Accelerated Ammonia Synthesis Holds
Promise for Conversion of Renewable Energy
Research by
Japanese scientists at Hiroshima University reveals a way to make
ammonia from its constituent molecules of nitrogen and hydrogen at
ambient pressure.
The new study, published on
Feb. 2 in the Journal
of Physical Chemistry C, demonstrates a process with
potential for use in renewable energy storage and transfer, which
relies on a dispersed and fluctuating network of resources, such as
sun and wind. “The ultimate goal of this work is to establish the
small-scale NH3 production process to effectively utilize
renewable energy” said study author and associate professor Hiroki
Miyaoka from Hiroshima University’s Natural
Science Center for Basic Research and Development.
Ammonia (NH3) has
recently been recognized as an outstanding energy carrier molecule. In
1918, German chemist Fritz Haber won the Nobel Prize for synthesis of
ammonia from its elements,
paving the way for ammonia’s significant role in industrial
fertilizers. However, use of ammonia in renewable energy applications
has been limited by the processes available to synthesize it. The
Haber-Bosch process,
used in industrial production of ammonia, requires high temperature
and pressure, conditions not typically available in renewable energy
storage and transport infrastructure.
The NH3 synthesis
process via chemical looping using lithium hydride (LiH) starts by
combining LiH with N2 (molecular nitrogen) at ambient
pressure and temperatures up to 500°C to yield a lithium imide product
(LiNH2). The lithium imide then reacts with hydrogen gas (H2)
to yield ammonia. The reaction time for ammonia synthesis from its
constituent molecules in this process is more than 1000 minutes. Its
speed is limited by the clumping up (agglomeration) of the products of
the reaction into large particles (more than 200 μm) that don’t have
much surface area exposed to the hydrogen gas. For its practical
application in distributed renewable energy, this prolonged reaction,
requiring extreme conditions, is an impediment to ammonia production.
In the new study,
researchers experimented with using lithium oxide (Li2O) as
a molecular scaffold to synthesize ammonia under ambient pressure and
temperatures below 400°C, conditions easy to mimic in nonindustrial
settings. They combined the reactant lithium hydride with lithium
oxide and found that the lithium hydride prevented clumping, leaving
smaller particles (less than 50 μm) with more surface area exposed for
chemical reactions. Using these non-agglomerated reactants and adding
the gaseous hydrogen used in the final step of ammonia synthesis, they
were able to produce ammonia more quickly; the reaction substantially
sped up.
If ammonia can be produced
quickly with relatively simple equipment under modest temperature and
pressure conditions, it paves the way for smaller-scale ammonia
production.
“The chemical looping
process is useful to establish the small-scale NH3
synthesis process, which can be operated under lower pressure and
temperature with higher conversion yield than the conventional
catalytic process,” said Miyaoka. The new process also obviates the
need for expensive metal catalysts — such as elemental ruthenium (Ru)
— used in industrial synthesis of ammonia.
The results of this study
are relevant to renewable energy generation, which tends to be more
distributed than industrial production. The process pioneered in the
Hiroshima lab to produce ammonia efficiently under near-ambient
conditions is the foundation for such applications.
“As a next step, the
practical reaction processes to effectively control the above NH3 synthesis
should be considered from chemical engineering points of view,”
Miyaoka said.
Authors of the paper
include Kentaro Tagawa, Hiroyuki Gi, Keita Shinzato, Hiroki Miyaoka,
and Takayuki Ichikawa in Hiroshima University’s Graduate School of
Advanced Science and Engineering.
The Japan Society for
the Promotion of Science funded this research.
About the study:
-
Journal: The Journal
of Physical Chemistry C
-
Title: Improvement of
Kinetics of Ammonia Synthesis at Ambient Pressure by the Chemical
Looping Process of Lithium Hydride
-
Authors: Kentaro
Tagawa, Hiroyuki Gi, Keita Shinzato, Hiroki Miyaoka & Takayuki
Ichikawa
-
DOI: 10.1021/acs.jpcc.1c09902
Profile of Associate Professor Hiroki Miyaoka.
Courtesy of
Hiroshima University
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