Σύνθεση καταλυτών κοβαλτίου για τη καύση πτητικών οργανικών ενώσεων (V.O.Cs)

Abstract

In this work we attempted to contribute to the effort done in the last years for a deeper understanding of the adsorption mechanism of the ions in the interfacial region developed between the surface of the γ-alumina particles, being in aqueous suspension containing the aforementiοned ions and the aqueous solution. In the frame of this target we studied the kinetics of the adsorption as well as the influence of various experimental parameters on the extent of adsorption. Moreover, we studied the surface dissolution of the carrier during the adsorption experiments, the behaviour of the impregnating solution in the presence and absence of the carrier, as well as the reversibility of the adsorption. Finally we studied the adsorption using diffuse reflectance spectroscopy (DRS). It was found that the mechanism of adsorption does not involve surface dissolution of the γ-alumina, formation and deposition of a mixte Co-Al phase of hydrotalcite structure nor bulk precipitation of the cobalt phase induced by cobalt particles produced through dissolution of the supported cobalt phase. It has been confirmed, using DRS, that Co(II) has octahedral symmetry in the adsorbed cobalt species. Moreover, it was observed for a first time that the adsorption brings about important changes in the adsorption bands, appear in the visible region, strongly suggesting the formation of inner sphere surface Co(ΙΙ) complexes in agreement with the EXAFS results reported in the literature. On the other hand the "S" form of the adsorption isotherm achieved indicated that the initially formed inner sphere Co complexes promoted additional deposition of the species. This presumably takes place via reaction of an already deposited Co (H2O, O-)6 2+ species, part of a surface Co(ΙΙ) oligomer, with two species being in the interfacial region in agreement with our kinetic results. This presumably results to the formation of supported bidimentional islands containing a number of the Co (H2O, O-)x 2+ units which increases with the Co(ΙΙ) surface concentration which are transformed above a critical Co(ΙΙ) surface value to a tridimentional Co(ΙΙ) surface precipitate similar to Co(ΟΗ)2. The formation of this precipitate corroborated by the observation that the γ-alumina surface absorbs Co(ΙΙ) even for Co(ΙΙ) loading corresponding to more than 9 theoretical monolayers of supported . The reversibility experiments indicated the deposition of two, at least, Co(ΙΙ) phases: one strongly bounded to the support surface presumably included the two dimentional islands of the Co(ΙΙ) inner sphere complexes and one weakly bounded to the surface probably corresponding to the tridimentional Co(ΙΙ) surface precipitate The kinetic study of adsorption, performed for a first time, suggested that two species, being in the interfacial region, are involved in the adsorption process, irrespectively of the value of the surface Co(II) surface concentration. However, there is no, for the moment, known the number and the kind of the receptor sites involved in the rate determined step. In the present study we investigated, moreover, the influence of the deposition technique on the physicochemical characteristics and the catalytic activity, with respect to the catalytic combustion of benzene, of the supported "cobalt oxide"/γ-Al2O3 catalysts. To do that we prepared and studied four series of catalysts. The catalysts of the first two series were prepared using the Equilibrium Deposition-Filtration (EDF) technique. The catalysts of the other two series have been prepared using simple or successive pore volume impregnation in the absence (PVI) or presence of the nitrilotriacetic acid (PVIN). The Co loading in each series was varied from 0.8 to 21% wt. The samples were characterized using various techniques: BET for the determination of the specific surface area, diffuse reflectance spectroscopy (DRS), X-ray diffraction powder analysis (XRD), X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). The above study showed that the deposition technique influences remarkably the characteristics of the supported cobalt phase even before drying, after drying but mainly after calcination at 3000C. Thus, the catalysts prepared by EDF exhibited remarkably greater dispersity of the supported Co as it is compared to that of the corresponding catalysts prepared by PVI. This may be easily attributed to the fact that in the EDF catalysts Co(II) is deposited via adsorption, interfacial or surface bidimentional oligomerization and polymerization as well as through formation of tridimentional oligolayer surface precipitate but only at relatively high Co(II) surface concentration. In contrast, in the PVI catalysts the deposition takes mainly place through bulk precipitation during drying, resulting thus to the relatively large supported crystallites of <<cobalt oxide>>. Although the EDF catalysts were proved to be generally more active than the corresponding PVI catalysts, the differences in dispersity only partly are reflected in catalytic activity. A plausible explanation is that the active centers are mainly located on the relatively large and weakly bounded supported crystallites and not on the strongly bounded Co(II) and Co(III) supported species produced during the calcination of the EDF catalysts by the transformation of the deposited Co(II) bidimentional species mentioned above.The PVI.N catalysts are, generally, proved to be the most active, though in the EDF catalysts the dispersity was somewhat greater. The TPR results indicated that this could be attributed to the fact that the nitrilotriacetic acid complexes Co(II) during impregnation inhibiting presumably the formation of the sorbed Co(II) bidimentional species mentioned above and thus the formation, after calcination of the strongly bounded and rather inactive Co(II) and Co(III) supported species. Moreover, in the PVIN catalysts is precipitated the complex mentioned above instead of the Co(ΟΗ)6(NO3)2 precipitated, during drying, in the PVI catalysts. As the volume of the first is greater than the second, the Co(II) is probably diluted in the resulting precipitate resisting thus to sintering during calcination. Therefore the "cobalt oxide" supported crystallites formed in the PVIN catalysts are expected to be smaller than the corresponding ones formed in the PVI catalysts. These are experimentally confirmed by the TPR results and may be taken into account for explaining the relatively high dispersity and the highest activity found in the case of the PVIN catalysts.