Spatial-temporal phenomena on surface structure libraries

Catalytic hydrogen oxidation on noble metals is a technologically important surface reaction, particularly for fuel cells, catalytic heat production, elimination of hydrogen via catalytic recombination, and hydrogen sensors. Despite its importance for future technologies, the molecular-level details and the structure- sensitivity of this reaction still hold many unanswered questions. In the present project, the hydrogen oxidation reaction will be studied on the micrometer-scale and nanometer-scale using surface microscopies: photoemission electron microscopy (PEEM) and field electron/ion microscopy (FEM/FIM). As catalysts, polycrystalline rhodium foils consisting of micrometer-sized crystalline grains will be used. Such grains are usually randomly oriented on the foil surface presenting different crystallographic structures. Therefore, such a mosaic-like surface can be considered as surface structure library and is well suited for revealing the role of surface structure in a catalytic reaction. The ongoing reaction will be visualized in real time by PEEM and recorded digital video-files will be computer-processed in order to obtain kinetic data (kinetics by imaging approach). Employing this approach, we have recently discovered a new phenomenon in catalytic hydrogen oxidation: multifrequential oscillations on micrometre-sized grains of polycrystalline rhodium. The reaction oscillates in a self-sustaining way, but each grain has its own frequency. Detection of this effect has opened a new research field but has also raised a series of open issues, which will be elucidated in the present project. It appeared, e.g., that grain boundaries serve as frequency transformers and oxygen diffuses during the oscillations under the topmost surface rhodium layer forming subsurface oxygen. Formation and depletion of such subsurface oxygen layer governs the oscillation of the reaction, playing the role of a feedback mechanism. To understand the function of this mechanism in detail and to reveal the role of surface structure represent the main goals of this project. Apart from the micrometer-sized grains of a polycrystalline foil, the reaction will be studied on nanometer-sized apexes of extremely sharp rhodium tips. Such half-spherically shaped apexes resemble the nanoparticles of industrially used catalysts. Using FEM/FIM, the reaction will be visualized in real time with nanometer resolution and using the kinetics by imaging approach, kinetic data will be obtained. Comparison of the data obtained on a micrometer and nanometer scale will provide insights into the size effect in this societally important catalytic reaction.

Catalytic hydrogen oxidation on noble metals is a technologically important surface reaction, particularly for fuel cells, catalytic heat production, elimination of hydrogen via catalytic recombination, and hydrogen sensors. Despite its importance for future technologies, the molecular-level details and the structure-sensitivity of this reaction still hold many unanswered questions. In the present project, the hydrogen oxidation reaction was studied on the micrometer-scale and nanometer-scale using surface microscopies: photoemission electron microscopy (PEEM) and field electron/ion microscopy (FEM/FIM). As catalysts, polycrystalline rhodium foils consisting of micrometer-sized crystalline grains were used. Such grains are usually randomly oriented on the foil surface presenting different crystallographic structures. Therefore, such a mosaic-like surface can be considered as surface structure library and is well suited for revealing the role of surface structure in a catalytic reaction. The ongoing reaction was visualized in real time by PEEM and recorded digital video-files were computer-processed in order to obtain kinetic data ("kinetics by imaging" approach). Employing this approach, we have discovered a new phenomenon in catalytic hydrogen oxidation: multifrequential oscillations on micrometre-sized grains of polycrystalline rhodium. The reaction oscillates in a self-sustaining way, but each grain has its own frequency. Detection of this effect has opened a new research field but has also raised a series of open issues, which were elucidated in the present project. It appeared, e.g., that grain boundaries serve as frequency transformers and oxygen diffuses during the oscillations under the topmost surface rhodium layer forming subsurface oxygen. Formation and depletion of such subsurface oxygen layer governs the oscillation of the reaction, playing the role of a feedback mechanism. To understand the function of this mechanism in detail and to reveal the role of surface structure were major achievements of this project. Apart from the micrometer-sized grains of a polycrystalline foil, the reaction was studied on nanometer-sized apexes of extremely sharp rhodium tips. Such half-spherically shaped apexes resemble individual nanoparticles of industrially used catalysts. Using FEM/FIM, the reaction was visualized in real time with nanometer resolution and using the kinetics by imaging approach, kinetic data were obtained. Comparison of the data obtained on a micrometer and nanometer scale provided insights into the size effect in this technically important catalytic reaction. The development of correlative microscopy for the observation of coupling phenomena and the promotion of catalytic surface reactions, which even showed chaotic states, were further "highlights" of this very successful project.