Methanol Decomposition on Co(0001): Influence of the Cobalt

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Methanol Decomposition on Co(0001): Influence of the Cobalt Oxidation State on the Reactivity Jun Chen, Qing Guo, Jiawei Wu, Wenshao Yang, Dongxu Dai, Maodu Chen, and Xueming Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00727 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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The Journal of Physical Chemistry

Methanol Decomposition on Co(0001): Influence of the Cobalt Oxidation State on the Reactivity

Jun Chen,1), 2) Qing Guo,2),3), *) Jiawei Wu,2) Wenshao Yang,4) Dongxu Dai,2) Maodu Chen,1) and Xueming Yang 2), *) 1

Key Laboratory of Materials Modification by Laser, Electron, and Ion Beams (Ministry of Education), School of Physics, Dalian University of Technology, Dalian, Liaoning, 116023, P. R. China 2

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, 457 Zhongshan Road, Dalian, Liaoning, 116023, P. R. China 3

4

*)

Department of Chemistry, Southern University of Science and Technology, Shenzhen, Guangdong, 518055, P. R. China.

Hangzhou Institute of Advanced Studies, Zhejiang Normal University, 1108 Gengwen Road, Hangzhou, Zhejiang, 311231, P. R. China

To whom all correspondence should be addressed. Correspondence Emails:

[email protected] and [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Reaction of methanol (CH3OH) on metal surfaces has received lots of concerns due to its potential application in hydrogen (H2) production and fuel cells. In this work, we have studied the decomposition of CH3OH on Co(0001) and its oxide surfaces using temperature programmed desorption (TPD) to understand the effect of surface oxidation on the production of H2 via CH3OH decomposition. On the clean Co(0001) surface, CH3OH molecules decompose into CO and H atoms easily in the temperature range of 280-350 K, leading to a maximum yield of 0.27 ML for H2 production. With O atoms on the Co(0001) surface, methoxy groups can be easily formed. As the surface temperature rises to about 370 K, methoxy groups begin to decompose followed by an immediately desorption of H2 and CO at the same temperature, resulting in a maximum yield of H2 increases to 0.42 ML on the ~0.25 ML O atoms covered Co(0001) surface. However, on the CoO and Co3O4-like surfaces, CH3OH is selectively decomposed to CH2O and CO2 respectively, the H2 production is depressed significantly. While, the activities of CoO and Co3O4-like surfaces are much lower than that of O atoms covered Co(0001) surface. Therefore, to avoid the formation of oxide surfaces is helpful for H2 production from CH3OH decomposition.

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1. INTRODUCTION The development of hydrogen fueled proton-exchange membrane fuel cell (PEMFC) is limited by the difficult of the storage and transportation of hydrogen.1 Producing hydrogen from methanol (CH3OH) is a promising method to circumvent these problems, since CH3OH has a high hydrogen to carbon ratio and is easy to store and transport at atmospheric conditions.2-3 Moreover, CH3OH can be used in the direct methanol fuel cell (DMFC) without converting to H2 first. In the DMFC, the crucial step is catalytic CH3OH decomposition.4 Thus, fundamental understanding of the decomposition of CH3OH with active materials is important for the development of fuel cells and H2 production. In the last decade, much attention has been focused on the decomposition of CH3OH on various metal surfaces. On the inert metal surfaces (such as, Cu(110),5-9 Ag(111),10-11 and Au(110)12), only a tiny part of adsorbed CH3OH molecules dehydrogenate to formaldehyde (CH2O) and H2. Whereas, on the relatively active transition metal surfaces (such as Pt(111),13-14 Pd(111),15-16 Ni(111)17), CH3OH mainly decomposes into CO and H2 with a high yield. While, initial C-H bond scission in CH3OH decomposition is more favorable on the Pt(111)14 and Pd(111)16 surfaces. Conversely, O-H bond activation is more competitive than C-H bond activation on the surface of Ni(111).17 In addition, CH3OH decomposition starts with O-H bond scission to produce CO and H2 has also been found on the Ru(0001),18-19 Fe(110),20 and Ir(111)21-22 surfaces. While, a small amount of C-O bond scission product was also observed on the Ru(0001), Fe(110) and Ir(111) surfaces. 3



