CoO-MgO Solid Solutions Research Articles

Cobalt–Magnesium Oxide Catalysts for Deep Oxidation of Hydrocarbons

Co–Mg catalysts for methane combustion were synthesized and studied, revealing the transformation of MgCo2O4 spinel into a CoO–MgO solid solution with oxygen release from the spinel lattice as the calcination temperature increased. Repeated heat treatment of the calcined solid solution at lower temperatures led to spinel regeneration with segregation of the solid solution phase. A TPR of the samples showed the presence of two characteristic peaks, the first of which relates to the transition of Co3+Oh spinel to the Co2+Oh structure of CoO, and the second to the reduction of CoO to Co°. The second peak was observed at 540–620 °C for samples calcined at temperatures below spinel decomposition, and for high-temperature samples at 900–1100 °C. Taking into account the identity of the structure of phases obtained in both cases, the formation of not a true CoO–MgO solid solution, but rather a mixture of ordered oxides (“pseudo-solid solution”) in the low-temperature region, was postulated. A study of the activity of the samples showed the high activity of the spinel systems and a linear relationship between the activation energy of methane oxidation and the heat treatment temperature.

NiO-MgO and CoO-MgO Thin-Film Solid Oxide Solutions on a Mo(100) Support: Formation, Reduction, and Influence of the Support

We report on the formation, structure, composition, and reduction of NiO-MgO and CoO-MgO solid oxide solution thin films supported on a Mo(100) substrate. By use of a combination of grazing and normal emission X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory calculations we show that the Mo(100) support can exert a marked influence on the composition profile and reduction of the NiO-MgO solid oxide solution thin film. We find that the underlying Mo(100) support increases the stability of the Ni solute at the oxide film/Mo(100) interface relative to its stability in the MgO matrix. Further, we find that NiO is more easily reduced at the oxide film/Mo(100) interface than in the solid oxide solution film. These effects are the result of the large affinity of the Ni metal to the Mo support. XPS and STM measurements show a similar behavior for Mo(100)-supported CoO-MgO solid oxide solution thin films. The implications of these findings on the use of these thin films as model catalysts or novel catalytic materials are discussed.

Solid-solution catalysts for CO 2 reforming of methane

One of the important developments in CO 2 reforming of methane is highly effective MgO-based solid-solution catalysts. Herein, their high activity and selectivity as well as excellent stability for CO 2 reforming of methane were briefly reviewed. How to inhibit carbon deposition via the formation of NiO–MgO or CoO–MgO solid solution was discussed. The reduction of NiO (or CoO) in the NiO–MgO (or CoO–MgO) solid solution is much more difficult than that of pure NiO (or CoO), which contributes to the formation of very small Ni particles to inhibit carbon deposition. It is generally recognized that the reduction of a metal oxide is determined by its metal–oxygen bond strength. Herein, however, it was showed that the reduction of a metal oxide is strongly dependent on both metal–oxygen bond strength of the metal oxide and the metal–metal bond strength of its metal product. Furthermore, it was proposed that a critical factor to control the reduction of NiO (or CoO) in the solid solution is the isolation effect that NiO (or CoO) is isolated by MgO, which inhibits the metal–metal bond formation during the reduction.

Thermodynamics of CoO–MgO solid solutions

Enthalpies of mixing in Co x Mg 1 − x O solid solutions at T=298 K have been determined using high temperature drop solution calorimetry in molten sodium molybdate (3Na 2O · 4MoO 3) solvent at T=973 K. Slightly positive enthalpies of mixing were observed, which conform to a regular solution model with interaction parameter W=(5.1 ± 0.3) kJ · mol −1, which is similar to the interaction parameter calculated from previously measured activity-composition relations. Standard molar entropies S m 0 at T=298 K calculated from subambient C p data for CoO, MgO are (52.8 ± 0.1) J · K −1 · mol −1 and (28.1 ± 0.1) J · K −1 · mol −1, respectively. Integration of the C p,m/ T of the Co 0.50Mg 0.50O mixture yields an entropy of (40.3 ± 0.1) J · K −1 · mol −1. This value does not include configurational entropy from the Co/Mg site disorder. The excess entropy of Co 0.50Mg 0.50O is zero within experimental error. Heat capacities were also measured between T=298 K and T=973 K. The enthalpies of mixing do not depend on temperature in this range.

