TG measurement of reactivity of candidate oxygen carrier materials

This study examined several candidate raw materials for use as the reactive agents in developing new oxygen carriers for chemical looping combustion. A thermogravimetric analyzer, Mettler TGA/DSC1, was used to measure oxygen capacity and relative reaction rates during oxidation and reduction cycles. The reactive gases used were 4 % hydrogen in inert gas for the reduction cycle and air for the oxidation cycle, with a nitrogen purge between reduction and oxidation cycles. Samples were typically tested for at least ten cycles to study any change in reactivity or oxygen capacity. Reaction temperatures tested ranged from 700 to 900 °C. Materials tested included an iron oxide ore, iron-based tailings from a metals extraction process, a nickel oxide supported on nickel aluminate and a copper oxide plus inert material system. The materials varied in their oxygen capacity, reactivity and the change in properties with repeat cycles. Of the samples tested, the NiO–NiAl₂O₄ oxygen carrier demonstrated the fastest reaction in reduction and oxidation and had stable properties over ten cycles. The iron oxide ore sample performance declined significantly with repeat cycles. The performance of the iron-based tailings declined slightly over the ten cycles. The addition of inert second phase materials to CuO improved the performance by inhibiting sintering of the oxide at the operating temperature. Although the reactivity of the tailings and iron hydroxide samples was not as high as the NiO based oxygen carrier, they are promising carrier materials due to their low cost and lower toxicity relative to nickel. Future experiments will look at CO and CH₄ reduction reactions using the TG, surface characterization using SEM, XRD, and cyclic testing in a batch fluidized bed reactor.     

Effects of doping with CeO2 and calcination temperature on physicochemical properties of the NiO/Al2O3 system

The effects of doping with CeO₂ and calcination temperature on the physicochemical properties of the NiO/Al₂O₃ system have been investigated using DTA, XRD, nitrogen adsorption measurements at −196°C and decomposition of H₂O₂ at 30–50°C. The pure and variously doped solids were subjected to heat treatment at 300, 400, 700, 900 and 1000°C. The results revealed that the specific surface areas increased with increasing calcination temperature from 300 to 400°C and with doping of the system with CeO₂. The pure and variously doped solids calcined at 300 and 400°C consisted of poorly crystalline NiO dispersed on γ-Al₂O₃. Heating at 700°C resulted in formation of well crystalline NiO and γ-Al₂O₃ phases beside CeO₂ for the doped solids. Crystalline NiAl₂O₄ phase was formed starting from 900°C. The degree of crystallinity of NiAl₂O₄ increased with increasing the calcination temperature from 900 to 1000°C. An opposite effect was observed upon doping with CeO₂. The NiO/Al₂O₃ system calcined at 300 and 400°C has catalytic activity higher than individual NiO obtained at the same calcination temperatures. The catalytic activity of NiO/Al₂O₃ system increased, progressively, with increasing the amount of CeO₂ dopant and decreased with increasing the calcination temperature.     

Improvement of stability of out-layer MgAl2O4 spinel for a Ni/MgAl2O4/Al2O3 catalyst in dry reforming of methane

Summary The stability of Ni/-Al₂O₃ catalyst in dry reforming of methane was found to be improved by the addition of MgO into the catalyst, probably due to the formation of out-layer MgAl₂O₄ spinels, which can effectively suppress the phase transformation to form NiAl₂O₄ spinel phases, stabilize the tiny Ni crystallites and suppress carbon deposition in dry reforming of methane.     

Formation of NiO/YSZ functional anode layers of solid oxide fuel cells by magnetron sputtering

The decrease in the polarization resistance of the anode of solid-oxide fuel cells (SOFCs) due to the formation of an additional NiO/(ZrO₂ + 10 mol % Y₂O₃) (YSZ) functional layer was studied. NiO/YSZ films with different NiO contents were deposited by reactive magnetron sputtering of Ni and Zr–Y targets. The elemental and phase composition of the films was adjusted by regulating oxygen flow rate during the sputtering. The resulting films were studied by scanning electron microscopy and X-ray diffractometry. Comparative tests of planar SOFCs with a NiO/YSZ anode support, NiO/YSZ functional nanostructured anode layer, YSZ electrolyte, and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were performed. It was shown that the formation of a NiO/YSZ functional nanostructured anode leads to a 15–25% increase in the maximum power density of fuel cells in the working temperature range 500–800°C. The NiO/YSZ nanostructured anode layers lead not only to a reduction of the polarization resistance of the anode, but also to the formation of denser electrolyte films during subsequent magnetron sputtering of electrolyte.     

