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Abstract
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Researchers
examined the intrinsic hydrogenation activities of homologous series of
noble-metal-containing zeolites Y using in situ solid-state NMR spectroscopy
under semibatch conditions. For the hydrogenation of acrylonitrile, reaction
rates in the sequence Pd/H,Na–Y > Rh/H,Na–Y > Pt/H,Na–Y > Ir/H,Na–Y
were determined. The dehydrogenation of propane at these zeolites gave a
sequence of the turnover frequencies of Ir/H,Na–Y > Rh/H,Na–Y >
Pd/H,Na–Y, while Pt/H,Na–Y zeolites showed significantly higher activities.
The temperature-programmed desorption of hydrogen (H2-TPD) was utilized for
studying the strength of H2/metal interactions. The positions of the
high-temperature peaks were arranged according to 2.8Pd/H,Na–Y (723 K) >
2.3Rh/H,Na–Y (713 K) > 4.7Ir/H,Na–Y (663 K). Comparison of these data
indicates that strong H2/metal interactions are accompanied by a preferred
formation of surface hydrogen atoms, which are the reason for the high
hydrogenation activity of Pd/H,Na–Y zeolites compared with Rh/H,Na–Y and
Ir/H,Na–Y zeolites.
For
propane dehydrogenation, the strong H2/Pd interactions in Pd/H,Na–Y zeolites
hinder the desorption of the reaction product H2, explaining the lower
dehydrogenation activity of these zeolites compared with Rh/H,Na–Y and
Ir/H,Na–Y zeolites. For the high catalytic activities of the Pt/H,Na–Y
zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt
clusters is discussed.
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