Enhanced Activity of Titanocene Complex for Electrocatalytic Nitrogen Reduction Reaction

Karol J Fijalkowski

doi:10.3390/catal11030389

Abstract

Enhanced titanocene (Cp2TiCl2) based electrocatalytic system for nitrogen reduction was shown, comprising glassy carbon electrode, high level of the catechol redox mediator, optimized binary THF/MeOH solvent and unique design of the reactor having ammonia permeable membrane at the outlet, which allowed constant nitrogen flow through the working solution during entire electrolysis without risk of evaporation of the solvent. Catalytic activity was observed in the potential range of (−1.5)–(−2.3) V, reaching TON of 2.83%, corresponding to the production of 0.566 μmol NH3 (9.64 μg) in 24 h hydrolysis at −2.3 V using 0.02 mmol TiCp2Cl2 (5 mg).

Highlights

The first reports on titanium complexes as catalysts for selective nitrogen reduction were published in 1966 by Vol’pin and Shur, who investigated the use of a variety of transition metal compounds (e.g., TiCl4 ) for stoichiometric reduction of nitrogen to ammonia [13]
Mechanistic studies suggested that titanium dimer may be an intermediate in the nitrogen reduction process
More than four times higher performance was achieved by changing the electrode material into glassy carbon, optimization of catechol level and solvent composition, and major modifications in the setup by closing the outlet of the reactor by ammonia permeable membrane, which allows constant nitrogen flow through the working solution during entire electrolysis without risk of evaporation of the solvent, which was kept in the reactor by the membrane

Summary

Introduction

The natural process of nitrogen reduction has been intensively studied to determine the mechanism of nitrogenase catalyzed reaction, which utilizes 16 equivalents of ATP upon production of two ammonia molecules [1,7]: N2 + 10H+ + 8e− + 16ATP → 2NH4 + + 2H2 + 16ADP + 16Pi (2). Due to the high energy demand of the reaction, a sustainable process of ATP recycling would be needed for the enhanced design of a nitrogenase-based nitrogen fixation system [8,9]. Various biomimetic iron complexes have been designed to mimic nitrogenase active center using tripodal phosphene ligands, known to catalytically reduce nitrogen at −78 ◦ C when using an excess of strong reductants and strong acids, such as KC8 or HBArF 4 ·2Et2 O [18,19,20,21,22]. This opens possibilities towards immobilization of the titanocene complex on the surface of carbon electrodes for further development of the system

Results and Discussion

Influence tests of of aqueous ammonia

Results

Experimental

Electrochemical

Conclusions