Enhanced electrochemical reduction of rare earth oxides in simulated oxide fuel via co-reduction of NiO in Li2O–LiCl salt

Rare earth oxides in spent oxide fuel from nuclear plants have poor reducibility in the electrochemical reduction process due to their high oxygen affinity and thermodynamic stability. Here, we demonstrate that the extent of their reduction can be enhanced via co-reduction of NiO in a Li₂O–LiCl electrolyte for the electrochemical reduction of a simulated oxide fuel (simfuel). First, the electrochemical behaviors of Nd₂O₃, NiO, and Nd₂O₃–NiO were studied by cyclic voltammetry and voltage control electrolysis. Then, the electrochemical reduction of the simfuel containing UO₂ and rare earth oxides (Nd₂O₃, La₂O₃, and CeO₂) was conducted in molten LiCl salt with 1 wt.% Li₂O via the co-reduction of NiO. The extent of reduction of the rare earth oxides was found to be significantly improved.     

Solid Reactions of Egyptian Kaolin towards Ni(II). II

The solid reactions between Kaolin of particle size 37 and 74μ and NiO in different mole ratios were performed at two temperatures, viz., 900 and 1100°C. The effect of the three parameters: concentration, particle size and temperature was studied spectrophotometrically and by x-ray diffractometry. The identified resulting phases are mainly the green spinel NiAl₂O₄, together with free SiO₂ and mullite Al₆Si₂O₁₃.     

Oxidation behavior of NiAl alloy at low temperatures

Oxidation behavior of NiAl alloy at low temperatures was studied. A NiAl plate was oxidized by exposure to ambient atmosphere at room temperature, heated at 473 K in air, and heated at 773 K in air. The oxide formed on the NiAl surface was investigated by angle‐resolved X‐ray photoelectron spectroscopy (AR‐XPS). Chemical composition and atomic concentration in the oxide layer were analyzed with factor analysis of XPS spectra. Exposure of the NiAl plate to the ambient atmosphere resulted in the formation of an Al₂O₃ layer along with a small amount of NiO. Oxidation of the NiAl plate at 473 K in air formed a film of double‐layered oxide; the top layer consisted of NiAl₂O₄ and a small amount of NiO, and the second layer was Al₂O₃. Successive oxidation at 773 K only changed the oxide‐layer thickness without changing the structure. Formation of oxide observed in the present study corresponds to the thermodynamic prediction for the oxidation behavior of NiAl at 1373 K. Copyright © 2007 John Wiley & Sons, Ltd.     

Plasma Assisted Synthesis and Physicochemical Characterizations of Ni–Co/Al2O3 Nanocatalyst Used in Dry Reforming of Methane

To assess the effects of plasma treatment a Ni–Co/Al₂O₃ nanocatalyst (10 % Ni and 3 % Co) was prepared via impregnation method followed by treatment with a non-thermal plasma to be investigated in a catalytic dry reforming of methane. The impregnated and plasma-treated nanocatalysts were characterized using XRD, FESEM, EDX, TEM, BET, FTIR, and XPS techniques. The XRD patterns confirmed the presence of nickel as NiO and NiAl₂O₄ and cobalt as Co₃O₄ on alumina support. Small NiO, NiAl₂O₄, and Co₃O₄ crystals observed in plasma-treated nanocatalyst, exhibited a good dispersion of active phase in this catalyst. The average particles size in plasma-treated sample obtain by FESEM micrograph were shown to be smaller than that of impregnated sample and the morphology was more homogenous and relatively agglomeration-free in plasma-treated Ni–Co/Al₂O₃ nanocatalyst. According to BET analysis, specific surface area of plasma-treated sample was 58 % higher than the non-treated catalyst. TEM analysis showed that particles of active phase were fairly small and well-dispersed on Al₂O₃ as a result of the plasma treatment. Better dispersion of active metal on the surface of plasma-treated sample was confirmed by XPS analysis. The plasma-treated sample showed higher yield and conversion at all temperature ranges investigated and was more resistant to coke formation compared to the non-treated sample. The results from the characterization and reaction studies suggests that plasma treatment may be a promising method for obtaining more active and stable nanocatalysts for dry reforming of methane.     

