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

The nanoscale Er2O3 and 30 mol% BaCl2/Er2O3 catalysts prepared by a modifie d sol-gel method have been compared with their large-size counterparts for the oxidative dehydrogenation of ethane (ODE) to ethylene. The nanoscale ca talysts showed better catalytic performance than the corresponding large-si ze ones, especially at lower reaction temperatures and shorter contact time s. X-ray diffraction results revealed that the extent of cubic Er2O3 lattic e 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 detect ed 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 resul ts of X-ray photoelectron spectroscopic investigation demonstrated that aft er 40 h surface chlorine composition on nanoscale 30 mol% BaCl2/Er2O3 incre ased slightly, while that on the large-size catalyst decreased. O-2 tempera ture-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 w as beneficial for the activation of oxygen molecules. Taking into considera tion the catalytic activity and Raman results, we suggest that dioxygen spe cies such as O-2(delta-) (0<delta<1), O-2(-), O-2(n-) (1<n<2), and O-2(2-) are responsible for the selective oxidation of C2H6 to C2H4. (C) 2000 Elsev ier Science B.V. All rights reserved.