One has to look very closely to understand exactly what is happening on the surface of the catalyst. Solid catalysts are usually finely structured materials made of tiny crystals. There are a variety of microscopes that can monitor chemical processes on these surfaces – for example, they use ultraviolet light, X-rays or electrons. But none of the methods alone provide the complete picture.
That’s why a research team from TU Wien and the Fritz Haber Institute in Berlin has developed a new method that allows a “three-eye” approach to catalytic reactions – using three different surface microscopes. In this way, they were able to demonstrate that during the catalytic conversion of hydrogen and oxygen to water, the reaction fronts on the crystal surface not only form striking geometric patterns, but also uncover a new mechanism for the propagation of these fronts. Especially for climate-relevant technologies such as ecologically clean hydrogen energy production, a comprehensive understanding of these processes is crucial.
Take different measurements in one instrument
“Many scientific questions can only be answered by combining different microscopy methods on the same sample, which is called correlation microscopy,” says Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien. “However, this often has limitations.” You have to remove the sample from one instrument and perform the same experiment again in another microscope. Often, for methodological reasons, the experimental conditions are quite different – some measurements are made in vacuum, others in air. Usually the temperature is different. Also, you may not be looking at the same spot on the sample with different instruments – this will also affect the results. Therefore, it is difficult to combine the results of different measurements in a reliable way.
Ultraviolet, X-ray and Electron
However, it is now possible to combine three different microscopes to examine the same spot on the same sample under the same environmental conditions. Three different electron microscopes were used: two different variants of photoelectron microscopy (PEEM), UV-PEEM and X-PEEM, and low energy electron microscopy (LEEM).
In UV-PEEM and X-PEEM, the sample surface is irradiated with UV light and X-rays, respectively. In both cases, it causes electrons to be emitted from the surface. Similar to how a beam of light is focused in an optical microscope, the electron beam forms a real-time image of the surface and the processes taking place there. In X-PEEM, the emitted electrons can also be filtered based on their energy to determine the chemical composition of the sample surface. The Berlin Synchrotron (HZB BESSY II) provided the research team with the necessary high-energy, high-intensity X-rays. In LEEM technology, the surface is irradiated with an electron beam. Electrons backscattered from the surface create real-time images of the sample surface and ongoing processes such as catalytic reactions.
Professor Yuri Suchorski, who has been working on surface microscopy since 1974, said that since all three microscopes use different imaging mechanisms, different aspects of catalytic hydrogen oxidation can be studied at sites with the same structure of the sample. “Furthermore, the X-PEEM technique provides chemical contrast and therefore allows us to correlate pattern formation on the surface with the chemical composition of the surface and the reactants present on the surface, hence the term correlation microscopy.”
Watch how hydrogen oxidizes into water
Therefore, it is possible to study in real time hydrogen oxidation on well-defined microscopic regions of rhodium foil structure (structure determined by researchers at USTEM at TU Wien).
The reaction spread across the surface like a wave, revealing a new type of pattern formation never encountered before. “In front of the diffusion reaction front, new small islands of catalytically active regions are formed, accelerating the propagation of the reaction,” says Professor Rupprechter. In computer simulations that provided virtual reaction microscopy, the team was able to simulate and explain the formation of these islands.
With related methods, the specific intensities (spatial and energy resolution, field of view, magnification to the nanometer range) of each microscopy method can now be efficiently utilized to image ongoing catalytic reactions in unprecedented detail.
The oxidation of hydrogen to water via solid catalysts is one of the important processes that can generate energy without combustion and pollution (the exhaust gas consists of pure water), for example in fuel cells. For the future development of new green energy production technologies, it is important to observe the ongoing catalytic reactions with multiple eyes to gain insight into the details of the catalytic process.