Nickel Crystallites Research Articles

A Monte Carlo model for the sintering of Ni/Al 2O 3 catalysts

A mathematical model was developed to describe the sintering of nickel crystallites in Ni/Al 2O 3 catalysts used for steam reforming. Atom migration and particle migration were considered along with many contributions from phenomenological models previously described in the literature. The model combines both mechanisms according to a Monte Carlo approach. Size-dependent surface and interface properties and the influence of surrounding atmosphere and temperature were accomplished. Model predictions qualitatively describe experimentally observed behavior.

The structural transitions during leaching of Ni2Al3 phase in a Raney Ni-Al alloy

The microstructure transitions during leaching of a rapidly solidified Ni-Al alloy have been investigated by means of X-ray diffraction, transmission electron microscopic (TEM) and high resolution electron microscopic (HREM) techniques. Ni2Al3 was the main phase in the starting Ni-Al alloy. The microstructure of the Raney nickel catalyst consists of nano-scale nickel crystallites, residue of source phases surrounded by nano-scale boundary regions. The transformation during leaching of Ni2Al3 phase was an advancing interface type process. Clusters of AuCu-structure type face-centrered tetragonal Ni3Al2 as an intermediate phase seems to appear in the reaction front. Based on an analysis of the atomic configurations of phases Ni2Al3, Ni3Al2 and nickel, a reasonable explanation for the transition mechanism during leaching of Ni2Al3 phase and the arc characteristic of diffraction spots was proposed. The nickel crystallites generated during leaching obey an orientation relationship with the source Ni2Al3 phase, which is consistent with the Delannay's orientation relationship proposed for nickel and NiAl phases. The nano-scale structural characteristic of the Raney nickel catalyst, especially its porous structure at the boundary regions, provides an excellent hydrogenation catalytic activity and selectivity of the catalyst. (C) 2001 Kluwer Academic Publishers.

Characterization of rice husk ash-supported nickel catalysts prepared by ion exchange

The rice husk ash-supported nickel catalysts prepared by ion exchange technique and the support itself have been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). In addition to BET surface area, total pore volume and average pore diameter were also measured. The metal surface area and the crystal size of nickel were measured by the selective chemisorption of hydrogen. The effect of calcination temperature on the catalyst reduction was investigated by the temperature-programmed reduction (TPR) technique. The results show that nickel silicate with a layer structure formed after drying step. The BET surface area increases with the nickel loading of the catalyst. The nickel surface area increases with nickel loading up to 16.7 wt.% Ni and then decreases with further increase in nickel loading. The dispersion of nickel gradually decreases with nickel loading. Furthermore, the mean size of nickel crystallites increases with nickel loading. The reduction temperature increases with the increase of calcination temperature. Reduction of NiO from the thermal decomposition of the layered nickel silicates is found to be unusually difficult.

Deactivation of steam reforming catalysts by sintering: experiments and simulation

Sintering of coprecipitated Ni/Al 2O 3 catalysts was studied in a microreactor at 500 and 800°C under steam reforming reaction. Changes in microstructure (crystalline phases, average metal crystallite sizes and size distributions) were investigated by temperature programmed reduction (TPR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Differences between particle size distributions determined by XRD and TEM are discussed in view of their specific limitations. Low temperature calcination of unpromoted catalysts may have a beneficial stabilizing effect upon reduction. The rate of sintering of small nickel crystallites increases as a result of the combined action of steam and high temperature, particularly for narrow initial crystallite size distributions and high initial dispersions. This behavior suggests a shift, from crystallite migration in the initial stages of heat treatment, to atomic migration in the latter stages. The lanthanum promoter seems to inhibit metal crystallite growth under reduction and reaction conditions. Predictions of a developed Monte Carlo based model are in good qualitative agreement with experimental observations.

