Systematic Study of Adsorption and the Reaction of Methanol on

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Systematic Study of Adsorption and the Reaction of Methanol on Three Model Catalysts: Cu(111), Zn-Cu(111) and Oxidized Zn-Cu(111) Takanori Koitaya, Yuichiro Shiozawa, Yuki Yoshikura, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09598 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

Systematic Study of Adsorption and the Reaction of Methanol on Three Model Catalysts: Cu(111), Zn-Cu(111) and Oxidized Zn-Cu(111) Takanori Koitaya†, Yuichiro Shiozawa, Yuki Yoshikura, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu* The Institute for Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba, 277-8581, JAPAN

*)

Corresponding author: [email protected]

†

Present address: Graduate School of Arts and Sciences, The University of Tokyo,

3-8-1, Komaba, Meguro-ku, Tokyo, 153-8902, JAPAN

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Abstract The adsorption and reaction of methanol on Zn-modified Cu(111) surfaces were investigated by synchrotron-radiation X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Dehydrogenation of methanol is not observed on clean Cu(111) or clean Zn-Cu(111) surfaces. The amount of more stable methanol molecules at defect sites is increased on a Zn-Cu(111) surface, because surface roughening occurs as a result of surface-alloy formation, which leads to an increase in step density. All of the methanol is desorbed intact by heating to ~220 K. On the other hand, when a Zn-Cu(111) surface is pre-oxidized by oxygen, methanol is partially dehydrogenated to methoxy species at 130-160 K. The reactivity of methoxy depends significantly on the adsorption sites. In particular, some reactive methoxy species are found, which are dehydrogenated to formaldehyde at 220-290 K, on a pre-oxidized Zn-Cu(111) surface. Methoxy species on the surface of a zinc-oxide island are dehydrogenated to formaldehyde at 340-490 K. Dehydrogenation of methoxy above room temperature has been reported on both zinc oxide surfaces and oxidized Cu surfaces. Therefore, we conclude that there exists a synergetic effect between the zinc-oxide islands and the partially oxidized Cu substrate for the partial oxidation (dehydrogenation) of methanol. 2 ACS Paragon Plus Environment

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1. Introduction Methanol is an indispensable chemical as a solvent, a chemical feedstock and a fuel.1 Hydrogen production through steam reforming or partial oxidation of methanol is an important subject, since hydrogen is a candidate future energy carrier.2 Cu/ZnO catalysts are widely used in methanol steam reforming.3 In-situ characterization of the catalyst has shown that the interaction between Cu and ZnO plays a key role in improving catalytic reactivity.4, 5 Due to its industrial importance, adsorption and reaction of methanol on Cu surfaces have been intensely studied by many researchers over the last few decades.6-29 On a Cu(111) surface, adsorbed methanol molecules form two types of superstructures (1D chain and hexamer) via intermolecular hydrogen bonds.24 As for dehydrogenation of methanol on a clean Cu(111) surface, a few studies have reported that a small amount of methanol is dehydrogenated into methoxy,17, 19 whereas only molecular desorption without dehydrogenation was observed in other studies.7, 9 Thus, there is no consensus on the reactivity of methanol on a clean Cu(111) surface even under well-defined ultra-high vacuum (UHV) conditions. Pre-adsorption dehydrogenation

of

oxygen

reactivity

of

on

Cu

methanol.7,

surfaces 9,

17,

significantly 19


According

changes to

the

previous


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density-functional theory (DFT) studies,27,

28

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the activation energy for methanol

dehydrogenation to methoxy is significantly decreased in the presence of a chemisorbed oxygen atom. The formed methoxy on oxidized Cu(111) was desorbed from the surface as formaldehyde and hydrogen by heating to around 400 K.19 On the other hand, co-adsorbed water molecules also promote the dehydrogenation reaction.26,

30

The

results clearly indicate that the interaction between methanol and other adsorbates, such as atomic oxygen and water, affects the reactivity of methanol as well as the molecule-substrate interaction. Methanol adsorption on zinc oxide is also attracting attention as a model system of catalytic methanol activation.31-44 Reactions of alcohols and carboxylic acids on zinc oxide surfaces are comprehensively reviewed by Vohs.43 In the case of methanol adsorbed on zinc oxide single-crystal surfaces, a variety of reaction intermediates in methanol dehydrogenation is observed: methoxy,32,

34,

35

para-formaldehyde,35

methyl/methylene groups,35 hydroxyl,35 and formate.34 Adsorbed methanol is finally oxidized to CO and CO2. The reactivity depends significantly on surface orientation and termination, which largely affect the acid-base pair interaction between molecules and the ionic ZnO surface.45, 46 A recent study has elucidated that polar ZnO surfaces are more reactive than non-polar surface due to a larger amount of surface defects which are 4 ACS Paragon Plus Environment

