The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al2O3 system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxid e levels increase with the addition of various metal oxides to the basic co pper oxide/alumina system. Addition of sulfur dioxide to the reaction strea m sharply reduces the level of ammonia conversion, but has a beneficial eff ect on selectivity to nitrogen. Added water vapour has a lesser effect on a ctivity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al2O3 is also active for the selective catalytic reduction of nitric ox ide by ammonia, but this reaction is not effected by sulfur dioxide additio n. A mechanism for ammonia oxidation to nitrogen is proposed wherein part o f the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is establi shed. N-2 or N2O are formed depending upon the average oxidation state of t his pool. An abundance of labile lattice oxygen species on the catalyst sur face leads to overoxidation and to N2O formation. On the other hand, reduce d lability of surface lattice oxygen species favours a lower average oxidat ion state for the monoatomic surface nitrogen pool and leads to N-2 formati on. (C)2000 Elsevier Science B.V. All rights reserved.