Characterization of nanosized nickel prepared by surface reactions of Ni‐σ‐organyl complexes on silica: an electron paramagnetic and ferromagnetic resonance study

Dispersed atomic nickel(0) is formed during the reaction of the nickel-σ-organyl complexes with silanol groups at temperatures below 373 K. That nickel is oxidized to Ni(I) by protons of the silanol groups in a consecutive step. The Ni(I) portion amounts to about 70% w/w of the Ni used. Six different Ni(I) species are detected by electron paramagnetic resonance. They are stabilized by interaction with the silica surface and the organic moieties; they act as anchor ions for the Ni(0) atoms. Ni(0) crystallites stabilized in this way are about 0.5 nm in diameter after a treatment at 373 K. The influence of the Ni(I) ions on the collective, magnetic properties of the clusters is revealed by calculation of ferromagnetic resonance (FMR) spectra using the independent-grain approach according to Schlömann and Kotyukov. A strain of about 10 GPa is brought about in the nickel crystallites by the interaction with Ni(I) ions. © 1998 John Wiley & Sons, Ltd.

Interaction between nickel and molybdenum in Ni–Mo/Al 2O 3 catalysts: I : CO 2 methanation and SEM-TEM studies

A series of alumina supported Ni, Mo and Ni–Mo catalysts were prepared for investigating the interaction between Ni and Mo in terms of their structural properties and catalytic activities. The catalysts containing 0–15 wt% Ni and/or Mo were prepared by impregnation or coimpregnation and were tested for BET surface area and total pore volume. CO chemisorption was used for metal surface area measurements. The catalytic activities of the samples were tested by CO 2 methanation. Selected Ni/Al 2O 3 and Ni–Mo/Al 2O 3 catalysts with Ni and Mo loadings in the 5–15 wt% range were also examined by TEM and SEM-EDX. The results show that Mo changes the structural properties and the methanation activities of the catalysts. TEM and SEM-EDX studies support the synergetic interaction between Ni sites and MoO x species and show that the nature of the Ni–Mo interaction changes with the level of Ni and Mo loading. At low Mo loadings, Mo species act as a diluent of the matrix, promoting Ni particle distribution. At higher Mo loadings, partial coverage of the nickel particles is observed especially on catalysts with high total metal loading. SEM backscattered composition images indicate the presence of molybdenum in the oxide form which means that the active sites are the nickel crystallites.

Catalytic Behavior of Graphite Nanofiber Supported Nickel Particles. 1. Comparison with Other Support Media

The hydrogenation of 1-butene and 1,3-butadiene has been carried out over a series of supported nickel catalysts at 80 °C and atmospheric pressure. It was found that not only the activity but also the selectivity of nickel crystallites could be dramatically altered when the metal was dispersed on graphite nanofibers compared to the performance obtained with more conventional support materials, such as active carbon and γ-alumina. Transmission electron microscopy examinations showed that the metal was evenly distributed over the graphite nanofiber surfaces, and in general the particles adopted a well-defined thin flat hexagonal shape. In contrast, the crystallites formed on active carbon and γ-alumina did not acquire the same well-defined morphological features; however, on average, they were considerably smaller than those generated on the nanofiber surfaces. The most dramatic feature was the fact that in the oxide supported nickel system the average particle size was about 5 times smaller than that for the same metal loading on the graphite nanofibers. Consideration of the particle size distributions in conjunction with the catalyst reactivity data indicates that hydrogenation of either 1-butene or 1,3-butadiene is not directly related to metal dispersion. It is suggested, instead, that differences in the behavioral patterns of the catalyst systems are related to the observed modifications in metal particle morphological characteristics induced by the chemical and structural properties of the support materials. In this context, consideration must also be given to the possibility that the support can induce electronic perturbations in the metallic component, and this feature could be most prominent with a conductive material such as graphite nanofibers.

Structural Properties of Chemically Synthesized Nanostructured Ni and Ni:Ni3C Nanocomposites

We have used a reductive technique known to produce highly reactive metals to fabricate nickel and nickel-based nanostructured materials. The strong dependence of the magnetic, chemical, electrical, and optical properties of nanostructured materials are intimately correlated with material structure; thus, thorough knowledge of the effect of synthesis parameters on the structure is critical for the refinement of fabrication techniques. X-ray diffraction and electron microscopy are used to determine the effect of the synthetic conditions and subsequent processing on the material structure. Characteristic lengths of these materials range from 3 to 50 nm, depending on synthesis and annealing conditions. Annealing produces a metastable Ni3C phase that forms only in the presence of active carbon, suggesting that not only active nickel but also active carbon results from this process. The addition of P(Ph)3 affects the time and temperature dependence of the nickel crystallite growth, the temperature at which Ni3C crystallites are first observed and the maximum temperature to which Ni3C can be retained.

