Effect of support on reactivity and selectivity of Ni-based oxygen carriers for chemical-looping combustion

Different Ni-based oxygen carriers were prepared by dry impregnation using γ-Al₂O₃ as support. The reactivity, selectivity during methane combustion, attrition rate and agglomeration behavior of the oxygen carriers were measured and analyzed in a thermogravimetric analyzer and in a batch fluidized bed during multi-cycle reduction-oxidation tests.Ni-based oxygen carriers prepared on γ-Al₂O₃ showed low reactivity and low methane combustion selectivity to CO₂ and H₂O, because most of the impregnated NiO reacted to NiAl₂O₄. To avoid or to minimize the interaction of NiO with alumina some modifications of the support via thermal treatment or chemical deactivation with Mg or Ca oxides were analyzed. Thermal treatment of γ-Al₂O₃ at 1150 °C produced the phase transformation to α-Al₂O₃. Ni-based oxygen carriers prepared on α-Al₂O₃, MgAl₂O₄, or CaAl₂O₄ as support showed very high reactivity and high methane combustion selectivity to CO₂ and H₂O because the interaction between the NiO and the support was decreased. In addition, these oxygen carriers had very low attrition rates and did not show any agglomeration problems during operation in fluidized beds, and so, they seem to be suitable for the chemical-looping combustion process.     

Green and facile synthesis of cube-like γ-AlOOH and its usage for fabricating Mg stabilized Na-β″-Al2O3 powders

In this work, we describe the discovery of synthesizing γ-AlOOH particles via direct reaction of Al(OH)₃ and H₂O under a green, facile and cost-effective hydrothermal treatment, in which no addition of other additives is required. The as-synthesized γ-AlOOH is in cube-like morphology with micrometres size, which is subsequently used as a precursor to fabricate Mg stabilized Na-β″-Al₂O₃ powder with high β″-Al₂O₃ phase content through solid-state reaction. Time-dependent experiments are carried out to investigate the microstructure and phase evolutions of the γ-AlOOH precursor during the hydrothermal treatment process, and the sintering temperature on the β″-Al₂O₃ phase content and microstructure of the terminal Na-β″-Al₂O₃ powder are also studied. The present approach inspires a new and facile way in the fabrication of γ-AlOOH and Na-β″-Al₂O₃ on a large scale.     

Influence of ZrO2 and WO3 doping additives on the thermal properties of porous mullite ceramics

Porous mullite-corundum refractory ceramics were produced by a patented slurry slip casting method from compositions based on commercially available α-Al₂O₃ and γ-Al₂O₃, fused SiO₂ and kaolin. Pores were formed as a result of a chemical reaction of aluminium with water. The influence of usage of raw materials and doping additives such as micro-size ZrO₂ and WO₃ on the sintering temperature, formation of crystalline phases, linear thermal expansion, thermal conductivity and thermal shock resistance of mullite-corundum ceramic was studied. The best thermal shock resistance and, simultaneously, lower thermal conductivity was achieved for the samples doped with WO₃. This was due to the influence of micro-sized WO₃ on the change in γ-Al₂O₃ modification to α-Al₂O₃ and on the structure of mullite ceramics.     

Morphology control of α-Al2O3 platelets by molten salt synthesis

It is of great importance to control the morphology of α-Al₂O₃ plate-like powders since α-Al₂O₃ platelets with different shapes are needed in various applications. This paper was focused on how to control the morphology of α-Al₂O₃ platelets by molten salt synthesis. Results show that the morphology of α-Al₂O₃ platelets is affected by the heating temperature, heating time, the molten salts species, the weight ratio of salt to powders, additives and the addition of nano-sized seeds. Especially, it is very effective to control the morphology of α-Al₂O₃ platelets by adjusting the addition of additives such as Na₃PO₄·12H₂O and TiOSO₄. α-Al₂O₃ flakes with irregular shape are obtained by the addition of Na₃PO₄·12H₂O, while thick α-Al₂O₃ particles with hexagonal shape are obtained by the addition of TiOSO₄. The combination addition of Na₃PO₄·12H₂O and TiOSO₄ makes it possible to obtain thin α-Al₂O₃ platelets with discal shape. A small amount of nano-sized seeds addition also has a strong effect on the size of α-Al₂O₃ platelets. However, if the seeds are added too much, the overlapping and abnormal crystal growth of α-Al₂O₃ platelets occur, and the size distribution becomes nonuniform. The effect mechanism of additives and seeds on the morphology of α-Al₂O₃ platelets was also discussed in this paper.     