Recently, cobalt has also been proposed as a promising candidate catalyst for H2 production from CH3OH decomposition.23 Habermehl-Cwirzen and coworkers23 found that part of CH3OH molecules dissociatively adsorb on the clean Co(0001) surface to produce H atoms and stable methoxy (CH3O) intermediate at 165 K as evidenced by XPS results. At higher surface temperature (> 280 K), CH3O groups further decompose into CO and H atoms, and no C-O bond scission product is detected on this surface. Whereas, the surface state variation by oxidation significantly affects reactivity and reaction channels. For example, on the cobalt thin films,24 the presence of oxygen favors partial oxidation of CH3OH to CH2O over decomposition to CO and H2. While, surface oxidation not only inhibits the C-H bond breaking of CH3O, but also decreases the activity of the Co surface toward C-O and O-H bond scissions, leading to a reduction in total reactivity. Similarly, Marta and coworkers25 found that CH2O, carbon oxides, and several decomposition and fragmentation products could be produced via CH3OH decomposition on the CoO and Co3O4 surfaces. In addition, Zafeiratos and coworkers26 reported that CH3OH is totally oxidized to CO2 and H2O via an HCOOads intermediate on the Co3O4 surface, and partial oxidation of CH3O to CH2O is more preferred on the CoO surface. Although these studies clearly show that the oxidation state of cobalt can significantly change the decomposition channels of CH3OH, the fundamental processes about how the reaction channels involved in CH3OH decomposition, especially the H2 formation channel, are affected by different oxidation states of cobalt are still lack. The oxidation of Co(0001) is dependent on the oxygen (O2) exposures and the 4


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surface temperature.27-32 Castro et al.27 reported that O2 molecules dissociatively adsorb on the Co(0001) surface at 120 K. When the chemisorbed O atom layer reaches 0.5 ML, the formation of Co3O4 starts, while the formation of CoO can't be ruled out. At room temperature, O atoms dissolves into the bulk even at very low O atom coverage and CoO is formed. Only high O2 exposures lead to the formation of Co3O4.27 Similar results were also found by Getzlaff and coworkers,28 and the coexistence of chemisorbed O atoms (Oa) and antiferromagnetic CoO islands on the surface was reported. Theoretical studies further proposed that the oxidation processes occur on the Co(0001) surface as the following sequence with increasing O2 exposures: O2 → Oa/Co(0001) → CoO → Co3O4.30-31 In this work, we have systematically investigated the decomposition of CH3OH on Co(0001) with different oxidation states under ultrahigh vacuum (UHV) conditions. On the clean Co(0001) surface, CO and H2 are the only products from CH3OH decomposition. When the Co(0001) surface is covered with Oa atoms, the H2 production via CH3OH decomposition is largely enhanced. However, CH3OH molecules prefer oxidation into CH2O and HCOO- over decomposition to CO and H2 on the CoO and Co3O4-like surfaces, respectively.

2. EXPERIMENTAL METHOD The experiments were conducted in an UHV system equipped with sputter ion gun, low energy electron diffraction (LEED) optics for sample preparation, and a quadruple mass spectrometer (SRS RGA100) for TPD spectra measurement, the chamber base pressure is better than 2 × 10-10 Torr. The Co(0001) single crystal 5



