Selectivity-directing factors of ammonia oxidation over PGM gauzes in the Temporal Analysis of Products reactor:: Primary interactions of NH3 and O2

The Temporal Analysis of Products (TAP) reactor has been applied to study selectivity-directing factors of the high-temperature NH3 oxidation to NO, N2O, and N-2 over commercial knitted Pt and woven Pt-Rh alloy gauzes at 973-1173 K. The unique features of the TAP technique enable investigation of the mechanism of this highly exothermic process under isothermal conditions over catalysts of industrial relevance, and the applicaton of much higher peak pressures as compared to surface science techniques in ultrahigh vacuum. This article focuses on the investigation of primary interactions of NH3 and O-2. The overall reaction mechanism was found to be very similar over both Pt and Pt-Rh gauzes. NH3 activation is favored over O-2-pretreated gauzes, while the as-received gauzes are virtually inactive for NH3 decomposition to N-2. The selectivity to NO primarily depends (in the concentration of adsorbed oxygen species. A high ratio of adsorbed O/NHx species favors NO formation, confirming that undesirable secondary reaction paths are minimized at a high O coverage. Besides, our results suggest that the nature of oxygen species influences the distribution of NO and N-2 in the product. It is put forward that weakly bounded oxygen species lead to a high NO selectivity, while strongly bounced oxidize NH3 into N-2. The interaction of NH3 and NO also contributes to N-2 formation, while direct NO decomposition is practically suppressed over the oxidized gauzes. Application of isotopically labeled (NH3)-N-15 and high peak pressures were essential for detecting N2O formation during ammonia oxidation. Analysis of secondary interactions of NH3 and NO in the authors' next project is required to further unravel the origins of reaction by-products like N2O and N-2. (C) 2004 Elsevier Inc. All rights reserved.