AVS 66th International Symposium & Exhibition
    Surface Science Division Thursday Sessions
       Session SS+AS+HC+TL-ThM

Paper SS+AS+HC+TL-ThM10
An Investigation on Active Sites of La2O3 Catalyst for OCM Reaction: A Combined Study of in situ XRD, XPS and Online MS

Thursday, October 24, 2019, 11:00 am, Room A220-221

Session: Surface Science of Energy Conversion and Storage
Presenter: Yong Yang, ShanghaiTech University, China
Authors: Y. Yang, ShanghaiTech University, China
C. Guan, ShanghaiTech University, China
E.I. Vovk, ShanghaiTech University, China
Z. Liu, ShanghaiTech University, China
X. Zhou, ShanghaiTech University, China
J.P.H. Liu, ShanghaiTech University, China
Y. Pang, ShanghaiTech University, China
Correspondent: Click to Email
Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly to valuable C2 products (ethane and ethylene). Previous results suggested that the bulk structure change of the La2O3 catalyst was related to the performance of the reaction. In this work, a designed in situ XRD-MS coupled characterization setup coupled with online MS instrument are used for measuring both the reaction products and the bulk structure of the catalyst in real time and under simulated industrial conditions. This allows for the more detailed study in order to relate information from of bulk structure change vs. CO2 related treatment and quantitative analysis of the reaction products, thus for a further connection and understanding of the conversion rate of CH4 and the selectivity of C2. The work presented focused on online characterization of the OCM reaction on La2O3 catalyst, covering different parameters including: 1. La2O3 pretreatment under different CO2 concentrations, 2. Consecutive OCM reactions, comparing the behavior of a clean surface La2O3 catalyst with a La2O3 catalyst after OCM, 3. OCM performed after La2O3 has undergone pretreatment with pure CO2. Results indicate that carbonates formation on La2O3 is two step, surface carbonates formation at below 500°C and bulk formation at 500-700°C. In situ TPD performed in a high pressure gas cell (HPGC) and XPS measurement results confirm the above.

The results showed that bulk CO32- formation under CO2 exposure, results in higher light-off temperature of CO2 and C2 than the clean surface during OCM reaction. There is carbonate formation on commercial La2O3 during OCM reaction and CO2 desorption after OCM reaction by in situ XRD-MS, and it influences the light-off temperature of CO2 and C2 up to 65°C higher than the clean surface. It is proposed that CO32- may perform as a catalyst poison in this reaction. This result provides an important insight of the active site for OCM reaction. Based on this result, a brief XPS study of the carbonate free sample surface, which may be only preppared from the HPGC vacuum connected further reveals an oxide feature related with methane activation. Additional DFT calculations based upon the experimental data indicates a carbonation mechanism which occurs in the subsurface, which in turn could be related to La2O3 activity.