Influence of the preparation method on the structure-activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation

In the present work we studied the influence of the methodology used for mounting Co(II) species on the gamma-alumina surface on the physicochemical properties and the catalytic activity of the 'cobalt oxide'/gamma-alumina catalysts for complete oxidation of benzene. Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotri acetic acid (ma) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta-pvi >> pvi >> edf, nta-pvi >> edf approximate to pvi and edf > nta-pvi > pvi. The catalysts prepared were characterized using various techniques (BET, UV-vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on CO3O4-supported crystallites, loosely or moderately interacting with the gamma-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the gamma-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned CO3O4 crystallites: the ratio 'amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step' closely related with the ratio 'amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step' as well as the composition of the precipitated phase. The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)(6)(2+) (.) 2NO(3)(-). Upon calcination it is transformed into loosely bounded CO3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts. The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800 degrees C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content. The application of the nta-pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(Il)-nta](-) and [Co(Il)-2nta](4-) complexes, in which the Co(H2O)6 2, complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta-pvi catalysts a precipitated phase containing the [Co(II)-nta](- .) NH4+(or H+) and [Co(II)-2nta](4- .) 4NH(4)(+) (or 4H(+)) complex salts predominates. Upon calcination these are transformed into CO3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity. (c) 2004 Elsevier B.V. All rights reserved.