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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V-Oxide Promoted Rh(110) Surface Bernhard von Boehn, and Ronald Imbihl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00852 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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The Journal of Physical Chemistry

Chemical Wave Patterns and Oxide Redistribution during Methanol Oxidation on a V-Oxide Promoted Rh(110) Surface Bernhard von Boehn, and Ronald Imbihl* Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstrasse 3A, D-30167 Hannover, Germany.

ABSTRACT

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 VOx films (θV ≤ 1 MLE) 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 (PEEM). At ≈ 1000 K a redistribution of VOx 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)/VOx almost no catalytic activity in formaldehyde production is found on Rh(110)/VOx.

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Introduction

Reactions of the automotive catalytic converter like CO oxidation or NO reduction, which are catalyzed by the noble metals Pt, Pd, Rh and Ir, have demonstrated a wealth of interesting nonlinear phenomena including kinetic oscillations, chemical wave patterns, chaos and turbulence1–3. Single crystal studies of catalytic CO oxidation on Pt revealed that the origin of the rate oscillations and chemical wave patterns at low pressure (p < 10-3 mbar) lies in adsorbateinduced reconstructions of the metallic substrate1,2. In contrast to reactions on metal surfaces, no chemical wave patterns on oxidic surfaces have been observed so far.

In recent years, the dynamics of ultrathin V oxide layers on Rh(111) in the O2 + H2 and in the partial oxidation of methanol have been studied with photoemission electron microscopy (PEEM) and low energy electron microscopy (LEEM) as spatially resolving methods4–6. It has been found that the distribution of the initially homogeneous V oxide layer changes under reaction conditions. The V-oxide condenses into stripe patterns or patterns of circular VOx islands. Surprisingly, the macroscopic VOx islands (20 – 200 µm) move and coalesce under reaction conditions thus undergoing a process similar to Smoluchowski ripening6–8. In order to see whether this coalescence mechanism can be generalized to other systems, catalytic methanol oxidation was investigated on a Rh(110) surface with a submonolayer coverage of V-oxide9,10. The Rh(110) surface, as an open surface, is thermodynamically less stable than Rh(111). In contrast to Rh(111)/VOx where a large number of well characterized 2Dphases exist11,12, no such phases were known for Rh(110)/VOx at the beginning of the reaction studies.


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After deposition of 0.1 – 0.4 MLE (monolayer equivalents) VOx onto Rh(110), we found in catalytic methanol oxidation rather unusual chemical wave patterns which bear a certain resemblance to the patterns seen in Rh(110)/NO + H213–15. At elevated temperatures also macroscopic VOx islands develop on Rh(110)/VOx under reaction conditions. As will be shown below, the Rh(110)/VOx system behaves quite differently than the Rh(111)/VOx catalysts in catalytic methanol oxidation. We can attribute these differences to the fact that the interaction of V-oxide with Rh(110) is more complex than the interaction with Rh(111). On the densely packed Rh(111), the metal surface can be considered as a rigid support for V-oxide11,12. As least as long as oxidizing conditions prevail, no indication for alloying of V with the metallic substrate exists. The deposition of metallic V on Rh(110) has been investigated16. As was demonstrated convincingly by photoelectron diffraction, the V atoms penetrate the Rh bulk forming a subsurface Rh-V alloy at elevated temperature (T > 800 K). Prior to the reaction studies we have investigated the deposition of V and of V-oxide onto Rh(110) with LEED, LEEM and XPS10. From these investigations we obtained indirect support that subsurface alloy formation also plays a role in the redistribution of VOx and in the chemical wave patterns under reaction conditions. In this article we focus exclusively on the phenomenology of the chemical wave patterns and of V-oxide redistribution during catalytic methanol oxidation on Rh(110)/VOx. A specific part of these patterns, namely the traveling interface modulations, have been published separately. At low VOx coverages (0.1 MLE) in the bistable range of the reaction, amplitude excursions from the average interface position travel along the interface near the equistabilitypoint, separating the oxidized and reduced state of the surface9. For a mechanistic interpretation, first the different phases we observe in PEEM need to be identified chemically. Such an identification is in


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principle feasible with so-called “spectromicroscopic” methods, which utilize synchrotron radiation and which allow one to take x-ray photoelectron spectra or LEED from small selected areas (1 µm)17. Unfortunately, at least at present, these methods cannot be applied in the 10-4 mbar range, which is the pressure range in which the chemical wave patterns reported here take place. We expect, however, that the application of such methods will be possible in the near future. It should be added, that the study of chemical wave patterns on complex surfaces not only adds new features to the phenomenology of chemical waves on surfaces, but a mechanistic understanding provides us also with detailed insights into the dynamics of supported catalysts under reaction conditions.

