Raman spectroscopic and EPR investigations of oxygen species on SrCl₂-promoted Ln₂O₃ (Ln=Sm and Nd) catalysts for ethane-selective oxidation to ethene

The SrCl₂-promoted Ln₂O₃ (Ln=Sm and Nd) catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. With the doping of SrCl₂ into Ln₂O₃, the C₂H₄ selectivity and C₂H₆ conversion were enhanced considerably. We also found that the addition of SrCl₂ to Ln₂O₃ could markedly reduce the deep oxidation of C₂H₄. The 40 mol% SrCl₂/Ln₂O₃ catalysts were stable for 60 h of on-stream ODE reaction. Under the reaction conditions of temperature=640°C and space velocity=6000mlh^-1 g^-1, 40 mol% SrCl₂/Sm₂O₃ showed 80.3% C₂H₆ conversion, 70.9% C₂H₄ selectivity, and 56.9% C₂H₄ yield while 40 mol% SrCl₂/Nd₂O₃ gave 63.8% C₂H₆ conversion, 74.3% C₂H₄ selectivity, and 47.4% C₂H₄ yield. X-ray photoelectron spectroscopic and chemical analysis of chloride indicated that the Cl^- anions were evenly distributed in the 40 mol% SrCl₂/Ln₂O₃ catalysts. We observed that Cl leaching was insignificant. The results of temperature-programmed desorption of oxygen and temperature-programmed reduction studies demonstrated that the addition of SrCl₂ to Ln₂O₃ enhanced the activation of oxygen molecules. We believe that such improvement is closely associated with the defects formed during the exchanges of ions between the SrCl₂ and Ln₂O₃ phases. X-ray powder diffraction results revealed that the Ln₂O₃ lattices were enlarged, whereas the SrCl₂ lattices contracted in the 40 mol% SrCl₂/Ln₂O₃ catalysts. In situ Raman results indicated that there were dioxygen adspecies such as O₂^2-, O_2n- (1<n<2), O₂^-, and O₂^δ- (0<δ<1) on the 40 mol% SrCl₂/Ln₂O₃ catalysts. Electron paramagnetic resonance (EPR) results indicated that there were dioxygen O₂^- and mono-oxygen O^- adspecies present on the SrCl₂-doped catalysts. Based on the results of in situ Raman and EPR studies as well as the catalytic activity data, we suggest that the O₂^2-, O₂^n-, O₂^-, and O₂^δ- adspecies favor the selective oxidation of C₂H₆ to C₂H₄, whereas the O^- adspecies is responsible for the deep oxidation of C₂H₆.