Enhanced catalytic activity

The technological appHcations of molecular sieves are as varied as their chemical makeup. Heterogeneous catalysis and adsorption processes make extensive use of molecular sieves. The utility of the latter materials Hes in their microstmctures, which allow access to large internal surfaces, and cavities that enhance catalytic activity and adsorptive capacity. 

The catalytic properties of the shock-modified rutile whose defect properties have been reported in previous sections of this chapter have been studied in a flow reactor used to measure the oxidation of CO by Williams and coworkers [82G01, 86L01]. As shown in Fig. 7.7 the effect of shock activation is substantial. Whereas the unshocked material displays such low activity that an effect could only be observed at the elevated temperature of 400 °C, the shock-modified powder shows substantially enhanced catalytic activity with the extent of the effect depending on the shock pressure. After a short-time transient is annealed out, the activity is persistent for about 8 h. Although the source of the surface defects that cause the activity is not identified, the known annealing behavior of the point defects indicates that they are not responsible for the effect. 

Fig. 7.7. Studies of the catalytic activity of shock-modified rutile in the oxidation of CO shows greatly enhanced catalytic activity, which is strongly influenced by the shock conditions [86G01].

There are, however, cases when the hydrogen pretreatment results in an enhanced catalytic activity of an alloy. The phenomenon may be explained also in connection with a metal-metal hydride transformation, namely as a post-hydride effect. 

These results lead us to the conclusion that nano-sized metallic gold and strong interaction between gold and ceria contribute enhanced catalytic activity for CO oxidation in the absence and presence of water vapor. 

It is very seldom that a commercial catalyst consists of only a single chemical compound or element. Often the active constituent is supported on a carrier material that may or may not possess catalytic activity of its own. Enhanced catalytic activity, selectivity, or stability may also be achieved by the addition of other materials referred to as promoters or inhibitors. 

Thus, the preheating of a specimen in an atmosphere of hydrogen (leading to the absorption and adsorption of hydrogen) enhances catalytic activity, as demonstrated by numerous works (30,33-38). 

Zeolites though possess high hydrothermal stability and enhanced catalytic activity, the merits are not fully exploited for industrial applications because micropores hinder the diffusion of bulkier molecules. This led to the development of new kind of silicious materials like that of MCM and SBA with large pore volumes which can be accessed by bulkier molecules. But these suffer with the limitation of week catalytic activity and poor hydrothermal stability which hampers their use for various industrial applications... 

Jacobsen et al. reported enhanced catalytic activity by cooperative effects in the asymmetric ring opening (ARO) of epoxides.[38] Chiral Co-salen complexes (Figure 4.27) were used, which were bound to different generations of commercial PAMAM dendrimers. As a direct consequence of the second-order kinetic dependence on the [Co(salen)] complex concentration of the hydrolytic kinetic resolution (HKR), reduction of the catalyst loading using monomeric catalyst leads to a sharp decrease in overall reaction rate. 

Tong, X., et ah, Enhanced catalytic activity for methanol electro-oxidation of uniformly dispersed nickel oxide nanoparticles - carbon nanotube hybrid materials. Small, 2012. 

High metal dispersion results in exposure of metal sites of enhanced catalytic activity. I. Stranski et al. used the Gibbs-Wulff principle " ... 

For the metals on the left leg of the volcano plot, an increase of the heat of adsorption will ensue enhanced catalytic activity. This consequence applies most obviously to gold which in its standard state is a poor catalyst because heats of adsorption of most molecules are very low on gold. Indeed, Hamta and other authors confirmed that gold becomes very active when present as nanoparticles. 

Transition metal complexes encapsulated in the cavities of zeolites and meso-porous materials exhibit enhanced catalytic activity, compared to their neat analogs. " We had earlier found that Cu(II)-acetate exhibited enhanced regiose-lective orf/zo-hydroxylation of phenols using atmospheric oxygen as the oxidant on encapsulation in molecular sieves Y, MCM-22 or VPI-5. Rao et al. had also found a similar enhancement for encapsulation in Al-MCM-48. 

Enhanced Catalytic Activity of Nanoparticles Supported on Mesoporous Materials... 

Some Pt/Ru-based trimetallic electrocatalysts, such as Pt/Ru/Mo, give enhanced catalytic activity leading to a power density, in an elementary single DMFC, at least twice that of Pt/Ru catalyst. 

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