Nickel Crystallite Size Research Articles

Characterization of [formula omitted] catalysts by TEM, X-ray diffraction, and chemisorption techniques

Nickel crystallite sizes calculated from H 2, CO, and O 2 chemisorption are compared to those determined from X-ray diffraction (XRD) and transmission electron microscopy (TEM). Good agreement is obtained among all techniques for typical Ni SiO 2 and Ni Al 2 O 3 catalysts, whereas sizes calculated from H 2 and CO chemisorption are far too large for Ni TiO 2 catalysts. In contrast, oxygen chemisorption provides values in excellent agreement with those from XRD and TEM, and indicates that monolayer coverages of CO and H 2 are much lower on TiO 2-supported nickel. This agreement also implies that oxygen monolayer coverage on TiO 2-supported nickel does not exceed unity regardless of crystallite size, whereas coverages of two or higher are observed on SiO 2- and Al 2O 3-supported nickel, in agreement with previous studies. Isobars for CO and H 2 adsorption on Ni TiO 2 clearly show that activated adsorption is not responsible for the low CO and H 2 uptakes at 300 K. Low-contrast nickel crystallites on titania are observed in the TEM micrographs, indicating that nickel may exist with a raft-like morphology on this support. These results provide evidence that strong metal-support interaction behavior exists in the Ni TiO 2 system.

Determination of metal crystallite size and morphology in supported nickel catalysts

The application of H 2 chemisorption, X-ray diffraction (XRD) line broadening, and transmission electron microscopy (TEM) to the determination of metal crystallite size and size distribution in Ni SiO 2 , Ni Al 2 O 3 , and Ni TiO 2 catalysts having wide ranges of nickel loadings and dispersions was investigated. Average crystallite diameters estimated from H 2 chemisorption and TEM were found to be in very good agreement over wide ranges of metal dispersion and loading in the Ni SiO 2 system and in good agreement for a 15% Ni Al 2 O 3 ; poor agreement was evident in the Ni TiO 2 system, the results suggesting that H 2 adsorption was suppressed. In the few samples where it was possible to obtain information from XRD, the estimates of crystallite diameter were generally in good or fair agreement with those from H 2 chemisorption or TEM. The specific limitations of these three techniques in determination of nickel crystallite size and their application to the study of sintering and metal support interactions in supported nickel catalysts are presented and discussed.

Crystal structure of platinum and nickel foils as a function of substrate temperature during deposition

The empirically determined conditions for making strong, self-supporting foils, 100–300 μg/ cm 2, of nickel and platinum by electron-gun vacuum evaporation include substrate temperatures of 250–350°C. X-ray diffraction studies have been made to determine what changes in crystal structure with increasing temperature are correlated with foil strength. Nickel crystallite size was found to be nearly infinite at the optimum substrate temperature but orientation remained relatively random. Platinum exhibited increasing crystallite size with temperature and also a highly oriented structure whose planar alignment was determined by the substrate.

Structure effects in the interaction of hydrogen with benzene, cyclopropane and carbon monoxide over silica supported nickel catalysts

AbstractStudies on the catalytic interaction of hydrogen with three substrates on a series of silica supported nickel catalysts with average crystallite sizes ranging from 5 to 50 A are reported. On benzene the specific activity is almost constant down to about 12 A crystallite size. For smaller crystallite sizes the activity drops sharply. For hydrogenation and hydrogenolysis of cyclopropane a similar drop in activity is observed for very small crystallites and these also tend to have lower selectivity towards hydrogenolysis. These findings are discussed In terms of the required ensemble size, inspired by work on alloys elsewhere. For methanation of carbon monoxide an almost linear increase of specific activity with crystallite size over the entire range has been found, which implies almost constant activity per unit mass of nickel metal for all catalysts. In the course of many hours methanation heavy carbon deposition and growth of average nickel crystallite size occur, without significant change in activity. Interpretation of these findings in terms of required ensemble size does not appear very satisfactory. Much further work is needed to arrive at a satisfactory explanation

