Publication

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

Mickael Montandon-Clerc
2018
EPFL thesis
Abstract

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.

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Related concepts (35)
Hydrogen storage
Several methods exist for storing hydrogen. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of ammonia. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs.
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be represented as . The word aqueous (which comes from aqua) means pertaining to, related to, similar to, or dissolved in, water. As water is an excellent solvent and is also naturally abundant, it is a ubiquitous solvent in chemistry.
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