Redistribution dynamics of ultrathin vanadium oxide layers under catalytic conditions and activation of diffusion by surface acoustic waves

The reaction-induced redistribution of vanadium oxide supported on noble metal single crystal surfaces (inverse model catalysts), and the influence of surface acoustic waves (SAW) on the composition of a bimetallic Rh/Pt surface are studied. Previously, the movement and coalescence of macroscopic, two-dimensional vanadium oxide islands on Rh(111) during catalytic methanol oxidation was explained with a polymerization / depolymerization mechanism. To investigate how general this mechanism is, the reaction dynamics of vanadium oxide on Rh(111), Rh(110) and Pt(111) are investigated in a number of catalytic reactions. Island formation and coalescence are observed in ammonia, CO, and methanol oxidation on VOx/Rh(111) in the 0.0001 mbar range. Exchanging oxygen by NO as oxidizing agent results in an inverse pattern, i. e. holes in a dense vanadium oxide film instead of vanadium oxide islands surrounded by bare Rh surface. Spectroscopic LEEM reveals, that NO influences the width of the interface vanadium oxide island / bare Rh(111), thus indicating a change in the line tension. The line tension possibly explains the complementary types of pattern formation. Indications for a Rh surface oxide under reaction conditions are found. In the 0.0001 mbar range on Rh(111), oscillating vanadium oxide islands occur. A tentative mechanism is proposed, based on phase transitions inside the vanadium oxide islands, which result from gradients in the oxygen coverage. With near ambient pressure LEEM, turbulent redistribution dynamics are observed during methanol oxidation at 0.02mbar. On VOx/Rh(110) island formation occurs, but no island coalescence is seen. Instead, a wealth of chemical wave pattern is found: traveling interface modulations (TIMs), traveling wave fragments and target pattern, as well as chemical waves propagating over both, the bare Rh(110) substrate and macroscopic vanadium oxide islands. TIMs are explained by a mechanism based on the reversible creation of surface defects at the interface. The system VOx/Pt(111) is characterized by the reversible diffusion of V into the Pt bulk under reaction conditions. As a consequence, no pattern formation occurs. A strong effect of the metallic support on the behavior of VOx catalysts is demonstrated by the different types of pattern formation in VOx/Rh(111), VOx/Rh(110), and VOx/Pt(111). In addition to different types of pattern formation, also the selectivity and catalytic activity is strongly influenced by the support. Whereas formaldehyde is the main product in catalytic methanol oxidation on VOx/Rh(111), no formaldehyde production is detected on VOx/Rh(110) and VOx/Pt(111). The influence of SAWs on the diffusive intermixing of a Rh/Pt surface is investigated by laterally resolved X-ray spectroscopy. The results are compared to Auger spectroscopy measurements on the thermal diffusion of Rh into the Pt bulk on a Pt(100) single crystal and on polycrystalline Pt. At 445 K, a SAW-induced intermixing of Pt and Rh is detected. In thermal diffusion experiments, the onset of Rh diffusion into the Pt bulk is found to occur around 500 to 550 K. The experiments are a first step towards verifying the working hypothesis, that structural defects caused by SAWs are the main reason for a SAW-induced increase in catalytic activity reported in literature

Reactivity and Stability of Ultrathin VOx Films on Pt(111) in Catalytic Methanol Oxidation

The growth of ultrathin layers of VOx (< 12 monolayers) on Pt(111) and the activity of these layers in catalytic methanol oxidation at 10−4 mbar have been studied with low-energy electron diffraction, Auger electron spectroscopy, rate measurements, and with photoemission electron microscopy. Reactive deposition of V in O2 at 670 K obeys a Stranski–Krastanov growth mode with a (√3 × √3)R30° structure representing the limiting case for epitaxial growth of 3D-VOx. The activity of VOx/Pt(111) in catalytic methanol oxidation is very low and no redistribution dynamics is observed lifting the initial spatial homogeneity of the VOx layer. Under reaction conditions, part of the surface vanadium diffuses into the Pt subsurface region. Exposure to O2 causes part of the V to diffuse back to the surface, but only up to one monolayer of VOx can be stabilized in this way at 10−4 mbar