(MaTecK, 6 mm × 6 mm × 1 mm) was cleaned by cycles of Ar+ sputtering (1.7 keV) at 600 K for 15 min and UHV annealing at 630 K for 30 min. CH3OH and CD3OH samples were purified by cycles of pump-freeze-thaw processes, and then were introduced to the surfaces at 100 K through a home-built calibrated molecular beam doser. O2 gas with high purity of 99.999% was dosed to the clean Co(0001) surfaces at 100 K and 250 K for preparation different oxidation states of Co surfaces. TPD spectra were collected with a ramping rate of 2 K/s. Because the hcp-fcc phase transition occurs in cobalt at about 700 K,33 the TPD spectra were only collected to 630 K to avoid phase change. On the oxidized Co(0001) surfaces, part of adsorbed methanol (CH3OH) will react with Oa on the surfaces, leading to a decrease of the coverage of Oa on the surfaces or a change of surface structure for the oxidized surfaces. In this case, it is very difficult for us to determine the amount of O2 that is needed to restore the oxidized surfaces after CH3OH reaction to the initially prepared ones. To make sure the accuracy of our experiments, the oxidized surfaces used for every experiments need to be newly prepared via the oxidation of the clean Co(0001) surface before CH3OH adsorption. However, when the clean Co(0001) surface is oxidized by O2 to form the Oa covered surface, heating the Oa covered Co(0001) surface to 630 K under UHV condition or in the presence of CO can not remove the Oa atoms.32 Similar results were also found in our previous study that focused on low temperature CO oxidation on Co(0001), in which cycles of TPD processes heating up to 630 K do not remove the oxidation states of Co(0001).37 While, restoring the cobalt surface from 6


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oxidation states by heating probably need to heat up to ~1000 K.27 At such a high temperature, the phase change has occurred. Therefore, between every TPD measurement, a clean Co(0001) surface needed to be prepared by two cycles of Ar+ sputtering and annealing before the preparation of the oxidized surfaces.

3. RESULTS 3.1 CH3OH Decomposition on Co(0001) Figure 1 shows the TPD spectra collected at mass-to-charge ratios (m/z) of 2 (H2+) and 28 (CO+) after exposing the clean Co(0001) surfaces to various coverages of CH3OH at 100 K. At low CH3OH coverage (< 0.18 ML (1 ML = 1.8×1015 molecules/cm2, is equal to the amount of Co atoms in the unit area)), the desorption of H2 and CO are only detected in the TPD spectra of m/z = 2 and 28 at high temperatures (> 300 K). While, no low temperature peaks in the TPD spectra of these two masses indicates that CH3OH molecules are completely decomposed at these coverages. When the CH3OH coverage increases to 0.18 ML, the H2 desorption peak (@377 K) saturates with a yield of 0.27 ML, and a peak at 170 K grows in the TPD spectra of m/z = 2 and 28. As the CH3OH coverage keeps increasing, an unsaturated peak develops at around 150 K. According to previous works about CH3OH adsorption on Co(0001),23, 34 the low-temperature peaks (< 200 K) come from the desorption of CH3OH molecules. While, as shown in the inlet of Figure 1b. The TPD profiles of m/z = 2, 28 and 31 collected on the 0.39 ML CH3OH covered Co(0001) surface are the same, indicating the signals observed in the TPD traces of m/z = 2, 28 below 200 K are due to cracking of the CH3OH in the ionizer of the mass 7



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spectrometer. However, because the RGA used in this work has been modified to be suitable for TPD measurements, leading to the detecting efficiency at different masses changed significantly. The largest signal in the CH3OH cracking pattern is m/z = 28, not m/z = 31. Unlucky, we didn’t find the reason. In addition, based on previous works of CO and H2 desorption on Co(0001),35,36 no desorption peaks of CO and H2 were observed at temperature < 200 K as evidenced by TPD measurements. Therefore, the low-temperature peaks (< 200 K) could only come from the desorption of CH3OH molecules. As reported by Habermehl-Cwirzen and coworkers,23 CH3OH could easily dissociate into CH3O and H atom via O-H bond scission as the surface temperature is lower than 165 K. CH3OHa → CH3Oa + Ha

< 165 K

(1)

