Decomposition of Methanol on Mixed CuOâCuWO4 Surfaces - The

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Decomposition of Methanol on Mixed CuO – CuWO Surfaces Matthias A. Blatnik, Carl Drechsel, Nataliya Tsud, Svetlozar Surnev, and Falko P. Netzer J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06233 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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The Journal of Physical Chemistry

Decomposition of Methanol on Mixed CuO – CuWO4 Surfaces M. Blatnik1, C. Drechsel1, N. Tsud2, S. Surnev1* and F.P. Netzer1* 1

Surface and Interface Physics, Institute of Physics, Karl-Franzens University Graz,

Universitätsplatz 5, 8010 Graz, Austria 2

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, Prague 18000, Czech Republic

ABSTRACT Mixed CuO(2x1)-CuWO4 layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2x1) surface oxide with adsorbed (WO3)3 clusters. The adsorption and decomposition of methanol on these well-defined CuO-CuWO4 surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200 - 300 K or decompose into CHx and possibly CO. The most reactive surfaces are mixed CuO-CuWO4 phase, with CuWO4 coverages 0.5-0.8 monolayer, thus pointing at the importance of oxide phase boundary sites. In a minority reaction channel, a small amount of formaldehyde is detected on the CuWO4 surface. The CuWO4 oxide phase becomes modified as a result of reduction and a morphology transition triggered by the methanol decomposition, but the pristine surface state can be recovered by a post-oxidation treatment with oxygen.

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INTRODUCTION Methanol (CH3OH) has a wide range of applications in the chemical industry1 and is a promising potential alternative for automobile fuel.2,3 Due to the importance of Cu/ZnO catalysts in the methanol synthesis from syngas4,5, the reaction of methanol on clean and oxygen-covered Cu(110) surfaces has been intensively investigated over the last three decades.6-18 The main conclusion from these studies is that the catalytic oxidation of methanol on copper starts with its decomposition to surface methoxy (CH3O) species, followed by their dehydrogenation to formaldehyde (CH2O). The partial oxidation of methanol occurs via the formation of formate (HCO2) that eventually decomposes into CO2 and H2, which readily desorb. The chemical state and concentration of oxygen on the Cu(110) surface plays a decisive role in the oxidation of methanol, with the partially covered CuO(2x1)-reconstructed surface found as the most reactive, whereas the fully O-covered surface is almost unreactive. This suggested that the Cu-CuO boundary may be decisive for the chemical reactivity.

Methanol, as the simplest alcohol, has been considered as an ideal probe molecule for characterizing the catalytic reactivity and product selectivity of metal oxide surfaces.19,20 This renders methanol particularly suitable for investigating the catalytic behavior of mixed metal oxides, which may expose several surface sites with different chemical and electronic properties.21 The methanol oxidation products on mixed oxide surfaces should reflect the nature of the surface active sites: strong acidic sites generate dimethyl ether (CH3OCH3), basic sites yield dehydrogenation to CO2, whereas redox sites produce formaldehyde.19 However, correlating the catalytic performance to the surface characteristics, such as the chemical composition, morphology, coordination number and oxidation states of the metal cations, requires a precise knowledge of the structure of mixed oxide surfaces, which is rarely achieved in practical catalyst systems. Here, catalytic model systems with well-defined atomic arrangements are required. In this study, we present such a model system for a mixed Cu-W oxide surface.

Ternary tungsten oxides, such as metal tungstates (MWO4), belong to an important class of materials with diverse technological applications, amongst them in catalysis, photocatalysis and photoelectrochemistry.22-29 Recently, the fabrication of a two-dimensional (2D) Cu tungstate (CuWO4) ternary oxide monolayer on Cu(110) by a novel on-surface synthesis 2 ACS Paragon Plus Environment

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method has been reported.30 The method employs the thermally induced surface chemical reaction between the Cu(110)(2x1)-O surface oxide layer with a monolayer of (WO3)3 clusters, the latter deposited from the gas phase, leading after reaction to a well-ordered 2D CuWO4 phase.30 This method of preparation allows us to generate CuO-CuWO4 model surfaces with well defined structure and variable composition, ranging from pure CuO to pure CuWO4 via mixed CuO-CuWO4 surfaces. In the present paper, we address the adsorption and decomposition of methanol on these CuO-CuWO4 surfaces using a multi-technique approach involving STM and LEED for surface structural information, XPS and HREELS for the characterization of surface species, and temperature programmed desorption (TPD) for complementary detection of desorption products. The structure of the 2D CuWO4 phase on Cu(110) is particular and quite unrelated to the wolframite structure of its parent bulk compound, with Cu-O stripes in the top surface layer but with tetrahedrally O coordinated W atoms in the second layer (see Fig. 1 in the next section);30 the latter may become accessible to adsorbates, as discussed in this paper. We find that the partial oxidation of methanol to surface formaldehyde is not observed on the CuO(2x1) surface, but that adsorbed formaldehyde species can be detected on the CuWO4 surface to a limited extent. The CuWO4 phase becomes modified by the interaction/reaction with methanol, but can be reconverted to its initial state by a subsequent oxidative treatment.

