Solution combustion synthesis of metal nanopowders: Copper and copper/nickel alloys

Based on a general methodology for the preparation of metal–nanopowders by solution combustion synthesis (SCS), the reaction pathways for SCS of pure copper and copper–nickel alloy nanopowders are investigated. It is confirmed that the necessary condition for SCS of metals in a metal‐nitrate oxidizer–glycine system is the property of the oxidizer to decompose with formation of HNO₃ species. In this case, for compositions with excess of glycine, a hydrogen reducing atmosphere develops in the reaction front, leading to the formation of reduced metals. The proposed reaction pathways are supported by X‐ray diffraction analysis of the quenched samples and DTA–TGA studies of the Cu(NO₃)₂·6H₂O–glycine and Ni(NO₃)₂·6H₂O/Cu(NO₃)₂·6H₂O–glycine systems. The results show that the formation of Cu₂O and CuO oxide phases takes place at early stages in the reaction front followed by their reduction to pure Cu phase in the postcombustion zones. However, in a Cu–Ni alloy, a fraction of intermetallic Cu–Ni phase appeared directly in the combustion front, whereas the rest of the oxygen‐free alloy formed through reduction of oxide phases. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Formation of triple alloys and their hydrides in the Ti-Zr-Hf-H system

A novel technique is suggested for synthesis of refractory metal alloys by dehydrogenation of compacted mixtures of transition metal hydrides. This technique is applicable to preparation of not only binary but also multi-component alloys, intermetallics, and alloys of III, IV, V, and VIII Group metals with various additives. In this work, the results of detailed investigation on the conditions for formation of Ti-Zr-Hf triple alloys from the TiH₂, ZrH₂, and HfH₂ are presented. The alloy structure was found to depend on the ratio of components in a green charge. The triple alloys containing the α- and ω-phases have been prepared. Demonstrated was the ability of Ti-Zr-Hf triple alloys to undergo reaction with hydrogen in a combustion mode yielding respective hydrides.     

Efficient photocatalysts by hydrothermal treatment of TiO2

The aim of this work was to identify the optimum synthesis conditions and the most effective technique for noble metal deposition in a perovskite/palladium-based catalyst for natural gas combustion. The solution combustion synthesis (SCS) of perovskite/zirconia-based materials was investigated, by starting from metal nitrates/glycine mixtures. Characterization and catalytic activity tests were performed on as-prepared powders and then repeated after calcination for 2 h at 900 °C in calm air. Calcination appeared to be beneficial in that, despite lowering the specific surface area, it promoted the simultaneous crystallization of both LaMnO₃ and ZrO₂ and the half-conversion temperature (T₅₀), regarded as an index of catalytic activity, was lowered. Two phases, both active towards methane oxidation – lanthanum manganate and palladium oxide – were combined so as to evaluate their synergism in terms of catalytic activity. Pd was therefore added either via incipient wetness impregnation on LaMnO₃·2ZrO₂ or through a one-step SCS-based route. Characterization and catalytic activity tests followed suit. Optimal composition and preparation routes were found: T₅₀ was lowered from 507 °C – pure LaMnO₃ prepared via SCS – to 432 °C attained with a 2% (w/w) Pd load on pre-calcined LaMnO₃·2ZrO₂.     

Solution combustion synthesis of CeFeO3 under ambient atmosphere

Polycrystalline CeFeO₃ was prepared with almost single phase purity of 93% by a solution combustion synthesis (SCS), but contrarily to all other reports, CeFeO₃ could be obtained under ambient atmosphere. The perovskite-type phase was gained at 400 °C from a solution containing stoichiometric amounts of metal nitrates and by using glycine as fuel. For comparison, CeFeO₃ was also prepared under inert atmosphere by a conventional, oxalate-based high-temperature solid-state reaction. The products were structurally characterised by X-ray powder diffraction (XRD), and the lattice parameters were determined from subsequent Rietveld refinements. Since solution combustion is often referred as the method delivering nano-sized ceramics, the morphological characteristics of the products obtained through the two different synthesis methods were compared by scanning electron microscopy (SEM). Furthermore, the particle sizes were determined by laser diffraction analysis.     