When the surface temperature rises to ~280 K, CH3O starts to decompose to surface CO and H atoms, and CH3O groups are totally decomposed at ~350 K.23 CH3Oa → COa +3Ha

280-350 K

(2)

low temperature channel

Then, the surface H atoms and CO will desorb as H2 and CO gases at moderate temperatures.23 2Ha → H2(gas)

> 350 K

(3)

COa → CO(gas)

> 375 K

(4)

Our result is consistent with the observations.23 While, no C-O bond scission channel that maybe occurs at the defects of the cobalt surface34 is detected, demonstrating that CH3OH could decompose into H2 and CO efficiently on Co(0001) at low coverages. 8


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Figure 1. TPD spectra collected at m/z = 2 (H2+) and 28 (CO+) after exposure of different coverages of CH3OH to the clean Co(0001) surface at 100 K.

When the coverage of CH3OH increases, the H2 desorption peak increases in intensity and shifts from 405 K to 377 K (Figure 1a). However, the desorption peak of H2 via CH3OH decomposition is much narrower than that from the H atoms covered Co(0001) surfaces produced by dissociative adsorption of H2.35 On the basis of previous works,36 the repulsive interaction between CO and D atoms could make the D2 desorption peak shift to lower temperature significantly and narrow the desorption peak after exposing D atoms pre-covered Co(0001) surfaces to CO molecules.

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Similarly, the desorption peak of H2 observed in Figure 1a is also due to the repulsive interaction between CO and dissociated H atoms. Accompanied by the desorption of H2, the CO desorption peak at 438 K is observed (Figure 1b), and also saturates at 0.18 ML CH3OH coverage. 3.2 CH3OH Decomposition on O/Co(0001) To unravel how the reaction channels in CH3OH decomposition are affected by different oxidation states of cobalt, the reaction of CH3OH on oxidized Co(0001) surfaces was investigated. The different oxidation states of Co(0001) surfaces were prepared by exposing the Co(0001) surfaces to different exposures of O2 gas at 100 K. Figure 2 shows the TPD spectra collected at m/z = 2 (H2+), 18 (H2O+), 28 (CO+), and 30 (CH2O+) after exposure of 0.39 ML CH3OH to the 0-1.17 L O2 pre-oxidized Co(0001) surfaces at 100 K. As the exposure of pre-dosed O2 increases from 0 to 0.83 L, the H2 (m/z = 2) desorption peak at 377 K gradually increases in intensity (Figure 2a), slowly shifts to higher temperature and becomes broader. When O2 exposure is bigger than 0.83 L, the H2 desorption peak gradually changes from a nearly symmetric desorption peak to an asymmetrical desorption peak and quickly shifts to 400 K, which may be due to the desorption of two peaks. A high temperature peak locates at 400 K and grows rapidly with increasing O2 exposure. While, a shoulder peak at 377 K decreases significantly.

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Figure 2. TPD spectra collected at m/z = 2 (H2+), 18 (H2O+), 28 (CO+), and 30 (CH2O+) after exposure of 0.39 ML CH3OH to various amount (0 L - 1.17 L) of O2 pre-exposed Co(0001) surfaces at 100 K.

Concomitant to the changes of the H2 desorption peak, the CO desorption peak at 438 K increases in intensity and shifts to lower temperature with increasing O2 exposure (< 0.50 L) (Figure 2b). As O2 exposure is bigger than 0.5 L, a new desorption peak at ~390 K appears. Then, the peak grows rapidly and shifts to 400 K with increasing O2 exposure. Likewise, the 438 K peak decreases very fast, similar to the decrease of the H2 desorption peak at 377 K. At 1.17 L O2 exposure, the 400 K desorption peak becomes the main desorption feature for both the H2 and CO TPD spectra. The two peaks of CO desorption clearly indicates that a new reaction channel in CH3OH decomposition may exist on the O2 pre-dosed Co(0001) surfaces. According to recent works about H2 and CO desorption on the Oa atoms pre-covered

