Copper Aluminates Research Articles

Precursor engineering of hydrotalcite-derived redox sorbents for reversible and stable thermochemical oxygen storage

Chemical looping processes based on multiple-step reduction and oxidation of metal oxides hold great promise for a variety of energy applications, such as CO2 capture and conversion, gas separation, energy storage, and redox catalytic processes. Copper-based mixed oxides are one of the most promising candidate materials with a high oxygen storage capacity. However, the structural deterioration and sintering at high temperatures is one key scientific challenge. Herein, we report a precursor engineering approach to prepare durable copper-based redox sorbents for use in thermochemical looping processes for combustion and gas purification. Calcination of the CuMgAl hydrotalcite precursors formed mixed metal oxides consisting of CuO nanoparticles dispersed in the Mg-Al oxide support which inhibited the formation of copper aluminates during redox cycling. The copper-based redox sorbents demonstrated enhanced reaction rates, stable O2 storage capacity over 500 redox cycles at 900 °C, and efficient gas purification over a broad temperature range. We expect that our materials design strategy has broad implications on synthesis and engineering of mixed metal oxides for a range of thermochemical processes and redox catalytic applications.

Photocatalytic degradation of oxytetracycline with the REMs (Er, Tm, Yb)-doped nickel and copper aluminates

Oxytetracycline (OTC) is an antibiotic drug. It is widely used in veterinary, animal husbandry, and agriculture. Antibiotics are emerging environmental pollutants. We propose an efficient photocatalytic method for remediation of aqueous reservoirs from OTC. We synthesized erbium-, thulium- and ytterbium-doped nickel and copper aluminates. The doped samples have better dispersion, higher specific surface areas (188 m2 g−1 for Er-NA) and decreased energy band gaps. The doping of nickel and copper aluminates (NA and CA) significantly improved the photocatalytic performance. Er-CA was the most photoactive. The Er-doping of NA and CA increased photodegradation efficiency from 27 to 73 % and from 65 to 90 %, respectively. The LC-MS analysis allowed to identify the major intermediates of OTC photodegradation. The photocatalytic metabolites were hemocompatible and did not present cytotoxicity towards monocytic cells. We envisage a great potential of the rare-earth-metal-doped semiconductors in the removal of emerging environmental pollutants.

Effect of Sc and Zr on Precipitation Behaviour of Wrought Al- Bronze

Aluminium bronze doped with Sc and Zr were prepared by casting followed by cold-rolling with varied degrees of deformation. Then, the cold-rolled samples were aged for one hour at various temperatures, isochronally up to 500°C and isothermally at 300°C and 350°C up to four hours respectively. To better understand the precipitation behavior of Al-bronze with Sc and Zr additions, microhardness and electrical resistivity of aged alloys were investigated. The formation of copper aluminates precipitate resulted in a significant hardening improvement in aged alloys particularly at 300 °C. The addition of both Sc and Zr improved the thermal stability of the aged alloys by forming Al3Sc and Al3Zr precipitates. Sc, on the other hand, had proven to be more effective at improving aging behaviour of Al-bronze alloy. Electrical resistivity was also observed to increase with increasing aging temperature, then decrease slowly due to the production of fine precipitates and coarsening of this precipitate by over ageing, respectively. The DSC investigation supported the precipitation response by demonstrating the varied peaks of the heating curve at various temperatures. The reflectance spectroscopy study revealed that the addition of Sc and Zr exhibited greater optical response owing to grain refinement. Optical microscopy of cold rolled samples exhibited multiple phases of elongated grain along the rolling direction, and Sc and Zr additions were capable of refining alloy grains by precipitation of primary Al3Sc and Al3Zr particles formed as heterogeneous nuclei during solidification. In the meantime, recrystallization and grain growth development were induced in Al-bronze by ageing at 500°C for an hour. During ageing treatment, the production of secondary Al3Sc and Al3Zr precipitates efficiently inhibited the mobility of dislocation and sub-grain boundaries, preventing the Sc and Zr doped alloys from recrystallization.