One-step solution combustion synthesis of K0.5Na0.5NbO3 powders at a large-scale

In this work, fine powders of K_0.5Na_0.5NbO₃ (KNN) are produced by a single step of solution combustion synthesis (SCS) using glycine as the reductant fuel and potassium & sodium nitrates as the oxidizers, which are mixed with niobium oxide. Single phase of KNN products can be successfully prepared in a one-step SCS by adjusting the glycine-nitrate ratio. It is concluded that the stoichiometric or slightly fuel-excess condition is good for the single step production of KNN. The one-step SCS of KNN also shows good scaling-up productivity, which can be ignited by both electric heating and microwave heating. This work may provide a feasible large-scale production of various functional oxides by a one-step SCS process.     

Al2O3–W nanocomposites obtained by colloidal processing and in situ synthesis of nano-metal particles

The preparation process and the properties of Al₂O₃-W nanocomposites obtained by gelcasting are reported. The novelty of the synthesis path is the formation of nano-tungsten particles in an in situ reduction of water-soluble precursor during pressureless sintering. The use of water-soluble salt as a tungsten precursor ensured highly homogenous distribution of reinforcing particles and good adhesion between ceramic and metal phases. The maximum content of tungsten in the composites was approximately 0.5 wt%. The size of the reinforcing particles was less than 100 nm. Presence of metallic tungsten and tungsten carbide (W₂C) in the composites lead to the improvement of mechanical parameters: an increase of hardness by about 10 % and of fracture toughness by about 20 %, compared to the reference sample of pure aluminum oxide.     

Solution combustion synthesis for preparation of structured catalysts: A mini-review on process intensification for energy applications and pollution control

Solution combustion synthesis (SCS) is a preparation technique that can be used to synthesize a variety of inorganic nanomaterials and structured catalysts. It is based on a self-propagating exothermic redox reaction between organic salts and a fuel mixed together in an aqueous solution, which results in the formation of nanocrystalline and highly pure solid nanomaterials. SCS can be considered as an attractive synthesis method for catalysts due to the simple nature of the synthetic route and short reaction times. The process is easily scaled up to any kind of application which makes it economically attractive. This mini-review provides a short overview on the synthesis of structured catalysts by SCS and their recent utilization for energy applications and pollution control.     

Solution Combustion Synthesis of nanoscale Cu-Cr-O spinels: Mechanism,properties and catalytic activity in CO oxidation

Nano-structured Cu-Cr-O-based catalysts were successfully prepared by Solution Combustion Synthesis (SCS), and it has been found that their physical properties and atomic structure depend (in a complex way) on the initial composition of SCS processing, that the temperature of combustion and composition of initial mixture influences metal concentration in the nanocomposite, and that metal formation is going through metal oxide reduction by NO, H₂, CO. Presence of CO, NH₃ and H₂ in the gas phase, during SCS, was determined chromatographically. Connection between structural changes during reaction and properties of final nanocomposite material was determined. The catalysts were characterized by XRD, SEM/EDX, and their catalytic activity has been determined in CO oxidation. The pore structure of the samples was calculated with the Barrett-Joyner-Halenda (BJH) academic model, and the specific surface area was calculated with the Brunauer-Emmett-Teller (BET) adsorption equation. The specific surface area of the catalysts is varied between 10 m²/g and 37 m²/g. Understanding the interrelationships between activity of catalyst and the ensuing atomic structure has allowed a degree of optimization of the catalytic properties of the new catalysts.     

An investigation of fuel precursors and calcination temperature to obtain mayenite (Ca12Al14O33) powders by solution combustion synthesis

The influence of the fuel (glycine, urea, citric acid and sucrose) and calcination temperature used to obtain calcium aluminate in the mayenite phase assisted by solution combustion synthesis (SCS) is the central point of this work. Thermal gravimetric analysis helps to establish the calcination temperatures used (1100, 1200 and 1300 °C). Using the X-ray diffraction technique (XRD) and complementary analyses, such as Raman spectroscopy, the specific surface area and laser granulometry, it was possible to elucidate the behavior and relationship of the fuel and heat treatment on the phase formation, crystallite size and powder crystallinity. Glycine showed better performance than other fuels, with the lowest calcination temperature, obtaining pure mayenite with nanometric crystallite size in all calcination ranges. Thus, it was observed that the type of fuel has an influence on obtaining pure mayenite, as well as the calcination temperature, and glycine reveals the best performance.     