Effects of Ethanol Impregnation on the Catalytic Properties of Silica-Supported Cobalt Catalysts

Silca-supported Co₃O₄ (6 wt% as Co) catalysts were prepared by pore volume impregnation of ethanol or aqueous cobalt nitrate solutions, and calcined in vacuo to 300 °C. The catalytic performances of these catalysts for oxidation and hydrogenation of CO were examined. All Co₃O₄/SiO₂ catalysts were found to be very active in catalyzing oxidation of CO to CO₂ as compared to a commercial 1 wt% Pt/Al₂O₃. The ethanol-prepared catalysts exhibited higher activity than those of the aqua-prepared catalysts. Pre-calcination of the ethanol-prepared catalysts in oxygen at 600 °C resulted in a dramatic decrease in the activity. Temperature programmed oxidation indicated the presence of carbon deposits on the surface of used catalysts. Infrared spectra showed the continuous generation of CO₂ when these catalysts were exposed to CO. These indicate the primary role of CO disproportionation in catalytic oxidation of CO on Co₃O₄ at low temperature and explain the sharp decrease in activity in the initial period. After reduction at 400 °C, the ethanol-prepared catalysts were also found to be more active in catalyzing hydrogenation of CO, and produced less methane and olefin (C2-C4) fraction. Higher turnover frequencies were observed after high temperature reduction (600 °C) as well, at which ethoxyl groups were removed from silica surface. In both reactions, the enhanced activity for the ethanol-prepared catalysts can not be fully accounted for by the increase in the dispersion of Co₃O₄ or CO metal. This suggests that the surface structures of Co₃O₄ or CO were further modified by the carbonaceous species derived from ethanol.     

Activities and their relation to cation distribution in NiAl2O4MgAl2O4 spinel solid solutions

The activity of NiAl₂O₄ in NiAl₂O₄MgAl₂O₄ solid solutions has been measured by using a solid oxide galvanic cell of the type, Pt, Ni + NiAl₂O₄ + Al₂O₃(α)/CaOZrO₂/Ni + Ni_xMg_1?xAl₂O₄ + Al₂O₃(α). Pt, in the temperature range 750–1150°C. The activities in the spinel solid solutions show negative deviations from Raoult's law. The cation distribution in the solid solutions has been calculated using site preference energies independent of composition for Ni²⁺, Mg²⁺, and Al³⁺ ions obtained from crystal field theory and measured cation disorder in pure NiAl₂O₄ and MgAl₂O₄, and assumi g ideal mixing of cations on the tetrahedral and octahedral positions. The calculated values correctly predict the decrease in the fraction, α, of Ni²⁺ ions on tetrahedral sites for 1>x>0.25, observed by Porta et al. [J. Solid State Chem.11, 135 (1974)] but do not support their tentative evidence for an increase in α for x < 0.25. The measured excess free energy of mixing can be completely accounted for by using either the calculated or the measured cation distributions. This suggests that the Madelung energy is approximately a linear function of composition in the solid solutions. The composition of NiOMgO solid solutions in equilibrium with NiAl₂O₄MgAl₂O₄ solid solutions has been calculated from the results and information available in literature.     

On the rate of oxidation of co on La2O3 doped NiO/Al2O3 catalysts: an artificial neural network approach

Summary The rate constants of the oxidation of CO on a number of pure and La₂O₃ doped NiO/Al₂O₃ solid catalysts were correlated with the mole percent of dopant, calcinations temperature, surface area, pore volume and pore mouth diameter by an artificial neural network simulator. The cross validation method had to be used due to the scarcity of the data. A three-layer network with 3 nodes in the hidden layer was found to simulate the system well.     

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

Fe₂O₃/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.     

Facile Green Synthesis of NiO/NiCo2O4 Nanocomposite as an Efficient Electrochemical Platform for Determination of Dopamine

The synthesis of NiO/NiCo₂O₄ nanoparticles by an eco-friendly, fast, simple and cost-effective approach employing Urtica extract is reported in this study. The NiO/NiCo₂O₄ nanocomposite were characterized using VSM, FTIR, XRD, and SEM techniques. Moreover, to construct a modified carbon paste electrode, NiO/NiCo₂O₄ were employed and this sensor was used for dopamine (DA) detection. Using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques, the electrochemical behavior of dopamine at the NiO/NiCo₂O₄/CPE was investigated. Analysis of dopamine, with a limit of detection (LOD) equal to 0.04 μM, in the concentration range of 0.1–100.0 μM, was facilitated by NiO/NiCo₂O₄/CPE. Moreover, the satisfactory selectivity for DA determination in the presence of uric acid (UA) and ascorbic acid (AA), was obtained. The suggested new sensor displayed a good reproducibility, sensitivity, and stability for determination of DA in drug and biological samples.     

