Hydrocarbons metal particle size effects

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

It is well admitted that the first step of the hydrocarbons synthesis from syngas is the CO dissociation on a metallic center (Co°, Fe°, or Co-Fe in the present case). CO dissociation into Cgurf and CO2 has been studied on the partially Lai j,Coo.4Feo,603 5 solids [43]. The results clearly show that the rate of CO dissociation increases almost linearly with the lanthanum deficiency. This is associated with the increased amount of reduced metal (2.1 wt% for y = 0-10.9 wt% for y=0A). The metal particles size has also an effect. For y = 0.4, CO dissociation is lower when catalysts were initially calcined at 900 °C (12% of CO conversion for 14.1 wt% of metal of average particle size of 28 nm) compared to those calcined at 750 °C (19% of CO conversion for 10.9 wt% of metal of average particle size of 10 nm). The larger size of particles led to a lower surface/volume ratio and to a decrease of CO dissociation. 

The subject of particle size effects in heterogeneous catalysis, in particular the dependence or otherwise of catalytic activity and selectivity on metal particle size in the range 1-10 nm, is of interest to both industrial and academic circles. General trends that have emerged suggest that transformations such as those involving the reactions of unsaturated hydrocarbons, e.g., catalytic hydrogenation and isomerisation, are independent of metal particle size, i.e., are... 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Some aspects of the particle size, alloying effect, and metal-support interaction in nano-sized supported metal particles are presented for the oxidation of ethylene, the hydrogenolysis of alkanes, and the hydrogenations of unsaturated hydrocarbons and a,j8-unsaturated aldehydes. The influence of these phenomena is highlighted on the... 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

Although such effects are well known for hydrocarbon reactions over platinum and other metals,it is unlikely to apply to the present situation where considerable surface reconstruction, corresponding to the formation of an oxide film, is likely. The properties of such a layer would change with particle size simply because of the increase in surface energy with decreasing size. The smaller particles would tend more towards the composition Ag20 than the larger ones. Since it is known that silver(i) oxide is not a selective catalyst for ethylene oxidation such a model could explain these size effects. 

Even more spectacular results in terms of the increasing importance of nanocatalysis for bulk industrial processes have recently been reported by Kuipers and de Jong [32, 33]. By dispersing metallic cobalt nanoparticles of specific sizes on inert carbon nanofibers the authors were able to prepare a new nano-type Fischer-Tropsch catalyst. A combination of X-ray absorption spectroscopy, electron microscopy, and other methods has revealed that zerovalent cobalt particles are the true active centers which convert CO and H2 into hydrocarbons and water. Further, a profound size effect on activity, selectivity, and durability was observed. Via careful pressure-size correlations, Kuipers and de Jong have found that or cobalt particles of 6 or 8nm are the optimum size for Fischer-Tropsch catalysis. The Fischer-Tropsch process (invented in 1925 at the Kaiser-Wilhelm-Institute for... 

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