NiAl2O4 Spinel Research Articles

Magneto-Optical and Catalytic Properties of Recyclable Spinel NiAl 2 O 4 Nanostructures Using Facile Combustion Methods

Spinel nickel aluminate (NiAl 2 O 4) nanospheres (NSPs) and nanoplatelets (NPLs) were successfully synthesized using a facile microwave combustion method (MCM) and conventional combustion method (CCM), respectively, using urea as the fuel. The as-prepared samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED), UV–Visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and vibrating sample magnetometer (VSM) analysis. Powder XRD, FT-IR, EDX, and SAED results showed that the samples were pure phase spinel NiAl 2 O 4 nanocrystals without any other secondary phase impurity. UV–Vis DRS and PL spectroscopy results were used to calculate the energy band gap (E g) value of the samples which is 3.41 and 3.08 eV for NiAl 2 O 4 NSPs and NiAl 2 O 4 NPLs, respectively. VSM results of the NiAl 2 O 4 NPLs sample show lower M s (49.16 × 10 −3 emu/g) values compared with the NiAl 2 O 4 NSPs sample (73.42 × 10 −3 emu/g), which confirm that both the products have ferromagnetic behavior. NiAl 2 O 4 NSPs were found to have higher surface area than NiAl 2 O 4 NPLs, which in turn leads to the improved performance towards the selective oxidation of benzyl alcohol into benzaldehyde, with 100 % selectivity, and it was found that the NiAl 2 O 4 NSPs sample show higher conversion (96.25 %) than NiAl 2 O 4 NPLs (85.37 %). The as-prepared samples show high activity, good reusability, remarkable stability, and environmentally friendly materials for industrial and technological applications.

Bioethanol combustion with CO2 capture in a 1 kWth Chemical Looping Combustion prototype: Suitability of the oxygen carrier

The use of a renewable liquid fuel, such as ethanol, in a Chemical Looping Combustion (CLC) process, in the context of carbon capture and sequestration (CCS) technologies can lead to carbon negative power system. This work presents experimental results obtained in a continuously operating CLC unit (1kWth) using ethanol as fuel during more than 100h. Complete ethanol conversion was reached under all operating conditions. The combustion efficiency as a function of the oxygen carrier to ethanol ratio was determined for four oxygen carriers: Cu14–γAl, Ni21–γAl, Ni18–αAl and Fe20–γAl. The oxygen carriers were prepared by impregnation on γ- and α-Al2O3. The performance of these materials was evaluated on the basis of their reactivity with ethanol and its intermediate gaseous products. The suitability of the use of the oxygen carriers was finally decided by taking into account oxygen transport capacity and hydrodynamic criteria. Cu14–γAl and Ni18–αAl fulfill all the requirements to successfully conduct a CLC process and almost 100% CO2 capture efficiency was obtained. On the contrary, the Ni21–γAl oxygen carrier was discarded due to the low reactivity of the spinel NiAl2O4 at normal operating conditions. Iron-based materials are suitable for the CLC process, although higher Fe-content or higher operating temperatures (≈950°C) would be necessary to make use of the Fe20–γAl material feasible.

Effect of Hf 4+ Concentration on Oxygen Grain‐Boundary Diffusion in Alumina

The kinetics of oxygen grain‐boundary diffusion in alumina was studied as a function of HfO2 concentration. The oxidation of Ni marker particles to NiAl2O4 spinel was utilized to delineate the position of the oxidation front. The HfO2 doping levels spanned the solubility limit, ranging from 100 to 2000 ppm (Hf:Al concentration). For each dopant level, the parabolic rate constant (k) was determined for oxidation anneals carried out at 1400°C. Relative to undoped alumina, HfO2 doping slowed the oxygen transport kinetics by a factor of approximately 3–8, depending on the dopant concentration. At 1400°C, the solubility limit of HfO2 was found to be between 100 and 200 ppm. The results showed that the level of benefit saturated at the dopant level corresponding to the solubility limit (sollim), where 100 ppm < sollim < 200 ppm. The results of the transport experiments were also examined with respect to the fractional grain‐boundary coverage (f), as opposed to overall HfO2 content. An approximate linear relationship between the rate of oxygen transport and f was observed, which can be rationalized in terms of a site‐blocking model.

