The effect of cluster thickness on the adsorption of CH 4 on Pd n

Abstract

Adsorptions of methane on Pd n clusters were studied using PW91 functional with a plane wave basis set to understand the role of Pd n clusters in detecting trace amounts of CH 4 in air and the observations made in the temperature-programmed desorption experiment on the CH 4 physisorption on Pd clusters supported by Al 2O 3. Fifteen Pd n clusters consisting of up to 13 atoms were chosen to form various CH 4 · · · Pd n adducts. Dispersion correction to the adsorption energy was also calculated. Our results show that the interaction between CH 4 and Pd n cluster depends strongly on the adsorption site and CH 4 orientation. The most preferred adsorption occurs (1) along the C ∞ axis of the linear Pd n clusters, (2) perpendicular to the main C n axis of the planar Pd n clusters, and (3) at the atop site of the three-dimensional Pd n clusters. All four coordination modes of σ−bonds, η 1, η 2–C,H, η 2–H,H, and η 3, are found in these adducts. Additionally, the η 3–C,H,H, η 3-Pd,Pd,Pd, and η 1-Pd,Pd modes are identified. The η 1 σ−bond is found to be dominant in the CH 4 · · · Pd n adducts with Pd n clusters larger than trimer. Our results show that the CH 4 does not adsorb on top of the monolayer Pd clusters. Furthermore, the adsorption strength is more dependent on the thickness of the Pd clusters than the cluster size.