Faraday Discussions最新文献

Nitrogenases catalyse the 6-electron reduction of dinitrogen to ammonia, passing through a series of redox and protonation levels during catalytic substrate reduction. The molybdenum–iron nitrogenase is the most well-studied, but redox potentials associated with proton-coupled transformations between the redox levels of the catalytic MoFe protein have proved difficult to pin down, in part due to a complex electron-transfer pathway from the partner Fe protein, linked to ATP-hydrolysis. Here, we apply electrochemical control to the MoFe protein of Azotobacter vinelandii nitrogenase, using europium(III/II)-ligand couples as low potential redox mediators. We combine insight from the electrochemical current response with data from gas chromatography and in situ infrared spectroscopy, in order to define potentials for the binding of a series of inhibitors (carbon monoxide, methyl isocyanide) to the metallo-catalytic site of the MoFe protein, and the onset of catalytic transformation of alternative substrates (protons and acetylene) by the enzyme. Thus, we associate potentials with the redox levels for inhibition and catalysis by nitrogenase, with relevance to the elusive mechanism of biological nitrogen fixation.

Templated synthesis is an intriguing strategy for the length-controlled synthesis of oligomers. Traditionally, such reactions require stoichiometric amounts of the template with respect to the product. Recently we reported catalytic macrocyclic templates that promote oligomerization of a small molecule substrate with a remarkable degree of length control. Herein we present our efforts toward creating linear templates for catalytic length-controlled oligomer synthesis.

With the increase in the greenhouse effect and reduction of fossil fuel resources, it is urgent to find a feasible solution to directly convert power to chemicals using renewable energy and achieving zero carbon emissions targets. It is necessary to convert renewable energy (i.e., solar, wind, water, etc.) into electrical power replacing fossil-fuel-fired power. Therefore, the power-to-chemicals approach is gaining more and more attention. In the past two decades, non-thermal plasma, electro-catalysis, photo-catalysis, and their hybrid approaches have shown great potential for the power-to-chemicals solution. This paper introduces the application of plasma technology in energy conversion, focusing on three main routes for plasma-enabled ammonia synthesis, and analyses the state-of-the-art. Research results of ammonia synthesis based on plasma technology are discussed. The application of advanced in situ diagnostics evidences the importance of specific intermediate species and reaction pathways. Electrons, vibrationally-excited species, free radicals, and surface-adsorbed species play important roles in plasma-catalytic ammonia synthesis. Combined with experiments and simulations, the mechanisms of plasma-catalytic ammonia synthesis are examined. Vibrationally-excited species can effectively reduce the catalytic surface energy barrier. The techno-economics of the plasma-enabled ammonia synthesis technology is discussed in view of its competitive advantages. It is emphasized that the power-to-chemicals approach can be adapted for most chemical manufacturers, and these methods would play crucial roles in reducing carbon emissions and environmental pollution. Finally, suggestions are provided for the sustainable development of the power-to-chemicals industry in the future.

Photoredox catalysis is a valuable tool in a large variety of chemical reactions. Main challenges still to be overcome are photodegradation of photocatalysts and substrates, short lifetimes of reactive intermediates, and selectivity issues due to unwanted side reactions. A potential solution to these challenges is the pre-organization of the photosensitizer, substrate and (co)-catalyst in supramolecular self-assembled structures. In such architectures, (organic) dyes can be stabilized, and higher selectivity could potentially be achieved through pre-organizing desired reaction partners via non-covalent interactions. Perylene diimide (PDI) is an organic dye, which can be readily reduced to its mono- and dianion. Excitation of both anions leads to highly reducing excited states, which are able to reduce a variety of substrates via single electron transfer. The incorporation of PDI into a heteroleptic [M₄L^a₂L^b₂] supramolecular square has been recently demonstrated. Herein we investigate its photophysical properties and demonstrate that incorporated PDI indeed features photocatalytic activity. Initial results suggest that the pre-organisation by binding positively affects the outcome.

N₂ dissociative adsorption is commonly the rate-determining step in thermal ammonia synthesis. Herein, we performed density functional theory (DFT) calculations to understand the N₂ dissociation mechanism on models of unsupported Ru(0001) terraces, Ru B5 sites, and polar MgO(111)-supported Ru₈ cluster mimicking a B5 site geometry, denoted (Ru₈(B5-like)/MgO(111)). The activation energy of N₂ dissociative adsorption on the Ru₈(B5-like)/MgO(111) model (E_a = 0.33 eV) is much lower than that on the unsupported Ru(0001) terrace (E_a = 1.74 eV) and Ru B5 (E_a = 0.62 eV) models. The lower N₂ dissociation barrier on Ru B5 sites is facilitated by the enhanced σ donation and π* back-donation between N₂(σ, π*) and Ru(d) orbitals resulting in the stronger activation of the molecular side-on N₂* dissociation precursor. The Ru₈(B5-like)/MgO(111) also exhibits enhanced σ donation because of the B5-like cluster geometry. Furthermore, the Ru cluster of the bare Ru₈(B5-like)/MgO(111) model is positively charged. This induced an unusual π donation from N₂(π) to Ru(d) orbitals as revealed by analyses of the density of states and partial charge densities. The combined σ and π donation resulted in an increased synergistic π* back-donation. The total interactions between N₂(σ, π, π*) and Ru(d) resulted in an overall electron transfer to the adsorbed N₂ from the Ru atoms in the B5-like site with no direct involvement of the MgO(111) substrate. Analyses of bond stretching vibrations and bond lengths show that the N₂(σ, π, π*) and Ru(d) interactions lead to a weaker N–N bond and stronger Ru–N bonds. These correspond to a lower barrier of N₂ dissociation on the Ru₈(B5-like)/MgO(111) model, where the highest red-shift of N–N vibration and the longest N–N bond length were observed after side-on N₂* adsorption. These results demonstrate that an electron-deficient Ru catalyst are not always inhibited from donating electrons to adsorbed N₂. Rather, this study shows that the electron deficiency of Ru can promote π* back-donation and N₂ activation. These new insights may therefore open new avenues to design supported Ru catalysts for nitrogen activation.

Ruthenium–NHC based catalysts, with a chelated iminium ligand trans to the N-heterocyclic carbene (NHC) ligand, that polymerize dicyclopentadiene (DCPD) at different temperatures are monitored using Density Functional Theory calculations to unveil the reaction mechanism, and subsequently how important are the geometrical and electronic features vs. the non-covalent interactions in between. The balance is very fragile and H-bonds are fundamental to explain the different behaviour of latent catalysts. This computational study aims to facilitate future studies of new generations of latent initiators for olefin metathesis polymerization, with the 3D and mainly the 2D Non-Covalent Interaction plots the characterization tool for H-bonds.