Nickel dispersion

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica 32). When introduced to a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C—H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p) and a deformation associated with a methylene group (6.9 p). A weak band at 3.3 p was attributed to an ole-finic C—H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of... 

Our third hypothesis, i.e., that the activity enhancement involves the proximity of the zeolite s acid sites, appears to be consistent with the hydrocarbon adsorption experiments, but may also be due to differences in the nickel dispersion arising from surface area differences between the two types of particles. Clearly, the adsorption of hexane at lower temperature on the nickel contaminated zeolitic particles suggests a significantly altered environment from both the uncontaminated and the non-zeolitic materials. 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Maid et al. [40] developed a nickel dispersed carbon membrane catalyst about 100 pm thick. Between the membrane plates, the gas flow occurred in gaps of various thickness between 200 and 1 500 pm. The plates were mounted into a stack-like testing device for methanol decomposition to carbon monoxide and hydrogen. 

Several catalysts on the market today contain special vanadium traps or vanadium scavengers in order to protect the active ingredients against poisoning and/or destruction by Vanadium. These "Metal Traps" limit the mobility of the vanadium pentoxide compounds under FCC conditions (2, 10). The nickel problem needs to be approached differently and more recently, progress has been made towards reducing the dehydrogenation activity of nickel dispersed on FCC catalysts (11). 

Transmission electron micrographs and XPS results have been used to show that a catalyst, with a high silica content in the matrix, prevents nickel dispersion (16,18). In fact, in a FCC with a Si-rich (Si/Al = 4.3) surface, XPS data has indicated that calcination and steaming cause nickel (and vanadium) migration to the cracking catalyst surface where nickel sinters. As a result, nickel crystallites 50... 

Earlier studies in nickel catalysts resistance to coking in steam reforming showed that the carbon deposition rate depends not only on such direct factors as nickel dispersion [10] or the support composition [11], but also on indirect factors, connected with the preparation and pretreatment conditions of the systems, the latter influence the coking rate by causing changes of the direct factors [12]. 

The dramatic increase in irreversible CO adsorption on presulfided supported nickel catalysts at moderate pressures (162) has significant, practical implications in regard to the use of CO chemisorption to measure nickel dispersion. For example, it is often desirable to determine nickel surface areas for catalysts used in a process where sulfur impurities are present in the reactants. Substantial differences in the measurements of nickel surface area by H2 or CO adsorption are possible depending upon the catalyst history and choice of adsorption conditions. In view of the ease with which catalysts may be poisoned by sulfur contaminants at extremely low concentrations in almost any catalytic process, and since large CO uptakes may be observed on supported Ni not necessarily representative of the unpoisoned nickel surface area, the use of CO adsorption to measure nickel surface areas is highly questionable under almost any circumstance. 

In order to control the mechanism by which nickel is loaded onto sodium titanate and to control the degree of nickel dispersion, we need to understand the ion-exchange properties of the sodium titanate support, the hydrolysis chemistry of the dissolved nickel, and how the different nickel species are expected to interact with the titanate support. 

Notwithstanding the lack of consistency of the data, there is no doubt that large amounts of phosphorus in the catalyst formulations cause the decrease of the molybdenum and nickel dispersions and favor the formation of bulk compounds such as M0O3, Al2(Mo04)3, and NiO (60, 67, 80, 84, 91, 92). McMillan et al. (93) reported, however, that the formation of A]2(Mo04)3 is suppressed in the presence of nickel. 

Eijsbouts et al (94) reported XPS data showing that the nickel dispersion tends to decrease as a result of sulfidation. Lopez-Agudo et al (69) observed that the dispersion of nickel in NiP/Al samples decreases as a result of sulfidation, although it is not affected by addition of phosphorus to the oxide form of the catalyst. 

The ESC deposit contained appreciable levels of potential catalytic elements. These appeared to have promoted gasification, as the oxidation of the ESC coke was nearly five orders of magnitude greater than that of effectively pure graphite (SP-1), with an ash content of 1 ppm (Table 3). Such a wide discrepancy would be less likely to be attributable to structural or textural differences. Also, metallic nickel dispersed in the SP-1 graphite at concentrations of 303 ppm and 1.4 wt.% increased its gasification rate by factors of 10 and 4 x 10 respectively. 

