Angewandte Research Articles Chemie How to cite: Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 Catalyst Activation Hot Paper International Edition: doi.org/10.1002/anie.202005883 German Edition: doi.org/10.1002/ange.202005883 On the Promotion of Catalytic Reactions by Surface Acoustic Waves Bernhard von Boehn, Michael Foerster, Moritz von Boehn, Jordi Prat, Ferran Maci/, Blai Casals, Muhammad Waqas Khaliq, Alberto Hern#ndez-M&nguez, Lucia Aballe, and Ronald Imbihl* Abstract: Surface acoustic waves (SAW) allow to manipulate neous catalysis a strong promotion effect by bulk and surface surfaces with potential applications in catalysis, sensor and acoustic waves has been shown for several oxidation and nanotechnology. SAWs were shown to cause a strong increase decomposition reactions on metal and metal oxide cata- in catalytic activity and selectivity in many oxidation and lysts.[2, 3] In a surface acoustic wave (SAW) periodic distortions decomposition reactions on metallic and oxidic catalysts. of the surface with an amplitude of a few nm and a wavelength However, the promotion mechanism has not been unambigu- of several mm propagate across the surface with the velocity of ously identified. Using stroboscopic X-ray photoelectron sound in a solid, i. e., with about 4000 m s@1 in the case of spectro-microscopy, we were able to evidence a sub-nano- LiNbO3.[4] The amplitude of SAWs decays exponentially in second work function change during propagation of 500 MHz the bulk. In ethanol oxidation over palladium, copper or silver SAWs on a 9 nm thick platinum film. We quantify the work metal catalysts rate-increases by SAWs up to a factor of five function change to 455 meV. Such a small variation rules out have been found.[5] In UHV type experiments SAWs were that electronic effects due to elastic deformation (strain) play found to increase the rate of catalytic CO oxidation over a major role in the SAW-induced promotion of catalysis. In a Pt(110) single crystal by up to a factor of six.[6, 7] a second set of experiments, SAW-induced intermixing of a five The promotion of catalytic reactions by SAWs is of monolayers thick Rh film on top of polycrystalline platinum interest, first of all, because a non-thermal activation of was demonstrated to be due to enhanced thermal diffusion catalytic reactions might be favorable for the optimization of caused by an increase of the surface temperature by about 75 K yield and selectivity. Secondly, the mechanistic understanding when SAWs were excited. Reversible surface structural changes of the promotion effect represents a challenging scientific are suggested to be a major cause for catalytic promotion. problem. Finally, a detailed mechanistic understanding of the processes occurring during the excitation and propagation of Introduction SAW might help to develop novel nano-patterning ap- proaches that could be used for sensors and nanoscale design In sonochemistry chemical reactions are initiated by of surfaces.[8] acoustic waves, an effect successfully demonstrated both in Soon after the discovery of the catalytic promotion effect the liquid phase and in solid state reactions.[1] In heteroge- by SAWs, studies with surface analytical techniques were started, aiming at elucidating the mechanism of the promo- [*] B. von Boehn, M. von Boehn, Prof. Dr. R. Imbihl tion effect.[3] Infrared absorption spectroscopy (IRAS) re- Institut ffr Physikalische Chemie und Elektrochemie, vealed that SAWs can induce a site change of CO adsorbed on Leibniz Universit-t Hannover copper surfaces.[9] On/off experiments with photoelectron 30167 Hannover (Germany) emission microscopy (PEEM) demonstrated that SAWs cause E-mail: imbihl@pci.uni-hannover.de reversible changes of the work function (WF) on the metal Dr. M. Foerster, J. Prat, M. W. Khaliq, Dr. L. Aballe surfaces of copper, palladium and gold.[10] Based on these ALBA Synchrotron Light Source results, it was concluded, that the catalytic promotion effect is 08290 Cerdanyola del Valles (Spain) due to electronic effects induced by SAWs.[3] Strain is known Dr. F. Maci/ Dept. of Condensed Matter Physics, University of Barcelona to affect the reactivity of solid surfaces, but in order to cause 08028 Barcelona (Spain) significant changes in the activation barriers of surface Dr. B. Casals processes, the lattice constant should change by at least Institut de CiHncia de Materials de Barcelona (ICMAB-CSIC) 10@3.