Researchers Apply Nanotechnology to Explain Unresolved Processes Behind Catalysis

The fact that catalytic processes are employed for producing nearly 80% of products in the chemical industry makes catalysis highly obligatory for energy conversion and treatment of exhaust gases. Moreover, to save time, to conserve resources, and to protect the environment, it is extremely important that these processes are carried out in a fast and efficient manner. Testing of new substances and arrangements that can give rise to new and better catalytic processes is continually ongoing in this field. Recently, a new technique has been developed by a research team from the Paul Scherrer Institute (PSI) in Villigen and ETH Zurich to enhance the accuracy of such experiments, thus enabling the speeding up of the search for optimal solutions. Simultaneously, their technique has provided a solution to an almost 50-year-old scientific controversy. Their approach has been reported in the journal Nature.

The new process discovered by the Swiss scientists has enabled the chemical industry to easily investigate and optimize the catalytic processes.

We have found a way to construct catalytic model systems—that is, experimental set-ups—accurate to one nanometre and then to track the chemical reactions of individual nanoparticles. This makes it possible to selectively optimise the efficiency of catalytic processes.

Waiz Karim, affiliated with the Institute for Chemical and Bioengineering at ETH Zurich as well as the Laboratory for Micro and Nanotechnology at the PSI.

Catalysis is a fundamental chemical process in which reactions of substances are triggered or accelerated by using a catalyst. It has a crucial role in the treatment of exhaust gases; in the production of acids, synthetic materials, and other chemical products; and in energy storage. This is the main reason why the industry shows a great intent toward optimization of catalytic processes. “To do that, you need a deeper understanding of what is going on at the molecular level,” stated Jeroen van Bokhoven who led the study. He is professor of Heterogeneous Catalysis at ETH Zurich as well as head of the Laboratory for Catalysis and Sustainable Chemistry at the PSI.

Model experiment with unprecedented precision

The new approach provides an in-depth knowledge: The research team developed a model system that enables them to study catalysis in the most minute detail. The experimental part was mainly conducted at the PSI, and the team worked on theoretical basis at ETH Zurich. The team of Karim and van Bokhoven used iron oxide for the model experiment, where iron oxide was converted to iron by adding hydrogen and by using platinum as the catalyst. The addition of platinum splits molecular hydrogen, H₂, into elemental hydrogen, H, which readily reacts with iron oxide.

The salient feature of their model is the use of state-of-the-art electron-beam lithography (chiefly used only in semiconductor technology). This enables the researchers to position miniscule particles on a support, where each particle included only a few atoms. The iron oxide particles had a size of only 60 nm. The size of the platinum particles was even smaller at 30 nm, i.e. about two-thousandths of the diameter of a human hair. These particles were positioned in pairs on a grid-like model at 15 different distances from each other. In the first grid segment, the platinum particle was positioned accurately on top of the iron oxide particle. In the 15th segment, the particles were positioned 45 nm from each other. In the 16th segment, the iron oxide was positioned entirely alone. “Thus we were able to test 16 different situations at once and control the size and spacing of the particles with one-nanometre accuracy,” explained Karim. Then, the model was vaporized with hydrogen and observations were made.

For performing observations at the molecular level, in an earlier project, the team had employed a technique known as “single-particle spectromicroscopy” to analyze minute particles by using X-rays. The instruments required to perform the observations are available at the Swiss Light Source SLS of the PSI, which is an expansive research facility that generates high-quality X-ray light. Not only is the accuracy of positioning the particle new, but the corresponding precise observation of chemical reactions (which includes simultaneous observation of a number of particles in different situations) was not possible earlier: “In previous studies, placement of the nanoparticles of two different materials could be off by up to 30 nanometres,” explained Karim.

Distance-dependent spillover of hydrogen

Nevertheless, certain chemical phenomena occur on an even smaller scale. One among these is the so-called hydrogen spillover effect, which was examined by the PSI and ETH researchers with the new model.

The effect unquestionably contributes to the efficiency of catalysis carried out using hydrogen. This fact was found out in 1964. However, a detailed interpretation or visualization was not possible till date. Consequently, the conditions under which it really occurs have been controversial.

As a maiden attempt, the team headed by Karim and van Bokhoven was victorious in analyzing the required precision: When the hydrogen molecules encountered the platinum particle, they split at once; consequently, the elemental hydrogen flows down the sides onto the support material, and spread around, the same manner in which water streams out of a spring. According to the researchers, the hydrogen atoms encounter the iron oxide particles, “reducing” them to iron. “We were able to prove that how far the hydrogen flows depends on the support material,” reported Karim. The farther hydrogen flows, the higher the contribution of the spillover to the catalysis. For instance, in case of presence of aluminum oxide in the support, which itself is not reduced, the hydrogen flows not beyond 15 nm. On the contrary, in case of reducible titanium oxide, it flows over the entire surface. “Clearly, for some support materials it is important how tightly the particles sit on them.”

Advancing chemical science

Consequently, the new nanotechnology process developed by the PSI and ETH researchers has clarified the conditions under which the hydrogen spillover effect occurs.

Our method rests on three pillars. The nanofabrication of the model system, the precise measurement of the chemical reactions, and last but not least the theoretical modelling: In accordance with the experiments we were able to describe the process down to the molecular level. He further suggested that this can pave the way for expansive discoveries overall in the field of chemical science: With this we are opening up a whole new dimension for the investigation and understanding of catalytic processes. And with this understanding, industrial production processes can be optimised in a much more targeted way.

Jeroen van Bokhoven