Catalytic hydrogenation of alkenes under biphasic conditions

Ruthenium(II)-arene diphosphine complexes are shown to be effective precursors for the catalytic hydrogenation of the C=C bond in an aqueous biphase using molecular hydrogen as the hydrogen source. A mechanistic study performed in water, utilizing high gas pressure NMR spectroscopy and electrospray ionization mass spectrometry, and experiments involving the variation of pH, enables indirect identification of the catalytically active species resulting from the substitution of the chloride with the dihydrogen ligand. Similar catalytic experiments are carried out in ionic liquids (ILs), such as [bmim][BF4] and [bmim][TfO] using an analogous precursor, but no conversion is observed and the ruthenium(II) species does not undergo any chemical change during the reaction. The dissociation of chloride, leading to the catalyst, is inhibited in these ILs due to a lower solvation of chloride. An estimate of the bmim-chloride interaction, which is lower than the water-chloride interaction, is determined by 1H NMR spectroscopy. To ensure the formation of the active species in such ILs, the coordinated chloride is abstracted beforehand and replaced by acetonitrile, which restores the catalytic activity. An alternative approach to overcome the chloride dissociation problem consists of improving the solvation of the chloride in the IL medium. To accomplish this, water is added as a co-solvent in the IL phase and, under these conditions, the activity is restored. In order to assess the reactivity of chloride ions in ILs, a relative scale for the solvation of chloride is given for a series of [cation][Tf2N] ILs. This scale, based on the reaction enthalpy for the dissociation of a chloride-templated metallacage, yields the following order of chloride solvation: [C5e3am][Tf2N] < [bmpy][Tf2N] < [bmim][Tf2N] ≤ [bdmim][Tf2N] < [bpy] [Tf2N] ≪ [mimeOH][Tf2N]. In Tf2N-based ILs, the catalytic activity of hydrogenation reactions using the same Ru(II) precursor in the presence of chloride is totally modified. In a series of [cation][Tf2N] ILs, the rate of formation of the catalyst, which is promoted by chlorides, is shown to depend on the solvation of chloride.

Hydrogen storage in the carbon dioxide/formic acid system, using homogeneous iron(II)-phosphine catalysts in aqueous solution

Mickael Montandon-Clerc

The valorization of carbon dioxide and its transformation into useful chemicals is essential as mankind consumes more and more fossil fuels, thus producing equivalent wastes. The storage of hydrogen, a promising energy carrier, is also of great interest in developing a sustainable future. Here, we present our results on aqueous phase formic acid (FA) dehydrogenation reaction using non-noble metal, iron based pre-catalysts. We have synthesized the m-trisulfonated-tris[2-(diphe nylphosphino)ethyl]phosphine sodium salt (PP3TS), a water soluble polydentate ligand. The catalysts, with iron (II), were formed in situ and were active in homogeneous catalytic, selective formic acid dehydrogenation resulting in H2 and CO2 from aqueous formic acid solutions. This required no organic co-solvents, bases or any additives. Manometry, multinuclear NMR and FT-IR techniques were used to follow the dehydrogenation reactions, calculate kinetic parameters, and analyze the gas mixtures for purity. The iron (II) catalyst is entirely selective and the H2 and CO2 gas mixture is free from CO contamination. To the best of our knowledge, these represent the first examples of first row transition metal based catalysts that dehydrogenate quantitatively formic acid in aqueous solution. The reverse reaction, the direct CO2 hydrogenation using an iron (II) phosphine catalyst is also presented here. In water, using the same iron (II) catalyst precursor with PP3TS as the ligand, up to 0.5 M of formic acid can be produced without any additives, i.e. in acidic aqueous solutions. These results were obtained at room temperature and under hydrogen and carbon dioxide pressures. The system is not sensitive to oxygen or air exposure. Therefore, his carbon dioxide reduction and formic acid dehydrogenation cycle can be repeated several times without the loss of the catalyst activity. Hydrogen is regarded as one of the future energy carriers. Using this Fe(II)-PP3TS catalyst, a reversible hydrogen storage system can be realized by a battery-like charge/discharge mechanism. This allows for a clean storage of energy in the form of formic acid as well as the safe delivery of hydrogen gas to proton exchange membrane fuel cells.

EPFL2018

Switching the Mechanism of Catalyst Activation by Ionic Liquids

Corinne Daguenet, Paul Joseph Dyson

In ionic liqs. (ILs), the activation of a ruthenium(II) catalyst precursor occurs via an unexpected mechanism, which is different from the activation mechanism that takes place in water. The effectiveness of a biphasic catalytic hydrogenation reaction using the ruthenium(II) precatalyst is highly dependent on the nature of both anion and cation of the IL. Evidence is provided that chloride ions are required to generate the active catalyst. Moreover, the influence of the IL cation on the rate of hydrogenation is correlated with the differences in capability of the IL to solvate chloride. A kinetic study provides insights into the mechanism of the formation of the active catalytic species.

2006

Nanostructured Electrocatalysts for Water Splitting Reactions

Lucas-Alexandre Stern

At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.