Synthesis of CuAl2O4 Nanoparticle and Its Conversion to CuO Nanorods

The molten salt approach was used to convert CuAl2O4 nanoparticles to CuO nanorods in this study. Molten hydroxide (NaOH) synthesis was chosen over molten salts (NaCl/KCl) for removing aluminium oxide from copper aluminate at low temperatures. The molten salt process is environmentally beneficial. Polymeric precursors were used to make nanosized copper aluminates. Alginic acid polymer is used to gel aqueous solutions of copper acetate and aluminium nitrate, yielding precursor after further heating. The precursor provides 14 nm nanosized copper aluminates after being heated at 900°C for 5 hours. XRD, FTIR, SEM, and TEM were used to characterize the nanosized copper aluminate powder. Solid state mixing and solution technique were used to investigate molten hydroxide treatment of spinel CuAl2O4. The products of the reaction were identified using XRD. FTIR and SEM are also used to analyze the sample. Using UV-DRS absorbance spectrum analysis, the optical characteristics of CuAl2O4 and CuO nanorods were examined. Using the Tauc plot method, the band gaps of CuAl2O4 and CuO were calculated to be 4.3 and 3.93 eV.

CuAl2O4–CuO–Al2O3 catalysts prepared by flame-spray pyrolysis for glycerol hydrogenolysis

The catalysts of copper aluminates (CuAl2O4), copper oxide (CuO), and alumina (Al2O3) synthesized via one-step flame-spray pyrolysis (FSP) with various mole ratios of Cu to Al have been studied in the hydrogenolysis of glycerol, a byproduct from the biodiesel industry, to 1,2-propanediol (1,2-PDO) at 220 °C under 50 bar of H2 for 6 h. The studies on the CuAl2O4 nanoparticles reveal that their formation and particle size depend upon the mole ratio of Cu to Al, and they possess a structure analogous to that of the bulk material. The X-ray absorption near edge spectroscopy (XANES) data along with X-ray diffraction (XRD) patterns confirmed the formation of a CuAl2O4–CuO–Al2O3 mixed phases. The characteristic CuAl2O4 species was dominant during the flame process when a low content of Cu to Al, such as a 1:5 mol ratio, was supplied. It was observed that the CuAl2O4 combined with the CuO and Al2O3 phases exerts a strong positive effect on the catalytic activity. A selectivity to 1,2-PDO of 91.7% was observed for the synthesized catalyst with a 1:1 mole ratio of Cu to Al. The developed catalyst was superior to the conventional CuO-based ones in the hydrogenolysis of glycerol process.

A kinetic study on the mechanisms of metal leaching from the top surface layer of copper aluminates and copper ferrites.

Recent studies have reported the potential copper immobilization in aluminates (CuAl2O4 and CuAlO2) and ferrites (tetragonal CuFe2O4 and cubic CuFe2O4) and suggested a reliable method to stabilize metals in reusable ceramic products. In this study, copper immobilization effect was further analyzed in the leaching solutions with pH close to environmental conditions. The results from the chemical equilibrium model Visual MINTEQ illustrated that almost all copper, aluminum, and iron formed complexes with CH3COO- ions in the leachates. The dissolution behavior on sample surface was further explicated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The weight percentage of leached copper was lower than 0.1% even after 22-day leaching, indicating the successful copper stabilization in aluminates and ferrites. The results showed the highest copper concentrations in CuAlO2 leachates and the smallest leached copper amount from tetragonal CuFe2O4, respectively. The incongruent dissolution with Al-O or Fe-O bonds still remaining on the solid surface may be beneficial for preventing further leaching of copper. Furthermore, the modeling of reaction kinetics found that copper leaching from the CuAl2O4 and CuAlO2 obeyed the second-order reaction with correlation coefficients higher than 0.99. Moreover, the shrinking core model was chosen to analyze the leaching mechanisms of both CuFe2O4 ferrites, and the diffusion through product layer model acted as the rate-controlling step in their leaching process.