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Experimental

The experiments presented in this article were conducted in a standard 100 L UHV system equipped with a photoemission electron microscope (PEEM), a cylindrical mirror analyzer for Auger electron spectroscopy (AES), a low-energy electron diffraction (LEED) optics and a differentially pumped quadrupole mass spectrometer (QMS) for reaction rate measurements. The Rh(110) surface was cleaned prior to each experiment by repeated cycles of Ar ion bombardment (1 kV, 2.5 µA, 15 min, 300 K) and oxygen treatment (1·10-6 mbar, 1000 K, 20 min) followed by a final flash annealing to 1300 K. The series of oxygen induced reconstructions observed with LEED resulting in a c(2x8) at high exposures18 (1·10-7 mbar, 570 K) was taken as evidence for a clean surface. To form VOx overlayers, V was deposited on three different Rh(110) single crystals by electron beam evaporation of a high purity V rod (Goodfellow) at a sample temperature of 670 K in an oxygen atmosphere of 2·10-7 mbar (reactive evaporation11). In order to obtain ordered VOx overlayers the Rh sample was heated up


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in oxygen 10 minutes before the deposition was started. In this way, V was deposited onto a c(2x8)-O structure on Rh(110). The VOx coverage is given in monolayer equivalents (MLE), with 1 MLE corresponding to the number of Rh atoms that are present in the topmost layer of Rh(110). A coverage calibration was achieved using the XPS/LEED results for Rh(110)/V and Rh(110)/VOx obtained at the Nanospectroscopy beamline of the synchrotron light source Elettra, where V coverages could be assigned to the V-oxide structures observed in LEED10. However, the V coverages thus determined should be considered as tentative, since the calibration relies on the assumption of a layer-by-layer growth for Rh(110)/V, what needs to be verified experimentally. The Rh(110) single crystal (MaTeck GmbH) was heated either resistively (up to 1000 K) by passing current through two Ta wires spot welded to the sample or indirectly by electron bombardment from the backside (up to 1400 K). The sample temperature was measured with a K-type thermocouple spot-welded to the backside of the sample. The total pressure was measured with an ionization gauge (uncorrected). The spatiotemporal dynamics of the surface were followed in situ with a spatial resolution of about 1 µm in video frequency (20 ms) using PEEM. In PEEM the sample is illuminated by UVlight emitted from a deuterium discharge lamp (5 – 6.5 eV photon energy). The ejected photoelectrons are projected onto a micro channel plate detector. PEEM images primarily the local work function variation and yields only indirect chemical information. All PEEM experiments were conducted in a gas atmosphere in the 10-4 mbar range at temperatures between 300 and 1000 K.


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Figure 1. Temperature programmed reaction measurements during catalytic methanol oxidation on a clean Rh(110) surface and on a Rh(110) surface covered with 0.4 MLE VOx. The measurements were conducted with a fixed methanol pressure of 3·10-4 mbar and a variable oxygen pressure. a) Reaction rates measured on the clean Rh(110) surface with a methanol to oxygen ratio of 1:3 and 1:5. Shown is the heating branch of the reaction rates. b) Heating and cooling branch of the second run of a reaction rate measurement on the VOx covered Rh(110) surface. Second run means, that the initially homogeneously distributed VOx has already been condensed into macroscopic VOx islands. The oxygen to methanol ratio was 1:3 and the heating/cooling rate β = 0.5 K/s. Note that the rates in (a) and (b) are plotted with the same y-scale.

For reaction rate measurements the chamber was used as a continuous flow reactor and linear heating/cooling ramps ranging from 300 to 1000 K (heating rate β = 0.5 K/s) were applied. The sample was placed directly in front of a cone (6 mm diameter opening) which connects the main chamber to a differentially pumped QMS. In this way it was ensured that all detected reaction products were coming directly from the sample surface.

3. Results a) Catalytic activity.


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On the unpromoted Rh(110) surface, methanol reacts with oxygen to the main products CO, H2O and some CO2 as shown by Fig. 1a. This diagram displays the reaction rates of temperature programmed reaction (TPR) experiments for two different ratios p(O2)/p(MeOH) with a fixed methanol partial pressure of 3·10-4 mbar. Under oxidizing conditions (1:3 ratio, left graph in Fig. 1a) the reaction sets in at ≈ 400 K and after going through a maximum at 530 K it decays to a very low value beyond ≈ 1000 K. Under reducing conditions (p(O2)/p(MeOH) = 1:5) ignition of the reaction is delayed to ≈ 500 K and the rate remains high when 1000 K are reached. The addition of up to 0.4 MLE VOx does not alter the principle behavior of the reaction but the catalytic activity is reduced by roughly 40 % for 0.4 MLE as demonstrated by Fig. 1b. The experiment was conducted under the same oxidizing conditions as the one shown in the left graph of Fig. 1a. A temperature window for high catalytic activity extends from ≈ 500 K to ≈ 850 K. During cooling down the reaction sets in at 850 K by taking a steep increase at ≈ 700 K. Compared to Rh(111)/VOx the most remarkable feature of the measurements is that almost no formaldehyde is produced. On Rh(111)/VOx formaldehyde was the dominant reaction product in catalytic methanol oxidation at temperatures above 800 K6. We interpret the TPR results as follows: the catalytic activity is mainly determined by the part of the surface that is not covered by VOx (60%), whereas the VOx covered part of the surface (40%) is catalytically less active. Accordingly the same rate curves as for the unpromoted Rh(110) surface should result but with a scaling factor of 0.6. A comparison of Figs. 1a and b shows that overall this explanation is in agreement with the data. The absence of a substantial formaldehyde production indicates that the VOx on Rh(110) is in a different state than on Rh(111), where the VOx islands were shown to be the catalytically active phase. As will be shown below the VOx islands on Rh(110) respond dynamically to changes in the reaction conditions but apparently


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their contribution to the overall activity is so small that it does not contribute significantly to the overall catalytic activity.

