Mechanism of the Surface Hydrogen Induced Conversion of CO2 to

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Mechanism of the Surface Hydrogen Induced Conversion of CO to Methanol at Cu(111) Step Sites 2

Yeonwoo Kim, Tran Si Bui Trung, Sena Yang, Sehun Kim, and Hangil Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02083 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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Mechanism of the Surface Hydrogen Induced Conversion of CO2 to Methanol at Cu(111) Step Sites

Yeonwoo Kim,† Tran Si Bui Trung,† Sena Yang,† Sehun Kim, † and Hangil Lee‡,*

†

Molecular-Level Interface Research Center, Department of Chemistry, KAIST, Daejeon

34141, Republic of Korea ‡

Department of Chemistry, Sookmyung Women's University, Seoul 04310, Republic of

Korea

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ABSTRACT Cu/ZnO/Al2O3 is an industrially important heterogeneous catalyst for the conversion of CO2 to methanol, which is in world-wide demand and to solve the activation mechanism of catalytically inactive CO2. Recent studies have achieved numerous improvements in active site of catalysts for this process which can be described as 'active copper with step sites' decorated with ZnOx. In spite of these improvements, the mechanism of this process is still unknown, and even its initial stage remains unclear. In this study, we simplified the catalytic system to bare Cu(111) and Cu(775) surfaces in order to systematically determine the mechanistic effects of step sites. The reaction was conducted by using a CO2/H2 gas mixture at 1 Torr at various temperatures and characterized with infrared reflection absorption spectroscopy (IRRAS). The initial activation of CO2 was found to occur only with the coadsorption of hydrogen; it cannot on its own be converted into other activated species. This co-adsorbed hydrogen induces the dissociation of CO2 and converts it into CO, surface oxygen (O*), and surface hydroxyl (HO*). These species are subsequently converted to carbonate (CO3*), bicarbonate (HCO3*), and formate (HCOO*). One significant observation is that the number of these formate species on step sites continuously decreases with increases in the number of CH2 species during step wise heating. And continuous reaction is obtained from formate transfer from terrace to step. In addition, instantaneous feature of methoxy (CH3O*) was also observed during the evacuation process. These phenomena strongly indicate that formate is an essential intermediate, especially on steps, for the conversion of CO2 to methanol, and that the reduction in its level during this process is due to step-by-step hydrogenation.


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KEYWORDS. Methanol synthesis, CO2 hydrogenation, formate on step, Cu(775) surface, dioxymethylene, formate transfer 1. INTRODUCTION Cu/ZnO/Al2O3 is currently used as an industrial catalyst for methanol synthesis from CO2/CO/H2 syngas mixtures under the conditions pressure = 50 to 100 atm and temperature = 473 to 573 K.1-6 This catalytic system can be utilized not only in methanol synthesis1-3 but also in water-gas shift reactions7-9 and steam reforming10-12 and its activation of carbon dioxide can be extended to the green catalytic conversions of other chemical feedstocks.13 Despite its industrial importance and numerous investigations, the mechanism on the molecular level of the conversion of syngas to methanol is not well understood. There are two important aspects of such a mechanism: the nature of the active sites and the routes of reaction. The systematic approach of J. Nakamura et al. well describes the active sites using the turnover frequency (TOF) of reaction on the low index single crystal surfaces. The results show that the Cu(111) surface decorated by 19% of Zn indicates the 12 times promotional effect for methanol synthesis in comparison with bare Cu(111) surface. That phenomena only observe on Cu(111) surface unlike on the Cu(110) and Cu(100) surface.14,15 More recently, it has been shown that for a series of catalysts with controlled planar defects such as stacking faults and steps, the number of planar defects is linearly proportional to their intrinsic activities. Thus, stepped Cu(111) with Zn decoration contains Cu-ZnOx special active sites that make significant contributions to the catalyst’s activity in methanol synthesis.1,16-18 From the mechanistic point of view, the most important intermediate in the synthesis of methanol is formate (HCOO*) according to extensive observations with X-ray photoelectron


