Co3Mo3N is
one of the most active catalysts
for ammonia synthesis; however, little is known about the atomistic
details of N2 adsorption and activation. Here we examine
whether N2 can adsorb and activate at nitrogen surface
vacancies. We have identified the most favorable sites for surface
nitrogen vacancy formation and have calculated vacancy formation free
energies (and concentrations) taking into account vacancy configurational
entropy and the entropy of N2 at temperature and pressure
conditions relevant to ammonia synthesis (380–550 °C,
100 atm) via a semiempirical approach. We show that 3-fold hollow
bound nitrogen-containing (111)-surfaces have surprisingly high concentrations
(1.6 × 1016 to 3.7 × 1016 cm–2) of nitrogen vacancies in the temperature range for ammonia synthesis.
It is shown that these vacancy sites can adsorb and activate N2 demonstrating the potential of a Mars–van Krevelen
type mechanism on Co3Mo3N. The catalytically
active surface is one where 3f-hollow-nitrogens are bound to the molybdenum
framework with a hexagonal array of embedded Co8 cobalt
nanoclusters. We find that the vacancy-formation energy (VFE) combined
with the adsorption energy can be used as a descriptor in the screening
of materials that activate doubly and triply bonded molecules that
are bound end-on at surface vacancies.
Dispersion-corrected
periodic DFT calculations have been applied
to elucidate the Langmuir–Hinshelwood (dissociative) and an
Eley–Rideal/Mars–van Krevelen (associative) mechanism
for ammonia synthesis over Co3Mo3N surfaces,
in the presence of surface defects. Comparison of the two distinct
mechanisms clearly suggests that apart from the conventional dissociative
mechanism, there is another mechanism that proceeds via hydrazine
and diazane intermediates that are formed by Eley–Rideal type
chemistry, where hydrogen reacts directly with surface activated nitrogen,
in order to form ammonia at considerably milder conditions. This result
clearly suggests that via surface defects ammonia synthesis activity
can be enhanced at milder conditions on one of the most active catalysts
for ammonia synthesis.
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