Chemical Engineering June 2018 - 11

Biologically inspired denitrification catalyst
T
ever,
he development of denitrification
catalysts that can reduce nitrate
and nitrite to N2 is critical for sustaining
the nitrogen cycle. Howregulating
the catalytic
selectivity
has proven to be a challenge, due to the
difficulty of controlling complex multi-electron/proton
reactions. Now, Ryuhei Nakamura
and coworkers at Riken (Wako City,
Japan; www.riken.jp) have developed an
artificial catalyst that imitates the denitrification
enzyme of microorganisms, and
succeeded in converting nitrite ions efficiently
into harmless N2.
The researchers focused on the microorganisms
that perform multi-step reactions
under mild conditions using four enzymes
that contain metals, such as Fe, Cu and
Mo as the active center of the enzyme.
They found that the catalyst composed of
the enzyme containing Mo as the active
center of the enzyme in the shape of a
pterin-like structure, coordinated with the
oxygen and sulfur, efficiently detoxifies the
nitrite ions into N2 under mild conditions
and without using a large-scale drainage
treatment installation. Using a hydrotherCMS-6208-Whiteboard-CE-May.pdf
1
mal
synthesis method, they fabricated the
catalyst with the pterin-like structure and
confirmed that this catalyst contained a
MoS4 structure similar to the active site of
the enzyme.
They also showed that utilizing sequential
proton-electron transfer (SPET)
pathways is a viable strategy to enhance
the selectivity of electrochemical reactions.
The selectivity of an oxo-molybdenum
sulfide electro-catalyst toward
nitrite
5/3/18 9:56 AM
reduction to dinitrogen exhibited
a volcano-type pH dependency with a
maximum at pH 5. The pH-dependent
formation of the intermediate species
(distorted Mo(V) oxo species), identified
using operando electron paramagnetic
resonance (EPR) and Raman spectroscopy,
was in accord with a mathematical
prediction that the pKa of the reaction
intermediates determines the pH-dependence
of the SPET-derived product. By
utilizing this acute pH dependence, they
achieved a Faradaic efficiency of 13.5%
for nitrite reduction to N2, which is the
highest value reported to date under
neutral conditions.
n
clear Systems Division, will lead the
investigation into a chloride salt CSP
heat-transfer fluid. Robb's nuclearfocused
research uses the Liquid Salt
Test Loop, a one-of-a-kind test facility
that heats fluoride salts to around
700°C, and then pumps the salts
through the loop. The loop is helping
researchers develop and analyze
technologies that industry could one
day use in fluoride salt-cooled hightemperature
reactors, a type of MSR.
Robb and Gen3 CSP project collaborators
from the University of
Utah, Virginia Tech and Argonne
National Laboratory will develop the
solar-based loop, with salts and components
different from those found in
the Liquid Salt Test Loop.
The facility will operate at temperatures
above 725°C. This increased
temperature will increase the efficiency
of heat-to-electricity conversion
to provide an economic advantage
over current CSP systems that
normally operate below 600°C. The
end result could be a CSP system
with greater electricity output at lower
costs. This work could put the solar
industry closer to reaching DOE's
goal of reducing the cost of CSP energy
to $0.05/kWh by 2030. ❑
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11
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Chemical Engineering June 2018

Table of Contents for the Digital Edition of Chemical Engineering June 2018