b) Patterns on a homogeneously VOx covered surface Pattern formation in catalytic methanol oxidation has been studied mainly at two V-oxide coverages, θV = 0.1 MLE and θV = 0.4 MLE. Macroscopic V-oxide islands visible in PEEM only develop when the sample is heated up beyond 920 K. Below that temperature PEEM shows a spatially uniform surface. We can therefore distinguish between pattern formation on a homogeneously VOx covered surface and patterns involving macroscopic VOx island formation. The term homogeneous here refers to the macroscopic length scale of PEEM (> 1 µm) while on a meso-scale an island structure may prevail. At low V coverage, at θV = 0.1 MLE, the surface exhibits bistability. In a bistable system reaction fronts initiate transitions between two stable kinetic states of the system19; here between an oxygen covered and a nearly adsorbate free state of the system. The behavior is therefore not much different from the behavior of the unpromoted Rh(110) surface with one exception. One observes traveling interface modulations with an amplitude strongly enhanced compared to the unpromoted surface9. At a medium V coverage, at θV = 0.4 MLE, novel type of chemical wave patterns appear which are described in the following. All these experiments are conducted in the 10-4 mbar range. While heating up the VOx covered surface in a methanol/oxygen atmosphere the surface remains homogeneous in PEEM until oxidation fronts develop at ≈ 700 K. This marks the beginning of a temperature range of so-called “double metastability” or “dynamic bistability” which extends from ≈ 700 K to ≈ 850 K. In dynamic bistabilty, the originally stable states of the system both


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Figure 2. Dynamic bistability in catalytic methanol oxidation on Rh(110)/VOx. A propagating oxidation front (black) collides with a surface defect, causing the nucleation of a reduction front (bright) that transforms the oxygen covered surface into a refractory state. FOV: 530 x 615 µm2, T = 790 K, p(O2) = 1·104 mbar, p(MeOH) = 3·10-4 mbar, θV = 0.4 MLE.

become unstable20. Effectively, such a system of dynamic bistability behaves very similar to an excitable system, with the main difference being that the pulses in a system of dynamic bistability are more irregular20. On an anisotropic surface like Rh(110) pattern formation will be anisotropic since the diffusivity of adsorbed particles will vary depending on the crystallographic direction. The anisotropy of diffusion will not only depend on the geometric corrugation and thus vary with the type of reconstruction, but it may also change depending on the adsorbate coverages. The anisotropy thus becomes state-dependent14,15,21–23. As illustrated by Fig. 2 we observe a different anisotropy for the oxidation and the reduction front, which indicates a statedependent anisotropy. In the PEEM images in Fig. 2 first an elliptically shaped oxidation front evolves with the long axis of the ellipse being oriented in the [11̅0]-direction.


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The assignment of the dark area in PEEM to an oxygen covered surface is based on the fact that adsorbed oxygen in general increases the work function of metals and on the observation that in the limit of a very small methanol partial pressure the surface becomes uniformly dark in PEEM. Conversely, a bright area in PEEM marking the absence of oxygen is assigned to the reduced surface. As soon as the expanding oxidation front hits a surface defect, marked by an arrow in the first PEEM image of Fig. 2, a reduction front nucleates converting the dark area into bright area. In the PEEM images one can clearly identify three distinct intensity levels: medium gray, dark and bright, which we can assign to the resting state, the excited state and the refractory state of an excitable system, respectively. A chemical interpretation of the three grey levels will be given in the discussion section. The oxidation front propagates with a velocity of v[11̅0] = 6.9 ± 0.6 µm/s in the [11̅0] direction and v[001] = 2.1 ± 0.8 µm/s in the [001] direction, whereas the corresponding velocities of the reduction front are v[11̅0] = 7.3 ± 1.1 µm/s and v[001] = 5.9 ± 1.4 µm/s. Due to its higher propagation speed, the reduction front eats up the oxygen island created by the oxidation front. Interestingly, the reduction front exhibits a different anisotropy v[11̅0] / v[001] than the oxidation front. Fast and slow direction have switched their roles in the reduction front. Similar observations have been made in the system Rh(110)/H2 + O2 where the front anisotropy was found to vary with temperature and p(H2)14,21,24. After the transformation of the oxidized area (black) into reduced surface (bright), the bright area slowly returns in a refractory period to the original grey level of the surface. The assignment of this process to a refractory period is well supported by the observation that an expanding oxygen front cannot enter this region before the resting state is established again, as demonstrated by the


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Figure 3. Activation of a surface defect through collision with chemical waves. Top: PEEM images of an oscillating surface defect. Bottom: Plot of the local PEEM intensity as a function of time. The PEEM intensity has been integrated over the area of the defect. The oscillation frequency decreases after each collision with a traveling wave fragment (black arrows). FOV: 62 ⨯ 77 µm2, T = 790 K, p(O2) = 1·10-4 mbar, p(MeOH) = 3·104 mbar, θV = 0.4 MLE. The time between the first and last PEEM image is 21 s, the time between the individual images varies between 1 and 9 s.

