Nature of the First Electron Transfer in Electrochemical Ammonia Activation in a Nonaqueous Medium

Abstract

Decreasing costs of renewable sources of electricity will increase the viability of electrochemical processes in chemical manufacturing. To this end, improved understanding of electrochemical N-H bond activation is essential to develop electrochemical routes for nitrogen-containing chemicals. In this work, we investigate electrochemical ammonia activation in acetonitrile, a prototypical non-aqueous solvent for electro-organic syntheses. Non-aqueous environments are desirable for electroorganic syntheses due to large electrochemical stability windows and high solubility for organic products. We find that ammonia oxidation in acetonitrile proceeds through an outer-sphere mechanism involving an initial electron transfer as the rate-determining step, likely producing an ammonia radical cation. Density functional theory calculations explain a low transfer coefficient and suggest possible subsequent reaction steps. Structural factors involved in lowering of the transfer coefficient provide insights that are applicable to wider range of small-molecule activation chemistries.

Modeling Interfacial Electron Transfer in the Double Layer: The Interplay between Electrode Coupling and Electrostatic Driving

“…This phenomenon provides a plausible explanation for the unusually high Tafel slopes observed for certain outer-sphere reactions such as ammonia oxidation. 57…”

Acetonitrile Transition Metal Interfaces from First Principles

“…1,2 Among nonaqueous solvents, acetonitrile is commonly used. 1−29,29−32 The use of acetonitrile to enhance or control the performance of complex electrochemical reactions such as oxygen reduction, 1,4−9,30 carbon dioxide reduction, 2,[14][15][16][17][18][19][20][21][22][23][23][24][25][26]31 and various oxidation reactions [10][11][12][13]28,32 is of active research interest. Understanding the effects of nonaqueous solvents, including acetonitrile, on surface electrochemistry is thus a crucial step toward engineering catalytic and electrochemical systems for optimal performance.…”

“…Metal–solvent interfaces are important in a variety of technological applications. The composition of the solvent can critically influence the performance of electrochemical processes. , Among nonaqueous solvents, acetonitrile is commonly used. − ,− The use of acetonitrile to enhance or control the performance of complex electrochemical reactions such as oxygen reduction, ,− , carbon dioxide reduction, ,− ,− , and various oxidation reactions − ,, is of active research interest. Understanding the effects of nonaqueous solvents, including acetonitrile, on surface electrochemistry is thus a crucial step toward engineering catalytic and electrochemical systems for optimal performance.…”

Chemical and Electrocatalytic Ammonia Oxidation by Ferrocene

“…Computational Analysis of Ammonia Oxidation by Ferrocenium. Recognizing that the Fc + /Fc couple can serve as a redox mediator capable of oxidizing substrates in an outer-sphere manner, 29,30 1-electron oxidation thermodynamics for ammonia 31 were evaluated by ab initio and DFT methods to compare these energies against the experimentally determined ionization energy of Fc to Fc + in MeCN (+114.8 kcal/mol). 32 MP2/aug-cc-pVTZ/gas//MP2/aug-cc-pVTZ/SMD-MeCN calculations reveal a substantial lowering of the ionization energy of discrete NH 3 molecules from +234.9 kcal/mol in the gas phase (exp: 232.2(5) kcal/mol) 33 S6).…”

“…S14A). 31 Controlled potential electrolysis (CPE) for 13 h using a 10 cm 2 reticulated vitreous carbon (RVC) electrode in a 10% ammonia: MeCN (v:v) solution (ca. 4.6 M) at -10 °C with 1.0 mM Fc produced N 2 with a Faradaic yield of 44% (Fig.…”