Study on the stability of different refractories during the melting process of a K4169 Ni-based superalloy

The composition of the refractory strongly affects the cleanliness of the alloy. K4169 Ni-based superalloys were melted in different types of refractories in this study. The cleanliness of the Ni-based superalloy and phase transformation of the refractory were observed by X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy energy dispersive spectroscopy (SEM‒EDS). The high-temperature stabilities of a Y₂O₃-based refractory, MgO-based refractory, and Al₂O₃-based refractory during melting with a Ni-based alloy were compared. The oxygen content was also lowest, and no Y₂O₃-containing inclusions were observed in the Ni-based alloy melted with the Y₂O₃-based refractory at 1823 K. Inclusions with 21%–29% MgO and a phase composed of Al, Mg and O with an area of approximately 1300 μm² were observed in the alloy. This indicates that the dissolution and erosion of the Y₂O₃-based refractory were weak, and obvious physical erosion and chemical dissolution of the MgO-based refractory occurred during the melting process of the Ni-based alloy. The width of the refractory phase adhered to the boundary of the Ni-based alloy increased in the order Y₂O₃-based refractory (15 μm- 23 μm)< Al₂O₃-based refractory (93 μm- 285 μm)< MgO-based refractory (3.5 mm–3.6 mm), indicating that the adhesive strength of the MgO-based refractory with the Ni-based alloy was strongest. The interaction between the refractory material, Ni-based alloy and inclusions was analyzed based on thermodynamic calculations by Factsage software. The effects of dissolution of the three refractory types on the formation and transformation of the new phases and inclusions were estimated. The thermodynamic results were in good agreement with the experimental results.     

Growth mechanism of single-crystal α-Al2O3 nanofibers fabricated by electrospinning techniques

Crystal-growth-related microstructures and the length-to-diameter ratio of a single-crystal-type α-Al₂O₃ nanofiber were examined using HR-TEM techniques. The fibers exhibited diameters ranging from 50 to 100 nm and lengths of several tens of micrometers. During thermal treatments, the alumina fiber went through phase transformations similar to boehmite. Therefore, the phase evolution, especially the final θ- to α-Al₂O₃ stage of the phase transformation, may be the determining factor in the microstructural evolution of the nanofibers. HR-TEM techniques were utilized to demonstrate that the single crystals were formed by the coalescence of well-elongated α-Al₂O₃ colonies. The fibers grew in the [1 1 0] or [1 1 2] direction instead of [0 0 1]. A thermodynamic analysis revealed that if the α-Al₂O₃ nanofiber that transformed from θ-Al₂O₃ behaved in a stable manner, there could be a size ratio limit for the length and diameter of each α-Al₂O₃ colony. The smallest potential diameter was calculated to be around 17 nm.     

Surface coating of α-Al2O3 nanoparticles with poly(vinyl alcohol) as biocompatible coupling agent for improving properties of bio-active poly(amide-imide) based nanocomposites having l-phenylalanine linkages

Nanocomposites (NCs) containing metal oxide nanoparticles (NPs) as fillers are used in a wide range of applications and in various fields. Poly(vinyl alcohol) (PVA) is an attractive polymer because of its many desirable applications and characteristics. In this investigation, at first, the surface of alumina (α-Al₂O₃) NPs was modified with PVA as a biocompatible modifier. Then, the optically active poly(amide-imide) (PAI) nanostructure was prepared by using molten tetrabutylammonium bromide as a molten ionic liquid and triphenyl phosphite as the condensing agent. Finally, the modified α-Al₂O₃ (α-Al₂O₃-PVA) NPs were incorporated into the PAI matrix for the preparation of PAI/Al₂O₃-PVA NCs (PAPNCs). To investigate effect and nature of coating on the surface of the α-Al₂O₃ NPs and preparation of PAPNCs, the samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. The surface morphology examination demonstrated the monodispersed characteristics of α-Al₂O₃ NPs after surface modification with the PVA and incorporation into the PAI matrix.     

