Investigation of the Mechanism of Ammonia Oxidation and Oxygen Exchange over Vanadia Catalysts Using N-15 and O-18 Tracer Studies

Abstract

Isotopic labeling studies in ammonia oxidation and oxygen exchange have been performed over unsupported vanadia catalysts having preferential exposure of different crystal planes. All catalysts were characterized using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, 3-D imaging, and temperature-programmed adsorption/desorption/reduction techniques. Isotopic labeling studies have been performed under steady-state conditions by using 16O 2 → 18O 2, NH 3 + 16O 2 → NH 3 + 18O 2, and 14NH 3 + O 2 → 15NH 3 + O 2 switches. The experimentally obtained transients have been compared to those calculated through a mathematical simulation. Results of the oxygen exchange experiments show that, although there is no formation of the crosslabeled oxygen, gaseous oxygen actively exchanges with lattice oxygen on all crystal planes of V 2O 5. The oxygen located on the (010) basal 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 V 2O 5 crystals. The kinetic experiments performed for ammonia oxidation reaction studies suggest that there are at least two types of sites on the (010) plane responsible for this reaction, one leading to NO formation and the other one leading to N 2 and N 2O formation. The type of adsorbed ammonia species formed are thought to be controlled by the immediate environment of the V = O species on the surface.