Coupling N2 and CO2 in H2O to synthesize urea under ambient conditions

TLDR: An electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets has been shown to directly couple N2 and CO2 in H2O to produce urea under ambient conditions.

Abstract: The use of nitrogen fertilizers has been estimated to have supported 27% of the world’s population over the past century. Urea (CO(NH2)2) is conventionally synthesized through two consecutive industrial processes, N2 + H2 → NH3 followed by NH3 + CO2 → urea. Both reactions operate under harsh conditions and consume more than 2% of the world’s energy. Urea synthesis consumes approximately 80% of the NH3 produced globally. Here we directly coupled N2 and CO2 in H2O to produce urea under ambient conditions. The process was carried out using an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets. This coupling reaction occurs through the formation of C–N bonds via the thermodynamically spontaneous reaction between *N=N* and CO. Products were identified and quantified using isotope labelling and the mechanism investigated using isotope-labelled operando synchrotron-radiation Fourier transform infrared spectroscopy. A high rate of urea formation of 3.36 mmol g–1 h–1 and corresponding Faradic efficiency of 8.92% were measured at –0.4 V versus reversible hydrogen electrode. Conventionally, urea is synthesized via two consecutive processes, N2 + H2 → NH3 followed by NH3 + CO2. Now, an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets has been shown to directly couple N2 and CO2 in H2O to produce urea under ambient conditions.

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1
Coupling N
2
and CO
2
in H
2
O to synthesis urea under ambient
conditions
Chen Chen
1†
, Xiaorong Zhu
2†
, Xiaojian Wen
3†
, Yangyang Zhou
1†
, Ling Zhou
1
, Hao
Li
1
, Li Tao
1
, Qiling Li
1
, Shiqian Du
1
, Tingting Liu
1
, Dafeng Yan
1
, Chao Xie
1
, Yuqin
Zou
1
, Yanyong Wang
1
, Ru Chen
1
, Jia Huo
1
, Yafei Li
2
*, Jun Cheng
3
*, Hui Su
4
, Xu
Zhao
4
, Weiren Cheng
4
, Qinghua Liu
4
*, Hongzhen Lin
5
, Jun Luo
6
, Jun Chen
7
*,
Mingdong Dong
8
, Kai Cheng
9
, Conggang Li
9
and Shuangyin Wang
1
*
1
State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082,
P. R. China
2
College of Chemistry and Materials Science Nanjing Normal University Nanjing,
Jiangsu 210023, P. R. China
3
State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R.
China
4
National Synchrotron Radiation Laboratory, University of Science and Technology
of China, Hefei, Anhui, 230029, P. R. China
5
i-LAB, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of
Sciences, Suzhou 215123, P. R. China
6
Tianjin Key Laboratory of Advanced Functional Porous Materials and Center for
Electron Microscopy, School of Materials Science and Engineering, Tianjin
University of Technology, Tianjin 300384, P. R. China
7
Intelligent Polymer Research Institute, Australian Institute of Innovative Materials,
Innovation Campus, University of Wollongong, Northfields Avenue, Wollongong,
NSW 2500, Australia
8
Interdisciplinary Nanoscience Center, Aarhus University, Denmark
9
Key Laboratory of Magnetic Resonance in Biological Systems, State Key Laboratory
of Magnetic Resonance and Atomic and Molecular Physics, National Center for
Magnetic Resonance in Wuhan Collaborative Innovation Center of Chemistry for Life
Sciences, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences,
Wuhan 430071, P. R. China

