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The subject of the research is features of synthesis of alumina cements on the basis of compositions of system CaO – CoO – Al2O3 – MoO3 with use of cobalt‐molybdenum‐containing waste for reception of refractory materials with a complex of the set properties. The processes of phase formation in raw mixtures have been investigated. X‐ray phase studies of clinkers fired at different temperatures and holding times proved that as a result of the interaction of the raw materials of the mixture in the material synthesizes a mixture of hydraulically active calcium mono‐ and di‐aluminate and refractory cobalt spinel, which provides the resulting complex high strength, accelerated curing time, fire resistance. The absence of phases corresponding to molybdenum compounds is explained by the fact that they are part of hydraulically active aluminates as limited solid solutions, deforming the crystal lattice and increasing hydraulic activity. Thus, it was found that the high strength of cement is due to the presence of calcium hydroaluminates such as C2AH8, aluminum hydroxide, as well as unhydrated grains of calcium aluminates, which will contribute to further long‐term strength. It is this coexistence of phases in both the crystalline and colloidal states provide high strength cement stone. [ABSTRACT FROM AUTHOR]

Cesium-promoted cobalt spinel is promising as a catalyst for the low-temperature decomposition of nitrous oxide for use in the second stage of a single-reactor unit for the combined removal of nitrogen oxides. This work studied the effect of the conditions for the preparation of granular and block bulk catalysts based on Co3O4 by extrusion molding. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrospun CeO 2 , Co 3 O 4 and Co 3 O 4 -CeO 2 nanofibers were synthesized. • Co 3 O 4 -CeO 2 fibers were also prepared by sequential impregnation of cobalt. • Nanofibers resulted active both for soot and CO oxidations. • Co 3 O 4 -CeO 2 was more active than single oxide fibers for diesel soot oxidation. • Co 3 O 4 was the most active fiber catalyst for CO oxidation. Co, Ce and Co-Ce nanofibers were synthesized by the electrospinning method. To obtain mixed fibers, Co was incorporated in two ways: one step synthesis and sequential synthesis. Nanofibers were applied to two reactions: gas-solid (CO oxidation) and solid-solid-gas (combustion of diesel soot). Different activity trends were observed in both reactions. For soot combustion, Co-Ce nanofibers were more active either than Co or Ce nanofibers. For CO oxidation instead, the activity order was Co > Co-Ce > Ce. Moreover, in the case of soot combustion, for all samples the temperature of maximum combustion rate occurred at a relatively narrow range (350−390 °C). On the contrary, for CO oxidation the temperature range for T 50 was notably wider (150−325 °C). The high activity shown for all the samples for soot oxidation is related to the multiple contact points between the fibers and the soot particles, and the trend observed for CO oxidation related to the higher redox capacity of Co, as observed from CO-TPR experiments. [ABSTRACT FROM AUTHOR]

• K promoter is responsible for activity of Co 4 MnAlO x in direct NO decomposition. • Surface reaction intermediates were identified as NO x −. • O 2 and/or NO x −desorption as O 2 and NO were identified as the rate determining step. • N 2 formation is fast. • Description of reaction mechanism for the first time. The reaction mechanism of direct NO decomposition over Co 4 MnAlO x mixed oxides prepared by co-precipitation method and promoted by 0–3.1 wt.% of potassium was studied. Temperature programmed desorption of NO and in-situ diffuse reflectance infrared Fourier transform spectroscopy confirmed the presence of several nitrogen species (adsorbed NO, NO-, NO 2 -, and NO 3 -) present on the catalyst surface and revealed that O 2 desorption occurs at the high temperature region together with the NO desorption as the product of NO 2 - and/or NO 3 - decomposition. This region corresponds with the temperature of launching NO conversion curve, which together with the results from the transient state experiments proved that O 2 desorption is the slowest step while the reaction steps including NO x - formation and those leading to N 2 release are fast. The Langmuir-Hinshelwood mechanism was proposed as a plausible way of direct NO decomposition over K/Co 4 MnAlO x catalysts and NO 2 - and NO 3 - were determine as the main reaction intermediates. [Display omitted] [ABSTRACT FROM AUTHOR]

