A frequency–response study of the acidity of mordenite catalysts

Abstract

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.