Formation Of NiAl2O4 Research Articles

Effects of Steam on Ni/Al2O3 Catalysts for Ammonia Decomposition

Decomposition of gaseous NH3 from ammonia (NH3)-containing wastewater was explored using Ni-loading Al2O3 catalysts. The thermochemical decomposition of an NH3/steam mixture (wet-NH3) with different steam contents at 873, 923, and 973 K using a fixed-bed reactor under ambient pressure. The present results indicated that the catalysts can be deactivated by the formation of NiAl2O4, which can be thermodynamically generated, and confirmed that the extent of deactivation was greatly affected by the partial pressure of the steam (PH2O). The catalytic activities at 873 K decreased with increasing PH2O, whereas the activity was constant above PH2O of 25 kPa. However, the NH3 conversion was almost independent of the NH3 flow rate and temperature, and ∼30% of the NH3 was decomposed at each temperature. This study indicated that, even in the presence of steam, this catalyst could decompose NH3 from NH4+-containing water.

Aluminum oxide as a dual-functional modifier of Ni-based anodes of solid oxide fuel cells for operation on simulated biogas

Al2O3 and SnO2 additives are introduced into the Ni–YSZ cermet anode of solid oxide fuel cells (SOFCs) for operation on simulated biogas. The effects of incorporating Al2O3/SnO2 on the electrical conductivity, morphology, coking resistance and catalytic activity for biogas reforming of the cermet anode are systematically studied. The electrochemical performance of the internal reforming SOFC is enhanced by introducing an appropriate amount of Al2O3 into the anode, but it becomes worse with excess alumina addition. For SnO2, a negative effect on the electrochemical performance is demonstrated, although the coking resistance of the anode is improved. For fuel cells operating on biogas, stable operation under a polarization current for 130 h at 750 °C is achieved for a cell with an Al2O3-modified anode, while cells with unmodified or SnO2-modified Ni–YSZ anodes show much poorer stability under the same conditions. The improved performance of the cell with the Al2O3-modified anode mainly results from the suppressed coking and sintering of the anode and from the formation of NiAl2O4 in the unreduced anode. In sum, modifying the anode with Al2O3 may be a useful and facile way to improve the coking resistance and electrochemical performance of the nickel-based cermet anodes for SOFCs.

CO methanation over ZrO2/Al2O3 supported Ni catalysts: A comprehensive study

A series of ZrO2/Al2O3 composite oxides with different ZrO2 contents were prepared by impregnation method. The NiO/ZrO2/Al2O3 catalysts for CO methanation were prepared and tested in a tubular microreactor under the condition of T=300–650°C, P=1.5MPa, GHSV (gas hourly space velocity)=10,000h−1, and H2/CO=3. The results revealed that adding ZrO2 restrains the formation of NiAl2O4, weakens the Ni–Al2O3 interaction, and makes the catalyst more easily reduced, enhancing significantly the activity of catalyst. Especially, among these catalysts, 20Ni/40ZA catalyst with ZrO2 content of 40wt.% exhibits high CO conversion and CH4 selectivity in a broad temperature region of 300–550°C. Moreover, 20Ni/40ZA performs superior stability than 20Ni/Al2O3. The added ZrO2 also improves the anti-carbon deposition and anti-sintering performance of Ni/Al2O3 catalyst.

Preparation, characterization and catalytic properties of nickel aluminate nanoparticles: A comparison between conventional and microwave method

In the present work, synthesis of nickel aluminate using Opuntia dilenii haw as plant extract by a microwave combustion method (MCM) and its comparison with the conventional combustion method (CCM) is investigated. O. dilenii haw plant extract simplifies the process, provides an alternative process for a simple and an economical synthesis. The absence of surfactant has led to a simple, cheap and fast method of synthesis of NiAl2O4 nanoparticles. The as-synthesized NiAl2O4 nanoparticles were characterized by X-ray diffraction (XRD) studies, Fourier transform infrared spectroscopy (FT-IR) studies, high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray analysis (EDX), high resolution transmission electron microscopy (HR-TEM), diffuses reflectance spectroscopy (DRS), and Brunauer Emmett Teller (BET) surface area analysis. The XRD results confirmed the formation of the cubic phase NiAl2O4. The formation of pure nickel aluminate phase was confirmed by FT-IR. The formation of NiAl2O4 nanoparticles was confirmed by HR-SEM and HR-TEM and their possible formation mechanisms were also proposed. MCM could produce NiAl2O4 with uniform size and well-defined shape with crystallinity. The optical property was determined by DRS. NiAl2O4 prepared by the microwave combustion method was found to possess a higher surface area, lower crystallite size than the NiAl2O4 nanoparticles prepared by the conventional combustion method, which in turn has led to the improved performance toward the selective oxidation of benzyl alcohol to benzaldehyde.

