Reactive Phase Separation during Methanol Oxidation on a V-Oxide

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Reactive Phase Separation during Methanol Oxidation on a V‑OxidePromoted Rh(110) Surface Bernhard von Boehn,† Tevfik O. Menteş,‡ Andrea Locatelli,‡ Alessandro Sala,§ and Ronald Imbihl*,† †

Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstrasse 3A, D-30167 Hannover, Germany Elettra Sincrotrone Trieste S.C.p.A., Strada Statale 14, Km 163.5 in Area Science Park, 34149 Basovizza, Trieste, Italy § CNR-IOM - Istituto Officina dei Materiali, Headquarters (Trieste, TASC), c/o Area Science Park − Basovizza, Strada Statale 14, Km 163.5, 34149 Basovizza, Trieste, Italy ‡

S Supporting Information *

ABSTRACT: The distribution of ultrathin layers of vanadium oxide on Rh(110) (θV ≤ 1 MLE, one monolayer equivalent corresponds to the number of Rh atoms in the topmost Rh(110) surface layer) after exposure to catalytic methanol oxidation in the 10−4 mbar range has been investigated with x-ray photoelectron spectroscopy and spectroscopic low-energy electron microscopy (SPELEEM). The reaction is shown to cause a macroscopic phase separation of the VOx film into VOx-rich and into V-poor phases. For θV = 0.8 MLE compact VOx islands develop whose substructure exhibits several ordered phases. At θV = 0.4 MLE the VOx-rich phase consists of many small VOx islands (0.1−1 μm). Laterally resolved x-ray photoelectron spectroscopy of V 2p3/2 shows an oxidic component at 515.5 eV binding energy (BE) and a component at 513.0 eV BE attributed to metallic or strongly reduced V. On the V-poor phase only the reduced/metallic component is present. The results are compared with the distribution of ultrathin layers of vanadium oxide on Rh(111) after catalytic methanol oxidation. The presence of the metallic V on Rh(110) is at variance with the behavior of Rh(111), where V is found to be present only in high oxidation states during methanol oxidation.

1. INTRODUCTION Ultrathin V-oxide layers on a Rh(111) substrate have been considered a well-defined model system for supported V-oxide catalysts.1−5 At submonolayer coverages, V oxide forms a large number of ordered phases on Rh(111), which are strictly twodimensional. During catalytic methanol oxidation the V-oxide condenses into domains giving rise to macroscopic striped or circular patterns. Under reaction conditions, neighboring circular V-oxide islands undergo a coalescence process that is controlled by a chemical polymerization/depolymerization equilibrium between small VOx clusters and compact VOx islands.6,7 Though mechanistically different, this process results in the formation of larger islands similar to Ostwald or Smoluchowski ripening.8,9 At variance to the case of Rh(111)/VOx, where no chemical waves are seen during catalytic methanol oxidation, V oxide on Rh(110) behaves differently. In fact, one can observe here a rich variety of chemical waves including wave fragments traveling along well-defined crystallographic directions, varying front geometries, traveling interface modulations, dendritic growth of the VOx phase, and a reactive phase separation into macroscopic VOx islands and V-poor surroundings.10,11 During VOx deposition onto Rh(110), a number of ordered phases have been identified with LEED, but in contrast to Rh(111), no structural analysis has been so far performed for any of them.12 There is evidence supporting the idea that VOx on Rh(110) cannot be described as a simple oxide supported on a metal but is structurally more complex due to alloying and, possibly, © XXXX American Chemical Society

