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 Ln(2)O(3) (Ln=Sm and Nd) catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. With the doping of SrCl2 into Ln(2)O(3), the C2H4 selectivity and C2H6 conversion were enhanced considerably. We also found that the addition of SrCl2 to Ln(2)O(3) could markedly reduce the deep oxidation of C2H4. The 40 mol% SrCl2/Ln(2)O(3) catalysts were stable for 60 h of on-stream ODE reaction. Under the reaction conditions of temperature=640 degrees C and space velocity=6000 ml h(-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/Ln(2)O(3) 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 Ln(2)O(3) 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 Ln(2)O(3) phases. X-ray powder diffraction results revealed that the Ln(2)O(3) lattices were enlarged, whereas the SrCl2 lattices contracted in the 40 mol% SrCl2/Ln(2)O(3) catalysts. In situ Raman results indicated that then were dioxygen adspecies such as O-2(2-), O-2(n-) (1<n<2), O-2(-), and O-2(delta-) (0<delta<1) On the 40 mol% SrCl2/Ln(2)O(3) catalysts. Electron paramagnetic resonance (EPR) results indicated that there were dioxygen O-2(-) 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 O-2(2-), O-2(n-), O-2(-), and O-2(delta-) adspecies favor the selective oxidation of C2H6 to C2H4, whereas the O- adspecies is responsible for the deep oxidation of C2H6 (C) 2000 Elsevier Science B.V. All rights reserved.