PREPARATION OF (Ni/W)-γ-Al2O3 MICROSPHERES AND THEIR APPLICATION IN ADSORPTION DESULFURIZATION FOR MODEL GASOLINE

A kind of desulfurization adsorbent, (Ni/W)-γ-Al₂O₃ microsphere, was prepared by a new method of in situ chemical reduction. The adsorbent consists of active components (transition metals Ni and W) and a carrier (γ-Al₂O₃). Ni and W in γ-Al₂O₃ microspheres are fine in size and can be distributed homogeneously on the surface and inside of the γ-Al₂O₃ carrier. The desulfurization of the adsorbent made by the in situ chemical reduction method was carried out in model gasoline. Its desulfurization capacity increases 23% in comparison with that made by the conventional impregnation method. The composition and configuration of adsorbents were analyzed by scanning electron microscopy (SEM), electron energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The in situ chemical reduction method offers a new and promising method for preparation of desulfurization adsorbents containing active components.     

Methods and Approaches of the Sol–Gel Technology for the Surface Modification of Aluminum Oxide Powders

The results of studies, sol–gel synthesis, and the sedimentation stability of complex multicomponent sol–gel systems of the “silica sol modified with Co(NO₃)₂ · 6H₂O, Al(NO₃)₃ · 9H₂O with α-Al₂O₃ or γ-Al₂O₃ as highly dispersed filler” type are generalized. The physical–chemical processes accompanying the formation of modifying layers on the powder oxide particles are examined. The promising prospects of applying α-Al₂O₃ powders modified with a silicate layer of the composition (wt %) 1.2K₂O · 3Al₂O₃ · 3.2CaO · 12.5Na₂O · 28.1B₂O₃ · 52SiO₂ in the fabrication of ceramic materials with improved strength characteristics are demonstrated.     

Melt differentiation and crystallization of clinker minerals in a CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₃SiO₅ [C₃S(ss)], Ca₂SiO₄ [C₂S(ss)], Ca₂(Al_xFe_1 − x)₂O₅ with 0.40 ≤ x ≤ 0.57 (ferrite) and Ca₃Al₂O₆ [C₃A(ss)] were crystallized out of a complete melt with 52.9 mass% CaO and Al₂O₃/Fe₂O₃ = 0.70. When the melt was cooled from 1673 K at 80 K/h, the crystals of ferrite with x = 0.40, C₃S(ss) and C₂S(ss) would start to nucleate from the melt at 1630 K. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.57 at 1613 K. The resulting ferrite crystals showed a zonal structure, the x value of which successively increased from the cores toward the rims. Actually, the x values of 0.43 and 0.52 were confirmed for, respectively, the cores and rims by EPMA. As the simultaneous crystallization of zoned ferrite, C₃S(ss) and C₂S(ss) proceeded, the coexisting melt would become progressively enriched in the Al₂O₃ component. After the termination of the ferrite crystallization, the C₃A(ss), C₃S(ss) and C₂S(ss) crystallized out of the differentiated melt. The end result was the four phase mixture of ferrite, C₃A(ss), C₃S(ss) and C₂S(ss), being free from the nucleation of Ca₁₂Al₁₄O₃₃ solid solution.     

NiO/Al2O3 oxygen carriers for chemical-looping combustion prepared by impregnation and deposition-precipitation methods

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using γ-Al₂O₃ and α-Al₂O₃ as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a thermogravimetric analyzer and methane combustion selectivity towards CO₂ and H₂O, attrition rate, and agglomeration behavior were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests.XRD and TPR analysis showed the presence of both free NiO and NiAl₂O₄ phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl₂O₄ that affected negatively to the OC reactivity and methane combustion selectivity towards CO₂ and H₂O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on γ-Al₂O₃.The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process. OC prepared by the AI and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on γ-Al₂O₃ with NiO content lower than 30 wt%, and OC prepared by the DP method on α-Al₂O₃ with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using γ-Al₂O₃ as support were higher than the ones prepared using α-Al₂O₃ as support, and in general the attrition rates of all the OC were low.The OC prepared by AI, HI or DP methods on α-Al₂O₃ as support had appropriated characteristics to be used in the chemical-looping combustion process.     

Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al2O3@amorphous alumina nanocomposite core/shell structure

In this study, α-Al₂O₃@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl₃ and the commercial α-Al₂O₃ nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al₂O₃ phases after the calcination process and the shell material was completely converted to γ-Al₂O₃ at 1000?°C. However, for the α-Al₂O₃@amorphous alumina core-shell nanoparticles were completely converted to α-Al₂O₃ at 1000?°C. It can be concluded that α-Al₂O₃ core particles, as the seed crystalline, help to transforming of γ-Al₂O₃ phase as the shell material directly without forming transitional phases to α-Al₂O₃. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al₂O₃@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ～99.8% and the in-line transmittance value is ～80% within the IR range.     

Iron in carbonate containing AFm phases

One of the AFm phases in hydrated Portland cement is Ca₃(Al_xFe_{2 − x})O₆.CaCO₃.nH₂O. It is based on hexagonal and platey structural elements and the interlayer structure incorporates CO₃²⁻. The solid phases were experimentally synthesized and characterized by different techniques including X-ray techniques (XRD and EXAFS) and vibrational spectroscopy techniques (IR, Raman). Fe-monocarbonate (Fe-Mc) and Al-monocarbonate (Al-Mc) were found to be stable up to 50 °C, while Fe-hemicarbonate (Fe-Hc) was unstable with respect to Fe-Mc in the presence of calcite. Fe-Mc has a rhombohedral symmetry which is different from the triclinic of the Al analogue. Both XRD and thermodynamic modelling of the liquid compositions indicated that Al-Mc and the Fe-Mc phases do not form solid solution. The solubility products were calculated experimentally at 20 °C and 50 °C. Under standards condition the solubility products and other thermodynamic parameters were estimated using temperature-solubility product extrapolation and found to be logK_S0 (Fe-Mc) = −34.59 ± 0.50, logK_S0 (Fe-Hc) = −30.83 ± 0.50 and logK_S0(Al-Mc) = −31.32 ± 0.50.     

Synthesis of plate-like α-Al2O3 single-crystal particles in NaCl–KCl flux using Al(OH)3 powders as starting materials

Plate-like α-Al₂O₃ single-crystal particles were successfully synthesized in NaCl–KCl flux using Al(OH)₃ powders as starting materials, and the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders was focused. When Al(OH)₃ powders are used as starting materials, the synthesized product at 900 °C is mainly composed of α-Al₂O₃ and κ-Al₂O₃, and most synthesized particles show alveolate morphology. At 1100 °C, single-phase α-Al₂O₃ powders are developed, in which there are many aggregations of intensively bound plate-like particles. In contrast, using porous amorphous Al₂O₃ powders obtained by pre-calcining Al(OH)₃ powders at 550 °C for 3 h as the starting material, plate-like α-Al₂O₃ single-crystal particles can be well developed above 900 °C. The reason of the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders is also discussed in the paper.     

Modelling of catastrophic stress development due to mixed oxide growth in thermal barrier coatings

In thermal barrier coatings (TBCs) of heavy-duty gas turbines, thermally grown oxide (TGO) develops in two stages, i.e. firstly, a thin layer of dense protective α-Al₂O₃ forms slowly, and then, a layer of porous detrimental mixed oxide (MO) between top coat (TC) and α-Al₂O₃ appears. During long-term isothermal oxidation at high temperature, the failure of TBCs usually occurs when a critical thickness of MO is reached, but the exact failure mechanism is still largely unclear, let alone the related stress development. In this paper, we analyze the stress evolution and the resultant failure modes due to the whole-layer growth of uniform MO. The results show that it is MO, rather than α-Al₂O₃, that is mainly responsible for the micro-cracking and/or delamination in TBCs. The fast growth of expansive MO induces catastrophic stresses, which leads to micro-cracking in the α-Al₂O₃ layer. The cracking of α-Al₂O₃ layer reduces the oxidation resistance and further accelerates the MO growth. Our theoretical analysis provides a reasonable explanation of the experimental results.     

