Inexhaustible Energy Carrier Hydrogen

Methanol, water and a copper-zinc catalyst may be used to produce carbon monoxide depleted hydrogen, a power source for PEM (polymer-electrolyte-membrane) fuel cells, with high efficiency. By identifying the copper-zinc phase, which generates particularly clean hydrogen, Innsbruck scientists have cleared a hurdle for cutting-edge energy use.
Image: On a catalyst, methanol is used to produce hydrogen in a technically simple and quick way.

Hydrogen (H2) is a component of water and the most common element in the universe and, thus, seems to be a nearly inexhaustible energy carrier. However, the colorless and odorless gas is also highly flammable and, in combination with oxygen, produces a highly explosive gas mixture. To avoid technical and safety problems of hydrogen storage and transport, scientists have to find an efficient and sustainable way for using chemically bound hydrogen in power applications. Over the last few years, methanol (CH3OH) has turned out to be one of the most promising hydrogen carriers for mobile power applications: Hydrogen can be generated from this simple alcohol in a fast and technically simple way by a process called catalytic methanol steam reforming. Bernhard Klötzer’s research group has been working on optimizing the process where methanol reacts with water since 2008. “When generating hydrogen from methanol, we aim to get hydrogen (H2) of high purity. At the same time, the production of carbon monoxide has to be avoided because it poisons the fuel cell anodes, which must not happen in practical applications,” says Bernhard Klötzer about the challenges if the reforming process is to be applied to PEM fuel cells. These fuel cells may be used in future mobile applications such as in motor vehicles, battery chargers and are already used in submarines and space craft.

Practical use – a challenge

Two years ago the scientists showed ideal structural preconditions for clean hydrogen generation using a palladium zinc catalyst. Now they have studied the process with a copper zinc catalyst, which is considerably less costly. “Many systems are under discussion. From the point of view of basic research, the palladium zinc catalyst is considered the most suitable catalyst due to its thermal stability. However, it is also a very expensive choice. When we talk about practical application, not only technical questions have to be considered but also costs,“ explains Klötzer. That is why he is currently interested in methanol steam reforming on copper zinc catalysts. These catalysts, used for technical industrial applications already, were originally designed for methanol synthesis, the reverse reaction. “What works in one direction, also works, in principal, in the other direction. But considerably higher temperatures and a higher water vapor pressure are required for the reforming process and that is why the industrial methanol synthesis catalyst is not stable in the reforming application,“ explains the researcher. His research group works on the principles and preconditions under which the reforming process works in a stable and selective manner.

Amount of zinc crucial for hydrogen generation

For research purposes a model catalyst is used, which consists of a clean copper foil onto which monolayers of zinc are deposited. The amount of zinc used is crucial for hydrogen generation: “When you use too much zinc, an inactive zinc oxide layer entirely blocking the copper metal surface is produced, which we don’t want,” explains Klötzer. In collaboration with scientists at the Fritz-Haber Institute in Berlin the scientists, using the model catalyst, were able to perform in-situ photoelectron spectroscopy analysis of the surface under ‘real world’ reforming conditions at the BESSY II (Berlin Electron Storage Synchrotron - X-ray source for materials science and surface science investigations) – a decisive advantage to applied technical catalyst systems. On this basis, the researchers were able to follow the state of zinc oxidation and spatial distribution live. “Under reaction conditions, zinc oxide islands were produced on one part of the catalyst surface while a copper zinc bimetal surface still existed. At exactly this moment of coexistence, the activity of the catalyst increases by the factor 1000. At this phase, hydrogen generation is particularly efficient due to the capability of the metal-oxide interface to split water,“ explains Klötzer one of the most important results of his research work. This result presents a crucial theoretical but also practical step towards a better understanding of methanol steam reforming. Klötzer is convinced: “Catalytic water splitting, in general, is an important topic for many future applications in energy technologies.“

Bernhard Klötzer’s research group at the Institute of Physical Chemistry, University of Innsbruck, has been working on this project since 2008 and is funded by the Austrian Science Fund. They have just published their research results in the journal Angewandte Chemie International Edition.