Effects of calcination program and rehydration on palladium dispersion in zeolites NaY and 5A
1992; Elsevier BV; Volume: 89; Issue: 2 Linguagem: Inglês
10.1016/0926-860x(92)80230-a
ISSN1873-3875
AutoresZ. Zhang, Gerhard Mestl, Helmut Knözinger, Wolfgang M.H. Sachtler,
Tópico(s)Ammonia Synthesis and Nitrogen Reduction
ResumoThe calcination program in preparing palladium supported in zeolite Y or 5A has a strong effect on the metal reduction and the metal dispersion. UV-VIS diffuse reflectance and Raman spectroscopy of Pd/NaY and Pd/5A and temperature-programmed reduction profiles reveal significant effects due to calcination at different heating rates. Maximum concentration ofPd2+ ions is achieved by calcination at a heating rate of 0.5 Kmin, which leads to monoatomically dispersed palladium in zeolites after reduction. Calcination at higher heating rates produces PdO particles due to autoreduction followed by subsequent oxidation. Migration of Pd2+ ions from sodalite cages to supercages in calcined samples occurs during hydration in moist air. Recalcination of the hydrated samples leads to hydrolysis of palladium ions in supercages. As a result, PdO particles are formed which, upon reduction, produce large palladium particles. Due to the very small cage apertures in 5A, supply of oxygen from the atmosphere surrounding the zeolite is insufficient to maintain an oxidizing atmosphere inside the cages when oxygen consumption by ammonia oxidation is rapid. As a consequence, autoreduction of palladium takes place in 5A under conditions where it is negligible in Y.
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