EXPERIMENTAL The experiments have been performed in three different ultrahigh vacuum (UHV) chambers (base pressures < 1x10-10 mbar), all equipped with LEED optics, and facilities for sample cleaning and manipulation, and physical vapor deposition sources. The STM experiments have been conducted in a custom-designed UHV system equipped with a variable-temperature STM (Omicron). The STM has been operated at room temperature using electrochemically etched W tips. The XPS experiments with high spectral resolution have been measured at the Materials Science beamline at the Elettra-Sincrotrone Trieste, Italy, with a hemispherical electron analyzer (Phoibos 150, SPECS). The photoemission spectra were taken at photon energies of 130 eV, 410 eV and 640 eV to excite photoelectrons from the W 4f, C1s, and O 1s core level regions, respectively. The total energy resolution in these experiments ranged between 100 meV (W 4f) and 450 meV (O 1s). All core level spectra were collected at normal 3 ACS Paragon Plus Environment


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emission angle with the sample held at 110 K. The binding energy scale was calibrated with respect to the Fermi edge of the metal substrate, and all spectra were normalized to the secondary electron background at a few eV lower binding energy than the respective core level peak. The W 4f, C 1s and O 1s spectra have been decomposed into several spectral components, with a Donjach-Šunjić line shape convoluted with a Gaussian distribution.31 A linear background was subtracted from the spectra prior to the decomposition analysis. The HREELS and TPD experiments have been conducted in a UHV system equipped with a highresolution electron energy loss spectrometer (Delta 0.5, SPECS) and quadrupole mass spectrometer (Prisma, Pfeiffer Vacuum). The HREELS spectrometer is capable of a total energy resolution of ∼1 meV and was operated in specular reflection geometry at 60° incidence from the surface normal and with a primary energy of 4.0 eV. The instrumental resolution used in the present HREELS experiments was typically set to 3 – 5 meV. For the TPD measurements, the sample was positioned directly in front of a skimmer cone, encapsulating the differentially pumped mass spectrometer.

The Cu(110) surface has been cleaned by cycles of Ar ion sputtering at 1.0 keV and annealing in UHV to 850 K. An atomically flat CuO surface has been obtained after the oxidation of the Cu(110) surface in 5x10-8 mbar O2 at 570 K (Fig. 1a), which yields the well-known (2x1) reconstruction,32 as evident from the atomically resolved STM image (inset of Fig. 1a) and the sharp (2x1) LEED pattern (Fig. 1d). Well-ordered 2D CuWO4 layers were prepared by deposition of (WO3)3 clusters onto the CuO(2x1) surface at room temperature followed by annealing in UHV to 570 K.30 The (WO3)3 clusters have been generated by sublimation of WO3 powder at ~ 1300 K in a thermal evaporator (Createc, Germany); the evaporation flux was monitored by a quartz microbalance. The deposited amount of (WO3)3 clusters is given in monolayers (ML), whereby 1 ML is defined as the (WO3)3 coverage resulting in the formation of a complete CuWO4 wetting layer on the Cu(110) surface, as exemplified in the large-scale STM image in Fig. 1c. Here, three Cu(110) terraces, separated by monatomic steps, are fully covered by the CuWO4 layer, which exhibits a stripe pattern with the stripes running parallel to the [001] substrate direction and separated by a distance of ~ 13 Å (see insert). The latter corresponds to ~ 5 Cu lattice vectors in the [110] direction and is the result of a Moiré effect between the Cu substrate and the CuWO4 overlayer. The insert of Fig. 1c shows an atomic-resolved STM image of the CuWO4 structure, which displays a quasirhombic (angle of ~97°) unit cell with a size of 4.8 Å (indicated on the image). The 4 ACS Paragon Plus Environment

