Low-temperature CO oxidation on Co(0 0 0 1)

Accepted Manuscript Research paper Low-Temperature CO Oxidation on Co(0001) Jiawei Wu, Jun Chen, Qing Guo, Dongxu Dai, Xueming Yang PII: DOI: Reference:

S0009-2614(17)30205-1 http://dx.doi.org/10.1016/j.cplett.2017.02.085 CPLETT 34598

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

31 December 2016 24 February 2017 27 February 2017

Please cite this article as: J. Wu, J. Chen, Q. Guo, D. Dai, X. Yang, Low-Temperature CO Oxidation on Co(0001), Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.02.085

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Low-Temperature CO Oxidation on Co(0001) Jiawei, Wu1), 2), a), Jun Chen1), 3), a), Qing Guo1), *), Dongxu Dai1), Xueming Yang 1), *) 1

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

2

3

a)

University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing 100049, P. R. China School of Physics and Optoelectric Engineering, Dalian University of Technology, Dalian, Liaoning 116023, P. R. China

Authors who made similar contributions to this work.

*)

To whom all correspondence should be addressed. Correspondence Emails: [email protected] and [email protected]

1

ABSTRACT Low-temperature oxidation of CO, perhaps the most extensively studied reaction in the history of heterogeneous catalysis, is becoming increasingly important in the context of cleaning air and lowering automotive emissions. Here, we have studied low-temperature CO oxidation on Co(0001) using temperature programed desorption method. We show that chemisorbed O adlayer on Co(0001) does not promote CO2 formation. However, when a Co3O4-like surface is formed at 90 K, large amount of CO2 is produced at 90 K in the presence of weakly bound oxygen species, demonstrating that low-temperature CO oxidation can be rationalized on O2-saturated Co3O4-like oxide surfaces. However, the formation of carbonate species via the CO2 product and weakly bound oxygen species reduces the yield of low temperature CO2. Thus, how to block the formation of carbonate plays a key role in enhancing the low-temperature CO2 production on the Co 3O4-like oxide surfaces.

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1. Introduction Low-temperature catalytic conversion of carbon monoxide (CO) to carbon dioxide (CO2) via oxidation with transition-metal nanoparticles supported on transition metal oxides has attracted enormous attentions in the last three decades because of its potential applications in many important processes, such as the removal of CO from hydrogen (H2) gas for fuel cell applications and emission control in automobiles with catalytic converters [1-18]. While, due to its relative simplicity, the reaction has been widely considered as a prototype for addressing general scientific questions in heterogeneous catalysis. Due to important applications of CO oxidation, the reaction on single transition-metal surfaces has become one of the most extensively studied reactions. Up to date, CO oxidation has been carried out experimentally on noble metals, such as Pt [19-23], Au [24-27], Pd [28-31], and other bimetallic surface alloys with noble metals [14,32,33]. With these studies, it is believed that CO oxidation on transition metals proceeds via the traditional Langmuir-Hinshelwood mechanism, where coadsorbed CO and O react to form CO2 under ultrahigh vacuum (UHV) conditions. However, experimental investigations for low-temperature CO oxidation on cheap metal surfaces (e.g., Ni [14,34] and Co [35,36]) have rarely been performed. Recently, using a combination of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and scanning tunneling microscopy (STM), Knudsen and coworkers [14] carried out a systematic investigation of low-temperature CO oxidation on Ni(111) and Au/Ni(111) surfaces. The authors 3

showed that nickel oxide was formed on both pure Ni(111) and Au/Ni(111) surfaces when the surfaces were exposed to large amounts of O2 at 100 K, and then CO can be oxidized at 100 K on both surfaces under these conditions. Similarly, when the pure Co(0001) surface was exposed to large amounts of O2 at or above 300 K, a Co3O4-like surface could be formed by a nucleation and growth mechanism [35,37]. TPD result showed that CO2 desorption occurred with a peak temperature at 470 K followed after CO exposure to the cobalt oxide (Co3O4-like) surface [35]. In addition, Kizilkaya and coworkers [36] found that cobalt oxide formation was negligible when the surfaces were predosed O2 at 250 K, leading to no observation of CO2 product. Up to date, no further experimental studies have explored low-temperature CO oxidation on the Co(0001) surface. Here we have studied low-temperature CO oxidation on the Co(0001) surface using TPD method. We find that the chemisorbed CO and O on Co(0001) do not react to form CO2 significantly. Very interestingly, when the clean Co(0001) surface oxidizes to the Co3O4-like oxide surface with 6 L O2 (1 L = 10

