Title : Selective catalytic oxidation of ammonia to dinitrogen over hydrotalcite-like derived multicomponent oxide systems
Ammonia (NH3) is a colourless gas with characteristic odour that belongs to a group of air pollutants whose emission to atmosphere is strictly monitored and restricted. These limitations result from its high toxicity and destructive impact on the environment (e.g. eutrophication, corrosion). At the same time, despite the significant impact on humans and the environment, ammonia is used as a reagent in many chemical processes (e.g. urea manufacturing, hydrodenitrification process, DeNOx process) or as a component of fertilizers. Presently it has been established that almost 75 % of total ammonia emission (about 4 200 kt y-1) is related to agriculture while only 25 % (about 1400 kt y-1) to industry and chemical processes. Because of the high toxicity and widespread sources of ammonia emission, NH3 efficient removal processes should to be further studied. Selective catalytic oxidation of ammonia to dinitrogen (NH3-SCO) is one of the most promising method. Possible application areas of NH3-SCO include the treatment of waste gases from gasification of biomass, chemical production processes and NH3 slip from the SCR or SNCR NOx/NH3 process. In case of processes related to ammonia slip, exhaust gases already contain oxygen – oxidizing agent, so further ammonia removal does not require any additional reactants into flue gases.
However, by-products like N2O, NO and NO2 could be released because of the possible side reactions and complex mechanism of ammonia oxidation. Formation of these products is related to reaction condition as well as to chemical composition and structure of catalysts. Therefore, the effective NH3-SCO catalysts should operate at relatively low temperature range and should selectively direct the reaction to the formation of dinitrogen and water vapour.
Our studies have shown that mixed metal oxides (MMO) obtained by calcination of hydrotalcite-like materials are characterized by high efficiency as catalysts of NH3-SCO process. Among studied catalysts, Cu-based oxide systems demonstrate the highest removal efficiency with the relatively high selectivity to the dinitrogen. In the presented studies, series of Cu-MMO catalysts were tested as catalysts of NH3-SCO reaction. Oxide systems were obtained by calcination of hydrotalcite-like materials at temperature 800 °C. Additionally, obtained catalysts were modified by wet impregnation by Ce (5 %wt.). These kinds of materials are characterized by relatively well-developed surface area, homogenous distribution of active centres and high thermal resistance. Samples were characterized by several techniques: X-ray diffraction (XRD), infrared spectroscopy (FT-IR), low-temperature nitrogen sorption, cyclic voltammetry (CV), temperature programmed reduction (H2-TPR), UV-vis diffuse reflectance spectroscopy (UV-vis-DRS) and scanning electron microscopy (SEM).