Periodic DFT Study of Benzene Adsorption on Pd(100) and Pd(110) at Medium and Saturation Coverage

Benzene adsorption on Pd(100) and Pd(110) has been investigated using periodic density functional theory (DFT) calculations. 4-Fold hollow geometries are preferentially adopted on both surfaces, and due to stronger repulsive interactions on Pd(100) a larger decrease in adsorption energy is calculated from medium to saturation coverage (∼120 kJ mol–1) compared to Pd(110) (∼15 kJ mol–1). On Pd(100), a slight energetic preference is calculated at saturation coverage for an adsorbate with two CC bonds parallel to the [011̅] direction. However, an adsorption geometry with alternately two types of benzene adsorbates, rotated azimuthally by 30° relative to one another, cannot be discarded since both geometries are compatible with ultraviolet photoemission spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS) observations. On Pd(110), there is a slight energetic preference for the hollow(0) site relative to the hollow(15) and hollow(30) at saturation coverage, and their calculated electronic features match UPS experiments. For the hollow(30), calculated vibrational features are not compatible with HREELS experiments, indicating that benzene does not populate hollow(30) sites at saturation coverage. Calculated STM images confirm that the experimentally observed two-lobed protrusion separated by a single depression oriented with its direction some 50° from [11̅0] can only correspond to the hollow(15) adsorbate. Inclusion of van der Waals interactions (vdW-DFT) increases adsorption energies by some 50 kJ mol–1, but the relative ordering of the various adsorption sites remains unaltered as compared to PW91.