penetration of V into the deeper layers of Rh. In this regard, the formation of a V subsurface alloy has already been demonstrated by photoelectron diffraction for the case of metallic V on Rh(110).13 One key observation further underlining the differences between the two systems is that no formaldehyde formation is observed on Rh(110)/VOx in catalytic methanol oxidation, whereas this is the dominant product on Rh(111)/VOx at temperatures above 800 K.10 A different behavior of Rh(110) toward a VOx layer as compared to Rh(111) is not unexpected because the very open and therefore thermodynamically less stable Rh(110) surface is prone to restructuring and alloying in order to reduce its high surface energy. This work is part of a series of investigations aimed at uncovering the excitation mechanism of chemical waves in methanol oxidation on Rh(110)/VOx. To do this, we will proceed as follows: (i) identify the relevant phases occurring during the formation of chemical waves and (ii) determine their different role in the excitation mechanism, based on their varying adsorption and catalytic properties. Both tasks require the use of in situ methods with structural and chemical sensitivity. Unfortunately, the pattern-forming parameter range for our system starts at 10−4 mbar, which is beyond the pressure range accessible in the spectroscopic low-energy electron Received: March 15, 2018 Revised: April 23, 2018 Published: April 25, 2018 A

DOI: 10.1021/acs.jpcc.8b02544 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

O 1s, V 2p, and Rh 3p peaks were fitted using Doniach−Šunjić (DS) line shapes convoluted with a Gaussian after a Shirley background was subtracted.17 Since the SPELEEM can only be operated (when using the beamline photons) at pressures lower than 1 × 10−6 mbar, the V-oxide structures were prepared in a separate high-pressure reaction chamber in the 10−4 mbar range. Transfer from this chamber to the microscope vessel was performed in UHV. The reaction was run for 30 min at ≈1000 K at an oxygen partial pressure of 1 × 10−4 mbar and methanol partial pressure of 3 × 10−4 mbar; subsequently, the sample was cooled in gas atmosphere down to 470 K, at which point the gas supply was stopped. Ex situ characterization with LEEM and μLEED was carried out at room temperature in vacuum. XPEEM measurements were performed at a temperature of 370 K in an oxygen atmosphere of 2 × 10−7 mbar in order to prevent beam damage effects.

microscopy (SPELEEM) during operation.14,15 In order to bridge this pressure gap, the following strategy has been applied. We will apply reaction conditions in the 10−4 mbar range in a dedicated high-pressure reactor, followed by a sample transfer in the analysis chamber where SPELEEM is used in an oxygen atmosphere in the 10−7 mbar range. As will be shown in the following, we can generate in this way a phase separation into a VOx-rich and a V-poor phase which is preserved under measurement conditions.

2. EXPERIMENTAL SECTION All experiments were performed using the spectroscopic photoemission and low-energy electron microscope (SPELEEM) at the Nanospectroscopy beamline of the Elettra synchrotron light source. The SPELEEM III microscope (Elmitec GmbH) allows laterally resolved x-ray photoelectron spectroscopy measurements (XPEEM = x-ray photoelectron emission microscopy) at an energy resolution of 300 meV and spatial resolution of 25 nm, using x-rays in the range of 20− 1000 eV photon energy. In the SPELEEM, the sample is illuminated by the x-ray beam at grazing incidence (16°), and the photoelectrons are collected in normal emission. The microscope can also probe the specimen with low-energy electrons, using the elastically backscattered electron beam for imaging. Such LEEM operation allows surface-sensitive realspace imaging with a best spatial resolution of about 10 nm. Structure identification is carried out using microprobe LEED (μ-LEED). In μ-LEED the illuminating electron beam is laterally restricted by apertures of 0.5, 1, or 5 μm diameter. The Rh(110) surface was cleaned by cycles of Ar ion sputtering (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 in LEED, with the c(2 × 8) pattern at high exposures (1 × 10−7 mbar, 570 K), was taken as evidence for a clean surface.16 V-oxide overlayers were prepared on the Rh(110) single-crystal surface by electron beam evaporation of a high purity V rod (Goodfellow) at a substrate temperature of 670 K in an oxygen ambient of 2 × 10−7 mbar (reactive evaporation3). To obtain ordered VOx overlayers, the Rh substrate was heated to 670 K in oxygen for 10 min before the deposition was started. In this manner V was deposited onto the c(2 × 8)-O structure on Rh(110). All VOx coverages reported in the following are given in monolayer equivalents (MLE) of the topmost Rh(110) surface layer. A coverage calibration was achieved using previous XPS and LEED results for the deposition of V and V oxide on Rh(110), where V coverages could be assigned to V-oxide structures observed in LEED.12 We note that the determined V coverages should be considered as tentative, since the calibration relies on the assumption of a layer-by-layer growth for Rh(110)/V, which has not been yet verified experimentally. All V-oxide structures previously studied on Rh(111) were grown using the same procedure, resulting in the well-known ( 7 × 7 ) R19.1° structure for coverages lower than 0.6 MLE.3 The photoelectron spectra shown in this work were recorded at photon energies of 650 eV (Rh(110)/VOx) and 652 eV (Rh(111)/VOx). In the case of Rh(110), the binding energies were referenced to the Fermi level evaluated from the valence band spectrum, which was measured separately. The binding energies of the spectra recorded on the Rh(111) substrate were referenced to the Rh 3d5/2 bulk peak (307.2 eV), instead. The

