Limitations of Electrochemical Nitrogen Oxidation toward Nitrate

Abstract

The electrocatalytic N 2 oxidation reaction (NOR) using renewable electricity is a promising alternative to the industrial synthesis of nitrate from NH 3 oxidation. However, breaking the triple bond in the nitrogen molecule is one of the most essential challenges in chemistry. In this work, we use density functional theory simulations to investigate the plausible reaction mechanisms of electrocatalytic NOR and its competition with oxygen evolution reaction (OER) at the atomic scale. We focus on the electrochemical conversion of inert N 2 to active *NO during NOR. We propose formation of *N 2 O from *N 2 and *O as the rate-determining step (RDS). Following the RDS, a microkinetic model is utilized to study the rate of NOR on metal oxides. Our results demonstrate that a lower activation energy is obtained when a catalyst binds *O weakly. We show that the reaction is extremely challenging but also that design strategies have been suggested to promote electrochemical NOR.

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“…Ko et al 102 also showed that >80% FE is obtained in the electrocatalytic reduction of NO on copper. N 2 oxidation to nitrogen oxides is also a faster process than the N 2 reduction, 103,104 as also discussed later.…”

“…Abundant literature exists on the electrocatalytic reduction of N 2 (often indicated as NRR -nitrogen reduction reaction), but the reaction of nitrogen oxidation (NOR) has been investigated much less. 103,104 Dai et al 105 showed the possibility of preparation by direct electrochemical oxidation of nitrogen on ZnFe x Co 2Àx O 4 spinel oxides in an alkaline electrolyte. They reported a nitrate production rate of B130 mmol h À1 g MO À1 at an applied potential of 1.6 V versus the reversible hydrogen electrode (RHE).…”

“…Abundant literature exists on the electrocatalytic reduction of N 2 (often indicated as NRR – nitrogen reduction reaction), but the reaction of nitrogen oxidation (NOR) has been investigated much less. 103,104…”

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“…This is true for the oxygen pathway (III). 110 In metal oxides with a weak *O adsorption energy, the energy barrier is low. As an example, potentials with activation energies below 1 eV are observed for metal oxides such as SnO 2 , PdO 2 , and TiO 2 .…”

“…8 (III) reveals that one of the possible reaction kinetic models that can be considered for N 2 oxidation is one where N 2 O production is the rate-determining step. 110 Consider the following reactions to keep the kinetic model straight while still capturing important chemistry:…”