The effect of cluster thickness on the adsorption of CH4 on Pdn

Adsorptions of methane on Pdn clusters were studied using PW91 functional with a plane wave basis set to understand the role of Pdn clusters in detecting trace amounts of CH4 in air and the observations made in the temperature-programmed desorption experiment on the CH4 physisorption on Pd clusters supported by Al2O3. Fifteen Pdn clusters consisting of up to 13 atoms were chosen to form various CH4 · · · Pdn adducts. Dispersion correction to the adsorption energy was also calculated. Our results show that the interaction between CH4 and Pdn cluster depends strongly on the adsorption site and CH4 orientation. The most preferred adsorption occurs (1) along the C∞ axis of the linear Pdn clusters, (2) perpendicular to the main Cn axis of the planar Pdn clusters, and (3) at the atop site of the three-dimensional Pdn clusters. All four coordination modes of σ−bonds, η1, η2–C,H, η2–H,H, and η3, are found in these adducts. Additionally, the η3–C,H,H, η3-Pd,Pd,Pd, and η1-Pd,Pd modes are identified. The η1 σ−bond is found to be dominant in the CH4 · · · Pdn adducts with Pdn clusters larger than trimer. Our results show that the CH4 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.