Electrochemical synthesis of urea at gas-diffusion electrodes III. Simultaneous reduction of carbon dioxide and nitrite ions with various metal catalysts

Simultaneous reduction of carbon dioxide and nitrite ions was examined at the gas-diffusion electrodes with various catalysts (Cr, Mo, Mn, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, In, Tl, Sn, and Pb). The formation of urea, CO, formic acid, and ammonia at the gas-diffusion electrodes with the catalysts of groups 11-14 was found on the simultaneous reduction. The maximum current efficiency (c.e.) of urea formation at Cd catalysts is approximately 55% at -1.0 V. The formation of urea at the gas-diffusion electrodes with the catalysts of groups 6-10, except Pd, was not found for the simultaneous reduction, f CO2 and nitrite. Relationship of the ability of urea formation to the ability of CO and NH3 formation was investigated at various catalysts. The c.e. of urea increases with increasing the c.e. of CO and NH3 on the reduction of CO2 alone and nitrite ions alone, respectively. Hence, the ability of urea formation at the catalysts depends on the ability of CO and NH3 formation. The catalysts with the high ability of CO and NH3 formation could form large amounts of CO-like and ammonia-like precursors. Urea would be formed from both the ammonia-like precursor formed from nitrite ions, and the CO-like precursor formed from CO2 at the groups 11-14 catalysts.