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The
production of ammonia amounts to about 200 megatons per year. This
makes it the second-most produced chemical in the world, surpassed
only by sulfuric acid. There are several means of producing ammonia,
but the Haber-Bosch process remains the most prevalent, accounting for
about 90% of total production. Haber-Bosch and the other processes
involved in industrial-scale production require high temperatures
(more than 400°C) and high pressure (more than 150 bar). Those
conditions are needed to break the strong bonds in nitrogen and react
with hydrogen to form ammonia (NH3).
These
processes, taking up around 1% of global energy consumption, are
largely fossil fuel-based. Hence, ammonia is the most greenhouse
gas-intensive chemical-making reaction globally, totaling roughly 1.5%
of total global CO2 emissions.
In addition, demand for ammonia is only expected to increase in the
coming years—mainly due to its use in synthetic fertilizers needed to
feed an increasing global population, although it is also being
considered as a marine fuel.
One of the major challenges on the climate front, and the energy and
food front, is the production of ammonia. Today it is made in some of
the largest factories in the world. The only really efficient way to
make ammonia is under high temperatures and high pressure and using a
carbon-based feedstock. Nature is very good at making ammonia at
ambient pressure and temperatures in enzymes like nitrogenase.
However, the process is very slow and impossible to scale to
industrial production.
—co-corresponding author Professor Tejs Vegge, DTU Energy and the
VILLUM Center for the Science for Sustainable Fuels and Chemicals
(V-Sustain)
Professor Vegge led the research along with Professor Ping Chen from
the Dalian Institute of Chemical Physics (DICP), Chinese Academy of
Sciences.
Together with the team from DICP, Tejs Vegge and his colleagues from
DTU, Dr Jaysree Pan and Associate Professor Heine A. Hansen, have
introduced a new class of complex metal hydride catalysts that enable
mild-condition ammonia synthesis.
Conventional heterogeneous catalysts based on metallic iron or
ruthenium mediate dinitrogen dissociation and hydrogenation through a
relatively energy-expensive pathway. Here we report the ternary
ruthenium complex hydrides Li4RuH6 and
Ba2RuH6 as
an alternative class of catalysts, composed of electron- and
hydrogen-rich [RuH6]
anionic centres, for non-dissociative dinitrogen reduction, where
hydridic hydrogen transports electrons and protons between the centres,
and the Li/Ba cations stabilize NxHy (x = 0–2,
y = 0–3) intermediates. The dynamic and synergistic involvement of all
the components of the ternary complex hydrides facilitates an
associative reaction mechanism with a narrow energy span and perfectly
balanced kinetic barriers for the multistep process, leading to
ammonia production from N2 + H2 with
superior kinetics under mild conditions.
—Wang et al.
Their
process allows them to synthesize ammonia at temperatures as low as
300 °C (573 K) and at pressures as low as 1 bar. Practical application
of these catalysts shows promise concerning small-scale production of
ammonia based on renewable energy. Such systems would generally
require catalysts operating under pressures around 50 bar and
temperatures below 400 °C.
We
believe our research stands out in that this new class of catalysts
actually lies somewhere between the biological and the industrial
processes. It has something from the human, artificial
process—heterogeneous catalysis—and something from what goes on in
enzymatic and homogenous catalysis. It is an entirely new way of
making ammonia, and we’re using the best of both worlds allowing us to
lower the temperature and pressure significantly.
—Professor Vegge
The
reduction of nitrogen is realized via multiple ruthenium hydride
complexes, [RuH6]4-,
which are rich in electrons and hydrogen. The hydrogen transports
electrons and protons between the centre and the nitrogen. At the same
time, the alkaline metals lithium or barium (Li/Ba) stabilize the
reaction intermediates. However, the process is highly dynamic;
several parts of the complex also serve other functions. The
calculations alone have taken years to complete.
Everything is different from what we’ve seen before. For example,
although ruthenium is a well-known component in ammonia catalysis, it
is present in a different form and behaves differently. It is
surrounded by hydrogen atoms and forms a hydride complex, allowing it
to transfer hydrogen in a novel way. You could picture this catalyst
as a symphony orchestra, where every part has to function together to
make it work. The fascinating part is that it does work – there are no
false notes.
Ammonia catalysis is arguably the best-studied catalytic system in the
world. To find a truly new mechanism that opens a door into a new
world is very satisfying as a scientist. However, it may also open up
new possibilities for ammonia production to take place in a less
energy-intensive way. The large factories of today are needed to make
the production profitable. Our catalysts or similar concepts may
enable production in smaller, decentralized factories. This would also
cut down on transport, which adds substantially to the price and CO2 emissions
of ammonia today.—Professor
Vegge
Resources
Wang, Q., Pan, J., Guo, J. et
al. (2021) “Ternary ruthenium complex hydrides for ammonia
synthesis via the associative mechanism.” Nat Catal 4,
959–967 doi: 10.1038/s41929-021-00698-8