[11] A SAW with a wavelength of 8 mm (500 MHz) and 08193 Bellaterra (Spain) a high power density of 165 W m@1, however, produces Dr. A. Hern#ndez-M&nguez a maximum strain below 5 X 10@4. Paul-Drude-Institut ffr Festkçrperelektronik, The catalytic promotion effect of SAWs poses a challeng- Leibniz-Institut im Forschungsverbund Berlin e.V. ing problem for theory, since the frequency v of SAWs is with 10117 Berlin (Germany) 106–109 Hz about five to eight orders of magnitude below the Supporting information and the ORCID identification number(s) for frequency required for energy quanta, hv, to have an the author(s) of this article can be found under: activating effect on surface processes. However, molecular https://doi.org/10.1002/anie.202005883. dynamics (MD) and kinetic Monte Carlo simulations pre- T 2020 The Authors. Published by Wiley-VCH GmbH. This is an dicted that SAWs can enhance surface diffusion by a factor of open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribu- 4.5 X 103.[12, 13] Besides an adiabatic temperature rise and strain tion and reproduction in any medium, provided the original work is caused by SAWs, the enhancement was attributed to the properly cited, and is not used for commercial purposes. coupling of high frequency harmonics generated due to the 20224 T 2020 The Authors. Published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 Angewandte Research Articles Chemie nonlinear sharpening of SAWs to vibrational modes of the mechanisms for SAWs and provide a physical basis for further adsorbates. Experimentally, a smaller enhancement of the research on possible promotion mechanisms. surface diffusion coefficient of Au clusters on a Si(111) surface by a factor of 19 was found.[13] Similarly, in a MD simulation the non-linear properties of SAWs were made Results responsible for the promotion of catalytic CO oxidation on a (110) oriented Pt surface.[14] The promotion effect was In order to measure the influence of SAW on the local attributed to shock spikes caused by SAWs, which would work function and on the surface composition, we perform enhance the diffusion and desorption of adsorbed CO two different types of experiments. The experimental set-ups molecules. are sketched in Figure 1 a. In both cases, LiNbO3 crystals None of the above cited explanations can explain the time equipped with interdigital transducer electrodes (IDTs) serve scale with which the reaction rate responds to SAWs. as substrate and source for SAWs. SAWs are excited by Electronic effects should cause an immediate response of feeding an AC signal in the MHz range to the IDTs. For the catalytic surface, but experimentally the rate responds measuring SAW-induced work function changes, a micro- with a delay of several tens of seconds to minutes.[3] This structured Pt film of 20 nm thickness (reduced to roughly delayed response is also observed in UHV-type experiments 9 nm during surface cleaning before the experiments) is on Pt(110), where transport limitations play no role. This deposited on top of the LiNbO3 crystal. A photo of the Pt/ unsolved question was the primary motivation for us to look LiNbO3 sample surface is shown in Figure 1 b. For probing the into the mechanism of catalytic promotion by SAWs in effect of SAWs on the surface composition, an ordered array a systematic manner. Besides an electronic effect, SAW might of five monolayers thick, square Rh patches (approximately also induce changes in the arrangement of surface atoms, i. e. 50 X 50 mm2 size each square) is deposited in situ onto the Pt cause surface restructuring, which could enhance the catalytic stripe, as can be seen in the LEEM image in Figure 1 c. In this activity. We assess the electronic effects by taking advantage way, laterally resolved X-ray photoemission spectra can be of the time structure of synchrotron radiation that allows us to recorded simultaneously on Rh-covered and uncovered parts take stroboscopic photoelectron emission images of the waves of the Pt stripe. A schematic plot showing regions of with a time resolution below 100 ps,[15] and thus detect compressive and tensile strain during propagation of a SAW changes in the local WF as a function of the SAW