PEEM images in the bottom half of Fig. 2. The refractory period is quite long, extending over roughly 300 s. In an excitable system, surface defects play an important role because they typically provide the stimuli required to trigger a chemical wave. The PEEM images in Fig. 3 show such an oscillating surface defect of ~20 µm diameter. This defect is large enough so that one can see that the transformation from an oxidized state into a reduced state occurs via a reaction front which nucleates at the edge of the defect. In contrast to that, the transition from a reduced into an oxidized state starts in the center of the elliptically shaped defect. The behavior of the oscillating defect is modified through the interaction with chemical waves, as demonstrated by a plot of the intensity of the oscillations inside the defect against time in Fig. 3. At t ≈ 50 s the defect region becomes oscillatory through collision with a traveling wave fragment (first arrow). Two subsequent collisions with oxidation fronts at t = 450 s and t = 950 s (second and third arrow)


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Figure 4. A traveling wave fragment during catalytic methanol oxidation on Rh(110)/VOx. The TWF propagates along the [11̅0]-direction. FOV: 173 x 173 µm2, θV = 0.4 MLE, T = 740 K, p(O2) = 1·10-4 mbar, p(MeOH) = 3·10-4 mbar.

cause a decrease of the oscillation frequency while simultaneously the amplitude grows and the oscillation becomes more regular. Apparently collisions with the chemical waves cause a structural and/or chemical modification of the defect. In the region of dynamic bistability one also observes so-called traveling wave fragments (TWF), as shown in Fig. 4. In an isotropic excitable medium the free ends of a pulse fragment would curl in, forming a pair of counter-rotating spiral waves19. This curling-in can be prevented if the medium exhibits a state-dependent anisotropy. Then wave fragments travelling along a certain crystallographic direction can be generated, as has been shown quite generally15. In the system Rh(110)/NO + H2 where TWF are seen, a state-dependent diffusional anisotropy is caused by the various O,N-induced reconstructions of Rh(110) which are connected with different reconstruction geometries18,25. The wave fragment in Fig. 4 nucleates at a small surface defect. Initially, only an oxidation front (black) develops. After growing to a certain size a reduction front (bright) nucleates and the


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fragment travels with constant velocity of ~2 µm/s along the [11̅0]-direction. For the particular parameter values of p(MeOH) = 3·10-4 mbar and p(O2) = 1·10-4 mbar chosen here, TWF have been observed in a temperature window from 720 K to 850 K, what is nearly identical with the range of dynamic bistability. TWF have before been observed in catalytic CO oxidation on Pt(110) and in the NO + H2 reaction on Rh(110)15,22,23 but there the TWF were travelling along the [001]-direction what is perpendicular to the direction in which the TWF are propagating here.

c) Patterns involving formation of macroscopic VOx islands. The following experiments have all been conducted with a vanadium coverage of θV = 0.4 MLE and with p(MeOH) ≈ 3·10-4 mbar and p(O2) = 1·10-4 mbar. Macroscopic VOx islands visible in PEEM develop when the sample is heated above ≈ 920 K in the MeOH/O2 atmosphere. The islands nucleate at surface defects, but in order to establish a sufficiently high growth rate the temperature is raised to 1020 K. As demonstrated by Fig. 5 (see Video S1 in SI) within a couple of minutes elongated dark islands form with a width of 30 – 70 µm and a length of several hundred µm. As time progresses the islands undergo a ripening process: the contrast island vs. surroundings becomes stronger and some of the small islands vanish while simultaneously the larger islands expand. This is what is expected for Ostwald ripening. At the end, after 29 min, islands of similar size with about equal spacing between them remain, but there is still too much irregularity to qualify the pattern as a periodic structure. During the ripening process the integral PEEM intensity continuously grows. Since the Rh(110) surface is anisotropic, one should expect an orientation of the elongated V-oxide islands along the crystallographic axes, but this is not what one observes. Evidently, the orientation of the VOx islands is initially dictated by macroscopic surface scratches as has been verified. The orientation of the white stripes in the


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first PEEM image in Fig. 5a coincides with the direction of macroscopic surface scratches visible in PEEM images of the clean Rh(110) surface.


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Figure 5. Condensation of VOx into macroscopic islands and island ripening during catalytic methanol oxidation at 1020 K. a) PEEM images showing the condensation and ripening under reaction conditions. b) PEEM intensity profile taken along the white line in the last PEEM image in (a), showing an island substructure consisting of a bright core and a dark boundary. FOV: 650 µm, θV = 0.4 MLE, p(O2) = 1·10-4 mbar, p(MeOH) = 3·104 mbar.

The assignment of the dark islands to VOx is based on their behavior with respect to changes in the gas phase composition. As will be shown below (Fig. 7) the VOx islands respond sensitively to changes in the gas-phase, whereas the surrounding metal surface behaves rather insensitive. As demonstrated by the PEEM intensity profile in Fig. 5b taken along the white line in the last


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Figure 6. PEEM images and local PEEM intensity evolution acquired during cooling the sample with a constant cooling rate of 0.5 K/s under reaction conditions. The cooling is stopped at 680 K, where the islands start to brighten. The PEEM intensity was integrated over an area of 35 µm2 size inside and outside a VOx island. FOV: 650 µm, same experimental conditions as in Fig. 5.

