Orientation-Dependent Interaction between CO2 Molecules Adsorbed on Ru(0001).

Determining the molecular structure of CO2 adsorbed on metal surfaces and its mutual interactions is important to understand its catalytic conversion reactions. Here, we study CO2 adsorption on Ru(0001) using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Adsorbed at 77 K, the CO2 molecules form mainly disordered structures at submonolayer coverage, except for small (2 × 2) domains. The adsorbed molecules are no longer linear as in the gas phase, but instead, they adopt a "V"-shape geometry with the carbon atom occupying three-fold hcp hollow sites and possess three symmetry-equivalent orientations. Annealing to 250 K causes partial desorption of the molecules, while the remaining molecules form trimers of three different configurations with different interaction energies determined by their relative orientations. The "strong"-interacting trimer shows a cyclic structure, about 40 meV more stable than the "weak"-interacting trimer that is composed of three parallel molecules.