Continuous-flow electrosynthesis of urea and oxalic acid by CO_(2)-nitrate reduction and glycerol oxidation

电催化CO_(2)和硝酸盐还原耦合甘油氧化生成尿素和草酸

Urea and oxalic acid are critical component in various chemical manufacturing industries.However,achieving simultaneous generation of urea and oxalic acid in a continuous-flow electrolyzer is a challenge.Herein,we report a continuous-flow electrolyzer equipped with 9-square centime-ter-effective area gas diffusion electrodes(GDE)which can simultaneously catalyze the glycerol oxidation reaction in the anode region and the reduction reaction of CO_(2)and nitrate in the cathode region,producing oxalic acid and urea at both the anode and cathode,respectively.The current density at low cell voltage(0.9 V)remained above 18.7 mA cm^(-2)for 10 consecutive electrolysis cycles(120 h in total),and the Faraday efficiency of oxalic acid(67.1%)and urea(70.9%)did not decay.Experimental and theoretical studies show that in terms of the formation of C-N bond at the cathode,Pd-sites can provide protons for the hydrogenation process of CO_(2)and NO3-,Cu-sites can promote the generation of*COOH and Bi-sites can stabilize*COOH.In addition,in terms of glycerol oxidation,the introduction of Cu and Bi into Pd metallene promotes the oxidation of hydroxyl groups and the cleavage of C-C bond in glycerol molecules,respectively.

随着全球环境问题日益突出,寻找可持续的能源和化工生产方式成为迫切需求.CO_(2)作为一种主要的温室气体,其再利用和减排成为全球关注的焦点.同时,硝酸盐是一种常见的水污染物,探索有效的处理方式对环境保护至关重要.利用电催化技术,一方面可以将CO_(2)和硝酸根还原生成尿素,实现对CO_(2)和硝酸盐的资源化利用;另一方面,在另一反应中,甘油可以被氧化生成草酸.这两个过程都能直接生产重要的化工产品,具有重要的经济价值和环保意义.研究电催化CO_(2)和硝酸根还原生成尿素的技术,为可持续能源和化工生产开辟了新的途径.然而,在连续流动的电解槽中,如何高效地实现CO_(2)和硝酸根还原生成尿素,同时优化甘油氧化生成草酸的反应条件,是一个需要克服的挑战.本文报道了一种简单的水热法一步合成了三金属烯Pd70Cu12Bi18.高分辨透射电子显微镜和X射线粉末衍射测试结果证明了Pd,Cu和Bi原子均匀地分布在金属烯中.X射线光电子能谱和同步辐射分析结果表明,Pd原子周围电子密度增加,Bi原子周围电子密度降低.值得注意的是,具有特殊电子结构的Pd70Cu12Bi18在甘油氧化反应以及CO_(2)和硝酸根共还原反应中展示出较好的性能.将Pd70Cu12Bi18应用在配备了9 cm2有效面积的气体扩散电极的连续流动电解槽中,实现了同时催化阳极区的甘油氧化生成草酸以及阴极区的CO_(2)和硝酸盐共还原生成尿素.结果表明,该电解槽可以在低电池电压(0.9 V)下获得18.7 mA cm^(-2)的电流密度,草酸和尿素在连续流动的电解槽中的法拉第效率分别为67.1%和70.9%.此外,稳定性测试结果表明该电解槽经过120小时反应后,电解槽的电流密度和产物的法拉第效率没有发生衰减.同位素标记实验结果表明,生成的尿素来源于CO_(2)和电解液中的硝酸根.原位红外测试和理论计算研究表明,*COOH和*NH_(2)是C?N键形成的关键中间体.此外,对于阴极C?N键的形成,Pd位点可以为CO_(2)和NO3?的加氢过程提供质子,Cu位点促进了*COOH的产生,Bi位点可以稳定*COOH.此外,对于甘油氧化,在Pd金属烯中引入Cu和Bi分别促进了甘油中羟基的氧化和C?C键的断裂.综上所述,本文发展了一种双功能金属烯电催化剂,并实现了在连续流动的电解槽中同时生产尿素和草酸的目标,为推动绿色环保的电催化技术在化学工业中的应用提供新思路和参考.