NiO Crystallites Research Articles

Characterization of microstructures in nickel alumina catalysts by analytical electron microscopy

Evolution of microstructures of Ni on γ- and α-alumina during thermal treatment in air and hydrogen atmospheres was studied using TEM, selected area diffraction and microdiffraction. Oxidation at 775 K led to the formation of nickel aluminate both in the case of γ- and α-alumina. Microdiffraction analysis showed an oriented growth of the NiO microcrystallites with respect to the γ-alumina substrate. The NiO phase was present even after reduction at 775 K. Nickel crystallites produced during reduction at 775 K were covered by nickel aluminate. The presence of the nickel aluminate phase in the close vicinity of NiO crystallites influences strongly the texture of NiAl 2O 3 microstructures during the reduction of the catalysts. Recrystallization or/and reduction of the nickel aluminate shells during reduction at temperatures ⩾ 875 K leads to the formation of voids between the nickel and the nickel aluminate (alumina) phases. The epitaxial relationship of the Ni microcrystals with the (100), (110), (111) and (112) surfaces of the γ-alumina has been established from electron diffraction and microdiffraction patterns. An increase of the Ni lattice parameter by 1% after reduction at 1075 K is interpreted by diffusion of aluminum atoms into the Ni lattice to form a solid solution.

Structural modifications of a spent molten carbonate fuel cell

Several performance and endurance tests have been carried out on molten carbonate fuel cells (MCFC) operating at different temperatures (873, 893 and 923 K). Electrochemical performance parameters, morphological characteristics and chemical analyses on samples of spent electrodes and tiles were evaluated. An output power decrease of 10% from the original value was taken to be the largest acceptable decay. Cell operation was stopped upon reaching this point. Electrolyte losses ranging from 18.6 to 24.5 of the original content were the determining factor in the MCFC performance decrease. Scanning electron microscopy showed that post-test anodes retained a satisfactory microstructure, while the tiles appeared to be very sensitive to the effect of operating temperature. The cathode structure, after an initialin situ oxidation, appeared to consist of agglomerates of 0.9 μm NiO crystallites. Long-term operation produces some modification of the pore size distributions of the used components, resulting in a final electrolyte distribution that was far from optimal. Due to the opposite effect of temperature on cell lifetime and power output, the highest power output and shortest cell lifetime were obtained for the 923 K test.

Structural steps to oxidation of Ni(100)

In this paper, we emphasize the temperature- and exposure-dependent development of low-energy electron diffraction patterns,measured quantitatively during oxidation of Ni(100) at 80 to 400 K. We find a strong temperature dependence in the development of LEED patterns associated with NiO. NiO(111) is favored by adsorption temperatures below 300 K, whereas a (7×7)-like structure is favored by adsorption temperatures of 300 to 400 K. Room temperature is a ‘‘crossover’’ point between these two forms of the oxide. The final oxide depth is independent of adsorption temperature and, therefore, of epitaxial orientation, between 80 and 400 K. When the sample is heated in vacuum after adsorption, massive rearrangements take place above 500 K. Some of the nickel reverts to metallic nickel covered by a c(2×2) oxygen overlayer, and some forms NiO crystallites which are probably deeper than the initial oxide skin. Effectively, the parent oxide disproportionates into a less-oxygen-rich phase and a more-oxygen-rich phase. This is again independent of the orientation of the initial oxide epitaxy.

In-situ HVEM studies of the early stages of oxidation of nickel and nickel-chromium alloys

In-situ oxidation of Ni, Ni-5at%Cr and Ni-25at%Cr single crystals of 〈100〉 orientation was studied by electron diffraction and contrast experiments both in a high voltage electron microscope environmental cell, and in conventional 100 keV transmission electron microscopes. Environmental cell studies were conducted at temperatures between 500 and 700 ° C, and at oxygen partial pressures of 2.7 Pa and 2.4 kPa Torr for periods up to 90 min. The oxidation process was characterized using bright-field, dark-field imaging and selected-area diffraction techniques, as well as X-ray energy-dispersive and electron energy loss spectroscopy. Oxidation was observed to begin within 40 s. Simultaneous nucleation of small NiO crystallites of 5 to 10 nm diameter and growth of bright rectangular facets were observed, eventually resulting in non-uniform sample thickness. The epitaxial relationships were determined for Ni 〈100〉 and Ni-5at%Cr 〈100〉. These results and the conditions for the formation of Cr 2O 3 are discussed and compared with those for oxidation processes in bulk alloys.