Solid–solid interactions in Co 3O 4–MoO 3/MgO system

Cobalt/magnesium mixed oxide solids and cobalt–molybdenum/magnesium mixed oxide solids were prepared by thermal decomposition of basic magnesium carbonate pretreated with different proportions of cobalt nitrate and then with calculated amounts of ammonium molybdate. The proportions of cobalt expressed as Co 3O 4 were 0.1, 0.2 and 0.3 mol while the concentrations of molybdenum expressed as mol% MoO 3 were 2.5 and 5.0. The prepared mixed solid specimens were calcined in air at 400–1000 °C. The solid–solid interactions in Co 3O 4–MoO 3 were investigated using DTA, TG and X-ray powder diffraction (XRD) techniques. The results obtained revealed that MgO dissolved cobalt oxide in its lattice forming CoO–MgO solid solution. The amount of cobalt dissolved increases by increasing the temperature in the range 800–1000 °C. This finding was confirmed by X-ray diffractograms in which all the diffraction lines of cobalt oxide disappeared at 1000 °C. MoO 3 present interacted readily with MgO and cobalt oxide by heat treatment at temperature starting from 400 °C producing MgMoO 4 and CoMoO 4 which remained stable by heating at 1000 °C. The impregnation of basic magnesium carbonate with cobalt nitrate much enhanced its thermal decomposition yielding MgO, which decomposed completely at 395.5 °C instead of 525 °C. The formation of magnesium cobaltite (MgCo 2O 4) has been ruled out via XRD investigation at relatively high diffraction angles.

Catalytic combustion of methane on Co/MgO: characterisation of active cobalt sites

A series of Co/MgO catalysts with 3–12wt.% Co were prepared by impregnation and calcined at 1073K for 10h. The catalytic behaviour of these samples toward CH4 combustion was found to increase with cobalt loading, though a plateau was reached at ca. 9wt.% Co content. Bulk characterisation was carried out using XRD, TPR and Raman spectroscopy, and showed that the solids were made up of a CoO–MgO solid solution and a MgO phase. A detailed examination of their surfaces was achieved through FTIR spectroscopy of adsorbed CO probe molecules, which indicated that at low cobalt loadings only a small proportion of the Co going into the solid solution was present on exposed faces as either Co2+ oxo-species or pentacoordinated Co2+. However, as the cobalt content of the samples increased, a larger amount was exposed on the surface. This effect levelled off at 9wt.% Co, after which the increase in exposed Co2+ sites was countered by the masking effect of islands of MgO. In addition, at high cobalt loadings (9 and 12wt.%) Co formed small clusters which showed bulk CoO-like behaviour. Consequently, the benefit of having surface Co2+ species was balanced by the clustering effect of these species and the presence of MgO islands, negating their contribution to the overall catalytic activity of the samples.

High and low surface area NiO–MgO and CoO–MgO solid solutions: a study of XPS surface composition and CO oxidation activity

Oxide solid solutions NiO–MgO of high surface area were studied by XPS. The surface Ni2+ concentration was found to be equal, within experimental errors, to the bulk concentration. The result is analogous to that found previously for the low surface area NiO–MgO system and for both the high and low surface area systems of CoO–MgO. The catalytic oxidation of CO by O2, on high and low surface area NiO–MgO and CoO–MgO materials, was investigated with the aim of relating the catalytic activity with transition metal ion nature and concentration. Turnover frequency data (CO2 molecules produced per second per surface atom) show that the activity is due primarily to the transition metal ions and is not subject to the ions being in special configurations (dimers or trimers) or in special positions (edges, corners). The activity of CoO–MgO is higher than that of NiO–MgO solid solution.

EPR study of the mobility of paramagnetic species on the surface and in the bulk of solids

The temperature dependence of EPR spectra provides information on the mobility of paramagnetic species at the gas (liquid)/solid interface and in the bulk of solids. Changes in the environment of molecules on solid surfaces caused by their motion occurring upon thermal treatment at various temperatures are observed. Superoxide radical can migrate from Co(III) to Mg(II) surface sites of the CoO–MgO solid solutions. In aqueous solutions transition metal ions coordinate water molecules, forming aquacomplexes which are usually free to tumble within the liquid medium. Their mobility is, however, strongly modified in the vicinity of the solid surface or inside the narrow pores. In solids the migration of paramagnetic species from the surface into the bulk is controlled by the temperature of thermal treatment. In the case of V 2O 5–ZrO 2 catalyst this process is strongly influenced by the phase transitions occurring in the solid matrix and by the presence of alkali metals.

Solid state galvanic cell studies of interdiffusion between NiO and CoO-doped MgO

In the present work, interdiffusion between NiO and CoO-doped MgO has been investigated by the solid state galvanic cell method in the temperature range 1273–1439 K. The compositional as well as temperature effects of doping on the rate of interdissolution were examined during the measurements. Evaluation of the interdiffusivity data from the cell EMF values required a knowledge of the activity of NiO in the ternary oxide system CoOMgONiO. An assessment of the thermodynamics of this system was made from the literature data on the relevant binary systems using a two sublattice model. From the EMF values and the thermodynamic information, the interdiffusivities were calculated using a theoretical model developed in the division earlier. The results indicate that, at low concentrations of the dopant, the changes in the interdiffusivities are not significant. On the other hand, at 10 mol% CoO in the starting CoOMgO solid solution in contact with 50 mol% NiO, an increase in the rate of diffusion was noticed.