Sulfur adsorption and release properties of bimetallic Pd–Ni supported catalysts

The formation of hydrogen sulfide in car exhaust is undesirable due to unpleasant odor and toxicity of H2S gas. H2S release can be suppressed by the addition of a NiO scavenger to a three-way catalyst (TWC). In this work, Pd–Ni bimetallic catalysts were prepared by the co-addition of Pd and Ni to γ-Al2O3 or Al2O3-La2O3 support, by the impregnation method. Different concentrations of a propionic acid aqueous solution were used as the impregnation solvent. The structure of prepared catalysts was characterized by Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature-programmed reduction (TPR) techniques. Catalyst poisoning by SO2 was simulated under lean conditions and H2S release under rich conditions. XRD and TPR measurements revealed the effect of the impregnation solvent concentration on the ratio between NiO and NiAl2O4 spinel species and the reducibility of Ni species. Co-addition of Pd with Ni was proven to be beneficial for H2S suppression. Prepared bimetallic catalysts released considerably less H2S compared to physical mixtures of Pd/Al2O3 with NiO. The presence of bulk and well dispersed NiO on Pd–Ni catalysts assisted in sulfur release in the form of sulfur oxides rather than H2S. Bimetallic catalysts supported on Al2O3-La2O3 were found to release more H2S compared to catalysts on γ-Al2O3. The use of diluted solvent in bimetallic catalysts preparation decreased H2S release from Pd–Ni catalysts.

Multiscale model of metal alloy oxidation at grain boundaries.

High temperature intergranular oxidation and corrosion of metal alloys is one of the primary causes of materials degradation in nuclear systems. In order to gain insights into grain boundary oxidation processes, a mesoscale metal alloy oxidation model is established by combining quantum Density Functional Theory (DFT) and mesoscopic Poisson-Nernst-Planck/classical DFT with predictions focused on Ni alloyed with either Cr or Al. Analysis of species and fluxes at steady-state conditions indicates that the oxidation process involves vacancy-mediated transport of Ni and the minor alloying element to the oxidation front and the formation of stable metal oxides. The simulations further demonstrate that the mechanism of oxidation for Ni-5Cr and Ni-4Al is qualitatively different. Intergranular oxidation of Ni-5Cr involves the selective oxidation of the minor element and not matrix Ni, due to slower diffusion of Ni relative to Cr in the alloy and due to the significantly smaller energy gain upon the formation of nickel oxide compared to that of Cr2O3. This essentially one-component oxidation process results in continuous oxide formation and a monotonic Cr vacancy distribution ahead of the oxidation front, peaking at alloy/oxide interface. In contrast, Ni and Al are both oxidized in Ni-4Al forming a mixed spinel NiAl2O4. Different diffusivities of Ni and Al give rise to a complex elemental distribution in the vicinity of the oxidation front. Slower diffusing Ni accumulates in the oxide and metal within 3 nm of the interface, while Al penetrates deeper into the oxide phase. Ni and Al are both depleted from the region 3-10 nm ahead of the oxidation front creating voids. The oxide microstructure is also different. Cr2O3 has a plate-like structure with 1.2-1.7 nm wide pores running along the grain boundary, while NiAl2O4 has 1.5 nm wide pores in the direction parallel to the grain boundary and 0.6 nm pores in the perpendicular direction providing an additional pathway for oxygen diffusion through the oxide. The proposed theoretical methodology provides a framework for modeling metal alloy oxidation processes from first principles and on the experimentally relevant length scales.

New insight into electrochemical-induced synthesis of NiAl2O4/Al2O3: Synergistic effect of surface hydroxyl groups and magnetism for enhanced adsorptivity of Pd(II)

A new promising adsorbent, Ni supported on γ-Al2O3 was prepared in a simple electrolysis system (Ni/Al2O3-E) in minutes and was compared with the sample prepared by a physical mixing method (Ni/Al2O3-PM). The adsorbents were characterized by XRD, TEM, FTIR, 27Al MAS NMR, XPS, and VSM. The results showed that besides NiO nanoparticles, a NiAl2O4 spinel was also formed in Ni/Al2O3-E during the electrolysis via the dealumination and isomorphous substitution of Ni2+ ions. In contrast, only agglomerated NiO was found in the Ni/Al2O3-PM. Adsorption test on removal of Pd2+ ions from aqueous solution showed that the Pd2+ ions were exchanged with the hydrogen atoms of the surface–OH groups of both adsorbents. Significantly, the Ni/Al2O3-E demonstrated a higher adsorption towards Pd2+ ions than Ni/Al2O3-PM due to its remarkably higher degree of magnetism, which came from the NiAl2O4. The use of 0.1gL−1 Ni/Al2O3-E gave the maximum monolayer adsorption capacity (qm) of 40.3mgg−1 at 303K and pH 5. The Ni/Al2O3-E showed high potential for simultaneous removal of various noble and transition metal ions and could be also used repetitively without affecting the high adsorptivity for Pd2+ ions. This work may provide promising adsorbents for recovery of various metals as well as other materials for such related applications.

