Understanding the Site‐Selective Electrocatalytic Co‐Reduction Mechanism for Green Urea Synthesis Using Copper Phthalocyanine Nanotubes

Abstract

Green synthesis of urea under ambient conditions by electrochemical co‐reduction of N2 and CO2 gases using effective electrocatalyst essentially pushes the conventional two steps (N2 + H2 = NH3 and NH3 + CO2 = CO(NH2)2) industrial process at high temperature and high pressure, to the brink. The single step electrochemical green urea synthesis process has hit a roadblock due to the lack of efficient and economically viable electrocatalyst with multiple active sites for dual reduction of N2 and CO2 gas molecules to urea. Herein, copper phthalocyanine nanotubes (CuPc NTs) having multiple active sites (such as metal center, Pyrrolic‐N3, Pyrrolic‐N2, and Pyridinic‐N1) as an efficient electrocatalyst which exhibits urea yield of 143.47 µg h–1 mg–1cat and faradaic efficiency of 12.99% at –0.6 V versus reversible hydrogen electrode by co‐reduction of N2 and CO2 are reported. Theoretical calculation suggests that Pyridinic‐N1 and Cu centers are responsible to form CN bonds for urea by co‐reduction of N2 to NN* and CO2 to *CO, respectively. This study provides the new mechanistic insight about the successful electro‐reduction of dual gases (N2 and CO2) in a single molecule as well as rational design of efficient noble metal‐free electrocatalyst for the synthesis of green urea.

Advanced insights towards electrochemical urea synthesis: Strategic design and techno–commercial compatibility

“…The subsequent hydrogenation to the *NCON* intermediate leads to two probable reaction pathways which are namely distal and alternating. 26,29 In distal mechanistic route, the (H + + e -) reacts with the distal N atom whereas in alternating mechanistic route the (H + + e -) alternatively attacks the N atom on both sides (figure 6 20 (d, e) alternating and distal mechanism. 26 (Reproduced with permission) (b) Carbon dioxide gas and Nitric oxide gas: As nitrogen gas has very low solubility in aqueous phase, so to address the issue Huang et al 40 proposed direct electrolytic urea synthesis by using carbon dioxide gas and nitric oxide gas, as nitric oxide has higher solubility in water.…”

“…(v) H-type cell: The most common and conventional electrochemical cell 19,20,25,29 employed for electrocatalytic urea synthesis, the two chambers are separated by a selectively permeable membrane (Nafion membrane generally) which allows only proton exchange (Figure 8a). The two electrodes are placed on two chambers separately; concerned gas or gases are purged in the chamber of the working electrode, the catalyst mounted on the working electrode which adsorbs the reactant molecules and the potential is applied to perform the electrochemical reaction.…”

“…17 As United Nations Emissions Gap Report from 2020 reveals that the average greenhouse gas emission have increased by 2.6% in 2019 and COVID-19 pandemic have made a small yet significant impact by reducing the CO2 emission by 7% and the effect will not outlast from 2030 if the entire World does not financially integrate the decarbonization strategy into the existing system. 18 Therefore significant research have been carried out to reduce the carbon footprint by developing energy efficient, economical and environment friendly way to produce urea using electrochemical process, 9,10,19,20 which generally involves simultaneous reduction process of the carbon dioxide (CO2RR) and nitrogen sources (NRR). 3,4,[21][22][23] However, there exists a number of hurdles in the progress route of electrochemical urea production which needs to be resolved, these are as follows, (a) The adsorption of the reactants (N2 and CO2) to the surface of the electrocatalyst is of very feeble nature, 20,24 (b) Simultaneous reduction process (N2 and CO2) often competes against one another which results in various sorts of product formation apart from urea, 25 (c) higher overpotential required to cleave the extremely stable N≡N and C=O bond, 26 (d) effectively supressing the competing HER.…”

“…18 Therefore significant research have been carried out to reduce the carbon footprint by developing energy efficient, economical and environment friendly way to produce urea using electrochemical process, 9,10,19,20 which generally involves simultaneous reduction process of the carbon dioxide (CO2RR) and nitrogen sources (NRR). 3,4,[21][22][23] However, there exists a number of hurdles in the progress route of electrochemical urea production which needs to be resolved, these are as follows, (a) The adsorption of the reactants (N2 and CO2) to the surface of the electrocatalyst is of very feeble nature, 20,24 (b) Simultaneous reduction process (N2 and CO2) often competes against one another which results in various sorts of product formation apart from urea, 25 (c) higher overpotential required to cleave the extremely stable N≡N and C=O bond, 26 (d) effectively supressing the competing HER. 19,20 In this article we concisely present the inception and the continuous steady growth of electrochemical green urea synthesis.…”

Descriptors and graphical construction forin silicodesign of efficient and selective single atom catalysts for the eNRR

“…Further, we computed the N-N bond length after adsorption to analyse the activation of N2 by each SAC83 (see TableS3in the SI). It is seen that the weak activation of N2 (N-N bond length close to 1.11 Å) obtained mostly on Ni-, Cu-and Zn-based SACs leads to a higher eNRR overpotential (> 1V)17,84 . Also, we observed that the mainly TM-N3 and Mo-center based SACs shows a large expansion in N-N bond length.…”