3. RESULTS 0.4 MLE V Oxide. In the first experiment 0.4 MLE V-oxide was deposited on the Rh(110) surface, and the sample was heated in the reaction chamber in a MeOH + O2 atmosphere with p(MeOH) = 3 × 10−4 mbar and p(O2) = 1 × 10−4 mbar to 1000 K for 30 min. After the sample was transferred to the microscope chamber, LEEM and XPEEM were performed. The images shown in Figure 1a reveal that the reaction has induced

Figure 1. LEEM (a) and XPEEM (b) images of a macroscopic VOx island formed on Rh(110) upon methanol oxidation in a reaction atmosphere of 1 × 10−4 mbar oxygen and 1 × 10−4 mbar methanol acquired ex situ. The images were recorded at 370 K in an oxygen atmosphere of 2 × 10−7 mbar, θV = 0.4 MLE. The field of view and electron energies are indicated by labels on top of each image.

a phase separation leading to the formation of microscopically extended surface regions exhibiting different V and O concentration (namely, V poor and VOx rich). Interestingly, the VOx-rich phase displays numerous islands that appear dark in LEEM at the chosen electron energy, their size being typically below 1 μm. The resulting morphology is quite different from that observed in our previous PEEM experiment of pattern formation during catalytic methanol oxidation, which was characterized by the formation of large VOx islands showing uniform composition.10 This different morphology may be related to a condensation process occurring in the VOxrich phase upon cooling after reaction. B


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The Journal of Physical Chemistry C The XPEEM images of O 1s and V 2p3/2 in Figure 1 show that the islands correlate with high V and O coverage. Analysis of the intensity of the XPEEM data demonstrates that, outside the VOx islands, a substantial concentration of V and O persists. Evidently, the phase separation does not lead to the complete separation into V-free and VOx-covered regions. For the spectrum of the V-poor region data were extracted from the whole V- and O-depleted area visible in the second and third XPEEM image of Figure 1b. The spectrum of the VOx-rich region was extracted from all V- and O-rich areas visible in the second and third XPEEM image of Figure 1b. The local XP spectra of the VOx-rich and V-poor regions extracted from the XPEEM data are displayed in Figure 2. An

Figure 2. XP spectra taken ex situ on V-oxide-poor and -rich regions on Rh(110)/VOx after macroscopic islands have formed upon catalytic methanol oxidation at 1020 K. The spectra were extracted from the XPEEM data shown in Figure 1. The Arabic numbers refer to the different core levels as described in the legend. The different components are summarized in Table 1.

Figure 3. Comparison of VOx island formation on Rh(110) and Rh(111) during catalytic methanol oxidation. The XP spectra are all taken ex situ from V-oxide-poor and V-oxide-rich regions on Rh(110)/ VOx and Rh(111)/VOx surfaces. The macroscopic islands have formed upon catalytic methanol oxidation at 1020 K. (a) and (b) XP spectra containing the V 2p3/2 and Rh 3d5/2 levels of the V-rich and Vpoor region on Rh(110), respectively. (c) LEEM image showing macroscopic V-oxide islands on Rh(110). (Beam energy 13 eV, field of view: 30 μm). (d) and (e) V 2p3/2 and Rh 3d5/2 levels measured inside a macroscopic VOx island and on the surrounding surface on Rh(111), respectively. (f) PEEM image illustrating the substructure of a VOx island on Rh(111). All XP spectra are taken in an oxygen atmosphere of 2 × 10−7 mbar at 370 K in order to prevent beam damage effects.

