Effect of Co-content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation

Abstract

The present investigation was undertaken in an endeavor to study the effect of the cobalt content on the structure and activity of Co–Al hydrotalcite-like materials as catalyst precursors for CO oxidation by varying the Co 2+/Al 3+ atomic ratio, thermal treatment of the samples and the reaction temperature. The samples (Co 2+/Al 3+ = 0.5, 1.5, 3.0) have been synthesized by the co-precipitation method. The unsupported Co 3O 4 has been prepared according to the same procedure as the reference compound in order to reveal the role of Al 3+ ions presence. The physicochemical characterization of the uncalcined, hydrothermally treated, calcined and tested samples has been accomplished appropriately by ICP-AES, N 2 adsorption, Powder X-ray diffraction technique and Diffuse Reflectance Spectroscopy and H 2-TPR measurements. The samples were examined by a number of heating–cooling cycles during the activity tests as a procedure to screen the most active catalyst precursor. It was established that the hydrotalcite-like structure of all uncalcined samples had been completely destroyed during the CO oxidation reaction. A concomitant phase transformation into poorly crystallized spinel-type Co 2+(Co 3+,Al 3+) 2O 4 mixed oxide occurred. This spinel-like mixed oxide phase is better organized in all samples after their calcination at 500 °C. The TPR examinations reveal concomitant presence of high-temperature reduced non-stoichiometric CoAl 2O 4. It was found out that the Co–Al mixed oxide, derived from the sample with the highest cobalt loading (Co 2+/Al 3+ = 3.0) preserves a complete and prolonged CO oxidation ability even after cooling down to ambient temperature. On the contrary, the samples with ratios Co 2+/Al 3+ = 0.5 and 1.5 as well as the Co 3O 4 oxide deactivate more rapidly during the cycles. A hypothetic scheme is proposed for activation/deactivation of the catalysts. It is related to the oxygen ion-radicals O 2 x– stabilization by Al 3+ cation association with the Co 2+/Co 3+ redox couple via anionic vacancy.