On the Promotion of Catalytic Reactions by Surface Acoustic Waves

• Published in: Angewandte Chemie International Edition 2020-11, Vol.59 (45), p.20224-20229
• Main Authors: Boehn, Bernhard, Foerster, Michael, Boehn, Moritz, Prat, Jordi, Macià, Ferran, Casals, Blai, Khaliq, Muhammad Waqas, Hernández‐Mínguez, Alberto, Aballe, Lucia, Imbihl, Ronald
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Catalytic activity; Decomposition reactions; Deformation effects; Elastic deformation; heterogeneous catalysis; LEEM; Nanotechnology; Oxidation; PEEM; Photoelectrons; Platinum; Saws; Selectivity; Strain; Surface acoustic waves; Surface temperature; Thermal diffusion; work function; Work functions
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202005883

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis, sensor and nanotechnology. SAWs were shown to cause a strong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However, the promotion mechanism has not been unambiguously identified. Using stroboscopic X‐ray photoelectron spectro‐microscopy, we were able to evidence a sub‐nanosecond work function change during propagation of 500 MHz SAWs on a 9 nm thick platinum film. We quantify the work function change to 455 μeV. Such a small variation rules out that electronic effects due to elastic deformation (strain) play a major role in the SAW‐induced promotion of catalysis. In a second set of experiments, SAW‐induced intermixing of a five monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be a major cause for catalytic promotion. Surface acoustic waves (SAWs) are dynamic lattice distortions, which can be excited on piezoelectric materials. For many years a promotion of the catalytic activity and selectivity by SAWs has been known, but no mechanistic understanding has been established yet. We quantify the effect that SAWs have on the electronic and geometric structure of a model catalyst, providing a basis for discussing the promotion mechanism of SAWs in catalytic surface reactions.