Ni Sintering Research Articles

Evaluation of Ni/Y2O3/Al2O3 catalysts for hydrogen production by autothermal reforming of methane

Ni/xY(2)O(3)-Al2O3 (x = 5, 10, 15, 20 wt%) catalysts were prepared by sequential impregnation synthesis. The catalytic performance for the autothermal reforming of methane was evaluated and compared with Ni/gamma-Al2O3 catalyst. The physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), Transmission electron microscope (TEM), X-Ray Photoelectron Spectrometer (XPS), Thermo Gravimetric Analyzer (TGA) and H-2-temperature programmed reduction techniques (TPR). The decrease of nickel particle size and the change of reducibility were found with Y modification. The CH4 conversion increased with elevating levels of Y2O3 from 5% to 10%, then decreased with Y content from 10% to 20%. Ni/xY(2)O(3)-Al2O3 catalysts maintained high activity after 24 h on stream, while Ni/Al2O3 had a significant deactivation. The characterization of spent catalysts indicated that the addition of Y retarded Ni sintering and decreased the amount of coke. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The effect of impregnation strategy on structural characters and CO2 methanation properties over MgO modified Ni/SiO2 catalysts

Mg-modified Ni/SiO2 catalysts with different MgO contents were prepared by two impregnation methods. The catalysts prepared by co-impregnation method could show better activity and stability than those prepared by sequential impregnation method. The modification of MgO acted as a key factor in enhancing the capacity of CO2 adsorption and accelerating the activation of CO2. Moreover, modified MgO could also increase Ni species dispersion and suppress the metallic Ni sintering and oxidation.

Phase composition and microwave dielectric properties of (Zn, Ni)TiNb2O8 solid solution

The Zn1−xNixTiNb2O8 (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized by the conventional solid-state reaction method. The crystal structure and microwave dielectric properties were investigated by XRD, SEM and dielectric measurements. The XRD patterns of Zn1−xNixTiNb2O8 ceramics showed that (Zn, Ni)TiNb2O8 phase and (Ni, Zn)0.5Ti0.5NbO4 phase were obtained. With the increase of sintering temperature and Ni content, the amount of the second phase [(Ni, Zn)0.5Ti0.5NbO4] enhanced, resulting in the increase of dielectric constant and the decrease of Q × f value. Moreover, the temperature coefficient of resonant frequency (τf) shifted to a positive value with increasing Ni content. In conclusion, the excellent microwave dielectric properties were obtained for Zn0.7Ni0.3TiNb2O8 ceramics sintered at 1,125 °C for 4 h: er = 41.36, Q × f = 31,760 GHz, τf = −9.2 ppm/°C. Furthermore, to realize the zero value for τf, a proper adjustment of Ni doping and sintering temperature can be feasible in this system.

Steam reforming of model gasification tar compounds over nickel catalysts prepared from hydrotalcite precursors

Nickel catalysts derived from hydrotalcite-like compounds, with 10 and 20wt.% NiO loading, were prepared and tested in steam reforming of different tar model compounds: benzene, toluene and naphthalene. Naphthalene was used in toluene solution. The catalysts were evaluated in terms of activity with temperature and deactivation with time, using a steam/carbon ratio of 1.5. The catalysts presented similar behavior, with slightly higher conversions for the catalyst with 10% NiO in benzene and toluene reforming. H2 formation was much lower than that predicted by thermodynamics, with CO2 as the main product at low temperatures and CO at high temperatures. Naphthalene is much more difficult to be converted, and inhibits the toluene reforming. Both catalysts showed good stability with time on stream despite the high amount of carbon deposit and Ni sintering. The amount and morphology of the coke are dependent on the nature of aromatic compound.

Methanation over Ni/SiO2: Effect of the catalyst preparation methodologies

We confirmed here that the catalyst preparation methodologies have a significant effect on the activity and stability of Ni/SiO2 catalyst for methanation of syngas (CO + H2). Catalyst characterizations using X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR) and transmission electron microscope (TEM) were performed to investigate the structure and performance of the catalysts. The activity and stability of catalysts prepared by thermal decomposition and dielectric-barrier discharge (DBD) plasma decomposition of nickel precursor were compared. The plasma decomposition results in a high dispersion, an enhanced interaction between Ni and the SiO2 support, as well as less defect sites on Ni particles. Enhanced resistance to Ni sintering was also observed. In addition, the plasma prepared catalyst effectively inhibits the formation of inactive carbon species. As a result, the plasma prepared catalyst exhibits significantly improved activity with enhanced stability.

