Theoretical study of CH4 photodissociation on the Pt(111) surface

Abstract

The photodissociation of CH4/Pt(111) is studied by density functional theory and the state-averaged complete active space self-consistent field (SA-CASSCF) method using a cluster model Ptn (n=1,4,6,7,10). With the small clusters (n⩽4), the equilibrium molecule–surface distances (H3CH–Pt) are less than 2.3 Å and the binding energies are 4–14 kcal/mol, the order of the chemisorption. With larger clusters, the molecule–surface distance and the binding energy are calculated to be 3.00 Å and 0.67 kcal/mol, respectively, of the order of the physisorption, which coincides with the experiments. The SA-CASSCF calculations verify that, in spite of the weak interaction between CH4 and Ptn in the ground state, the first excited state of CH4 (Rydberg type) interacts with Ptn unoccupied states strongly, resulting in the charge-transfer state and finally leading to the dissociation to CH3+H(−Pt); on the Pt(111) surface, the excitation energy to the Rydberg state of CH4 decreases by ∼3 eV compared to that in an isolated CH4 molecule. These results support the experimental results that the direct excitation of CH4 is invoked on the Pt(111) surface by irradiation of the 193 nm photon, leading to the dissociation to CH3 and H.