Molecular adsorption of oxygen on transition-metal carbide

Room-temperature oxygen adsorption on NbC(111) and WC(<10(1)over bar 0>) was studied by high-resolution electron energy loss spectroscopy and ultraviolet photoelectron spectroscopy. On NbC(111), oxygen is adsorbed dissociatively at the first stage. Above 2 x 10(-4) Pa . s of exposure, a molecular species appears, which shows O-O stretching and M-O-O bending vibrations at 122 and 89 meV. These peaks disappear at 300 degrees C. When the NbC(111) surface is saturated by dissociative N atoms prior to the oxygen adsorption, the molecular adsorption occurs from the first stage. This suggests that the molecular adsorption is caused by a site-blocking mechanism. On the other hand, oxygen is adsorbed molecularly on WC(<10(1)over bar 0>) from the first. The UPS shows a broad peak at - 6.5 eV from the Fermi level, which is consistent with the 1 pi(u) and 3 sigma(g) orbitals of the molecular oxygen species.