-6

Torr s) at 90 K, low-temperature CO oxidation occurs efficiently on the Co3O4-like oxide surface at 90 K, leading to the formation of CO2.

2. Experimental Methods The TPD measurements were carried out in an ultrahigh vacuum (UHV) chamber with base pressure better than 7 × 10-11 Torr，equipped with low energy electron diffraction (LEED) optics, a quadruple mass spectrometer (SRS RGA100), and an ion gun for sample cleaning. The Co(0001) single crystal (MaTecK, 10 × 10 × 1 mm3) 4

was cleaned by cycles of Ar+ sputtering (1 keV) at 570 K for 15 min and UHV annealing at 630 K for 30 min. All of the gases (CO and O2) were dosed to the sample at 90 K through a home-built calibrated molecular beam doser. Between every TPD measurement, the surface was cleaned by Ar+ sputtering and UHV annealing to heal and clean the Co (0001) surface. TPD signals were collected with a ramping rate of 2 K/s.

3. Results and discussion Similar to the work done by Kizilkaya and coworkers [36], low energy electron diffraction (LEED) measurements have been carried out to establish the ordering of the chemisorbed oxygen atom adlayer (Oad) (Fig. S1 in Supplementary Materials). A (2 × 2) LEED pattern develops with increasing O2 exposure at 250 K, and reaches a maximum intensity around 1.5 L. As the exposure of O2 keeps increasing, the (2 × 2) diffraction spots become fuzzy and no clear order is observed for the O2 exposure ≥ 2 L. While, CO2 desorption spectrum is obtained after a saturation dose of CO onto the Oad covered surface at 90 K. No obvious CO2 product is detected (Fig. S2), indicating that the chemisorbed Oad atoms on Co(0001) do not promote CO oxidation to form CO2 significantly at low temperature, which is consistent with previous works [36]. Previously, Xie and coworkers [13] found that CO can be oxidized to CO2 efficiently at ~200 K on Co3O4 nanorods. Whereas, based on the work done by Knudsen and coworkers [14], CO oxidation on the Ni(111) surface occurs even at 100 K when the surface is oxidized with large amount of O2 at 100 K, but the efficiency of 5

CO oxidation is low. Therefore, it is interesting to investigate whether CO oxidation on the Co(0001) surface occurs at about 100 K. As shown in Fig. 1, a series of CO desorption spectra acquired at a mass-to-charge ratio (m/z) of 28 (CO+) as a function of CO exposure on both the clean and oxidized Co(0001) surfaces. The oxidized Co(0001) surfaces were prepared by exposure of 6 L O2 to the clean Co(0001) surfaces at 90 K. On the clean Co(0001) surfaces (Fig. 1a), different desorption features gradually appear in the TPD spectra as the exposure of CO increases. At 2 L CO exposure, three desorption peaks around 350, 270 and 210 K are observed. As reported by desorption of

, the 350 K peak can be assigned to on the bridge/hollow sites. With increasing exposure, the repulsion

between CO molecules becomes stronger, CO molecules begin to adsorb at less stable sites (on-top or almost on-top sites), and then desorb at ~270 and ~210 K, respectively. When

the surface to 2 L CO. However, with the same exposure of CO on the oxidized Co(0001) surfaces, the intensities of CO TPD peaks decrease significantly compared with that on the clean Co(0001) surfaces (Fig. 1b), and the peaks shift to lower temperature by about 200 K, suggesting that CO molecules adsorb weakly on the oxidized Co(0001) surface. A

the

saturation coverage of CO on the clean Co(0001) surface is about 0.67 ML (1 ML = 1.8 × 1015 molecules / cm2). Thus, the absolute coverage of CO on clean and oxidized Co(0001) surfaces can be roughly estimated (Fig. 2). On the clean Co(0001) surfaces, 6

the coverage of CO increases linearly with the exposure of CO.