phase. is depicted in Figure 1 d. For the detection of restructuring effects, we designed a bimetallic model catalyst with a vertically layered structure that enables measuring vertical intermixing induced by SAWs. Our results allow ruling out one of the proposed promotion Figure 1. Experimental set-up for the excitation of SAWs on microstructured metal films on top of a piezoelectric material. a) LiNbO3 single crystals with IDTs and metal films. Left: Schematic view of a micro-structured Pt film used for detection of SAW-induced WF changes. Right: Schematic view of a micro-structured Pt/Rh film constructed for the detection of SAW-induced intermixing of the metallic constituents; Pt film thickness: & 9 nm (left) and & 40 nm (right), Rh film thickness: & 5 ML (right). b) Photography of the micro-structured Pt film. c) LEEM image of the microstructured Pt/Rh film. The area corresponding to the LEEM image is highlighted by a red circle in a. d) Schematic plot showing regions of compressive and tensile strain during propagation of a SAW. Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 T 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 20225 Angewandte Research Articles Chemie Figure 2. Determination of the local WF change during propagation of a & 500 MHz SAW on a Pt film at 300 K. a) Stroboscopic XPEEM image of a Pt domain (upper part) and the adjacent LiNbO3 surface area (lower part) recorded with a photon energy of 300 eV at @0.5 eV bias voltage. In the raw data the SAW is visible only on the free LiNbO3 surface where it causes a variation of the surface potential of 1.3 eV amplitude. The red frame marks the window used for subsequent data analysis. b) Differential image of two stroboscopic XPEEM images phase-shifted by Df = p. The total integration time is 70 min for the image. c) Photoelectron spectra of a Pt domain around the cut-off energy as obtained from stroboscopic XPEEM image stacks for opposite SAW phases (0 and p). The vertical line marks the bias voltage at which the images in a and b were taken. d) Local WF change as function of the position in a propagating SAW, and numerical fit with a sine function. For this plot the data between the two white lines in b were averaged. SAW-Induced Work Function Changes the LiNbO3 surface potential are completely shielded by the equipotential metal surface (for details on the electrical The design of the IDTs on the LiNbO3 crystal is tuned to characterization and film morphology please refer to the generate SAWs with a wavelength of 8 mm at a frequency supplementary information). exactly matched to the repetition rate of the X-ray pulses In order to highlight small brightness variations caused by generated by the ALBA Synchrotron in the multi-bunch local WF changes induced by SAWs, expected to be at most in mode (499.654 MHz). For determining the local WF, spectro- the meV range, two stroboscopic images with opposite SAW scopic XPEEM images are taken with the electron kinetic phase (i. e., shifted by p) were recorded for every measure- energy close to zero, i. e. at the low energy photoelectron cut- ment. By subtracting the two phase-shifted images, each off. As shown in Figure 2 c, the photoelectron yield varies integrated over 35 min, all other inhomogeneities cancel out, steeply at the chosen sample bias voltage of @0.5 eV and thus whereas only the amplitude of brightness variations caused by the local image brightness sensitively depends on shifts of the SAWs should double. An example of one of the two recorded cut-off energy, i. e., on variations of the WF. phases is shown in Figure 2 a, and the result of such an image On the free LiNbO3 surface the SAWs are readily visible subtraction in Figure 2 b. in PEEM because, being an insulating piezoelectric material, From the slope of the spectra in Figure 2 c, the XPEEM the acoustic waves are accompanied by a variation of the intensity variations in Figure 2 b can be converted into local surface potential of up to 3 V amplitude.