Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber-Bosch process where dihydrogen and dinitrogen form ammonia at a very high pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.
We have investigated the reaction mechanism of the electrochemical reduction of carbon dioxide to hydrocarbons on copper electrodes. This reaction occurs via two pathways: a C 1 pathway leading to methane, and a C 2 pathway leading to ethylene. To identify possible intermediates in the reduction of carbon dioxide we have studied the reduction of small C 1 and C 2 organic molecules containing oxygen. We followed the formation and consumption of intermediates during the reaction as a function of potential, using online mass spectrometry. For the C 1 pathway we show that it is very likely that CHO ads is the key intermediate towards the breaking of the C-O bond and, therefore, the formation of methane. For the C 2 pathway we suggest that the first step is the formation of a CO dimer, followed by the formation of a surface-bonded enediol or enediolate, or the formation of an oxametallacycle. Both the enediol(ate) and the oxametallacycle would explain the selectivity of the C 2 pathway towards ethylene. This new mechanism is significantly different from existing mechanisms but it is the most consistent with the available experimental data. ExperimentalAll experiments were carried out in an electrochemical cell using a three-electrode assembly at room temperature. The cell and
One of the key challenges in designing light-driven artificial photosynthesis devices is the optimisation of the catalytic water oxidation process. For this optimisation it is crucial to establish the catalytic mechanism and the intermediates of the catalytic cycle, yet a full description is often difficult to obtain using only experimental data. Here we consider a series of mononuclear ruthenium water oxidation catalysts of the formand its derivatives). The proposed catalytic cycle and intermediates are examined using density functional theory (DFT), radiation chemistry, spectroscopic techniques and electrochemistry to establish the water oxidation mechanism. The stability of the catalyst is investigated using Online Electrochemical Mass Spectrometry (OLEMS). The comparison between the calculated absorption spectra of the proposed intermediates with experimental ones, as well as free-energy calculations with electrochemical data, provides strong evidence for the proposed pathway: a water oxidation catalytic cycle involving four proton-coupled electron transfer (PCET) steps. The thermodynamic bottleneck is identified in the third PCET step involving the O-O bond formation. The good agreement between the optical and thermodynamic data with DFT predictions further confirms the general applicability of this methodology as a powerful tool in the characterisation of water oxidation catalysts and for the interpretation of experimental observables.2
Here we showcase the synthesis and catalytic response of the anionic iridium(III) complex [IrCl 3 (pic)(MeOH)] − ([1] − , pic = picolinate) toward the evolution of oxygen. Online electrochemical mass spectrometry experiments illustrate that an initial burst of CO 2 due to catalyst degradation is expelled before the oxygen evolution reaction commences. Electrochemical features and XPS analysis illustrate the presence of iridium oxide, which is the true active species.
When exposed to a potential exceeding 1.5 V versus RHE for several minutes the molecular iridium bishydroxide complex bearing a pentamethylcyclopentadienyl and a N-dimethylimidazolin-2-ylidene ligand spontaneously adsorbs onto the surface of glassy carbon and gold electrodes. Simultaneously with the adsorption of the material on the electrode, the evolution of dioxygen is detected and modifications of the catalyst structure are observed. XPS and XAS studies reveal that the species present at the electrode interface is best described as a partly oxidized molecular species rather than the formation of large aggregates of iridium oxide. These findings are in line with the unique kinetic profile of the parent complex in the water oxidation reaction.
Based on previous work that identified iridium(III) Cp* complexes containing a C,N‐bidentate chelating triazolylidene‐pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C5Me5–) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron‐donating substituents into the pyridyl unit of the chelating ligand (H, a; 5‐OMe, b; 4‐OMe, c; 4‐tBu, d; 4‐NMe2, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure‐activity relationship with electron‐donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOFmax = 2500 h–1 for complex 10 with a MeO group on pyr and a OEt‐substituted triazolylidene, compared to 700 h–1 for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN‐mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation.
a b s t r a c tThe activation processes of [Cu II (bdmpza) 2 ] in the water oxidation reaction were investigated using cyclic voltammetry and chronoamperometry. Two different paths wherein CuO is formed were distinguished. [Cu II (bdmpza) 2 ] can be oxidized at high potentials to form CuO, which was observed by a slight increase in catalytic current over time. When [Cu II (bdmpza) 2 ] is initially reduced at low potentials, a more active water oxidation catalyst is generated, yielding high catalytic currents from the moment a sufficient potential is applied. This work highlights the importance of catalyst pre-treatment and the choice of the experimental conditions in water oxidation catalysis using copper complexes.
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