XPS and EPR study of high and low surface area CoO–MgO solid solutions: surface composition and Co2+ion dispersion

Solid solutions CoO–MgO (MCo) of low and high surface area (LSA and HSA) have been studied by XPS and EPR analysis to evaluate the surface composition and the dispersion of cobalt ions in the systems. Different preparation routes were adopted: wet impregnation (Iw), dry impregnation (Id) and coprecipitation (C). LSA solid solutions (As= 10–20 m2 g–1) were obtained after treatment in air at 1173 K (1173 LSA) or at 1473 K (1473 LSA), HSA materials (As= 200–300 m2 g–1) after treatment under vacuum at 1173 (1173 HSA). XPS intensity ratios, ICo/IMgvs. xCo/xMg molar fraction showed no appreciable differences between the bulk and surface compositions of LSA and HSA solid solutions from Id and C precursors, calcined at 1173 and at 1473 K. MCo 1173 LSA Iw samples had evident cobalt enrichment, due to Co3O4 segregation, whereas Iw HSA did not. NO2 or O2 treatments led to the formation of CoIII. The Co2+ bulk dispersion in LSA and HSA was studied by EPR.

ChemInform Abstract: Electronic Structure and Orientation of Dioxygen Species on the Surface of CoO‐MgO Solid Solutions.

AbstractThe adsorption of oxygen on the CoO‐MgO surface is studied by EPR using 17O‐enriched oxygen.

An XPS and TPR study of the reduction of promoted cobalt-kieselguhr Fischer-Tropsch catalysts

Reduction of unpromoted Co- and promoted Co-ThO 2-MgO-Kieselguhr catalysts has been studied with X-ray photoelectron spectroscopy and temperature-programmed reduction techniques. A mechanism of reduction is proposed based on spectroscopic measurements of the surface phases before and after H 2 treatment at 400 °C. Cobalt oxide Co 3O 4 is present on all air-calcined catalysts, and is reduced rapidly to CoO at temperatures >300 °C in hydrogen. CoO is then more slowly reduced to Co metal on the unpromoted catalysts at 400 °C. If the promoters MgO and ThO 2 are present, a reaction between CoO and promoter can occur resulting in new phases which are more resistant to reduction than CoO alone. XPS results suggest one phase as a CoO-MgO solid solution, but the CoO-ThO 2 phase could not be identified. The cobalt in magnesia-promoted CoO phases is reduced completely to the metal at much higher temperatures (500–700°C) but at 400°C the surface reduction is limited to 55% ± 5 after 1 h. Variability in the degree of reduction of various promoted cobalt catalysts is therefore dependent on the nature and loading of the promoter and the reduction temperature. Promotion of a Co-Kieselguhr catalyst with ThO 2 or MgO does allow a higher degree of control over the CoO reduction kinetics than is found with unpromoted catalysts.

Electron spin resonance spectroscopy of surface species formed upon adsorption of nitrogen oxides and oxygen on high-surface-area Nio–MgO and CoO–MgO solid solutions

Solid solutions of high-surface-area (h.s.a.) NiO–MgO (Ni content 0.1–5 atoms per 100 Mg atoms) were activated by heating under vacuum at 1173 K for 5 h before exposure to O2, NO2, NO or N2O at temperatures in the range 298–815 K. With all gases, depending on the temperature of adsorption, two species were observed by electron spin resonance spectroscopy. Both are axial signals of nickel species on the surface of MgO, as shown by broadening experiments with O2. The e.s.r. analysis, the chemical conditions under which the species are formed and their thermal stability allow the assignment of the two species to (i) a coordinatively unsaturated Ni3+(Nis3+) and (ii) Ni3+ ions in a distorted octahedral surface complex (Nis3+⋯L). Adsorption of NO at 178–298 K produces a (Ni⋯NO)2+ adduct, NO2–2 and Mg2+⋯NO.Solid solutions of h.s.a. CoO–MgO (Co content 0.1–5 atoms per 100 Mg atoms) activated as for NiO–MgO were exposed to NO2 or NO. The labile cobalt nitrosyl adduct formed is tentatively assigned to Co2+⋯(NO)2; NO2–2 species and Mg2+⋯NO are also formed.Some total and reversible adsorptions (removed by evacuation at the temperature of adsorption) were determined volumetrically. Reflectance spectra were taken under the same conditions for the formation of the paramagnetic species. Surface processes by which the various species were formed are discussed.

