INVESTIGATION OF THE MECHANISM OF AMMONIA OXIDATION AND OXYGEN-EXCHANGE OVER VANADIA CATALYSTS USING N-15 AND O-18 TRACER STUDIES

Isotopic labeling studies in ammonia oxidation and oxygen exchange hav e been performed over unsupported vanadia catalysts having preferentia l exposure of different crystal planes. All catalysts were characteriz ed using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron micr oscopy, 3-D imaging, and temperature-programmed adsorption/desorption/ reduction techniques. Isotopic labeling studies have been performed un der steady-state conditions by using O-16(2) --> O-18(2), NH3 + O-16(2 ) --> NH3 + O-18(2), and (NH3)-N-14 + O-2 --> (NH3)-N-15 + O-2 switche s. The experimentally obtained transients have been compared to those calculated through a mathematical simulation. Results of the oxygen ex change experiments show that, although there is no formation of the cr osslabeled oxygen, gaseous oxygen actively exchanges with lattice oxyg en on all crystal planes of V2O5. The oxygen located on the (010) basa l plane seems to be more reactive whereas the replenishment of oxygen from the catalyst bulk appears to be faster towards the side planes of the V2O5 crystals. The kinetic experiments performed for ammonia oxid ation reaction studies suggest that there are at least two types of si tes on the (010) plane responsible for this reaction, one leading to N O formation and the other one leading to N-2 and N2O formation. The ty pe of adsorbed ammonia species formed are thought to be controlled by the immediate environment of the V = O species on the surface. (C) 199 4 Academic Press, Inc.