Interaction between CO and NH3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia (¹⁵NH₃ or ND₃). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θ_CO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θ_CO = 0.5. The dissociation of ammonia in the β?(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the ?(2 × 2) structure. The HREEL spectra indicated that the C_3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β?(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH₃ interaction on metal surfaces is presented.