Mechanistic aspects of the Andrussow process over Pt-Rh gauzes. Effect of gauze morphology and oxygen coverage on primary O2-NH3-CH4 interactions

Primary O-2-NH3-CH4 interactions in the Andrussow process were investigated in the temporal analysis of products (TAP) reactor over three commercial Pt-Rh gauzes having been applied for different times on-stream in this reaction (fresh, activated and spent catalysts). The gauzes were characterized by scanning electron microscopy combined with energy dispersive X-ray analysis. It was established that they underwent severe morphological changes including deposition of various impurities with increasing time on-stream. Content and type of these impurities possess local variations. However, the near-to-surface Pt/Rh ratio of the deposit-free areas does not change with time on-stream. These areas are strongly faceted on the spent Pt-Rh gauze. Despite the gauzes differ in their morphology and in the content of impurities, the overall scheme of HCN production via oxidative coupling of CH4 and NH3 is valid for all the gauzes studied. Nitric oxide primarily formed via ammonia oxidation reacts further with methane and ammonia yielding hydrogen cyanide and nitrogen, respectively. The formed HCN is consecutively converted to N-2. The surface restructuring, coverage by oxygen species and/or the presence of iron oxide influence the interplay between the reaction pathways leading to HCN and its further transformations. (C) 2010 Elsevier B.V. All rights reserved.