Adsorption of small gas molecules on a single Pt atom supported by pristine graphene: diffusion Monte Carlo study

We have used diffusion Monte Carlo (DMC) method to study adsorption of small gas molecules on a single Pt atom supported by pristine graphene. Firstly, we obtain Pt-graphene binding energy curves as a function of the vertical distance between Pt and graphene for three different adsorption sites (bridge, on-top, hollow), from which the equilibrium binding energies and distances are determined. Our DMC results are compared with DFT results based on several different density functionals, which reveals that PBE and rVV10 results show good agreement with our DMC results for both equilibrium energies and distances. For molecular adsorption on the Pt-graphene complex, we observe significant overestimation in DFT binding energies compared to the DMC ones. In the case of O₂ adsorption, the spin degrees of freedom are considered along with geometries. While DFT predicts the lowest-energy structure of a spin-triplet side-on configuration where the molecular axis of O₂ is parallel to the graphene sheet, DMC finds that the spin-singlet side-on configuration with the molecular axis being slightly tilted toward graphene is the lowest-energy state. The DMC calculations are currently in progress to examine diffusion path of O₂ and catalytic performance of Pt-graphene complexes.