Effects of Y2O3-modification to Ni/γ-Al2O3 catalysts on autothermal reforming of methane with CO2 to syngas

The effects of Y2O3-modification to Ni/γ-Al2O3 catalysts on autothermal reforming of methane to syngas were investigated. It was found that the introduction of Y2O3 (5%, 8%, 10%) lead to significant improvement in catalytic activity and stability, and the H2/CO ratio could be adjusted via controlling the O2/CO2 ratio of the feed gas. According to the characterization results of catalysts before and after reaction, it was found that the Y2O3·γ-Al2O3 supported Ni catalysts had higher NiO reducibility, smaller Ni particle size, higher Ni dispersion and stronger basicity than those of the Ni/γ-Al2O3 catalysts. The analysis of catalysts after reaction showed that the addition of Y2O3 inhibited the Ni sintering, changed the type of coke and decreased the amount of coke on the catalysts. All the experimental results indicated that the introduction of Y2O3 to Ni/γ-Al2O3 resulted in excellent catalytic performances in autothermal reforming of methane, and Y2O3 played important roles in preventing metal sintering and coke deposition via controlling NiO reducibility, Ni particle size and dispersion, and basicity of catalysts.

Electrical conductivity of Ni–YSZ composites: Variants and redox cycling

Short-term changes in the electrical conductivity of different Ni–YSZ composites (cermets) were measured by an in-situ 4-point DC technique. The isothermal reduction was carried out in dry, humidified or wet hydrogen at temperatures from 600 to 850°C. The cermets reduced at 600°C showed a stable conductivity of about 1100S/cm, which increased to an enhanced ~2000S/cm upon re-oxidation and subsequent re-reduction cycling at the same temperature. At 850°C, a rapid initial conductivity loss was observed; upon re-reduction after the re-oxidation both the conductivity and its loss rate were largely the same as in the initial reduction. The presence of steam had an accelerating effect on the conductivity loss at 850°C. In addition to cermets with a typical microstructure, different modified microstructures and compositions were tested. In the modified cermets, Al, Mg, and Ce were used as Ni dopants alongside with undoped Ni. Scanning electron microscopy of cermets reduced in different conditions showed increasing particle size and loss of metal-to-metal percolation in the samples reduced at higher temperatures, and a very fine microstructure in the high conductivity sample re-oxidised at 600°C. Short-term conductivity changes due to microstructural changes in both the standard and modified cermets with different Ni doping were compared by re-oxidation at 600°C and subsequent thermal excursions up to 1000°C by normalising the conductivity to a constant temperature. Modified cermets show reduced conductivity loss on both isothermal heating and high temperature ramping. Most stable cermets were the ones with undoped NiO and modified microstructure, as well as modified microstructures using Ti and Mg, or Ce secondary oxide coatings. Master sintering curve approach was successfully implemented to analyse the conductivity loss data. The MSC analysis yielded apparent activation energies for Ni sintering within the composite of 375kJ/mol when heated in dry H2 and 440kJ/mol when under wet H2.

Hydrogen production by methane cracking using Ni-supported catalysts in a fluidized bed

Nickel, supported on porous alumina (γAl2O3), non-porous alumina (αAl2O3), and porous silica, was used to catalyze methane cracking in a fluidized bed reactor for hydrogen production. The effects of temperature, PCH4, and particle diameter, and their interactions, on methane conversion were studied with each catalyst. Temperature was the dominant parameter affecting the hydrogen production rate for all catalysts and particle diameter had the strongest effect on the total amount of carbon deposited. Maximum methane conversion as a function of support type followed the order Ni/SiO2 > Ni/αAl2O3 > Ni/γAl2O3. Nonetheless, better fluidization quality was obtained with Ni/γAl2O3. Methane conversion was increased by increasing temperature and particle size from 108 to 275 μm due to better fluidization achieved with 275 μm particles. Increasing the flow rate and methane partial pressure (PCH4) caused a drop in methane conversion. Tests were also run in a fixed bed reactor, and at constant weight hourly space velocity (WHSV), higher conversion was achieved in the fixed bed, but at the same time faster deactivation occurred since a higher methane conversion led to increase in carbon filament and encapsulating carbon formation rates. A critical problem with the fixed bed was the pressure build-up inside the reactor due to carbon accumulation. Finally, a series of cracking/regeneration cycle experiments were carried out in the fluidized bed reactor. The regeneration was performed through product carbon gasification in air. Ni/αAl2O3 and Ni/γAl2O3 activity decreased significantly with the first regeneration, which is attributed to Ni sintering during exothermic regeneration/carbon oxidation. However, Ni/SiO2 was thermally stable over at least three cracking/regeneration cycles, but mechanical attrition was observed.

