Reducing the world’s dependence on fossil fuels sits high on the short list of global research priorities. One strategy calls for powering vehicles with fuel cells that use hydrogen extracted from methanol to generate electricity. But the extraction requires high temperatures, and that limits efficiency. Those limits soon may be loosened, however, thanks to a new catalyst that drastically lowers the temperature required to liberate hydrogen.
Methanol is an attractive energy carrier because it is rich in hydrogen, a fuel that does not produce greenhouse gases upon oxidation. In addition, methanol remains liquid over a wide temperature range, allowing it to be handled by manufacturers, shippers, and motorists via existing infrastructure developed for gasoline and other liquid petroleum products.
A key barrier to exploiting methanol in this way—often referred to as the methanol economy—is the high energy input required by methanol converters, also known as reformers. These devices vaporize liquid methanol and react the vapor with solid catalysts to generate the hydrogen supply for the fuel cell. The reformers typically work best at high pressures and at temperatures well above 200 °C.
Martin Nielsen, Elisabetta Alberico, and Matthias Beller of Rostock University in Germany and coworkers now report that they have developed soluble ruthenium-based metal-organic compounds that readily catalyze hydrogen extraction from water-methanol mixtures at temperatures below 100 °C (Nature, DOI: 10.1038/nature11891).
On the basis of NMR spectroscopy and other analytical methods, the team found that the active form of the catalyst is generated in strongly alkaline solutions from a catalyst precursor. In the resulting pincer complex, ruthenium is bound to nitrogen and phosphorus atoms that form a tridentate ligand.
Hydrogen liberation takes place in a multistep process in which water serves as a reactant. First, the active form of the catalyst forms a complex with methanol, then it frees a hydrogen molecule, momentarily generating formaldehyde. Formaldehyde forms a diol-type species that loses another hydrogen molecule as it transforms to a formate intermediate. That species sheds CO2 and a hydrogen molecule, the third one produced. The last step regenerates the active catalyst.
The researchers evaluated catalyst performance under a variety of conditions. In one test on a 9:1 methanol-water mixture in strongly basic solution, they found that 1.8 ppm of the catalyst generated 2,700 equivalents of hydrogen per hour per equivalent of catalyst at 91 °C—which is considered a good level of efficiency. The value rose to 4,700 in tests on pure methanol.
In tests on less alkaline 4:1 methanol-water mixtures, which are closer to conditions needed for real-world fuel-cell applications, the team observed that the hydrogen production value decreased during the initial stages of the experiment to just 800 equivalents. They point out, however, that the gas output and catalyst system remained stable over the course of the next three weeks.
Describing the work as “a seminal finding,” University of Toronto chemistry professor Douglas W. Stephan, a specialist in organometallic chemistry, remarks that the study demonstrates “the viability of soluble catalysts for efficient and long-lived generation of hydrogen from methanol.” Chemists will now certainly apply their considerable skills in designing solution-phase catalysts to further improve these systems, he adds.
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