2
*Correspondence to: shuangyinwang@hnu.edu.cn; liyafei@njnu.edu.cn;
chengjun@xmu.edu.cn; qhliu@ustc.edu.cn; junc@uow.edu.au.
†
These authors contributed equally to this work.
Abstract
The use of nitrogen fertilizers has been estimated to have supported 27% of the
world’s population over the past century. Urea (CO(NH
2
)
2
) is conventionally
synthesized through two consecutive industrial processes, N
2
+ H
2
→ NH
3
followed
by NH
3
+ CO
2
→ urea. Both reactions operate under harsh conditions and consume
more than 2% of the world’s energy. Urea synthesis consumes approximately 80% of
the NH
3
produced globally. Here we directly coupled N
2
and CO
2
in H
2
O to produce
urea under ambient conditions. The process was carried out using an electrocatalyst
consisting of PdCu alloy nanoparticles on TiO
2
nanosheets. This coupling reaction
occurs through the formation of C–N bonds via the thermodynamically spontaneous
reaction between *N=N* and CO. Products were identified and quantified using
isotope labelling and the mechanism investigated using isotope-labelled operando
synchrotron radiation Fourier transform infrared spectroscopy. A high urea formation
rate of 3.36 mmol g
-1
h
-1
and corresponding Faradic efficiency of 8.92% were
measured at -0.4 V versus reversible hydrogen electrode.
Over the past century, nitrogen fertilization has supported approximately 27% of the
world’s population
1
. As urea is one of the most important nitrogen fertilizers with a
high nitrogen content, the development of the urea industry is of great significance to
meet the demands of an ever-increasing population. Currently, the synthesis of urea is
dominated by the reaction of NH
3
and CO
2
under harsh conditions (150-200 °C,
150-250 bar) with large energy consumption. Moreover, complex equipment and
multi-cycle synthetic processes are required to enhance the conversion efficiency
2,3
.
The production of urea consumes approximately 80% of the global NH
3
4
, which is
mainly derived from artificial N
2
fixation.
The fixation of earth-abundant N
2
is challenging both scientifically and
technologically due to the high inertness of this molecule
5
. The industrial fixation of
N
2
and H
2
to obtain ammonia is always dominated by the Haber-Bosch process,
operating at a high temperature and high pressure due to the high bonding energy of
the N–N triple bond (940.95 kJ mol
-1
) and consuming approximately 2% of world’s
energy annually
5-15
. Thus, there have been extensive research activities to reduce the
activation energy of the N
2
-to-NH
3
reaction under milder conditions
16-19
.

3
Electrocatalytic N
2
fixation under ambient conditions combines the advantages of
the utilization of clean energy and protons directly from water
20-23
.
However, current
research mainly focuses on sole N
2
electrochemical fixation to generate NH
3
, and the
post treatment of primary products—further applications have seldom been
considered
2,3
. Actually, the separation of NH
3
in an aqueous electrolyte and its further
purification to obtain gaseous NH
3
with high purity would make the subsequent urea
synthesis complicated and unpractical.
For CO
2
fixation, carbon capture and sequestration (CCS) is mainly attributed to
the formation of strong covalent bonds (C–N)
24-27
, but the high energy consumption,
high cost and leakage risk of the captured CO
2
prohibit further application of this
method
28
. Compared with the direct electrolysis of CO
2
, the electrolysis of CO tends
to create multi-carbon products via more efficient C–C coupling
29-31
. On this basis,
Jiao and co-workers creatively introduced ammonia as a source of nitrogen to realize
C–N coupling and the production of acetamides with a high rate and selectivity (40%)
under ambient conditions
32
. In consideration of the hurdles in N
2
and CO
2
fixation and
the increasing interest in photo- or electro-driven C–N bond formation
32-34
, we
hypothesize that the simultaneous electrocatalytic coupling of N
2
and CO
2
would
enable the formation of C–N bonds and thus realize the conversion to urea under
ambient conditions.
Herein we demonstrate an approach for the electrochemical coupling of N
2
and
CO
2
in H
2
O to form urea with a carefully designed electrocatalyst consisting of PdCu
alloy nanoparticles on TiO
2
nanosheets. Apart from the design of catalysts, the
electrolyte utilized also plays the key role in the evaluation of catalytic
performance
20,35
. Feng et al. pointed out that the hydrogen evolution reaction (HER)
can be efficiently suppressed in a neutral electrolyte to achieve the ammonia synthesis
with a high Faradic efficiency
20,36
, which is also applicable to this work. Following
this guidance, the efficient urea synthesis was achieved with a urea formation rate of
3.36 mmol g
-1
h
-1
and a corresponding Faradic efficiency of 8.92% at -0.4 V versus
reversible hydrogen electrode (RHE) in a flow cell.
Theoretical calculations are of great significance to the selection of catalysts and
the understanding of reaction mechanisms. Medford and co-workers have carried out
significant and pioneering research on this topic, developing fundamental
understanding of the molecular-scale processes that underlie the important and
potentially transformative N
2
and CO
2
reduction processes
37-40
. Inspired by their
instructive theory that the carbon radicals on the catalyst surface can interact strongly
with N
2
molecules to facilitate the catalytic process
33
, possible mechanisms for C–N

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Coupling n2 and co2 in h2o to synthesize urea under ambient conditions" ?

