Preparation and structural characterization of Co/Al2O3 catalysts for the ozonation of pyruvic acid

A series of Co/Al2O3 catalysts were prepared by the incipient wetness impregnation method using γ-Al2O3 support and (CH3COO)2Co·4H2O solutions, followed by calcination at 500–800°C. Characterization of catalysts was accomplished by several techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), physisorption of nitrogen, mercury and helium-based pycnometries, Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and pH of zero charge (PZC). Impregnation of support produced a moderate decrease of its surface area and pore volume and also led to minor changes of its PZC. Depending on preparation conditions (i.e., calcination atmosphere and temperature and metal loading), one or more of the following Co-containing compounds were identified: CoO, Co3O4 and CoAl2O4. The support and prepared Co/Al2O3 catalysts were tested to catalyze the ozonation of aqueous pyruvic acid at pH 2.5. Pyruvic acid was shown refractory towards single ozonation but the use of γ-Al2O3 and Co/Al2O3 catalysts resulted in 56–96% pyruvic acid conversion and 41–78% decrease in DOC after 2h of ozonation of phosphate-buffered solutions. In the absence of the buffer, conversion rate was enhanced likely as a result of pH increase during the course of the process thus giving rise to the indirect way of ozonation through hydroxyl radicals. Acetic acid was found as the main by-product of pyruvic acid ozonation. Depending on the catalyst used, yield of acetic acid varied from 32 to 49%, values noticeably lower that that obtained from the control non-catalytic ozonation experiment (73%). Differences in catalytic activity amongst the various Co/Al2O3 catalysts investigated were attributed to the different Co active phases deposited on the γ-Al2O3 surface. The following sequence of increasing activity can be inferred from experimental results: CoO, CoAl2O4 and Co3O4. All the Co/Al2O3 catalysts prepared showed good stability as the percentage of cobalt leached out was rather low.