Large NiO Research Articles

Ordered Mesoporous Silica to Study the Preparation of Ni/SiO2 ex Nitrate Catalysts: Impregnation, Drying, and Thermal Treatments

In this contribution, we investigated the preparation of Ni/SiO2 catalysts with aqueous [Ni(OH2)6](NO3)2 solutions via the impregnation and drying method using ordered mesoporous silica SBA-15 (mesopore diameter of 9 nm) as model support to study each step in the preparation: impregnation, drying, calcination, and reduction. After impregnation, not all the mesopores of SBA-15 appeared filled with precursor solution. Consecutive drying led to formation of 9 nm Ni3(NO3)2(OH)4 crystallites exclusively within the mesopores. During air calcination, severe sintering and redistribution took place, resulting in a low NiO dispersion, including large NiO crystals outside of the mesopores and rodlike NiO particles inside the mesopores. The degree of sintering depended on the concentration of Ni3(NO3)2(OH)4 decomposition products (NO2, N2O, O2 and H2O), and in particular NO2 and O2 were found to promote sintering and redistribution. Therefore, maintaining low concentrations of the latter components during the thermal...

Solution Precursor Plasma Spray of Nickel-Yittia Stabilized Zirconia Anodes for Solid Oxide Fuel Cell Application

In conventional plasma spray of SOFC components, the large NiO and YSZ particles used, about 50-150 microns for high porosity coating, reduce the density of three-phase sites for electrode reaction. In this article, the SPPS process was used to synthesize and deposit Ni-YSZ anodes. The results show that several process parameters have significant effects on the microstructure and phase composition of the deposited material. The deposits were composed of tower-like, irregularly shaped agglomerates and smooth surface deposits. The sizes of the agglomerates increase with the decrease of the plasma-torch power and most are not completely molten during the impact. After heat treatment to reduce the NiO present in the as deposited coatings, the coatings were found to contain spherical YSZ particles about 0.5 μm in diameter distributed in a continuous Ni matrix, which is verified by both SEM observation and electrical resistance measurement. The coatings have 30-50% porosity.

Study of the growth of NiO on highly oriented pyrolytic graphite by X-ray absorption spectroscopy

In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10 −5 Torr) at room temperature. We paid special attention to the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni 2+ form, as supported by multiplet calculations. In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by atomic force microscopy (AFM) in this system.

Direct observation of magnetic instabilities in NiO thin films epitaxially grown on Fe(0 0 1)

We study, by means of X-ray magnetic linear dichroism (XMLD) and photoemission electron microscopy (PEEM), the magnetic instabilities in the NiO/Fe(0 0 1) system, as a function of the NiO film thickness. For NiO films thinner than a critical thickness of about 10 ML the NiO AFM moments align in-plane perpendicular to the Fe substrate magnetization. Just above the critical thickness the coupling turns out to be collinear. The behavior for very large NiO thicknesses (up to about 80 ML), studied by applying an external magnetic field, shows a complete reversal of the NiO domains, for a 90° rotation of the substrate magnetization. Furthermore, a decreasing trend of the NiO XMLD signal as a function of the AFM film thickness is shown. These findings are discussed and tentatively explained in terms of defects occurring at the interface and/or in the NiO volume.

Photocatalytic Water Splitting on Ni-Intercalated Ruddlesden−Popper Tantalate H2La2/3Ta2O7

A series of Ruddlesden−Popper-type hydrous layered perovskites, ATa2O7 (A‘ = H or K, A = La2/3 or Sr), were presented as novel catalysts for photocatalytic water splitting into H2 and O2 under UV irradiation. These hydrous perovskites showed higher activity than anhydrous perovskites (KTaO3, La1/3TaO3) for overall splitting of water. Results of photoluminescence spectroscopy and H2 evolution from aqueous n-butylamine solution support the hypothesis that the high activity of the hydrous perovskites results from their hydrated layered structure, where the photogenerated electrons and holes can be effectively transferred to the intercalated substrates (H2O and n-butylamine). Addition of Ni cocatalyst to H2La2/3Ta2O7 via an ion-exchange reaction increased the activity, while Ni addition did not improve the activity of H2SrTa2O7. Ni K-edge EXAFS/XANES, UV−vis spectroscopy, and TEM results for Ni-loaded catalysts indicate that Ni2+ ions and small NiO clusters are intercalated into the layers of H2La2/3Ta2O7, while relatively large NiO particles at the external surface of the perovskite are the main Ni species in H2SrTa2O7. Thus, the highly dispersed Ni(II) species at the interlayer space are shown to act as effective sites for H2 evolution in the photocatalytic water splitting.

