Nickel particle size

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. 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

Nickel particle size and supporting material size after the endurance tests in the stream of CH4(15mL/min)+CO2(15)+He(70). 

The thermal treatment is one of the factors which controls the properties of the final catalyst [56]. The total surface area (in the range between 100 and 300m2g l) decreases with increasing reduction temperature however, the nickel surface area (typically 20-50 m2g l) increases which is probably due to a higher degree of reduction. The best precursor with respect to a high surface area is the hydroxycarbonate. The surface areas of catalysts prepared from hydroxy-chlorides and nitrates are smaller by about a factor of two. Nickel particle sizes are in the order of 5nm for such catalysts. 

Table 4. Mean nickel particle sizes (nm) in EUROPT-1 determined by various methods [27],...

The whisker carbon has a higher energy than graphite which is reflected in lower equilibrium constants for the reversible decomposition reaction of methane and carbon monoxide (8,13) as shown in Fig. 2. This was observed early (14) and later it was found that the deviation from graphite to the thermodynamics depends on the nickel particle size and that the deviation could be explained by the extra energy... 

The thermodynamic carbon limit B is a function of the composition of the feed gas (atomic ratio 0/C and H/C) and total pressure. An example of thermodynamic limits (16) is given in the diagram in Fig. 6. This calculation should apply the thermodynamics of whisker carbon pushing the carbon limits to more critical conditions. It means that in principle the carbon limits depend on the nickel particle size of the crystal (17,18) as illustrated in Fig. 7. 

Electron microscope (EM) examination of plasma solids evidences a very wide distribution of nickel particle size. In the greater part of the examined areas, nickel particle diameter lies within the range of the detection limit of EM (0.3 nm). EDX analyris performed in selected areas where apparently no nickel particles are visualized indicates the presence of nickel. On the other hand, some very large particles exist in some areas their apparent diameter can be as large as SOO nm. 

The nickel particle sizes were measured from electron micrographs obtained with a Transmission Electron Microscope (TEM, JEOL lOOCXII UHR). The average particle... 

XRD provides a quick and easy method to determine the volume-averaged rrickel particle size, but generally smaller nickel particle sizes are indicated by XRD than by other methods. One reason is that nickel particles are polycrystalline. The presence of twin planes in the TEM images (Figtrre 4.4) of rrickel particles further supports this conclusion. 

The nickel particle size has an impact on the nucleation of carbon. The smaller the crystals, the more difficult the initiation of carbon formation. This result was demonstrated in TGA experiments summarised in Table 5.3 with two catalysts having the same activity but different dispersions of nickel [415] [425]. The catalysts were heated at a fixed rate, and it was shown that the onset temperature was approximately 100°C higher for the catalyst with small nickel crystals (approximately 7 nm) than for that with large crystals (approximately 100 nm). 

The impact of nickel particle size on the equilibrium constant is also illustrated in Figure 5.11 showing data for various metal catalysts [396], Ni-a and Ni-b curves represent data for catalysts with particle sizes of 300 nm and 10 nm, respectively. The noble metals have even smaller equilibrium constants. This is due to the metal particle effect or to other structural effects (p in Equation 5.3). Palladium shows a different behaviour than the other metals, probably due to the formation of an interstitial solid solution and mobility of carbon in palladium [540]. 

Figure 5.16 Carbon limits at various nickel particle sizes. Prereformer conditions. Thermodynamic potential for carbon to the left of the curves [451]. Reproduced with the...

A Materials Analysis Company electron beam microprobe was used for the analysis of nickel. Characteristic radiation emitted by the nickel in the specimen was resolved by a properly positioned lithium fluoride crystal and the intensity was measured with a proportional detector. A motor-driven gear mechanism moved the sample in a step-wise fashion relative to the electron beam. Integrated counts were taken at various intervals along the radius of an oxidized nickel sphere and into the glass sufficient to give a smooth curve for the concentration as a function of distance. The electron beam diameter was 1 ji and the depth of penetration was a maximum of 3 fi. These values are small compared with the 100 // oxidized nickel particle size. The electron beam microprobe was equipped to take photographs of the X-ray image for any element from an oscilloscope. In order to minimize the... 

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. 

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 increased activity of the catalysts can probably be related to the small Nickel particles size formation at the surface with the promoter. The XRD results showed Ni crystallite size rather smaller than the non-promoted Raney Ni. 

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