Comparative investigation of nickel aluminate (NiAl2O4) nano and microstructures for the structural, optical and catalytic properties

Nickel aluminate (NiAl2O4) nano and microstructures were synthesized by both microwave combustion method (MCM) and conventional combustion method (CCM) using aloe vera as the plant extract. The synthesized NiAl2O4 nanoparticles were characterized by X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR) studies, high resolution scanning electron microscopy (HR-SEM), high resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDX), diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) studies. The XRD results confirmed the formation of a cubic NiAl2O4. The formation of NiAl2O4 nano and microstructures were confirmed by HR-SEM and HR-TEM. The optical absorption and photoluminescence emissions were determined by DRS and PL spectra, respectively. The effect of catalyst and solvent on the catalytic oxidation of benzyl alcohol using the NiAl2O4 catalyst prepared by both CCM and MCM was also investigated.

Relevance of the catalytic activity on the performance of a NiO/CaAl2O4 oxygen carrier in a CLC process

This paper presents a detailed investigation related to the physical and chemical changes underwent by a low Ni content oxygen carrier during 90h of continuous operation in a CLC plant [1]. The oxygen carrier reached a combustion efficiency of 99% with a metallic Ni inventory of 17kg/MWth, which was the lowest value found in literature for any kind of Ni-based oxygen carrier. The present study was carried out in order to find the reasons by which this material exhibited a different combustion behavior in relation to other Ni-based oxygen carriers. Fresh and after-used particles were characterized by several techniques. XRD, TPR-TPO and XPS techniques proved that the use of CaAl2O4 as oxygen carrier support avoided the formation of NiAl2O4, which is a less reactive Ni-based compound than NiO and always formed in other Ni-based materials. The fact that all the NiO impregnated was found as free NiO in the oxygen carrier particles implied that this material maintained a high reactivity after 90h of continuous CLC operation. The results obtained in the batch fluidized bed reactor have revealed the great relevance of the catalytic activity of the metallic Ni on the global fuel gas combustion process. The low Ni content present in this material makes necessary the use of very high reduction solid conversion to have enough metallic Ni in the fuel reactor to catalyze the CH4 reforming reaction, and therefore to obtain high combustion efficiencies. This result will become very relevant for the design of a CLC unit using this Ni-based material as oxygen carrier.

An investigation on the role of ytterbium in ytterbium promoted γ-alumina-supported nickel catalysts for dry reforming of methane

Addition of low quantities of ytterbium to sol–gel prepared Ni/γ-Al2O3 catalysts has been shown to lead to significant increases in catalytic activity and long term stability in the catalytic conversion of CO2 and CH4 into syngas (H2 and CO). The role of ytterbium in these catalysts was investigated in this study through detailed investigations on the structure and composition of ytterbium promoted Ni/γ-Al2O3 catalysts using the following techniques: synchrotron X-ray diffraction, X-ray Photoemission Spectroscopy, Transmission Electron Microscopy, Scanning Electron Microscopy/Energy Dispersive X-ray analysis, Temperature Programmed Reduction techniques and N2 adsorption–desorption isotherms. The results obtained indicated that ytterbium, at small quantities (up to 2 wt%), interacted strongly with the support which in turn altered the interaction between nickel and the support (most notably it was found to completely inhibit the formation of NiAl2O4). This decreased interaction between Ni and the support also led to a higher quantity of Ni being present in the catalyst in the form of Ni.