We report a comparative study of the electronic structure and magnetic properties of two cobalt compounds Co 3 O 4 and Co 3 S 4 , through first-principles Hubbard- U calculations. Our results indicate that Co 3 O 4 and Co 3 S 4 have similarities in crystal structure (normal spinel), magnetic order (antiferromagnetism), Co spin configuration (high spin Co2+ and low spin Co3+), and comparable band-gap energy. However, the U -dependence on electronic structure in two materials are different from each other. With a change in the applied U values, the band dispersion and the type of band gap are significantly changed in Co 3 O 4 , while the band-gap energy only is affected in Co 3 S 4. Image 1 [ABSTRACT FROM AUTHOR]

Highlights • Au/Cu-doped Co oxides are compared with graphite, BDD and SnO 2 -Sb for CN− oxidation. • The dopant nature plays a key role on the electrocatalytic behavior of Co spinels. • Cu is incorporated into the spinel structure while Au segregates as particles. • Inexpensive "active" Cu-Co spinels exhibit higher efficiencies than "non-active" BDD. • Anode design is crucial for the feasibility of electrochemical wastewater treatment. Abstract The feasibility of the electrochemical technologies for wastewater treatment greatly relies on the design of efficient but inexpensive electrocatalysts. It is generally accepted that the so-called "non-active" anodes (like the boron-doped diamond (BDD) or SnO 2 -based anodes), producing highly oxidizing hydroxyl radicals, are the most promising candidates for pollutants abatement. In this work, the electrocatalytic performance of various cobalt oxides, pure and doped with Cu or Au, for CN− oxidation has been studied and compared with that of conventional graphite, BDD, SnO 2 -Sb and SnO 2 -Sb-Pt. The metal oxide electrodes were prepared by thermal decomposition of the salt precursors onto Ti. For the M-doped Co 3 O 4 electrodes, the nominal M/Co ratios were Cu/Co = 0.07–1.00; and Au/Co = 0.05–0.20. The electrodes were characterized by different techniques (XRD, SEM, EDX, XPS) and their electrocatalytic response was studied by cyclic voltammetry and galvanostatic electrolysis in a H-type cell in aqueous 0.1 M NaOH. The obtained results show that the nature of the dopant plays a key role on the electrocatalytic behavior of cobalt spinels. Thus, while Cu catalyzes the CN− electro-oxidation, Au declines it. This is explained by the fact that, unlike Au (which segregates as Au-rich particles), Cu is effectively incorporated into the spinel structure by forming a solid solution (Cu x Co 3-x O 4). In this solid solution, atomic scale Cu (spinel) -CN− specific interactions occur to catalyze the reaction, whereas in segregated Au particles the oxidation is hindered probably by a too-strong adsorption of cyanide and/or its inaccessibility to oxide active sites. Electrolysis runs have revealed that "active" over-saturated Cu-doped spinels (Cu/Co = 1.00) exhibit higher current efficiencies than conventional graphite and "non-active" BDD and SnO 2 -based anodes. Hence, we hereby demonstrate that an inexpensive "active" electrocatalyst can show even higher efficiency than the most powerful BDD anode. These results highlight the significance of anode design in the application of the electrochemical technique for wastewater treatment. [ABSTRACT FROM AUTHOR]

In this work a series of mixed iron-cobalt spinel catalysts was synthesized by solution combustion synthesis and doped with 3 wt% of palladium via incipient wetness impregnation. The catalysts were synthesized with different Co:Fe ratios, characterized from the structural and surface point of view (XRD, XRF, μRS, BET, FESEM), and tested toward the oxidation of methane in lean conditions, in gas mixtures containing from 0.5 to 2 vol% of CH. The temperature window for the catalytic activity of the undoped mixed iron-cobalt spinels spread from 350 to 650 °C. While lower iron substitution levels lead to a decrease in the spinel's activity, the catalyst with / of the cobalt in the CoO substituted with iron shows similar activity to the parent CoO. Although the materials were proved to be heterogeneous from the crystal phase point of view (a mixture of cobalt-iron spinels), the observed reactivity still depends on the amount of iron introduced during the synthesis. The addition of Pd lowered the temperature window for the catalytic activity to the range of 250-550 °C. The promoting effect of Pd is most evident for the pure CoO catalyst. In contrast to the Pd-free catalysts, the best activity of Pd-promoted samples was observed for the catalysts with the highest iron content. [ABSTRACT FROM AUTHOR]