Alkali promotion of nickel catalysts for carbon monoxide methanation

Differential reactor studies of carbon monoxide hydrogenation have been carried out on a series of 8% Ni SiO 2Al 2O 3 catalysts in which the supports have been impregnated with increasing amounts of sodium. Magnetic measurements show that the extent of reduction and the nickel crystallite size are not influenced by the sodium on the support. Hydrogen adsorption measurements reveal that the exposed surfaces are in agreement with the crystallite sizes and are not significantly poisoned by the alkali. The turnover number increases with sodium content and passes through a maximum at 0.3% Na with an enhancement by a factor of 6. Kinetic data best fit the surface reaction mechanism for which rate = kK CO P CO P H 1 2 H (1 + K CO P CO ) 2 For K CO, the pre-exponential factor and heat of adsorption are unchanged by Na, but the parameters of k are not. The pre-exponential factor passes through a maximum and the activation energy decreases as the Na level increases. These results are best explained by an initial decrease in acidity, which keeps the nickel surface clean, followed by a poisoning via direct metal interaction or support modification by the alkali.

Nickel crystallite size and net activity of hydrogenation catalysts

Abstract and SummaryAmong supported nickel‐based hydrogenation catalysts, the Ni crystallite size apparently plays a secondary role in net hydrogenation activity for undistilled tallow fatty acids and nonselective hydro‐genation of oxidized soybean oil. The nickel crystal‐lite size measured by the x‐ray diffraction profile broadening technique of Scherrer varied between 55 A and 150 A. The commercial catalyst with the small‐est nickel crystallite size, in the samples studied, was not the most active for hardening soybean oil, while fatty acid hydrogenation showed a large crystallite catalyst to have the highest activity. Since the percent reduced nickel used in catalytic hydrogenation is not well known if the Ni/NiO ratio is poorly defined, relative activities were then correlated with qualita‐tive x‐ray diffraction measurements of the Ni/NiO values. Again, there was no trend in activity as a func‐tion of Ni/NiO. This apparent puzzle is probably due to real differences in the micro structure of the catalyst support. A series of experimental reductions using a common green catalyst led to very good cor‐relations between net activity for fatty acid hydro‐genation and the crystallite size and Ni/NiO ratio. On a given support, the crystallite dimension can be modified by the reduction treatment and is not sharply fixed by the selection of nickel salt and sup‐port. If the stoichiometric ratio of hydrogen is lowered, the crystallite dimension is reduced, but so is the qualitative efficiency of reduction (Ni/NiO), with the result that an exceptionally small crystallite size catalyst may be less active than one with larger crystals, but with more reduced Ni/unit weight.

The catalytic synthesis of hydrocarbons from [formula omitted] mixtures over the group VIII metals : IV. The kinetic behavior of CO hydrogenation over Ni catalysts

The catalytic behavior of unsupported nickel and nickel dispersed on a variety of supports has been studied in the CO H 2 synthesis reaction. The catalysts were characterized by chemisorption and X-ray diffraction measurements. Specific activities and product distributions were determined and kinetic parameters for the methanation reaction were obtained. The specific activity is sensitive to the support material but the variation in activity is less than an order of magnitude. There is also evidence that maximum activity may occur over a certain nickel particle size range. The kinetic parameters are relatively insensitive to the support; however, dispersing nickel on any of the supports studied enhanced the formation of higher molecular weight hydrocarbons compared to unsupported nickel. These differences in catalytic behavior are attributed to changes in the adsorbed state of CO on nickel surfaces which are a result of metal-support interactions or variations in nickel crystallite size.

Ultrahigh magnetic field measurements of nickel crystallite size distributions

Dispersed nickel supported on silica and alumina has been characterized with ultrahigh field magnetization measurements. A superconducting solenoid operable up to 100 kOe was used for this purpose. The samples were superparamagnetic so that the computation of crystallite size distributions from the magnetic data was possible. A computational technique was developed for this purpose and tests on model distributions carried out. Much better results were obtained than hitherto possible. The catalysts, containing 40 wt% Ni, were initially bidisperse, with both low and high radii grouped distributions. Evidence for both crystallite migration in the case of the small particles and interparticle transport for the larger radii was detected. Measured and calculated surface areas were in agreement.