On the promotion of catalytic reactions by surface acoustic waves

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis,sensor and nanotechnology.SAWswere shown to cause astrong 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 asub-nano-second work function change during propagation of 500 MHz SAWs on a9nm thick platinum film. We quantify the work function change to 455 meV.Such asmall variation rules out that electronic effects due to elastic deformation (strain) play amajor role in the SAW-induced promotion of catalysis.In asecond set of experiments,SAW-induced intermixing of afive 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 amajor cause for catalytic promotion

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V‑Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10^–4 mbar range during catalytic methanol oxidation on ultrathin VO_<i>x</i> films (θ_V ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [11̅0] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VO_<i>x</i> leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VO_<i>x</i>, almost no catalytic activity in formaldehyde production is found on Rh(110)/VO_<i>x</i>

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V‑Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10^–4 mbar range during catalytic methanol oxidation on ultrathin VO_<i>x</i> films (θ_V ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [11̅0] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VO_<i>x</i> leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VO_<i>x</i>, almost no catalytic activity in formaldehyde production is found on Rh(110)/VO_<i>x</i>

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V‑Oxide Promoted Rh(110) Surface

Chemical wave patterns and the formation of macroscopic vanadium oxide islands have been investigated in the 10^–4 mbar range during catalytic methanol oxidation on ultrathin VO_<i>x</i> films (θ_V ≤ 1 monolayer equivalent) supported on Rh(110). At temperatures around 800 K, wave fragments traveling along the [11̅0] direction and oxidation/reduction fronts exhibiting different front geometries are observed with photoemission electron microscopy. At ≈1000 K, a redistribution of VO_<i>x</i> leads to the growth of macroscopic oxide islands under reaction conditions. On these macroscopic V-oxide islands chemical waves including traveling wave fragments propagate. Under conditions close to equistability of oxidized and reduced phase, a dendritic growth of the V-oxide islands is observed. In contrast to Rh(111)/VO_<i>x</i>, almost no catalytic activity in formaldehyde production is found on Rh(110)/VO_<i>x</i>

On the Promotion of Catalytic Reactions by Surface Acoustic Waves

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.The authors thank Werner Seidel for technical support in the fabrication of the IDTs, Rolf J. Haug, Hannover, for help in preparation of the samples, and Leo Zhigilei for carefully reading the manuscript. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. LA and MF acknowledge support from Spanish MINECO through Grant No. RTI2018‐095303‐B‐C53. FM and BC acknowledges support from Spanish MINECO through Grants No. RYC‐2014‐16515, No. MAT2015‐69144‐P, No. SEV‐2015‐0496 and No. MAT2017‐85232‐R. Open access funding enabled and organized by Projekt DEAL.Peer reviewe

On the importance of the structure in the catalytic reactivity of Au-based catalysts

Au-based materials are remarkably efficient catalysts in the domain of partial oxidation reactions. Nonetheless, questions remain about the physico-chemical phenomena involved at the molecular level. In this work, the catalytic properties of Au-Ag samples, in the form of ultrathin Ag layers on a Au(1 1 1) surface and Au-based model nanoparticles, have been investigated with different microscopy techniques. Using photoemission electron microscopy (PEEM), the exposure of a Au(1 1 1) single crystal doped with various amounts of Ag (0 to 3 monolayers) to O2/H2 and O2/CH3OH gas mixtures did not lead to any specific spatiotemporal pattern formation. In contrast, the use of curved nanoscopic Au and Au-8.8 at.% Ag tip-samples analysed by field emission microscopy (FEM) under similar experimental conditions indicates the presence of catalytic activity. The influence of the silver concentration and of the morphology on the reactivity is discussed. This work highlights the necessity of different experimental approaches aimed at bridging the materials gap often encountered between surface science studies and applied catalysis.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Understanding Fundamental Processes on Au-Ag Catalysts during Oxidation Reactions: a Correlative Microscopy Approach

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Understanding Fundamental Processes on Au-Ag Catalysts during Oxidation Reactions: a Correlative Microscopy Approach

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