PEEM image of Fig. 5a, the islands are not uniformly dark but exhibit an inner bright core and a dark rim. A similar substructure was observed in the VOx islands on Rh(111) during catalytic methanol oxidation under similar conditions. On Rh(111) the VOx islands consisted of a bright core region, which is surrounded by a dark outer ring in PEEM6. However, in contrast to Rh(111)/VOx, the islands on Rh(110) do not move under reaction conditions and they do not undergo a coalescence process. As one cools down the sample from 1020 K the surface first uniformly darkens in PEEM, as demonstrated by the PEEM images in Fig. 6. This darkening can be explained by an enhanced oxygen adsorption at lower temperature. When 680 K are reached the VOx islands rapidly brighten, resulting in a reversal of brightness island vs. brightness surroundings, as compared to the starting situation at 1020 K. This phenomenon is accompanied by an increase of the catalytic activity, visible in the sharp rise of the reaction products H2O, CO and CO2 during cooling in the


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temperature programmed reaction measurement in Fig. 1b (last plot). Interestingly, the change in PEEM intensity is much more pronounced on the VOx covered regions than on the surrounding surface. This change in brightness indicates a certain catalytic activity of VOx in the total oxidation of methanol.


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Figure 7. Reduction and oxidation of VOx islands initiated by variation of the methanol partial pressure. PEEM data are shown, recorded at T = 680 K and with fixed oxygen partial pressure of 1·10-4 mbar. a) The reduced surface is uniformly oxidized as the methanol partial pressure is lowered from 3·10-4 mbar to 1·10-4 mbar. Time between images: 3 and 14 s. b) Upon increasing the methanol pressure back to its initial value, the VOx islands are transformed into the reduced state again. Time between images: 5 and 24 s. c) x,t-plot demonstrating the propagation of a reduction front in the experiment reproduced in (b). The plot is constructed by taking intensity profiles perpendicular to an island boundary and stacking them on top of each other. The vertical white lines mark the initial position of the VOx island boundary. Experimental conditions: FOV: 650 µm, T = 680 K, θV = 0.4 MLE. For better visibility, the contrast of the PEEM images of the oxidized surface is enhanced.

The VOx islands and their surroundings react sensitively to variations in the composition of the gas atmosphere as demonstrated by the PEEM images in Fig. 7. At 680 K first p(MeOH) is lowered from 3·10-4 mbar to 1·10-4 mbar keeping p(O2) fixed at 1·10-4 mbar. Remarkably, as shown in Fig. 7a, the VOx islands turn dark as oxidation fronts, which nucleate at the island boundaries, propagate towards the center of the island until the whole surface has been turned into an oxidized state. The oxidation front is quite fast and from intensity profiles one can estimate a velocity of the oxidation front of about 12 µm/s. By increasing p(MeOH) back to its


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original value, the initial state of the surface is restored as depicted in Fig. 7b. The reduction of the VOx islands takes place via propagating reduction fronts which nucleate at the rim of the islands. The progress of a reduction front is clearly visible in the x,t-plot in Fig. 7c. The plot has been constructed by taking PEEM intensity profiles perpendicular to a long VOx islands. A front velocity of 2.2 µm/s is obtained. The reduction process of the VOx islands on Rh(110) is thus markedly different from Rh(111)/VOx where the reduction process in catalytic methanol oxidation started from the interior of the VOx islands6,26.

Dendritic growth. After depositing 0.4 MLE V-oxide on Rh(110) the sample with the freshly deposited and macroscopically homogeneous VOx film was heated up in a reaction atmosphere of p(MeOH) = 3·10-4 mbar and p(O2) = 1·10-4 mbar. Different from the preceding experiments the surface is prevented from completely switching to an oxidized state in the region of dynamic bistability by continuously readjusting p(MeOH) at T > 800 K. The system is thus kept in a reduced state but very close to the transition to an oxidized state. In case that a too high methanol partial pressure is adjusted, no V-oxide can form and metallic V starts to diffuse into the Rh bulk. In PEEM a homogeneously bright surface appears then. Therefore only a very narrow parameter space exists, in which the following growth behavior can be observed.


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Figure 8. PEEM images showing the dendritic growth of Voxide islands starting at 960 K during heating to 1020 K under reaction conditions. The solid white line in the second PEEM images indicates the position of intensity profiles shown in Fig. 9. FOV: 500 ⨯ 400 µm2, p(O2) = 1·10-4 mbar, p(MeOH) ≈ 3·10-4 mbar, θV = 0.4 MLE.

As shown in Fig. 8, during heating up a new phase nucleates at T = 960 K which has grown to an island of about 275 by 180 µm size as 1020 K is reached (see Video S2 in SI). When the heating schedule is stopped at this point a dendritic structure evolves covering the whole imaged area within 4 min. The main axis of the dendritic structure is oriented along the [11̅0]-direction. A second dendritic island growing from the upper right corner approaches the main island but shortly before the two islands make contact, the growth stops leaving a 10 to 20 µm wide gap between the islands. Similar to the VOx islands in Fig. 5, the dendritic islands exhibit a dark outer rim. This dark boundary layer actually constitutes the dendritic structure visible in the PEEM images. The kinetics of the dendritic growth are visualized in an x,t-plot constructed from intensity profiles taken perpendicular to the boundary of a VOx island shown in Fig. 9. Over a period of roughly 3 min one observes linear growth with a constant velocity of 0.4 µm/s. A couple of times the interface snaps back by 5 – 12 µm reaching thereby a velocity of up to 6 µm/s. These events


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Figure 9. x,t-plot showing the growth of a dendritic island during the experiment depicted in Fig. 8. The plot was constructed by taking intensity profiles perpendicular to the boundary of a VOx island. The position of the intensity profile is indicated in Fig. 8, experimental conditions as in Fig. 8, T = 1020 K.

showing up as spikes in the x,t-plot take place seemingly randomly. After such an event the interface returns close to its initial position and continues to grow as before. Interestingly, the dendritic VOx islands are surrounded by a thin bright region which separates the island from the surrounding homogeneous surface, which is characterized by a lower intensity value. Upon growth of the dendritic islands this homogeneous surface region shrinks until finally a bright gap of roughly constant width remains separating neighboring VOx islands. The three black arrows in Fig. 9 indicate the interface between this two regions of lower and higher PEEM intensity.