The system NiAl2O4Ni2SiO4 at high pressures and temperatures: Spinelloids with spinel-related structures

Phase relations in the system NiAl₂O₄Ni₂SiO₄ were studied in the pressure range 1.5 ～ 13.0 GPa and in the temperature range 800 ～ 1450°C. Two new phases, IV and V, were found in regions of pressure higher than 4 GPa. Phase V disproportionates into a mixture of Ni₂SiO₄-spinel, NiO, and Al₂O₃ at approximately 9.5 GPa and 1100°C. Phases III, IV, and V form a solid solution in some compositional range: phases IV and V have a composition around NiAl₂O₄·Ni₂SiO₄, whereas phase III spreads from NiAl₂O₄·Ni₂SiO₄ to the NiAl₂O₄-rich side. All the phases I ～ V are structurally considered to be spinel derivatives, “spinelloids,” with three kinds of tetrahedral groups; isolated tetrahedra TO₄, linked ones T₂O₇, and triply linked ones T₃O₁₀. The ratios of isolated tetrahedra to linked ones are large in the higher-pressure phases and small in the lower-pressure phases. The difference of compositional range of phase III from that of phases IV and V is possibly explained by the avoidance of linked tetrahedra such as O₃AlOAlO₃.     

Magnetron formation of Ni/YSZ anodes of solid oxide fuel cells

Physico-chemical and structural properties of nanocomposite NiO/ZrO₂:Y₂O₃ (NiO/YSZ) films applied using the reactive magnetron deposition technique are studied for application as anodes of solid oxide fuel cells. The effect of oxygen consumption and magnetron power on the discharge parameters is determined to find the optimum conditions of reactive deposition. The conditions for deposition of NiO/YSZ films, under which the deposition rate is maximum (12 μm/h), are found and the volume content of Ni is within the range of 40–50%. Ni-YSZ films reduced in a hydrogen atmosphere at the temperature of 800°C have a nanoporous structure. However, massive nickel agglomerates are formed in the course of reduction on the film surface; their amount grows at an increase in Ni content in the film. Solid oxide fuel cells with YSZ supporting electrolyte and a LaSrMnO₃ cathode are manufactured to study electrochemical properties of NiO/YSZ films. It is shown that fuel cells with a nanocomposite NiO/YSZ anode applied using a magnetron sputtering technique have the maximum power density twice higher than in the case of fuel cells with an anode formed using the high-temperature sintering technique owing to a more developed gas-anode-electrolyte three-phase boundary.     

氢分离与CO催化甲烷化双功能型钯复合膜

以多孔Al₂O₃陶瓷为基体材料, 采用浸渍法担载NiO后用2B铅笔修饰NiO/Al₂O₃表面, 通过化学镀法沉积约5 μm厚的金属钯, 还原后成功制得Pd/Pencil/Ni/Al₂O₃膜. 为进行对比, 还制备了未担载镍的Pd/Pencil/Al₂O₃膜. 膜的表面和断面形貌分别采用扫描电镜和金相显微镜观测, 膜的透氢动力学通过H₂/N₂单气体法测试, 并以成分为H₂ 77.8%, CO 5.2%, CO₂ 13.5%和CH₄ 3.5%的原料氢测定了膜的氢分离效果. 结果表明, 未载镍的Pd/Pencil/Al₂O₃膜只具有氢分离作用, 而Pd/Pencil/Ni/Al₂O₃膜还可以有效地将钯膜泄漏的CO和CO₂转化为甲烷, 因而成为双功能型钯膜. 这种双功能膜尤其适用于面向质子交换膜燃料电池(PEMFC)的氢气分离, 既有效解决了PEMFC对氢燃料中CO格外敏感的难题, 又提高了对钯膜缺陷的容忍度, 因而延长了钯膜的使用寿命.     

