RESEARCH HIGHLIGHTSNITROGEN CHEMISTRY

Taking steps to rebalance the nitrogen cycle
A catalyst with an unusual step-by-step mechanism could turn troublesome fertilizer runoff
into harmless nitrogen gas© PATRICE LATRON/LOOK AT SCIENCES/SCIENCE PHOTO LIBRARY

A

catalyst inspired by nature
may help bring the global
nitrogen cycle back into balance,
research by RIKEN scientists
suggests1. It promises to offer away to remove excess nitrogenfrom wastewater, which iscurrently upsetting the nutrientbalance of natural ecosystems.
Nitrogen comes in many forms
in nature—from biologicallyinert molecular nitrogen (N2)in the air to ammonia andnitrates in soils and water,which plants depend on forgrowth. While various microbes
in the soil convert nitrogenfrom one form to another,
the industrial production ofammonia for fertilizer has
overwhelmed nature’s ability
to cycle ammonia, nitrates andnitrites back into nitrogen gas.
Algal blooms are one symptomof excess nitrites and nitrates inmarine ecosystems (see image).
To rebalance the nitrogen
cycle, researchers have beendeveloping catalysts thatturn nitrates into

nitrogen gas. But it has provenvery challenging to funnel thenitrogen through the series ofelectron- and proton-transfer
steps required for this transformation without generating
undesired by-products.
Now, inspired by nature’s
denitrification microbes, Ryuhei
Nakamura of the RIKEN
Center for Sustainable Resource
Science and his team havedeveloped an oxomolybdenum
electrocatalyst with unusual
reactivity that overcomes theselectivity problem.
Whereas proton and electrontransfer occurs together for
most denitrification catalysts,
Nakamura’s team realized theircatalyst operated stepwise, via
sequential proton−electrontransfer (SPET).
The key observation was thatthe catalyst’s activity depended
highly on the pH, which would
be expected only if
protontransfer was a separate step.
“SPET function is very rare, andso it came as a big surprise that
our oxomolybdenum catalysts
showed catalytic activity thatclearly depended on the pH,”says Nakamura.
The catalyst’s pH sensitivity
meant that the researchers could
adjust the pH to maximize
the rate of N2 formation andminimize by-products. Usinga technique called operando
spectroscopy, they analyzed thepH dependence and verified
that the catalyst operated via theSPET pathway.
The catalyst’s selectivity
needs to be enhanced evenfurther before it can be used
for wastewater treatment,
Nakamura says. “We will try
to improve the selectivity ofnitrogen gas generation byregulating temperature andso on,” he says. “Inparallel, wewill deepen our physicochemical
understanding of SPET, as it
has potential to broaden thehorizon of catalytic chemistry,”
he adds. For example, reactionselectivity issues have hampered
efforts to convert emissions ofcarbon dioxide, the main agentof climate change, back into
useful chemicals and fuels. SPET
catalysis could allow researchers
to selectively turn carbon
dioxide into products desired by
the chemical industry.
Reference

1. He, D., Li, Y., Ooka, H., Go,
Y. K., Jin, F., Kim, S. H. &Nakamura, R. Selective
electrocatalytic reductionof nitrite to dinitrogenbased on decoupled
proton−electron transfer.
Journal of the American
Chemical Society 140,
2012−2015 (2018).

A beach in Brittany, France, covered by sea lettuce. Many beaches in Brittany
are regularly affected by huge algal blooms due to fertilizers, primarilynitrates, leaching from farms into rivers. The algae pose a health hazard dueto the poisonous fumes produced as they decay. A catalyst created by RIKEN
researchers could be used to convert excess nitrates into benign nitrogen gas
and hence reduce the occurrence of such algal blooms.

FALL 201821