ROLE OF AMMONIA OXIDATION IN SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE OVER VANADIA CATALYSTS

The role of direct oxidation of ammonia in SCR reactions and the catalytic anisotropy of V2O5 catalysts in this reaction network were investigated. The catalysts were characterized using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and 3-D imaging. Temperature-programmed reduction experiments using ammonia and hydrogen as reducing agents and temperature-programmed desorption studies were also performed. A steady-state fixed-bed reactor system was used for activity and selectivity measurements. The product analysis was carried out by a combination of techniques including chemiluminescence NOx analysis, on-line gas chromatography and on-line mass spectrometry, and chemical titration methods. The results obtained in this study, when combined with our previous studies in SCR reactions, indicate that over vanadium pentoxide catalysts, ammonia oxidation and NO reduction reactions are closely coupled. The results suggest that ammonia adsorption takes place on at least two types of sites located on the (010) and (100) planes of V2O5. The sites that selectively reduce NO to N2 are the V-O-V sites located on the side planes, whereas V=O sites promote direct oxidation of ammonia to NO and the formation of N2O from NO and NH3 interaction more readily. It is found that the V=O sites are more easily reduced with both hydrogen and ammonia. © 1993 Academic Press, Inc.