Nickel catalysts particle size distribution

Morikawa et al. (42) suggest that nickel aluminate itself undergoes hydrogen reduction only to a superficial extent, and then produces extremely small nickel particles as the reduction product. In this circumstance, the nickel particle size distribution in a reduced nickel/alumina catalyst will obviously be much dependent on the preparative details that control the proportions nickel oxide and nickel aluminate and the size of the particles in which these substances exist before reduction. 

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. ... 

One further difference exists between HDS and HDM. Bridge [37] has shown, very clearly, that HDS is not limited by diffusion while HDM is. Using a nickel-molybdate based catalyst with a unimodal microporous size distribution, the demetalation of Arabian heavy atmospheric residuum was found to be affected by catalyst particle size, while HDS was not. As the diameter of the pore was decreased, the maximum in the metals deposition profile moved closer to the external surface of the pellet, agmn indicating difiusional limitations for FIDM. 

Still another application of thermomagnetic analysis to nickel catalysts relates to the addition of other components, such as copper, which may be thought to have a favorable influence on catalyst behavior. Nickel has a magnetic moment corresponding to 0.6 unpaired electron per atom in the d-band. Alloys of nickel and copper become progressively less magnetic until, at 60 atom % copper, the magnetic moment becomes zero. It is, therefore, a simple matter to determine to what extent solid solution has taken place if, say, some copper nitrate is added to the nickel solution used in preparation of the catalyst. Similarly, any influence of the copper on particle size distribution is readily observed. 

The disintegration of the nickel particles should be comparable for the type 1 and 2 electrodes because the same catalyst is used in both electrodes. Because of the size selection of the nickel catalyst in the type 3 electrodes, the nickel particles disintegrate differently from those of the other electrode types. Eor all of the electrode types, the concentration of small nickel particles increases however, because of the disintegration of large nickel particles in the type 1 and 2 electrodes, nickel particles of different sizes are formed. Therefore, the change in the size distribution is not as significant as that for the nickel catalyst in the type 3 electrodes only the large nickel catalyst particles are removed from the particle-size distribution in the type 1 and 2 electrodes. In the type 3 electrodes, the initial particle-size distribution is shifted to smaller particle sizes. 

The great difference in the conversions of promoted and non-promoted catalysts can be attributed, firstly, to the leaching procedure and to the nickel particle size distribution. Using the same leaching procedure for different alloys the difference was marked. 

Influence of Metal Particle Size in Nickel-on-Aerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity... 

The results obtained with nickel raised the question whether the relation found between rate of exchange and particle size holds also for other metals of group VIII. We therefore carried out the benzene-D2 reaction on some iridium catalysts widely differing in particle size. We chose iridium because we knew from earlier experiments that iridium black gives a very characteristic cyclohexane isotopic distribution pattern with a maximum for C6H4Ds, whereas the patterns of Ni, Ru, Pd, and Pt show a maximum for the d6 compound. 

Van Hardeveld, R., and, F. Hartog, Influence of metal particle size in nickel-on-aerosil catalysts on surface site distribution, catalytic activity, and selectivity, Adv. Catal. 22,75 (1972). 

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