Electrochemical Synthesis Of Urea By Simultaneous Reduction Of NO₃～- And CO₂

The burning of an enormous volume of fossil fuels gives rise to aconstant evolution of carbon dioxide （CO₂） and nitrogen oxides （NO_x）,which causes serious problems for the global environment and the humanbody. It is interesting that CO₂ and NO_x are simultaneously removed,because they are simultaneously formed at most factories, e. g. steelworks. In the present work, simultaneous reduction of CO₂ and nitrateions （NO₃^-） was investigated in 0.2 mol/L KHCO₃ aqueous solutions atCu/Zn alloy electrode under different temperature and pressure. Theeffects of temperature and pressure on the yield of reduction productswere evaluated and the mechanism for the formation of urea waselucidated.The formation of urea, CO and ammonia （NH₃） at Cu/Zn alloyelectrode were found in the simultaneous reduction. The current-potentialcurves shifted towards positive potentials at Cu/Zn alloy electrode with the increase of temperature and potential. The current efficiency of NH₃formation and urea formation increased with increasing temperaturewhile the current efficiency of CO formation decreased with increasingtemperature. The current efficiency of NH₃ formation under ambientpressure was much higher than that under higher CO₂ pressure.Nevertheless, the current efficiency of CO formation under ambientpressure was much lower than that under higher CO₂ pressure. Anincrease of applied pressure led to higher current efficiency of ureaformation. Both the linear relationship between overpotential and thelogarithm of partial current density for NH₃ and the linear relationshipbetween overpotential and the logarithm of partial current density for COwere in accord with Tafel plot.The relationship of urea formation to the ability for CO and NH₃formation was examined under different temperature and differentpotential. The current efficiency of urea increased with increasing currentefficiency of CO and NH₃ in the reduction of CO₂ alone and nitrite ionsalone, respectively. Hence, the ability for urea formation at variousconditions depended on the ability for CO and NH₃ formation. With thehigh ability for CO and NH₃ formation, large amounts of CO-like andammonia-like precursors could be formed. Urea was supposed to beformed from the ammonia-like precursor formed from nitrite ions and theCO-like precursor formed from CO₂. The present work will promote the technique on the removal of CO₂and NO_x in the field of electrochemistry and electrochemical synthesis ofurea by simultaneous reduction of NO_x and CO₂ is accessible.