Adsorption and dissociation of the HCl and Cl2 molecules on W(111) surface:: A computational study

The adsorption and dissociation of Cl-2 and HCl molecules on W(111) surface have been studied at the density functional theory (DFT) level in conjunction with the projector augmented wave (PAW) method. The molecular structures and surface-adsorbent interaction energies of W(111)/Cl, W(111)/H, W(111)/Cl-2, and W(111)/HCl systems are predicted. In these studies, four adsorption sites, such as top (T), bridge (B), shallow (S), and deep (D) sites, of the W(111) surface are considered. It is shown that the Cl-2 and HCl molecules adsorb to the W(111) surface by the end-on manner (by their Cl-Cl or H-Cl bonds perpendicular to the W surface), and their dissociative adsorptions occur without intrinsic energy barriers and are exothermic by 80.46 and 53.72 kcal/mol, for Cl-2 and HCl, respectively. Molecular dynamics studies show that the dissociation of Cl-2 and HCl molecules on the W(111) surface occur in asymmetric fashion: at the beginning adsorbate forms a strong bond between one of their atoms and W centers, followed by the dissociation of the Cl-Cl (and/or H-Cl) bond and formation of the second bond between the atoms of adsorbate and the W center. For the Cl-2 molecule, both Cl atoms are preferred to adsorb at the top W centers. For the HCl molecule, after the dissociation of the H-Cl bond the Cl atom still occupies the top adsorption site, but the H atom prefers to move to the position between the top and shallow W centers. The rate constants for the dissociative adsorption of Cl-2 and HCl have been predicted with variational RRKM theory.