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Fig. 1 (a) Series of CO (m/z = 28) TPD spectra from the clean Co (0001) surfaces with different CO exposures; (b) Series of CO (m/z = 28) TPD spectra from the oxidized Co (0001) surfaces with different CO exposures.

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clean Co(0001) surface Pre-dose 6 L Oxygen

CO Coverage / ML

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Fig. 2 The amount of CO that can be accommodated on the clean and oxidized Co (0001) surfaces as a function of CO exposure, derived from the data in Fig. 1a and 1b.

Whereas, on the oxidized surfaces, the coverage of CO is much smaller than that on 7

the clean surface with the same exposure of CO. For such dramatic changes of CO spectra on the oxidized Co(0001) surface, there are two possible reasons. First, on the basis of previous studies [39-42], the clean Co (0001) surface oxidizes immediately to form a Co3O4-like surface with weakly bound oxygen species at the temperature lower than 120 K with O2 exposure. In our work, after exposure of 6 L O2 on the clean Co(0001) surface at 90 K, the oxygen rich phase Co 3O4-like may be formed on the Co(0001) substrate. Therefore, the dramatic changes of CO spectra on the oxidized surfaces can be attributed to the formation of Co3O4-like oxide. The other possible explanation is that the interaction between surface bound CO and O2 on the Co3O4-like surface is rather strong, which may exist another channel for the “missing” CO on the surface−CO may react with O2 to form CO2, similar to low-temperature CO oxidation on the Ni(111) surface [14].

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Fig. 3 (a) Series of CO2 (m/z = 44) TPD spectra from the oxidized Co (0001) surfaces with different amounts CO; (b) CO2 (m/z = 44) TPD spectra collected on three different types of surfaces after 8

exposure of 2 L CO2: i. the clean surface; ii. the surface with 6 L O2 exposure at 90 K; iii. the surface with 6 L O2 exposure at 90 K followed by flashing to 200 K.

To find out whether CO2 product could be formed, TPD spectra at m/z = 44 (CO2+) were measured after exposing the oxidized Co(0001) surfaces to various exposures of CO (Fig. 3a). Without CO adsorption, a small peak at about 440 K is still observed, this may be due to the background adsorption of CO 2 on the oxidized surface. As the coverage of CO increases, two desorption peaks at 120 and 440 K are clearly detected in the TPD spectra of CO2. While, the intensities of these two desorption peaks increase and reach to the maximum at 2 L CO exposure. The remarkable CO2 product strongly demonstrates that low-temperature CO oxidation occurs on the oxidized Co(0001) surface. The two desorption peaks of CO2 product indicate different desorption channels of CO2 product. To confirm how the CO2 product desorbs from the Co3O4-like surface, a separate experiment of CO2 adsorption on the clean and oxidized Co(0001) surfaces has been carried out. Two types of oxidized Co(0001) surfaces were prepared. The first type was formed by oxidizing the clean surface with 6 L O2 at 90 K. The second one was prepared by flashing the first one to 200 K to remove weakly adsorbed O2 molecules. On the clean Co(0001) surface, only a small peak is observed at ~125 K in the TPD spectrum of m/z = 44 after exposure of 2 L CO2 (Fig. 3b, line i). However, with the same CO2 exposure, the TPD signals of CO2 on the oxidized surfaces (Fig. 3b, line ii and iii) are rather bigger than that on the clean surface, indicating that the adsorption of CO2 on the clean Co(0001) surface is very weak. For the first type of oxidized Co(0001) surface, two desorption peaks at 120 and 440 K are detected in the 9