approximately 1 μm wide stripe around the interface VOxrich/V-poor region was excluded from the analysis. Best curve fits to the experimental data are also shown. The spectra confirm the enrichment of V and O inside the VOx islands, but the enrichment factor amounts only to 7 for V and 3.4 for O as calculated by numerical integration of the backgroundcorrected V 2p3/2 (Figure 3a) and O 1s (not shown) intensity. The V 2p peak was fitted with one component in the case of the V-poor regions and with two components at the VOx islands. The binding energies of V, Rh, and O for the different phases are summarized in Table 1, which also contains the values of VOx islands on Rh(111) for comparison. In the vanadium-poor regions a ratio of oxygen to vanadium atoms, NO/NV, of 0.3 was estimated from the peak area ratio of the O 1s and V 2p core levels (for details see Supporting Information). The binding energy (BE) of the V 2p3/2 emission recorded in this region, 513.0 eV, appears rather low for the Vpoor region. The low BE component of the VOx-rich region displays the same BE of 513.0 eV. This value is rather close to that of metallic V, i.e., 512.2 eV.18 The slightly higher BE value we measured could be attributed to the formation of dispersed V particles (with size well below the microscope lateral resolution) in a low oxidation state or alloyed with Rh.13 We note that a BE of 513.2 eV was measured for the so-called “wagon-wheel” structure of VOx on Rh(111) and assigned to V being in a V2+ state.5 A likely explanation for the presence of a detectable amount of oxygen on the vanadium-oxide-poor region is that an ambient oxygen atmosphere of 2 × 10−7 mbar was maintained through the XPEEM measurements in order to

prevent x-ray induced reduction of the vanadium oxide structures. Focusing now on the VOx-rich regions, we find that the high BE component of the V 2p3/2 emission is peaked at 515.5 eV. The oxygen to vanadium ratio, NO/NV, is here estimated to be 0.8. This BE of 515.5 eV corresponds to V in the oxidation state +3, the binding energies for V+3 in bulk oxides reported in the literature varying in the range from 515.2 to 515.9 eV.19−22 The binding energies of the O 1s peaks in the V-poor and V-rich areas were both determined to be 530.0 eV. In growth experiments of VOx on Rh(110), the formation of a (12 × 1) LEED pattern was observed after deposition of 3 MLE, followed by annealing in 2 × 10−7 mbar oxygen at 700 K.12 The XP spectra of this (12 × 1) show V 2p3/2 components at 513.4 and 516.3 eV and an O 1s peak at 530.0 eV BE. These signals are also present in the spectrum of the VOx-rich areas shown in Figure 2, but both V 2p3/2 components are shifted by 0.4 and 0.8 eV BE, respectively. In the following, we compare the photoelectron spectra of VOx on Rh(110) with those of VOx on Rh(111).6 The C


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Table 1. XPS Binding Energies Measured on Different Phases Appearing in Phase Separation during Catalytic Methanol Oxidation on Rh(110)/VOx and Rh(111)/VOxa Rh(110)

Rh(111)

a

VOx rich V poor (12 × 1)-VOx12 VOx island core VOx island rim uncovered Rh(111)

O 1s

V 2p3/2

Rh 3d5/2

Rh 3p3/2

530.0 530.1 530.0 529.9 530.0 529.9

515.5/513.0 513.0 516.3/513.4 516.2/514.8/513.8 516.2/514.5/513.5 -

307.3 307.3 307.1 307.2 307.2 307.2

496.0/495.0 496.0/494.6 496.0 496.4 496.1 496.3

The spectra were acquired in an oxygen atmosphere of 2 × 10−7 mbar at 370 K, and all binding energies are given in eV.