Transmission Electron Microscopy of Meta-stable Al2O3 Scale Formed on NiCoCrAl Coated Layer During Isothermal Oxidation

Metastable Al2O3 scale formation on NiCoCrAl coated layers was investigated during initial-stage reaction for up to 100 h exposure at 800 °C in air. NiCoCrAl was diffusion-coated onto low carbon steel by electroplating a NiCo layer followed by pack-cementation deposition of Cr and Al. The microstructural characterization of as-coated and oxidized samples was carried out at the micro- and nano-scales. The XRD and diffraction pattern in TEM analysis revealed the presence of metastable Al2O3, spinel NiAl2O4 and cubic (Cr,Al)2O3 in the oxide. The phase identification using XRD and electron diffraction analysis associated with the coating composition was plotted on an isothermal oxidation map.

Study of the dry reforming of methane and ethanol using Rh catalysts supported on doped alumina

Various MxOy–Al2O3 oxides (M=Zr, Mg, Ni, Ce, La), prepared by a sol–gel method, were used as supports for rhodium catalysts in methane and ethanol dry reforming. After calcination, the Rh/MxOy–Al2O3 catalysts present BET surface areas between 85 and 208m2g−1. For the CH4 dry reforming, the highest yields in hydrogen are obtained in the following ranking: Rh/NiO–Al2O3>Rh/Al2O3>Rh/MgO–Al2O3≈Rh/CeO2–Al2O3>Rh/ZrO2–Al2O3>Rh/La2O3–Al2O3, not very different from the one for ethanol dry reforming at 800°C which is: Rh/NiO–Al2O3≫Rh/Al2O3≈Rh/MgO–Al2O3≈Rh/CeO2–Al2O3>Rh/ZrO2–Al2O3≈Rh/La2O3–Al2O3, although the reaction temperature is different. For the two reactions, the best catalyst is Rh/NiO–Al2O3 even if it deactivates during the reaction due to carbon deposition. This high activity may be explained by both the presence of the NiAl2O4 spinel phase, avoiding the deactivation of rhodium by migration in alumina, and the high dispersion of Rh favored by the presence of nickel particles at the support surface. Moreover, Ni particles on the support surface also contribute to the activity as demonstrated by the high hydrogen yield obtained in the presence of NiO–Al2O3, which is similar to the one obtained with Rh/Al2O3.

Highly active and stable Ni-based bimodal pore catalyst for dry reforming of methane

Bimodal pore NiCeMgAl catalysts were synthesized via the refluxed co-precipitation method. Methane reforming using carbon dioxide over this bimodal pore NiCeMgAl catalyst for syngas production was systematically studied by optimizing the active component NiO-loading, calcination temperature, reduction temperature and gas hourly space velocity (GHSV). The Ni15CeMgAl sample with 15wt% NiO loading, was found to be active enough at 750°C with a high CH4 conversion of 96.5%. The proper reduction temperature for the NiCeMgAl catalyst is either 550–650°C or 850°C. Higher calcination temperature favors the formation of NiAl2O4 and MgAl2O4 spinel structures. The Ni active sites derived from the NiAl2O4 spinel structure had longer stability than those from the free NiO. Compared with non-bimodal pore NiCeMgAl catalyst, bimodal pore NiCeMgAl catalyst has a longer stability in the feed gas without dilution. The large pores in the bimodal pore Ni15CeMgAl catalyst were supposed to contribute to the quick molecule transfer during the dry reforming of methane (DRM) reaction when the GHSV was less than 96,000h−1. The evolution of the Ni15CeMgAl catalyst before and after the DRM reaction was investigated by BET, XRD, TEM, and TGA techniques. A schema of the DRM reaction on the bimodal pore Ni15CeMgAl catalyst was proposed, and the correlation between the structure evolution and catalytic performance change was also discussed.

