New low-temperature catalytic process for producing hydrogen from methanol; potential future application for fuel cell vehicles
|(a) Schematic pathway for a homogeneously catalyzed methanol reforming process via three discrete dehydrogenation steps. (b) Best performing catalysts. Nielsen et al. Click to enlarge.|
Researchers from Germany and Italy have developed an efficient low-temperature catalytic process to produce hydrogen from methanol. Hydrogen generation by this method proceeds at 65–95 °C (149-203 °F) and ambient pressure with excellent catalyst turnover frequencies (4,700 per hour) and turnover numbers (exceeding 350,000). This could make the delivery of hydrogen on mobile devices—and hence the use of methanol as a practical hydrogen carrier—eventually feasible, the team suggests in a paper published in the journal Nature.
One of the challenges to hydrogen fuel cell vehicles is the efficient on-board storage of adequate amounts of the hydrogen gas required for fuel cell operation due to the properties of the gas. Methanol conceptually is an interesting alternative, as it is a liquid at room temperature (easier transportation and handling) and contains 12.6% hydrogen. However, current methanol reforming technologies for the production of hydrogen are conducted at high temperatures (> 200 °C) and high pressures (25–50 bar), limiting potential mobile applications of “so-called reformed methanol fuel cells”, they note.
...the state-of-the-art efficiency of reformed methanol fuel cells is only approximately 40%. Moreover, very low concentrations of carbon monoxide are tolerated in the fuel cell (< 10 parts per million, p.p.m.), which is challenging for current catalytic systems. In general, methanol reforming occurs in the steam phase using heterogeneous catalysis and high temperatures.
...Our long-term interest in efficient low-temperature hydrogen generation led us to identify active molecular-defined catalysts for a low-temperature (< 100 °C) aqueous-phase methanol dehydrogenation process. We envisioned a one-pot stepwise process including initial dehydrogenation of methanol to hydrogen and formaldehyde, water-promoted dehydrogenation of formaldehyde to formic acid and hydrogen, and final dehydrogenation of formic acid to hydrogen and carbon dioxide. This should lead to an overall yield of three molecules of hydrogen and one of carbon dioxide, thus resembling classic methanol reforming.—Nielsen et al.
The researchers’ efficient low-temperature aqueous-phase methanol dehydrogenation process is facilitated by ruthenium complexes—a central ruthenium clamped by a nitrogen and two phosphoruses, themselves attached to organic groups.
All experiments on the catalysts were performed under inert atmosphere (argon) with exclusion of air. A solution of MeOH and H2O in a given ratio, containing a defined amount of base, was heated to a certain temperature and let equilibrate for 30 min. Then an amount of one of five catalysts was added to the solution.
The team found that several factors can significantly affect the catalyst activity: the nature of the base (which mediates the initial reforming sequence), its concentration, the water content, and the temperature. They found that 8.0 M potassium hydroxide (KOH) gave optimal results in a 4:1 MeOH/H2O solution.
The optimized system is stable under aqueous alkaline conditions and remains active for more than three weeks. Once the relative concentrations of the reagents and possible intermediates competing for the catalyst have reached suitable steady-state values, the ratio of H2 to CO2 expected for MeOH aqueous reforming (H2/CO2 3:1) is observed. Moreover, this system represents full conversion of all ‘available’ hydrogen atoms in the substrates to hydrogen by homogeneous catalysis. An intrinsic drawback is the base needed for this reaction to stay active. Nevertheless, using this system it might be possible to combine the advantages of methanol as the ‘hydrogen carrier’ and the superior efficiency of proton-exchange membrane fuel cells compared to methanol fuel cells.—Nielsen et al.
Martin Nielsen, Elisabetta Alberico, Wolfgang Baumann, Hans-Joachim Drexler, Henrik Junge, Serafino Gladiali & Matthias Beller (2013) Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature doi: 10.1038/nature11891