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Co(0001) surfaces done by Kizilkaya and coworkers,32 the Oa atoms on the Co(0001) surfaces will lower the desorption energies of H2 and CO, leading to the desorption of H2 and CO at lower temperatures. As shown in Figure 2b, the CO desorption peak gradually shifts to lower temperature with increasing O2 exposure, similar to Kizilkaya’s observation.32 Whereas, the H2 desorption peak shifts to higher temperature as the O2 exposure increases, contrary to Kizilkaya’s result, demonstrating that the high temperature desorption peak of H2 at 400 K could not be attributed to the recombination of dissociated H atoms that are produced via CH3O decomposition in the temperature range of 280-350 K. More interestingly, at 1.17 L O2 exposure, the main desorption peaks of H2 and CO appear at the same temperature (400 K), and the shapes of the H2 and CO desorption peaks are very similar to each other. This indicates that the desorption of H2 and CO at 400 K is most likely due to a new reaction channel─the decomposition of a stable intermediate at about 400 K followed by an immediately recombinative desorption of H2 and CO. Otherwise, the H2 desorption should be detected at a lower temperature that on the clean Co(0001) surface. Similar desorption feature of CO TPD was also reported by Habermehl-Cwirzen and coworkers,23 the CO desorption spectrum from CH3OH decomposition on Co(0001) shows a remarkable shoulder peak at the left side of the CO desorption peak at ~370 K, which is similar to CH3OH decomposition on the oxidized Co(0001) surfaces in this work, indicating that the Co(0001) surface used in Ref. 23 may be polluted by O2. In addition, the yields of H2 and CO increase with O2 exposure, 12


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suggesting that CH3OH decomposition is enhanced after O2 exposure.

Figure 3. TPD spectra collected at m/z = 2 (H2+), 3 (HD+), 4 (D2+), 18 (H2O+), 19 (HOD+), 20 (D2O+) and 28 (CO+) after exposure of 0.39 ML CD3OH on the clean and 1 L O2 pre-covered Co(0001) surfaces at 100 K.

To confirm the possible intermediate, an isotopic labeling experiment was performed on the clean and 1 L O2 pre-covered Co(0001) surfaces. Figure 3 shows the TPD spectra collected at m/z = 2 (H2+), 3 (HD+), 4 (D2+), 18 (H2O+), 19 (HOD+), 20 (D2O+) and 28 (CO+) after exposure of 0.39 ML CD3OH on the clean and 1 L O2 pre-covered Co(0001) surfaces. On the clean Co(0001) surface, H2 (m/z = 2), HD (m/z = 3), and D2 (m/z = 4) (Figure 3a) are detected. Due to the isotopic effect, the desorption temperatures of H2, HD, and D2 gradually elevate. No signals of products H2O, HOD and D2O are detected. However, on the 1 L O2 pre-dosed Co(0001) surface, H2 and HD desorption peaks disappear during the TPD process (Figure 3b).

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Correspondingly, an obvious H2O signal desorbed at 158 K appears, and the CD3OH desorption peak decreases significantly. The results suggest that surface oxygen species could react with CD3OH molecules facilely to produce H2O and CD3O at < 158 K, leading to no observation of HD, H2, HOD and D2O signals. 2CD3OHa + Oa/(1/2)O2a→ 2CD3Oa + H2Oa

< 158 K

(5)

After H2O desorption, only CD3O groups will exist on the 1 L O2 oxidized surface. Upon heating, a small part of CD3O group will decompose into CO and D atoms in the temperature range of 280-350 K, resulting in the desorption of D2 at ~ 380 K (Figure 3b, the low temperature shoulder in TPD trace of m/z = 4), similar to the desorption temperature of D2 on the clean Co(0001) surface. As the surface temperature rises to about 370 K, the stably adsorbed CD3O groups begin to decompose followed by an immediately desorption of D2 and CO. 2CD3Oa → 2CO(gas) +3D2(gas)

