16:49 May 1, 2008
The frequency-response (FR) technique has been used to follow the dynamics of ad-/desorption of NH3 on Na+/, H+/ and Pt0/H+-mordenites. FR spectra were recorded, as a function of NH3 pressure, in the range 0.4-1.0 Torr and temperatures between 373 and 873 K. The results suggest that intercrystalline diffusion of NH3 is the rate-controlling process in all the pelletized, strongly acidic samples irrespective of the other conditions. However, this rate was influenced by the underlying sorption interactions. On well-dispersed, powdered samples two parallel sorption processes could be distinguished. At high NH3 coverage the NH4+ ions generated in the H-form are the effective sorption sites for NH3 below 600 K. At higher temperatures and/or at lower pressures, i.e., at lower coverages, the signal of the slower processes attributed to sorption involving protonation and deprotonation of NH3 becomes more pronounced. Pt incorporation affects the acidity and sorption properties significantly. |