Heterogeneous Electrochemical Ammonia Oxidation with a Ru-bda Oligomer Anchored on Graphitic Electrodes via CH−π Interactions

Abstract

Molecular catalysts can promote ammonia oxidation, providing mechanistic insights into the electrochemical N 2 cycle for a carbon-free fuel economy. We report the ammonia oxidation activity of carbon anodes functionalized with the oligomer {[Ru II (bda-κ-N 2 O 2 )(4,4′bpy)] 10 (4,4′-bpy)}, Rubda-10, where bda is [2,2′-bipyridine]-6,6′dicarboxylate and 4,4′-bpy is 4,4′-bipyridine. Electrocatalytic studies in propylene carbonate demonstrate that the Ru-based hybrid anode used in a 3-electrode configuration transforms NH 3 to N 2 and H 2 in a 1:3 ratio with near-unity faradaic efficiency at an applied potential of 0.1 V vs Fc +/0 , reaching turnover numbers of 7500. X-ray absorption spectroscopic analysis after bulk electrolysis confirms the molecular integrity of the catalyst. Based on computational studies together with electrochemical evidence, ammonia nucleophilic attack is proposed as the primary pathway that leads to critical N−N bond formation.

Rapid Aqueous Ammonia Oxidation to N2 Using a Molecular Ru Electrocatalyst

“…This may suggest a rate-determining ammonia nucleophilic attack (ANA) step at a putative high-valent Ru-nitrido species, akin to previous reports. 19,20 We further note that while the current in the cyclic voltammograms of 1 in the absence of NH 3 is limited by diffusion of 1 and obeyed the Randles−S ̌evcǐ ́k dependence on the scan rate (Figure S6), the amplified anodic current under catalysis conditions was not linearly dependent on the square root of scan rate (i.e., not determined by the semi-infinite diffusion of 1 to the electrode). Instead, at scan rates above 600 mV s −1 , the peak catalytic current was independent of the scan rate (Figure 4c) suggesting the current was no longer determined by the bulk diffusion of 1 or NH 3 but by the rate of regeneration of 1 at the electrode, as NH 3 reduced the electrogenerated 1 + .…”

“…In weakly protic media (pK a > 13), these further decompose to 19 In the second instance published by the same group, they immobilized oligomers of [Ru II (bda-κ-N 2 O 2 )] (bda 2− : 2,2′-bipyridine-6,6′-dicarboxylate)-4,4′-bipyridine onto carbon paper electrodes. 20 Upon electrolysis in aqueous NH 3 solutions, they found that the oligomers detached from the electrode and did not report an FE for N 2 under aqueous conditions. A purported 100% FE for AO to N 2 was reported in propylene carbonate; however, we found this value to likely be erroneous due to calculation errors (see reference note).…”

“…We note the unimolecular mechanism makes this catalyst amenable to grafting on electrode surfaces without affecting TOF. Surface grafting might even enhance the catalyst stability by suppressing any bimolecular autocatalytic decomposition pathways we have discovered in this work . This work demonstrates the necessity of DAFCs in aqueous media rather than organic media and the viability of achieving high performance with molecular catalysts under aqueous conditions.…”

“…The first of these studies by Llobet and co-workers tested [Ru­(tda-κ-N 3 O)] (tda 2– : 2,2′:6′,2″-terpyridine-6,6′′-dicarboxylate) and determined a turnover number (TON) of 1 with a low N 2 faradaic efficiency (FE) of 30–45% . In the second instance published by the same group, they immobilized oligomers of [Ru II (bda-κ-N 2 O 2 )] (bda 2– : 2,2′-bipyridine-6,6′-dicarboxylate)-4,4′-bipyridine onto carbon paper electrodes . Upon electrolysis in aqueous NH 3 solutions, they found that the oligomers detached from the electrode and did not report an FE for N 2 under aqueous conditions.…”

Electrochemical Ammonia Oxidation with Molecular Catalysts

“…Llobet et al described the first Ru molecular AO catalyst 6 that operates in aqueous conditions, demonstrating the chemoselectivity of AO over WO . In 2023, they reported AO by the ruthenium oligomer {[Ru II (bda-κ- N 2 O 2 )­(4,4′-bpy)] 10 (4,4′-bpy)} (4,4′-bpy = 4,4′-bipyridine) heterogenized to carbon anodes that achieved a TON of 7500 . Other molecular AO catalysts, including [Cp*Ru II (P t Bu 2 N Ph 2 )­(NH 3 )] + (P tBu 2 N Ph 2 = 1,5-di­(phenylaza)-3,7-di­(tert-butylphospha)­cyclooctane) and Ru­(II) porphyrins, but not limited to Ru complexes, operate via hydrogen atom abstraction chemistry to form N 2 or C–N bonds and are beyond the scope of this text.…”