Travelling wave fragments on VOx islands. At T = 680 K, in an atmosphere with p(MeOH) = 3·10-4 mbar and p(O2) = 1·10-4 mbar, the surface is in a reduced state. The starting configuration is therefore a reduced surface with large VOx islands, as shown in Fig. 5. As the temperature is now continuously increased into the region of dynamic bistability, elliptically dark islands nucleate at T = 790 K on the area


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Figure 10. Traveling wave fragments on macroscopic VOx islands during catalytic methanol oxidation at 830 K. a) Wave fragments nucleating at the borders of the V-oxide islands and travelling across the V-oxide covered regions. The white arrows mark the initial growth direction. b) Magnified section of the PEEM images in (a) showing details of the traveling wave fragments. Experimental conditions: p(O2) = 1·10-4 mbar, p(MeOH) = 3·10-4 mbar, ∆t = 10 s between frames 1 - 2 and 3 – 4 and ∆t = 20 s between frames 2 – 3 in b). FOV: 650 µm (a) and 174 µm (b).

surrounding the VOx islands. At T = 830 K the heating ramp is stopped. As soon as the elliptical dark islands touch the boundaries of the bright VOx islands, dark elliptically shaped oxygen islands nucleate at the boundary and grow into the interior of the VOx islands, as demonstrated by the PEEM images in Fig. 10a and b (see Video S3 in SI). The long axis of these oxygen islands is oriented in the [11̅0]-direction. Quite similar to the behavior seen on the homogeneous surface in the region of dynamic bistability (Fig. 2), the oxidation front is followed by a reduction front with different anisotropy and faster propagation speed. Fig. 10b shows a magnified section of the PEEM images shown in Fig. 10a, marked by a white, dashed square. In the first frame the nucleation of an elliptical oxidation front can be seen. The direction of fastest propagation is indicated by a white arrow in the first two frames and coincides with the [11̅0]– direction. As soon as the oxidation front has crossed the VOx island and touches its boundary, a reduction front nucleates there. The reduction front propagates only on the oxygen covered area,


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thus turning the black surface into a bright area, which marks the refractory state (black arrows in the third and fourth frame of Fig. 10b). The bright area then gradually returns to the initial grey. Quite generally, reduction fronts are observed to always nucleate at boundaries of VOx islands. As seen before on a homogeneously VOx covered surface (Fig. 4), one observes now traveling wave fragments propagating along the [11̅0]-direction on the VOx islands. The average velocity of the oxidation fronts in the [11̅0]- and in the [001]-direction is 5.9 ± 1.1 µm/s and 2.8 ± 0.8 µm/s, respectively. For the reduction fronts, the corresponding propagation speeds are 5.9 ± 0.9 µm/s in the [11̅0]-direction and 9.3 ± 2.4 µm/s in the [001]-direction.

4.

Discussion

On a V-oxide promoted Rh(110) surface we observed chemical wave patterns during catalytic methanol oxidation and, at elevated temperatures, a reaction-induced redistribution of V-oxide into macroscopic V oxide islands. Chemical wave patterns have so far not been observed on any oxidic surface including V-oxide supported on Rh(111). The chemical wave patterns shown here are of a kind rarely seen in surface reactions: dynamic bistability, different front geometries under identical conditions and traveling wave fragments are observed. These patterns exhibit a striking similarity with the NO + H2 reaction on Rh(110) where the same types of chemical wave patterns were found13–15. One can therefore safely assume that a state-dependent anisotropy, which was shown to be responsible for the unusual wave patterns in the NO + H2 reaction on Rh(110), is also present here. As was shown in a preceding LEED/LEEM (low-energy electron microscopy) study, quite a number of VOx-induced surface reconstructions exist in the system Rh(110)/VOx10. They might be responsible for the state-dependent anisotropy suspected here but


