REDOX CHEMISTRY OF HIGHLY DISPERSED RHODIUM IN ZEOLITE NAY

NaY zeolite exchanged with [Rh(NH3)5Cl]2+ ions have been studied using temperature programmed oxidation (TPO), temperature programmed reduction (TPR), and Fourier transformed infrared spectroscopy. The TPO profiles show that ammine ligands in NaY encaged [Rh(NH3)5Cl]2+ are destroyed above 300 °C, whereas the Rh precursor ion remains intact after calcination at 200 °C. TPR profiles in conjunction with the COads IR spectra show that the reducibility of Rh by H2 is largely controlled by the concentration of the surface protons, i.e. Rh3++H2⇌Rh++2H+ Rh+ + 1/2H2⇌Rh0+H+ In the presence of ammonia, the protons are neutralized and Rh3+ is reduced to Rh0. However, reduction remains incomplete when the concentration of protons is high. The ammonia was provided either by NH3 admission or by conservation of ammine ligands by controlled calcination. CO adsorption does not lead to reoxidation of Rh0 particles to Rh+ ions. © 1990 J.C. Baltzer A.G. Scientific Publishing Company.