Influence of metal oxides on the performance of Pd/Al2O3 catalysts for methane combustion under lean-fuel conditions

Pd/Al2O3 catalysts modified by different metal oxides (Mn, Fe, La, Mg and Ni oxides) were prepared and tested for fuel-lean methane/air catalytic combustion. It was found that the support material influenced the combustion performance significantly. The addition of NiO or MgO effectively improved the activity and hydrothermal stability of catalysts. X-ray diffraction (XRD) results showed the presence of spinel phase in the Pd/Al2O3–NiO and Pd/Al2O3–MgO catalysts. Temperature-programmed desorption of ammonia (NH3-TPD) measurements indicated that the supports became weakly acidic because of the formation of NiAl2O4 or MgAl2O4 spinel. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed the well dispersion of Pd particles on the Al2O3–NiO and Al2O3–MgO supports. It was concluded that the formation of the NiAl2O4 or MgAl2O4 spinel phase during preparation would be beneficial for the methane catalytic performance of Pd catalysts.

Steam Reforming of Glycerol for Hydrogen Production Over Supported Nickel Catalysts on Alumina

The experiment was carried out to produce hydrogen through steam reforming of glycerol over nano-sized Ni catalysts supported on alumina (Al2O3). The catalysts were characterized by BET surface area, metal dispersion, XRD, TPR, NH3-TPD and SEM. 15 wt% Ni/Al2O3 catalysts presented carbon nano fiber after the catalyst was used. However, when the Ni loading was higher than that of 15 wt%, the catalytic activity reduced, and the increase of the Ni particle size and the formation of graphitic carbon occurred. The Ni/SiO2(70)-Al2O3 with the high surface area and the small Ni particle size promoted the catalytic activity and could easily reduce from NiO to Ni, inhibiting the formation of NiAl2O4.

Reaction between NiO and Al2O3 in NiO/γ-Al2O3 catalysts probed by positronium atom

NiO/γ-Al2O3 catalysts with NiO content of 9wt% and 24wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100°C to 1000°C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ3 and τ4, which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ4 and its intensity I4, which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600°C, abnormal increase of the lifetime τ4 is observed. This is due to the formation of NiAl2O4 spinel from the reaction of NiO and γ-Al2O3. The generated NiAl2O4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ4. Formation of NiAl2O4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.

Methane cracking using Ni supported on porous and non-porous alumina catalysts

Porous and non-porous alumina catalysts were used as nickel supports to catalyze methane cracking. Different operating parameters were studied in a thermal gravimetric analyzer, including methane and hydrogen partial pressures, temperature and flow rate. During CH4 cracking, carbon builds up on the catalyst surface and therefore the catalyst requires periodic regeneration. Cycling tests were performed, using air during the regeneration phase to burn off the carbon. The results showed that the non-porous catalyst performed better than the porous catalyst in terms of cracking during the first cycle. Full regeneration of the catalysts by oxidizing the deposited carbon was achieved at 550 °C, while oxidation was very slow at 500 °C. After full regeneration, the performance of the porous catalyst became considerably better than the non-porous. The porous catalyst kept its activity for 24 cracking/regeneration cycles, while the non-porous catalyst lost half of its activity by the second cracking cycle and almost all of its activity after six cycles. NiAl2O4 formation and Ni sintering caused the non-porous catalyst activity loss. TPO results showed that two carbon types were deposited on the catalysts, namely Cβ and Cγ, where Cβ is more active than Cγ.

10.2478/s11814-009-0227-2

Two types of CeO2-modified Ni/Al2O3 catalysts were prepared by a consecutive impregnation method with different sequences in the impregnation of Ni and CeO2, and their performance in autothermal reforming (ATR) of isooctane was investigated. Catalysts prepared by adding CeO2 prior to the addition of Ni, Ni/CeO2-Al2O3, produced larger amounts of hydrogen than those obtained using catalysts prepared by adding the two components in an opposite sequence, Ni-CeO2/Al2O3. The results of H2 chemisorption and temperature-programmed reduction revealed that added CeO2 increased the dispersion of the Ni species on Al2O3 and suppressed the formation of NiAl2O4 in the catalyst such that large amounts of Ni species were present as NiO, the active species for the ATR. The elemental and thermogravimetric analyses of deactivated catalysts indicated that Ni/CeO2-Al2O3, which showed a longer lifetime than Ni-CeO2/Al2O3, contained lesser amounts and different types of coke on the surface.