Influence of metal–support interactions on the stability of Ni/SiO2 catalysts during cyclic oxidation–reduction treatments

Ni/SiO2 catalysts prepared by impregnation and deposition–precipitation have been subjected to cyclic treatments in hydrogen at 500 °C and oxygen at 450 °C to test their resistance to sintering. The activity of the catalysts in benzene hydrogenation was tested after each cycle. The resulting behaviour was found to depend upon the method of preparation and whether the initial treatment of the precursor was reducing or oxidizing. For a catalyst prepared by impregnation and reduced initially, a strong reduction in catalytic activity was recorded as the number of cycles was increased; this behaviour is ascribed to facile sintering which occurs in the absence of an interaction between metal particles and the support. When the same precursor was calcined initially large NiO crystallites were formed on the external surface of the support; subsequent reduction–oxidation cycles acted to change their texture, making them more irregular or porous and resulting in an increased metallic surface area and catalytic activity. Catalysts prepared by deposition–precipitation were very resistant to sintering owing to strong interactions between the metal and support; at the precursor stage, a nickel hydrosilicate was formed which had a turbostratic morphology. Direct reduction of this compound gave rise to finely divided particles of metallic nickel trapped between the SiO2 support which retained its turbostratic morphology. Calcination of this precursor caused some nickel to enter the support, further anchoring the active phase and rendering it very resistant to sintering.

Surface structure and NiO dispersion for NiO/γ‐Al2O3 catalysts

AbstractChemisorption of CO at temperatures below 100°C has been used to evaluate the dispersion of NiO in a series of NiO/γ‐Al2O3 catalysts. CO adsorption measurements were supplemented by i.r. spectroscopy to establish the CO/NiO stoichiometry. From monolayer coverage data derived from Freundlich's equation, the percentage of NiO dispersion and the average NiO crystallite size were calculated. The crystallite size of supported NiO was found to be practically constant for NiO loadings from 1 to 8 wt%, experimenting an abrupt increase for higher NiO contents. These results were consistent with additional information on the surface structure of the catalysts obtained by IR spectroscopy, X‐ray diffraction, magnetic susceptibility, and reduction with H2.

Formation of epitaxial NiO by Oxygen implantation in [100] Ni

18O has been implanted at room temperature up to dosesof 4.4 × 10 17/cm 2 in a [100] Ni crystal. The implantation energy was 60 keV. Backscattering and channeling analyses at various implantation steps give evidence for oxygen migration towards the surface. After the overall implantation, X-ray analysis reveals the presence of NiO in epitaxy [100]//[100] with the substrate. Backscattering and channeling experiments tend to indicate that between the surface and R p the sample is formed of NiO epitaxial crystallites and of pure nickel leading to a mean stoichiometry NiO 0.75, constant in depth up to R p. The misfit between the Ni and NiO lattice parameters induces strains and extended defects formation. Between R p and R p + ΔR p the oxygen concentration decreases, and the channeling results show no preferential direction. This region corresponds probably to a highly disordered NiO alloy. This hypothesis is confirmed by annealing experiments: the oxygen present between R p and R p+ ΔR p migrates towards the surface at 400°C and a reordering of this region is observed. There is also an improvement of the structural quality of the NiO crystallites.

Physico-chemical characterisation of impregnated and ion-exchanged silica-supported nickel oxide

X-ray analysis, reflectance spectroscopy, analytical electron microscopy and X.p.s. measurements have been used to investigate the influence of the preparation method on the dispersion and interaction of nickel oxide on silica. Two series of solids containing up to 10 wt % NiO were prepared, respectively, by pore volume impregnation and ion-exchange methods. It was found that impregnated samples are poorly dispersed and contain essentially bulk-like NiO crystallites or aggregates. Only in the sample with the lowest nickel content (2 wt % NiO) may the presence of a second nickel species be deduced. Conversely, ion-exchanged samples are well dispersed and give no evidence of the presence of NiO, but rather contain a surface layer of nickel hydrosilicate. The results were used to account for the dramatic difference in the reducibility of impregnated and ion-exchanged samples.