As potential substitutes for precious metals, cobalt spinel has been considered as a promising alternative catalyst for the low-temperature catalytic oxidation of biofuels. This work reports the one-step synthesis of cobalt spinel by a more elaborated approach named pulsed-spray evaporation chemical vapor deposition with a new feedstock of Co(acac) 3 and toluene. The obtained samples were comprehensively characterized in terms of structure and morphology (XRD, FTIR, Raman and SEM), chemical composition (XPS and EDS) and the redox properties (TPR/TPO). The structural and morphological analysis indicates that the synthesized thin films are pure cubic Co 3 O 4 with homogeneous matrix with well-defined facets and straight edges. Compared to the commonly used feedstock of Co(acac) 2 /ethanol, Co(acac) 3 /toluene could result in a much higher growth rate, more lattice oxygen on the surface and better re-oxidation property. The Co 3 O 4 film exhibits attractive performance towards the deep oxidation of n -butanol by preventing the formation of CO and acetaldehyde. The relationship between the peculiar structure and properties of Co 3 O 4 is highlighted. The surface basic sites, specific morphology and the redox properties are proposed to play synergistic roles in the reaction sequence. [ABSTRACT FROM AUTHOR]

A series of supported catalysts of the Co 3 O 4 spinel active phase dispersed over alumina extrudates (9 × 25 mm) was prepared by several methods (incipient wetness impregnation with Co(NO 3 ) 2 and CoCl 2 , with glycerol-assisted impregnation with Co(NO 3 ) 2 , combustion synthesis, and two variants of spray deposition with Co(NO 3 ) 2 on pristine and ammonia soaked extrudates). The catalysts were characterized by XRF, XRD, RS, UV–vis, SEM/TEM/EDX, and their catalytic deN 2 O activity was investigated in the temperature programmed surface reaction (TPSR) mode. The relation between the spinel active phase particle size and its radial dispersion over the alumina extrudate and the deN 2 O activity was revealed and quantified. For the assessment of the active phase utilization, the N 2 O concentration profile across the extrudates was calculated using Thiele modulus and compared with the radial distribution of the spinel. It was shown that the dispersion of spinel active phase exhibits optimal profile when the sample is obtained in the presence of the organic components of the precursor mixture (glycerol or urea). The obtained results were discussed in the context of practical implications for the development of an efficient, low-cost catalyst for the N 2 O abatement. [ABSTRACT FROM AUTHOR]

A series of potassium or cesium doped ZnCoO|AlO catalysts with different alkali loadings were prepared, characterized with respect to chemical composition (XRF), structure (XRD, RS) morphology (TEM), and the alkali promoter thermal stability. A strong beneficial effect on the deNO activity of the ZnCoO|AlO catalyst (decrease in the T by about 80 °C) was observed for both promoters at different surface coverages. It was found that in comparison to a rather narrow range of optimal cesium loading (0.5-2 atoms/nm) a comparable promotional effect of potassium doping was observed for a slightly wider surface concentrations (0.5-3 atoms/nm). Such difference was attributed to surface dispersion of potassium over the alumina support and the spinel active phase, while cesium was found to be located mainly on the spinel phase. For practical applications, the superiority of potassium over cesium consist in fact that a similar beneficial effect is associated with much higher thermal stability in the temperature range of the catalyst deNO operation and lower price of the promoter precursor. [ABSTRACT FROM AUTHOR]

A series of Pd/Co 3 O 4 catalysts with increasing palladium loading in the range of 0.5–5 wt.% was prepared by incipient wetness impregnation of Co 3 O 4 . Solution combustion synthesis with urea as a fuel was used to optimize the Co 3 O 4 reactive support for palladium in the combustion of methane in lean conditions. The obtained catalysts were thoroughly examined by XRD, XPS, XRF, RS, FESEM, H 2 -TPR, TGA, and N 2 -BET techniques. The catalytic tests of CH 4 combustion were performed for 0.5, 1, and 2 vol.% CH 4 , with constant lambda value. The obtained results revealed that a sub-stoichiometric fuel-to-oxidizer ratio, 0.75, results in the most catalytically active Co 3 O 4 phase. The important differences in the catalysts’ activity were apparent for the highest CH 4 concentration, with the 3% Pd/Co 3 O 4 being the most active catalyst. The observed activity was explained considering the physicochemical, spectroscopic, and microscopic characterization of the catalysts with the PdO nanocrystals surface distribution being the determining factor for the catalysts’ reactivity. A simple model accounting for the observed dispersion effect is proposed. The model is based on a two types of the interaction of the surface PdO active phase with the Co 3 O 4 support. Firstly, cobalt oxide helps to remove hydroxyl species from the PdO surface, thus making the active sites more available for methane activation. Secondly, cobalt spinel provides lattice oxygen to the PdO phase, again helping to recreate active sites thereon. [ABSTRACT FROM AUTHOR]