> 370 K

(6)

high temperature channel

With increasing O2 exposure, H2O desorption at ~160 K appears and grows in intensity (Figure 2c), which is due to the reaction of surface oxygen species and CH3OH molecules. Correspondingly, the desorption peak of molecular CH3OH on Co sites (172 K peak) decreases and disappears with increasing O2 exposure (Figure 2d). While, the multilayer peak also decreases, demonstrating that more CH3O groups are produced on the oxidized surfaces, compared to that on the clean surface. In addition, no CH2O was found in the range of O2 exposure, which is different from previous works about CH3OH decomposition on CoO and Co3O4 that CH2O is one of the main products,24-26 demonstrating that CoO oxidation state is not formed after exposing the 14


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Co(0001) surface to 1.17 L O2. While, theoretical studies concluded that the oxidation of the Co(0001) surface with O2 exposure occurs in the following processes: O2 → Oa/Co(0001) → CoO → Co3O4.30-31 Therefore, as the O2 exposure is less than 1.17 L, the Co(0001) surface is mostly likely covered with Oa atoms.

Figure 4. TPD spectra collected at m/z = 2 (H2+), 18 (H2O+), 28 (CO+), and 30 (CH2O+) after exposure of 0.39 ML CH3OH to various amount (1 L - 2 L) of O2 pre-exposed Co(0001) surfaces at 250 K.

As reported by Weststrate et al.,32 adsorption of moderate amounts of O2 at 250 K on Co(0001) leads to the formation of p(2 × 2)-O structure with 0.25 ML O atom coverage. Similarly, a 0.25 ML Oa atoms covered Co(0001) surface was also prepared after exposing the clean Co(0001) surface to 2.0 L O2 at 250 K. Then, TPD measurements were performed after adsorbing 0.39 ML CH3OH on the oxidized surfaces that were prepared by exposure clean Co(0001) surfaces to different amount

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of O2 (1.0, 1.5, 2.0 L) at 250 K (Figure 4). At 2.0 L O2 exposure, the desorption temperatures and the peak shapes of CO and H2 are very similar to those collected on the 1.17 L O2 pre-dosed Co(0001) surface when O2 molecules adsorb at 100 K, further illustrating that the Oa atoms covered Co(0001) surfaces are formed when O2 molecules adsorb on Co(0001) at 100 K in the exposure range of 0-1.17 L. Then, CH3OH molecules could react with Oa atoms to produce H2O and CH3O, reaction (5) can rewrite as 2CH3OHa + Oa → 2CH3Oa + H2Oa

< 158 K

(7)

The Oa atoms could enhance the production of CH3O groups on the Oa/Co(0001) surfaces, and stabilize the adsorption of CH3O groups. As a result, the yields of H2 and CO from CH3OH decomposition is largely improved. 3.3 CH3OH Decomposition on CoO and Co3O4 When O2 exposure increases from 1.17 L to 1.67 L, the low temperature shoulder at ~377 K in the H2 TPD spectra (Figure 5a) and the high temperature shoulder at ~438 K in the CO TPD spectra (Figure 5b) gradually decrease and nearly disappear. While, the 400 K desorption peak for both H2 and CO increases slowly. This suggests that the low temperature channel (reaction (2)) for CH3O decomposition is further inhibited

by

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Figure 5. TPD spectra collected at m/z = 2 (H2+), 18 (H2O+), 28 (CO+), and 30 (CH2O+) after exposure of 0.39 ML CH3OH to various amount (1.17 L - 2.33 L) of O2 pre-exposed Co(0001) surface at 100 K.

Figure 6. TPD spectra collected at m/z = 2 (H2+), 18 (H2O+), 28 (CO+), and 30 (CH2O+) after exposure of 0.39 ML CH3OH to various amount (2.33 L - 8.33 L) of O2 pre-exposed Co(0001) surface at 100 K.