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spectroscopy (XPS), scanning tunneling microscopy (STM), and in situ infrared (IR) spectroscopy.15,19,20 These formate species arise from the direct hydrogenation of CO2 by preadsorbed surface hydrogen in an Eley-Rideal (E-R) type reaction.4,8,21 Thus the mechanism of the conversion of CO2 to methanol is thought to include step by step hydrogenation from formate to methanol (CH3OH) via the transformation of dioxymethylene (H2CO2*), formaldehyde (H2CO*), and methoxy (CH3O*).22,23 However, the only observable intermediates are formate and methoxy except on ZrO2-supported Cu catalysts. 24 Thus, most of the currently accepted mechanism relies on DFT calculations.8,25-27 Recently, kinetic studies of the formation of formate and methanol have demonstrated that the formation of formate is 100 times faster than the release of methanol. So the formation of methanol cannot result from the hydrogenation of bidentate formate by adsorbed H species alone.28,29 And a water-activated carboxyl mechanism has been proposed instead of formate-mediated hydrogenation.27 Thus the initial process and the reaction intermediate have not been identified. In addition, there have been no spectroscopic observations of intermediates except of formate species. So the route between CO2 and methanol is not well characterized. In this study, we obtained spectroscopic evidence that provides insight into the reaction pathway from CO2 to methanol, which could be used to optimize the catalysis of methanol synthesis. This paper focuses on the identification of the routes of the conversion of CO2 to methanol, and on the determination of the sequence of reactions and the role of Cu step sites as active sites. We tried to simplify the system, so conducted the step-sensitive reactions on a Cu(775) surface, which is cut at an oblique angle of 8o with respect to the [112] direction, and compared these results with those for a flat Cu(111) surface. We also present the first detailed experimental investigation of the mechanism of the formation of formate and its


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transformation to methanol.

2. EXPERIMENTAL Our IRRAS (infrared reflection absorption spectroscopy) experiments were carried out in an ultrahigh vacuum (UHV) chamber (p < 1×10-10 Torr). The IRRAS measurements were performed by focusing an IR beam onto the surface at a grazing incidence angle of 8o via ZnSe windows. The spectra were recorded at various temperatures with 2.0 cm-1 resolution (500 scans) by using a FT-IR spectrometer (Thermo Scientific, Nicolet 6700) equipped with a liquid nitrogen cooled MCT detector. All spectra were obtained from a single reflection from the surface of the Cu single crystal. The Cu(775) and Cu(111) single crystals for the IR experiments (MaTeck, size: 10 mm (diameter) × 1 mm (thickness) disk) were welded onto tantalum foil with two spot-welded tantalum wires. The heating and positioning of the samples were achieved with these two tantalum wires. The temperature of each sample was measured with an alumel/chromel thermocouple (type K) spot welded onto the back of the sample. Each Cu surface was cleaned by sputtering it with several cycles of 1 keV Ar+ ions for 30 minutes at 500 K followed by annealing at 900 K for 15 minutes. The pressure of the chamber was maintained below 3.0×10-10 Torr during the annealing procedure. In order to perform the hydrogenation reaction and adsorption experiments in the UHV chamber, two separated gas dosers with high precision leak valves were used to supply the gases with minimal contamination. Hydrogen (99.999%) and carbon dioxide (99.999%) gases were injected at the same time into the chamber at the designated pressure with selected inlet


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speeds. After backfilling up to the desired pressure, spectra were collected by raising the temperature while maintaining the pressure. Temperature measurements during the heating process were obtained from the average value within ± 5 K ranges.

3. RESULTS AND DISCUSSION

Figure 1. IRRAS spectra recorded at 300 K by backfilling gaseous CO2 at 250 mTorr onto (a) the Cu(111) surface and (b) the Cu(775) surface, and by backfilling a H2 (750 mTorr) and CO2 (250 mTorr) gas mixture onto (c) the Cu(111) surface and (d) the Cu(775) surface.

