A Viable Hydrogen Storage and Release System Based on Cesium Formate and Bicarbonate Salts: Mechanistic Insights into the Hydrogen Release Step

Andrew Francis Dalebrook, Gabor Laurenczy, Katerina Stefania Sordakis
Wiley-VCH Verlag Berlin, 2015
Article

Résumé

Aqueous solutions of cesium formate and bicarbonate repre- sent an effective hydrogen storage–delivery couple that under- goes either release or take up of hydrogen in the presence of {RuCl2(mTPPTS)2}2 (TPPTS = triphenylphosphine trisulfonate) and excess mTPPTS ligand, with no other additives required. Cesium salt solutions offer the advantage of improved volu- metric and gravimetric H2 density compared to their sodium and potassium analogs, owing to their high water solubility. Details of the equilibrium between formate and bicarbonate, which constitutes an important parameter for the applicability of this H storage/release cycle, were determined. H production is readily tunable by controlling the operating pressure. This behavior was also rationalized through the identification of catalytic intermediates under various conditions. High con- centration formate and bicarbonate solutions were used during the tests and the bidirectional catalytic system could be recycled without loss of activity or replacement of solvent. A tentative mechanism is proposed for the formate dehydrogen- ation step. Among the identified hydride species, the penta- coordinated [RuH(H2O)(TPPTS)3] complex was indispensable for promoting the formate dehydrogenation reaction.

Official source

https://infoscience.epfl.ch/record/210495?ln=fr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés

Chargement

Publications associées

Chargement

Securing our energy future while minimizing the associated environmental impacts is a challenging endeavor and the fundamental aspect of sustainable development. The choice of energy resource and the reduction of greenhouse gas emissions, primarily carbon dioxide (CO2), are key parameters in this process. Hydrogen gas (H2) is an energy carrier with the potential of "green" production and utilization, in addition to having attractive inherent fuel properties. However, complications during its storage arise from its intrinsically light nature. Chemical H2 fixation within liquid formic acid (FA) and its derivatives is therefore a promising approach. In the present dissertation, reversible H2 storage cycles based on homogeneous FA/formate dehydrogenation and CO2/bicarbonate hydrogenation, as well as the related CO2 transformation to methanol are investigated. The first chapter provides an overview of challenges underlying energy sustainability, prospects for integration of H2 with our energy infrastructure, the employment of FA as a liquid organic hydrogen carrier and state of the art homogeneous catalysts for selective FA dehydrogenation and CO2 hydrogenation. The combination of these reactions to form a closed CO2 loop is the basic concept behind a reversible H2 storage system as it is envisaged by our group. In the second chapter, FA dehydrogenation/formation in the presence of triethylamine is discussed. The results are summarized as equilibrium positions within this couple, which can be controlled by adjustment of the operating pressure and temperature. The basic additive promotes H2 fixation but also allows for on-demand H2 release from a triethylammonium formate substrate. The third chapter addresses H2 storage in basic media, i.e. aqueous bicarbonate hydrogenation, with a Ru(III) catalyst precursor and a number of water-soluble phosphine ligands. The latter are introduced to tailor the exhibited activity by affecting the steric and electronic parameters of the catalyst. All phosphines provide high bicarbonate conversions, albeit with low reaction rates. Chapter four summarizes the optimization and mechanistic studies of the reversible formate dehydrogenation reaction, in the presence of a Ru(II)-mTPPTS catalyst. Cesium is chosen as the cation for the formate/bicarbonate salts because it leads to their high solubility in the aqueous solvent and therefore an increase in the hydrogen storage capacity. The involved equilibria are determined and a reaction mechanism for the formate dehydrogenation step is proposed, based on NMR studies. Advantages of the presented approach include the utilization of water as the solvent, the absence of CO2 throughout the reactions and the successful in situ recycling of the environmentally benign hydrogen carrier. In Chapter five the feasibility of producing both FA and methanol directly from CO2, at room temperature, in a "one-pot" reaction using aqueous solvent is demonstrated. Formic acid formation occurs in water without additives or organic solvents with a homogeneous iridium catalyst. Formic acid also undergoes disproportionation into methanol, with the same complex. Under optimized conditions, FA conversions of 98% and methanol selectivities of 96% are achieved in the disproportionation reaction. These reactions are relevant to the field of sustainable H2 storage, but might also provide an alternative approach to the commercial fossil-fuel-dependent production of formic acid and methanol.

EPFL2016

Chargement

The reactions of (eta(6)-C6H6)RuCl2 and (eta(6)-p-cymene)RuCl2 With hydrogen in the presence of the water-soluble phosphines tppts (meta-trisulfonated triphenylphosphine) and pta (1,3,5-triaza-7-phosphaadamantane) afforded as the main species (eta(6)- C6H6)RuH(tppts)(2), (eta(6)-C6H6)RuH(pta)(2), (eta(6)-p-cymene)RuH(tppts)(2) and (eta(6)-p-cymene)RuH(pta)(2). This latter complex was also formed in the reaction of [(eta(6)-p-cymene)RuCl2(pta)] and hydrogen with a redistribution of pta. In addition, prolonged hydrogenation at elevated temperatures and in the presence of excess of pta led to the formation of the arene-free [RuH(pta)(4)CI], RuH(pta)(4)(H2O), [RuH2(pta)(4)] and RuH(pta)(5) complexes. Ru-hydrides, such as (eta6-arene)RuH(L)(2), catalyzed the hydrogenation of bicarbonate to formate in aqueous solutions at p(H-2) = 100 bar, T = 50-70 degreesC. (C) 2004 Elsevier B.V. All rights reserved.

2004

Chargement

Homogeneous catalytic decomposition of formic acid for high pressure hydrogen generation

Céline Fellay

The main obstacles in the widespread use of hydrogen as energetic alternative to fossil fuels are storage problems intrinsically linked to its extremely light and explosive nature. Since no ideal storage systems currently exist, reversible fixation of hydrogen is a very active field of research. Formic acid has previously been proposed as hydrogen storage material using heterogeneous catalysts to decompose it. These systems, however, lose activity over time and/or have low selectivity. Formic acid decomposition has been revisited during the course of this thesis in attempt to provide a catalytic system that efficiently and selectively decomposes formic acid into H2 and CO2. Accordingly, a homogeneous catalytic system to decompose formic acid in aqueous phase has been developed. The catalytically active species are formed in situ from [Ru(H2O)6]2+ or RuCl3, and sulfonated phosphine ligands. The decomposition takes place under mild conditions, over a large range of temperatures (25-170°C) and pressures (1-750 bar, at least). The robustness of the catalytic systems allows a very large number of catalytic runs to be performed without any deactivation, hence, continuous hydrogen production could be achieved. Mechanistic investigations allowed the characterisation of several intermediates and a catalytic cycle has been proposed. The selective catalytic process developed affords hydrogen essentially free of CO, at desired rates and pressures, overcoming previous limitations in the use of formic acid as a hydrogen storage material. The research undertaken in this thesis is presented in three parts: (i) development and optimisation of the catalytic system in batch mode, (ii) mechanistic investigations on the optimised catalysts, and (iii) development of a continuous hydrogen production system.

EPFL2008

Chargement

EPFL2016