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

Abstract 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 HAl-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 Al 3+ 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 HAl-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 NH 3 reveals that H 2 adsorbed in Pd modified HAl-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 HAl-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.