Nickel crystallite size

The activity loss measured here is caused by recrystallizations. This was demonstrated by using scanning electron microscopy to determine nickel crystallite size in the same catalyst samples. These tests revealed that the catalyst used in demonstration plants has only a slight tendency to recrystallize or sinter after steam formation and loss of starting activity. 

The catalytic hydrogenation of benzene has been carried out as a model reaction to increase the hydrogenated cyclic compounds from aromatics. Catalyst samples containing nickel on different supports were prepared and tested. It was found that a-Al203 supported nickel showed the best activity for benzene conversion reaction. Nickel metal area, its dispersion and nickel crystal size were determined. The best activity is obtained with 40% nickel concentration (as oxide) and at the optimum nickel crystallite size of 196A° and optimum metal area of 10.8m2/g. 

Nevertheless, the results showed that the selectivity was not altered for different conditions. This fact indicates that the reaction was not influenced by electronic effects but by geometric effects. The main factor of distinction of the catalysts was the nickel particle size, which for the promoted catalysts were about threefold smaller. Indeed, the reduction of the Nickel crystallite size with the introduction of Cr or Mo in the Raney-Ni catalyst increased the Ni metallic superficial area, which is effectively the active phase for hydrogenation reaction. Despite of the less total nickel amount of prepared samples compared to commercial sample, the available nickel amount for reaction is larger than in the promoted catalysts. 

The three catalysts were subjected to various reduction and pretreatment conditions and subsequently their BET surface area, metal surface area, nickel crystallite size and degree of reduction were compared. 

Nickel crystallite size and degree of reduction of all three catalysts studied after "super" reduction and the same treatments as in figure 5. + =... 

The barium ferrite was found to have an increase in magnetic anisotropy, as in the nickel ferrite, but its overall effect on magnetization was less because of greater magnetocrystalline anisotropy. The shock modification caused reduced crystallite size and local damage that resulted in increased microwave absorption. 

This conclusion was additionally confirmed by Palczewska and Janko (67) in separate experiments, where under the same conditions nickel-copper alloy films rich in nickel (and nickel films as well) were transformed into their respective hydride phases, which were proved by X-ray diffraction. The additional argument in favor of the transformation of the metal film into hydride in the side-arm of the Smith-Linnett apparatus consists of the observed increase of the roughness factor ( 70%) of the film and the decrease of its crystallite size ( 30%) after coming back from low to high temperatures for desorbing hydrogen. The effect is quite similar to that observed by Scholten and Konvalinka (9) for their palladium catalyst samples undergoing the (a — j8) -phase transformation. 

Fig. 15. Kinetics of the ethylene hydrogenation on Ni and 0-Ni-hydride film catalysts m denotes mass of films, which as known is connected with the thickness and crystallite sizes of the films involved. Blank points—rate of reaction proceeding on Ni film catalysts black points—rate of reaction proceeding on nickel previously exposed to the atomic hydrogen action, i.e. transformed to some extent into /3-Ni-hydride.

Skeletal nickel consists of highly-dispersed nickel with a large surface area [68, 91-96], the structure often being likened to a sponge [51,74], The activity of the catalyst is proportional to the surface area and hence the degree of nickel crystallite dispersion [26,76,91], The nickel crystallites are about 1-20 nm in size [24,92,94-96], and decrease in size with decreasing temperature... 

Table 5.3 Summary of Crystallite Size, Surface Area, and Pore Volume of Skeletal Nickel Leached under Different Conditions...

In comparison to skeletal nickel, skeletal copper has a significantly larger crystallite size of about 10-100 nm [32,46,92,96,100,101], Fasman and coworkers [46,100,101] examined the crystal structure more closely and found that it consisted of copper crystals that had agglomerated into granules or precipitated onto oxides. The copper crystal grains and subgrains were of about 10-13 nm in size, while the copper agglomerates were 50-80 nm. 

A marked effect of the Ce02/Zr02 composition (in samples containing 40 wt.% NiO) on the catalytic activity was noticed. The catalysts with Ce Zr =1 1 (6A) were not only more active (than 7A and 8A) but were also stable during the reaction. Sample 8A containing no zirconia in the support showed a low activity. The NiO crystallite size (Table 11.2) in these compositions varied in the order 7A < 6A < 8A. It may be recalled that on ceria-based catalysts the crystallite size of nickel metal was similar to that of NiO. The higher activity for 6A than 7A indicates that in addition to accessibility of... 

The exchange of a number of compounds in this category with deuterium has been examined by Burwell and his colleagues. n-Heptane has been exchanged over nickel-keiselguhr (43), reduced nickel oxide (29), a series of nickel catalysts of varying crystallite size (37), and over palladium supported on 7-alumina (43). Less extensive studies were also made with 2,3-dimethylbutane (29, 43) and n-hexane (42). 

AIF3 catalyst (240 g, crystallite size 200 A radius, prepared from A1C13 and HF) was added to a nickel reactor tube 19 mm in diameter and 914 nun long. The reactor was electrically heated externally. The tube ends were fitted with pipe connections for the inlet and outlet of a gas stream and for the insertion of a suitable thermocouple into the nickel tube and catalyst bed. Gaseous CHC1F, (173 g h l) was fed... 

Table 1 gives the average sizes of nickel crystallites measured by X-ray line broadening analysis on (111) reflections, before and after the five hydrogenation runs. They increase moderately and even decrease for RNiFe. This confirms that the BET area loss could be due in part to a poisoning which reduces the capacity of nitrogen adsorption. However, measurements of the metallic surface area should also be done to confirm possible surface poisoning. 

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. 

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