Subsurface Nitrogen Dissociation Kinetics in Lithium Metal from Metadynamics

The
dissociation of molecular nitrogen in lithium is of interest
for several promising technologies, such as the catalytic synthesis
of ammonia in ambient or mild conditions. In this work we simulate
nitrogen dissociation in the lithium BCC (110) surface at ambient
and elevated temperatures using density functional theory (DFT) metadynamics
simulations. The rate constants at temperatures of 300, 400, and 500
K are calculated by statistical analysis of the reaction time distributions
from the accelerated simulations. This approach finds and estimates
rate constants for transition pathways out of the initial state; the
required input is the stable initial state and a reasonable choice
of collective variable. A single collective variable is used in this
case: the N–N distance. The results are robust to changes in
metadynamics parameters, and the reaction time distributions follow
the expected exponential distribution. We show that the metadynamics-derived
rate constants are in agreement with results from the conventional
harmonic approximation approach using a climbing image nudged elastic
band (NEB) transition state search. The reaction barriers from metadynamics
and the NEB/harmonic approximation agree to within 0.02–0.04
eV at all temperatures studied. This work demonstrates that the harmonic
approximation provides an accurate description of the rate constants
for nitrogen dissociation in lithium metal, even at temperatures near
or above the melting point of lithium, lending credence to previous
and future theoretical studies using this approximation. Moreover,
this work demonstrates a step toward the automated exploration and
discovery of reaction mechanisms and associated rate constants for
elementary surface-catalyzed reactions using DFT-based metadynamics.