Influence of Method of Introduction of Cu- and Zn-Based Modifiers on the Properties of Chromia–Alumina Catalysts

Three methods of introduction of modifiers based on Cu and Zn compounds into the CrOx/Al2O3 catalysts for dehydrogenation of light paraffin hydrocarbons are considered: Introduction from sol, introduction using successive impregnation technique and introduction of modifiers by impregnation along with precursor of chromium oxide. The obtained samples are studied by a complex of physical-chemical methods (XRD, UV-Vis spectroscopy, temperature-programmed reduction with hydrogen (TPR-H2), X-ray fluorescent (XRF) spectrometry, low-temperature N2 sorption). The catalytic properties of the samples are studied in kinetic mode in isobutane dehydrogenation. Cu- and Zn-modifiers are shown to influence on the peculiarities of reduction of Cr6+ and, hence, specify the state of active surface of CrOx/Al2O3 catalysts formed in the reductive reaction medium. Not only do the states of modifiers influence on the initial activity of the catalyst, but also on its activity after oxidative regeneration. Introduction of modifiers by successive impregnation method results in formation of copper and zinc aluminates or defective spinels on the Al2O3 surface. When the active component is introduced, the modified surface of the support promotes formation and stabilization of Cr6+ sites that can undergo reversible reduction–oxidation and provide high activity and selectivity towards formation of isobutylene (>98%).

Evaluation of a hierarchically-structured CuO@TiO2-Al2O3 oxygen carrier for chemical looping with oxygen uncoupling

High redox reactivity as well as sufficient physical durability of the oxygen carrier (OC) is of critical importance for the successful implementation of chemical looping processes. In this work, CuO@TiO2-Al2O3 OC with hierarchical structure, which is a kind of CuO-rich, Al2O3-supported and TiO2-stablized ternary combined metal oxides system, was prepared by the self-assembly template combustion synthesis (SATCS) method. Performance of the rationally designed OC was cyclically tested with H2 in a thermogravimetric analyzer (TGA), as well as with coal in a batch fluidized bed reactor. TGA results showed that both the reduction and oxidation reactivity of the OC exhibited a reinforcing process during the first few cycles and then stabilized within the next dozens of cycles, and no obvious reactivity degradation phenomenon was observed. Reaction characteristics of the OC were further examined by reacting with coal in a fluidized bed reactor. Several critical performance indexes, like carbon conversion rate, oxygen release rate and combustion efficiency at different temperatures were calculated and compared. Mutual promotion relationship between carbon conversion rate of the coal and oxygen release rate of the OC was observed. Moreover, 20 cycles of redox process with anthracite were further conducted at 900°C to evaluate both the physical and chemical stability of the OC. Stable and high combustion efficiency (higher than 97.2%) was achieved during the cyclic fluidized bed test, and the subsequent physicochemical characterization indicated that neither CuAl2O4 phase was formed nor serious sintering problem occurred within the used OC samples. Thermo-mechanical analysis (TMA) results demonstrated relatively high transformation temperature of the OC, which explains well its superior anti-sintering property. All in all, the SATCS-derived CuO@TiO2-Al2O3 OC exhibited distinguished advantages of preventing the side reaction between CuO and Al2O3 to form copper aluminates as well as inhibiting the OC sintering, which could be a suitable OC candidate for commercial application in chemical looping with oxygen uncoupling (CLOU) processes.

