A comparison of mixed-conducting oxygen-permeable membranes for CO2 reforming

SrFeCo0.5Ox (SFC) and Ba0.5Sr0.5Co0.8Fe0.2Ox (BSCF) ceramic membranes have been evaluated for the CO2 reforming of CH4 at 800 °C over a Pt/ZrO2 and a Pt/CeZrO2 (17.5 wt% Ce) catalyst. The SCF membrane was a dense membrane approximately 2 mm thick while the BSCF membrane had a 400 μm dense layer supported on a 2.2 mm porous BSCF layer. Both membranes exhibited good mechanical integrity and stability under reaction conditions, and the membrane effect on methane conversion was roughly proportional to the apparent oxygen production of the membranes. Methane conversion and water production increase simultaneously in the presence of the membrane and the membranes improve catalyst longevity as well as activity. Results suggest that membrane oxygen reacts with hydrogen at the membrane surface to form water that can then participate in the reaction set via steam reforming. Among the combinations of membrane and catalyst tested, the Pt/CeZrO2 catalyst on the BSCF membrane produced the highest methane conversion and H2:CO ratios as well as the most water. This test also yielded the greatest net membrane oxygen production, with more than three times the oxygen production of the BSCF test with the Pt/ZrO2 catalyst. The SFC membranes exhibited steady state net oxygen production of zero or less. Negative membrane oxygen production values are attributed to the reduction of CO2 to CO on the membrane surface with incorporation of the oxygen into the membrane material.