Selective catalytic oxidation of ammonia to nitrogen over Mg-Al, Cu-Mg-Al and Fe-Mg-Al mixed metal oxides doped with noble metals

Abstract

Mg-Al, Cu-Mg-Al and Fe-Mg-Al mixed oxides, were obtained by thermal decomposition of synthetic hydrotalcite-like materials and, in the next step, were modified with selected noble metals (Pt, Pd, Rh) by incipient wetness impregnation method. The process of thermal decomposition of hydrotalcite-like materials into metal oxide systems was studied by thermogravimetry method combined with the on-line analysis of gaseous products of the sample decomposition (TG–DTA-QMS). The obtained catalysts were studied with respect to chemical composition (EDS), structure (XRD, UV–vis–DRS), morphology (STEM), surface area (BET) and redox properties (H2-TPR). Metal oxide catalysts obtained from the hydrotalcite-like precursors were characterized by high dispersion of transition metals (Cu, Fe), which were present mainly in the form of monomeric or small aggregated species dispersed in the Mg-Al oxide matrix. Noble metals, as it was shown by STEM studies, were rather uniformly dispersed on the surface of the samples. The obtained samples were tested as catalysts of the selective catalytic oxidation (SCO) of ammonia to nitrogen. The Cu-Mg-Al oxide catalyst was active in the low-temperature SCO process, while the Fe-containing sample was found to operate in the high temperature range. Modification of these catalysts with noble metals significantly decreased temperature of the ammonia oxidation but also decreased the selectivity to nitrogen. The best catalytic properties were obtained for the Cu-Mg-Al oxide catalyst modified with small amount of platinum (0.2wt%), which operated at relatively low temperature with high selectivity to nitrogen.