Study of Ce0.7Sn0.3O2 supported PdO catalysts for CO oxidation

Palladium supported by Ce_0.7Sn_0.3O₂ has been prepared by an impregnation method, and used for low temperature carbon monoxide oxidation. They were characterized by means of XRD and H₂-TPR techniques. For PdO/Ce_0.7Sn_0.3O₂ catalyst, three reduction peaks (α, β and γ) are observed. The β peak contributes to the reduction of PdO species and Sn⁴⁺ species on the surface of Ce_0.7Sn_0.3O₂; β peak to the reduction of bulk SnO₂ and surface Ce⁴⁺and the γ peak to the reduction of bulk CeO₂. The increase of Pd loading from 0 to 0.75% enhances oxidation of CO, further increase of the Pd content affects the catalytic activity but slightly. XRD and TPR results show that highly dispersed Pd on the surface of the support is the active species for CO oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of Calcination Temperature on Surface Oxygen Vacancies and Catalytic Performance Towards CO Oxidation of Co3O4 Nanoparticles Supported on SiO2

Co₃O₄/SiO₂ catalysts for CO oxidation were prepared by conventional incipient wetness impregnation followed by calcination at various temperatures. Their structures were char-acterized with X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and X-ray absorption fine structure (XAFS) spectroscopy. Both XRD and Raman spectroscopy only detect the ex-istence of Co₃O₄ crystallites in all catalysts. However, XPS results indicate that excess Co²⁺ ions are present on the surface of Co₃O₄ in Co₃O₄(200)/SiO₂ as compared with bulk Co₃O₄. Meanwhile, TPR results suggest the presence of surface oxygen vacancies on Co₃O₄ in Co₃O₄(200)/SiO₂, and XAFS results demonstrate that Co₃O₄ in Co₃O₄(200)/SiO₂ con-tains excess Co²⁺. Increasing calcination temperature results in oxidation of excess Co²⁺ and the decrease of the concentration of surface oxygen vacancies, consequently the for-mation of stoichiometric Co₃O₄ on supported catalysts. Among all Co₃O₄/SiO₂ catalysts,Co₃O₄(200)/SiO₂ exhibits the best catalytic performance towards CO oxidation, demon-strating that excess Co²⁺ and surface oxygen vacancies can enhance the catalytic activity of Co₃O₄ towards CO oxidation. These results nicely demonstrate the effect of calcination temperature on the structure and catalytic performance towards CO oxidation of silica-supported Co₃O₄ catalysts and highlight the important role of surface oxygen vacancies on Co₃O₄.     

Activities of δ-Al2O3-supported bimetallic Pt?NiO and Co?NiO catalysts for methane autoreforming: Oxidation studies

The methane oxidation activities of Pt−NiO and Co−NiO bimetallic catalysts have been investigated as part of a larger research program on the autothermal reforming of methane (combined methane oxidation and steam reforming) in a fluidized bed reactor. Experiments at atmospheric pressure and 783–1023 K for both catalysts showed that the reaction was more selective towards H₂ production at CH₄∶O₂ ratios greater than unity. Light-off temperature increased with decreasing CH₄∶O₂ ratios, but increase in gas velocity (beyond minimum fluidization) increased the light-off temperature. Co−NiO was as promising as the more expensive Pt−NiO catalyst for the oxidation.     

Co3O4−CeO2 Nanocomposites for Low-Temperature CO Oxidation

In an effort to combine the favorable catalytic properties of Co₃O₄ and CeO₂, nanocomposites with different phase distribution and Co₃O₄ loading were prepared and employed for CO oxidation. Synthesizing Co₃O₄-modified CeO₂ via three different sol-gel based routes, each with 10.4 wt % Co₃O₄ loading, yielded three different nanocomposite morphologies: CeO₂-supported Co₃O₄ layers, intermixed oxides, and homogeneously dispersed Co. The reactivity of the resulting surface oxygen species towards CO were examined by temperature programmed reduction (CO-TPR) and flow reactor kinetic tests. The first morphology exhibited the best performance due to its active Co₃O₄ surface layer, reducing the light-off temperature of CeO₂ by about 200 °C. In contrast, intermixed oxides and Co-doped CeO₂ suffered from lower dispersion and organic residues, respectively. The performance of Co₃O₄-CeO₂ nanocomposites was optimized by varying the Co₃O₄ loading, characterized by X-ray diffraction (XRD) and N₂ sorption (BET). The 16–65 wt % Co₃O₄−CeO₂ catalysts approached the conversion of 1 wt % Pt/CeO₂, rendering them interesting candidates for low-temperature CO oxidation.