[15] Collecting strobo- WF changes (please refer to the supplementary information scopic images with 2 ns separation, the propagating SAWs are for details). The results are displayed in Figure 2 d. An directly imaged on the uncovered LiNbO3, as shown at the intensity cross section averaged over the area highlighted by bottom of Figure 2 a. For a single stroboscopic image, long the two vertical lines in Figure 2 b, reveals a variation of the integration times can be used because between two synchro- local WF with an 8 mm periodicity, equal to the SAW tron light pulses, the synchronized SAW advances by exactly wavelength. The SAWs thus clearly cause a local change of one wavelength, overlaying multiple identical partial images the WF, but the amplitude of these WF changes is with 455 : on the detector. In contrast, on the Pt film, the variations in 13 meV very small (: 13 meV representing the statistical 20226 www.angewandte.org T 2020 The Authors. Published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 Angewandte Research Articles Chemie error). In order to make sure that the dependence shown in SAW-Induced Structural Changes Figure 2 c is not an artifact due to an electrostatic effect caused by potential variations in the adjacent area of Pt-free The question on the nature of the reversible and slow WF LiNbO3, line scans were taken at varying distances from the changes caused by SAWs finds a convincing answer if one domain boundary (please refer to the supplementary infor- takes into account the possibility of induced structural mation). In fact, a strong intensity variation with the SAW changes. Surface roughening is associated with a WF de- periodicity is seen at the border of the Pt stripe. The phase of crease, a phenomenon known as the Smoluchowski effect.[19] this intensity variation is, however, shifted by approximately p The creation of structural surface defects like vacancies, with respect to the SAW as visible on the LiNbO3 crystal, and atomic steps or of structural disorder is a slow process and its intensity decreases fast with increasing distance from the since surface roughening in general is known to increase the stripe border, until finally only a very weak intensity variation reactivity of surfaces, the catalytic promotion effect of SAWs is visible in phase with the SAW on LiNbO3, which is the true may find a simple answer. Severe morphological changes, signal of the WF modulation. including destructions visible by eye, were found on the The zero in the plot of Figure 2 d represents areas of zero Pt(110) single crystal exposed to SAW.[20] However, the intensity in the difference image, and thus no WF difference. authors concluded that these dramatic effects, which by Compressive and tensile strain of the SAWs should cause nature were irreversible, were not responsible for the main asymmetric changes in the WF, as the effect on electron part of the promotion of catalytic CO oxidation by SAWs. density is opposite. DFT calculations conducted for a Pt(100) In order to detect restructuring effects, a model system is surface demonstrated that compressive strain increases the created by depositing a few atomic layers thick ( & 5 layers) WF while tensile strain decreases it.[16] Since the essential Rh film on top of a 40 nm Pt stripe. As shown in Figure 1 a parameter is the change in the electron density, one can (right side), a micro-structured design is used, in which an estimate the WF change from the relative volume change. uncovered part of the Pt film serves as reference to monitor Using the linear relation given in ref. [16] one arrives at the degree of Rh/Pt intermixing. If the SAWs promote a theoretical estimate of approximately 13.4 meV per 1 % diffusion in the vertical direction, then we should observe volume change. In our experiment the strain, and thus the a change in the microspot-XPS ratio of the Pt signal between relative volume change, can be computed easily (please refer an area covered with a Rh patch and the surrounding to the supplementary information for details). From the SAW uncovered area. Just for entropic reasons the two metals surface potential amplitude of 1.3 V we calculate a volume should mix but also enthalpy provides a driving force for change of 1.6 X 10@4, which translates to an estimated segregation and therefore intermixing, since the surface 28.4 meV work function change per 1 % volume change, in energy of Pt is lower than that of Rh.