TPD spectrum of CO2 (Fig. 3b, line ii). Whereas, for the second type, only the 120 K desorption peak is observed (Fig. 3b, line iii). The result indicates that the 120 K peak is due to the direct desorption of weakly bound CO2 on the oxidized surfaces, and the high temperature CO2 desorption may be related to molecular O2 on the Co3O4-like surface. Compared with the result obtained on the oxidized Ni(111) surface [14], the high temperature desorption peak of CO2 is most likely due to carbonate decomposition, which is formed by further reaction of CO2 and surface weakly adsorbed oxygen species. As shown in Fig. 3a, at low CO exposure (≤ 0.5 L), the weakly adsorbed oxygen species is mildly consumed to oxidize CO to CO2 product, and further reaction of CO2 and surface weakly adsorbed oxygen could occur easily. As a result, the 440 K peak is the main desorption feature, and increases very fast. And the 120 K desorption peak is rarely observed. With increasing CO exposure (≥ 0.5 L), the depletion of weakly adsorbed oxygen species for CO2 production is becoming more and more, thus, the

Yield of CO2 / ML

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Fig. 4 Yield of CO2 product as a function of CO dosage, derived from the data in Fig. 3a.

increase of 440 K peak becomes slower and slower. Conversely, the 120 K peak increases rapidly. In order to evaluate the relative importance of the CO oxidation reaction on the oxidized Co(0001) surface, the yield of CO2 product has been estimated (Fig. 4) after calibrating the detecting efficiency of our mass spectrometer at m/z = 28 and 44. It is obvious that most of the CO2 product desorbs via carbonate decomposition at high temperature. At low CO exposure, the high temperature CO2 product grows very fast, the low temperature CO2 product is rarely formed. As the CO exposure is larger than 0.5 L, the rate of the low temperature CO2 product grows faster than that of the high temperature one. However, the formation of carbonate species on the oxidized surface is initiated from low-temperature CO oxidation. Thus, the total efficiency of the CO oxidation on the oxidized Co(0001) surfaces (or O2/Co3O4-like surface) can be estimated, which is defined as YCO2 = [CO2]/ ([CO2] + [CO]) ([CO2], [CO] are the coverages of CO2 and CO on the oxidized surface). The efficiency of the CO oxidation is nearly a constant (about 46%) as the CO exposure increases from 0.25 L to 2 L (Fig. 5), implying that the CO oxidation reaction on the oxidized Co(0001) surface occurs very efficiently at 90 K. Based on previous studies of CO oxidation [43,44,45,46], the oxidation of CO on a Co3O4 substrate and other transition metal oxides is generally believed to follow the Mars–van Krevelen (MvK) mechanism: CO molecules react with the neighboring lattice oxygen ions of the Co 3O4 substrate rather than the chemisorbed O atom, and 11

the consumed lattice O atom is subsequently refilled by the O2 from the gas phase. Especially, Jiang and coworkers [47] proposed that in order to complete the catalytic cycle of CO oxidation by O2 on Co3O4, the process of lattice O ions abstracted by CO

The probability of CO2 production / %

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Fig. 5 The probability of CO2 production as a function of CO exposure.

is the rate limiting step for CO oxidation, and that the O 2 molecule can easily dissociate without a barrier between two neighboring oxygen vacancies further heal the vacancy. Further, by virtue of extensive density functional theory (DFT) calculations, Wang and coworkers [46] found that surface Co3+ is the active site for low-temperature CO oxidization rather than Co2+. And the three-coordinated surface oxygen bonded with three Co 3+ may be slightly more reactive than the other two kinds of lattice oxygen, that is, the two-coordinated O bonded with one Co2+ and one Co3+ and the three-coordinated O bonded with one Co2+ and two Co3+. Thus, the weakly adsorbed O2 molecules will not react with CO on Co3O4 directly. 12