Influence of surface support properties on the liquid-phase selective hydrogenation of phenylacetylene on supported nickel catalysts

The role of textural and acid–base properties of supports on metal–support interaction effects has been evaluated through the results obtained in the liquid-phase selective hydrogenation of phenylacetylene carried out on 20 wt% supported nickel catalysts. SiO2, Al2O3, AlPO4, active carbon and a natural sepiolite were used as supports. The effects of Ni–Cu alloying were also studied. The relative adsorption constants, K_T,D, and the relative reactivities, RT,D, obtained in the consecutive hydrogenation process, were used to follow the effects of any electron transfer between the nickel atoms and acid–basic sites on the support surface. Structural defects of supported nickel crystallites were determined by X-ray diffraction line broadening analysis and used for measuring the participation of geometric effects of supported nickel crystallites in the catalytic activity. Results obtained may be better explained within a framework of simultaneous electronic and structural metal–support interaction effects.

Iron-silica and nickel-silica nanocomposites prepared by high energy ball milling

Metal-silica nanocomposites with different metal volume fractions have been prepared via solid state exchange reactions induced by ball milling followed by a reduction treatment in H2flux. In nickel-containing mixtures oxygen transfers directly from NiO to Si while NiO is reduced to Ni. When NiO is present in a large ratio, its excess can be reduced by a thermal treatment in H2flux. Nickel crystallites are obtained with nanometer size in the milling process and there is no significant growth during thermal treatment. Similar process conditions applied to Fe-containing mixtures give rise to a more complex reaction path which prevents the complete conversion of Fe(III) to Fe. Nickel-silica and iron-silica nanocomposites are also produced by ball milling mixtures of either nickel or iron with amorphous silica.

The reactivity of carbon-containing species on nickel. Effects of crystallite size and related phenomena

A pronounced variation of the reactivity of sub-monolayers of carbon-containing species deposited from synthesis gas with the nickel crystallite size is observed. Nickel crystallites of about 4 nm display the presence of a carbon-containing ad-layer of maximum reactivity. The rate of methanation is discussed in terms of the reactivity of the carbonaceous fragments, and the catalyst preparation procedure.

Nickel catalysts for internal reforming in molten carbonate fuel cells

Natural gas may be used instead of hydrogen as fuel for the molten carbonate fuel cell (MCFC) by steam reforming the natural gas inside the MCFC, using a nickel catalyst (internal reforming). The severe conditions inside the MCFC, however, require that the catalyst has a very high stability. In order to find suitable types of nickel catalysts and to obtain more knowledge about the deactivation mechanism(s) occurring during internal reforming, a series of nickel catalysts was prepared and subjected to stability tests at 973 K in an atmosphere containing steam and lithium and potassium hydroxide vapours. All the catalysts prepared showed a significant growth of the nickel crystallites during the test, especially one based on α-Al 2O 3 and a coprecipitated Ni/Al 2O 3 sample having a very high nickel content. However, this growth of nickel crystallites only partially explained the very strong deactivation observed in most cases. Only a coprecipitated nickel/alumina catalyst with high alumina content and a deposition-precipitation catalyst showed satisfactory residual activities. Addition of magnesium or lanthanum oxide to a coprecipitated nickel/alumina catalyst decreased the stability. Adsorption and retention of the alkali was the most important factor determining the stability of a catalyst in an atmosphere containing alkali hydroxides. This is because the catalyst bed may remain active if a small part of the catalyst bed retains all the alkali.

