Electrochemical synthesis of urea at gas-diffusion electrodes V. Simultaneous reduction of carhon dioxide and nitrite ions with various boride catalysts

Abstract

Simultaneous reduction of carbon dioxide and nitrite ions was examined at gas-diffusion electrodes with various boride catalysts (TiBj, CrB, ZrBj, NbBî, MoB and WB). The formation of urea, CO, formic acid and ammonia at the gas-diffusion electrodes with ZrB^ NbB2 and MoB was found in the simultaneous reduction. The maximum current efficiency of urea formation on ZrB2 catalysts is approximately 33 % at -1.3 V. The formation of urea at the gas-diffusion electrodes with the catalysts of TiB^ CrB and WB was not found in the simultaneous reduction of CO2 and nitrite ions. Relationship of the ability for urea formation to the ability for CO and NH3 formation was investigated with various boride catalysts. The current efficiency of urea formation increases with increasing the current efficiency of CO and NH3 in the reduction of CO2 alone and nitrite ions alone, respectively. The ability for urea formation at the catalysts depends on the ability for CO and NH3 formation. Urea would be formed from both the ammonia-like precursor formed from nitrite ions and CO-like precursor formed from CO2 at the catalysts of ZrB2, NbB2 and MoB.