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active sites for methanol decomposition.47 In contrast to the cases of Cu and ZnO surfaces, less is known about adsorption states and the reactivity of methanol on binary surfaces, such as Zn-Cu surface alloy and the Cu-ZnO interface. Fu and Somorjai have shown a synergetic effect between the Cu(110) surface and ZnOx islands on methanol oxidation into formate.48 On the other hand, TPD measurements by Zhang et al. show no trace of methanol dehydrogenation on the Cu monolayer formed on a Zn(000 1 ) -O surface,49 whereas methoxy is formed on a Cu-deposited ZnO(0001)-Zn surface.50 In this study, we systematically investigated the adsorption and reaction of methanol on three model catalyst systems, i.e., clean Cu(111), clean Zn-Cu(111), and pre-oxidized Zn-Cu(111) surfaces by synchrotron-radiation X-ray photoelectron spectroscopy (SR-XPS) and temperature-programmed desorption (TPD). 2. Experimental All experiments were performed under UHV. The Cu(111) surface was cleaned by repeated cycles of Ne+ sputtering and annealing at 673 K. The temperature of the sample was measured by a K-type thermocouple attached to the side of the Cu crystal by a Ta foil. The measured temperature was calibrated by the desorption temperature of multilayer water.51 The cleanness of the surface was checked by low-energy electron 5 ACS Paragon Plus Environment


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diffraction (LEED) and XPS. Atomic carbon was observed as a residual contaminant after the cleaning procedure. The amount of atomic carbon impurity was estimated to be less than 0.02 ML (ML = atoms per one surface Cu atom). Methanol (Wako Chemicals, 99.8+%) was handled under a dry nitrogen atmosphere to avoid contamination during preparation. It was further purified by several freeze-pump-thaw cycles. The purified methanol was adsorbed on the sample at 82 K. SR-XPS measurements were performed using an UHV chamber (base pressure = 2 × 10-10 Torr) at a soft X-ray undulator beamline (BL-13B) of the Photon Factory in Tsukuba, Japan. All of the SR-XPS spectra shown in this paper were collected at room temperature using a hemispherical electron analyzer (SPECS, Phoibos 100) at a normal emission angle with a pass energy of 6 eV. C 1s and O 1s core-levels were measured at a photon energy of 630 eV. The overall experimental resolution was estimated to be 0.19 eV at hν = 630 eV. The TPD and conventional XPS measurements were performed in another UHV chamber (base pressure = 1 × 10-10 Torr) using a hemispherical electron analyzer (VG Scienta, R3000) at a normal emission angle. In TPD studies, desorbed species were detected by a quadrupole mass spectrometer (Balzers, QMS 200). The XPS spectra were also measured using an Al Kα X-ray (hν = 1486.6 eV) to estimate coverages of 6 ACS Paragon Plus Environment

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methanol, reaction products and deposited Zn. The obtained XPS spectra were fitted by Voigt functions with a linear background. The coverage of adsorbed methanol was calculated from the area intensity of C 1s peaks. The peak intensity was calibrated by the known coverage of saturated single-layer cyclohexane on Cu(111) (θC6H12 = 0.14 ML per surface Cu atom).52 Zn was deposited on the Cu(111) surface at room temperature using a modified home-built deposition source.53 A Zn wire (0.5 mm diameter, 99.999 % purity) was installed in a quartz cell. The cell was held and heated by a tungsten filament. The deposition rate of Zn was monitored by a quartz microbalance. The actual coverage of deposited Zn was estimated from the intensity ratio between Zn 2p3/2 and Cu 2p3/2 core-levels that were measured by the Al Kα light source or the SR light at hν = 1100 eV.54 For the experiments on the Zn-Cu(111) surface, the Zn-deposited sample was annealed at 475 K for 1 min. The annealing leads to desorption of multilayer Zn and the formation of a well-dispersed Zn-Cu(111) surface alloy.55 The oxidized Zn-Cu(111) surface was prepared by exposure of 590 L oxygen at room temperature, followed by annealing at 500 K for 10 min. According to previous STM studies, a two-dimensional zinc oxide island is formed on the Cu(111) surface through surface segregation of Zn atoms when the surface is oxidized by O2 at a sample temperature of 500 K,56 and of 7 ACS Paragon Plus Environment


