MAGNETIC-RESONANCE STUDIES OF AMMONIA ADSORPTION AND DECOMPOSITION ON TITANIA-SUPPORTED VANADIA CATALYSTS

In-situ proton and deuteron single-resonance NMR techniques were used to study the adsorption and decomposition of ammonia as a function of temperature on a series of V2O5/TiO2 catalysts with the weight loading of vanadia ranging from 0 to 6.1 wt % (approximately one monolayer). After analysis of magnetic resonance line shapes, several different molecular species were found to be present on the catalyst surfaces. At low adsorption temperatures on the low-loading samples, the predominant adsorbed species was intact, coordinated ammonia rotating about its C3 axis. As the weight loading or adsorption temperature was increased, the adsorbed, intact ammonia exhibited an increased H-N-H angle; the total number of protons associated with intact ammonia per gram of sample decreased. At the point where intact ammonia was no longer observed, a new species was observed and assigned to NH2 moieties. On all samples, ammonium ions were observed at low adsorption temperatures (100-degrees-C) but were found to be easily removed by heating. Their concentration was greater on the higher weight loading samples. The formation of short-lived VOH groups from the protons of dissociated NH3 was inferred on the higher weight loading samples. Multiple-quantum spin-counting experiments were done to further investigate the nature of the adsorbed species and their groupings on the surfaces.