A Viable Hydrogen‐Storage System Based On Selective Formic Acid Decomposition with a Ruthenium Catalyst

Abstract

Hydrogen represents an important alternative energy feedstock for both environmental and economic reasons, and when combined with fuel-cell technology, very efficient energy conversion can be achieved.[1] Although the advantages of hydrogen over fossil fuels are numerous, the actual use of hydrogen as a transportation fuel is limited mainly because of storage and delivery problems. Conventional hydrogen-storage methods, such as high-pressure gas containers and cryogenic liquid/gas containers, have weight and safety issues.[2] Consequently, a great deal of research is being undertaken to develop new materials, such as metal hydrides [2,3] and carbon nanostructures, [4] that store hydrogen efficiently, although no entirely satisfactory options have been found so far. Formic acid containing 4.4 wt % of hydrogen, as well as its conjugate base, formate salt, are well known sources of hydrogen [5][6][7] and have previously been reported as potential hydrogen-storage material.[8] Formic acid has the advantage over other substrates that only gaseous products are formed (H 2 /CO 2 ), hence preventing the accumulation of by-products, which is a limitation for mobile applications. However, until now potential applications have been limited by catalyst regeneration requirements, by harsh reaction conditions, and by poor selectivity. We present herein an efficient, completely selective, and robust system for hydrogen production from formic acid using water-soluble homogeneous catalysts. [9] Decomposition of formic acid was carried out in aqueous solution using hydrophilic ruthenium-based catalysts, generated from the highly water-soluble ligand meta-trisulfonated triphenylphosphine (TPPTS) with either [Ru(H 2 O) 6 ] 2+ or, more conveniently, commercially available RuCl 3 .The catalysts were activated prior to use by reaction with sodium formate and formic acid and the catalytic decomposition of formic acid performed under a wide range of pressures and temperatures. The generated H 2 /CO 2 pressure was typically between 1 and 220 bar, but no inhibition of catalytic activity was observed up to a pressure of 750 bar (see the Supporting Information for details). The rate of formic acid decomposition increased with temperature, and at all temperatures a conversion of 90-95 % can be achieved (Figure 1). The total conversion does not reach 100 % because the formate salt added for the activation of the catalyst is not converted; however, all the formic acid is consumed.[10] In addition, the catalyst was stable up to 170 8C and remained active after one year in solution.The thermal decomposition of formic acid into CO and H 2 O, which depends on the temperature and the formic acid concentration, becomes nonnegligible at elevated temperatures. [11] No traces of CO, which is known to poison some fuel cells, [12] could be detected by FTIR spectroscopy (detection limit of 3 ppm) in a sample of gases generated by using this catalyst system at 100 8C (see the Supporting Information), because of the rapidity of the reaction and, hence, the ...

Selective and mild hydrogen production using water and formaldehyde

“…Previous reports about formic acid and methanol showed that the presence of at least catalytic amounts of base is crucial for the successful dehydrogenation of the hydrogen carrier 8,15,16,18 . In contrast, we found that the dehydrogenation of methanediol derived from FA or pFA is possible also in absence of base additives ( Supplementary Fig.…”

“…Therefore, one alternative is chemical hydrogen storage in molecular materials in which the dihydrogen is stored in the form of hydridic and protic hydrogen. Examples are metal hydrides [4][5][6] , ammonia borane derivatives 2,3,7 , formic acid 2,[8][9][10][11][12][13][14][15][16][17] and methanol [18][19][20][21] . Ammonia borane derivatives have a good weight efficiency (r19.4 wt% H 2 ) 2,3,7 , but the solid spentfuels are hardly recyclable.…”

“…Ammonia borane derivatives have a good weight efficiency (r19.4 wt% H 2 ) 2,3,7 , but the solid spentfuels are hardly recyclable. Formic acid-based systems are robust 8,15,16 , but the H 2 weight efficiency is quite low (4.4 wt% H 2 ), and so far low-temperature methanol-based systems (12.5 wt% H 2 ) 18 are limited to inert conditions, as the nature of the catalysts is sensitive to air and might decompose under oxidative conditions. Moreover, the formic acid and methanol dehydrogenation require additional base, which is a drawback for the weight efficiency.…”

Decomposition of Formic Acid Catalyzed by a Phosphine‐Free Ruthenium Complex in a Task‐Specific Ionic Liquid

“…[1] In particular for portable applications, the use of liquid hydrogen has several disadvantages due to its continuous evaporation. Among various storage materials and methods currently under investigation, molecular hydrogen adsorption on materials of large surface area [5][6][7][8] and clathrate hydrates, [9] the use of bonded hydrogen atoms in hydrocarbons, [10] metal hydrides [11,12] or formic acid (FA) [13][14][15][16][17] show considerable promise.…”

Application of Ionic Liquids in Hydrogen Storage Systems

“…The application of ammonia borane and its derivatives for hydrogen storage is matter of research in the last decade as well as a compound as simple as formic acid, which is in this sense the hydrogenationproduct of carbon dioxide. (Fellay et al, 2008;Loges et al, 2008;Scholten et al, 2010;Stephens et al, 2007) The application of ammonia borane itself is more efficient than elemental hydrogen, but nevertheless accompanied by some problems as well. Since there is a worldwide infrastructure suitable for the deployment of liquid fuels, a solid fuel bears certain disadvantages competing with a liquid fuel.…”