Cobaltous oxide catalysts characterization

A. A. Mirzaei, R. Habibpour, M. Faizi, and E. Kashi, Characterization of iron-cobalt oxide catalysts effect of different supports and promoters upon the structure and morphology of precursors and catalysts, Applied Catalysis A, vol. 301, no. 2, pp. 272-283, 2006. 

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

In recent years increasing use has been made of an alternative procedure involving the oxidation of hydrocarbon substrates in polar solvents, usually acetic acid, in the presence of relatively large amounts of metal catalysts, usually the metal acetate. These reactions are characterized by high rates of oxidation, high conversions, and more complete oxidation of the substrate. For example, the classic autoxidation of cyclohexane is carried out to rather low conversions and affords mainly cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone. Autoxidation of cyclohexane in acetic acid, in the presence of substantial amounts of cobalt acetate catalyst, results in the selective formation of adipic acid at high conversions (see Section II.B.3.c). 

This phenomenon has also been observed for catalysts prepared using an aqueous route (182). Both the iron and cobalt promoters led to an increase in selectivity. The iron-promoted catalyst was characterized by an increase in activity, but the cobalt-promoted catalyst was characterized by a decrease in activity. The decrease in activity of the cobalt-doped catalyst was attributed to the formation of VOPO4 in the final catalyst. The VOPO4 is formed by the oxidation of V0HP04 1 H20 during the introduction of the promoters in the incipient wetness technique. A similar effect was reported for catalysts doped with indium and tetraethy-lorthosilicate (TEOS) (181). The improved performance was observed only with both promoters in the catalyst. It was proposed that the... 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

Our work is a tentative to rationalize the influence of the preparation parameters over the composition, active phase and catalytic activity. Modifying the pH of precipitation between 6.0 and 8.5, the surface area, the bulk, the surface composition and the active phases of the catalysts were modified significantly. Raman spectroscopy has revealed the presence of M0O3 phase only in the catalyst prepared at pH 6.0. From the literature data, we know that the precipitation of the Ni(OH)2 and Co(OH)2 occurs at pH 7 and 7.5, respectively. At pH higher than 7.5 our calculations suggest the presence of nickel or/and cobalt oxides but these phases were not detected with any of the characterization techniques used. 

In Fig. 2, TPR profile of the unreduced dried catalyst is compared with the profile of a sample prepared from this catalyst by calcination at 500°C in air. The reduction peak at 320°C in the profile of the calcined sample may characterize the reduction of Co O /7/ while that at 470°C is caused by the reduction of mixed oxides,e.g. xCoO.yMgO.The peak at 320°C in the catalyst profile originates very probably from CO evolved during the decomposition of cobalt hydroxycarbonate (compare with Fig. 3b). The peak at 370°C may be ascribed to the reduction of cobalt oxides originating from the decomposed hydroxycarbonate. From the comparison of both TPR profiles it clearly follows that the composition of unreduced forms differs and that the metallic phase originated from different precursors. 

In this study, the CVD process is used to deposit cobalt oxide as planar model layers and as working monolithic catalysts to enable accurate characterization and test of performance. 

The catalyst was well-characterized by a combination of methods. By TPR, the cobalt oxide species were found to interact strongly with the alumina support. A TPR carried out after reduction in hydrogen at 350°C for 16 hours still showed that a considerable fraction of Co oxide species remained unreduced. The extent of reduction, quantified by tallying the munber of pulses of O2 that were consumed by the catalyst, was found to be 53%. The dispersion... 

Cobalt oxide and soluble aquo or hydroxo complexes have been known as water oxidation catalysts since the early 1980s, under heterogeneous and homogeneous conditions [52,53]. However, cobalt ions in aqueous solution present poor stability, showing detrimental separation as insoluble oxides, these latter being characterized by very low activity. 

Oxidation of cyclohexene by polymeric cobalt-containing catalysts [132,133] has many significant features. In the presence of Co(AcAc)2 (AcAc = acetyl-acetonate) the process is characterized by an induction period, after which the rate rapidly reaches its highest value and decreases thereafter (Fig. 12-10, plot 1). 

L.A. Boot, 1996, Preparation, characterization and catalytic testing of cobalt oxide and manganese oxide catalysts supported on zirconia, Appl. Catal. A, 137,69-86... 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Saib, A. M., Borgna, A., van de Loosdrecht, J., van Berge, P. J., Geus, J. W., and Niemantsverdriet, J. W. 2006. Preparation and characterization of spherical Co/Si02 model catalysts with well-defined nano-sized cobalt crystallites and a comparison of their stability against oxidation with water. J. Catal. 239 326-39. 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

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