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corresponding LEED pattern (Fig. 1f) contains a large number of sharp diffraction spots, which are due to the superposition of the rhombic unit cell and Moiré reflections. The DFT structure model (Fig. 1g) consists of a tri-layer slab with an O layer at the interface to the Cu substrate, a W layer in the middle and a mixed Cu-O layer at the surface, forming chains in the [110] direction.30 The W atoms in this 2D CuWO4 layer have a tetrahedral oxygen coordination: two O are at the surface and two O at the Cu interface; this is different to the bulk CuWO4 phase (with a distorted wolframite structure), where W is octahedrally coordinated to oxygen. Notably, the 2D CuWO4 surface is quite open: the distances between the surface Cu and subsurface W atoms (which are located ~ 1 Å below the surface Cu atoms) are 3.35 Å in the [001] and 3.76 Å in the [110] substrate directions. This makes the subsurface W atoms potentially accessible for adsorption and reaction of methanol. At submonolayer CuWO4 coverages, mixed CuO-CuWO4 surfaces can be created (see Fig. 1b and 1e for 0.6 ML), where the CuO and CuWO4 phases are separated by sharp (rough) boundaries in the [001] ([110]) directions. Various CuO-CuWO4 surfaces, with the CuWO4 coverage varying between 0 and 1 ML, have been prepared and characterized by XPS and HREELS (see Supporting Information, Fig. S1 for further details).

Figure 1. Large scale STM images (100 x 100 nm2, 2.0 V, 10 pA) of the CuO(2x1) (a), 0.6 ML CuWO4 (b) and 1.0 ML CuWO4 (c) surfaces on Cu(110). The images were differentially filtered to enhance the topographic contrast. The insets in (a) and (c) are high-resolution STM images of the CuO(2x1) (7.5 x 7.5 nm2, 1.0 V, 40 pA) and CuWO4 (7.5 x 7.5 nm2, 1.4 V, 50 pA) surfaces, respectively; the (2x1) and rhombic unit cells are indicated. LEED pictures of the CuO(2x1) (d), 0.6 ML CuWO4 (e) and 1.0 ML CuWO4 (f) surfaces on Cu(110). The electron energy was 70 eV; (g) DFT derived structure model of the 2D CuWO4 phase30, viewed from different directions (Cu-green, Wblue, O-red). 5 ACS Paragon Plus Environment


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Methanol (CH3OH, purity > 99.9%, Sigma Aldrich) has been dosed from a leak valve at a background pressure of 1x10-8 - 5x10-8 mbar in the XPS experiments and from a directional doser in the HREELS-TPD experiments. The CH3OH was purified using repeated freezepump-thaw cycles prior to use. In all experiments, the sample was held between 90 and 110 K during methanol dosing. In the TPD experiments the sample temperature was ramped at a rate of ∼3 K/s and multiple m/e channels were monitored in the quadrupole mass spectrometer (QMS).

RESULTS AND DISCUSSION The surface species following exposure of the surfaces to methanol at 110 K and after various temperature treatment steps have been monitored by XPS via their characteristic core level binding energies and a quantitative impression of their concentrations is obtained from the intensities of the respective core level peaks. Figure 2, top panels, shows series of C 1s spectra taken from the pristine CuO(2x1) surface (a) and from three different CuWO4 covered surfaces with coverages of 0.4 ML (b), 0.75 ML (c) and 1.0 ML (d), following a methanol dose of 20 L at 110 K and subsequent flashes in UHV to the indicated temperatures. The C 1s spectra have been decomposed into different core-level components, corresponding to different methanol adsorption and reaction products, and their intensities have been plotted as a function of the temperature in the bottom panels, Fig. 2(e-h). The respective binding energy (BE) values are listed in Table 1. For all surfaces, the methanol exposure at 110 K results in a multilayer of condensed CH3OH. The C 1s spectra display an intense and broad peak with a binding energy of 287.0 ± 0.1 eV, which is in good agreement with literature values.14

Table 1. Binding energies (in eV) of the different C 1s core level components for the CuO(2x1), 0.6 – 0.75 ML CuWO4 and 1 ML CuWO4 surfaces.

CuO(2x1) 0.6 – 0.75 ML CuWO4 1 ML CuWO4

CH3OHcond 287.0

CH3OHads 286.5

CH3O 285.5

CH3O‘‘ -

CH2O -

CHx 284.3

287.0

286.5

285.5

286.0

288.4

283.9 – 284.1

287.0

286.5

285.5

-

288.4

283.4 – 284.5


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Figure 2. Top panels: C 1s spectra taken from the pure CuO (a) and 1ML CuWO4 (d), and mixed CuO-CuWO4 surfaces, 0.4 ML (b) and 0.75 ML (c), following a methanol dose of 20 L at 110 K and subsequent flashes in UHV to the indicated temperatures. The spectral features associated with the different CH3OH adsorption and reaction products are indicated. Bottom panels: Intensities of the different C 1s core level components for the CuO (e), 0.4 ML (f), 0.75 ML (g) and 1.0 ML (h) CuWO4 surfaces plotted as a function of the temperature.

