Isolation of Oxygen Formed during Catalytic Reduction of Carbon Dioxide Using a Solid Electrolyte Membrane

Abstract

We studied the separative recovery of oxygen formed during the hydrogenation of carbon dioxide using a composite system consisting of yttria-stabilized zirconia (YSZ) membrane-equipped Ag electrodes as a pump and a nickel/zeolite catalyst to initiate the hydrogenation of carbon dioxide. The methanation of carbon dioxide proceeded efficiently in the presence of the nickel/zeolite catalyst even at 873 K, the lowest functional temperature at which YSZ has ionic conductivity. Carbon dioxide conversion and methane yield reached 100% and 80%, respectively, at H2/CO2 = 10, space velocity < 6200 h-1, and 873 K. Carbon dioxide was adsorbed by the crystal structure of the zeolite even at 873 K, and methanation of the adsorbed carbon dioxide (CO2 ad) proceeded by the following one-step reaction: CO2 ad + 3H2 → CH4 + H2O. The rate constant of methanation was estimated to be 1.6 × 10-2 cm3 g-cat-1 s-1 for the pseudo-first-order reaction. Electrochemical isolation of oxygen formed during hydrogenation of carbon dioxide was carried out under galvanostatic conditions. It was assumed that the oxygen was converted to water by the nickel/zeolite catalyst and then transported through the YSZ electrolyte after the water was ionized. We assumed that the oxygen was not formed by the dissociation of carbon dioxide directly on the cathode electrode. It appeared that the charge-transfer process was the rate-determining step for isolation of oxygen on the cathode electrode in the presence of the nickel/zeolite catalyst.