N2O formation and dissociation during ammonia combustion: A combined DFT and experimental study

N2O decomposition over a 95%Pt/5%Rh knitted catalyst under the industrial conditions of ammonia combustion has been studied. It is found that N2O is not significantly decomposed over the catalyst surface or in the gas phase, in contrast to reported observations of N2O destruction in laboratory reactors at lower temperatures. Since the surface morphology of platinum catalysts is uncertain, we have employed Density Functional Theory (DFT) to shed light on the N2O formation and decomposition pathways that occur on different Pt surfaces. We studied the flat Pt(1 1 1) and Pt(1 0 0) surfaces and the stepped Pt(2 1 1) and Pt(1 1 0) and found that all could support N2O formation via (1) NO* + N* ⇌ N2O(g). Although the dimerization of two NO molecules NO* + NO* ⇌ (NO)2* ⇌ N2O(g) + O* (3) is energetically favuorable relative to (1) on Pt(2 1 1), dimerization is kinetically unfavourable because of the very low surface concentration of NO under typical operating conditions.DFT showed decomposition of N2O via N2O ⇌ N2 + O* to be facile (energy barriers 29 and 32 kJ mol−1, respectively) on Pt(1 0 0) and Pt(1 1 0) surfaces and we infer that these facets are essentially absent from the surface of Pt-based catalysts during ammonia combustion. The Pt(1 1 1) and Pt(2 1 1) surfaces provide better bases for the development of fundamental surface microkinetic models for the ammonia combustion on platinum.