Thermodynamic stability of solid phases in the system Cu–O–Al2O3 by means of the EMF and DSC-TGA techniques

Thermodynamic properties of solid phases in the Cu–O–Al2O3 system were measured by means of the EMF method with oxygen concentration galvanic cells based on stabilized zirconia solid electrolytes. The standard Gibbs energies of formation of pure copper oxides and copper aluminates from their component oxides were determined. Copper aluminates were also investigated calorimetrically by the DSC-TGA combined techniques. Based on the calorimetric measurements, the enthalpies and temperatures of spinel decomposition and delafossite melting were determined. The obtained results were discussed and compared with the available literature data.

The interaction between CuO and Al2O3 and the reactivity of copper aluminates below 1000 °C and their implication on the use of the Cu–Al–O system for oxygen storage and production

This work investigates the interaction between CuO and different aluminas (and precursors) which are commonly used as support materials at temperatures between 700 and 1000 °C, as well as some chemical properties of the resulting copper aluminates.

Production of Copper and Cobalt Aluminate Spinels and Their Application As Supports for Inulinase Immobilization

Copper and cobalt aluminates were obtained through the use of chitosan as template. In this synthesis route, chitosan is eliminated by heating, and a porous material is produced. These oxides were used as supports for inulinase immobilization by adsorption process. Physical properties of produced particles were analyzed by X-ray diffraction (XRD) and nitrogen adsorption-desorption isotherms. Both oxides presented particles containing mesoporous characteristics and high surface area, which is desirable for applications in enzyme immobilization processes. The results revealed that the copper and cobalt aluminates exhibit high inulinase immobilization efficiencies, which makes them promising supports for enzyme immobilization.

Self-assembly template combustion synthesis of a core–shell CuO@TiO2–Al2O3 hierarchical structure as an oxygen carrier for the chemical-looping processes

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are the most promising technologies for capture of CO2 at a low cost. CLC and CLOU involve cyclic reduction and oxidation of a solid oxygen carrier (OC) to transfer active oxygen from air to carbon-based fuels for combustion with the inherent feature of isolating CO2. One key issue with these technologies is to manufacture a high-performance, economical and practical OC material. In this paper, we propose a self-assembly template combustion synthesis (SATCS) method for preparing a hierarchical structure copper-based OC, Cu-rich, Al2O3-supported and TiO2-stabilized in a core–shell microarchitecture. The selection of raw materials and the rational design of OCs are based on the density functional theory (DFT) calculations and the classical Derjaguin–Landau–Verwey-Overbeek (DLVO) theory, respectively. The spontaneous aggregation between α-Al2O3 microparticles (μm-Al2O3) and rutile TiO2 nanoparticles (nm-TiO2) is driven by the van der Waals attractive and electrostatic attractive forces to form core(Al2O3)–shell(TiO2) hard templates in the wet precursor containing copper nitrate and urea. Upon calcination of the dried precursor in air, fewer nitrogen oxides (NOx) are released, demonstrating an environment-friendly fabrication process, whereby a highly dispersed and high-loading Cu-based crystalline layer is deposited on the nanostructured shell. A representative CuO@TiO2–Al2O3 OC (17.5wt% Al2O3, 5wt% TiO2) possesses hierarchical porosity and an interconnected framework and exhibits excellent performance of the reactivity, stability, oxygen release/uptake capacity and mechanical strength in the high-temperature redox cycles. The proper addition of TiO2 nanoparticles is effective in preventing the formation of copper aluminates (CuAl2O4 and CuAlO2) and inhibiting the sintering of active component grain. In addition, the rational design method and novel synthesis technique allow a mass production of various metal oxide composites used for chemical looping processes with CO2 capture and clean utilization of energy.