In this work, a series of CoO spinels were produced by different synthesis routes (precipitation, solution combustion synthesis, and hard template method), and were used as non-noble catalysts for the oxygen evolution reaction (OER) under basic conditions. The investigated catalysts have a proportional relation between electrochemical activity, surface roughness, and specific surface area. The hard template synthesis method resulted in the most active catalyst compared in this work, which we ascribe to its highly porous structure, and concomitant Co/Co redox couple at a lower potential, attributed to the OER. The most performant catalyst was compared with a commercial catalyst (Ni@NiO, Alfa Aesar) showing only 0.01 V overpotential difference, evaluated at 10 mA cm (overpotential 0.44 V). Graphical Abstract: [ABSTRACT FROM AUTHOR]

In this work a series of cobalt spinel catalysts was synthesized by different methods and doped with 1 wt% of palladium via incipient wetness impregnation or solution combustion synthesis (SCS). Both undoped and Pd-doped spinels were characterized from the structural and surface point of view (XRD, XRF, μRS, BET, FESEM) and tested toward the oxidation of methane in lean conditions, in gas mixtures containing from 0.5 to 2 vol% of CH. These methane concentrations are typical of processes involving combustion of CH in lean conditions as the after-treatment of unconverted CH in compressed natural gas-fed vehicle exhausts, or the treatment of fugitive CH from leaks in coal mines. The temperature window for the catalytic activity of the undoped CoO materials spread from 350 to 600 °C. The preparation method affected the catalytic activity of undoped CoO, with the best catalytic activity related to the spinel prepared by SCS employing urea. The addition of 1 wt% of Pd lowered the temperature window for the catalytic activity to the range of 250-500 °C, with insignificant differences for the various preparation methods. The beneficial effect of Pd-doping was assigned to the enhancement of the reduction of cobalt in the oxide structure. [ABSTRACT FROM AUTHOR]

A series of supported Co 3 O 4 /α-Al 2 O 3 catalyst synthesized by glycerol-assisted method was thoroughly characterized (X-ray micro-tomography, XRD, RS, FTIR, SEM/TEM/EDX, TG/DTA, TPR), and their reactivity was evaluated in N 2 O decomposition in 5% N 2 O/He and 1.5% N 2 O, 1% NO x , 1 vol.% H 2 O, 2.0 vol.% O 2 . Their superior catalytic activity in comparison to the benchmark Co 3 O 4 /α-Al 2 O 3 catalyst, obtained via incipient wetness impregnation with the aqueous solution, was accounted for by differences in cobalt spinel dispersion on the alumina support, spinel nanoparticle size and faceting. Whereas for the samples prepared from the aqueous solution the spinel active phase exhibits a broad bimodal size distribution (with the maxima for 10–50 nm crystallites and 300–600 nm agglomerates) in the case of glycerol-assisted synthesis a monomodal distribution is featured by isolated spinel nanocrystallites of 10–30 nm. At the same time, the presence of glycerol changes the spinel crystallites morphology from rhombicuboctahedron with abundant exposition of the less favorable (111) planes into the truncated cubic shape with the dominant more active (100) facets. It was shown that the beneficial effect of glycerol is sustained in typical gaseous residuals (H 2 O, NO x , O 2 ) present in the tail gases of nitric acid plants. The observed effect of glycerol-assisted synthesis was accounted for by complexation of cobalt by glycerol, leading to the formation of an intrapore cobalt glycerolate, which upon calcination is transformed into well dispersed Co 3 O 4 cubic nanocrystals. [ABSTRACT FROM AUTHOR]