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600-673 K.57

3. Results and discussion 3.1 Methanol adsorption and desorption on clean Cu(111) and Zn-Cu(111) surfaces Figures 1(a) and (b) show O 1s and C 1s core-level spectra of adsorbed methanol on clean Cu(111) as a function of the heating temperature, respectively. Methanol molecules were adsorbed on the surface at 82 K. At 82 K, the C 1s spectrum of adsorbed methanol shows two peaks. The peak positions of the two components were determined to be 286.08 eV and 287.04 eV at 82 K. The peak at lower (higher) binding energy can be assigned to monolayer (multilayer) methanol. The difference in the binding energies of the two peaks mainly results from the final state effect. The core-hole screening by substrate electrons strongly depends on the distance between the adsorbed molecule and the metal surface. Insufficient screening of multilayer methanol leads to a core-level shift to a higher binding energy.58 The observed peaks of C 1s have an asymmetric shape with a tail structure at higher binding energy, which results from vibrational excitation in an ionized final state.59 The obtained C 1s spectrum was fitted by two components with fine vibrational structures for the C-H stretching mode. The 8 ACS Paragon Plus Environment

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fitting parameters for methyl groups in ethane were applied for the present peak fitting;60 the vibrational energy was fixed at 0.40 eV, and the vibrational branching ratio was determined assuming a linear coupling model with an S factor of 0.37. The fitting result agrees well with the experimental spectrum.



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Figure 1. O 1s and C 1s spectra of methanol on (a, b) clean Cu(111) and on (c, d) Zn(0.18 ML)-Cu(111) surface alloy as a function of heating temperature. Methanol was adsorbed on the surface at 82 K, followed by heating to the temperatures shown in the figure. All the spectra were measured after cooling down to 82 K. (e) Methanol coverage as a function of heating temperature estimated from C 1s peak intensity.


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On the other hand, the O 1s spectrum apparently consists of a broad peak centered at 533.1 eV. It can also be deconvoluted into two components at 532.81 eV and 533.28 eV. As in the case of the C 1s core-level, these are attributed to be monolayer and multilayer methanol, respectively. After heating to 132 K, the multilayer peaks were decreased in intensity, while most of the monolayer methanol remained on the surface. When the sample was annealed at higher temperatures, the amount of adsorbed monolayer methanol decreased continuously, and all of the adsorbed methanol desorbed from the surface without any reaction by heating to 204 K. This is consistent with previous studies that have shown intact desorption on a clean Cu(111) surface. Note that a significant promotion of dehydrogenation only occurred with co-adsorbate, such as atomic oxygen and water.7, 9, 26 In fact, the coverage of the oxygen-containing impurities in this study is very small (below 10-3 ML) as evidenced by no O 1s peak of the contaminant in Fig. 1(a). A series of XPS measurements of methanol adsorbed on the Zn-Cu(111) surface alloy (θZn = 0.18 ML) was also performed (Figs. 1(c) and (d)). The results are basically the same as those on a clean Cu(111) surface. At 82 K, the C 1s (O 1s) peaks of monolayer and multilayer methanol were observed at 286.17 eV (532.84 eV) and 287.07 eV (533.32 eV), respectively. By heating the sample, multilayer methanol first 11 ACS Paragon Plus Environment


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desorbed, followed by desorption of monolayer methanol. As in the case of methanol on a clean Cu(111) surface, no reaction products from dehydrogenation of methanol were detected. This indicates that a Zn-Cu(111) alloy surface is not reactive for methanol dehydrogenation. Although adsorbed methanol does not react on both surfaces, its thermal stability is changed by alloying with Zn. Figure 1(e) shows methanol coverage estimated from the area intensities of the C 1s spectra. At 82 K, monolayer (multilayer) coverage on a clean Cu(111) and Zn-Cu(111) alloy was 0.22 ML (0.39 ML) and 0.23 ML (0.38 ML), respectively. At 132 K, the coverage of monolayer methanol was nearly constant (0.22 ML). However, the thermal stability of multilayer methanol was different; 0.17 ML of multilayer methanol remained on Cu(111) after heating to 132 K, whereas methanol coverage on a Zn-Cu(111) surface was only 0.06 ML. At 147 K, most of the multilayer methanol was desorbed from the surface. The coverage of monolayer methanol was larger on Cu(111) (0.11 ML) than on Zn-Cu(111) (0.04 ML) after heating to 147 K. Thus, the desorption rate of monolayer methanol is faster on Zn-Cu(111) than on Cu(111). However, the amount of methanol on Zn-Cu(111) (0.03 ML) after heating to 163 K is larger than that on the Cu(111) surface (0.01 ML). As discussed below, methanol remaining on the surfaces at 163 K can be 12 ACS Paragon Plus Environment