The Uses of Copper and Zinc Aluminates to Capture and Convert Carbon dioxide to Syn-gas at Higher Temperature

The uses of copper and zinc aluminates to capture and convert the CO2 to syn-gas were studied at higher temperatures. The samples of copper and zinc aluminates were prepared by solid-solid fusion method by calcining in air at 900 oC for 3 h. Those samples were characterized by acidity/alkalinity, surface area, XRD pattern, IR, SEM images and screening to capture CO2 at the different temperatures. The phases Cu2O, CuO, ZnO, CuAl2O4 and ZnAl2O4 were found to be in the samples of zinc and copper aluminates. Acidity and surface area of the samples of copper and zinc aluminates were found to be in the ranges from 0.063 to 9.37 mmol g-1 and 3.04 to 11.8 m2 g-1, respectively. The captured CO2 by the samples of copper and zinc aluminates was found to be 19.92 to 31.52 wt% for the temperature range 40 to 850 oC. The captured CO2 at 550 oC by variable Zn/Al and Cu/Al mol ratio from 0.5 to 6 of the samples of copper and zinc aluminates was found to be 12.81 to 18.04 wt%. The reduction of carbon dioxide by zinc and copper aluminates was observed. The conversion of CO2 by methane over variable mol ratio of Cu/Al and Zn/Al in copper and zinc aluminates, respectively, at 500 oC showed the production of syn-gas by using the gas hourly space velocities (GHSV) 12000, 12000 and 6000 ml. h-1. g-1 of helium, CO2 and methane. The conversions of CO2 by methane over the samples of zinc and copper aluminates were studied at different mol ratios of CO2 to methane. © 2014 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

A high performance oxygen storage material for chemical looping processes with CO2capture

Chemical looping combustion (CLC) is a novel combustion technology that involves cyclic reduction and oxidation of oxygen storage materials to provide oxygen for the combustion of fuels to CO2 and H2O, whilst giving a pure stream of CO2 suitable for sequestration or utilisation. Here, we report a method for preparing of oxygen storage materials from layered double hydroxides (LDHs) precursors and demonstrate their applications in the CLC process. The LDHs precursor enables homogeneous mixing of elements at the molecular level, giving a high degree of dispersion and high-loading of active metal oxide in the support after calcination. Using a Cu–Al LDH precursor as a prototype, we demonstrate that rational design of oxygen storage materials by material chemistry significantly improved the reactivity and stability in the high temperature redox cycles. We discovered that the presence of sodium-containing species were effective in inhibiting the formation of copper aluminates (CuAl2O4 or CuAlO2) and stabilising the copper phase in an amorphous support over multiple redox cycles. A representative nanostructured Cu-based oxygen storage material derived from the LDH precursor showed stable gaseous O2 release capacity (∼5 wt%), stable oxygen storage capacity (∼12 wt%), and stable reaction rates during reversible phase changes between CuO–Cu2O–Cu at high temperatures (800–1000 °C). We anticipate that the strategy can be extended to manufacture a variety of metal oxide composites for applications in novel high temperature looping cycles for clean energy production and CO2 capture.

Vacuum infiltration of copper aluminate by liquid aluminium

This paper studies attained microstructures and reactive mechanisms involved in vacuum infiltration of copper aluminate preforms with liquid aluminium. At high temperatures, under vacuum, the inherent alumina film enveloping the metal is overcome, and aluminium is expected to reduce copper aluminate, rendering alumina and copper. Under this approach, copper aluminate toils as a controlled infiltration path for aluminium, resulting in reactive wetting and infiltration of the preforms. Ceramic preforms containing a mixture of Al 2O 3 and CuAl 2O 4 were infiltrated with aluminium under distinct vacuum levels and temperatures, and the resulting reaction and infiltration behaviour is discussed. Copper aluminates stability ranges depend on vacuum level and oxygen partial pressure, which determine both CuAl 2O 4 and CuAlO 2 ability for liquid aluminium infiltration. At 1100 °C and 0.76 atm vacuum level CuAl 2O 4 is stable, indicating pO 2 above 0.11 atm. Reactive infiltration is achieved via reaction between aluminium and CuAl 2O 4; however, fast formation of an alumina film blocking liquid aluminium wicking results in incipient infiltration. At 1000 °C and 3.8 × 10 −7 atm vacuum level, CuAlO 2 decomposes to Cu and Al 2O 3 indicating a pO 2 below 6.0 × 10 −7 atm; infiltration of the ceramic is hindered by the non-wetting behaviour of the resulting metal alloy. At 1000 °C and 1.9 × 10 −6 atm vacuum level CuAlO 2 is stable, indicating pO 2 above 6.0 × 10 −7 atm. Extensive infiltration is achieved via redox reaction between aluminium and CuAlO 2, rendering a microstructure characterised by uniform distribution of alumina particles amid an aluminium matrix. This work evidences that liquid aluminium infiltration upon copper aluminate-rich preforms is a feasible route to produce Al–matrix alumina-reinforced composites. The associated reduction reaction renders alumina, as fine particulate composite reinforcements, and copper, which dissolves in liquid aluminium contributing as a matrix strengthener.