© 2020, The Author ( s ), under exclusive licence to Springer Nature Limited. The process was carried out using an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets. 

Q2. What is the role of TiO2 in the urea synthesis?

The TiO2 has been reported to stabilize the intermediates to promote CO2RR compared with carbon support48, and the application of TiO2 especially the OV-rich one is crucial to promote the urea synthesis in this work. 

Q3. What is the effect of side-on adsorption on the N2 bond?

The side-on adsorption facilitates back feed of electrons from the d orbitals of Pd and Cu to the π* orbitals of N2, reducing the N–N bond order60,61. 

Q4. How much urea formation rate was obtained at -0.4 V versus RHE?

A maximum urea formation rate of 0.12 mmol g-1 h-1 and a corresponding Faradic efficiency of 0.66% were acquired at -0.4 V versus RHE. 

Q5. What is the effect of OVs on the urea synthesis?

The introduction of OVs enhanced the ability for activation on N2 and CO2 for Pd1Cu1/TiO2-400, also facilitating the generation of urea. 

Q6. What is the effect of flow cells on the urea synthesis?

The utilization of flow cells has been reported to improve electrocatalytic performance towards NRR12,49 and CO2RR46-48,50,51 respectively and was introduced to boost the performance for electrochemical urea synthesis in this work. 

Q7. How was the chemisorption of the catalysts measured?

As the chemisorption of N2 is the initial step towards NRR, the N2 chemisorption ability of the catalysts was measured by temperature-programmed desorption (TPD) with a mass detector. 

Q8. What is the reason for the decline in the yield rate of urea?

the amount of CO needs to be well controlled, as once the potential exceeds -0.4 V, the decline in the yield rate for urea is ascribed to the excessive release of CO and its occupation of adsorptive sites for N2 and CO2 to some extent. 

Q9. What is the role of the electrolyte in the urea synthesis?

Apart from the design of catalysts, the electrolyte utilized also plays the key role in the evaluation of catalytic performance20,35. 

Q10. What is the effect of the presence of N2 on the urea?

The computational results reveal that the presence of *N2 can facilitate CO2 reduction, and the reduced CO can further react with *N2 to form urea with ultrahigh activity and selectivity. 

Q11. What is the important process for urea synthesis?

Urea (CO(NH2)2) is conventionally synthesized through two consecutive industrial processes, N2 + H2 → NH3 followed by NH3 + CO2 → urea. 

Q12. What is the reason for the high electrocatalytic activity towards urea synthesis?

It can be assumed6that the high electrocatalytic activity towards urea synthesis is derived from the simultaneous improvements for NRR and CO2RR. 

Q13. What is the effect of OVs on the structure of TiO2?

The formation of OVs could narrow the band gap of TiO2-400 (Supplementary Fig. 5) but did not change the crystalline phase (Supplementary Fig. 6). 

Q14. Why is the fixation of earth-abundant N2 challenging?

The fixation of earth-abundant N2 is challenging both scientifically and technologically due to the high inertness of this molecule5. 

Q15. What is the urea formation rate of Pd1Cu1/TiO2?

Pd1Cu1/XC72R exhibits a low urea formation rate (Supplementary Fig. 20), which might be ascribed to the weakened interaction between the metal and carbon support. 

Q16. What is the problem with the fixation of gaseous molecules?

The fixation of gaseous molecules, particularly that of N2, has attracted extreme attention but is challenging due to the intrinsic inertness of this molecule. 

Q17. What is the advantage of the electrochemical fixation of N2 and CO2 under ambient conditions?

there have been extensive research activities to reduce the activation energy of the N2-to-NH3 reaction under milder conditions16-19.3Electrocatalytic N2 fixation under ambient conditions combines the advantages of the utilization of clean energy and protons directly from water20-23.