Nickel Electro- crystallization: Influence of Unsaturated Organic Additives on the Mechanism of Oriented Crystal Growth

The addition of organic compounds to the electrolytic baths during the electrodeposition of metals affects the structure and the properties of the deposits, leading to the preparation of coatings with defined characteristics. In this work, a complete study of the action of the organic additives on the crystalline orientation of the nickel electrodeposits was carried out. The selection of the organic compounds was made in order to investigate the effect of the stereochemical structure and unsaturated bonds on the nickel electrocrystallization process. It was found that the presence of acetylenic compounds (butyne-2-diol 1,4 and propyne-2-ol) in nickel plating baths leads to the predominance of high inhibited modes of crystal growth. Propyne-2-ol seems to be more active, as the geometry of its molecule favours the adsorption of the additive on the metallic surface and, thus, its faster hydrogenation. With the additon of phenylpropyl alcohol, a strong perturbation of the oriented crystal growth of the nickel crystallites occurs. The adsorption of the phenyl group on the metallic surface and its hydrogenation is responsible for the predominance of high inhibited textures. In the case of coumarin, the complexity of its molecule makes its adsorption more difficult, resulting in the development of [100] textured deposits even at high concentrations of the additive. Finally, when sodium benzene sulphonate or saccharin are added to the bath, the presence of sulphur in their molecules influences the nickel electrodeposition process through a different mechanism.

Surface structure of bulk nickel catalysts, active in the gas-phase hydrodechlorination reaction of aromatics

Several bulk nickel catalysts obtained from different procedures have been structurally characterized using BET, XRD, SEM, TPR and TPD techniques. XRD and SEM clearly show diffentiated crystalline morphologies on the oxidized and reduced nickel phases obtained from the three catalysts studied, in agreement with the BET surface areas. TPR and TPD results show the reducibility behaviour, and that mainly atomic hydrogen chemisorbs onto the catalysts with the better crystallized and larger nickel crystallites. The catalytic activities depicted by this study for the hydrodechlorination reaction of ortho-dichlorobenzene show that all the bulk nickel catalysts tested yield conversions and selectivities towards benzene of >80% at temperatures >493 K. Also, the catalysts with larger octahedral crystallites showed higher TOF values and selectivities toward benzene than those with poorly crystallized structures at any reaction temperature in the range 523–393 K. The behaviour is interpreted in terms of a structure-sensitive reaction. Also, a reversible/irreversible temperature-dependent mechanism of nickel chloride formation is proposed.

In situ scanning tunnelling microscopic study of nickel electrodeposition on HOPG

The early stages of the electrodeposition of nickel on highly oriented pyrolytic graphite (HOPG) were investigated by in situ scanning tunnelling microscopy, scanning electron microscopy and electrochemical measurements. Experimental results showed that the electrodeposition of nickel on HOPG followed an island growth mechanism. An individual island was composed of some larger grains which could be observed by both STM and SEM. However, a number of small grains, so-called subgrains, could be observed only by STM. In situ and real-time observation of the island growth revealed that the formation of the subgrains resulted from the progressive nucleation and 3D growth around the kinks of nickel crystallites. It was found that the nuclei were formed in a specific crystallographic direction, which led to the propagation of the long-range order crystallites. During the continuous tracing of the progress of nickel deposition, the formation of new islands was not observed. This reproducible phenomenon meant that the nuclei acting as the “growth centres” of the islands were formed instantaneously in the initial period of electrodeposition.

Reduction of model steam reforming catalysts: NiO/α-Al2O3

The effect of NiO loading (5–21 wt.-%) on the reduction of NiO/α-A2O3 catalysts, prepared by multiple impregnation of nickel nitrate solution followed by calcination at 650°C has been characterized usingtemperature-programmed reduction, isothermal hydrogen consumption, magnetization, X-raydiffraction, and electron microscopy. X-ray diffraction analysis of fresh catalysts indicated normal NiO crystallites about 30 nm in size. Studies from 270°C to 350°C show hydrogen consumption at lower temperatures is faster than the subsequent growth of nucleated clusters of nickel atoms into crystallites, with the rates of the two processes approaching each other at higher temperatures. As NiO loading increases, chemical reduction becomes more difficult but nickel crystallite growth is not affected. This decreased reducibility is believed due to Al3+ ion incorporation into NiO surface layers during impregnation. Isothermal hydrogen consumption from 270°C to 450°C follows a shrinking core model with NiO crystallites decreasing progressively in size. Magnetic measurements show crystallite growth is dependent on diffusion-controlled nucleation. Decreasing hydrogen flow has a profound effect on chemical reduction and nucleation but not on growth. Similar results are found with added water vapor. X-ray diffraction and transmission electron microscope measurements reveal 23 nm nickel crystallites with some evidence for Al-Ni alloy formation. A mechanism is proposed in whichadsorbed water inhibits chemical reduction and nucleation, and foreign ions such as A13+ increase this effect.