Electropolishing of metals in alcoholic solution of sulphuric acid

The potentiodynamic polarization curves of iron, nickel, cobalt, molybdenum, copper and iron-nickel and cobalt-nickel alloys were measured in the polishing solution of 1M H₂SO₄ in CH₃OH. The above mentioned metals and alloys may be divided into three groups: metals and alloys with high dissolution rate in the active state in which the polishing follows immediately behind the active dissolution region (Fe, Co and Ni-Fe and NiCo alloys with a low nickel content), metals and alloys with low passivating current density that become passive at first and the polishing takes place behind the region of localized corrosion (nickel and NiFe and NiCo alloys with a high nickel content) are metals that are polished in the transpassive region (molybdenum).     

SHS metallurgy of refractory inorganic compounds of chrome at atmospheric pressure

Laws of synthesis of cast refractory inorganic compounds and alloys in the regime of combustion at atmospheric pressure are studied. It is demonstrated that the combustion parameters and the composition of the synthesis products can be controlled by varying the ratio of the high-exothermic and low-exothermic components of the initial mixture. In domains of optimal parameters, it is possible to perform gravitational separation of the metal and oxide phases of combustion products and to obtain cast materials.     

Electrochemical oxygen reduction on oxide catalysts

The process of oxygen electroreduction and adsorption in alkaline medium on nickel and cobalt oxides has been studied by the potentiodynamic and the disc-ring electrode methods. The oxide films were formed on the surface of metal specimens by thermal or electrochemical oxidation.After oxidation, the surface was examined by X-ray diffraction and electron diffraction. It was found that, depending on the oxidation conditions, simple or complex oxides are formed on the surface of metal specimens. Nickel and Cobalt oxide films obtained by thermal oxidation at t° ?300°C are more active than pure metals. The oxides of spinel structure Co₃O₄ and NiCo₂O₄ have an optimum activity. It is shown that oxygen adsorbed during the heat treatment of the specimens accelerates the process of the molecular oxygen reduction. On the oxides of spinel structure the oxygen reduction reaction proceeds to OH^?, whereas on simple nickel oxides the final product of oxygen reduction is HO^?₂.     

Analysis of the aluminum reaction efficiency in a hydro-reactive fuel propellant used for a water ramjet

A high-pressure combustor and a metal/steam reactor are used to simulate the two-stage combustion of hydro-reactive propellants used for a water ramjet. Raw metal powders added to the propellants are the aluminum power, magnesium powder, 50/50 aluminum-magnesium alloy (AM), and ball-milled 50/50 aluminum-magnesium alloy (b-AM), which are characterized by using scanning electron microscopy (SEM), x-ray diffraction (XRD), and simultaneous thermogravimetric analysis (TGA). The efficiencies of the Al reaction in the raw metal in heated steam and in the propellants during the two-stage combustion are calculated. The results indicate that both Mg and Al in the alloys, whether b-AM or AM, can react completely in air when heated up to 950°C. The XRD patterns for the combustion products of the AM and b-AM alloys in heated steam contain magnesium oxide MgO, spinel Al₂MgO₄, and Al diffraction peaks. The Al reaction efficiencies of the AM and b-AM alloy powders in heated steam are much higher than that of the Al powders. The hydroxyl-terminated polybutadiene (HTPB)-ammonium perchlorate (AP)-(b-AM)-Mg and HTPB-AP-AM-Mg propellant systems exhibit good performance in terms of the Al reaction efficiency, which are better than that of the HTPB-AP-Al-Mg and HTPB-AP-Al systems.     