Heating to 160 K causes the desorption of the CH3OH multilayer, as revealed by the vanishing of the CH3OH multilayer peak in the C 1s spectra (some readsorption of CH3OH during the cooling to 110 K may occasionally take place, leading to a remnant emission at the multilayer CH3OH peak position). The C 1s spectra at 160 K are significantly broadened, due to the appearance of several new spectral components. The most intense component has a BE of ~ 286.5 ± 0.2 eV and is associated with the monolayer of intact methanol molecules.14,33 The latter coexists on all surfaces with methoxy (CH3O) species, with a characteristic C 1s binding energy of 285.5 eV,16 and some hydrocarbon (CHx) species at a lower BE of around 284 eV are also present. Importantly, on the CuWO4 surfaces (Fig. 2b-d) a weak emission structure with a higher BE of 288.4 eV is present in addition, which is characteristic of formaldehyde (CH2O).34 The formaldehyde C 1s peak intensity grows linearly with increasing CuWO4 coverage, which clearly indicates that this low-temperature formaldehyde formation is promoted only at the surface of the CuWO4 phase. The oxidation of methanol to 7 ACS Paragon Plus Environment


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formaldehyde on the CuWO4 surface may be attributed to the presence of catalytically active redox sites, which we will associate with subsurface W cations. The adsorbed CH2O surface species desorb on increasing the temperature to 210 K, as evidenced by the vanishing intensity of the corresponding C 1s (Fig. 2f-h) peaks. At this temperature, the dominant species on all CuO-CuWO4 surfaces is methoxy, which coexists with a small rest of intact surface methanol and CHx decomposition products. The intensity of the CH3O peak decreases continuously with further increasing the temperature and vanishes completely above 310 K. The signature of the CHx species with BEs between 283-284 eV disappears beyond 450 K, indicating oxidative removal of C from the surfaces.

Interestingly, the C 1s spectra from CuO-CuWO4 surfaces with a CuWO4 coverage between 0.5 and 0.8 ML (Fig. 2c) show a particular behavior. Here, the CH3O component is broader and more intense (Fig. 2g) than on the other CuO-CuWO4 surfaces and needs to be fitted by two CH3O-related components: CH3O’ at 285.5 eV, i.e. at the same BE position as the CH3O peak on the other surfaces, and a second component CH3O” at 286.0 eV. The CH3O’’component forms first at 160 K, followed by the growth of the CH3O’ component as the temperature increases to 210 K. The CH3O’ peak vanishes (as the CH3O component) on increasing the temperature above 310 K, whereas the CH3O’’ peak is more stable and can still be detected at 450 K; it is removed only after heating to 550 K. Two different methoxy related desorption features have been detected in TPD spectra of methanol dosed O-Cu(110) surfaces, corresponding to two different formaldehyde desorption states at 330 K and 375 K.10 It has been argued that the stronger bound CH3O species are stabilized at higher oxygen coverage by their interaction with the boundary of Cu-O islands, whereas the less stable ones correspond to a mobile methoxy phase.10 Based on the structural information of our surfaces, it is suggested that the CH3O’’ peak is related to methoxy species stabilized at sites at the boundaries of the CuWO4 and CuO surface phases, which are abundant at the submonolayer CuWO4 – CuO surfaces – see Fig. 1(b).

Figure 3 shows two series of HREELS spectra taken from the CuO (a) and CuWO4 monolayer (b) surfaces after dosing ≥ 2 L of methanol at 90 K and subsequent heating steps to the indicated temperatures. The spectra of the CuO and CuWO4 surfaces prior to the methanol exposure are described in the Supporting Information (SI, Fig. S1) and are shown at the 8 ACS Paragon Plus Environment

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bottom of Fig. 3 for comparison. Following the methanol dose at 90 K a condensed layer of CH3OH forms, as indicated by the presence of the characteristic CH3-O stretch (130 meV), OH bend (95 meV) and methyl (rock-140 meV and bend-184 meV) vibrations.8 On heating to 160 K, methanol decomposes first into methoxy, which is bound via its oxygen end to Cu atoms on the CuO surface, giving rise to a Cu-OCH3 stretch vibration at around 54 meV (Fig. 3a).8 On CuWO4, there is an asymmetric vibrational signature in this range, broadened to lower energy (Fig. 3b); this may indicate a different adsorption site. Further heating causes a progressive decrease of the methoxy surface concentration, and above 300 K it disappears from the surface. The CuO surface is almost fully recovered after the heating step at 300 K, in line with results of previous studies, reporting a weak reaction of methanol with the fully Ocovered Cu(110) surface.9 In contrast, the methanol reacts stronger with the CuWO4 surface, which is still somewhat modified after the heating step at 400 K, as inferred from the reduced intensity of the characteristic W-O stretching loss peak at 107 meV and the modified losses at 44 and 120 meV. These modifications are even more pronounced at submonolayer CuWO4 surfaces (see SI, Fig. S2).

