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 ...

Monitoring of Sonochemically Released Carbon Dioxide from a Methanol-Water System

“…It is also well documented that CO 2 and its derivatives such as bicarbonate (HCO 3 − ) and carbonic acids (H 2 CO 3 ) are readily formed from HCO 2 H via the following steps (1) HCO 2 H  H 2 + CO 2 (decarboxylation) and (2) HCO 2 H + (O)  H 2 CO 3 (oxidation). However, the former path proceeds only minimally in the absence of catalysts such as platinum or ruthenium [13], the latter path involving hydrogen abstraction by HO˙ is the likely route for production of CO 2 in our system as predicted elsewhere [14]. Such a view is quite consistent with our present data in support of the involvement of HO˙ in USW-driven MeOH-derived CO2 production ( Figure 4).…”

Formic Acid Dehydrogenation on Au‐Based Catalysts at Near‐Ambient Temperatures

“…[1][2][3][4] Recent studies have shown that HCOOH decomposition is catalyzed with Ru-based complexes in the aqueous phase at near-ambient temperatures. [5,6] HCOOH decomposition reactions are used frequently to probe the effects of alloying and cluster size and of geometric and electronic factors in catalysis [7][8][9][10] . These studies have concluded that Pt is the most active metal for HCOOH decomposition, at least as large crystallites and extended surfaces [9] .…”

Dehydrogenation of Formic Acid over a Homogeneous Ru-TPPTS Catalyst: Unwanted CO Production and Its Successful Removal by PROX

“…Moreover, FA dehydrogenation gas mixtures contain much more CO2 (~50%) which could possibly effect the conversion of CO. Recently, FA has sparked interest again, because catalysts highly effective in FA dehydrogenation in reasonable conditions have been found [5][6][7][8][9]. This fact in combination with easy and safe handling opens a route of using FA in applications such as energy storage and transport [10].…”

“…Although very high catalyst selectivities have been reported in literature [7,9,11], a side-reaction of FA dehydration takes usually place, next to FA dehydrogenation, due to a still insufficient catalyst selectivity and/or thermal decomposition of FA:…”