Nano-sized catalytic system

Further exploration of the higher activity of the Ni complexes compared to Pd analogs led to the discovery of a novel nano-sized catalytic system with superior performance for hydrothiolation and hydroselenation reactions of alkynes [ 152,153]. Furthermore, it was found that with a simple catalyst precursor - Ni(acac)2 - the reaction was carried out with excellent yields and excellent selectivity, even at room temperature. Both terminal and internal alkynes were successfully involved in the addition reaction. This catalytic system was tolerant to various functional groups in alkynes and was easily scaleable for the synthesis of 1-50 g of product (Scheme 3.85) [152, 153]. The proposed mechanism of the catalytic reaction involved (i) catalyst self organization with nano-sized particles formation, (ii) alkyne insertion into the Ni—Z bond and (iii) protonolysis with RZH (Scheme 3.86). 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

In this study we summarize the recent developments in catalyst development in which nano-porous catalytic sites are accessible through a network of arterial micro-pores. These catalysts are obtained through a solution deposition of metals on a micro-porous polymeric template which is subsequently heat-treated to obtain porous metallic structures where the size of the pores ranged from tens of micrometers to tens of nanometers thus eliminating the problems of accessibility and rapid pore fouling and closure. The technique differs fundamentally from the compression-based systems where the porosity is reduced as a result of compaction. It also differs from the well-known wash-coating or chemical vapor deposition techniques. Furthermore, the mechanisms of metal deposition within micro-pores and nano-structure formation are novel. The importance and current fabrication techniques of porous metallic systems can be found in Refs. l... 

The other way of utilization of catalytic propraties in small particles involves the extrrandy high free surface of small partide catalyst systems. A number of nano-size compodtes prepared from noble mmls exhibited enhanced catalytic activity and loi time stability. The Au/Pd Umetallic dustra catalysts in poly-(JV-vinyl-2-pyrrdid[Pg.100]

The possibility to obtain a uniformly dispersed composite powder was shown for the a-Fe-Al203 system where metal particles with an average size of 55 nm were formed in an amorphous/nano alumina matrix.18 Other studies attempting to obtain dense bulk composites based on the sol-gel route using conventional pressure-assisted sintering ( 1400°C and an applied force of 10 MPa) resulted in a coarse microstructure.16 However, if reaching theoretical density is not a necessary requirement, a porous ceramic microstructure containing nanometer-sized metal particles can be used as a catalytic material.19 Certain combinations of composite materials demand... 

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