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1 Polish Journal of Chemistry

1 2004

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Temperature Programmed Desorption of Pyridine and 2,6-Dimethylpyridine from Differently Pretreated Pd/Al2O3 Catalysts

Skotak M., Karpiński Z.

Polish Journal of Chemistry

2004

Vol. 78 / nr 10

1953-1959

The temperature programmed desorption (TPD) of pyridine and 2,6-dimethylpyridine from alumina and two alumina-supported palladium catalysts of different metal loadings (0.3 and 2.77 wt.% Pd) confirmed the presence of strong acid sites in the samples subjected to high temperature reduction at 600°C. Roughly similar amounts of both organic bases were desorbed from the catalysts which underwent similar pretreatments. However, 2,6-dimethylpyridine appears less strongly bonded than pyridine to Lewis acid sites in alumina, apparently because of some steric hindrance produced by the presence of methyl substituents in 2- and 6-position to the nitrogen lone pair. Thus, pyridine is better suited for probing evolution of Lewis acidity in alumina, brought about by high temperature reduction at 600°C.With increasing temperature during thermodesorption, both organic bases adsorbed on palladium-containing samples undergo transformation, leading to desorption of several products, among which hydrogen and nitrogen predominate. Introduction of increasing amounts of palladium to alumina makes the acidity probing difficult, because a considerable part of adsorbed organic base is decomposed on metal sites. Decomposition of pyridine and 2,6-dimethylpyridine may serve as a convenient probe of availability of palladium surface. After high temperature reduction of Pd/Al2O3 a considerable part of Pd surface is blocked by support species.