Figure 3. HREELS spectra of CuO (a) and 1.0 ML CuWO4 (b) surfaces, taken before (bottom curves), and after a ≥ 2 L CH3OH dose at 90 K, and subsequent flash-annealing to the indicated temperatures. The main vibrational modes of the CuO and CuWO4 surfaces, as well as those of methanol adsorption and reaction products are indicated.

In order to monitor the evolution of the CuWO4 oxide phase during/after the interaction/reaction with methanol, it is useful to analyze the W 4f core level spectra. Figure 4 shows two sets of W 4f core level spectra for two different CuWO4 coverages, 0.6 ML (a),



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and 1.0 ML (b), taken after a methanol dose of 20 L at 110 K and subsequent flashes in UHV to the indicated temperatures.

Figure 4. W 4f core level spectra of 0.6 ML (a) and 1.0 ML CuWO4 (b) surfaces, taken before (bottom curves), after a 20 L CH3OH dose at 110 K, and after subsequent flash-annealing to the indicated temperatures.

The pristine CuWO4 surfaces are characterized by sharp W 4f7/2-5/2 doublet line shapes (bottom spectra in Fig. 4), as described in more detail in the SI (Fig. S1). After the methanol dose at 110 K the W 4f spectra are strongly attenuated due to the thick condensed CH3OH layer. The latter desorbs on heating to 160 K and the W 4f peaks recover intensity, but become significantly broadened with an asymmetry towards higher binding energies: the W 4f7/2 peak FWHM increases from 0.4 eV for the pristine CuWO4 to ~ 1.1 eV for the methanoldosed surfaces. This significant intensity redistribution reflects the influence of the methanol adsorbate and suggests that the W atoms of CuWO4 are involved in the bonding to the adsorbate. Although a spectral shift of metal atom core levels to higher BEs is commonly associated with charge withdrawal and oxidation (given the absence of differential final state effects), it appears that more subtle effects must be operative here, since it is not obvious how the molecular adsorption of methanol could lead to the oxidation of the W atoms. The W 4f peak shapes remain broad upon heating to higher temperatures on the 0.6 ML CuWO4 surface (Fig. 4a), where decomposition of methanol to methoxy and beyond has been established by the C 1s spectra (Fig. 2). On the CuWO4 monolayer, the W 4f peak width decreases again upon further heating (Fig. 4b), reaching a value of 0.65 eV at 450 K, i.e. after the methanol reaction is completed, whereas at the 0.6 ML CuWO4 surface the broad W 4f spectrum still persists at 550 K (Fig. 4a). This XPS spectral evidence demonstrates that the CuWO4 surface 10 ACS Paragon Plus Environment

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becomes modified as a result of the methanol reaction process, with methanol-induced modifications stronger for the sub-monolayer surface (where the methanol interactions are stronger – see Fig. 2c). It is noted that the observed surface modifications are not caused by thermal effects, since the CuWO4 layer is stable upon heating in UHV up to 700 K.30 Furthermore, it is important to emphasize at this point that the pristine CuWO4 layer can be completely recovered by a reoxidation in 5x10-7 - 2x10-6 mbar O2 at 573 K, as confirmed by XPS and HREELS – see SI, Fig. S2.

Quantitative information on the methanol-induced modifications of the CuWO4 phase can be obtained by fitting the W 4f spectra of the various CuO – CuWO4 surfaces, taken after methanol decomposition, by different core-level components (Fig. 5a). The main component (red-filled) corresponds to the intact CuWO4 phase, i.e. its peak position and line shape were kept fixed to the values established from the W 4f spectra before the methanol adsorption, only its intensity was allowed to vary. A second component (blue-filled) was necessary in the analysis at a lower binding energy (W 4f7/2 at 34.2 eV) with respect to the CuWO4 component, which indicates some reduction of the CuWO4 phase due to a loss of oxygen. This is confirmed by the O 1s spectra (Fig. 5b) taken after the methanol reaction (solid lines), which show some intensity decrease in comparison to the spectra measured prior to methanol adsorption (dashed lines). To account for the asymmetrically broadened W 4f peak shape at the higher binding energy side, a third core level component with a W 4f7/2 BE of 35.2 eV and a broader line shape (green-filled) had to be considered in the peak decomposition analysis. As mentioned above, the higher BE component could suggest that it corresponds to W species in a formally higher oxidation state than in the CuWO4 phase. Such conclusion is however precluded by the O 1s spectra, which indicate some CuWO4 reduction, and indeed the lower BE W 4f component at 34.2 eV is in sympathy with this conjecture. The behavior of the higher BE W 4f core level component is thus contradictory: it is induced by the molecular adsorption of methanol already at low temperature (160 K), remains after the desorption of methanol reaction products, but can be removed by reoxidation of the surface. We propose as a tentative explanation that the appearance of the high BE W 4f feature is caused by two different effects: i) it is initially caused by the adsorption of methanol/methoxy at low temperature at the W sites of the CuWO4 surface; and ii), it reflects a morphology change of the reacted CuWO4 phase at higher temperature, which occurs as a secondary effect - the latter is supported by the following STM observations. 11 ACS Paragon Plus Environment


