Ammonia Oxidation with a Ruthenium Polypyridyl Complex: Mechanism-Guided Approach to Low Overpotential Electrocatalysis

High Resolution Image Download MS PowerPoint Slide The use of ammonia as a potential zero-carbon fuel has attracted high interest in the quest for novel sustainable energy carriers. This requires a deep understanding of the principles of the ammonia oxidation reaction (AOR) to guide the development of efficient catalysts. Here, we report the complex [Ru II (TPA)(NH 3 ) 2 ](PF 6 ) 2 ( 1(PF 6 ) 2; TPA = tris(2-pyridylmethyl)amine), and we demonstrate that its 1e – oxidation in MeCN is partially ligand centered and finally produces [Ru II (TPA)(MeCN)(NH 3 )](PF 6 ) 2 ( 2(PF 6 ) 2 ). Deprotonation of 1 3+ (p K A = 12.6) is derived as the rate-determining step, and bimolecular N–N coupling of the key intermediate [Ru(TPA)(NH 3 )(NH 2 )] 2+ ( 4 2+ ) is proposed based on 15 N labeling experiments and electrochemical studies. 1(PF 6 ) 2 is then investigated as a homogeneous AOR electrocatalyst, and the formation of N 2 and H 2 is confirmed during electrolysis at a moderate operating potential. DFT computations established a complete PCET map to evaluate potential reaction pathways and, in combination with spectroscopic and electrochemical studies as well as product analysis using 15 N labeling, allowed us to propose a catalytic cycle where N–N bond formation occurs via intermolecular coupling ( I2M ) at an early amido stage ( 4 2+ ) of the PCET sequence. Circumventing higher metal oxidation states gives rise to a relatively low AOR overpotential of 0.96 V for 1 2+ . Complex 2 2+ is identified as an important intermediate that is catalytically competent and initiates a second cycle, while [Ru II (TPA)(MeCN) 2 ] 2+ ( XI 2+ ) represents an off-cycle product leading to gradual loss of catalytic activity.