German researchers improve catalyst for steam reforming of methanol with salt coating; enabler for renewable energy storage systems
Researchers at the University of Erlangen-Nürnberg (Germany) report in the journal Angewandte Chemie their development of an enhanced platinum catalyst for the steam reforming of methanol to release hydrogen.
A central problem of renewable energy technology lies in the great variation of energy generated (i.e., intermittency). One proposed solution is methanol-based hydrogen storage. In this scenario, excess renewable electricity can be used to electrolyze water to produce hydrogen. The hydrogen, in turn, is then reacted with carbon dioxide to make methanol and water, thus allowing it to be stored as a liquid. The hydrogen can be released from the methanol at a later time to power a fuel cell.
This regeneration of hydrogen is achieved through the steam reforming of methanol, which is essentially a reversal of the methanol forming reaction. In this reaction it is necessary to avoid the formation of carbon monoxide because even the smallest traces of CO would poison the catalysts used in fuel cells.
Better catalysts are needed to allow the reforming reaction to work effectively and selectively under decentralized conditions in smaller reactors at the lowest possible temperatures.
To complete the storage cycle, more efficient catalyst systems for MeOH steam reforming are of great technical interest. Such decentralized hydrogen production from MeOH would greatly benefit from high catalyst selectivity towards H2 and CO2 at the lowest possible temperature. Formation of CO has to be avoided as much as possible, because CO acts as a strong poison for almost all fuel cell catalysts. Low temperature activity is highly desirable to leverage heat integration potentials between the endothermic steam reforming and the exothermic fuel cell operation.
Currently, the reported limit for high temperature PEM (proton exchange membrane) fuel cell systems is 180 °C. This is a temperature level too low for operating the MeOH steam reforming reaction with high hydrogen yields.
...Herein, we report that Pt on alumina catalysts exhibit an exceedingly enhanced activity and selectivity after surface modification with a thin-film coating of hygroscopic and basic inorganic salts.—Kusche et al.
A team headed by Peter Wasserscheid and Jörg Libuda at the University of Erlangen-Nürnberg developed an improved catalyst using platinum nanoparticles deposited onto an aluminum oxide support. Most importantly, the surface is coated with a thin film of basic salts—a mixture of lithium, potassium, and cesium acetate. Liquid salts have very low vapor pressures so that even under the conditions used for a continuous reaction in the gas phase, they remain on the surface of the catalyst.
The salt coating is so effective because the solubility of the resulting hydrogen in these salts is very low, so that it is rapidly removed from the reaction zone. In addition, the salt is hygroscopic, meaning that it attracts water, and therefore makes the water, which is required for the reaction, readily available at the active sites on the catalyst.
The alkali ions also cause the reactants to bind more strongly, while the basic properties of the salt increase the selectivity for CO2.
The coated catalyst has a significantly higher catalytic activity than the uncoated material, and a very significant increase in selectivity toward carbon dioxide to over 99 %.
According to our spectroscopic findings, alkali doping by potassium species certainly plays an important role but additional contributions from the hygroscopicity and basicity of the salt were also found. We anticipate that the modification of classical heterogeneous catalysts by molten salt coatings can be used in the future as a rational and general approach to optimize heterogeneous catalysts through surface modification or co-adsorption effects. Further exploring the potential of this method should cause advances in molten-salt chemistry, surface science, catalyst preparation, and reaction engineering.—Kusche et al.
Kusche, M., Enzenberger, F., Bajus, S., Niedermeyer, H., Bösmann, A., Kaftan, A., Laurin, M., Libuda, J. and Wasserscheid, P. (2013), Enhanced Activity and Selectivity in Catalytic Methanol Steam Reforming by Basic Alkali Metal Salt Coatings. Angew. Chem. Int. Ed.. doi: 10.1002/anie.201209758