Optimization of the preparation conditions of cordierite honeycomb monoliths washcoated with cryptomelane-type manganese oxide for VOC oxidation

Ceramic honeycomb monoliths were washcoated with cryptomelane-type manganese oxides and their catalytic performance was evaluated in the oxidation of ethyl acetate. The effect of a mixture of ethyl acetate with toluene and of the presence of water vapour was also assessed.Different coating parameters, namely size of catalyst particles, number of immersions in the washcoating solution, presence of an initial coating with alumina, calcination temperature of this coating, as well as the amount of binding agent and ethanol in the washcoating solution were studied and optimized based on the catalytic activity of the structured catalyst. Small particles are required for a correct impregnation; however, since the smallest particles are less active, an intermediate size achieved the best catalytic results. Increasing the number of immersions over 3 did not significantly increase the catalytic activity of the structured catalyst. The presence of an initial coating with alumina and a binding agent (colloidal alumina) in the washcoating solution was found essential to increase the activity, whereas increasing the calcination temperature after the initial alumina coating above 500°C decreased the activity of the catalyst. The presence of ethanol in the washcoating solution did not significantly improve the activity of the structured catalyst.The optimized structured catalyst presented high catalytic activity in the removal of ethyl acetate (90% conversion into CO₂ at 256°C) and high stability during 100 h of reaction. The addition of toluene or water vapour in the feed gas did not significantly affect the activity of the coated monolith.

Effect of ball milling on the catalytic activity of cryptomelane for VOC oxidation

Cryptomelane-type manganese oxides prepared by a solvent-free method were evaluated as catalysts for the oxidation of ethyl acetate, ethanol and toluene. The original catalyst (K-OMS-2) presented high catalytic activity for ethyl acetate and ethanol oxidation, achieving 90% conversion into CO₂ around 200°C for both pollutants. Toluene was much harder to oxidize, requiring a temperature near 270°C for the same conversion. The original catalyst was mechanically treated in a ball mill at different intensities, in order to decrease the particle size for subsequent impregnation onto structured supports, as small particle sizes are usually recommended. The catalytic activity of the materials decreases with the increase in the severity of this treatment, which is related to the decrease of the surface area of the catalysts, since the other properties (phase purity, thermal stability, surface oxygen, average oxidation state and reactivity of the oxygen species) are similar among the catalysts with different ball milling treatments. For comparison, a platinum-based catalyst (1%-Pt/Al₂O₃) was also tested, which exhibited a high activity for toluene, but much lower activities for the two other volatile organic compounds tested. A long-term experiment, using ethanol as model pollutant, showed that the cryptomelane catalyst was stable for more than 100 h.