La0.5Ce0.5O1.75-catalytic layer for methane conversion into C2 products using solid oxide fuel cell

Abstract

Methane (CH4), the major constituent of natural gas and biogas, is an abundant source to obtain value-added hydrocarbons. The oxidative coupling of methane (OCM) is a direct catalytic route to convert CH4 towards C2 hydrocarbons, ethane (C2H6) and ethylene (C2H4). Using a solid oxide fuel cell (SOFC) is a strategy to overcome some challenges of fixed-bed catalytic reactors. In this context, we have studied the La0.5Ce0.5O1.75 (LCO) oxide as a catalytic layer in a SOFC for methane conversion to ethylene. Single phase LCO was synthesized by the combustion method. X-ray diffraction (XRD) showed that LCO solid solution has a disordered fluorite crystalline structure. Raman spectroscopy data evidenced the presence of surface oxygen vacancies, which may benefit the OCM reaction. SEM images evidenced a microstructure composed of porous agglomerated particles with an irregular shape, expected from the combustion synthesis. Impedance spectroscopy (IS) measurements of sintered LCO pellets were performed in a wide range of both temperature and oxygen partial pressure (pO2). Thermally activated behavior of the total electrical conductivity revealed that LCO is predominantly an ionic conductor at 10-6 < pO2 < 0.21 atm, whereas at pO2 ~ 10-21 atm, n-type electronic conductivity leads to a mixed ionic electronic conductor behavior, due to the Ce4+/3+ reduction. The catalytic properties of LCO and the optimized OCM reaction conditions were investigated a fixed-bed reactor. The desired products were obtained (C2H6 and C2H4), as well the parallel products (COx, and traces of C3H8 and C4H10). The higher rates of CH4 conversion (22 %), C2 selectivity (54 %) and yield (12 %) were reached for the reaction developed with 4CH4:1O2 at 750 ºC. Electrolyte-supported single cells were prepared using a YSZ disk (diameter ~ 19 mm, thickness ~ 0.4 mm) as the electrolyte. The cathode (LSM) and anode (NiO/YSZ) layers were both deposited by screen-printing on each side of the YSZ disk. For the catalytic layer, LCO was deposited, using the spray-coating technique, on the anodic side. The electrochemical properties of such fuel cell tests were characterized under 10 % H2 between 750 – 800 ºC, and then, the fuel was switched to 10 % CH4, both using Helium as a carrier gas. The anode outlet gas was monitored by an online gas analyzer setup that showed formation of C2H4, C2H6 and COx, indicating the effectiveness of the catalytic layer.