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assigned to that adsorbed at defect sites, such as step and kink.16 A larger amount of methanol at the defect sites on Zn-Cu(111) than on Cu(111) is rationalized by the fact that defect density is increased by alloying with deposited Zn.61 The desorption process of methanol adsorbed on these model surfaces was further investigated by TPD. Figure 2(a) shows a series of TPD spectra of methanol (CH3OH) on clean Cu(111). These spectra were obtained by monitoring the strongest fragment signal (m/z = 31) of methanol. At very small methanol coverage (0.01 ML), the peak temperature was 173 K, whereas it was nearly constant (177 K) at coverage between 0.05-0.12 ML. According to the previous study, there are two superstructures of methanol on Cu(111): a cyclic hexamer at low coverage, and a 6×√3 structure with one-dimensional hydrogen-bond networks.24 The present results indicate that methanol desorption is a first-order process, and that 1D-chained methanol is slightly more stable than hexameric methanol. In addition, there is a small broad peak around 200 K, which is assigned to methanol adsorbed at defect sites.16 At a methanol coverage larger than 0.21 ML, two new peaks appeared at lower temperatures (α1 and α2 peaks). According to the previous studies of methanol on Pt(111),62 and Au(111),63 the α1 and α2 peaks can be attributed to desorption of crystalline multilayer methanol and second-layer amorphous methanol, respectively. Thus, the TPD results indicate that the second-layer 13 ACS Paragon Plus Environment


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methanol has a different adsorption state from multilayer methanol.

Figure 2. TPD spectra of methanol as a function of coverage measured on (a) clean Cu(111), (b) Zn(0.21 ML)-Cu(111), and (c) Zn(0.46 ML)-Cu(111). The present TPD spectra were obtained with a heating rate of 0.9 K/s. The peaks indicated by “Mono” and “α” correspond to molecular desorption from the monolayer and multilayer, respectively.


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Figures 2(b) and (c) show desorption spectra of methanol from a Zn-deposited Cu(111) surface at Zn coverages of 0.21 ML and 0.46 ML, respectively. The desorption peak of monolayer methanol on Zn(0.21 ML)/Cu(111) was observed at 170 K, which is 7 K lower than that on clean Cu(111). The peak temperature was further decreased to 164 K at a Zn coverage of 0.46 ML. These results indicate that methanol adsorbed at terrace sites becomes less stable by alloying with Zn. This is consistent with the XPS results that show molecular desorption at a lower temperature on the Zn-Cu(111) alloy than on the clean Cu(111) surface. However, Zn deposition causes an increase in intensity of the desorption peak from defect sites at ~200 K. This originates from the increase in the defect density due to surface roughening by the alloying.61 For the adsorption of methanol on Cu(111), intermolecular interactions, such as the intermolecular hydrogen bond and van der Waals force, play an important role. Thus, molecular desorption should be affected by intermolecular interactions as well as substrate-molecule interaction, which may lead to coverage-dependent desorption kinetics. A leading edge analysis around the desorption-threshold temperature can give coverage-dependent desorption parameters.64 However, in the present TPD spectra, the desorption peaks of methanol at terrace and defect sites coexist and overlap each other even at a low methanol coverage (0.01 ML), which makes it difficult to estimate the 15 ACS Paragon Plus Environment


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desorption energies of methanol at defect sites using leading edge analysis. In this study, we analyzed TPD spectra at monolayer coverages (shown in Fig. 3(a)) according to the following procedure. First, desorption energies and pre-exponential factors at methanol coverages given in Fig. 3(a) were estimated by leading edge analysis.64 Then, coverage-dependent desorption energies were calculated using an inversion analysis,65, 66

with the pre-exponential factors obtained from the leading edge analysis, assuming

that the pre-exponential factor is coverage-independent.


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Figure 3. (a) TPD spectra used for analysis of desorption kinetics. (b) Desorption energy of methanol as a function of coverage. The pre-exponential factors are evaluated by the leading-edge analysis of the TPD spectra in (a). Then, the desorption energy was estimated from the inversion analysis64,

65

, assuming the coverage-independent

pre-exponential factors. The shaded area represents an error in desorption energy arising from uncertainty in pre-exponential factor (νd).



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The obtained values of desorption energies as a function of coverage and pre-exponential factors are shown in Fig. 3(b). On the clean Cu(111) surface, the pre-exponential factor at θMeOH = 0.12 ML was estimated to be 1012.6±0.3 s-1. Using this pre-exponential factor, the coverage-dependent desorption energy was obtained. It steeply decreased from 58 kJ/mol to 46 kJ/mol at a methanol coverage of less than 0.02 ML, which corresponds to desorption from defect sites (a broad peak around 200 K). At larger coverage, the desorption energy was nearly constant (45-46 kJ/mol), indicating that the adsorption state of methanol on the terrace site is almost homogeneous. The desorption energies of methanol on the Zn-Cu(111) surfaces were also estimated using constant pre-exponential factors (1013.0±0.2 s-1 at θZn = 0.21 ML and 1013.8±0.5 s-1 at θZn = 0.46 ML) estimated by leading-edge analysis. The desorption energies at θMeOH < 0.05 ML were significantly higher than those on clean Cu(111). This means that a larger amount of methanol on Zn-Cu(111) was adsorbed on more stable defect sites compared to the case of clean Cu(111). As for the desorption from the terrace site, the desorption energy increased slightly with increasing Zn coverage, although the peak temperature became lower on the Zn-Cu(111) alloy surface than on the clean Cu(111) surface. The faster desorption on Zn-Cu(111) is a consequence of the larger pre-exponential factor. According to a 18 ACS Paragon Plus Environment