We firstly performed in-situ IRRAS to identify the adsorption structures of CO2 on the Cu surfaces. Figures 1a and b show the spectra for the adsorption of CO2 (250 mTorr) onto the Cu(111) and Cu(775) surfaces respectively. All assignments are shown in Table 1. The intensity of the signal due to carboxylate (CO2δ-) species on the Cu(775) surface is weak, and there are almost no adsorption features for the Cu(111) surface. In general, CO2 cannot


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adsorb on metallic Cu surfaces as a carbonate (CO32-) species.2,30 When CO2 (250 mTorr) is supplied at the same time as H2 (750 mTorr), there are broad features at the region between 900 and 1250 cm-1 which are assigned as atomic hydrogen (further description is shown below). There have small hydrogen features on the Cu(111) surface and broad intense features on the Cu(775) surface. Previous investigations show that the step sites are active adsorption sites and provide the more favorable pathway to dissociate the hydrogen and possess the larger number of hydrogen atom.31,32 In company with atomic hydrogen, carbonate (1418 and 1429 cm-1) species are observed on both surfaces with some byproducts (1362, 1350, 1338, 830, and 816 cm-1).2,33-35 Thus, hydrogen co-adsorption is essential for the activation of CO2. When hydrogen molecules collide with the Cu surface, they dissociate and are adsorbed as atomic hydrogen. This catalytic dissociation of hydrogen occurs even at low temperatures and is stabilized by its high recombination barrier (1.07 eV).8,36 After the adsorption of hydrogen, adsorption features of CO2 become evident even for the relatively inactive Cu(111) surface. Thus, the co-adsorption of CO2 and H2 enables the formation of carbonate and/or bicarbonate species on both flat Cu(111) and stepped Cu(775) surfaces. In order to synthesize carbonate and/or bicarbonate species from H2 and CO2, the initial release of one oxygen atom from CO2 is essential. Thus, pre-adsorbed surface hydrogen initially reacts with CO2 and converts it to CO and surface oxygen (O*) and/or hydroxyl (HO*). These species are converted to carbonate/bicarbonate species by reaction with CO2 on both step (1429 cm-1) and terrace (1418 cm-1) sites. The hydrogen-induced adsorption and catalytic dissociation of CO2 is very interesting considering that no adsorption of CO2 has been reported on any low index Cu surface.37-40 In addition, formate species (HCOO*) were also observed on both surfaces. These formate species have been widely observed in previous STM, IR spectroscopy, and XPS


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experiments, which has prompted the proposal of an E-R mechanism for the reaction of gas phase CO2 with surface hydrogen.8,15,19-21 However, the source of formate has remained unclear because of the experimental difficulties produced by the harsh reaction conditions. Figures 2 and 3 confirm the formation of formate species on both surfaces with different mechanism.

Figure 2. IRRAS spectra obtained by backfilling a mixture of H2 (750 mTorr) and CO2 (250 mTorr) gases on the Cu(111) surface with stepwise heating from (a) 300 K to (b) 310 K, (c) 338 K, (d) 363 K, (e) 383 K, and (f) 393 K.

Figure 2 shows the IRRAS spectra for H2/CO2 mixtures on the Cu(111) surface for various temperatures. At 300 K, a formate peak is evident near 1341 cm-1, which corresponds to formate on terrace sites in the bidentate form.41 In addition, carbonate (1418 cm-1) and carboxylate species (1254 cm-1) are also present. We then increased the temperature because the reaction occurs actively at higher temperatures.42 At 310 K, the intensity of the peak at 1418 cm-1 is reduced, while that at 1341 cm-1 has increased, which suggests that the


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conversion of carbonate to formate has occurred. Desorption of carbonate (described below) is significantly delayed on the Cu(775) surface, for which this peak is steadily maintained until 400 K. Thus the formate species form from the carbonate species. On the ZrO2 surface, formate is also obtained from carbonate.43 These two peaks disappear due to species desorption when the temperature is raised to 380 K, but the meanings of these two disappearances are quite different. The peak at 1341 cm-1 due to formate species on terraces disappears at 380 K, whereas the peak due to carbonate on terraces is not thermally desorbed at 380 K (Figure 4a).41

Figure 3. IRRAS spectra obtained by backfilling H2 (750 mTorr)/CO2 (250 mTorr) gas mixtures onto the Cu(775) surface with stepwise heating from (a) 300 K to (b) 310 K, (c) 314 K, (d) 328 K, (e) 349 K, (f) 393 K, and (g) 429 K. (The changes in intensity are shown in Figure 4.)