Figure 4. Characterization of a V-oxide island on Rh(110) with LEEM. (a) PEEM image of VOx islands and a scheme illustrating the different phases identified by LEEM and μLEED. (b) LEEM bright-field image recorded at 44 eV, showing a cross section through a big V-oxide island. Several images are put together to form the cross section. Four different phases can be identified by different gray levels. (c) μLEED characterization of the four phases visible in the LEEM image in (b). θV = 0.8 MLE, and the electron energy for the μLEED images was 40 eV (image 1), 48 eV (images 2 and 3), and 44 eV (image 4).

show that the Rh 3d5/2 peak height for Rh(111) underneath the island core and rim is reduced by 11−14% with respect to the value of the uncovered Rh surface. This difference, compared to the only 3% difference on Rh(110), supports the idea that the structure of the VOx islands on Rh(110) is quite different from the structure of VOx on Rh(111). The absence of an attenuation in the Rh 3d intensity by the VOx layer on Rh(110) can be interpreted as due to VOx, being incorporated into the Rh topmost layer, instead of being located on top of it. At the extreme, one also could envision the VOx being encapsulated by the Rh substrate, i.e., forming a mixed oxide. A comparison of the V 2p3/2 spectra recorded on Rh(110)/ VOx and Rh(111)/VOx is shown in Figure 3a and d. The change in intensity of the spectra recorded inside the island on Rh(111) and on the surrounding surface demonstrate that, in contrast to Rh(110)/VOx, a nearly complete phase separation takes place. Outside the V-oxide islands the V 2p3/2 intensity is close to the detection limit of the microscope, and only a small contribution around 516.5 eV can be identified on Rh(111). 0.8 MLE V Oxide. In a second experiment, 0.8 MLE Voxide are deposited on the Rh(110) surface, and the sample is heated in a MeOH + O2 atmosphere with p(MeOH) = 3 ×

corresponding spectra are summarized in Figure 3a−e. As shown in Figure 3f, VOx on Rh(111) arranges into circular domains (or islands) that exhibit a reduced core region appearing bright in PEEM and an oxidized rim showing up as a dark area in PEEM.6 As demonstrated by the PEEM image in Figure 4a, VOx islands on Rh(110) exhibit a quite similar structure to those seen on Rh(111). In the Rh 3d5/2 XPEEM images the VOx islands show up as dark areas in the first XPEEM image of Figure 1b, which may be explained by the attenuation of the emitted Rh 3d electrons by V oxide on top of the Rh atoms. Using the appropriate attenuation factor, one could in principle estimate the thickness of the V-oxide layer (assuming the existence of a V-oxide overlayer). A closer look at the spectra in Figure 3b shows that the intensity of the Rh 3d signal in the dark and bright areas is basically identical. A detailed inspection of the same spectra reveals that the reason for the contrast in the XPEEM images is the 80 meV shift of the peak toward higher BE in the VOx islands. This shift might be caused by additional oxygen being present in the VOx island or by alloying of Rh with V. Figure 3e shows Rh 3d5/2 spectra from Rh(111)/VOx, where the composition of the VOx islands is well-known. The spectra D


