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(639d) Nature of the First Electron Transfer in Electrochemical Ammonia Activation in a Nonaqueous Medium

Authors: 
Schiffer, Z., Massachusetts Institute of Technology
Manthiram, K., Massachusetts Institute of Technology
Lazouski, N., Massachusetts Institute of Technology
Corbin, N., Massachusetts Institute of Technology
Ammonia is one of the largest volume commodity chemicals due to its use in the synthesis and production of fertilizers, polymers, and specialty chemicals such as hydrazine. Electrochemical routes to ammonia utilization in industrial chemical synthesis are appealing due to increasingly available distributed sources of renewable electricity. In this work, we investigate electrochemical ammonia activation in acetonitrile, a prototypical non-aqueous solvent for electro-organic syntheses. While ammonia oxidation in aqueous environments is well studied, the mechanism is less understood in a non-aqueous environment, yet non-aqueous electrolytes are required for electro-organic syntheses due to their large electrochemical stability windows and high solubility for organic products.

We have found that electrochemical ammonia oxidation in acetonitrile occurs via an outer-sphere mechanism where the initial electron transfer is the rate-determining step, likely producing an ammonia radical cation. This reaction is first order in ammonia concentration in the potential windows explored. DFT calculations suggest a radical intermediate and show that there is a large solvent reorganization energy associated with ammonia oxidation (1.94 eV). This large reorganization energy can explain why the apparent transfer coefficient for ammonia oxidation (0.32±0.01) is lower than the expected value of one-half. Understanding the mechanism and intermediates involved in ammonia oxidation is a key step toward the usage of ammonia in electro-organic syntheses to form products with nitrogen bonds, and this work provides a starting place for electrochemical synthesis of compounds containing nitrogen bonds, such as amino acids and benzylic amines. We believe that ammonia oxidation in solvents other than acetonitrile could enable alternative mechanisms that would further enable the use of ammonia in electrochemical reactions.