Studies of supported metal catalysts: II. The effect of dispersion on methane-deuterium exchange over supported nickel

The exchange reaction between methane and deuterium has been studied over a series of nickel-silica catalysts. Meaningful reaction rates were obtained between 225 and 250 °C on impregnated catalysts which varied from 2 to 50% nickel by weight. Multiple exchange was promoted by increased reaction temperature and increased nickel crystallite size. The apparent activation energy of the stepwise and multiple exchange reactions proved to be a function of nickel crystallite size: for the stepwise reaction the apparent activation energy increased from 12 to 27 kcal/mole with increasing crystallite size; the multiple exchange activation energy showed a corresponding increase from 20 to 33 kcal/mole.

Equilibria of decomposition reactions of carbon monoxide and methane over nickel catalysts

The equilibria of the decomposition reactions of carbon monoxide and methane were studied in the temperature range 450–700 °C on various nickel catalysts by means of a thermogravimetric apparatus. As previously, equilibrium constants were obtained which imply deviations from constants based on graphite data. The equilibrium constants varied from catalyst to catalyst and were correlated with nickel crystallite size. Examination in the electron microscope showed a whisker-like structure of the carbon formed, and that the diameter of the whisker was very close to that of the nickel crystallite attached to it. Deviations from graphite data may be explained by a disordered structure of the carbon and by a higher surface energy depending on the whisker diameter. The chemical composition and the activity of the catalyst appeared to be of minor importance for the deviations from graphite data. The reactions were prevented by sulfur chemisorbed on the nickel surface.

Measurement of the state of metal dispersion on supported nickel catalysts by gas chemisorption

Gas chemisorption has been investigated as a technique for the estimation of the state state of nickel dispersion on supported nickel hydrocarbon conversion catalysts. Published chemisorption studies on nickel catalysts are critically reviewed. Catalysts which were examined included nickel on alumina and nickel zeolites. Gas chemisorption measurements were supplemented by X-ray diffraction line-broadening determinations to establish the average nickel crystallite sizes. Nickel areas estimated from hydrogen chemisorption at 250°C at a hydrogen vapor pressure of 100 mm mercury and from average nickel crystallite size determined by X-ray diffraction line-broadening were in good agreement. These results provide strong evidence that dissociative hydrogen adsorption occurs only on nickel crystallites of sufficient size to offer adjacent hydrogen adsorption sites. Carbon monoxide chemisorption at room temperature at a carbon monoxide vapor pressure of 100 mm mercury, after correction for carbon monoxide adsorption on the support, provides the more accurate measure of total nickel area including nickel in an atomic state of dispersion. A combination of hydrogen chemisorption or X-ray diffraction line-broadening measurement, plus carbon monoxide chemisorption, can be used to estimate the relative amounts of nickel in comparatively large crystallites (∼50 Å) and in an essentially atomic state of dispersion, as well as the specific surface area of the nickel in these two size ranges.

Catalysis over supported nickel. Effect of support at low metal levels on benzene hydrogenation

The effect of the support at low metal levels on the kinetics of the hydrogenation of benzene to cyclohexane have been studied over a series of nickel catalysts. Two different supports, silica and silica-alumina, were used, and the nickel concentration was varied in the 1–10 % range. Kinetic studies were made in a differential flow reactor at temperatures in the range 70–200°C. The activity and apparent activation energies decreased with decreasing nickel content. At the lower metal levels the nickel supported on silica-alumina was less active per unit area of nickel than was nickel supported on silica. The difference between the specific activity of the nickel on the two supports increased as the metal level decreased, indicating that the effect of the support is strongest at lower metal levels. The results do not support the contention that the effect of nickel concentration on catalyst activity results from the fact that a minimum nickel crystallite size is necessary for benzene hydrogenation.