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surface Oa atoms, and the high temperature channel (reaction (7)) is still enhanced slowly. As the O2 exposure is bigger than 1.67 L, both the yields of H2 and CO decrease significantly. In the O2 exposure range of 1.17-2.33 L, the desorption of CH2O locating at 400 K is observed and grows very fast with increasing O2 exposure (Figure 5d). According to previous studies,26 the production of CH2O via CH3O decomposition only occurs on the CoO surfaces. Thus, as the O2 exposure is bigger than 1.17 L, CoO state starts to form on the Oa/Co(0001) surface. More importantly, the desorption temperature of CH2O is the same as that for H2 and CO, suggesting that CH2O may come from CH3O dehydrogenation followed by an immediately desorption of CH2O and H2 at such a high surface temperature, 2CH3Oa → 2CH2O (gas) +H2 (gas)

(8)

In addition, the H2O desorption peak (Figure 5c) shifts to higher temperature as O2 exposure is larger than 1.17 L, and gives a new peak at 182 K, indicating that the surface property is changed remarkably after exposure of O2 larger than 1.17 L. At 2.33 L O2 exposure, the yield of CH2O reaches to maximum (Figure 6d). Further increasing O2 exposure, all of the H2O (182 K), H2 (400 K), CO (400 K) and CH2O (400 K) peaks decrease and eventually disappear at 7.33 L O2 exposure. However, high temperature products (> 500 K) are detected (such as products H2 (593 K), H2O (615 K), CO (593 K), and CH2O (576 K)), and increase with O2 exposure. After O2 exposure exceeds 7.33 L, the TPD signals do not change anymore, indicating the surface has been oxidized to a stable form. Based on previous works,26 the stable form is the Co3O4-like form. To identify the possible products, TPD traces at other 18


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masses

Figure 7. TPD spectra collected at m/z = 2 (H2+), 18 (H2O+), 28 (CO+), (CH2O+), 31 (CH3O+) and 44 (CO2+) after exposure of 0.39 ML CH3OH to 7.33 L O2 pre-exposed Co(0001) surfaces at 100 K.

were collected after exposure 0.39 ML CH3OH to the 7.33 L O2 pre-dosed Co(0001) surface, as shown in Figure 7. In TPD spectrum of m/z = 44, desorption peak at 593 K is observed, which has the same desorption temperature of CO and H2. According to our early work about CO oxidation,37 the desorption temperature of CO2 on the Co3O4-like surfaces will not be as high as 593 K. While, the CO signal from CO2 cracking is very small. In Figure 7, the CO signal is even bigger than the CO2 signal, determining that the peak at 593 K observed in the TPD traces of m/z =2, 28 and 44 cannot be attributed to desorption of CO2. However, the result is similar to the previous study of CH3OH oxidation on Co3O4 surface,26 which were proposed to

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come from the decomposition of the formate (HCOO-) intermediate. Therefore, on the Co3O4-like surfaces, CH3OH molecules are completely oxidized to HCOO- species. During the TPD process, HCOO- mainly decomposes into H2, H2O, CO, and CO2 accompanied with a little amount of CH2O.

4. DISCUSSION The results of decomposition of CH3OH on the clean Co(0001) surface are similar to previous studies.23, 34 However, as shown in Figure 1b and Figure 3a, a small shoulder on the left side of the CO peak is observed, the small shoulder is similar to that observed on the Oa atoms covered Co(0001) surface (Figure 2b), it can be assigned to the decomposition of CH3O via the high temperature channel (reaction (6)). Thus, the presence of the small shoulder in Figure 1b and Figure 3a is due to the pollution of the Co(0001) surface by a small amount of Oa atoms that is left after the surface clean processes. When the O2 exposure increases, the yields of CO and H2 change dramatically. As shown in Figure 8, the yields of CO and H2 measured in the temperature range of 350-500 K as a function of O2 exposures are estimated and summarized. The cracking fragments from CH2O at m/z = 2 and 28 have been subtracted. On the clean Co(0001) surface, CH3OH can completely decompose into CO and H2.23 A maximum yield of about 0.135 ML CO is produced after CH3OH adsorption. Correspondingly, about 0.27 ML H2 is produced with the ratio of the yields of CO and H2 (RCO/H2) to be 1:2. With increasing O2 exposure, the yields of H2 and CO begin to increase, and RCO/H2 starts to decrease. At about 1.0 L O2 exposure, the surface is covered with ~0.25 ML 20