[21] reasonable agreement with the theoretical estimate. We note Exposing the surface at 300 K to SAWs of varying here that, since the relative volume change caused by SAWs frequency (125, 250, and 500 MHz) and power (up to depends on the material, the SAW-induced work function approximately 1 W) for periods ranging from one to several variations might be substantially larger on a softer material. hours did not cause any detectable changes in the surface The WF changes caused by SAWs of less than 1 meV are composition. Only when we raise the nominal sample very small in comparison to those caused by adsorbates or temperature to 400 K and switch on SAWs at 125 MHz (the surface restructuring, mostly in the range 0.1–1 eV, and in the frequency at which the maximum power arrives at the case of alkali adsorbates even above 1 eV.[17] The small WF sample) we observe intermixing. Switching on the SAWs changes caused by SAWs rule out that strain induced causes a temperature rise to 445 K, as measured with electronic effects play a significant role in the promotion of a thermocouple attached to a supporting Ta sheet below the catalysis by SAWs. However, in on/off experiments Inoue LiNbO3 substrate. In these conditions over a period of et al. found WF changes induced by SAWs on copper, gold 13 hours, the Pt 4f signal normalized to the signal of the free and palladium surfaces.[10] These WF changes were not Pt surface increases by a factor of & 1.75, as shown in completely reversible—only to about 60–70 %—and had an Figure 3 a. associated time constant of several tens of seconds. The The expected Pt enrichment in the surface is found, but PEEM intensity had not been calibrated, but since PEEM is the question is whether the effect can be explained by able to detect WF changes + 1 meV,[18] one can estimate that thermally activated diffusion alone. Therefore, we prepared Inoue et al. observed WF changes of at least a few meV. If a LiNbO3 sample completely covered by 40 nm Pt with these were the result of changes in the electronic structure, a three monolayers thick Rh film on top, and followed the they should i) appear instantly, and ii) disappear as soon as changes in composition during heating up by means of Auger SAWs are turned off. We also found a permanent WF electron spectroscopy (AES). The results displayed in Fig- decrease of 30 to 75 meV in the Pt areas exposed to SAW for ure 3 b show that Pt segregation to the surface starts to several hours with respect to Pt areas not exposed to SAW. become significant at & 475 K, which is only 30 K above the However, we attribute such decrease to changes in the surface 445 K measured below the substrate bottom phase during the structure due to prolonged exposure to SAWs (for details SAW experiment. In fact, measuring the sample surface please refer to the supplementary information). temperature directly with a pyrometer during SAW applica- tion showed that the temperature measured by the thermo- couple below the substrate underestimates the SAW induced heating of the surface by up to 30 K. Thus the Rh/Pt Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 T 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 20227 Angewandte Research Articles Chemie needs, however, to be reconciled with the above result that SAWs leave the surface essentially intact.[22] The finding by Inoue et al. that SAWs cause slow reversible WF changes of metal surfaces held in vacuum strongly suggests that reversible structural changes of the surface take place and that these changes contribute to the catalytic enhancement by SAWs. If the SAWs generate unstable non-equilibrium configurations of the surface atoms, then, after switching off the SAWs, the surface atoms would return to their equilibrium configurations, explaining the long time constants observed in on/off experiments. This explan- ation is compatible with our segregation experiment, since only small structural changes can be involved if the process is to be reversible. Molecular dynamics simulation that are currently in progress in fact demonstrate that shock fronts which arise due to a nonlinear sharpening of SAWs can cause structural modifications of the surface.