An Artificial Biomimetic Catalysis Converting CO2 to Green Fuels
Li and Wang Nanoscale Research Letters
An Artificial Biomimetic Catalysis Converting CO to Green Fuels 2
Caihong Li 0
Zhiming Wang 0
0 The Institute of Fundamental and Frontier Science, University of Electronic Science and Technology of China , Chengdu 610051 , China
Researchers devote to design catalytic systems with higher activity, selectivity, and stability ideally based on cheap and earth-abundant elements to reduce CO2 to value-added hydrocarbon fuels under mild conditions driven by visible light. This may offer profound inspirations on that. A bi-functional molecular iron catalyst designed could not only catalyze two-electron reduction from CO2 to CO but also further convert CO to CH4 with a high selectivity of 82% stably over several days.
Molecular iron catalyst; CO2; CO; CH4; Visible light
Social development and the energy crisis have increased
the demand on chemical fuels. Furthermore, the
increasing concentrations of CO2 in the atmosphere owing to
human activities such as excessive combustion of fossil
fuel, exhaust gas emission and respiration have had a
series of terrible impacts including global warming,
desertification, and sea level rise. One of the greatest
innovations in mitigation of energy crisis and
greenhouse effect was converting greenhouse gases CO2 into
fuel chemical feedstock compounds such as CH4, CO,
and other small molecules with visible light (it is called
photoreduction, in scientific jargon) [
]. The most
remarkable superiority of photoreduction is that it can be
driven by visible light compared to electroreduction
which is activated by applied voltage or thermal
reduction with high temperature. In addition, approximately
half of solar light is located in the visible range.
However, the low production rate and selectivity because of
multiple reaction pathways and a variety of products
severely limit large-scale practical application of CO2
The challenges in catalytic reduction of CO2 to
valueadded fuels ideally based on cheap and earth-abundant
elements rather than on precious metals are efficiency,
stability, and selectivity [
]. So far, the main methods
addressing these challenges have fallen into three categories:
screening transition metals [
] with high catalytic activity as
active sites such as Fe, Co, and Ni; formation of organic
macrocyclic structures to enhance long-term stability [
and ligand modification [
] to strengthen the desired
product selectivity. In each approach, the selected metal element
and the structural design both contribute to the final
catalytic performance and product selectivity.
Organic macrocyclic structures (OMS) chaining
transition metal elements are very popular catalysts used in
CO2 reduction, where the metal elements act as catalytic
active sites to adsorb and bind the CO2 molecules [
Microporous OMS can offer larger specific surface area,
i.e., more active sites to support catalytic reactions.
Nevertheless, the original OMS may not possess the
optimized catalytic performance. Structure optimization
such as ligand modification would improve the catalytic
activity especially product selectivity by inducing internal
interactions like H bonds which can stabilize the specific
intermediates in favor of gaining desired products.
Inspired from the photosynthesis of plants, Rao et al. [
creatively designed a biomimetic photocatalytic system
based on a molecular iron catalyst which miraculously
produced CH4 from CO2 at ambient temperature and
pressure. Such a frontier and significant discovery was
published in Nature.
Rao and co-workers judiciously designed an iron (transition
metal element) tetraphenylporphyrin (organic macrocyclic
structure) complex functionalized with trimethylammonio
groups (ligand modification) as the catalyst to reduce CO2.
This catalytic system was operated in a CO2-saturated
acetonitrile (CH3CN) solution containing a visible-light
photosensitizer aiming to capture photons from light irradiation and
afford energy (hυ) for the redox reactions as well as a sacrificial
electron donor used to provide electrons on photo-induced
command by photosensitizer to reduce CO2. The whole
system was significantly stable and driven by visible light
(λ > 420 nm) at 1 atm and room temperature.
Furthermore, Rao et al. first reported that the above
catalytic system whose catalyst was known as the most
efficient and selective molecular electro-catalyst for
reducing CO2 to CO in two-electron process, could also
be applied for the eight-electron reduction [
] from CO2
to CH4. They found an entirely new function of this
molecular iron catalyst under moderate conditions.
Meanwhile the authors analyzed and verified the reaction
mechanism of two-step procedure that first reduced
CO2 to CO and then converted CO to CH4 with 82% of
the CH4 selectivity by isotope labeling experiments and
blank experiments for the first time. Besides, they also
found that a meta-acid condition could play a role of
proton donor as well as H bond donor towards
stabilized intermediates [
] but unwished by-product
hydrogen selectivity would increase either.
Greenhouse gas CO2 molecules were adsorbed on the
surface of catalyst or more precisely on the metal Fe
active sites and distorted from the linear structure to a
certain angle; thus CO2 molecules were activated [
and formed Fe–CO2 adduct. In addition, this adduct was
further protonated reacting with H+ from solution and
formed Fe–CO adduct dehydrating a H2O molecule.
The intermediates of CO could be obtained through
hydrogenation at this time. Then, CO molecules were
bound to the metal active sites again through
subsequent multistep protonation and electron transfer
process and proceeded to yield CH4 gas eventually
desorbing from the catalyst surface. Later, this catalyst reused
to next catalytic cycle of CO2 molecules (Fig. 1).
The catalytic system they designed was bi-functional, for
catalyzing not only relatively simple two-electron
reduction to CO but also eight-electron reduction to CH4
utilizing only one catalyst at very easily satisfied conditions.
This was a profound progress because a catalyst can
catalyze efficiently a certain reaction generally. The
uplifting discovery of Rao et al. aroused great interest on
photoreduction of CO2 to value-added CH4 and has
inspired future efforts in this field. A drawback of this
report is that the authors have not deciphered the
reduction mechanism in greater detail yet; otherwise, it will
help to develop more efficient catalytic system improved
from mechanism aspects. Cheaper gas fuel may be
produced when the productive rate is improved via
optimization of structure and conditions.
The catalytic system designed by Rao et al. has other
promising properties besides these described here. For
example, it can convert toxic gas CO to green fuel CH4
just by light irradiation. Such a simple but significant
conversion might guide a new craze to turn waste into
wealth environmentally and efficiently. The application
and development of their discovery might form the basis
of a new branch of CO2 photoreduction or toxic gas
The authors greatly acknowledge the financial support from the National
Basic Research Program (973) of China (2013CB933301) and National Natural
Science Foundation of China 51272038.
CL investigated and reviewed relevant papers and then finished this article.
ZHW has guided and corrected this work. All authors read and approved the
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional affiliations.
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