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since so far no structural identification of the different surface phases under pattern forming reaction conditions has been conducted, this claim still needs to be substantiated in future studies. It is instructive to compare the behavior seen here with catalytic methanol oxidation on Rh(111)/VOx because for that system structural models of the 2D phases exist, that are well supported by DFT calculations and by scanning tunneling microscopy (STM)11,12. For Rh(111)/VOx a whole zoo of different 2D-phases exist, but they can be explained by a simple structural principle. The 2D-phases can all be described as a network structure in which VO5 pyramids are connected by sharing corner oxygen atoms. Apparently these VO5 pyramids are absent in Rh(110)/VOx because we do not see any substantial formaldehyde production, as it was the case for Rh(111)/VOx in methanol oxidation. Several other indications exist, that the VOx on Rh(110) is structurally quite different from VOx on Rh(111). At elevated temperatures VOx on Rh(110) condenses into macroscopic islands. This was also seen on Rh(111) but, in contrast to VOx on Rh(111), these islands do not move and coalesce under reaction conditions6. A third difference is that we see chemical waves propagating over the VOx islands on Rh(110) (Fig. 10), whereas no chemical waves were observed moving across VOx on Rh(111). All these observations suggest that VOx on Rh(110) cannot be described as an oxide supported on a rigid metal substrate. Instead a more complex interaction of the oxide and the metallic support has to exist. Studies of Rh(110)/V have demonstrated unambiguously that the deposited V at temperatures above 800 K penetrates the Rh bulk forming a subsurface alloy16. From our own XPS data of Rh(110)/VOx acquired ex situ we also arrived at the conclusion that part of the V is probably located in subsurface sites10. If we assume that VOx on Rh(110) is not simply a supported oxide,


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but involves alloying with Rh including probably the population of subsurface sites, then the different behavior of VOx on Rh(111) and Rh(110) would find a plausible explanation. Irrespective of the exact chemical nature of the surface phases one can try to conceive a plausible excitation mechanism, which could involve the following stages:

•

Resting state (grey area in PEEM). We assume that most of the V is present as Rh/V subsurface alloy. The topmost surface layer consisting mainly of Rh atoms would be very reactive towards oxygen keeping in mind that the initial sticking coefficient for oxygen on Rh(110) is close to one27.

•

Excited state (dark area in PEEM). Fast oxygen adsorption onto the top layer of the Rh/V alloy converts the resting state into an oxygen covered surface that represents the excited state of the system.

•

Refractory state (bright area in PEEM). Since oxygen binds more strongly to V than to Rh an oxygen covered surface will stimulate the segregation of V to the surface, where a V-oxide forms. On V-oxide the oxygen sticking coefficient is rather low so that dissociative methanol adsorption now dominates against O2 adsorption. A reduction front develops in which the dissociation products of methanol, Had and COad reduce the Voxide. This reduced phase constitutes the refractory state of the system.

•

Return to the resting state. Since V on the surface is no longer stabilized by oxygen, the V will diffuse back into the Rh bulk forming again a Rh/V subsurface alloy. This step restores the resting state of the system thus closing the excitation cycle.


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It has been observed that V atoms deposited on Rh(110) require temperatures above or around 800 K in order to overcome the barrier for bulk diffusion16. This observation supports the above sketched excitation mechanism that is based on the reversible segregation of V because the chemical waves occur in a similar temperature range extending from ≈ 700 to 850 K. The steps that would be required for verifying the above sketched hypothetical excitation mechanism would involve firstly the development of suitable in situ methods that would allow to characterize structurally and chemically the different surface phases under pattern forming conditions. With spectroscopic LEEM (SPELEEM) such “spectromicroscopic” methods are available at some synchrotron radiation sources, but, at least at present, no in situ measurements are feasible in the 10-4 mbar range, where pattern formation occurs. A second important step would be a structural analyses of the surface phases in the system Rh(110)/VOx in order to be able to relate the chemical and structural properties of the different phases to their role in the mechanism.

5.

Conclusions

It was shown, that the deposition of submonolayer quantities of V-oxide on Rh(110) generates a reaction system with rich dynamic behavior in catalytic methanol oxidation. Chemical wave patterns and a reaction-induced redistribution of the V-oxide have been observed. Around 800 K the originally bistable system turns into a system exhibiting traveling wave fragments and dynamic bistability with varying front geometries. Both types of chemical waves require the existence of a state-dependent diffusional anisotropy. The most likely cause for such a statedependent anisotropy are V,O-induced reconstructions with varying substrate geometries.


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At elevated temperatures around 1000 K the originally homogeneous VOx adlayer condenses under reaction conditions into macroscopic VOx islands. Under suitable reaction conditions this may happen via fast dendritic growth. Cooling down the surface with the macroscopic islands to 800 K, traveling wave fragments and oxidation and reduction fronts with different front geometries are seen to propagate on the V-oxide islands. The behavior of the VOx islands on Rh(110) in catalytic methanol oxidation is markedly different from VOx on Rh(111): (i) no substantial formaldehyde production occurs, (ii) the VOx islands do not move and coalesce under reaction conditions, and (iii) chemical waves can propagate across their surface. These differences suggest that, in contrast to VOx on Rh(111), the VOx on Rh(110) cannot simply be described as a supported oxide, but probably involves alloying with the Rh substrate including the population of subsurface sites by V. Among the different surface reactions exhibiting chemical wave patterns, the present system belongs to the systems with the richest phenomenology comparable to catalytic CO oxidation on Pt(110) and to the NO + H2 reaction on Rh(110). What is new is the involvement of an oxide and with V oxide it is one of the most versatile and most important catalysts in chemical industry. Elucidating the complex interplay between V oxide and Rh(110) in catalytic methanol oxidation could play a paradigmatic role for understanding the dynamics of supported catalysts under reaction conditions.


