May 08, 2005
Text: Tim Schröder
Basically, it all comes down to a little bit of powder – fine crystals in small glass tubes with plastic caps, or black dust in bulbous Erlenmeyer flasks. Hundreds of these containers are neatly arrayed on the laboratory shelves, each with a small adhesive label reading Cu/ZrO2, or Ca(AlH4)2. The sight of the laboratory is unspectacular, the tide of glass jars inconspicuously tidy. But looks can be deceptive. Without powder, there is no chemistry.
Ferdi Schüth is Director at the Max Planck Institute for Coal Research in Mülheim an der Ruhr and head of the Heterogeneous Catalysis working group. He is an acknowledged expert on the substances that make chemical reactions boil and bubble. “90 percent of chemistry is done by catalysts,” says Schüth. What he means is, hardly a single chemical process works without catalytic assistance. In some cases, it is these molecular pacemakers that activate the input materials in order to create the preconditions for a reaction. Frequently, they lower the reaction temperature, allowing the process to proceed under moderated conditions.
In other cases, they control the reaction so that fewer undesired byproducts are created. This can be decisive for the economic efficiency of chemical manufacturing. Schüth aims to develop new, effective catalysts as well as to optimize familiar substances and processes – in order, for example, to enhance the efficiency of methanol manufacturing, now that the latter is under discussion as a major source of hydrogen for fuel cells; or to simplify the synthesis of propylene oxide, an important ingredient in the production of plastics.
What is the origin of the term “heterogeneous catalysis”? It refers to the fact that catalysts exist in a different state than that of the substances which react to them. Generally, these molecular pacemakers are used in powder form, scattered into chemical solutions or exposed to a flow of gas. Their effectiveness is not solely dependent on the material composition: their structure and surface are also of importance. The larger the latter, the more room they provide for chemical reaction – allowing raw materials to be more efficiently converted. Even powders with particularly fine particles have a large surface area.
Schüth and his team therefore approach the development of their catalysts from several directions: in test tubes they culture filigree structures with vast surface areas. They create tiny, uniform hexagonal or octagonal particles a few nanometers (billionths of a meter) in size. And they remix established catalyst ingredients and test their efficiency in steel reaction vessels.
It always takes a touch of alchemy, says Schüth. Even though chemists nowadays are familiar with hundreds of catalytic substances, the mechanisms by which these work have often not yet been precisely decoded. “If you want to accelerate specific reactions, there is a whole range of known suspects,” Schüth explains. Take for example the nitrides or oxides of around 60 chemical elements. “On the other hand, for all the technical assistance at our disposal, producing the right catalysts with specific desired properties is still, to some extent, a matter of instinct.”