Support effects on the catalytic behavior of NiO/Al 2O 3 for oxidative dehydrogenation of ethane to ethylene

Six representative Al 2O 3 supports with different specific surface areas and pore volumes were used to prepare NiO/Al 2O 3 catalysts with two NiO loadings. Oxidative dehydrogenation of ethane (ODE) to ethylene was investigated over these catalysts. The yield of ethylene was found to be approximately proportional to the pore volume/surface area ratio of the support used for that catalyst. X-ray diffraction analysis (XRD), TEM and H 2-TPR were employed to characterize their structure differences. It was found that the physical properties of the Al 2O 3 supports were crucial to the dispersion of NiO. More large crystal NiO was found on the Al 2O 3 supports with lower pore volume, while more highly dispersed NiO was formed on the Al 2O 3 supports with higher pore volume. An interpretation based on the pore volume of the supports and the physical properties of salt precursors was proposed.

Corrosion behavior of Fe-20Cr and Ni-20Cr alloys in Ar-H2O-HBr gas mixtures at 1000 K

Discontinuous mass-change measurements and corrosion-product analyses were made for Fe-20Cr and Ni-20Cr alloys after exposing them to Ar-H 2 O-HBr gas mixtures at 1000 K for 24 hours. Predominantly chromia scales formed on both alloys. Upon cooling, the scales remained adherent to the Fe-20Cr alloy but spalled extensively from Ni-20Cr samples. After tests in HBr-rich gas mixtures, bromine-rich corrosion products were found underneath chromia scales on both alloys while nickel evaporation was observed from Ni-20Cr samples. Preoxidation of the Ni-20Cr alloy prior to exposure to Ar-H 2 O-HBr gas mixtures increased chromia scale adherence but did not prevent nickel loss from the alloy. Chromia scales formed on the Fe-20Cr alloy were more protective due to the absence of iron oxides in the scale. Large NiO crystals formed over the Ni-20Cr alloy decreased chromia-scale adherence and increased nickel loss from the alloy due to the low stability of NiO in HBr-containing gas mixtures.

The Structure of Aggregated NiO Particles in NiO/SiO<sub>2</sub> Particles Prepared by CVD Method

The structure of NiO particles in the NiO/SiO2 agglomerates prepared by CVD was investigated by XRD, TEM, XPS and TPR. TEM observation and XRD analysis indicated that the NiO particles obtained were in the aggregated state, and that the size of them became large, while that of primary particles became small, with increasing preparation temperature. TPR and XPS analyses indicated that a trace amount of nickel silicate coexisted with the NiO/SiO2 particles, most likely at the outer region of the NiO aggregates near the interface with the SiO2 deposited on them. From these results and the hydrogen reduction experiments, we proposed the following structure model of the NiO/SiO2 agglomeration; at high preparation temperature small primary NiO particles grow into large secondary particles, followed by coating with thin SiO2 layers, while at low temperature large primary NiO particles grow and are surrounded by SiO2 matrix.

Electron beam radiation effects on NiO crystals

It is well known that various electron beam induced effects can occur under high energy electron beam irradiation on the studied material in an electron microscope. It has been reported that under electron beam irradiation small metallic Ni particles can be reduced from the bulk NiO crystals and these Ni crystals are always in epitaxial relationship with the NiO substrate. These Ni particles may be formed by the preferential knock-out of oxygen atoms by the incident electrons and the subsequent aggregation of Ni atoms to form the nucleii. These nuclei will grow with further electron beam irradiation, forming the small Ni islands on the surface of the NiO crystals. Recently, we observed that not only does the reduction process occur on the NiO surface but also other kinds of interactions occur at the same time. First, large NiO crystals can be melted and flow away from the radiated area in an electron microscope. The process is radiation dose dependent.

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.