Nickel dispersion, metal area and crystallite size were determined by hydrogen chemisorption method [5]. 

This indicates that increasing the concentration of nickel oxide in the catalyst beyond 40% will not have any positive effect. For any catalyst, the catalytic activity varied with the dispersion of the metal, more the dispersion, more active is the catalyst. With high nickel content agglomeration of nickel crystallites takes place and it forms larger nickel crystallites (Table-1) causing lower nickel dispersion. 

Chemical composition, nickel dispersion, nickel area and the crystallite size... 

Among four different supports, a - AI2O3, y - AI2O3, Kieselguhr and MgO, nickel on a-Al203 is the best choice for catalytic hydrogenation of benzene. The activity of benzene hydrogenation increases with the increase of nickel concentration upto 40% NiO, after which increase of concentration the opposite trend is noticed. Similarly, an optimum value of nickel dispersion, metal area and its crystallite size have also been observed. 

After fabrication, the density of membrane was measured by dimension calculation. The crystalline phases of cermet membrane were investigated by using X-ray diffraction (XRD model JEOL, JDX-3530). Scanning electron microscope (SEM JEOL, JSM-6301F) was used to investigate the microstructure of Ni/AbOj membrane. For analysis of nickel dispersion of Ni/AbOj membrane, energy dispersive X-ray spectroscope (EDS Oxford Inca 300 and 350) with X-ray dot mapping was used. In addition, the pore size and porosity were determined by mercury porosimetry. 

The XRD analysis of the reduced catalysts shows for Catalyst (Ni-I) the lines at 26 = 44.5 and 51.85 which correspond to Ni°. The absence of these peaks in (Ni-PD) catalyst suggest a high nickel dispersion. 

The nickel dispersion of the catalyst on alumina support was less than that on silica support. This may be due to the strong interaction between nickel and alumina and undeveloped support pore structure than that of silica support. However, high catalytic activity and resistance to carbon deposition were obtained on the nickel catalyst supported on alumina. This indicated that metal dispersion was not the decisive factor that influenced the catalyst performance. Actually, the catalytic performance of the catalysts were integrative effect of nickel loading, metal dispersion, support, promoter, preparation and activation. 

Table 1 presents the main XRD and TPR results for calcined catalysts. The time evolution of nickel dispersion (D/Dq) and size distributions (Figures 1 and 2) confirms sintering for all catalysts, albeit at different extents. D/Dq vs. time data was fitted to Equation 1. The results are summarized in Table 2. For runs E6 and E13 no satisfactory fitting was obtained, thus the initial sintering rate was calculated from the variation of D/Do up to 5 h.

The autocatalytic nickel deposition can also be used for alloy deposition (Ni-Co, Ni-Fe) and for the deposition of nickel dispersion coatings (Ni/SiC). The nickel film can be heat-treated, below or well above 280 °C, the transition temperature amorphous/crystalline. The hardness increases up to 400 °C (1000-1200 HV) and then decreases again. Especially the corrosion protection... 

E. Endoh, M. Nakao, and Y. Takechi, Raney Nickel Dispersion-plated Low Hydrogen Overvoltage Cathode, In Proc. Vol. 99-21, The Electrochemical Society Inc., Princeton, NJ (1999), p. 245. 

Methane or natural gas steam reforming performed on an industrial scale over nickel catalysts is described above. Nickel catalysts are also used in large scale productions for the partial oxidation and autothermal reforming of natural gas [216]. They contain between 7 and 80 wt.% nickel on various carriers such as a-alumina, magnesia, zirconia and spinels. Calcium aluminate, 10-13 wt.%, frequently serves as a binder and a combination of up to 7 wt.% potassium and up to 16 wt.% silica is added to suppress coke formation, which is a major issue for nickel catalysts under conditions of partial oxidation [216]. Novel formulations contain 10 wt.% nickel and 5 wt.% sulfur on an alumina carrier [217]. The reaction is usually performed at temperatures exceeding 700 °C. Perovskite catalysts based upon nickel and lanthanide allow high nickel dispersion, which reduces coke formation. In addition, the perovskite structure is temperature resistant. 

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