Chemical‐Looping Reforming of Methane Using Iron Based Oxygen Carrier Modified with Low Content Nickel

AbstractFe2O3/Al2O3 and Fe2O3/Al2O3 modified by low content of Ni (below 2% in weight) oxygen carriers were prepared by mechanical mixing and impregnation method. The synthesized oxygen carriers were characterized by means of X‐ray diffraction (XRD), X‐ray fluorescence (XRF), scanning electron microscopy (SEM), BET‐surface area and temperature programmed reduction (TPR). Besides, redox cyclic reactivity and the performance of chemical looping reforming of methane of the oxygen carriers were studied in a thermal gravimetrical analysis (TGA) and fixed bed at 850°C. It was observed that the redox reactivity of the oxygen carriers is improved by Ni addition because synergic effect may occur between NiO and Fe2O3/Al2O3 to form NiFe2O4 and NiAl2O4 spinel phases. However, the improvement was not apparent as Ni addition reached 1 wt% or more because more nickel loaded resulted in methane decomposition into H2 and carbon leading to carbon deposition. The SEM and BET analysis showed that NiFe2O4 and NiAl2O4 particles dispersed into the pores of the Fe2O3/Al2O3 particles in the course of preparation. In addition, the resistance to sintering of the modified samples increased with the Ni addition increasing. The results of successive redox cycles showed that the Ni modified Fe2O3/Al2O3 oxygen carriers have good regenerability. With integration of reactivity and carbon deposition, the content 1.04 wt% of nickel doping was an optimal amount in the three modified samples.

Ni-Mg-Al solid basic layered double oxide catalysts prepared using surfactant-assisted coprecipitation method for CO2 reforming of CH4

Ni-Mg-Al solid basic catalysts for CO2 reforming of CH4 were prepared using a surfactant-assisted coprecipitation method. The preferred orientations of the surfactants on the Ni(111) and Ni(200) crystal planes were investigated. The catalytic performance of the surfactant-modified catalysts was tested at 800 °C. The cetyltrimethylammonium bromide (CTAB)-modified catalyst (CB-LDO; LDO = layered double oxide) was further studied at various reaction temperatures. All the catalysts were characterized using Fourier-transform infrared spectroscopy, X-ray diffraction, temperature-programmed reduction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and temperature-programmed oxidation. The results show that growth of the Ni(200) plane is promoted by tetrapropylammonium hydroxide and restrained by P123, PVP, and CTAB. The crystallinity degree of Ni(200) plays a key role in the activation of CH4. The CB-LDO catalysts retain high activities and stabilities, because of the crystal phase transformation at high temperature during the reaction; this leads to the formation of spinel NiAl2O4 and exposure of the Ni(200) crystal plane.

In situ local environment and partitioning of Ni2+ ions during crystallization of an oxyfluoride glass

The local structure of Ni2+ ions during crystallization of an oxyfluoride aluminosilicate glass was examined by high-temperature in situ spectroscopic experiments coupled with in situ X-ray diffraction to characterize the different crystallization steps. We show that Ni2+ ions in the glass are located in the silicate glass network. After heat treatment Ni2+ ions do not partition into the fluorine crystallites as observed for rare-earth metals. Instead, we observed the crystallization of a NiAl2O4 crystalline phase, a largely inverse spinel. The in situ spectroscopic results (XRD, UV–Vis–NIR and Ni K-edge XANES) give new insights on the nickel partitioning between the supercooled liquid and the new crystals and show that the inversion degree of NiAl2O4 spinel during its crystallization depends not only on the temperature but also on the annealing timescale. We also show that the addition of fluorine into aluminosilicate systems favors the formation of spinel crystals at lower temperature than usually observed, thereby playing a role to promote nucleation.

Pyridine adsorption on NiSn/MgO–Al2O3: An FTIR spectroscopic study of surface acidity

The acid–base properties of MgO–Al2O3 supports and NiSn/MgO–Al2O3 catalysts were evaluated by IR spectroscopy using pyridine as a probe molecule. The results indicate that only Lewis acid sites were detected on the surface of the supports as well as on the catalysts. Nevertheless, Brønsted acid sites were not detected. In the support without MgO three kinds of coordinatively unsaturated acid sites were detected: Al3+ cations occupying octahedral, tetrahedral and tetrahedral with cationic vacancy in the neighbourhood. The last sites appear as the strongest. Moreover, they are able to activate the pyridine molecules leading to the formation of an intermediate α-pyridone complex. When MgO or NiO were added to the alumina, the number and strength of the Lewis acid sites decreased and significant changes were observed in the tetrahedral sites with adjoining cation vacancies. The incorporation of the Mg2+ cations into the alumina's structure takes place on the vacant tetrahedral positions, forming spinel MgAl2O4. As a result, the fraction of tetrahedral sites with adjoining cationic vacancy diminished and the intermediate α-pyridone complex in the support with the highest MgO loading was hardly detected. The addition of Ni2+ cations leads to the filling of the free octahedral positions, resulting in the formation of a NiAl2O4 spinel structure and the thermal stability of the α-pyridone species decreases. In the catalysts, the progressive reduction of the number and strength of the Lewis acid sites is due to a competitive formation of the two types of MgAl2O4 and NiAl2O4 spinels. In the catalyst NiSn/30MgO–Al2O3 no cationic vacancies were detected and the surface reaction with α-pyridone formation did not occur.

