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 O^2- ions. Raman results showed that there were dioxygen species such as O₂^2-, O₂^2-, O₂^-, and O₂^δ- 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 C₂ selectivity and C₂H₄/C₂H₆ ratio.