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.

Supported Ni catalysts prepared by intercalation of Layered Double Hydroxides: Investigation of acid–base properties and nature of Ni phases

Supported nickel catalysts were obtained by exchange of Mg/Al Layered Double Hydroxides (LDHs) compensated with NO 3 - ions with negatively charged Ni complexes, followed by thermal reduction. With this aim, suspensions of Ni complexes were prepared by controlled hydroxylation of Ni 2+ cations in the presence of citrate (obtaining [ Ni ( C 6 H 5 O 7 ) ( OH ) ] y 2 y - species) or chloride (obtaining [NiCl 4] 2− species) complexing ions. For comparative purposes, other two supported Ni catalysts were prepared starting from Mg/Al LDHs compensated with Cl − ions and thereafter exchanged with [ Ni ( C 6 H 5 O 7 ) ( OH ) ] y 2 y - species (nominal degree of exchange: 100% and 20%). XRD and HRTEM techniques were employed for studying nickel phases and dispersion. CH 3CN adsorption at RT followed by FT-IR spectroscopy gave information on surface acid–base properties. CO adsorption at 77 K followed by FT-IR spectroscopy was employed to deepen the characterization of the metal phase. On the basis of the results obtained, it is possible to evidence a correlation between the metal phase morphologies and the surface acid–base properties of the reduced catalysts. High surface basicity revealed by the presence of strongly basic O 2− sites stabilizes small nickel particles (<4 nm), in spite of the high metal loading (>11 Ni wt.%), whereas, when the basicity is suppressed by Cl − ions, a dramatic Ni sintering occurs even at low metal loading (in the range 3–10 Ni wt.%).

Theoretical Study for the Sintering of Nickel Anode in Solid Oxide Fuel Cell

Ni sintering is one of the main factors in SOFC degradation. In this study, we first investigated the surface orientation on Ni nanoparticles by using molecular dynamics (MD) method. It was identified that Ni nanoparticles mainly consists of four facets: (111), (100), (110), and (113). We also analyzed the stability of surface energy, which is dominant driving force of sintering phenomena, in various sizes of Ni nanoparticles. The MD simulation of two particles system was performed to investigate the sintering phenomena. We found that the diffusion coefficient is almost same at the temperature between 1373 and 1673 K, while contact surface orientation is different. It was indicated that dominant sintering mechanism of Ni nanopartisles is surface diffusion.

Steam reforming reactions over a metal-monolithic anodic alumina-supported Ni catalyst with trace amounts of noble metal

A trace amount of noble metal (Ru or Pt <0.1 wt%) was doped onto an anodic alumina-supported Ni catalyst, to investigate its performance in the steam reforming of methane (SRM), especially during DSS (daily startup and shutdown) operation. Although the steam purge treatment at high temperatures seriously deactivated the Ni catalyst because of the oxidation of metallic nickel with steam into Ni 2+, trace Ru assisted the regeneration of active metallic nickel by hydrogen-spillover. And, the Ni sintering was largely alleviated by the addition of Ru, and it was probably due to the formation of Ru–Ni alloy. In comparison with the Ru-doped Ni catalyst, the Pt-doped Ni catalyst showed a more favorable tolerance to steam oxidation, even at 900 °C. In a stationary SRM test of 3000 h and a DSS SRM test of 500 times where the town gas 13A was used as hydrogen source, no obvious deterioration was detected over the Pt-doped Ni catalyst. Especially, when electrically heating the plate-type Pt-doped Ni catalyst to 700 °C, the SRM reaction system could reach a stable state within ca. 10 min, which offered a strong possibility to shorten the startup time from the 1–2 h of conventional reformer to just few minutes. In addition, the noble metal-doped Ni catalyst also showed favorable activity and durability when being applied to other steam reforming systems, such as kerosene and ethanol.