Highly efficient NiAl2O4-free Ni/γ-Al2O3 catalysts prepared by solution combustion method for CO methanation

Ti-doped Ni/γ-Al2O3 catalysts with Ti loadings varying from 0wt% to 5wt% were prepared in glycol by solution combustion method for CO methanation. Compared to Ni/γ-Al2O3 catalyst, Ti-doped Ni/γ-Al2O3 catalysts exhibited higher activity. In particular, an optimal Ti-doped Ni/γ-Al2O3 catalyst with Ti content of 3wt% (G-3Ti) achieved almost 100% CO conversion and ∼98.7% CH4 selectivity at 300°C, 0.1MPa, and a WHSV of 12,000mLg−1h−1, and showed good stability at 600°C. On analysis of characterization results, Ti species was found effectively restrict the formation of NiAl2O4 spinel phase, leading to a higher utilization of Ni species and thus more exposed active surfaces, which enhanced the CO adsorption capacity. In addition, the electron cloud density of Ni was increased by electron transfer from TiOx, which could facilitate the dissociation of CO on the catalyst surfaces. Moreover, solvents (i.e., ethanol, n-propanol and glycol) also significantly affected the physical–chemical properties of the catalysts, and catalyst prepared in glycol exhibited better catalytic performance for CO methanation.

The Effect of MgO Precoating on the Perfomance of Ni/MgO-Al<sub>2</sub>O<sub>3</sub> Catalysts for Carbon Dioxide Reforming of Methane

Ni catalysts supported on γ?-Al2O3 supports modified by different MgO precoating weight were prepared by impregnation method. The surface area, pore diameter and Ni crystallite size were investigated by N2 adsorption/desorption (BET) and X-ray diffraction (XRD) methods. The reducing temperature and quantity information of reducible nickel species were obtained by H2-TPR experiments. The CO2 adsorption/desorption abilities of catalysts were investigated and the activity of catalysts were also measured. The coke resistance performance of catalysts for carbon dioxide reforming of methane (CDRM) was studied by CO2 temperature programmed surface reaction (CO2-TPSR). H2-TPR results revealed that there were three kinds of Ni species on the supports. Appropriate modification amount of MgO could increase the surface nickel active sites which could be easily reduced and prevented the formation of NiAl2O4 spinel species. CO2-TPD indicated that the addition of MgO increased the amount of CO2 adsorption at lower temperatures. N2 adsorption/desorption results disclosed that the excessive modification of MgO would decrease the surface area and pore size of the samples. The reforming reaction results showed that the activity was elevated from 58.62% to 65.95% over the Ni/MgO-Al2O3 when the Mg/Al ratio was 0.05 (wt.) and there was less coke evidenced by the CO2-TPSR results comparing to the Ni/γ?-Al2O3 catalyst.

Hydrogen production via methane decomposition over Al2O3–TiO2 binary oxides supported Ni catalysts: Effect of Ti content on the catalytic efficiency

Alumina–titania binary oxides with different TiO2 contents (15%, 25% and 50%) have been prepared via ammonia gas diffusion method. These oxides were applied as supports for 60wt% Ni which was also precipitated by the same method and tested for methane thermal decomposition to COx-free hydrogen production. Different analytical techniques such as XRD, BET, TPR, Raman spectra, TGA and TEM were performed for characterization the catalysts before and after the catalytic runs. Incorporation of TiO2 in the catalysts facilitates the reduction of the catalysts due to the low formation of NiAl2O4 spinel structure. The Ni/TiO2(25%)–Al2O3 exhibited better activity and stability for catalytic decomposition of methane where about 59% hydrogen yield is obtained at 390min. TEM images showed that the catalysts produced different types of carbon nanostructured materials such as carbon nanofiber, graphene nanosheets and carbon nanotubes depending on the concentration of TiO2 in the catalysts.

