Electrocatalyst Design for Carbon Dioxide Co-Reduction with Nitrate for Sustainable Synthesis of Urea

The electrochemical co-reduction of nitrate (NO3 -) and carbon dioxide (CO2) for the synthesis of urea has drawn much research focus recently because it creates value-added products from environmentally hazardous pollutants. Urea is the major ingredient of fertilizer, that is currently produced through the Bosch-Meiser process starting from ammonia (NH3) and carbon dioxide (CO2), in which ammonia is produced through the Haber-Bosch process, leading to significantly high energy consumption and CO2 emissions. Urea electrosynthesis is a complex 16e-, 12H+ transfer reaction with multiple possible by-products such as H2, CO, HCOOH, NO2 -, NH3, etc., making the electrocatalyst design toward urea selectivity a big challenge. To reliably quantify the urea produced, we present an updated method for urea quantification using the diacetylmonoxime-thiosemicarbazide (DAMO-TSC) method while excluding the interference of NO2 - by-product. We believe this method can guide the electrocatalysis community to establish an updated protocol and avoid false positive/overestimating results. We will then present a catalyst designed for urea electrosynthesis consisting of Pd nanoparticles in nanoscale proximity to atomically dispersed Fe-Nx sites on a carbon matrix (FeNC). Our group has previously presented Pd nanoparticles and FeNC materials as efficient catalysts for CO2 and NO3 - reduction respectively. Their association at the nanoscale demonstrates over 20% urea faradaic efficiency at -0.5 V (vs RHE) with 100% N-selectivity (All by-products contain nitrogen). These very promising results provide us with the baseline to elucidate the role of individual active sites on the C-N coupling mechanism. To this effect, reaction intermediates of CO2 and NO3 - co-reduction are investigated by in-situ vibrational spectroscopy (Raman, infrared). This study validates the approach of tuning electrocatalysts through mechanistic studies realizing CO2 / NO3 - co-reduction. Reference: Murphy, E., Liu, Y., Matanovic, I., Rüscher, M., Huang, Y., Ly, A., Guo, S., Zang, W., Yan, X., Martini, A., Timoshenko, J., Cuenya, B. R., Zenyuk, I. V., Pan, X., Spoerke, E. D., & Atanassov, P. Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites. Nat Commun 14, 4554 (2023). Guo, S., Liu, Y., Murphy, E., Ly, A., Xu, M., Matanović, I., Pan, X., & Atanassov, P. Robust palladium hydride catalyst for electrocatalytic formate formation with high CO tolerance. Applied Catalysis. B, Environmental, 316, 121659. (2022). Huang, Y., Wang, Y., Liu, Y., Ma, A., Gui, J., Zhang, C., Yu, Y., & Zhang, B. Unveiling the quantification minefield in electrocatalytic urea synthesis. Chemical Engineering Journal, 453, 139836. (2023).