The Cu(775) surface is regarded as more active than the Cu(111) surface:1,44 the


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adsorption processes on this surface produce stronger signals and some more complex features than on the Cu(111) surface. Figure 3 was created with exactly the same method as used for the Cu(111) surface. The spectrum at 300 K demonstrates that the adsorption of CO2 occurs as carbonate species, CO32-, on steps (1429 cm-1) and terraces (1418 cm-1) and as carboxylate, CO2δ-, on terraces (1256 cm-1), steps (818 cm-1), and terraces (815 cm-1).2,45-47 Thus these species initially adsorb on both terrace and step sites. The adsorptions of formate (HCOO-) species on steps (1362 cm-1) and bicarbonate species, HCO3- (from 1338 to 1350 cm-1, 830 cm-1) are also evident.33,48,49 All peaks on step sites are assigned by the peaks on Cu(775) surface in comparison to Cu(111) surface. Formate species are assigned by the adsorption of formic acid (Figure S1). The bicarbonate species possibly arise due to the reaction of CO2 with surface hydroxyl groups and/or the hydrogenation of carbonate species (Scheme 1, b2). Computations with a microkinetic model of the Cu(111) surface have shown that CO3* species are stabilized by the formation of HCO3* (-1.21 eV).26 Interestingly, formate species are only observed at the step sites even small portion of step density ~12%. Although the broad band between 1338 and 1350 cm-1 could partly be due to formate species on terraces (1350~1352 cm-1), this contribution is negligible when compared to the possible number of adsorption sites on the terraces. Thus, the formate species mainly form at step edges at the beginning of the reaction, which is in agreement with STM results for formate synthesis at low coverages.19 In order to characterize the active species at higher temperatures, the surface temperature was increased from 300 K to 429 K. At 310 K, the bicarbonate species have completely disappeared in conjunction with the appearance of sharp formate peaks for terrace sites (1350 and 1352 cm-1). This transformation could be due to the exchange of hydroxyl for hydrogen (Scheme 1, b3) because bicarbonate, which is regarded as more stable than carbonate, has


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completely disappeared. This kind of transformation has also been observed at the Cu-Zr interface of the catalyst Cu/ZrO2/SiO2, which produces 27 times more catalytic enhancement than Cu/SiO2, but was first observed on bare Cu surfaces.24 These step sites play a mechanistic role that is similar to that of the Cu-Zr interface, i.e. they enable asymmetric adsorption that results in the formation of formate species from bicarbonate.

Scheme 1. Proposed mechanism of CO2 adsorption and hydrogenation. (a) CO2 adsorption without hydrogen gas, (b) surface hydrogen induced transformation of CO2; the processes in (b) are shown in more detail in b1, b2, and b3. (b1) dissociation of hydrogen, (b2) the reaction of CO2 with hydrogen atom, and (b3) the formation of formate from surface species.

Thus, there have different possible mechanism to synthesize the formate species rather than E-R mechanism. Langmuir-Hinshelwood (L-H) mechanism is one of alternative. At room temperature (Figure 1), adsorbed carboxylates are observed with formate species during co-adsorption of hydrogen and no carboxylate without hydrogen, which means that surface species stabilize the CO2 molecules on a Cu surface as carboxylate. According to the I.A. Bonicke et al., the surface oxygen is one of candidates for these kind of stabilizers. They


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exposed the CO2 on Cu(332) surface at a temperature of 95 K and increased the temperature to obtain the desorption results. From CO2 desorption spectrum, a broad maximum was developed at TD = 376 K with ED = 97.6 kJ/mol which desorption was completely disappeared when the temperature reached to near 400K. They elucidated the adsorbed oxygen stabilizes the CO2 as CO2- state with carbonate-like structure, not a carbonate species. According to Figure 4d, these CO2- species show the similar desorption feature.30 The other source of formate species is transformation of carbonate and bicarbonate as described above. Scheme 1 summarizes the processes involved in the transformation of CO2 into formate. At the beginning of the reaction, atomic hydrogen is pre-adsorbed on the Cu surface. (b1) The atomic hydrogen dissociates incident CO2 to atomic oxygen/hydroxyl and CO molecules. This release of CO molecules is shown as very small features on Figure S2. CO could not adsorb on metallic Cu surface at higher than 300K under UHV condition. However, CO adsorption is shown under the condition of high pressure.50,51 (b2) These adsorbed species are converted to carbonate, bicarbonate, and formate species. The carbonate and bicarbonate species are finally converted to formate by exchanging oxygen and hydroxyl with hydrogen. 24,43

(b3) Figure 4 shows the changes in the intensity of four different peak regions in order to

compare the reactions that occur on the step and terrace surfaces at various temperatures. All spectra were collected by stepwise heating from 300 K to 430 K at approximately 10 K intervals. The black squares are the peaks on the Cu(111) surface. The peaks on the Cu(775) surface are shown as red circles and blue triangles.