A series of Co 3 O 4 /CeO 2 catalysts with increasing cobalt spinel loading in the range of 1–20 wt.% was prepared by incipient wetness impregnation of CeO 2 . The obtained catalysts were thoroughly examined by XRD, XPS, XRF, RS, TEM/EDX/EELS, TPR and BET techniques. The catalytic tests in deN 2 O reaction revealed that the 10 wt.% of cobalt spinel in supported system is able to reproduce the activity of bare Co 3 O 4 catalyst. However, it was found that the catalyst with the lowest content of Co 3 O 4 equal to 1 wt.% exhibits the highest apparent reaction rate per mass of the spinel active phase. The observed activity was explained basing on the transmission electron microscopy analysis in terms of the dispersion of spinel phase over ceria support. A simple model that accounts for the observed strong dispersion effect is proposed. It consists in a two-step mechanism, where N 2 O is dissociated on the spinel nanograins and the resultant oxygen species are preferentially recombined at the Co 3 O 4 /CeO 2 interface periphery. [ABSTRACT FROM AUTHOR]

Series of precursors was prepared by precipitation of cobalt nitrate in aqueous solutions using various precipitation agents (NH 3 ·H 2 O, NaOH, Na 2 CO 3 ) and reactions conditions (OH/Co molar ratio, aging time). Powder XRD showed different phase composition of the obtained precursors, in which β-Co(OH) 2 , Co II –Co III layered double hydroxide, and basic cobalt carbonate were identified. Only Co 3 O 4 spinel-like oxide was found in products after heating at 500 °C in air but different N 2 O conversions were observed over the examined oxide catalysts. Especially the catalysts obtained from β-Co(OH) 2 precursors showed high catalytic activity in N 2 O decomposition. The correlation between methods of preparation, phase composition of precursors, catalytic properties of the related oxide catalysts, and the role of the cobalt ions as catalytic active sites is discussed. [ABSTRACT FROM AUTHOR]

In this work a series of palladium doped cobalt spinel catalysts was synthesized via solution combustion synthesis plus incipient wetness impregnation and tested in lean CH 4 oxidation. The obtained catalysts were characterized from the structural and surface point of view (XRD, XRF, μRS, BET). The Pd doping level, calculated as PdO, ranged from 0.5 to 5 wt.%. The optimal palladium loading was determined on the basis of catalytic tests in gas mixtures containing from 0.5 to 2 vol.% of CH 4 . The temperature window for the catalytic activity of the Pd/Co 3 O 4 materials spread from 270 to 550 °C. The optimal Pd loading was found to be 3 wt.%, and it was rationalized in terms of favorable surface Pd dispersion, since above this critical level of 3% an appreciable amount of PdO phase was formed. The reactivity of the optimal 3 wt.% Pd/Co 3 O 4 catalyst from one-shot solution combustion synthesis was evaluated with the aim of optimizing the synthesis procedure. [ABSTRACT FROM AUTHOR]

A series of monolithic catalysts with the 0.3 wt.% loading of the (Co,Zn)Co 2 O 4 spinel active phase dispersed on bare and ceria and zincite washcoated cordierite substrates was prepared by impregnation method: (Co,Zn)Co 2 O 4 /cordierite, (Co,Zn)Co 2 O 4 /ZnO/cordierite and (Co,Zn)Co 2 O 4 /CeO 2 /cordierite. The catalysts were thoroughly characterized (XRD, RS, SEM/TEM/EDX, XRF), and their catalytic deN 2 O activity was investigated using model gas mixture (2000 ppm N 2 O/N 2 ), and tail gases (1400 ± 50 ppm N 2 O, 900 ± 100 ppm NO x , 0.8 ± 0.2 vol.% H 2 O, 2.0 ± 0.2 vol.% O 2 ) of the nitric acid pilot plant. The reported data points correspond to the measurements performed at possibly the closest tail gas composition. Morphological SEM analysis of the monolith cross-sections indicated the segregation of both ZnO and CeO 2 washcoats in the form of islands covered by the (Co,Zn)Co 2 O 4 active phase. The catalytic tests revealed that the monolithic catalysts exhibit high catalytic deN 2 O activity, reaching X > 96% at 400 °C (model gas) and 450 °C (tail gases) for the best (Co,Zn)Co 2 O 4 /CeO 2 /cordierite system. It was also found that the specific reaction rate per cobalt spinel weight loading for the monolithic catalysts is even two orders of magnitude higher than in the case of the optimized bulk spinel phase. Yet, the beneficial effect of ceria, dominant in the model gas mixture, is largely dumped in tail gases at low temperature. It is however restored above 400 °C, when the poisoning H 2 O and NO x molecules are desorbed. [ABSTRACT FROM AUTHOR]

