In situ characterization of interaction of ammonia with carbon surface in oxygen atmosphere

摘要

The adsorption and oxidation of ammonia over carbons differing in the chemical structure of surface functional groups have been investigated by ETIR spectroscopy. The reactions of NH_3 with carbons have been studied both in the presence and in the absence of oxygen. As a result of NH_3 chemisorption, in addition to ammonium salts, there are formed surface amide and imide structures. At the higher temperature surface isocyanate species are formed. Thermal stabilities of surface structures, formed as a result of NH_3 chemisorption have been determined by means of FTIR spectroscopy. The activity and selectivity of carbons for the selective catalytic oxidation (SCO) of NH_3 to N_2 with excess O_2 has been shown by microreactor studies at 295-623 K. Carbon catalysts are very active for NH_3 oxidation. Nitrogen is generally the predominant product of ammonia oxidation. The selectivity to N_2, N_2O and NO is determined by the surface oxygen coverage and reaction temperature. The data obtained indicate that the N_2 is formed via selective catalytic reduction (SCR) between NH_4~+ surface species and NO formed from NH_4~+ oxidation. This implies that ammonia is activated in the form of NH_4~+ species for both SCR and SCO processes.