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Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst
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Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst
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Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst
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Journal Article
Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst
2020
Overview
Ammonia (NH
3
) is pivotal to the fertilizer industry and one of the most commonly produced chemicals
1
. The direct use of atmospheric nitrogen (N
2
) had been challenging, owing to its large bond energy (945 kilojoules per mole)
2
,
3
, until the development of the Haber–Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts
4
–
6
via electron transfer from the promoters to the antibonding bonds of N
2
through transition metals
7
,
8
. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals
9
,
10
. This strategy has facilitated ammonia synthesis from N
2
dissociation
11
and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N
2
(refs.
12
–
15
). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N
2
. In addition, the nickel metal loaded onto the nitride dissociates H
2
. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements.
Ammonia is synthesized using a dual-site approach, whereby nitrogen vacancies on LaN activate N
2
, which then reacts with hydrogen atoms produced over the Ni metal to give ammonia.