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study by Campbell and Sellers,67 adsorbed hydrocarbons have much larger entropies at desorption temperatures than those calculated with harmonic approximation, which is generally applied to DFT, and there is a proportional relationship between the entropies of adsorbed molecules and gas-phase molecules regardless of the substrate. These results indicate that entropies of rotational and translational motions parallel to the surface are dominant near the desorption temperature.67 Thus, adsorbed molecules at the desorption-threshold temperature are in 2D gas or liquid states, rather than in a 2D crystal in which rotational and translational motions are frozen. Based on these facts, it is reasonable to think that translational or rotational partition function is significantly different between clean Cu(111) and Zn-Cu(111). (The vibrational and electronic partition functions of a small molecule are much smaller than those of other motions, and thus the contribution of these factors is negligible.68, 69) One possible explanation for the larger pre-exponential factor, i.e. smaller adsorption entropy, on a Zn-Cu(111) surface is that translational motions of adsorbed methanol are affected by Zn deposition as follows. At the desorption temperature of methanol at a terrace site, the step sites are still occupied by adsorbed molecules. In such a situation, molecular diffusion across the step is suppressed and adsorbed molecules are likely to be confined in the terrace area. By deposition of Zn, the step 19 ACS Paragon Plus Environment


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density was increased.61,

70

This indicates that the average terrace area becomes

narrower for Zn-Cu(111) than for clean Cu(111). Translational partition functions are smaller if molecules are confined in a smaller area. Thus, molecules on Zn-Cu(111) have smaller translational partition functions and entropies. This leads to larger pre-exponential factors, since entropy change upon desorption is larger for molecules that have smaller adsorption entropies.

3.2 Reaction of methanol on a pre-oxidized Zn-Cu(111) surface Next, the reaction of methanol on a pre-oxidized Zn-Cu(111) surface was investigated. Figure 4 shows TPD spectra of methanol and reaction products. In the TPD experiments, fully deuterated methanol (CD3OD) was used to differentiate reaction products from possible contamination by adsorption of residual gas in the chamber during the experiment. The Zn (θZn = 0.28 ML) was deposited on the Cu(111) surface at room temperature, followed by O2 gas exposure (590 L). The oxygen-adsorbed sample was annealed at 500 K for 10 min. Oxygen coverage after the annealing was estimated to be 0.26 ML by O 1s XPS intensity. According to previous studies on the oxidation of Zn-Cu(111),56, 70 a thin ZnO(0001) layer was formed on the Cu(111) surface after the oxidation. The segregation of Zn and the formation of a ZnO layer are also observed on 20 ACS Paragon Plus Environment

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brass single crystal surfaces.71, 72 As discussed later, the Cu substrate was also partially oxidized by oxygen exposure in addition to the formation of a ZnO layer. Methanol (θMeOD = 0.7 ML) was adsorbed on the oxidized Zn-Cu(111) at 82 K, and then TPD experiments were performed at a heating rate of 0.9 K/s. The desorption peaks at ~170 K in the spectra of m/z = 32 and 34 are attributed to fragment signals of molecularly desorbed methanol. In the m/z = 34 spectrum, there is a broad desorption peak between 180 K and 260 K, indicating that stable adsorption sites for methanol were created on a surface similar to the case of the Zn-Cu(111) surface alloy.

Figure 4. TPD spectra (m/z = 4, 20, 32, 34) of methanol-d4 on the oxidized Zn(0.28 ML)-Cu(111) surface. The Zn-Cu(111) surface was oxidized by O2 gas exposure (590 L) at room temperature, followed by annealing at 500 K for 10 min. The coverage of oxygen atoms was estimated to be 0.26 ML. After the preparation of pre-oxidized Zn-Cu(111) surface, methanol was adsorbed on the surface at 82 K (initial methanol coverage was 0.7 ML). The figure shows the results of individual TPD experiments at (a) 130 K ≤ Ts ≤ 300 K, and (b) 310 K ≤ Ts ≤ 520 K. Mass spectra of m/z = 32 include the molecular ion signal of desorbed formaldehyde (CD2O), in addition to fragment ions of methanol. Two broad desorption peaks of formaldehyde (colored green and orange) were observed at 220-290 K and 340-490 K.