The effect of surface and bulk alkali (Na, K) promotions on the cobalt-oxide catalyst activity in soot oxidation was investigated. While surface promotion does not alter the cobalt spinel structure, the introduction of alkali into the bulk leads to the formation of layered cobaltates (revealed by XRD, Raman spectroscopy and XPS). The catalytic activity was determined using the temperature programmed oxidation of model soot (Printex 80), both in tight and loose contact conditions. The alkali bulk promotion is much more effective than surface promotion in soot oxidation, lowering the soot ignition temperature by 340 °C for the most active catalyst K 0.25 CoO 2 . [ABSTRACT FROM AUTHOR]

Cobalt spinel catalysts promoted with different cesium loadings from various precursors (Cs 2 CO 3 , CsNO 3 , CH 3 COOCs, and CsOH) were prepared by incipient wetness impregnation. The catalysts were characterized by XRF, XRD, SEM, XPS and Raman Spectroscopy. The work function studies were used to optimized the surface doping and monitor the retention of surface oxygen. The stability of cesium promoter was evaluated by means of species resolved – thermal alkali desorption method. The catalytic activity of N 2 O decomposition over the synthesized catalysts was studied by temperature programmed reaction. The role of cesium precursor was evaluated in terms of dispersion and the nature of the counterion. The results show that the most active deN 2 O catalysts are obtained for cesium carbonate at the narrow loading range of 2–3 atoms/nm 2 (complete N 2 O conversion at T < 200 °C). The twofold promotional effect is discussed in terms of modification of the electronic properties of the spinel catalyst (stimulation of dissociation step: N 2 O + e = N 2 + O – ) and stability of the surface oxygen species (recombination step: O – = ½O 2 + e). [ABSTRACT FROM AUTHOR]

Highlights: [•] The series of Co substituted MgAl2O4 spinels were investigated. [•] Catalytic activity of cobalt containing spinels was tested for decomposition of N2O. [•] Co3+ ions in octahedral sites were identified as deN2O active centres. [Copyright &y& Elsevier]

Abstract: We report on the crystallographic structure and magnetism of 5-nm Co–Al–O spinel nanocrystals synthesized under supercritical hydrothermal conditions. Structural examination using powder X-ray diffraction and chemical analysis showed the composition of the sample to be Co0.47Al2.36O4 rather than the stoichiometric composition of CoAl2O4. The site occupancy of Co on the A-site forming the diamond lattice was 0.47, which is slightly larger than the site percolation limit. Magnetization measurements showed that magnetic clusters emerged below 40K. At temperatures below 40K, a Griffiths-phase-like inhomogeneous state appeared in the sample in which magnetic clusters and paramagnetic spins coexisted. The dc-paramagnetic and ac-susceptibilities exhibited an anomaly below 7K. [Copyright &y& Elsevier]

Abstract: Cobalt spinel Co3O4 catalysts promoted with different potassium precursors (K2CO3, KNO3, CH3COOK, KOH) were prepared and characterized by surface (XPS, work function, SEM) and structure (Raman, XRD, H2-TPR) sensitive techniques. The stability of potassium promoter was determined by means of Species Resolved-Thermal Alkali Desorption method (SR-TAD). The N2O decomposition mechanism on series of synthesized catalysts was studied by temperature programmed reaction and pulse experiments of isotopically labeled 15N2 18O. The role of the octahedral cobalt ions as catalytic active sites was revealed. The most dispersed state of alkali promoter, obtained from K2CO3 precursor, results in its highest stability and lowest work function, leading to the highest catalytic activity in N2O decomposition in the investigated K-Co3O4 series. The beneficial effect of potassium is discussed in terms of the facile activation of N2O via dissociative electron transfer (N2O +e− =N2 +O−) and promotion of the preferred fast suprafacial recombination of oxygen intermediates (revealed by the isotopic experiments). [Copyright &y& Elsevier]