Prolonged toxicity characteristic leaching procedure for nickel and copper aluminates

The toxicity characteristic leaching procedure (TCLP) is a regulatory testing method widely employed to evaluate the environmental friendliness of waste materials. TCLP analysis provides a fast, easy and economical way to determine the mobility of waste pollutants under simulated landfill conditions. Recent studies on metal stabilization have reported the potential for nickel and copper aluminates to form in thermal treatment conditions, and suggested a more reliable method of stabilizing hazardous metals, particularly when products are to be reused. There is thus an urgent need for a convenient and effective method of quantifying metal leachability and identifying the metal leaching behavior of sparingly soluble materials. In this study, standard TCLP analysis was modified into a prolonged leaching experiment to investigate the leaching behavior of nickel and copper oxides (NiO and CuO) and their aluminates (NiAl(2)O(4), CuAl(2)O(4) and CuAlO(2)). The results demonstrate the difficulty of differentiating the leachability of highly insoluble phases, such as NiO and NiAl(2)O(4), using the standard TCLP. The prolonged TCLP method, however, confirmed NiAl(2)O(4) to have a lower degree of intrinsic leachability than NiO and that it could be expected to undergo congruent dissolution under landfill conditions. For the more soluble copper system, the aluminates were still found to possess a much lower degree of leachability, and their leaching behavior to follow an incongruent dissolution pattern. The results of this study prove prolonged TCLP analysis to be a convenient and effective way to evaluate the environmental friendliness of metal waste and to identify the leaching behavior of waste materials.

NO reduction with CO on Cu–Cr and Co–Cr oxide catalysts supported on Al2O3 and Al2O3 + SiO2

Cu–Cr and Co–Cr oxide systems supported on Al2O3 and Al2O3 + SiO2 were studied in order to find the appropriate combination as a catalyst for the reduction of NO with CO with respect to the active phase and the nature of the support. The catalysts were prepared by successive impregnation of the support with solution mixtures of tartaric acid and nitrate. The catalysts were characterized by atomic absorption spectroscopy (AAS), Brunauer, Emmett, Teller (BET), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods. The copper–chromium catalysts are more active than the cobalt–chromium ones. The catalysts supported on Al2O3 + SiO2 are more active than the supported on pure alumina. The probable reasons are the higher content of copper and cobalt on the surface of Al2O3 + SiO2 and a formation of less active in the studied reaction copper and cobalt aluminates on the alumina supported catalysts.

Defects in Divided Zinc−Copper Aluminate Spinels: Structural Features and Optical Absorption Properties