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Oa atoms. While, the yields of H2 and CO reach their maximum to be 0.42 ML and 0.28 ML, respectively. This demonstrates that the decomposition of CH3OH is largely enhanced on the Oa atoms covered Co(0001) surface. While RCO/H2 decreases to 1:1.5, further demonstrating that H2 and CO are produced from the decomposition of CH3O at this Oa atom coverage.

Figure 8. The yield of H2 and CO as well as the ratio of H2/CO as a function of the exposure of O2 at 100 K.

As O2 exposure increases from 1.0 L to 2.33 L, the yields of CO and H2 begin to decrease. There may be two reasons for the decrease of yields. First, more and more Co sites on the Co(0001) surface are occupied by Oa atoms with increasing Oa atom coverage. Then, the Co sites left for CH3OH adsorption becomes less and less, leading

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to the reduction of CH3OH decomposition. The other one is the formation of CoO islands on the Co(0001) at these O2 exposures, which shows a lower activity than clean Co(0001) for CH3OH decomposition as illustrated by Chen and Friend.24 As shown in Figure 8, although the yields of CO and H2 decrease very fast and a small amount of CH2O is produced when O2 exposure increases from 1.0 L to 1.67 L (Figure 8), RCO/H2 nearly keeps a constant value of 1.5, indicating that the decreased Co sites for CH3OH adsorption is the main reason for lowering the yields of CO and H2. As a result, the CH3O groups mainly decompose on the Oa/Co(0001) surface. However, as O2 exposure increases from 1.67 L to 7.33 L, the oxidation processes on the Oa/Co(0001) surface may occur as follow: Oa/Co(0001) → CoO → Co3O4. Then, the reactivity of CoO or Co3O4-like or CoO and Co3O4-like mixed surfaces for CH3OH decomposition to produce CO and H2 decreases significantly. More importantly, RCO/H2 increases very fast when O2 exposure increases from 1.67 L to 7.33 L, demonstrating that oxidation state variation affects the decomposition channels of CH3O or CH3OH significantly. For instance, CH3O groups prefer to decompose into CH2O and H2 on the CoO surface, whereas, CH3OH or CH3O is facile to be oxidized to HCOO- on the Co3O4-like surface.

5. CONCLUSIONS In summary, CH3OH decomposition on various oxidation states of Co(0001) surfaces have been investigated by TPD method. Reaction mechanisms of CH3OH decomposition on the Co(0001) surfaces with different oxidation states have been emerged in this work. A moderate coverage of Oa atoms on the Co(0001) surface 22


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could largely enhance CH3OH decomposition to produce CO and H2 by reacting with CH3OH to produce more surface CH3O groups. However, when the Co(0001) surfaces oxide into CoO or Co3O4-lilke surfaces, the surface reactivity drops obviously. While, the reaction channels also change dramatically, and the H2 formation becomes more difficult. The results suggest that oxidation states of metal surfaces affect the H2 production via CH3OH decomposition or CH3OH reforming processes significantly.

AUTHOR INFORMATION Corresponding Authors

[email protected] , [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21673235, NSFC Center for Chemical Dynamics), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17000000), Zhejiang Provincial Natural Science Foundation of China (LQ18B030003) and the Youth Innovation Promotion Association CAS.

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Methanol Decomposition on Co(0001): Influence of the Cobalt

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