Decoration of Nickel and Magnesium Oxide Crystallites with Spinel-Type Phases

The structure of the Al promoted magnesium and nickel oxides obtained by calcination of hydrotalcite-type coprecipitates was investigated by X-ray diffraction, 27 Al MAS-NMR, EXAFS, and alkaline leachings. In addition to the reflections of the MgO rock-salt type structure, the diffraction patterns of the Al modified MgO exhibited an additional, well-defined reflection at d=0.253 nm, attributable to cations on tetrahedral sites. The tetrahedrally coordinated cations were likely to belong to a nonstoichiometric spinel-type phase including an excess of magnesium. The same behavior was observed when Ga(III) were substituted for Al(III) ions in the materials

The Effect of Solution pH and Heat‐Treatment on the Properties of Electroless Nickel Boron Films

Electroless nickel boron films were investigated using metallizing solutions of various pH's and heat‐treatments at different temperatures. Solution pH controls the boron content in the as‐deposited films which in turn acts as a crystallization inhibitor. Heat‐treatments at different temperatures all result in the formation of face centered cubic nickel crystallites of varying dimensions and causes the migration of boron to the surface. Using radial distribution function techniques, the amount of residual stress in these films was also determined.

Characterization of several γ-alumina-supported nickel catalysts and activity for selective hydrogenation of hexanedinitrile

Studies of the Chemical preparation, Temperature-programmed reduction (TPR), powder X-ray diffraction (XRD), X-ray photoelectron (XP) spectra and catalytic activities of several Nickel/γ-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. The TPR measurements detect the inhibiting effect of γ-alumina on NiO reduction and also the formation of some kind of nickel aluminate at the NiO/γ-alumina interface at calcination temperatures > 623 K, which affects the final degree of reduction of the catalysts. The XRD detects the nickel phases present except that of the non-crystalline nickel aluminate. The XPS results indicate the presence of surface Ni2+ and reduced nickel, presumably in the form of nickel aluminate underneath the reduced surface nickel crystallites, particularly for higher nickel loadings. Catalytic conversions increase significantly with increase in nickel content and degree of reduction of the catalysts; the latter was highly resistant to metallic sintering. Selectivities of 100% towards azacycloheptane were obtained, where nickel aluminate may play a role in a decoration mechanism.

Metal—support interaction effects in the liquid-phase selective hydrogenation of 1,4-butynediol with nickel catalysts supported on AlPO 4 and on other conventional non-reducible compounds

The liquid-phase selective hydrogenation of 1,4-butynediol has been carried out on nickel catalysts at 20 wt.-% supported on different AlPO 4, AlPO 4Al 2O 3 and AlPO 4-SiO 2 systems. Furthermore, some conventional non-reducible compounds, such as SiO 2, Al 2O 3, active carbon or a natural sepiolite were also used as supports. In addition, the effects of nickel loading and NiCu alloying were studied using carbon as the metal support. The relative adsorption constants, K T,D, were also obtained from the relative reactivities, R T,D, obtained in the consecutive hydrogenation process and the corresponding individual hydrogenation rates of 1,4-butynediol and 1,4-butenediol. These values were used to follow the changes in the electronic structure of supported nickel crystallites. Besides, structural defects of supported-nickel crystallites such as ‘twin faults’, ‘stacking faults’ and ‘microstrains’, were determined by X-ray diffraction line broadening analysis. These parameters were used for measuring the participation of geometric support effects in the catalytic activity of supported nickel catalysts. TEM measurements coupled with digital image processing were also used to measure nickel particle sizes. Results obtained in the correlation between kinetic data and structural defects in supported-nickel crystallites as well as with textural and acid—basic properties of the supports indicated that electronic and geometric effects were simultaneously developed throughout metal—support interactions.