Zn1-xCuxAl2O4 (0 < or = x < 0.30) compounds have been synthesized by polyesterification using metallic salts and annealing at low temperatures as well as by conventional solid state. XRD-powder data refinements (Rietveld method) have demonstrated that both compound series crystallize in the spinel structure (Fd3m) and exhibit similar inversion rates. This low-temperature route lead to metastable phases with crystallite sizes around 40 nm whereas particle sizes are larger than 1 moicrom in the case of solid-state route. This preparative method largely described in the literature allows stabilizing reduced copper states thanks to the presence of reductive organic species, which are decomposed below T = 700 degrees C. The absorption spectra of the x = 0.15 composition exhibit strong differences depending on the synthesis route. These differences can be explained by the occurrence of Cu2+/Cu+ mixed valencies in compounds prepared by the low-temperature route; 33% of monovalent copper has been identified in the x = 0.15 composition prepared by low-temperature process, whereas the solid-state compound contains only divalent copper. Reductive properties of polyesterification reaction implying citric acid and low annealing temperature (T = 700 degrees C) are mainly responsible of the occurrence of the Cu2+/Cu+ mixed valencies. Actually, the annealing under air at T = 1000 degrees C of divided zinc-copper aluminates prepared at low temperatures (T = 700 degrees C) leads to the oxidation reaction Cu+ --> Cu2+ + e- confirmed by the evolution of magnetic measurements, ESR spectra, and optical absorption properties. Defects such as oxygen vacancies in the anionic network leading to reduction in the cations coordination number could also explain the strong evolution of optical absorption spectra especially around lambda = 700 nm where intervalencies transfer (Cu+/Cu2+) as well as intra-atomic d-d transitions (Cu2+ in a 5-fold coordination) can occur. Finally the occurrence of monovalent and divalent copper at the surface of such divided oxides, probably in tetrahedral sites, has been demonstrated by FTIR spectroscopy using the co-adsorption of CO and NO as probe molecules.

Catalytic combustion of methane on novel catalysts derived from Cu-Mg/Al-hydrotalcites

Novel Cu-Mg/Al mixed oxides (designated as i-CMAO-800) were prepared by calcinations of Cu-Mg/Al hydrotalcites [(Cu2+ +Mg2+)/Al3+= 3] at 800 °C. Their performance for the catalytic combustion of methane was investigated. The oxides and their precursors were characterized by XRD, TG-DSC, TPR and N2 adsorption/desorption techniques. The results showed that BET surface areas and the stability of the resultant oxides were greatly influenced by the copper contents in hydrotalcite precursors, bringing about difference in their activities for methane catalytic combustion. XRD results indicated that Cu was highly dispersed in hydrotalcite precursors in case of low copper contents, (Cu 40 wt%). For higher Cu contents, Cu(OH)2 was formed, and, consequently, a separate phase of CuO was detected in the oxide catalysts after calcination. As indicated by the TG-DSC results, different decomposition behaviors were observed for various hydrotalcites. Thermal calcination promoted the formation of copper aluminates and segregation of CuO from the bulk phases. TPR results showed 15CMAO-800 has the highest reduction rate, and the catalytic activities of iCMAO-800 mixed oxides depend on both the reduction rates and the amounts of copper ions in mixed oxides. The catalyst 15-CMAO-800 showed the best performance.

Dynamics of structural transformations in the reduction of copper aluminate

The dynamics of structural transformations during copper aluminate reduction in the temperature range used for catalyst activation was studied by high-temperature X-ray analysis under controlled conditions (hydrogen, 2O–4OO‡C). The techniques of neutron diffraction analysis, IR spectroscopy, chemical phase analysis, and electron microscopy were also used at particular stages. In the course of reduction, copper metal is deposited onto the surface of spinel crystals from the bulk. Spinel becomes cation-deficient with respect to copper. An analysis of powder diffraction patterns demonstrated that copper is reduced and released from tetrahedral positions of the spinel structure at temperatures below ~300‡C and from octahedral positions only at temperatures above 300‡C. In this case, a redistribution of aluminum ions was observed simultaneously. It is likely that the electrical neutrality is attained by the formation of OH groups, the appearance of which in reduced samples was detected by IR spectroscopy and confirmed by neutron diffraction analysis. At a reduction temperature of 400‡C, the oxygen framework was partially disintegrated. The structures of reduced copper aluminates and chromites were compared.