Water effect in hydrogen production from methane

Ni/MgO and Ni/Al 2O 3 catalysts were prepared, by wet impregnation, to compare their performance in hydrogen production from methane CPO, wet-CPO and SR. The catalytic activity was tested at 1073 K, 1 bar and 600–1200 h −1. Fresh and used catalysts were characterized by different techniques. Both supports, as expected, had a low surface area (27.1 m 2/g MgO and 6.2 m 2/g α-Al 2O 3), as determined by BET method. The images obtained with SEM and TEM revealed that the Ni was more dispersed in the MgO support than in the Al 2O 3 one. By XRD a strong interaction, as solid-solution, between NiO and MgO was found in the 30Ni/MgO and 40Ni/MgO catalysts. The fresh 40Ni/Al 2O 3 reduced catalyst was partially reduced. But after the activity tests the stability of the reduced Ni became bigger. Some Ni sintering was also observed in the 40Ni/Al 2O 3 after the wet-CPO and SR tests. The behaviour of the three catalysts was very good in CPO methane conversion (90–93%), but the gradual increase of the steam to carbon ratio, wet-CPO and SR, affected negatively the conversion.

The catalytic performance of Ni/MgSiO 3 catalyst for methane steam reforming in operation of direct internal reforming MCFC

Active and tolerant Ni-based catalyst for methane steam reforming in direct internal reforming molten carbonate fuel cell (DIR-MCFC) was developed. Deactivation of reforming catalysts by alkali metals from electrolyte composed of Li 2CO 3 and K 2CO 3 is one of the major obstacles to be overcome in commercialization of DIR-MCFC. Newly developed Ni/MgSiO 3 reforming catalyst showed activities of ca. 82% methane conversion for 240 min in out-of-cell test. In duration test, the unit cell containing Ni foam impregnated with Ni/MgSiO 3 in anode gas channel did not give performance degradation for more than 2000 h, while the unit cell assembled with Ni/MgSiO 3-coated anode showed a significant performance loss after an operation of 1200 h. Results obtained from X-ray diffraction and Brunauer–Emmett–Teller technique revealed that Ni sintering and support deterioration were decisive factors in decreasing the catalytic activity.

Effect of residual pressure on vacuum sintering of Ti–Ni alloys

In earlier work the authors examined the sintering of Ti–Ni alloys by means of dilatometry of mixed elemental powders. Some notable differences were observed when heat treatments were carried out using a vacuum tube furnace rather than the dilatometer: higher sintered density was achieved due to a combination of lower heating rate and lower residual pressure, and swelling during liquid phase sintering was greatly reduced. This observation is consistent with the idea that gas pressure within closed pores causes swelling during liquid phase sintering and retardation of shrinkage in solid state sintering. In addition to the results of measurements of density and open and closed porosity as a function of Ni content and sintering temperature, macrographs and optical micrographs of the sintered compacts are presented, and the effects of heating rate and compaction pressure are described.

Doped Ni Thin Layer Catalysts for Catalytic Decomposition of Natural Gas to produce hydrogen

Doped Ni-based Thin Layer Catalysts (Ni-TLCs) for Catalytic Decomposition of Natural Gas (CDNG) were investigated using a new Multilayer Catalytic Reactor (MCR). The influence of K, La, Mg and Cl on activity–stability patterns of a Ni-TLC catalyst was evaluated. An inverse straight-line relationship between carbon capacity (C/Ni, number of CH 4 molecules decomposed for Ni atom until complete deactivation) and reaction temperature was found independently of the dopant used. However, dopants significantly affect Ni specific activity (TOF, s −1), acting as either an electronic promoter or inhibiting Ni sintering phenomena. Reaction temperature is a key parameter in determining the evolution of coke. At low reaction temperature (<823 K), carbon whiskers form prevalently on “bare” and Mg, La, and Cl doped samples, whereas only the encapsulating carbon type formed on a K doped system. High hydrogen productivity was obtained on an Mg doped catalyst which, furthermore, proved to be the most stable system.

The effect of Al addition on the prevention of Ni sintering in bio-ethanol steam reforming for molten carbonate fuel cells

This paper investigates the effect of modifying an Ni catalyst on the prevention of sintering of the catalyst at high temperatures, without causing a reduction in catalytic activity. The Ni catalyst was modified by adding Al through a solid–gas–solid reaction at a low temperature to produce an Ni–Al solid solution. This process allows for low-cost production of the modified catalyst. An activity test of the catalysts was carried out at 650 °C and 1 atm to simulate direct internal reforming of a molten carbonate fuel cell (MCFC). Experimental results showed consistent activity of the Al-modified catalyst, even after aging under severe conditions (900 °C) to simulate accelerated sintering. TEM data did not show any significant physical changes even after aging. Addition of Al appeared to have successfully prevented Ni from sintering without reducing its catalytic activity in reforming bio-ethanol. In addition, an Ni–Al/MgO catalyst, integrated into the anode of an MCFC, was successfully tested for over 2000 h without any significant performance degradation.

