POMs are usually very low (—10 m2g). Another advantage of dispersion of POMs onto inert supports is improvement of the stability. Therefore, immobilization of POMs on a number of supports has been extensively studied. Silica and active carbon are the representative supports [25], Basic supports such as MgO tend to decompose POMs [101-104], Certain kinds of active carbons firmly entrap POMs [105,106], The maximum loading level of POMs on active carbons is 14 wt% [107], Dispersion of POMs onto other supports such as zeolites, mesoporous molecular sieves, and apatites, is of considerable interest because of their high surface areas, unique pore systems, and possibility to modify their compositions, morphologies, and sorption properties. However, a simple impregnation of POM compounds on inert supports often results in leaching of POMs. 

More recently, Koizumi et al. observed that Mn has an additional beneficial effect in unsupported Fe-based F-T catalysts. These authors studied the sulfur resistance of Mn-Fe catalysts and they observed superior catalysts stabilities, especially when the catalysts were pre-reduced in CO. This group also used IR spectroscopy in combination with CO as a probe molecule to compare Fe and Mn-Fe catalysts. It was found that the addition of Mn led to the appearance of several well-resolved bands upon CO adsorption. The appearance of the bands arising from bridged-bonded CO on Fe indicated that the size of the Fe particles were clearly larger than in the case of the unpromoted catalysts. They attributed the decreased reactivity towards H2S to the observed increase in Fe particle size. 

Supports. The spacing of supports is governed by the hot allowable stress of the piping materials stability, in the case of large-diameter thin-wall pipe deflection to avoid sagging or pocketing and the natural frequency of the unsupported length to avoid susceptibility to undesirable vibration. 

The establishment of the structures and thermal transformations of the catalytically active phases of bismuth molybdate resulted in research directed toward investigating the stability of the structure under reducing conditions. Fattore et al. (38) investigated an unsupported bismuth molybdate catalyst with composition Bi2032.66M0O3 during propylene... 

Solvated metal atoms can be dispersed in excess organic solvent at low temperature and used as a source of metal particles for the preparation of both unsupported metal powders and supported metal catalysts158,161. Alternatively, metal vapor is condensed into a cold solution of a stabilizing polymer to form crystallites of the order 2-5 nm in diameters159. Equation 17 illustrates the unique activity of a colloidal Pd catalyst in the partial hydrogenation of acenaphthene. 

Probably the most useful method for preparing polymeric materials through unsupported metallophilic interactions is an acid-base reaction. In these, basic gold(I) precursors react with metallic Lewis acids, forming supramolecular networks via acid-base stacking. The stability of these systems can be related to the ionic interactions nevertheless, the dispersion forces and relativistic effects can also be invoked to explain the formation of these systems, and in some occasions, these effects are even more important than the electrostatic attractions in determining the structural motifs. 

Nonetheless, the transposition of homogeneous catalytic reactions from unsupported to dendrimer-supported catalysts is still not straightforward. Various dendritic effects , positive and negative ones, on the activity, selectivity, stability and solubility of metallodendrimer catalysts have been observed in this respect. In our own research we have found that a high concentration of metal centers at periphery-functionalized metallodendrimers may translate into a decrease in the catalytic performance due to undesirable side-reactions between the catalytic sites at the dendrimer surface (Fig. 4 and Scheme 4). In contrast, when the exact same catalyst is located at the focal point of a dendron, this matter is avoided by isolating the active site, thereby providing a more stable albeit less active catalyst (Scheme 13). 

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