(Invited) Applications of Solid Electrolyte Membranes in Heterogeneous Catalysis. Electrochemical Reaction and Separation

In addition to their applications in sensors, separators and solid oxide fuel cells, solid electrolytes have been used in the construction of solid state cell-reactors, in which catalytic reactions were studied. Depending on the reaction system, there was a variety of goals in these studies: a) to take advantage of the selective conduction of ions, b) improve the yield to the desired product and c) cogenerate electricity and value-added chemicals. Two examples of applications of solid state cell-reactors in heterogeneous catalysis are presented here, a) the production of ethylene and hydrogen from methane with simultaneous separation of hydrogen from the reaction mixture and b) the electrochemical synthesis of NH₃.     

      The production of hydrogen with simultaneous conversion of methane to C₂ hydrocarbons was studied in a solid state oxygen ion (O^2-) conducting cell at temperatures between 700 ^oC and 850 ^oC. Methane, diluted in nitrogen was introduced over the anode (Ag) and steam, also diluted in nitrogen, was introduced over the cathode (Pt). When oxygen was electrochemically “pumped” from the cathode to the anode, steam was electrolyzed to produce gaseous H₂ while CH₄ was converted to C₂H₆, C₂H₄ and CO₂. At the anode, C₂yields exceeding 8% were obtained while an up to 65% conversion of steam was achieved at the cathode.

      The electrochemical synthesis of ammonia was studied in a proton conducting solid electrolyte cell. The BaZr_0.7Ce_0.2Y_0.1O_2.9 (BZCY72) proton conducting ceramic was used as the electrolyte with a Ni-BZCY72 cermet and a Pt film serving as cathodic and anodic electrodes, respectively. The reaction was studied at atmospheric pressure and at temperatures between 450 ^oC and 700 ^oC, under both, open- and closed-circuit conditions. A peculiar reaction rate enhancement was observed when the cell returned to open-circuit after operating under closed-circuit for a certain time. A possible explanation of this new phenomenon is that a fraction of protons electrochemically transported to the cathode, is "stored" in the Ni-BZCY72 electrode in the form of a highly reactive hydride which, upon current interruption, reacts with adsorbed N species to produce ammonia.