Speaker
Prof.
Karoliina Honkala
(Department of Chemistry, University of Jyväskylä Finland)
Description
`Electrocatalytic denitrification is a promising technology for the
removal of NOx species in groundwater. However, a lack of
understanding of molecular reaction pathways that control the
overpotential and product distribution have limited the advancement of
NO chemistry to the same level of well-understood electrocatalytic
processes like oxygen reduction reaction. Nitrate and nitrite
electroreduction can produce a variety of different products but
adsorbed NO has been considered a selectivity-determining species in
the process. Experimentally, NO electroreduction has been studied
under a variety of reaction conditions and the reaction demonstrates a
rich chemistry with several possible reaction steps and
intermediates. Thermodynamically, N$_2$ is the most favorable product but
other products such as NH$_4^{+}$ and N$_2$O are usually observed. Cyclic
voltammetry experiments on Pt(100) demonstrate that NH$_4^{+}$ is the only
product in acid electrolyte and the onset potential of the reaction is
0.25 VSHE. Furthermore, NO reduction was
proposed to proceed via the HNO intermediate together with the facile
formation of ammonia after chemical N-O dissociation.`
`The DFT calculations were performed with the VASP code to address the
thermodyncamic and kinetic aspects for various possible elementary
steps of NO electrochemical conversion to ammonia, nitrogen gas, and
nitrous oxide both at low and saturated NO coverage on Pt(100). DFT
results suggest that at low coverage a HNO intermediate dominates,
while at experimentally observed NO coverages there is a significant
thermodynamic and kinetic competition between two pathways proceeding
either via NOH or HNO intermediates. Our kMC program employs
the first reaction algorithm4 together with periodic boundary
conditions, and takes the needed input parameters, which are reaction
barriers and equilibrium constants for included elementary steps,
directly from DFT calculations. Furthermore, we permit the formation
of other products than NH$_4^{+}$ in kMC but they were not observed during
simulations. The potential-dependent kinetic Monte Carlo calculations
were performed to simulate NO stripping experiments. We demonstrate
how these profiles change as a function of the initial NO coverage
dosed on Pt(100) and the calculated electrochemical
NO stripping curve agrees well the measured one. Furthermore,
simulations provide a mechanistic interpretation for observed peaks:
high voltage results from NOH formation and conversion in simulations
and the second peak originates from the coverage-dependent activation
energy for NOH formation. Based on the large number of analyzed
reaction pathways from kMC simulations and the calculated Tafel slope,
we are able to identify a reaction mechanism, which consists of two
consecutive proton electron steps which takes place after the initial
NO adsorption. Along the most probable reaction pathway, NOH
protonates to HNOH, which undergoes a combined dissociation and
protonation step forming NH and water. Finally, NH transfers to NH3,
which spontaneously forms NH$_4^{+}$.`
`To conclude, the synergic DFT+kMC strategy provides detailed
microscopic information to future development of denitrification
technologies and it also offers a template to investigate and analyze
other electrochemical transformations.`
`The reduction of NO has been identified as a key step in the electrochemical denitrification of nitrites and nitrates. We combine density functional theory calculations and kinetic Monte Carlo simulations to study the reduction reaction on Pt(100). This approach describes the effects of coverage-dependent adsorbate-adsorbate interaction, reaction thermodynamics, water-mediated protonation kinetics, and transient potential sweeps on product rates and selectivities. We are able to predict electrochemical NO stripping curves in nice agreement with experiments and provide an elementary mechanistic interpretation of observed current peaks. The combined methodology provides a full reaction profile and reveals the sensitive balance between thermodynamics and kinetics in NO reduction. However, the computational approach is sufficiently general to be applicable to other electrocatalytic processes on metal catalysts that are technologically and environmentally important.`
Primary author
Prof.
Karoliina Honkala
(Department of Chemistry, University of Jyväskylä Finland)
