Unsupported synthesis

Hydrogenation of Acetylenes. Complete hydrogenation of acetylenes to the corresponding alkanes, which maybe requited to remove acetylenic species from a mixture, or as a part of a multistep synthesis, may be accompHshed using <5 wt % palladium or platinum on alumina in a nonreactive solvent under very mild conditions, ie, <100°C, <1 MPa (10 atm). Platinum is preferred in those cases where it is desired to avoid isomeri2ation of the intermediate olefin. Silver on alumina also can be used in this appHcation as can unsupported platinum metal. 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

In general, TPR measurements are interpreted on a qualitative basis as in the example discussed above. Attempts to calculate activation energies of reduction by means of Expression (2-7) can only be undertaken if the TPR pattern represents a single, well-defined process. This requires, for example, that all catalyst particles are equivalent. In a supported catalyst, all particles should have the same morphology and all atoms of the supported phase should be affected by the support in the same way, otherwise the TPR pattern would represent a combination of different reduction reactions. Such strict conditions are seldom obeyed in supported catalysts but are more easily met in unsupported particles. As an example we discuss the TPR work by Wimmers et al. [8] on the reduction of unsupported Fe203 particles (diameter approximately 300 nm). Such research is of interest with regard to the synthesis of ammonia and the Fischer-Tropsch process, both of which are carried out over unsupported iron catalysts. 

Table 1 Reversible water effects on an unsupported Co catalyst ratios of wet/dry results (from1 ). Reprinted from Journal of Catalysis, Vol. 210, C. J. Bertole, C. A. Mims and G. Kiss, The effect of water on the cobalt-catalyzed Fischer-Tropsch synthesis, pp. 84-96, Copyright (2002), with permission from Elsevier... 

In the very active field of unmodified nanoparticles recent discoveries have been made on size-selective Fischer-Tropsch catalysts that convert selectively CO and H2 into hydrocarbons there is a strong dependence of activity, selectivity and Hfetime on Co particle size. This topic of unmodified, supported or unsupported, nanoparticles is outside the scope of this chapter [74, 75]. Nevertheless, we mention discoveries made by Degussa, who have patented a process for H2O2 synthesis from molecular oxygen and molecular hydrogen with nanosized Pd particles (6 A) [76]. 

Our first study of these systems was the synthesis in 1998 of the polymeric complex [ 1( 6 5)2( )2] through the reaction between triphenylphosphine oxide, thallium nitrate and lithium bis(pentafluorophenyl)aurate(I) [71]. This complex consisted of an extended unsupported linear chain of alternate gold and thallium centers. These atoms displayed Au-Tl interactions of 3.0358(8) and 3.0862(8) A, and the thallium atoms showed a distorted pseudo-trigonal-bipyramidal environment, taking into account the stereochemically active inert pair of this atom. As described below, the environment around thallium is one of the main factors that affects the optical properties of these mixed systems. 

Although the synthesis and catalytic activity of unsupported and alumina-supported molybdenum nitride have been studied extensively, much less attention has been given to examining the surface structure and... 

Massive metals themselves are used as unsupported fixed-bed catalysts for example, Raney nickel is used in a variety of hydrogenation reactions. The synthesis of ammonia from N2 and H2 is carried out with reduced massive iron containing minor amounts of promoters. 

More recently, Aldridge further exploited this approach for the synthesis of [p-BMcs [(q S-CsI Is)Fc(CO)2 2] (10) [Eq. (5)] which represents a rare example of a structurally authentic complex with an unsupported borylene ligand B-R (R = alkyl, aryl, silyl). Interestingly, the formation of 10 is not accompanied by CO liberation although rather harsh conditions had to be applied.97... 

Relative rate constants for a-olefin readsorption decrease as follows kr c0>kr Ru> r Fe (7)- Although kr on Fe catalysts is smaller than on Ru or Co, the other parameters in Eq. (2), such as the low diffusivity of large hydrocarbon and the high site density on unsupported Fe catalysts, ultimately increase the probability of a-olefin readsorption therefore, pore diffusion effects also play a crucial role in Fe-catalyzed FT synthesis (Figures 3 and 4). Fe catalysts, however, give lower C20+ selectivity because of lower intrinsic values of kr- even though asymptotic chain termination probabilites are lower on Fe. 

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