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The Journal of Physical Chemistry C 10−4 mbar and p(O2) = 1 × 10−4 mbar to 1000 K for 30 min in the high-pressure cell. The PEEM image presented in Figure 4a shows a typical VOx distribution observed during reaction. Notably, the patterns are preserved after transferring the sample into the analysis chamber, as demonstrated by the LEEM image shown in Figure 4b. Four different phases can be identified by their different intensity. We note that LEEM and PEEM use different contrast mechanisms (the first technique exploits diffraction and the second differences in the local work function), so that the image intensities cannot be compared directly. As shown in Figure 4c, the different phases of the VOx island were characterized by μ-LEED, using an aperture which restricts the diameter of the illuminating electron beam to 1 μm: (1) the surrounding Rh(110) surface displaying a sharp (1 × 1) pattern (indicated by yellow circles), (2) a 20−30 μm wide bright zone which represents the dark boundary layer in PEEM and which is characterized by a (1 × 2) pattern, (3) a thin inner boundary layer which is only ∼5 μm wide and is hardly distinguishable in LEEM, displaying a (4 × 1) pattern superimposed with spots of a (2 × 2) structure, and (4) the core region which exhibits a (2 × 2) pattern. Due to the small width of phase (3), compared to the size of the probe, there might be a small overlap in the LEED patterns recorded from (3) and (4). Interestingly, a similar substructure was also found in the oxide islands in the system Rh(111)/VOx: a core which is separated from an outer rim by a thin boundary phase.6,7 Notably, all of the LEEM images of Figure 4b show tiny bright spots that are more or less uniformly distributed across various surface phases. These bright islands have been identified with μXPS and μLEED as SiO2 contaminations. Their hexagonal diffraction pattern is visible in some of the diffraction patterns in Figure 4c (red boxes). The SiO2 contamination was only present in the particular set of experiments shown in Figure 4 and not in the XPEEM measurements discussed before. At low pressure, the SiO2 particles are more or less inert so that their influence on pattern formation should be negligible.

for the mass transport over micrometer distances is the diffusion of small VnOm clusters.26 In catalytic methanol oxidation on Rh(111)/VOx the fast diffusion of clusters like V6O12 was shown to be responsible for the interaction of macroscopic VOx islands over distances as large as 100 μm.6 The high mobility of these clusters could therefore account for the necessary mass transport of V during reactive phase separation on Rh(110)/VOx. At a low V coverage on Rh(110), at θV = 0.4 MLE, no complete phase separation into a VOx-rich and V-poor phase occurred: the V-rich phase was not uniform but comprised many small VOx islands. At higher V coverage, namely, θV = 0.8 MLE, the VOx phase is laterally uniform. As revealed by μLEED, however, the VOx phases on Rh(110) exhibit a substructure of differently ordered phases. Such a substructure was also observed for VOx on Rh(111), in which the existence of different phases was due to an oxygen gradient across the island.6,7 The oxygen that is consumed by the “VOxmicroreactors” is provided by the surrounding metal surface (where O2 is adsorbing rapidly). Accordingly, the chemical potential of oxygen decreases from the rim of the VOx island to the core region. In the system under scrutiny, the catalytic activity of the different phases is not known, but a plausible assumption is that oxygen adsorbs more easily on the metallic rather than on the VOx phase.27 If this is indeed the case, the different phases in Figure 4 should show the presence of an oxygen gradient, reflecting in turn the chemical potential varying across the pattern. A cross section through a VOx island by μLEED revealed the presence of several ordered overlayers: a (1 × 2) in the outer ring, a (4 × 1) in the narrow boundary layer between the outer ring and island core, and finally, a (2 × 2) in the core of the island. These structures should be compared with the structures observed in the growth studies of V and VOx on Rh(110).12 With Rh(110)/V a (1 × 2), (1 × 4), and (2 × 1) have been reported; VOx deposition on Rh(110) led to the formation of a series of (1 × 3), (1 × 6), (2 × 2), (5 × 6), and a (12 × 1) overlayer structures. Thus, some of the periodicities observed with μLEED on the VOx island have already been seen in the growth study of V and VOx on Rh(110). This should make an identification of the surface phases in Figure 4 easier, keeping in mind that identical LEED patterns do not necessarily mean identical surface structures. A number of indications exist that the VOx islands on Rh(110) are structurally quite different from VOx islands on Rh(111). First of all, there is the above-mentioned absence of formaldehyde production on Rh(110)/VOx in catalytic methanol oxidation.10 Second, also a number of spectroscopic differences exist as evidenced by the results depicted in Figures 1−3. On Rh(110)/VOx a strongly reduced V state together with an oxidic component is present on the VOx-rich region. On the V-poor region only the reduced state is present. In contrast, on Rh(111)/VOx, the V 2p3/2 component measured on the surface surrounding the oxide islands has only intensity at a BE around 516.5 V corresponding to highly oxidized V, i.e., V4+. Furthermore, on Rh(110) we do not see a substantial damping of the Rh 3d signal by a VOx layer on top of the metallic substrate, as was the case for Rh(111)/VOx (Figure 3). An obvious solution to these problems is that VOx on Rh(110) involves alloying with the Rh substrate, probably including the population of Rh subsurface sites by V atoms. As for the case of the XPS experiments, the μLEED characterization has only been conducted after cooling down/pumping down but not in