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Table 1. Vibrational frequencies of carboxylate, carbonate, bicarbonate, and formate species CO2δ(Carboxylate) species

CO32(Carbonate) species

HCO3(Bicarbonate) species

δ(OCO)

Terrace 815 Step 818

840

νs(OCO)

1254-1256

1338-1350

νas(OCO)


HCOO-(Formate) species On Cu(775)

On Cu(111)


1341

1610

ν(CH)

2852

2830

ν(OCO)+ δip(CH)

2931

2917

When the temperature is increased above 328 K (Figure 3d), three distinct features become evident. First of all, the intensity of the peak due to formate on step sites continuously decreases above this temperature (Figure 4c, red circles). Secondly, a peak appears at 1336 cm-1 above 330 K (Figure 3e). Lastly, an additional peak appears at 1392 cm1

at 360 K and 1424 cm-1 at 400K (Figures 3f, 4b).

Figure 4. Variations in peak intensities for the Cu(111) and Cu(775) surfaces with stepwise heating at intervals of 10 K: (a) carbonate, (b) dioxymethylene, (c) formate, and (d) carboxylate.


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The decrease in the level of step formate with temperature is an interesting phenomenon that has previously been observed on Cu/SiO2; it was concluded that this phenomenon is the result of the desorption of monodentate formate, which was not found at higher temperatures.24 However, the previous assignment and desorption temperature of 1362 cm-1 formate peak (Figure S2) do not support this conclusion. In fact, the δip(CH) peak of monodentate formate on the Cu(110) surface arises at 1381 cm-1. If formate is adsorbed as monodentate formate, one C-O bond of O-C-O is aligned against from the surface which indicates strong C=O double bond at 1640-1670 cm-1.52,53 However, there is only a very weak 1612 cm-1 peak on the Cu(775) surface according to the results in Figure S1. In addition, the desorption temperature of monodentate formate is 150 K, which is much lower than the desorption temperature of the peak at 1362 cm-1. According to Figure S3, the desorption temperature of the peak at 1362 cm-1 is almost same as that of the 1352 cm-1 peak, and is also well matched with that of bidentate formate on the Cu(110) surface.15 Our DFT calculation results for the adsorption energies of formate (Figure S4) show that formate on step sites has a lower adsorption energy (0.36 eV) than on terrace sites, which also explains the slightly different fingerprint in the IRRAS spectra of the higher peak at 1362 cm-1. In contrast to the desorption of formate under UHV conditions, the intensity of the 1362 cm-1 peak continuously decreases but does not fully disappear and it fluctuates with variations in pressure from 0.5 Torr to 10.0 Torr even at temperatures much lower than the desorption temperature (Figure S5, black squares). This observation supports the hypothesis that formate species on step sites (1362 cm-1) participate dynamically in the reaction rather than thermally desorb from the surface. The disappearance of step formate coincides with the appearance of the peak at 1392 cm1

: the increase in intensity of this peak is shown as red circles in Figure 4b, whereas the


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intensity of the peak at 1362 cm-1 decreases (shown as red circles in Figure 4c). The peak at 1392 cm-1 is assigned to the CH2 wagging mode of dioxymethylene (H2CO2). H2CO2 has no experimental results on any surface but a similar structure of H2CI2 data using infrared spectroscopy and high-resolution electron energy loss spectroscopy was shown strong peaks in the range at 1359 cm-1 and 1350 cm-1 on TiO2 and Rh(111) surface, respectively.54,55 Most of all, the DFT results for H2CO2 on the Cu(111) surface predict that the chemisorbed species [H2CO2]2- on two top sites of Cu atoms (an aligned bridge) will produce a peak at 1398 cm1 56

.

