Graphene helps study catalysis: bridging the pressure gap

Catalysis is a process in which an added catalyst enhances the rate of a reaction, making chemical reactions quicker and less costly. Catalysis is a supporting pillar of today's industry, playing a role in 90% of all chemical production. Generating about a trillion USD in products worldwide, catalysis is used in energy processing (petroleum refining, steam reforming, automotive catalytic conversion, fuel cells), bulk and fine chemical manufacturing, food processing, and environmental protection. It has been known for a while that graphene can be used as the catalyst itself, however a new role for graphene in studying catalysis has recently emerged.

A key challenge in understanding catalysis is in-situ characterization of the surfaces involved, most commonly solid/gas interfaces. The surfaces change due to the reaction, and understanding their microscopic properties can yield important information about the reaction process itself. There has until now been a distinct lack of experimental techniques to measure surface structure in realistic conditions, i.e. at atmospheric pressures and above, as the reaction takes place – all measurement methods worked at low pressures, meaning that reaction results could be studied only before and after the reaction. This limitation has become known as the “pressure gap” in the catalysis community.

A novel method uses graphene in an atmospheric pressure X-ray photoelectron spectroscopy (XPS) device to bridge the pressure gap. This approach is based on separating the vacuum environment from the high-pressure environment by a silicon nitride grid that contains an array of micrometer-sized holes coated with a bilayer of graphene. X-rays are directed at the target material, causing electron emission from the material surface. Electron spectroscopy is performed on the ejected electrons to gain high resolution information about structural and chemical changes.

Figure: a) A sketch of the experiment; b) SEM image of bilayer graphene on silicon-nitride membrane with holes.

In a standard XPS device, the entire measurement is performed in moderate to high vacuum, to avoid loss of electrons to collisions with air molecules. In the atmospheric XPS setup, that is still the case, but a thin membrane of graphene separates the vacuum chamber from the atmosphere, while still allowing X-rays and electrons to penetrate and interact with the sample that lies on the atmospheric side of the experiment. The instrument was developed in a partnership of Graphenea with the Max Planck Institute for Chemical Energy Conversion and the Fritz-Haber-Institute in Germany, and the University of Cambridge in the UK. The instrument is described in full in the May issue of “Review of Scientific Instruments”, showcasing three examples of successful operation: the oxidation/reduction reaction of iridium and copper nanoparticles and with the hydrogenation of propyne on Pd black catalyst. The graphene bilayer was grown and transferred onto the silicon-nitride membrane at Graphenea.