Comparison of the adsorption of N2 on Ru(109) and Ru(001) -: A detailed look at the role of atomic step and terrace sites

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(00 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(I 09) as indicated by TPD and by a reduction of the singleton vibrational frequency, v(N-2), by similar to 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(001) surface, as indicated by a reduction of the terrace-N-2 singleton vibrational frequency by similar to 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 sigma-donor to Ru. Usina equimolar N-15(2) and N-14(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(00 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(I 09) 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. (c) 2007 Elsevier B.V. All rights reserved.