At temperatures above 400 K, the new peak at 1424 cm-1 appears with increases in

temperature. (Figure 4a, red circles) According to Figure 3g, its wavenumber is slightly redshifted from 1429 cm-1 to 1424 cm-1 and the peak also becomes sharper. In comparison with the peak at 1418 cm-1 that is present for the Cu(111) surface, the peak at 1429 cm-1 is also expected to shrink near 400 K because these two carbonate species are expected to exhibit similar behavior. Thus, the new 1424 cm-1 peak must be due to a different vibrational mode: one strong possibility is that this peak is due to the CH2 bending mode of formaldehyde, which appears at ~1420 cm-1 on a Cu surface.26,57 These two peaks at 1392 cm-1 and 1424 cm1

support the conclusion that the step by step hydrogenation of formate occurs via the

CH2O(O) species. Figure 4d shows the δ(OCO) values for the adsorbed carboxylate (CO2δ-) species at various temperatures.30 Interestingly, on the Cu(775) surface this peak appears as a doublet at 818 and 815 cm-1. Comparing the results for the Cu(775) and Cu(111) surfaces, 815 cm-1 is assigned to carboxylate on Cu(111) terrace sites and 818 cm-1 is assigned to carboxylate on the step edges of the Cu(775) surface. When the temperature is increased, the level of carboxylate on step sites dramatically decreases near 360 K. Thus CO2 cannot approach the step edge because at higher temperatures the step edge sites are occupied by molecules such


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as dioxymethylene and formaldehyde in spite of the decrease in the level of formate species. In other words, the residing two species on the step sites cause the early desorption and no observable step peaks of formate, and carboxylate on Figure 4c, d. The distinct feature at 1336 cm-1 that appears clearly at 349 K (Figure 3e) can be assigned to formate on Cu2+. Figure S6 shows the transformation and desorption features of the peaks at 1352 cm-1 and 1336 cm-1 under various pressure conditions. In the reaction at 10 Torr (blue triangles), there is an interesting abrupt increase in the intensity of the peak at 1352 cm-1 at 420 K, which coincides with a decrease in the peak at 1336 cm-1. This phenomenon can be understood as due to the discrepancy between the reaction rates of oxidation and reduction of the surface. At a pressure of 10 Torr, reduction is expected to be activated rather than oxidation. So the hydrogen molecules convert Cu2+ to Cu0, which results in a decline in the intensity of the peak at 1336 cm-1 and an increase in the intensity of the peak at 1352 cm-1. Figure 5 also supports the generation of Cu2+ phase by the oxidation of Cu0.

Figure 5. Desorption of carbonate from step sites (1429 cm-1) on the Cu(775) surface after CO2 hydrogenation due to exposure to CO2 (250 mTorr)/H2 (750 mTorr) followed by evacuation under 1×10-8 Torr.


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Figure 5 shows the surface oxidation process involving carbonate (1429 cm-1) species adsorbed at the step edge of the Cu(775) surface. According to these results, the desorption of carbonate species from the surface at 470 K occurs at the same time as the appearance of a peak at 2176 cm-1. This peak is assigned to CO molecules on Cu2+, where it arises at a higher wavenumber than on metallic Cu0 surfaces.58-62 Thus the adsorbed carbonate changes its surface oxidation state via a desorption process: it is converted into a CO molecule by the dissociation of two oxygens. These oxygen atoms remain on the Cu surface and trigger the transformation of Cu0 atoms to Cu2+ ions. Cu2+ is not stable under highly reduced conditions. However, the reaction is occurred from CO2 and H2, not from the H2-only. Formation and release of surface oxygen might be the same rate in equilibrium. Previous literatures also show the surface oxygen and hydroxyl species which were synthesized during the reaction.63 Thus, the possibility to form the local CuO region exists, although the pressure of hydrogen is high. The release of CO was conducted from the carbonate species. Chemical adsorption of CO2 forming the CuCO3 is very stable and can indicate the high desorption temperature in comparison to normal desorption temperature of CO.64

Figure 6. IRRAS spectra obtained after (a) backfilling H2 (750 mTorr)/CO2 (250 mTorr) gas


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mixtures onto the Cu(775) surface at 392 K for 20 minutes, (b) subsequent quenching to 300 K, (c) then pumping the gases out of the chamber, and (d) finally aging for 10 minutes to stabilize the molecules on the surface.