[23] The theoretical results could be further substantiated by scanning tunneling microscopy (STM) measurements, for example, before and after exposing a thin Pt crystal to strong SAW ex-citation. We briefly review the possible factors contributing to a promotion of catalytic reaction by SAWs: (i) Changes in the electronic structure (ii) Elastic strain modifying energy barriers (iii) Dynamic coupling effects of vibrational modes with Figure 3. SAW-induced and thermally-induced intermixing in a layered nonlinear components of SAWs Rh/Pt surface. a) SAW-induced Pt enrichment as a function of SAW (iv) Temperature rise due to energy dissipation of SAWs application time (125 MHz, & 0.1 W, 445 K substrate temperature). A (v) Adiabatic temperature changes Pt/Rh microstructure consisting of a 5 ML thick Rh film on top of (vi) Structural changes a 40 nm Pt film as displayed in Figure 1 c is used. Shown is the Pt 4f intensity of Rh covered Pt area normalized with respect to the Pt 4f intensity of the uncovered Pt area. b) Thermally induced Pt enrichment We can rule out electronic effects as significant factor on on the surface of 3 ML thick Rh film on top of & 40 nm Pt on LiNbO3. the basis of our results. Similarly, the elastic strain caused by The Rh/Pt Auger intensity ratio is followed while slowly heating up. SAW is estimated to be with less than 5 X 10@4 too small to Intermixing starts above about 475 K, as reflected by a steep decrease substantially reduce energy barriers. In contrast, the dynamic of the Rh/Pt signal. coupling of vibrational modes of the surface with nonlinear components of SAWs as well as the temperature increase resulting from dissipated energy of the SAWs may very well intermixing seen in Figure 3 a can be explained by thermal contribute to the promotion effect.[12] In addition, we have to diffusion alone. consider the adiabatic temperature variations caused by an expansion/contraction of the surface region during propaga- tion of SAWs. Simulations demonstrated that their amplitude Discussion can reach 5 % of the average temperature. Since positive and negative temperature variations do not cancel out in Arrhe- In previous experiments with a 500 nm thick Pt(110) nius law, this effect can significantly enhance a catalytic single crystal a temperature increase due to SAWs of 30–50 K rate.[12] Finally, a number of indirect evidences strongly was found, in agreement with our results.[7] This temperature suggest that reversible surface restructuring caused by SAWs rise will contribute to the promotion effect, but Mitrelias et al. plays an important role, in particular, in explaining the long clearly demonstrated that it alone could not explain the time constants that are often observed. Effects that rely on the increase of catalytic CO oxidation by SAWs. Based on the nonlinear components of SAWs such as dynamic coupling and finding that SAWs reversibly reduce the WF of a Pt(110) SAW-induced restructuring are expected to strongly depend crystal held in pure CO, it was concluded that SAWs induce on the SAW amplitude and on the surface structure. Further the desorption of CO.[6] The removal of CO from a CO- experiments are needed to fully quantify each of the remain- adlayer inhibiting O2-adsorption could certainly explain an ing possible contributions. activation of the CO + O2 reaction by SAWs, but the experimentally observed time constant of several tens of seconds does not agree with such a conclusion. On the other Conclusion hand, a SAW-induced roughening of the Pt(110) surface would also lead to activation and could explain the long time In summary, in order to disentangle different contribu- constants associated with activation. This latter explanation tions we have studied elementary surface processes that can 20228 www.angewandte.org T 2020 The Authors. Published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 Angewandte Research Articles Chemie underlie a promotion of catalytic reactions by SAWs. Using [3] Y. Inoue, Surf. Sci. Rep. 2007, 62, 305. stroboscopic imaging, we demonstrated that SAWs cause [4] a) B. A. Auld, Acoustic fields and waves in solids, Wiley, New a work function change in the range of hundreds of meV on York, 1973; b) M. E. Lines, A. M. Glass, Principles and Appli- cations of Ferroelectrics and Related Materials, Oxford Univer- a polycrystalline Pt film. Such small changes rule out sity Press, Oxford, 2001. electronic effects as cause for SAW enhancement of catalytic [5] Y. Inoue, Y. Watanabe, Catal. Today 1993, 16, 487. surface reactions. Using a layered bimetallic Rh/Pt film we [6] S. Kelling, S. Cerasari, H. H. Rotermund, G. Ertl, D. A. King, showed