Alumina Doped Ni/YSZ Anode Materials for Solid Oxide Fuel Cells

AbstractThe Al2O3–Ni–YSZ (Y2O3 stabilised ZrO2) anode materials with 0–6 wt% Al2O3 were prepared by tape casting method after being ball‐milled for 48 h. The influence of Al2O3 content on flexural strength, electrical conductivity, open porosity, relative density and thermal expansion coefficient (TEC) of Al2O3–Ni–YSZ anode was investigated. The introduction of Al2O3 significantly enhances the flexural strength of Al2O3–Ni–YSZ anode. The flexural strengths of 430 and 299 MPa are achieved for the specimen containing 0.25 wt% Al2O3 before and after reduction, respectively, while the flexural strengths are 201 and 237 MPa for the Ni–YSZ samples. The density decreases with increasing Al2O3 content and the open porosity increases correspondingly, after being sintered at 1350 °C for 4 h. The electrical conductivity at ambient temperature does not fall off when Al2O3 content is less than 1 wt%, but decreases rapidly when the content is above 3 wt% due to the formation of NiAl2O4. A maximum electrical conductivity of 1418 S cm–1 is obtained in the sample containing 0.5 wt% Al2O3. The TEC of the samples decreases with the introduction of Al2O3 in the temperature range of 20–850 °C.

Effect of CeO2-addition sequence on the performance of CeO2-modified Ni/Al2O3 catalyst in autothermal reforming of iso-octane

Two types of CeO2-modified Ni/Al2O3 catalysts were prepared by a consecutive impregnation method with different sequences in the impregnation of Ni and CeO2, and their performance in autothermal reforming (ATR) of isooctane was investigated. Catalysts prepared by adding CeO2 prior to the addition of Ni, Ni/CeO2-Al2O3, produced larger amounts of hydrogen than those obtained using catalysts prepared by adding the two components in an opposite sequence, Ni-CeO2/Al2O3. The results of H2 chemisorption and temperature-programmed reduction revealed that added CeO2 increased the dispersion of the Ni species on Al2O3 and suppressed the formation of NiAl2O4 in the catalyst such that large amounts of Ni species were present as NiO, the active species for the ATR. The elemental and thermogravimetric analyses of deactivated catalysts indicated that Ni/CeO2-Al2O3, which showed a longer lifetime than Ni-CeO2/Al2O3, contained lesser amounts and different types of coke on the surface.

Effect of alumina additions on the anode|electrolyte interface in solid oxide fuel cells

The longevity of a solid oxide fuel cell (SOFC) stack is curtailed by the fragility of its ceramic components. At Ceramic Fuel Cells Limited (CFCL), 15wt.% alumina is added to the commonly used 10mol% Y2O3–ZrO2 (YSZ) electrolyte to improve both the fracture toughness and grain-boundary conductivity of the electrolyte. This study investigates the effect of such addition of alumina on the anode|electrolyte interface; more specifically, which reactions occur with the Al2O3 at the interface and how these reactions influence fuel cell performance. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the formation of NiAl2O4 in the alumina regions in the electrolyte. The NiAl2O4 is observed to grow into the adjacent grain boundaries to form an interconnected NiAl2O4 network up to 4μm deep into the electrolyte. Impedance spectroscopy shows that the formation of NiAl2O4 does not affect the grain bulk ionic conductivity. The grain-boundary conductivity is markedly reduced at low temperatures. However, at the high SOFC operating temperature at CFCL (850°C) the contribution of the grain-boundary conductivity to the total conductivity is diminished, and the NiAl2O4 is found not to have an effect on the total electrolyte conductivity and is deemed not to be a detrimental reaction.

