Comparison of the adsorption of N 2 on Ru(1 0 9) and Ru(0 0 1) – A detailed look at the role of atomic step and terrace sites

Abstract

We present a direct side-by-side comparison of the adsorption and desorption of nitrogen on the atomically-stepped Ru(1 0 9) surface and the atomically-flat Ru(0 0 1) surface. Both infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) are employed in this study, along with density functional theory (DFT). We find that the chemisorptive terminal binding of N 2 is stronger on the atomic step sites than on the terrace sites of Ru(1 0 9) as indicated by TPD and by a reduction of the singleton vibrational frequency, ν(N 2), by ∼9 cm −1, comparing steps to terraces. In addition, we find that metal–metal compression effects on the terrace sites of Ru(1 0 9) cause stronger binding of N 2 than found on the Ru(0 0 1) surface, as indicated by a reduction of the terrace-N 2 singleton vibrational frequency by ∼11 cm −1 when compared to the singleton N 2 mode on Ru(0 0 1). These spectroscopic results, comparing compressed terrace sites to Ru(0 0 1) sites and confirmed by TPD and DFT, indicate that N 2 bonds primarily as a σ-donor to Ru. Using equimolar 15N 2 and 14N 2, it is found that dynamic dipole coupling effects present at higher N 2 coverages may be partially eliminated by isotopically detuning neighbor oscillators. These experiments, considered together, indicate that the order of the bonding strength for terminal-N 2 on Ru is: atomic steps > atomic terraces > Ru(0 0 1). DFT calculations also show that 4-fold coordinated N 2 may be stabilized in several structures on the double-atom wide steps of Ru(1 0 9) and that this form of bonding produces substantial decreases in the N 2 vibrational frequency and increases in the binding energy, compared to terminally-bound N 2. These highly coordinated N 2 species are not observed by IRAS.