Natural blue Co-bearing spinel crystals are rare and actively sought as gemstones, while synthetic blue Co-bearing spinel powders are largely used as ceramic pigments. High-quality spinel single crystals with compositions closely corresponding to the solid-solution series spinel sensu stricto (MgAl2O4)-cobalt spinel (CoAl2O4) were produced by flux growth method, with Na2B4O7 as flux. Low-cooling rates (2 °C/h) and linear temperature profiles were applied in the thermal interval 1200-800 °C, followed by rapid cooling. Thermal runs were performed in reducing atmosphere (fO2 10-8-10-15 bars) created by a continuous flow of a CO2:H2 mix with a ratio of 100:4 (cm³ /min). Ten experiments were successfully carried out and hundreds of inclusion-free gem-quality single crystals (up to 1 mm large) were produced in each of them, sometimes together with crusty aggregates and microcrystalline powder. Selected crystals were investigated by SEM/EDS X-ray mapping to check for compositional homogeneity and by electron-microprobe analysis to obtain the chemical formula. Crystals were found to be chemically homogeneous and entirely representing the MgAl2O4-CoAl2O4 solid-solution series, with the latter component ranging from 7 to 100%. With increasing Co2+ contents, the crystals vary in color from light blue to intensely dark blue in daylight. The unit-cell parameter a increases from 8.084 to 8.105 Å along the solid-solution series, and the observed increase is determined more by the inversion degree than by the variation in Co contents. The composition of crystal products does not correspond to the composition of the starting oxide mixture, being cobalt enriched in the crystals. A tentative explanation of this behavior is suggested by considering possible ionic potential as well crystal field stabilization effects. [ABSTRACT FROM AUTHOR]

Optical absorption spectroscopy and X-ray structural refinements were used to characterize short-range and long-range structures of 10 gem-quality, blue spinel single crystals synthesized on the (Mg1-xCox) Al2O4 solid solution (x = 0.07-1.00). The site distributions of Mg, Co2+, and Al show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Co2+, with a marked preference of Co2+ for tetrahedral coordination with respect to Mg, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a certain degree of inversion, i.e., occurrence of Al at T counterbalanced by the occurrence of divalent cations at M, which decreases from 0.24 to 0.13 with increasing Co2+ content. Short-range information based on the crystal field splitting parameter Dq derived from single-crystal optical spectra suggests that the local Co2+-O bond length at the T-site may increase marginally at increasing Co2+ content. An almost constant value for the Racah B-parameter, also derived from optical spectra, for tetrahedrally coordinated Co2+ suggests that any influence of substitutional second nearest neighbor cations on the ionicity of Co2+-O bonds at the T-site is very small. Long-range information shows that variations in the unit-cell parameter from 8.084 to 8.105 Å along the solid-solution series are mainly related to the ordering of Al at the M site as a result of the replacement of Mg by Co2+. Therefore, the spinel structure responds to the chemical variation by ordering of Al in such a manner that M-O remains almost constant and T-O increases. In this way, the lengths of shared octahedral edges are reduced and the destabilization effect due to the increased octahedral cation-cation repulsion is minimized. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure has also been shown to be valid in the MgAl2O4-CoAl2O4 solid-solution series. [ABSTRACT FROM AUTHOR]

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Abstract: Structured reactors based on metallic carriers for catalysts of highly enhanced transport properties can be an interesting alternative to monolithic converters. In this study the carriers based on wire gauze have been shown to improve the efficiency of the VOC combustion simultaneously decreasing the converter length. A successful application of metallic microstructures needs proper methods of catalyst layering. This study focuses on non-equilibrium plasma (NEP) deposition technique which is referred to Langmuir–Blodgett (LB) film deposition and to wet impregnation. The cobalt oxide catalyst deposited on wire gauze and steel sheets were characterised by XPS and Raman microscopy and tested in n-hexane oxidation in tubular and gradientless (jet-stirred) reactors. The model cobalt catalysts showed sufficiently high activity in relation with Pt reference catalysts. The most important factor affecting the catalyst activity was the type of the carrier. The best performance was achieved for the catalysts deposited on wire gauzes. [Copyright &y& Elsevier]

