A first-principles study on Pd modified ZSM-12 zeolites

摘要

The zeolites are usually modified by Pd before its application in the hydrogenation/dehydrogenation reactions. To understand the catalytic behavior of the Pd modified zeolites in catalytic reactions, the H-2 adsorption on Pd modified HA1-ZSM-12 zeolites were studied using dispersion corrected periodic density functional theory. It is interesting that the Pd could insert into the -OH groups to form the spillover structure of -O-Pd-H. The Bronsted acid H site of the zeolite reduced by Pd monomer to produce H-2 is thermodynamically favorable, while it is unfavorable by Pd dimmer. The modification of Pd weakens the Lewis acidity of Al3+ and Bronsted acidity of -OH. However, the Pd atoms could work as strong Lewis sites in the Pd modified zeolites. The modification of Pd also improves the H-2 activation/adsorption properties of HA1-ZSM-12. In addition, the acid properties of the Pd modified zeolites could be inevitably influenced by H-2 atmosphere, since the dissociative adsorbed H-2 could work as new Bronsted acid H and also influences the charge of Pd atoms. The calculated adsorption energy for NH3 reveals that H-2 adsorbed in Pd modified HAI-ZSM-12 results in weaker Lewis acidity of Pd sites. The new formed Bronsted acid sites after H-2 adsorption are weaker than the original HAI-ZSM-12. These results strongly demonstrates that the Pd modified ZSM-12 catalysts in real catalytic condition is much different from the prepared samples before it works in the catalytic reaction.