Figure 6 shows the IRRAS spectra before and after evacuation of the gases. After reaction for 20 minutes, there are some peaks that are similarly positioned to those in Figure 3f. Formate species on terraces (1352 cm-1) are dominant along with a Cu2+ (1336 cm-1) peak; very weak features at 1389 cm-1 are also evident. After quenching to 300 K, the spectrum is almost identical to that at 392 K, which means that the molecular arrangement is almost fixed at these temperatures. No peak at 1362 cm-1 has appeared yet. In contrast to the intensity of the 1352 cm-1 peak, the intensities of the 1336 cm-1 and 1389 cm-1 peaks have increased by approximately 29% and 39% respectively. These phenomena are due to the breaking of the equilibrium of reaction. Before the quenching process, the formation, reaction, and desorption processes are almost at equilibrium. However, during the quenching process desorption is completely restricted in contrast to formation and reaction.19,65 The increases in the intensities of the 1392 cm-1 and 1336 cm-1 peaks and the small increase in the intensity of the peak at 1352 cm-1 result from formation and reaction over the short quenching period. After evacuation of the gases from the chamber for 5 minutes (reaching a pressure below 1.0×10-8 Torr), intermediate dioxymethylene (1389 cm-1) desorbs from the surface step sites because of its weak adsorption and these step sites become available. This desorption triggers formate transfer from terrace to step, as shown by the shrinkage of the 1352 cm-1 and 1336 cm-1 peaks and the increase in the intensity of the 1362 cm-1 peak. Figure S2 shows the reverse transfer from step to terrace near the desorption temperature, which confirms that transfer between these two sites is facile. The terrace sites act as a reservoir of formate


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species and supply the formate from the terrace sites to the step sites after methanol is released by reaction on the step sites. This transfer makes consecutive reactions at step sites possible. After the quenching process, a peak appears at 2890 cm-1, which is assigned to the 2δas(CH3) mode of methoxy (Figure S7). This instantaneous feature is a result of the subsequent cooling, which enables the detection of an otherwise unstable molecule. But there is only the peak at 2890 cm-1, which is relatively small feature compared to other peaks of methoxy. It may be caused by the geometric contribution. After the hydrogenation process, methoxy species occupy on the terrace Cu atom, which methoxy species are possibly aligned in a configuration parallel to the surface because of the weak interactions between the methoxy hydrogen and the surface oxygen at the step edge. This configuration restricts all parallel components and thus gives rise to the peak at 2890 cm-1. According to the region between 900 cm-1 and 1250 cm-1, there have two broad peaks at 1182 cm-1 and 1072 cm-1. Both broad peaks are assigned to the first overtone of the deformation and stretching modes of the atomic hydrogen on the Cu(111) surface as saturation coverages, respectively. These results are equivalent with the adsorption of hydrogen-only (Figure S8) and almost the same shape and position as the hydrogen adsorption at 225 K.66 Relatively broader peaks than those at 225 K arise from the thermal contribution but the condition is still below the desorption temperature.67 This broad peak and geometric contribution of methoxy tarnish the small portion of v(CO) of methoxy at ~1072 cm-1.


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Scheme 2. Schematic diagram of CO2 hydrogenation during methanol synthesis: (a) formate on step sites, (b) formate on Cu2+ sites, (c) dioxymethylene, (d) methoxy, (e) methanol, and (f) formate transfer from terrace to step.

Scheme 2 illustrates the step-sensitive hydrogenation process. As mentioned above, the presence of formate species on step sites is crucial to the methanol synthesis reaction. According to the previous results of CO2 hydrogenation on bare Cu surfaces, the smaller coordination number (CN) showed greater activity as shown in surface tendency of Cu(111) < Cu(100) < Cu(110).68 This surface-dependent tendency is comparable with the proportion of dissociative adsorption to molecular adsorption of dioxygen on Cu surface.69 In other words, open surface of Cu(110) with 7 of CN can be more tightly bound to the adsorbed molecules such as dioxygen or formate, indicating the weakening the internal bonding of ] Cu atom. those species. From previous STM results, formate forms on the top site of a [011 ] direction is much larger than that of the [11 0] direction, The molecular protrusion of the [011

which suggests that formate does not align along the step edge.19 Thus formate species on a step site experience a totally different chemical environment. The CN of a Cu step site is 7, and thus it has a more positive charge than a terrace site (CN = 9) because of the missing nearest neighbor atoms. Asymmetric adsorption at step sites are also comprehensible as same