that SAWs leave the surface structure essentially Chem. Phys. Lett. 1998, 293, 325. intact, i. e., they usually do not lead to irreversible structural [7] T. Mitrelias, S. Kelling, R. I. Kvon, V. P. Ostanin, D. A. King, changes. The catalytic promotion effect by SAWs results from Surf. Sci. 1998, 417, 97. [8] a) S. Okuda, T. Ono, Y. Kanai, T. Ikuta, M. Shimatani, S. Ogawa, a combination of several factors. A major role we can K. Maehashi, K. Inoue, K. Matsumoto, ACS Sens. 2018, 3, 200; attribute to the increase in the average surface temperature b) Z. Ma, J. Guo, Y. J. Liu, Y. Ai, Nanoscale 2015, 7, 14047; by as much as 75 K, to which we have to add periodic c) J. B. Kinzel, D. Rudolph, M. Bichler, G. Abstreiter, J. J. Finley, temperature changes caused by adiabatic expansion/contrac- G. Koblmgller, A. Wixforth, H. J. Krenner, Nano Lett. 2011, 11, tion of the surface during SAWs. Moreover, we expect 1512; d) D. J. Collins, B. Morahan, J. Garcia-Bustos, C. Doerig, dynamic coupling of vibrational surface modes with higher M. Plebanski, A. Neild, Nat. Commun. 2015, 6, 8686; e) C. harmonics resulting from nonlinear sharpening of SAWs to Taillan, N. Combe, J. Morillo, Phys. Rev. Lett. 2011, 106, 76102; f) B. Paschke, A. Wixforth, D. Denysenko, D. Volkmer, ACS contribute. We hope our work triggers other quantitative Sens. 2017, 2, 740. studies in order to finally arrive at a detailed understanding of [9] a) H. Nishiyama, Y. Inoue, Surf. Sci. 2005, 594, 156; b) H. the mechanisms underlying the promising effect of catalytic Nishiyama, Y. Inoue, J. Phys. Chem. B 2003, 107, 8738. promotion by surface acoustic waves. [10] H. Nishiyama, Y. Inoue, Surf. Sci. 2006, 600, 2644. [11] a) M. Mavrikakis, B. Hammer, J. K. Nørskov, Phys. Rev. Lett. 1998, 81, 2819; b) D. J. Shu, F. Liu, X. G. Gong, Phys. Rev. B Acknowledgements 2001, 64, 441. [12] C. Wu, V. Y. Zaitsev, L. V. Zhigilei, J. Phys. Chem. C 2013, 117, 9252. The authors thank Werner Seidel for technical support in the [13] M. V. Shugaev, A. J. Manzo, C. Wu, V. Y. Zaitsev, H. Helvajian, fabrication of the IDTs, Rolf J. Haug, Hannover, for help in L. V. Zhigilei, Phys. Rev. B 2015, 91, 8. preparation of the samples, and Leo Zhigilei for carefully [14] Q. An, J. Qian, R. R. Nielsen, L. Sementa, G. Barcaro, F. R. reading the manuscript. The research leading to this result has Negreiros, A. Fortunelli, W. A. Goddard III, J. Mater. Chem. A been supported by the project CALIPSOplus under Grant 2016, 4, 12036. [15] M. Foerster, N. Statuto, B. Casals, A. Hern#ndez-M&nguez, S. Agreement 730872 from the EU Framework Programme for Finizio, A. Mandziak, L. Aballe, J. M. Hern/ndez Ferr/s, F. Research and Innovation HORIZON 2020. LA and MF Maci/, J. Synchrotron Radiat. 2019, 26, 184. acknowledge support from Spanish MINECO through Grant [16] X. F. Wang, L. He, S. Halas, T. Pieńkos, J. G. Lin, T. Durakiewicz, No. RTI2018-095303-B-C53. FM and BC acknowledges sup- Appl. Phys. Lett. 2013, 102, 223504. port from Spanish MINECO through Grants No. RYC-2014- [17] P. S. Bagus, D. K-fer, G. Witte, C. Wçll, Phys. Rev. Lett. 2008, 16515, No. MAT2015-69144-P, No. SEV-2015-0496 and No. 100, 126101. MAT2017-85232-R. Open access funding enabled and organ- [18] H. H. Rotermund, Surf. Sci. Rep. 1997, 29, 265. [19] R. Smoluchowski, Phys. Rev. 1941, 60, 661. ized by Projekt DEAL. [20] S. Kelling, N. Saito, Y. Inoue, D. A. King, Appl. Surf. Sci. 1999, 150, 47. [21] A. Christensen, A. V. Ruban, P. Stoltze, K. W. Jacobsen, H. L. Conflict of interest Skriver, J. K. Nørskov, F. Besenbacher, Phys. Rev. B 1997, 56, 5822. The authors declare no conflict of interest. [22] S. Ladas, R. Imbihl, G. Ertl, Surf. Sci. 1988, 197, 153. [23] M. V. Shugaev, C. Wu, V.. Y. Zaitsev, L. V. Zhigilei, J. Appl. Phys. 2020, 128, 045117. Keywords: heterogeneous catalysis · LEEM · PEEM · Surface acoustic waves · work function Manuscript received: April 23, 2020 [1] T. J. Mason, J. P. Lorimer, Sonochemistry. Theory, applications Revised manuscript received: July 8, 2020 and uses of ultrasound in chemistry, Wiley, New York, 1988. Accepted manuscript online: July 30, 2020 [2] Y. Inoue, MRS Bull. 2019, 44, 361. Version of record online: September 9, 2020 Angew. Chem. Int. Ed. 2020, 59, 20224 – 20229 T 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 20229