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ASSOCIATED CONTENT Supporting Information. A brief description of the experimental conditions of the three videos is included in the supporting information. (PDF S4) Video of the coarsening of macroscopic VOx islands. (Video S1) Video of the dendritic growth of macroscopic VOx islands. (Video S2) Video of traveling wave fragments on macroscopic VOx islands. (Video S3)

AUTHOR INFORMATION Corresponding Author *Corresponding Author: Ronald Imbihl ([email protected]) Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT The authors gratefully acknowledge Andrea Locatelli and Tefvik Onur Menteş for fruitful discussions. Funding Sources This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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(3) Imbihl, R. Chapter 9 Non-linear Dynamics in Catalytic Reactions. Dynamics; Handbook of Surface Science; Elsevier, Amsterdam, 2008; pp 341–428. (4) Lovis, F.; Hesse, M.; Locatelli, A.; Menteş, T. O.; Niño, M. Á.; Lilienkamp, G.; Borkenhagen, B.; Imbihl, R. Self-Organization of Ultrathin Vanadium Oxide Layers on a Rh(111) Surface during a Catalytic Reaction. Part II: A LEEM and Spectromicroscopy Study. J. Phys. Chem. C 2011, 115, 19149–19157. (5) Lovis, F.; Imbihl, R. Self-Organization of Ultrathin Vanadium Oxide Layers on a Rh(111) Surface during a Catalytic Reaction. Part I: A PEEM Study. J. Phys. Chem. C 2011, 115, 19141– 19148. (6) Hesse, M.; von Boehn, B.; Locatelli, A.; Sala, A.; Mentes, T. O.; Imbihl, R. Island Ripening via a Polymerization-Depolymerization Mechanism. Phy.Rev. Lett. 2015, 115, 136102. (7) Thiel, P. A.; Shen, M.; Liu, D.-J.; Evans, J. W. Coarsening of Two-Dimensional Nanoclusters on Metal Surfaces. J. Phys. Chem. C 2009, 113, 5047–5067. (8) Morgenstern, K. Fast Scanning Tunnelling Microscopy as a Tool to Understand Changes on Metal Surfaces: From Nanostructures to Single Atoms. phys. stat. sol. (b) 2005, 242, 773–796. (9) von Boehn, B.; Imbihl, R. Large Amplitude Excitations Traveling along the Interface in Bistable Catalytic Methanol Oxidation on Rh(110). Phys. Chem. Chem. Phys. 2017, 19, 18487– 18493. (10) von Boehn, B.; Menteş, T. O.; Locatelli, A.; Sala, A.; Imbihl, R. Growth of Vanadium and Vanadium Oxide on a Rh(110) Surface. J. Phys. Chem. C 2017, 121 (36), pp 19774–19785. (11) Schoiswohl, J.; Sock, M.; Eck, S.; Surnev, S.; Ramsey, M. G.; Netzer, F. P.; Kresse, G. Atomic-Level Growth Study of Vanadium Oxide Nanostructures on Rh(111). Phys. Rev. B 2004, 69, 155403. (12) Schoiswohl, J.; Surnev, S.; Sock, M.; Eck, S.; Ramsey, M. G.; Netzer, F. P.; Kresse, G. Reduction of Vanadium-Oxide Monolayer Structures. Phys. Rev. B 2005, 71, 86102. (13) Mertens, F.; Imbihl, R. Square Chemical Waves in the Catalytic Reaction NO + H2 on a Rhodium(110) Surface. Nature 1994, 370, 124–126. (14) Gottschalk; Mertens; Bär; Eiswirth; Imbihl. Chemical Waves in Media with StateDependent Anisotropy. Phy.Rev. Lett. 1994, 73, 3483–3486. (15) Mertens, F.; Gottschalk, N.; Bär, M.; Eiswirth, M.; Mikhailov, A.; Imbihl, R. TravelingWave Fragments in Anisotropic Excitable Media. Phys. Rev. E 1995, 51, R5193-R5196. (16) Píš, I.; Stetsovych, V.; Mysliveček, J.; Kettner, M.; Vondráček, M.; Dvořák, F.; Mazur, D.; Matolín, V.; Nehasil, V. Atomic and Electronic Structure of V–Rh(110) Near-Surface Alloy. J. Phys. Chem. C 2013, 117, 12679–12688. (17) Locatelli, A.; Aballe, L.; Mentes, T. O.; Kiskinova, M.; Bauer, E. Photoemission Electron Microscopy with Chemical Sensitivity: SPELEEM Methods and Applications. Surf. Interface Anal. 2006, 38, 1554–1557. (18) Comelli, G.; Dhanak, V. R.; Kiskinova, M.; Prince, K. C.; Rosei, R. Oxygen and Nitrogen Interaction with Rhodium Single Crystal Surfaces. Surf. Sci. Rep. 1998, 32, 165–231. (19) Mikhailov, A. S. Foundations of Synergetics I: Distributed Active Systems, [2nd ed.]; Springer Series in Synergetics, 0172-7389 51; Springer Berlin Heidelberg: Berlin, 1994. (20) Bär; Nettesheim; Rotermund; Eiswirth; Ertl. Transition between Fronts and Spiral Waves in a Bistable Surface Reaction. Phy.Rev. Lett. 1995, 74, 1246–1249. (21) Makeev, A.; Imbihl, R. Simulations of Anisotropic Front Propagation in the H2 + O2 Reaction on a Rh(110) Surface. J. Chem. Phys. 2000, 113, 3854–3863.


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Chemical Wave Patterns and Oxide Redistribution ... - ACS Publications

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