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Note that, in contrast to the cases of clean Cu(111) and Zn-Cu(111) surfaces, some of the adsorbed methanol reacted on the oxidized Zn-Cu(111) surface. Enhanced reactivity of methanol by pre-oxidation of the surface was also observed on the oxidized Cu(111)surfaces.7, 13, 17, 19, 25 Additional desorption signals were observed in the QMS signals of m/z = 4, 20 and 32; these desorbed species can be attributed to D2, D2O and formaldehyde (CD2O), respectively. Thus, adsorbed methanol is partially oxidized to formaldehyde (CD3OD CD2O + 2D). Most of the produced D atoms are desorbed as D2 at 320-500 K, whereas a small amount of desorbed D2O was detected at 213 K and 370 K, which is probably formed by the reaction of deuterium with preadsorbed oxygen. As for the desorption of formaldehyde, there are two desorption peaks at 220-290 K, and at 340-490 K. The mechanism of the adsorption reaction on the surface is discussed in detail later together with the SR-XPS results. Dehydrogenation of methanol on the pre-oxidized Zn-Cu(111) surface was further investigated by XPS. Figures 5(a) and (b) show a series of XPS spectra as a function of heating temperature. The sample was prepared according to a similar method to that of the TPD measurements; 0.19 ML of Zn was deposited on Cu(111), and then the sample was oxidized by O2 exposure (590 L) followed by annealing at 500 K for 10 min. The oxygen coverage of the sample was estimated to be 0.33 ML from 22 ACS Paragon Plus Environment

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the O 1s spectrum before methanol adsorption. Since the oxygen coverage is significantly larger than the Zn coverage, the adsorbed oxygen atoms bind to both Zn and Cu atoms. The O 1s spectrum of the oxidized Zn-Cu(111) surface was fitted by three peaks at 529.4 eV, 529.9 eV, and 531.4 eV. Oxidation of the Cu(111) surface leads to the formation of Cu2O surface oxide.73 Based on previous XPS results of ZnO,74 ZnOx film on Cu(111),75 and oxidized Cu(111),76 the observed O 1s peaks were attributed to ZnO (θO = 0.15 ML), Cu2O (θO = 0.09 ML), and defective ZnO (θO = 0.09 ML) in order of increasing binding energy. Zn is more easily oxidized compared with Cu, and thus more zinc oxide forms than copper oxide. The small oxygen coverage bound to Cu atoms indicates partial oxidation of the Cu substrate; metallic Cu sites and Cu2O sites coexist in the present condition.



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Figure 5. A series of (a) O 1s and (b) C 1s spectra of methanol and reaction products on oxidized Zn(0.19 ML)-Cu(111) surface as a function of heating temperature. Methanol was adsorbed on the surface at 82 K, followed by heating to the temperatures shown in the figure. All the spectra were measured after cooling to 82 K. (c) Methanol and methoxy coverages as a function of heating temperature estimated from C 1s peak intensity.


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Methanol was adsorbed on the pre-oxidized surface at 82 K. The O 1s spectrum of as-deposited methanol in Fig. 5(a) shows a broad peak at 533.2 eV with substrate oxide peaks that are attenuated by methanol adsorption. On the other hand, its C 1s spectrum can be fitted by two methanol components with fine vibrational structures. No peak of reaction products was observed at 82 K; all of the methanol was molecularly adsorbed. By heating to 132 K, multilayer methanol was desorbed from the surface, which is similar to the cases on Cu(111) and Zn-Cu(111). In addition, new peaks appeared at 530.5 eV in O 1s and 285.5 eV in C 1s. These peaks gradually increased in intensity up to 204 K, and disappeared almost completely after heating to 308 K, and can be attributed to thermal-reaction products. In the C 1s spectra, a peak around 287 eV is still observed at 147 K and 163 K. This peak is not assigned to the multilayer methanol, since the multilayer should be desorbed from the surface at lower temperature. We attributed the observed peak to a reaction product from methanol dehydrogenation. At 308 K, the C 1s and O 1s peaks of this species were observed at 286.3 eV and 531.1 eV, and disappeared after heating to 515 K. According to the TPD results, the reaction products from methanol were finally desorbed as formaldehyde. Thus, the observed reaction products in the XPS spectra might be assigned to formaldehyde itself, or some reaction intermediate species of the 25 ACS Paragon Plus Environment


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methanol oxidation (dehydrogenation). The reported values of C 1s binding energy of formaldehyde adsorbed on metal or metal oxide surfaces range between 287.3 and 288.6 eV,34, 77-80 which is slightly higher than those of the observed peaks in this study. The C 1s peak positions of the reaction products are close to the binding energy of adsorbed methoxy species (CH3O) listed in Table 1. Therefore, we attributed the XPS peaks of the reaction products to the methoxy intermediates.

