Ammonia synthesis from nitrogen and water at intermediate temperatures and elevated pressures by using an electrochemical hydrogen-membrane reactor with supported Ru catalysts and phosphate electrolytes.

Electrochemical synthesis of NH3 from N-2 and H2O with electrical power is a promising technology to convert redundant electricity to a chemical fuel. NH3 synthesis from N-2 and H2O using a combination of Cs-promoted Ru catalysts, a Pd-based hydrogen-membrane cathode, a CsH2PO4/SiP2O7 electrolyte, and a Pt anode was investigated in the temperature range of 200-250 degrees C and pressure range of 0.10.7 MPa. A maximum NH3 formation rate of 12.4 nmol s(-1) cm(-2) (759 mg h(-1) cm(-2)) was obtained at 250 degrees C and 0.7 MPa using 5 wt%-Ru/Cs+/MgO (Cs/Ru = 1 mol) for a current density of 30 mA cm(-2). The corresponding current efficiency for NH3 formation was estimated to be 12%, and the remaining part of the current was confirmed to be used for H-2 production. The NH3 formation rate increased upon increasing the total pressure, following thermodynamic equilibrium. 5 wt%-Ru/Cs+/CeO2 (Cs/Ru = 1 atom) was found to yield the highest formation rate of NH3 at 200-220(degrees)C and 0.1 MPa, while Ru/Cs+/MgO yielded a higher NH3 formation rate than Ru/Cs+/CeO2 under elevated pressure conditions. Because the dissociation of linearly adsorbed molecular N-2 on Ru is known to be the rate-determining step in NH3 synthesis, infrared spectroscopy was utilized to examine the linearly adsorbed N-2 on the Ru sites at the adsorption equilibrium at 30 degrees C.