Comparison of the dynamics and orientation of chemisorbed benzene and pyridine on molybdenum nitride (.gamma.-Mo2N)

Chemisorbed benzene and pyridine on high surface area (140-155 M2/g)gamma-Mo2N have been investigated by using temperature programmed desorption (TPD) and H-2-NMR Spectroscopy. NMR spectra obtained at 295 K indicate that a fraction of adsorbed benzene is static, a fraction rotates rapidly about the C6 axis of benzene, and a fraction translates rapidly in three dimensions. At 80 K benzene can no longer translate; that fraction which translates at 295 K undergoes only rapid rotational motion at 80 K. The uptake of benzene is 140 +/-50 mumol/g. Chemisorbed benzene is displaced completely by pyridine in a competitive adsorption experiment at 295 K. The uptake of pyridine at 295 K is 560 +/- 50 mumol/g, which is similar to the uptakes of ammonia and acetonitrile on gamma-Mo2N. Pyridine desorbs molecularly during a temperature ramp in two temperature ranges centered at 388 and 563 K. No other species desorb from gamma-Mo2N below 700 K. NMR experiments indicate that the majority of chemisorbed pyridine is static after adsorption at 295 K, while a fraction undergoes rapid motion. The absolute population of surface pyridine which is motional at 295 K remains constant after treatments at 295 and 493 K, while the static pyridine population falls after treatments above 295 K. The motion is consistent with incomplete rotations about the C2v axis of pyridine through an angle greater than 180-degrees. A model for the geometry of adsorbed pyridine consistent with this motion is proposed.