Effect of preparation method on catalytic properties of Co-Mn-Al mixed oxides for N2O decomposition

Abstract

Co-Mn-Al mixed oxides (Co:Mn:Al molar ratio of 4:1:1) were prepared by three different methods (i) calcination of hydrotalcite-like precursors (Co-Mn-Al-HT-ex), (ii) calcination of corresponding nitrates (Co-Mn-Al-nitr) and (iii) calcination of the product of mechanochemical reaction of Co, Mn, Al nitrates with NH4HCO3 (Co-Mn-Al-carb). The catalysts were characterized by AAS, XRD, SEM, Raman spectroscopy, FTIR, TPR-H2, TPD-N2O, step response experiments and tested for N2O decomposition in inert gas and simulated waste gas from HNO3 production. Different conditions of synthesis led to the formation of spinel-like phase with different structural properties leading to different catalytic activity. N2O conversions decreased with decreasing specific surface area in order Co-Mn-Al-carb > Co-Mn-Al-HT-ex > Co-Mn-Al-nitr. The synthesis from carbonate precursor led to the less ordered structure manifested itself in smaller crystallite size causing (i) higher surface area (ii) better reducibility and (iii) increase of mean (Co + Mn) valence. However, its highest catalyst activity was determined by the lowest bond strength of active site − oxygen while specific surface area and amount of active sites per unit surface were not decisive parameters for activities order.