A study of the mechanism of selective conversion of ammonia to nitrogen on Ni/γ-Al2O3 under strongly oxidising conditions

The activity and excellent selectivity (>90%) of gamma-Al-2-O-3-supported Ni for the selective catalytic oxidation (SCO) of NH3 to N-2 With excess O-2 has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N2H4 and NO have been used to model the intermediates of the SCO mechanism (direct formation of N-2 via the recombination of two NH, species) and of the in situ SCR mechanism (two-step formation of N-2 via the reduction of an in situ produced NO species by a NHx species), respectively. Two IR absorption bands appear during the TPO of NH3 in the temperature range of N-2 formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N-2 occurs via the in situ SCR mechanism. This also explains the sudden "NO jump" observed on various systems once the temperature is high enough to activate similar to 50% of the NH3 molecules fed to the catalyst. The fact that NO and NH3 are able to react to give N-2 at low temperature (from 100 degrees C) confirms that activation of NH3 is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism, (C) 2000 Elsevier Science B.V. All rights reserved.