Nickel catalysts particle size effect

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

In this system the average particle size depends on the BH4 Ni " ratio, the nickel salt concentration and the size of the inner water core of the reversed micelle. A BH4" Ni ratio of three gives the smallest size catalyst particles. Lower ratios lead to the formation of larger particles while higher ratios have no further effect on particle size. Micelles with smaller water cores produce smaller catalyst particles. The effect of nickel salt concentration on particle size is complex the smallest particles are formed with a concentration of 5x10"2M. Fig. 12.3 shows the relationship between the micelle composition, nickel salt concentration and the nickel boride particle size. For any given preparation the catalyst particles are essentially uniformly sized with only a 0.5 run distribution. ... 

Electron microscopic examination of these catalysts showed that the powders that exhibited Type A behavior had many particles less than 10 nm in diameter, whereas the powders showing Type B characteristics had relatively few of these small particles. 2 similar size effect was reported for Ni(P) catalysts prepared by the reduction of nickel orthophosphate.93 When the catalyst was reduced at temperatures near 400°C, Type A behavior was observed, whereas Type B behavior occurred with catalysts prepared at higher reduction temperatures. Here, too, a particle size effect was proposed to explain these results. Lower temperature reduction gives fine clusters of Ni interspersed with the Ni(P), and it was considered that these small particles were more effective for... 

This latter was accepted as evidence for possible aggregation, since some literature data indicate that hydrogenolysis, being a structure-sensitive reaction, involves multiple adsorption (/58-161). However, two recent review articles on particle size effects on metal catalysts (162, 163) emphasize the inconsistency of the rather scarce data on nickel and warn of the difficulties connected with this problem. These data, although scarce, support the conclusion that the structure in the precrystallization state is the most favorable for catalytic activity, but detailed understanding of the phenomenon requires further clarification. 

Fu L., Kung H.H., and Sachtler, W.M.H. (1987) Particle size effect on enantioselective hydrogenation of methyl acetoacetate over silica-sup-ported nickel catalyst, J. Mol Catal 42, 29-36. 

Catalysts of this type are of prime theoretical interest since they allow modeling of the size effects on the catalytic activity. They may be used to exemplify basic theories of the catalysis multiple theory, theory of active ensembles, and theory of skeletal catalysis. Many attempts were made to assess the influence of catalyst particle size on activity. For example, the size of nickel particles was varied from that of an individual atom to that of the bulk metal. Quantitative estimates were made of the effect of the Ni /Si02 and Ni Cu ,/Si02 particle size on the course of hydrogenation. Similar studies were performed of the behavior of deposited polynuclear complexes in the hydrogenation and isomerization of alkenes. ... 

Ermakova and co-workers manipulated the Ni particle size to achieve large CF yields from methane decomposition. The Ni-based catalysts employed for the process were synthesized by impregnation of nickel oxide with a solution of the precursor of a textural promoter (silica, alumina, titanium dioxide, zirconium oxide and magnesia). The optimum particle size (10 0 nm) was obtained by varying the calcination temperature of NiO. The 90% Ni-10% silica catalyst was found to be the most effective catalyst with a total CF yield of 375 gcp/gcat- XRD studies by the same group on high loaded Ni-silica... 

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

Measurements of Crystallite Disorder in Catalysts. - Many authors have speculated that the unusual activity of a particular catalyst preparation might be related to the presence of microstrain within individual catalyst particles. Experimental observations to support this speculation are few however, since in any highly dispersed material it is difficult to separate the effects of microstrain from other effects such as crystallite size and active site concentration. One careful study measured the microstrain in nickel and copper catalysts49 but failed to connect the results explicitly with activity data. 

Supported nickel catalysts are widely used for hydrogenation reactions. The most common supports are silica and activated carbon, however several studies have shown that clays are rather more effective in maintaining high metal surface area and can impart useful selectivity to the catalyst.18 Deposition of nickel(II) by the incipient wetness technique [in which the volume of nickel(II) solution used is equal to the pore volume of the clay and then dried] followed by H2 reduction was found to be the most effective method of generating a high dispersion of metal particles (< 10 nm in size) within the clay. 

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