Size effects on χ- to α-Al2O3 phase transformation

Nano-scaled χ-Al₂O₃ powders with d₅₀ mean particle sizes from 17 to 314 nm were prepared to investigate the size effect on their phase transformation. Structural properties and crystallization behavior as a function of thermal treatments of various-sized χ-Al₂O₃ particles were examined by DTA, XRD and TEM characterizations. It was confirmed that the decrease of particle size allows for stable α-Al₂O₃ formation at relatively low temperature. Furthermore, the phase transformation route of χ-Al₂O₃ to α-Al₂O₃ was also modified due to the decrease of particle size. A critical size of χ-Al₂O₃ that determines the phase transformation behavior was found to be around 40 nm. For particles larger than 40 nm, a transition phase of κ-Al₂O₃ is formed before obtaining final α-Al₂O₃ phase. Nevertheless, for those smaller than the critical size, starting χ-Al₂O₃ particles have to grow to 40 nm and then directly transform to α-Al₂O₃ bypassing κ-Al₂O₃ at a temperature as low as 1050 °C.     

Pt–Ni/γ-Al2O3 catalyst for the preferential CO oxidation in the hydrogen stream

The preferential CO oxidation (PROX) in the presence of excess hydrogen was studied over Pt–Ni/γ-Al₂O₃. CO chemisorption, X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and temperature-programmed reduction were conducted to characterize active catalysts. The co-impregnated Pt–Ni/γ-Al₂O₃ was superior to Pt/Ni/γ-Al₂O₃ and Ni/Pt/γ-Al₂O₃ prepared by a sequential impregnation of each component on alumina support. The PROX activity was affected by the reductive pretreatment condition. The pre-reduction was essential for the low-temperature PROX activity. As the reduction temperature increased above 423 K, the CO₂ selectivity decreased and the atomic percent of Ni in the bimetallic phase of Pt–Ni increased. This catalyst exhibited the high CO conversion even in the presence of 2% H₂O and 20% CO₂ over a wide reaction temperature. The bimetallic phase of Pt–Ni seems to give rise to high catalytic activity for the PROX in H₂-rich stream.     

Study on α-alumina precursors prepared using different ammonium salt precipitants

Nano-sized α-Al₂O₃ platelets have been produced by the precipitation method employing the starting material of Al(NO₃)₃·9H₂O and ammonium salt precipitants, such as the NH₄OH, (NH₄)₂CO₃ and NH₄HCO₃. The effects of chemical composition of ammonium salt precipitants and aging time of precipitated product on the formation of precursor and final product of α-Al₂O₃ particles were studied. The precursors with different crystal structures were formed depending on the chemical composition of precipitant and the agglomeration of final α-Al₂O₃ particles was found to be greatly affected by the precipitant. The aging time of precipitated precursor also influenced the agglomeration of final α-alumina particles.     

Tailorable magnetic properties of ε-Fe2O3/SiO2 hybrid via alkaline etching

In this study, conventional silicon alkaline-etching procedure was utilized to tailor magnetic properties of ε-Fe₂O₃/SiO₂ hybrid. It was found that the saturation magnetization, coercivity and exchange bias field can be readily changed and tailored by altering the etching time and frequency in a set of sodium hydroxide solutions. The relative quantity of ε-Fe₂O₃ phase, the proximity or pinning effect derived from SiO₂ phase as well as the phase transformation from ε-Fe₂O₃ to α-Fe₂O₃ during etching treatment were three main factors to its controllable magnetic properties. This work will shed new light on the development of functional ε-Fe₂O₃/SiO₂ composites with tailorable magnetism in practical magnetically-relevant applications.     

Influences of pH value on the microstructure and phase transformation of aluminum hydroxide

The effects of pH value on the composition, structure, morphology, and phase transformation of aluminum hydroxides prepared by chemical precipitation were studied. Aluminum hydroxide precipitated at the pH values of 5 and 6 is amorphous and transforms to α-Al₂O₃ at 950 °C via the amorphous aluminum hydroxide → amorphous Al₂O₃ → α-Al₂O₃ transformation path. Aluminum hydroxide precipitated at pH = 7 is boehmite and transforms to α-Al₂O₃ at 950 °C via the γ-AlOOH → γ-Al₂O₃ → α-Al₂O₃ path. Aluminum hydroxide precipitated at pH values in the 8 to 11 range is bayerite and transforms to α-Al₂O₃ at 1000 °C via the α-Al(OH)₃ → γ-Al₂O₃ → ε-Al₂O₃ + θ-Al₂O₃ → α-Al₂O₃ path. Moreover, the pH value affects not only the morphology of aluminum hydroxide particles which changes from ultrafine floccules through 50 nm blowballs then to 150 nm irregular agglomerates with increasing pH value but also the microstructures of final decomposition products of aluminum hydroxides.