Table 1. O 1s and C 1s binding energies of methoxy on Cu and ZnO surfaces Substrate

O 1s

C 1s

(eV)

(eV)

530.9-531.0

285.8-286.0

19

530.7

286.2

6

530.9

285.5

13

530.2

285.4

21

285.2

81

285.2

18

285.9

82

530.5

285.5

Present study

ZnO(101-0)

531.7-532.2

286.2-286.4

35

ZnO(0001-)

531.7-532.1

286.6-287.0

35

286.8

34

286.3-287.0

Present study

O-Cu(111) Cu(110) O-Cu(110), O-Cu(111), O-Cu(poly) Cu(110) Cu(poly) Cu(poly)

531.2

Cu2O Cu2O on Cu(111)

ZnO(0001) ZnO on Cu(111)

531.1


Reference

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Table 1 summarizes the reported binding energies of methoxy species on Cu and ZnO surfaces. The peaks of methoxy O 1s and C 1s core-levels on ZnO surfaces are observed at 531.7-532.2 eV and 286.2-287.0 eV, respectively.34, 35 The binding energies of methoxy on ZnO surfaces are higher than those observed on Cu surfaces (530.2-531.2 eV for O 1s and 285.2-286.2 eV for C 1s). In the present SR-XPS measurements on a pre-oxidized Zn-Cu(111) surface, O 1s and C 1s peaks of methoxy species are observed at 530.5 eV and 285.5 eV, respectively, at temperatures below room temperature. According to the previous XPS results of methoxy, these peaks are attributed to methoxy species on a Cu site. An STM study of methoxy on the pre-oxidized Cu(111) surface shows that the produced methoxy species is adsorbed on Cu2O islands.25 Thus, the methoxy species that are dehydrogenated to formaldehyde below room temperature reside at the Cu2O surface oxides. Another methoxy species on the pre-oxidized Zn-Cu(111) surface is stable above room temperature. The O 1s (C 1s) peak energies of the methoxy species were 531.08 eV at 308 K (287.0-286.3 eV at 147-308 K). These peaks have higher binding energies than those of methoxy on Cu2O, and are tentatively assigned to the methoxy species on the surface of ZnO island. Figure 5(c) shows the coverages of methanol and methoxy on the oxidized Zn-Cu(111) surface as a function of heating temperature. The initial coverage of 27 ACS Paragon Plus Environment


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monolayer methanol is 0.20 ML, which is close to the coverages on Cu(111) and Zn-Cu(111). The monolayer methanol desorbed gradually from the surface at a temperature above 147 K, and completely disappeared at 300 K. This behavior is consistent with the TPD result shown in Fig. 4(a). The methoxy coverage was increased when the sample was heated to 147 K. The methoxy on Cu2O sites reached maximum coverage (0.03 ML) at 204 K, whereas the maximum coverage of methoxy on ZnO is 0.07 ML at 147 K. The larger coverage of methoxy on ZnO sites than on CuO sites might result from a larger area of zinc oxide surface than that of copper oxide. The formation of methoxy and molecular desorption of methanol occurred at similar temperatures, indicating that the activation energy for methanol dehydrogenation to methoxy is close to the desorption energy of methanol. By heating to 308 K, methoxy on Cu2O desorbed from the surface, whereas 0.03 ML of methoxy remained on ZnO sites. The methoxy was completely desorbed as formaldehyde after heating to 515 K. The thermal reaction processes of methanol on the oxidized Zn-Cu(111) are summarized as follows. Molecularly adsorbed methanol at 82 K was desorbed from the surface or dehydrogenated to methoxy by heating to 140-200 K. The produced methoxy was further dehydrogenated to formaldehyde on both the Cu2O surface oxide and the ZnO islands. The formaldehyde desorbed from the surface immediately after the 28 ACS Paragon Plus Environment

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dehydrogenation of methoxy (reaction-limited desorption), since no adsorbed formaldehyde was detected by XPS. Note that methoxy on Cu2O reacted at a lower sample temperature (220-290 K) than on ZnO, indicating that the methoxy species on Cu2O is more reactive than those on ZnO. At a sample temperature between 340 K and 490 K, methoxy species on ZnO were desorbed from the surface. The produced hydrogen atoms were desorbed from the surface mainly as hydrogen molecules at a temperature between 310 K and 500 K. On the oxidized Cu(111) surface, some of the adsorbed methanol is dehydrogenated to methoxy at 105 K.17, 19 The produced methoxy from methanol is further dehydrogenated and desorbed as formaldehyde at a higher temperature between 370 K and 460 K.17, 19 On the other hand, methanol adsorbed on a Zn(0001) surface at 160 K is also partially dehydrogenated to methoxy.34 The formed methoxy species on ZnO(0001) is dehydrogenated to formaldehyde, which desorbs at 500 K, or reacts further with lattice oxygen to form formate,34 which is finally oxidized to produce CO, CO2, and H2O at 575 K.43 In the present TPD and XPS measurements, we clearly detected the reactive methoxy species that is dehydrogenated and desorbed as formaldehyde below room temperature. In addition, formate species, which is the intermediate in further oxidation 29 ACS Paragon Plus Environment