X‐ray photoelectron spectroscopy study of the surface composition of CoO‐MgO solid solutions

AbstractThe surface composition of CoO–MgO solid solutions has been investigated by XPS. The samples (prepared at 1200°C in air) were checked to be homogeneous by X‐ray diffraction analysis and by lattice parameter measurements. The study utilized peaks at different kinetic energies: Co 2p, O 1s, Co 3s, Mg 2s, Co 3p, Mg 2p and the Auger transition Mg KLL. It was shown that the specimens have a surface composition slightly different from the bulk, because of the formation of surface magnesium hydroxide. No segregation of cobalt was found. The results have a bearing on the catalytic activity of oxide solid solutions.

Surface dioxygen adducts on MgO-CoO solid solutions: Analogy with cobalt-based homogeneous oxygen carriers

Isolated Co 2+ on the surface of diluted CoOMgO solid solutions reversibly binds oxygen, forming molecular species characterized by OO stretching frequencies in the 1160 - 1015 cm −1 range. On the basis of isotope substitution experiments and by comparison with homogeneous analogues, it is concluded that O 2 is adsorbed in the O −1 2 superoxide form and that the diluted MgOCoO solid solutions are interesting examples of heterogeneous oxygen carriers and activators.

Coordination and dispersion of Co2+ ions in CoO—MgO solid solutions

CoO–MgO solid solutions of CoO concentrations 0.25–15.3 mol % have been investigated by e.s.r. spectroscopy in the temperature range 4–77 K. The distribution of isolated Co2+ ions between octahedral and tetrahedral sites has been estimated quantitatively. In the most dilute solid solutions (up to 3.00 mol % CoO) Co2+ ions occupy predominantly trigonally distorted tetrahedral sites. The exchange interactions extend up to the 3rd coordination sphere in CoO–MgO. The dependence of the number of paramagnetic O–2 radicals formed in the course of oxygen adsorption on CoO–MgO at room temperature is correlated with the analogous dependence of the number of isolated Co2+ ions of tetrahedral symmetry with trigonal distortion. Two kinds of adsorption centres are postulated in CoO–MgO solid solutions: tetrahedrally coordinated Co2+ ions with trigonal distortion isolated or coupled to the paramagnetic neighbours.

Effect of cobalt ion dispersion on H 2D 2 equilibration and other simple reactions catalyzed by CoOMgO solid solutions

Solid solutions of CoOMgO, with Co Mg ratios from 1 to 50, have been studied as catalysts for the H 2D 2 equilibration reaction. This paper presents data in the temperature range above 300 K (high temperature range, HTR). The catalytic activity per cobalt ion increases with cobalt content. A statistical analysis of the surface configurations shows that the turnover number (molecules per second per site) is a constant if the active site is a cluster containing at least a pair of cobalt ions. The mechanism of the HTR reaction is then discussed. Comparison of the concentration dependence for the N 2O decomposition reaction, and for the CO oxidation, shows that the former shows a decrease of activity per site with increasing Co 2+ content (“dilution effect”), which cannot be explained on the basis of configuration requirements only, while the second reaction shows a constant turnover number, implying a configuration-insensitive reaction. The data suggest details on the mechanisms.

IR-spectroscopic study of the effect of the CoO concentration of CoO-MgO solid solutions on the activation energy for oxidation of the surface forms of CO

1. Kinetic parameters for the oxidation of two surfaces forms of CO on CoO-MgO solid solutions have been determined by IR spectroscopy. The values obtained indicate that these forms differ in their reactivities with respect to oxygen. 2. Comparison of the effect of concentration changes on the activation energy for oxidation on cluster centers and on Weiss centers suggests that the exceptional properties of the cluster centers are due to weakening of the Co-to-surface bonding resulting from indirect exchange.

Catalytic Oxidation of Carbon Monoxide on CoO-MgO Solid Solutions

Catalytic Oxidation of Carbon Monoxide on CoO-MgO Solid Solutions

Magnetic and chemisorptional properties of dilute, highly dispersed CoO-MgO solid solutions

1. The CO adsorbed on CoO-MgO solid solutions with high specific surface areas is bound to three different types of surface centers. The first type is a pair of Co2+ ions, one in octahedral coordination and one in square pyramidal coordination, each weakly interacting with neighboring ions. The second type of center is an individual Co2+ ion in square pyramidal coordination and strongly interacting with neighboring Co2+ ions in the same coordination. The third type of center is a variant of the second, one ion Co2+ ion in square pyramidal coordination interacting with neighboring Co2+ ions in octahedral coordination. 2. The CO molecule bonds more firmly to centers of the first type than to centers of either of the other types.