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Figure 5. (a) W 4f spectra taken for different CuWO4 coverages after the methanol reaction. The spectra have been decomposed into different core level components, discussed in the text; (b) O 1s spectra taken for different CuWO4 coverages before (dashed line) and after (solid lines) the methanol reaction. Intensities of the principal CuWO4 W 4f component (red-filed in Fig. 5a) (c) and O 1s peaks (d), recorded before and after the CH3OH reaction, plotted against the CuWO4 coverage.

Figure 6 shows STM images of a 0.8 ML CuWO4 surface exposed to a large amount of methanol (600 L) at 300 K and flashed subsequently to 500 K. Image 6(a) shows that the CuWO4 surface has developed a nanostructured pattern, consisting of bright stripes running parallel to the [110] substrate direction with a width of 28 ± 4 Å, which are separated by narrow (5 – 10 Å) dark trenches. The high-resolution STM image of Fig. 6(b) confirms that the bright stripes correspond to narrow ribbons of the CuWO4 layer: the line scan in Fig. 6(c) displays the periodicity of the CuWO4 lattice. The CuWO4 ribbons with an average width of 4 ± 1 unit cells in the [001] of direction contain a large number of under-coordinated atoms at the stripe edges in the trenches – an impression of the structure at the stripe edges can be obtained from the CuWO4 DFT model along the [110] direction (Fig. 1g, middle part). The under-coordinated W atoms can lead to shifts in the core level binding energy, as it has been reported recently for a 2D WO3 layer on Pd(100), which displays a regular anti-phase domain boundary pattern, where a W 4f core level component has been found shifted to higher BE and attributed to the contribution of the edge atoms.35 Actually, the respective W 4f component in Ref. 35 was found at a very similar BE as the higher BE component observed 12 ACS Paragon Plus Environment

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here. Thus, in addition to some reduction the CuWO4 phase undergoes a morphology change as a result of the formation and decomposition of the methoxy species.

Figure 6. STM images of a 0.8 ML CuWO4 surface exposed to 600 L methanol at 300 K and flashed to 500 K. (a) 50 x 50 nm2, -1.7 V, 30 pA; (b) 20 x 20 nm2, -1.7 V, 10 pA; (c) line profile taken along the A-B line in (b). The fast-scan STM direction in (a) and (b) is along the vertical axis.

Fig. 5(c) compares the intensities of the principal W 4f component of the different CuWO4 surfaces (red in Fig. 5a) before and after the methanol reaction (note that the total W 4f intensity is conserved). The intensities are plotted against the nominal CuWO4 coverage, which has been determined from the evaporated amount of (WO3)3. The data points of the pristine CuWO4 surfaces (before CH3OH) lie on a straight line as expected, but there is a loss of the principal W 4f intensity after the methanol reaction, signaling the modifications of the CuWO4 phase (W 4f intensity has been redistributed to lower and higher BEs). This effect is most significant in the CuWO4 coverage range between 0.5 and 0.8 ML. The respective O 1s peak intensities, before and after methanol reaction, are plotted in Fig. 5(d) against the CuWO4, coverage. The data show a similar trend, with the highest oxygen loss, i.e. reduction, detected on the 0.5-0.8 ML CuWO4 surfaces.

