The enhancement of catalytic activity (i.e. how certain materials help chemical reactions to take place easier, faster, more directed or under more desirable conditions like lower temperature) by surface acoustic waves (SAW) is an established phenomenon, but its mechanism is still not well understood. Previous experiments showed that the electronic work function of model catalyst (e.g. Platinum, Pt) change within seconds to minutes when SAW are applied. This work function change was thought responsible for the SAW induced catalytic enhancement.
"Now, with our experiments, we can resolve at the nanosecond, scale work function variations caused by SAW", say researchers Ferran Macià (UB) and Blai Casals (ICMAB).
To investigate this hypothesis of an electronic effect being responsible for the catalytic enhancement, researchers from the Leibniz University Hannover, the ALBA Synchrotron, the Paul Drude Institut (Berlin), the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) an the Universitat de Barcelona (UB) performed two experiments. First, they measured the work function change in Pt caused by the elastic deformation of the SAW. This means they measured the direct and instantaneous (nanosecond scale) effect of the SAW itself on the electronic structure of the Pt model catalyst. The measured value was around 450 microelectronvolt in amplitude -which is a fantastic result considering how difficult it is to measure such a small value-, which is in pretty good agreement with preceding first principle theoretical calculations. This value is so small that it can be excluded a direct electronic effect being the reason for the promotion of catalysis by SAW.
Alternatively they propose that the SAW cause a surface roughening through some atomic or defect displacement. This roughening is in the researchers opinion responsible for the catalytic enhancement and can explain the work function changes described in the literature, which occur on the time scale of seconds to minutes. A work function change caused by surface roughening is known as the Smoluchowski effect. In the second experiment, it was observed that in a bimetallic system (Pt and Rhodium, Rh) atomic interdiffusion starts at a nominally lower temperature when SAW are applied, compared to pure thermally induced diffusion. However, in the end they found that the SAW cause a very surface specific heating, and therefore the interdiffusion is mainly, most likely caused by this additional heating. According to their knowledge this is the first time the atomic interdiffusion by SAW in this context is studied, although the heating effect of the SAW has been known before.
The present and proposed hypothesis to explain SAW induced promotion of catalysis is that the SAW cause a roughening of the metal surface through some atomic or defect displacement. However, since so far they had not been able to proof this, they performed the interdiffusion experiment. The expected slow timescale of this surface roughening is consistent with the previous and their results. Furthermore, it can now be exclude that a direct –fast- electronic effect is responsible for the high catalytic activity with SAW. The SAW induced work function chance measured is definitely too small to be the origin of the promotion of catalysis.
Scientists were able for the first time to measure the effect of SAW on the electronic structure of a Pt model catalyst and achieved a remarkable precision with the new experimental setup at the CIRCE beamline in the ALBA Synchrotron. Thus, they were able to rule out long debated mechanism for the SAW induced promotion of catalysis. Furthermore, scientists gained fundamental insights into the physical processes taking place during the propagation of SAW.
This study addresses an established phenomenon that is still not fully understood: By which mechanism do SAW promote the catalytic activity? Researchers have excluded possible explanations, and in particular provided the first experimental value of the intrinsic, direct electronic effect of the SAW in a metal (catalyst), which also may be useful in other context. The SAW promotion of catalysis is unlikely to make an impact on the industrial chemical catalysis, but is rather of fundamental interest. However, it could have future applications in a lab-on-a-chip environment or in gas sensing.
Cover Figure: Researchers from the ALBA Synchrotron working at the CIRCE beamline. (Inset) Work function oscillation in a thin Pt film imaged by stroboscopic X-ray photoemission electron microscopy (XPEEM). It is caused by the elastic deformation of the surface region by a Surface Acoustic Wave (SAW) in an underlying LiNbO3 substrate. The strong oscillation in the lower area is in the bare substrate and shows a large piezoelectric ampltiude. The periodicity serves as reference for the much smaller effect in the metallic Pt film.