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to CO and CO2,34 was not observed in the SR-XPS experiment. Thus, a special reaction site for selective partial dehydrogenation of methanol to formaldehyde is formed at the pre-oxidized Zn-Cu(111) surface. In this study, we assigned the reactive methoxy species to those adsorbed on the Cu2O surface oxide based on O 1s and C 1s binding energies. Note that the dehydrogenation of methoxy to formaldehyde below room temperature is not reported on the pre-oxidized Cu(111) surface without Zn deposition.19 Therefore, the chemical reactivity of the Cu2O surface oxide should be different between the pre-oxidized Zn-Cu(111) and Cu(111) surfaces. One of the possible origins of the enhancement of methanol reactivity on the pre-oxidized Zn-Cu(111) is the formation of special reactive sites at the boundary between Cu2O and ZnO islands on the surface. In fact, a recent STM study on the oxidized Zn-Cu(111) surface shows a presence of ZnO islands in contact with the Cu2O surface oxide.70 However, the microscopic mechanism of such synergetic effects between ZnO and Cu should be clarified in a future investigation using local probe methods and vibrational spectroscopy.

4. Conclusions Adsorption and reaction of methanol on Zn-modified Cu(111) surfaces were 30 ACS Paragon Plus Environment

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investigated by high-resolution XPS and TPD. Methanol does not react on either a clean Cu(111) nor a clean Zn-Cu(111) surface; all of the methanol is desorbed intact by heating. The desorption energy of molecular methanol on defect sites is significantly higher than that on terrace sites. The amount of methanol on more stable defect sites is increased by alloying with Zn, because surface roughening is induced by the alloy formation between the deposited Zn and Cu substrate, which leads to an increase of step density. On the other hand, methanol is partially decomposed to methoxy species when a Zn-Cu(111) surface is pre-oxidized. Two methoxy species are observed in XPS measurements as intermediates for dehydrogenation. The reactivity of these methoxy species depends significantly on the surface components. On ZnO islands, it is dehydrogenated at 340-490 K, whereas the more reactive methoxy species is found at Cu2O sites, which is dehydrogenated to formaldehyde at 220-290 K as desorption species. Such reactive methoxy to produce formaldehyde at a lower temperature was observed on neither the CuOy/Cu(111) surface nor the ZnO surface in previous studies. Therefore, we propose that there exists a synergetic effect between the zinc-oxide islands and the partially oxidized Cu substrate for the partial oxidation (dehydrogenation) of methanol.



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Acknowledgements This work was supported by JST ACT-C Grant Number JPMJCR12YU and by JSPS KAKENHI Grant Number 26105004 and 17H05212, Japan. The SR-XPS experiments were conducted under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2012S2-006 and 2015S2-008). We are also grateful to Prof. Kazuhiko Mase and staff members of Photon Factory for their technical support.


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UV photoemission-study. Surf. Sci. 1985, 160, 57-69. 63. Gong, J.; Flaherty, D. W.; Ojifinni, R. A.; White, J. M.; Mullins, C. B., Surface chemistry of methanol on clean and atomic oxygen pre-covered Au(111). J. Phys. Chem. C 2008, 112, 5501-5509. 64. Habenschaden, E.; Küppers, J., Evaluation of flash desorption spectra. Surf. Sci. 1984, 138, L147-L150. 65. Dohnalek, Z.; Kimmel, G. A.; Joyce, S. A.; Ayotte, P.; Smith, R. S.; Kay, B. D., Physisorption of CO on the MgO(100) surface. J. Phys. Chem. B 2001, 105, 3747-3751. 66. Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D., n-Alkanes on MgO(100). I. Coverage-dependent desorption kinetics of n-butane. J. Chem. Phys. 2005, 122, 164707. 67. Campbell, C. T.; Sellers, J. R. V., The entropies of adsorbed molecules. J. Am. Chem. Soc. 2012, 134, 18109-18115. 68. Niemantsverdriet, J. W., Spectroscopy in catalysis: An introduction, third edition. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2007 69. Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D., n-Alkanes on MgO(100). II. Chain length dependence of kinetic desorption parameters for small n-alkanes. J. Chem. Phys. 2005, 122, 164708. 70. Liu, B. H.; Groot, I. M. N.; Pan, Q.; Shaikhutdinov, S.; Freund, H. J., Ultrathin Zn and ZnO films

on

Cu(111)

as

model

catalysts.

Appl.

Catal.

A:

General

2017

DOI:

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Systematic Study of Adsorption and the Reaction of Methanol on

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