Pd-based (Ce,Zr)Ox-supported catalysts: Promoting effect of base metals (Cr, Cu, Ni) in CO and NO elimination

A series of Pd–M bimetallic three-way catalysts (M = Cr, Cu and Ni) supported on a (Ce,Zr)O_x material has been characterized using a combination of X-ray diffraction and Raman spectroscopy, and employing in situ diffuse reflectance Fourier transform infrared and X-ray near-edge structure spectroscopies to analyse the redox and chemical processes taking place during light-off conditions under CO, NO and O₂. The catalytic behaviour of these bimetallic systems was strongly affected by the degree of interaction between the noble and base metals in the calcined state. Among the base metals tested, Ni appeared to exert the least influence over the noble metal state/behaviour after calcination and under reaction conditions. Cr and Cu appear to interact with Pd in the calcined state, leading to a reduction in the temperature at which Pd was converted to Pd(0) with simultaneous formation of a binary PdM alloy during the reaction run. At high temperature, these alloy phases evolved into pure metallic Pd(0) particles and, in the case of the Cu-containing catalyst, result in a strong interaction with the support. The catalytic performance of these three Pd–M systems in the CO and NO elimination reactions are correlated with the nature and properties of the oxidized and reduced Pd-containing phases which are present in each case.     

Solution combustion synthesis of cerium oxide nanoparticles as corrosion inhibitor

In this study, combustion synthesis of cerium oxide nanoparticles was reported using cerium nitrate hexahydrate as starting material as well as urea, glycine, glucose, and citric acid as fuels. The influence of fuel type on structure, microstructure, band gap, and corrosion inhibition was investigated. X-ray diffraction (XRD) patterns and scanning electron microscopy micrographs showed that CeO₂ nanoparticles with different morphologies were obtained depending on the fuel type. Microstructural changes from unreacted gel to sponge-like morphologies were resulted by varying the fuel type from urea, glycine, and glucose to citric acid. In addition to Ce–O bonds, Fourier transform infrared analysis showed carbon bonds of carbonaceous compositions from incomplete combustion which were declined during combustion reaction. Furthermore, corrosion analyses showed that samples synthesized using urea fuel released the most Ce⁺⁴ ions and could have better protection than other samples.     

Competing chemical transformations in the combustion wave in the Fe2O3-Cr2O3-Al mixture

Laws of combustion of a thermit-type mixture Fe₂O₃-2Al-γCr₂O₃ are considered. The limits of liquid-phase combustion and separation of the oxide and metal phases in the final products are determined. Chromium oxide is demonstrated to weakly compete with iron oxide in the oxidation-reduction reaction with aluminum and to participate mainly in oxide-phase formation. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 89–91, May–June, 2006.     

Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts

Catalysts containing metals such as Cu, Ni, Fe, Co in their reduced state are often subjected to passivation procedures prior to characterization. Passivation with N₂O or O₂ to create a protective oxide layer also results in a certain degree of sub-surface oxidation. The heat released during oxidation is a critical parameter. The extent of bulk oxidation depends on the type of oxidant as well as on the size of the metal particles, as shown for copper catalysts. The final, meta-stable passivation layer requires a certain thickness to sustain exposure to ambient atmosphere. The encapsulation of metal particles in carbon is an efficient method for preserving the metallic state, as demonstrated for metallic nickel and iron with carbon nanofibers. The use of passivated samples for characterization of the active, i.e., reduced, catalyst has limited value.     

Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

Single frequency electrochemical impedance investigation of zero charge potential for different surface states of Cu–Ni alloys

Adsorption of sulphate anions onto polycrystalline pure copper, pure nickel and two copper–nickel alloys was explored by performing zero charge potential E _PZC measurements. E _PZC measurements were performed by differential capacitance analysis using single?frequency electrochemical impedance spectroscopy. E _PZC and surface charge of the studied materials were analysed for different surface states: bare—immediately after cathodic polarisation and after 5 h of immersion in a 0.1 M Na₂SO₄ solution. According to the surface state and nature of the studied materials, the choice of the appropriate measuring frequency was investigated. Specific methodologies dedicated to the study of electrode/electrolyte interfaces are then presented for bare metal, metals covered by corrosion products and metals covered by passive film. In the presence of passive film, the space charge capacitance interferes with the differential capacitance measurements, and its influence is discussed in the framework of semiconductor electrochemistry.     

On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl₂–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe₂O₃, Cr₂O₃, Co₃O₄, NiFe₂O₄) in pure CaCl₂ or CaCl₂–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe₂O₃ increased. Ni coated with NiO film had much higher stability during anodic polarization.