A b i n i t i o theoretical study on the reactions of a hydrogen molecule with small platinum clusters: A model for chemisorption on a Pt surface

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Abstract
Reactions of a hydrogen molecule with small platinum clusters Ptn (n=1,2,3) are studied theoretically by ab initio methods. This provides a cluster model study for hydrogen chemisorption on a Pt surface. The results suggest that the Pt atom and the linear Pt3 cluster will react with H2 and dissociatively adsorb it, making two Pt–H bonds, whereas the Pt2 cluster will not react with H2 because of a high energy barrier. The dissociative adsorption of a hydrogen molecule occurs at a side-on, on-top site of a surface Pt atom and molecular adsorption does not seem to occur. Essentially only one Pt atom is involved in the initial adsorption process. These findings are in contrast to the palladium case previously reported. Almost no energy barrier exists for the hydrogen migration from one Pt atom to an adjacent one, with a preference being shown for one H atom on each Pt atom rather than two H atoms on one Pt atom. The heat of adsorption, the stability of the catalytic surface, etc., are best represented by the Pt3–H2 system. Spin–orbit coupling effect is examined for the Pt–H2 system and found to be small for the singlet A1 state which is the most important state for the dissociative adsorption. This effect is important, however, to obtain natural potential curve in the large Pt–H2 separation.