Comparison of nanoscale and large-size BaCl₂-modified Er₂O₃ catalysts for the selective oxidation of ethane to ethylene

The nonoscale Er₂O₃ and 30 mol% BaCl₂/Er₂O₃ catalysts prepared by a modified sol-gel method have been compared with their large-size counterparts for the oxidative dehydrogenation of ethane (ODE) to ethylene. The nanoscale catalysts showed better catalytic performance than the corresponding large-size ones, especially at lower reaction temperatures and shorter contact times. X-ray diffraction results revealed that the extent of cubic Er₂O₃ lattice expansion in nanoscale BaCl₂/Er₂O₃ was larger than that in the large-size one and that more defects were formed in the nanoscale catalyst. We detected further enlargement of the cubic Er₂O₃ lattice in the nanoscale 30 mol% BaCl₂/Er₂O₃ catalyst during the first 40 h of on-stream reaction. The results of X-ray photoelectron spectroscopic investigation demonstrated that after 40 h surface chlorine composition on nanoscale 30 mol% BaCl₂/Er₂O₃ increased slightly, while that on the large-size catalyst decreased. O₂ temperature-programmed desorption studies illustrated that, compared to the large-size catalysts, there were more oxygen species desorbed from the nanoscale ones. In situ Raman studies indicated that the reduction in particle size was beneficial for the activation of oxygen molecules. Taking into consideration the catalytic activity and Raman results, we suggest that dioxygen species such as O₂^δ- (0<δ<1), O₂^-, O₂^n- (1<n<2), and O₂^2- are responsible for the selective oxidation of C₂H₆ to C₂H₄.