The concentration of the methoxy (CH3O) and formaldehyde (CH2O) surface species after methanol adsorption on the various Cu-O/CuWO4 surfaces is plotted against the CuWO4 coverage in Fig. 7. The relative amounts have been estimated from the maximal intensities of the corresponding components in the C 1s spectra (Fig. 2). Moreover, the fraction of the “reacted” (i.e. modified) CuWO4 phase, (CuWO4)R, is also included in Fig. 7. The latter has been determined from the difference of the principal W 4f component intensities before and after the reaction (Fig. 5c). The formaldehyde coverage scales linearly with the CuWO4 coverage, confirming that the low-temperature CH2O formation is promoted exclusively on the CuWO4 surface. The methoxy coverage reaches a maximum in the CuWO4 coverage 13 ACS Paragon Plus Environment


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range 0.5 - 0.8 ML, and this is correlated with the maximum of the (CuWO4)R “reacted” phase. Thus, the CuO-CuWO4 surfaces with CuWO4 coverages between 0.5 ML and 1 ML display clearly the highest activity for methoxy formation, suggesting that the CuO-CuWO4 boundary regions contain the most active sites. The CuWO4 islands expose at their edges undercoordinated Cu and W atoms (see Fig. 1g), with O atoms connected to both Cu and W. These “edge” sites may be particularly active in adsorbing and splitting methanol to methoxy and OH. This may be similar to what has been proposed for the methanol oxidation on OCu(110) surfaces, where the O atoms along the short [110] direction of (2x1)O-reconstructed islands were found most active for the formation of methoxy species.9 Some hints that the methanol reaction takes place preferentially near the CuO-CuWO4 phase boundary may be obtained from STM images (see SI, Fig. S3), where “reaction fronts” in the form of “reacted” (CuWO4)R – CuO boundary lines parallel to the [110] substrate direction can be recognized.

Figure 7. Concentration of surface species after methanol adsorption (left axis) and “reacted” (CuWO4)R fraction (right axis) plotted as a function of the CuWO4 coverage.

Finally, in Fig. 8 TPD spectra from three representative surfaces are reported after dosage of ∼1 L methanol at 90 K: CuO(2x1), 0.65 ML CuWO4, and 1 ML CuWO4 . Fig. 8(a) contains the desorption traces of mass spectrometer signals m/e = 31 and m/e = 30; while m/e = 31 and 30 represent molecular CH3OH, m/e = 30 contains in addition the signal from molecular formaldehyde, CH2O. A TPD trace (m/e = 31) of a > 3L methanol dose from the 1 ML CuWO4 surface (dotted curve at the top) illustrates the desorption of CH3OH from the condensed multilayer at ∼150 K. In the region 150 – 300 K, desorption of CH3OH occurs from the molecular methanol monolayer (≤ 200 K) and by recombination of methoxy species (200 – 300 K). The desorption from the molecular monolayer and the recombinative desorption of methoxy give rise to a two-peaked structure from the CuO surface (Fig. 8a, bottom), but molecular and recombinative desorption are less clearly separated on the CuWO4 14 ACS Paragon Plus Environment

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surfaces. The desorption of formaldehyde from the CuWO4 surfaces, occurring around 200 250 K according to the XPS analysis (Fig. 2), is overshadowed by the methanol desorption and cannot be clearly identified in the TPD spectra. However, there is an enhanced m/e = 30 desorption signal at 400 – 450 K, most pronounced on the 1 ML CuWO4 surface, which signals the reactive desorption of formaldehyde as a result of the oxidation of CHx species. The desorption of H2O (m/e = 18) after CH3OH dosing (1 L) at 90 K is shown in Fig. 8(b); for comparison, the respective TPD traces after H2O dosing are included in the figure (dashed lines). The desorption of H2O is via the recombination of surface OH, which are formed in the decomposition of adsorbed methanol to surface methoxy, formaldehyde and CHx. Water desorption is observed in the broad range from 200 – 325 K, with a larger tail towards higher temperatures on the CuWO4 surfaces. The removal of surface OH is the major channel of the surface reduction of the CuWO4 phase and is possibly the driving event inducing the surface restructuring leading to the “stripe phase” (see Fig. 6). CO (m/e = 28) is a possible product of methoxy decomposition19, but it can also be formed in the QMS as a methanol cracking product. m/e = 28 desorption traces after methanol adsorption show a broad peak in the 150 – 325 K range (see SI, Fig. S4). Careful comparison with m/e = 31 traces suggests that CO possibly desorbs as a result of some methoxy decomposition in the 200 - 325 K temperature range. Furthermore, a m/e = 28 desorption feature is seen at 400 – 450 K as oxidation products of CHx species desorb, but some QMS cracking of the desorbing formaldehyde cannot be excluded. CO2 (m/e = 44) desorption has not been observed from the CuWO4 surfaces, in agreement with the absence of formate surface species, but a small amount of desorbing CO2 is seen following the oxidation of CHx species at 450 K on the CuO surface (not shown).



