Vibrations, coverage, and lateral order of atomic nitrogen and formation of NH3 on Ru(101̄0)

Abstract

The dissociative chemisorption of nitrogen on the Ru(101̄0) surface has been studied using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). To prepare a surface covered by atomic nitrogen we have used ionization-gauge assisted adsorption. A saturation coverage of ${\theta }_N\text{=0.6}$ is achieved of which about 30% is in the subsurface region. At saturation coverage a ${(}_{\text{\hspace{0.5em}2\hspace{0.5em}1}}^{\text{−1\hspace{0.17em}1}})$ LEED pattern is observed. The ${\nu }_{\parallel }(\mathrm{Ru–N})$ mode at 41 meV and the ${\nu }_{\perp }(\mathrm{Ru–N})$ mode at 60 meV are identified. Upon exposing the nitrogen covered surface to hydrogen at 300 K we have observed the formation of ${\mathrm{NH}}_{\mathrm{3}}$ which is characterized by its symmetric bending mode ${\delta }_s({\mathrm{NH}}_{\mathrm{3}})$ at 149 meV. At 400 K, ${\mathrm{NH}}_{\mathrm{3}}$ could not be detected. The reaction intermediate NH is stable up to 450 K and has been identified by its vibrational losses ν(Ru–NH) at 86 meV, and ν(N–H) at 408 meV. The TD spectra of mass 14 show three desorption states of nitrogen, ${\mathrm{N}}_{\alpha }$ at 740 K (from subsurface N), ${\mathrm{N}}_{\beta }$ shifting from 690 to 640 K with increasing coverage, and ${\mathrm{N}}_{\gamma }$ at 550 K. The activation energy for desorption via the ${\mathrm{N}}_{\beta }$ state is $\text{120±10\hspace{0.17em}}\mathrm{kJ/mol}.$ The TD spectra of mass two showed three desorption states at 450, 550, and 650 K due to the decomposition of ${\mathrm{NH}}_x.$