Vapor-Phase-Mediated Phenomena Associated with High Temperature, High Water Content Oxidation of MCrAlX Bond Coats

Oxidation of MCrAlX (M = Ni and Co; X = Y or Re) bond coats was carried out at 1,125 °C in a range of N2–O2–H2O environments. A three-step process of (1) oxidation, (2) taper-polishing, and (3) re-oxidation was used to evaluate steady state development of thermally grown oxide (TGO). During initial oxidation, transient (Ni,Co)(Al,Cr)2O4 spinel formed above α-Al2O3. Following taper-polishing, no new spinel grew during 1–200 h of re-oxidation in any water vapor environment; spinel growth at the TGO surface by a steady state mechanism—owing to Al-depletion of the bond coat, as predicted elsewhere—was deemed unlikely. Observations of transient spinel volatilizing in wet environments were supported by measurements of nickel volatilizing from pre-fabricated NiAl2O4 spinel pellets as a function of humidity. In some cases, following volatilization, water promoted vapor phase-redeposition of spinel onto adjacent specimen surfaces. Spinel-related conclusions from past humid oxidation experiments for which volatilization phenomena were not considered—and especially for which Al-depletion of the bond coat is cited as the cause for spinel growth—should be reevaluated.

Use of a Ni Interlayer to Improve the Thermal Cycling Reliability of Cu/Sapphire Bilayers

The thermal cycling reliability of ceramic/metal laminates is critical for their applications in microelectronic modules. A eutectic bonding process is used to prepare copper/sapphire bilayers in this study. Due to elastic and thermal mismatches between alumina and copper, the Cu/sapphire bilayer cannot pass a thermal cycling test. The thermal cycling reliability can be improved through the use of a metallic nickel interlayer. During the bonding process, the nickel interlayer was oxidized first, reacted with alumina to form a NiAl2O4 spinel phase. The thermal diffusivity of the bilayer with spinel interphase remains high after the temperature cycling test.

The Bottom Up Approach is Not Always the Best Processing Method: Dense α‐Al2O3/NiAl2O4 Composites

The bottom up approach suggests that atomic scale mixing should permit optimal control of processing for many types of materials in terms of densification rates, final and average grain sizes; and, thereafter, global properties thereby minimizing processing conditions, capital equipment requirements, and energy consumption. The literature indicates that, to date, this axiom has not been tested although numerous researchers have whole‐heartedly adopted the concept. Liquid‐feed flame spray pyrolysis (LF‐FSP) provides atomically mixed NiO·3Al2O3 nanopowders(30–40 nm average particle size, APS) that are a single phase, spinel solid solution. Sintering 56 ± 1 wt% CIPped pellets to 95 ± 1% theoretical density using two different heating schedules produces α‐Al2O3/NiAl2O4 composites with grain sizes of 0.9 ± 0.2 µm. For comparative purposes, ball milled Al2O3 are synthesized from both highly atomically‐mixed single phase metastable spinel nanopowders and NiAl2O4 30–40 nm APS pellets of the same composition with a submicro­meter length scale mixing and similar green densities are also sintered to 95 ± 2% TD. In both instances, the same microstructures are realized despite the great difference in length scale of mixing. This contrasts greatly with the expectation that the atomically mixed materials would give finer grain sizes at the same densities and with faster sintering times, suggesting that the bottom up approach is not always valid.

New synthesis strategies for Ni/Al2O3-Sil-1 core–shell catalysts for steam reforming of methane

The catalytic performance of Ni/Al2O3-Sil-1 core–shell catalysts was investigated for the steam reforming of methane (SRM). The core–shell catalysts consisting of Ni/Al2O3 core catalyst and silicalite-1 (Sil-1) zeolite shell were prepared by different synthesis routes. Catalyst characterization indicates that repeated calcination at elevated temperatures during the catalyst preparation should be avoided to prevent the formation of the inactive NiAl2O4 spinel phase. Thus, the new preparation procedure for the core–shell catalyst involves first growing a Sil-1 shell on the alumina beads to obtain Al2O3-Sil-1 core–shell beads, followed by an incipient wetness impregnation to obtain a Ni/Al2O3-Sil-1 catalyst. The catalytic activity of the catalyst for the SRM reaction displayed ca. 10% improvement over core–shell catalysts prepared by traditional method. The prepared catalysts maintained high reactivity even when exposed to alkali vapor during reaction.