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Figure 8. TPD spectra recorded from Cu-O, 0.65 ML and 1.0 ML CuWO4 surfaces after exposure of ∼1 L methanol at 90 K. (a) m/e = 31(dashed) and e/m = 30 (solid) traces. The dotted specrum (e/m = 31, top) illustrates the desorption of the methanol multilayer after a methanol dose of 2-3 L at 90 K. (b) TPD spectra of water (e/m = 18) after methanol exposure. The dashed curves represent the desorption after the dosage of H2O.

The present study of the decomposition of methanol on well-defined mixed oxide CuOCuWO4 surfaces in a planar catalyst model geometry has allowed us to identify many details of the methanol-surface interactions in terms of molecular surface species and the evolution of the structure and morphology of the oxide surface during reaction. The quantification of the molecular surface intermediates by high-resolution XPS after adsorption of methanol at 90110 K followed by annealing steps up to 450 K has shown that methoxy is the dominant molecular surface species at T = 200-350 K. The mixed CuO-CuWO4 oxide surfaces are more active in supporting methoxy than the individual CuO and CuWO4 surfaces. The XPS data suggest that the boundary region between the CuO and CuWO4 oxide phases contains the most active sites: the methoxy species at these sites are distinguished by a particular C 1s binding energy and are thermally more stable than methoxy at other sites. The oxide surfaces are subjected to chemical and structural modification as a result of the methanol decomposition reaction: partial surface reduction and a morphology transition has been evidenced by XPS and STM. It is notable that these oxide surface modifications can be reversed by a reoxidation step. On the CuWO4 surface, the methanol/methoxy species are most likely adsorbed at the tetrahedrally oxygen-coordinated W sites, which are located in the subsurface layer but are accessible to adsorbates in the relatively open structure of the twodimensional CuWO4 phase. These sites display mainly redox activity; it should be noted that the formal charge on these W atoms is significantly less than in the respective CuWO4 bulk structure according to DFT Bader charge density analysis,30 and this may be at the root of the observed redox behavior. At last, it is worth emphasizing that the methodology of fabrication 16 ACS Paragon Plus Environment

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of the mixed CuO-CuWO4 surfaces, i.e. the on-surface synthesis by solid state chemical reaction between a metal-supported well-ordered surface oxide and the (molecular) precursors of a second oxide, is more generally applicable and may provide a suitable route for the preparation of mixed oxide model systems with tunable composition and structural properties.

CONCLUSIONS The interaction of methanol with CuO(2x1) and CuWO4 surfaces on Cu(110) has been investigated in a surface science approach by combining STM (LEED), high-resolution XPS, HREELS and TPD techniques to identify the molecular surface species and to assess the state of the oxide surface phases following methanol decomposition. The reaction of adsorbed methanol on CuO-CuWO4 surfaces leads to methoxy surface species as the primary decomposition products. On the surface of the CuWO4 phase, the formation of formaldehyde at low temperature is observed as a minority reaction channel. On the CuWO4 surface, the methoxy species are adsorbed at the W sites, which are located in the second layer but are accessible to adsorbate molecules in the open CuWO4 structure. The most active surfaces for methoxy formation are mixed phase CuO-CuWO4, with Cu tungstate coverages of 0.5-1 ML. This suggests that the W and O sites at the CuWO4 phase boundary are the most active sites. The methoxy species at these sites are distinguished in the XPS core level spectra and appear to be thermally stabilized. The methoxy species can be desorbed as methanol by recombination (≤ 300 K) or decompose to CHx, which eventually are oxidized to formaldehyde and CO around 450 K. The CuWO4 phase becomes modified by the interaction with methanol: some reduction (loss of oxygen) and a morphology transition is observed. The formation and removal of OH species (desorption as H2O) are most likely responsible for the latter effects. However, the CuWO4 modifications are reversible, and the pristine surface state can be recovered by a post-oxidation treatment in oxygen. This suggests that the CuO-CuWO4 surfaces may be robust in catalytic cycling situations. The mixed CuO-CuWO4 surfaces may therefore be of interest in promoting reactions where the surface methoxy species constitute important intermediates.



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Supporting Information Spectroscopic characterization of CuO-CuWO4 surfaces (Figures S1 and S2); STM of CuOCuWO4 surfaces after methanol reaction at 300K (Figure S3); TPD of CO (e/m = 28) (Figure S4).

AUTHOR INFORMATION *Corresponding authors: [email protected] [email protected]

ACKNOWLEDGEMENTS This work has been supported by the FWF Project P26633-N20. The technical support of the ELETTRA staff is acknowledged. N.T. acknowledges CERIC-ERIC consortium and Czech Ministry of Education (LM2015057) for financial support.


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