The effect of temperature gradients on thermal cycling and isothermal ageing of micro-tubular solid oxide fuel cells

Abstract Isothermal ageing and thermal cycling are performed on micro-tubular solid oxide fuel cells (SOFCs) in-order to understand degradation and failure mechanisms in micro-tubular SOFCs. For isothermal ageing, the effect of temperature gradients is investigated at 800 °C on two micro-tubular SOFC samples (1) 25 mm long and (2) 55 mm long. A temperature gradient is induced across the cells by passing 25% excess fuel for combustion at the cell outlet, thereby raising the temperature at this end to 950 °C. 25 mm long samples presented higher power density than the later ones and their rates of degradation were similar. Also, the effect of temperature gradients is investigated during the thermal cycling of micro-tubular SOFC to understand their contribution to electrochemical performance degradation. Two micro-tubes were characterized; one with optimum hydrogen flow rate and the other with 25% excess. No micro-cracking or de-lamination was observed in the micro-tube without a temperature gradient, whereas severe de-laminations and micro-cracking were observed when 25% excess hydrogen flow was used during thermal cycling. In conclusion, the effect of temperature gradients during isothermal ageing was marginal, and Ni sintering was found to dominate the degradation mechanism. On the other hand during thermal cycling, the temperature gradients were found to be contributive to degradation by opening micro-cracks and de-laminations.

Citrate or hydrotalcite?

Trace amounts of Pt- and Ru-doped Ni/Mg(Al)O catalysts were prepared by a citrate method and tested in the oxidative reforming of C 3 H 8 under daily start-up and shut-down (DSS) operation. The activity and the sustainability of the catalysts were compared with those of the Pt- and Ru-doped Ni/Mg(Al)O catalysts derived from hydrotalcite (HT) precursor. The DSS operation of C 3 H 8 reforming was carried out with O 2 gas or O 2 /H 2 O mixed gas between 200 °C and 600 °C or 700 °C under air purging conditions. The catalysts underwent steaming treatment with H 2 /H 2 O mixed gas at 900 °C for 10 h. This allowed us to test the effect of Ni sintering on the catalyst deactivation. Coking was significantly suppressed on both HT- and citrate-derived Ni catalysts. Although both preparations produced highly dispersed Ni particles on the catalysts, the HT-derived catalysts exhibited more finely dispersed Ni particles, resulting in higher activity values than those of the citrate-derived catalysts. The regenerative activity due to redispersion of sintered Ni particles was enhanced over the HT-derived catalysts compared with the activity over citrate-derived catalysts. Although a clear redispersion of Ni particles was not observed in the oxidative reforming, i.e., in the absence of steam, the size decrease in Ni particles was more significant over the HT-derived catalysts than over the citrate-derived catalysts. The Mg(Al)O periclase structure derived from Mg-Al HT likely plays an important role in the regenerative activity of Pt- and Ru-Ni/Mg(Al)O catalysts. Pt-doping was more effective than Ru for the catalyst sustainability in the oxidative reforming of C 3 H 8 . Trace amounts of Pt-doped Ni/Mg(Al)O catalysts derived from hydrotaclite exhibited higher and more sustainable activity than those prepared by the citrate method. Mg(Al)O periclase that originated from Mg-Al HT played an important role in the regenerative activity of the catalysts via reversible reduction-oxidation between Ni 0 and Ni 2+ on/in Mg(Al)O assisted by hydrogen spillover from Pt.

Basic Nickel Carbonate: Part I. Microstructure and Phase Changes during Oxidation and Reduction Processes

A significant industrial problem associated with the production of nickel from basic nickel carbonate has been identified. Fundamental studies of the change of phase, product surface, and internal microstructures taking place during oxidation and reduction processes at temperatures between 110 °C and 900 °C have been carried out. The various elemental reactions and fundamental phenomena that contribute to the change of the physical and chemical characteristics of the samples during the processes taking place in Ni metal production through gas/solid-reduction processes have been identified and thoroughly investigated. The following phenomena affecting the final-product microstructure were identified as follows: (1) chemical changes, i.e., decomposition, reduction reactions, and oxidation reactions; (2) NiO and Ni recrystallization and grain growth; (3) NiO and Ni sintering and densification; and (4) agglomeration of the NiO and Ni particles.