4. DISCUSSION A reactive phase separation should occur in bimetallic catalysts if: (i) one of the metallic constituents is bonded much more strongly to one adsorbate than to the other metal and (ii) if this constituent is sufficiently mobile.23 This concept has first been demonstrated with the highly reactive and highly mobile alkali promoters in the O2 + H2 reaction on Rh(110) and subsequently with Rh(110) alloyed with Au and Pd, also in the O2 + H2 reaction.24 In the present case, we do not see a periodic pattern as observed in the before mentioned examples, but surface defects and insufficient mobilities can easily cause irregularities in pattern formation. Nevertheless, the different chemical affinity of V and Rh to oxygen might well be responsible for a reactive phase separation to occur in catalytic methanol oxidation. According to Piś et al., who studied the deposition of V on Rh(110), temperatures around 820 K are required for bulk diffusion of V to occur.13 The same temperature range was reported for the subsurface alloy formation of 0.4 MLE V on Rh(111), whereas a higher temperature of up to 1023 K was found for a V coverage of 2.5 MLE.25 A diffusion length of a few Å is of course far away from the huge diffusion lengths on which macroscopic phase separation takes place, as evidenced by the images in Figures 1 and 4. A much more likely candidate E



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situ under reaction conditions. Therefore, a chemical or structural transformation during the transfer of the sample cannot be excluded. What is still missing is the connection between the reactive phase separation seen here and the dynamics of the macroscopic VOx islands during catalytic methanol oxidation on Rh(110)/VOx in the 10−4 mbar range.10 Unlike Rh(111)/ VOx, the VOx islands on Rh(110) do not move under reaction conditions, but they have a certain growth dynamics involving even dendritic growth patterns under suitable chosen conditions. Moreover, chemical waves are seen to propagate on the islands themselves as well as on the surrounding area. The study here demonstrates that the area surrounding the VOx islands is probably not simply metallic Rh(110) but an alloyed Rh(110)/V surface. Since in situ measurements with SPELEEM are currently not feasible in the 10−4 mbar range, a definitive proof that the phases appearing during chemical waves are identical with some of the phases characterized here needs to be provided by future studies.

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02544. S1: Details on the estimation of the amount of V and O from the XP spectra (PDF)

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ACKNOWLEDGMENTS

We wish to thank Matteo Lucian for technical support.

5. CONCLUSIONS We have observed that for VOx deposited on Rh(110) a reactive phase separation occurs during catalytic methanol oxidation in the 10−4 mbar range into a VOx-rich and a V-poor phase. For θV = 0.8 MLE, compact VOx islands of macroscopic size develop, which consist of several ordered phases, as shown by μLEED. At θV = 0.4 MLE the VOx-rich phase is not compact but consists of many small VOx islands. As demonstrated by μXPS the V contains an oxidic species, probably V3+, and a metallic or at least strongly reduced V species. On the V-poor phase only the reduced/metallic V species is seen. The reactive phase separation we observe is therefore apparently connected to V alloying with Rh. Compared to VOx on Rh(111), the interaction of VOx with the metallic substrate Rh(110) is more complex, involving alloying of both metals and probably the formation of subsurface V. The different nature of VOx on Rh(110) and Rh(111) is reflected by (i) a different catalytic activity in methanol oxidation, (ii) spectroscopy showing a metallic/strongly reduced V species on Rh(110), and (iii) the formation of chemical waves in the Rh(110)/VOx system and the absence of such waves on Rh(111).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bernhard von Boehn: 0000-0003-3722-5767 Andrea Locatelli: 0000-0002-8072-7343 Ronald Imbihl: 0000-0002-5155-7250 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest. F


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Reactive Phase Separation during Methanol Oxidation on a V-Oxide

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