Structure and Catalytic Properties of Ceria-based Nickel Catalysts for CO2 Reforming of Methane

Carbon dioxide reforming (CDR) of methane to synthesis gas over supported nickel catalysts has been reviewed. The present review mainly focuses on the advantage of ceria based nickel catalysts for the CDR of methane. Nickel catalysts supported on ceria–zirconia showed the highest activity for CDR than nickel supported on other oxides such as zirconia, ceria and alumina. The addition of zirconia to ceria enhances the catalytic activity as well as the catalyst stability. The catalytic performance also depends on the crystal structure of Ni–Ce–ZrO2. For example, nickel catalysts co-precipitated with Ce0.8Zr0.2O2 having cubic phase gave synthesis gas with CH4 conversion more than 97% at 800 °C and the activity was maintained for 100 h during the reaction. On the contrary, Ni–Ce–ZrO2 having tetragonal phase (Ce0.8Zr0.2O2) or mixed oxide phase (Ce0.5Zr0.5O2) deactivated during the reaction due to carbon formation. The enhanced catalytic performance of co-precipitated catalyst is attributed to a combination effect of nano-crystalline nature of cubic Ce0.8Zr0.2O2 support and the finely dispersed nano size NiO x crystallites, resulting in the intimate contact between Ni and Ce0.8Zr0.2O2 particles. The Ni/Ce–ZrO2/θ–Al2O3 also exhibited high catalytic activity during CDR with a synthesis gas conversion more than 97% at 800 °C without significant deactivation for more than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial pre-coating of Ce–ZrO2 resulting in the existence of stable NiO x species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. TPR results further confirmed that NiO x formation was more favorable than NiO or NiAl2O4 formation and further results suggested the existence of strong metal-support interaction (SMSI) between Ni and the support. Some of the important factors to optimize the CDR of methane such as reaction temperature, space velocity, feed CO2/CH4 ratio and H2O and/or O2 addition were also examined.

Characterization of Nickel Implanted α-Al<sub>2</sub>O<sub>3</sub>

Optical and structural properties of single crystalline α-Al2O3 were changed by the implantation of high fluences of Ni ions. Sapphire single crystals with <0001> orientation were implanted at room temperature with 150 keV nickel ions. Implantation fluences were in the range 0.3×1015 to 1.8×1017 cm-2. After implantation the optical absorption spectra reveal the presence of a band peaking in the region 300 - 500 nm, depending on the retained fluence. This is usually related to the presence of metallic particles. X-ray diffraction (XRD) studies show the presence of metallic Ni after implantation. Annealing in oxidizing atmosphere promotes the ecrystallization of the host matrix along with the formation of NiAl2O4 as deduced from Rutherford Backscattering Spectrometry (RBS) and confirmed through XRD. In vacuum the particles formed are metallic like with some Ni spinel also present. The control of the implantation fluence, temperature and annealing atmosphere allows tailoring the component phases.

Study on NiO excess in preparing NiAl2O4

When nickel aluminate spinel was prepared by solid-state reaction, an excess of NiO in products was observed, which was explained as the result of Al2O3 dissolving into spinel at high temperature. To reduce NiO and improve the formation of NiAl2O4, four factors were investigated in this study. The results showed that the milling time and starting materials had an important effect on the excess of NiO while the sintering temperature and sintering time had little effect on it.

A Highly Active and Stable Catalyst for Carbon Dioxide Reforming of Methane: Ni/Ce–ZrO2/θ-Al2O3

A novel catalyst, Ni/Ce-ZrO2/θ-Al2O3 has been designed and examined in carbon dioxide reforming of methane. It gives synthesis gas with CH4 conversion more than 97% at 800 °C and the activity was maintained during the reaction for longer than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial precoating effect of Ce-ZrO2 resulting in the existence of stable NiOx species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. From TPR results, it has been confirmed that NiOx formation is more favorable than NiO or NiAl2O4 formation, resulting in strong interaction between Ni and the support.

Oxidation behavior of sputtered Ni-3Cr-20Al nanocrystalline coating

The oxidation behavior of cast Ni-3Cr-20Al (at%) alloy and its sputtered nanocrystalline coating were investigated at 1000°C in air. Owing to the formation of the transient γ-Al2O3, θ-Al2O3 in the initial oxidation period, the mass gains were very rapid. With the transformation of the transition Al2O3 to stable α-Al2O3, the oxidation kinetics became nearly flat. α-Al2O3 and NiAl2O4 were both formed on the sputtered Ni-3Cr-20Al coating after 200-h oxidation. The formation of NiAl2O4 was probably related to the formation of Ni3Al during the oxidation process. The formation mechanism of NiAl2O4 is discussed.