Raman spectroscopic and TPR studies of oxygen species over BaO- and BaX2 (X = F, Cl, Br)-promoted Nd2O3 catalysts for the oxidative coupling of methane

The 30 mol% BaO- and 30 mol% BaX2(X = F, Cl, Br)-promoted Nd2O3 catalysts have been investigated for the oxidative coupling of methane (OCM). The addition of BaO or BaX2 to Nd2O3 could obviously enhance the catalytic performance. We observed leaching of halide during the OCM reaction. The stability of the halide ions follows the order: F- > Cl- > Br-. XRD and Raman results revealed that the addition of BaO or BaX2 has significantly altered the surface and bulk natures of Nd2O3. The enlargement of Nd2O3 lattice in the BaO- and BaX2-promoted Nd2O3 catalysts was due to ionic exchange between Ba2+ and Nd3+ or X- and O2- ions. Raman results showed that there were dioxygen species such as O-2(2-), O-2(n-), O-2(-), and O-2(delta-) on the surface when BaO or BaX2 was added to Nd2O3. We suggest that the lattice defects generated due to ionic exchanges could promote the formation of dioxygen adspecies. TPR results implied that, besides the dioxygen species, there were mono-oxygen species existing in these catalysts. By pulsing CH4 over the catalysts, we found that both dioxygen and mono-oxygen species could activate CH4 at high temperatures and the existence of dioxygen species could improve the catalytic performance, especially in the enhancement of C-2 selectivity and C2H4/C2H6 ratio. (C) 1998 Academic Press.