(Invited) Nitrogen Electrochemical Reduction Reaction Pathway on Noble Metallic Surfaces

Abstract

Nitrogen reduction reaction (NRR) has become a hot topic in recent years, since it allows the directly conversion of renewable electricity energy to chemical energy stored in ammonia, with the advantages of zero CO2 emission and potential energy saving. In this process, nitrogen molecules are reduced directly on the cathodic electrode, with ammonia as the final product. However, the achievements in this area are still far from meeting the requirements for its commercial application, majorly owing to the serious competing of the hydrogen evolution reaction (HER) on the cathodic electrocatalysts surfaces. Furthermore, this serious competing reaction significantly limits the nitrogen reduction reaction mechanism study on electrode surface, which is vital for the rational design of more advanced electrocatalysts.In this work, we employed surface-enhanced infrared absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS) to study the reaction mechanisms of nitrogen reduction on noble metal surfaces (Au, Pt, Ru, Rh). The N-N stretching (1109 cm-1) of the *N2Hy (3≤y≤4) was observed on Au surface, and the N=N stretching (~2000 cm-1) of the *N2Hx (0≤x≤2) was detected on Ru and Rh surface by surface-enhanced infrared absorption spectroscopy. In addition, the pulse signal of N2H2 (m/e = 29) was also detected with H2 on Rh surface by differential electrochemical mass spectrometry. Based on these results, we may deduce an overall NRR reaction pathway on metallic surfaces as depicted in Scheme 1. There is an equilibrium between the dissolution and adsorption of the nitrogen molecule in the three-phase interface (K0, K0’, and Kdiff). On the metallic catalyst surfaces, the nitrogen reduction may either follow a dissociative (dotted line) or an associative (solid line) pathway. The associative pathway mainly contains three procedures, involving two-electron transfer (k2), four-electron transfer (k3), and six-electron transfer (k4). On metallic surface, the exact reaction pathway that NRR follows on a certain metallic surface is determined by the rate determining step. For instance, if the energy barrier for N2H2 desorption is relatively lower than that of further reduction of N2H2, the NRR would follow a two-electron transfer procedure with N2H2 as the product, which would subsequently decompose into ammonia and nitrogen in the electrolyte (k7). The similar phenomenon would occur in the four-electron transfer procedure with N2H4 as the by-product. While in the dissociative pathway, only six-electron transfer procedure (k1) is involved with ammonia as the only product. After ammonia is formed on the metallic surfaces, there would be another equilibrium between the desorption of the ammonia from the surfaces into the electrolyte (Kdiff’) or the gas phase (K10’), and an ammonia dissolution equilibrium between the gas phase and the electrolyte (K10).Scheme 1. Electrochemical nitrogen reduction reaction pathway on metallic surfaces: associative pathway (solid line) and dissociative pathway (dotted line). Figure 1