The influence of Zn and K promoters on N2O decomposition over Co3O4 was investigated by work function measurement and temperature-programmed surface reaction. The beneficial effect of the promoters resulting in spectacular decrease in the temperature of 50% conversion by 200 °C was found to be essentially of electronic origin. The strong correlation between the catalyst work function and deN2O activity allowed for the optimization of the doping level of both additives. The pilot plant tests in real nitric acid tail gases revealed that the optimized double promoted (Zn, K) cobalt spinel catalyst maintained its remarkable activity in N2O decomposition (conversion >95% at the target temperature of 350 °C) for more than 60 h. [ABSTRACT FROM AUTHOR]

Abstract: The effect of alkali promoters on N2O decomposition in dry and wet (1% H2O) conditions over Co3O4 was investigated. The beneficial influence of dopants on the catalyst activity increases in the order: Li≪Na

The beneficial effect (decrease of the half conversion temperature by 100 °C) of potassium doping, in the range of 0–5 atoms/nm2, on N2O decomposition over Co3O4 was analyzed by work function measurements and DFT calculations. The optimal potassium surface loading was found to be 1.8 atoms/nm2. The effect was explained in terms of electronic promotion gauged by lowering of the catalyst work function by 0.48 eV (for K2CO3 precursor) and 0.44 eV (for KOH). The promotional effect is discussed in relation to the theoretical and experimental surface dipoles determined from Hirshfeld atomic charges and geometry of the postulated potassium adspecies and from the Topping model, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Highly uniform 3D sisal-like, dandelion-like and rose-like architectures of cobalt hydroxide carbonate with orthorhombic or monoclinic phase were synthesized through a facile selected-control hydrothermal process at 100, 140 and 180°C, respectively. In addition, cobalt spinel Co3O4 superstructures with broom-like, dandelion-like and rose-like morphologies were obtained by thermal conversion of the corresponding precursor of 3D architectures based on the thermal analysis results, and TEM images show that all Co3O4 superstructures were assembled by nanoparticles. The optical absorption properties of the Co3O4 superstructures were investigated and the results indicate that the superstructures are semiconducting with transitions corresponding to 775 and 530nm in the UV–vis spectroscopy. [Copyright &y& Elsevier]

Abstract: The characterisation of the surface charge of a cobalt spinel ultrafiltration membrane was investigated in the presence of electrolyte solutions by means of streaming potential measurements. The selectivity of a membrane towards different salts depends on the electrostatic interactions between the species in solution and the charge of the membrane surface. Firstly, the values of the streaming potential coefficient were measured during the filtration of NaCl, CaCl2, Na2SO4 and CaSO4 as a function of pH and ionic strength. The iso-electric point was determined for each electrolyte. Secondly, the rejection rates of the electrolytes were measured by ionic chromatography. The rejection was explained by a correlation with the surface charge of the membrane. [Copyright &y& Elsevier]

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N2 adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co based HTlc's improves considerably the catalytic activity. Co–Rh,Al HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex Co,Pd–La,Al HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh containing catalyst, both in a N2O containing stream and in one containing also SO2 and O2, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO2 and O2 are considered. [ABSTRACT FROM AUTHOR]

• Supported Co-Ni catalysts were evaluated for lean methane oxidation. • Bimetallic catalysts were found more active than the monometallic counterparts. • Addition of nickel enhanced lattice oxygen mobility and activity of the Co-catalysts. • Segregation of NiO was observed over the Ni-rich catalysts. • An optimal composition of 25 %wt.Co and 5%wt.Ni was found. This work deals with the extensive study of supported bimetallic Co-Ni catalysts for the complete oxidation of methane. The simultaneous incorporation of nickel and cobalt was found to enhance the redox properties by favouring the formation of nickel cobaltite-like species and partially inhibiting the interaction between Co 3 O 4 and alumina. In turn, this resulted in an increase of the amount of Co3+ cations in the catalysts, which led to a more notable presence of active lattice oxygen species in the samples. When the nickel loading was increased, the generation of less active NiO was observed. The improvement of the redox properties resulted in a promotion of the specific reaction rate and a shift of around 50 °C in the T 50 value over the bimetallic catalysts. The most active catalyst (25Co-5Ni) was found to be relatively stable during prolonged operation times, but suffered an appreciable and irreversible deactivation in the presence of water vapour. [ABSTRACT FROM AUTHOR]