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mechanistic hypothesis with tendency of activity on surfaces. Step sites have the same CN with Cu(110) surface as 7, which shows the asymmetric tight bonding with one of oxygen atom in formate. Thus, the 7-coordinated Cu atom at the step edge activates one of oxygen atom in formate for conversion. Figure S4 shows that formate species on step sites are more stable, as mentioned above. This asymmetrically strong adsorption weakens one internal C-O bond in formate, which can easily be converted to a C-H bond by hydrogenation and breaking the C-O bond. Cu-Zn sites are also comprehensible by using the same model. Zn sites are relatively easier to oxidize than Cu sites, thus those sites can more tightly bound to the adsorbed oxygen. In industrial condition, the state of Zn species is not fully oxidized ZnO and shows the reduced species as ZnOx which can more tightly bound to the one of oxygen atom in formate. As results of this process, Cu-Zn asymmetric adsorption of formate was also detectable in previous literatures.70,71 If formate adsorbs on the Cu surface in an asymmetric structure, the oxygen atoms in formate have slightly different chemical bonding. Step site oxygen is more tightly bound to the surface than terrace oxygen due to its CN. Such asymmetric bonding triggers oxygen dissociation from the formate intermediate (a, b) or dioxymethylene (c) to produce methanol (d, e). Continuous reaction is achieved by the transfer of formate from terrace to step (f). From the recent studies, SiO2 supported Cu showed the evolution of methanol with no formtate species under existence of water. They explained the CO2 react with water and transfer to the carboxyl and finally synthesize the methanol via carbene diol, methynol. Although ceria supported Cu/TiO2 described the reaction occur from the formate species to methanol via the formyl, formaldehyde, they could not observe the evidence of those intermediates and described from the DFT calculation. However, this study shows the shrinkage of formate with arise of CH2O(O) which can support the mechanism from the


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formate as a key intermediate and step-by-step hydrogenate to methanol. The main reason of activity enhancement at step sites is considered as asymmetric adsorption of formate species.

4. CONCLUSION In summary, the initial adsorption of CO2 cannot be activated on a bare copper surface. Thus, in order to convert CO2 to carbonate/bicarbonate and formate species, surface hydrogen from pre- and/or co-adsorption is essential (Scheme 1). This surface hydrogen induces the dissociation of CO2 and converts it to CO, which can be described as a reverse water-gas shift (RWGS) reaction, also producing surface oxygen (O*) and surface hydroxyl (HO*). These species are subsequently converted to carbonate (CO32-), bicarbonate (HCO3-), and formate (HCOO*). Formate is known to be produced by the reaction of gaseous CO2 and surface hydrogen in an E-R type reaction. However, it can also arise from the L-H mechanism of CO2 and H2 as well as the exchange of surface hydrogen with the oxygen/hydroxyl groups of carbonate/bicarbonate species at low temperature. The presence of formate species, especially on step sites, results in the formation of CH2O(O) as an intermediate during stepwise heating, which is converted to methoxy (CH3O*) and methanol (CH3OH). Furthermore, terrace sites act as formate reservoir to supply the formate to the step sites. This may decouple the formate coverage from the methanol formation rate. These conclusions confirm that 'step formate' is an essential intermediate and that its desorption occurs via the hydrogenation reaction.

AUTHOR INFORMATION


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Corresponding Author *Hangil Lee Tel.: +82 2 710 9409; fax: +82 2 2077 7321 E-mail: [email protected]

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (No. 20090083525 and No. 2015021156).

ASSOCIATED CONTENT Supporting Information. IRRAS spectra corresponding to the adsorption of formic acid and methanol; IRRAS spectra corresponding to the CO region during CO2 hydrogenation reaction; desorption plot of formate species; DFT calculations; additional IRRAS plot at various pressures; IRRAS spectra corresponding to adsorption of hydrogen-only. This material is available free of charge via the Internet at http://pubs.acs.org.

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