As shown in Fig. S3, when the clean surface is oxidized by 6 L O2 at 90 K to form the Co3O4-like surface, small amount of weakly bound O2 desorbs from the surface at 130 K during the TPD process. However, when the oxidized surface is flashed to 200 K to remove weakly bound O2 prior to CO adsorption, only a small amount of CO2 product is observed at 400 K. The result demonstrates that the weakly bound oxygen species is crucial for CO oxidation on Co 3O4. We could conclude that low-temperature CO oxidation is initiated by the oxidation the clean Co(0001) surface to the more reactive Co 3O4-like surface with O2 exposure at 90 K. Even without weakly bound O2 on the Co3O4-like surface, CO2 product can be still formed, demonstrating that part of CO molecules may abstract the lattice O atoms to form CO2 via Mvk mechanism [43,44,45,47,47]. However, the low yield of CO2 product on the Co3O4-like surface without weakly bound O2 (Fig. 6, line ii) also suggests that the direct oxidation of CO with weakly bound O2 may be possible.

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Temperature / K Fig. 6 CO2 (m/z = 44) TPD spectra collected on three different types of surfaces after exposure of 2 13

L CO: i. the surface with 6 L O2 exposure at 90 K; ii. the surface with 6 L O2 exposure at 90 K followed by flashing to 200 K.

Whereas, the formation of carbonate species will consume the surface weakly bound oxygen species (including O2 molecules), such a process will hinder O2 molecules to heal the oxygen vacancies and deplete the amount of surface weakly bound oxygen species, leading to reduce the efficiency of CO oxidation. Moreover, the formation of carbonate species largely decreases the yield of low-temperature CO2 production. This is consistent with the result obtained on the Ni(111) and Au/Ni alloy surfaces [14]. However, upon addition of Au on the Ni(111) surface, Au inhibits carbonate formation, and improves the formation of low temperature CO2. Thus, how to block the formation of carbonate plays a key role in improving the efficiency of the low-temperature CO2 production on the Co3O4-like oxide surface. It is well known that CO oxidation over Co 3O4 shows a size and morphology dependent effect [13, 46]. On the Co(0001) surface, Zimmermann and coworkers [41] found that oxide nucleation occurs via the early place-exchange of metal and oxygen atoms and the growth of an open, pseudoamorphous oxide structure with evident Co3O4-like features rather than of CoO in the low-temperature regime, which are in excellent agreement with the available low-temperature experimental data. Meanwhile, using the spin resolving photoelectron spectroscopy (SRPS) method, Getzlaff and coworkers [40] found that a thin Co3O4 film can be prepared on the Co(0001) surface at 80 K with a dosage of 20 L O2. Thus, small Co3O4-like clusters will be formed on Co(0001) at low O2 exposure in the low-temperature regime. With increasing O2 exposure, the clusters will grow bigger and bigger, and become a thin 14

Co3O4-like film eventually. To investigate the size and morphology dependent effect of Co3O4, a separate experiment on CO oxidation by controlling the O2 exposure at 90 K has been carried out to investigate the size and morphology dependent effect. As shown in Fig. 7a, the yield of CO2 is a monotonically increasing function of O2 dose. When O2 exposure is bigger than 6 L, the yield of CO2 does not increase any more. The result suggests that no significant size and morphology dependent effect of Co3O4 is observed in low-temperature CO oxidation. In addition, the O2 exposure is estimated by the pressure of O2 in the chamber in the work. However, a capillary array is used for O2 adsorption in our work, and the Co(0001) surface is facing the capillary array with a distance of 5 mm during the adsorption process, leading to that the actual O2 exposure is much higher than the estimation. Therefore, the Co(0001) surface is most likely to be covered by thin Co3O4-like film with 6-10 L O2 exposure at 90 K. Previous studies [39,40,41] showed that the surface O atom can diffuse into the bulk, cause Co3O4 to transform into CoO phase as the surface temperature is higher than 300 K. During the TPD process, the Co3O4-like film will transform into CoO film, which maybe affects the structural stability of surfaces. Thus, cycles of low temperature CO oxidation experiments have been carried out. During each cycle, no surface treatment war performed. As shown in Fig. 7b, for the first cycle, the clean Co(0001) surface was pre-oxidized by 10 L O2 at 90 K, and then TPD measurement of CO2 product was carried out after adsorbing 1 L CO on the oxidized surface. For the next two cycles, the surfaces was oxidized by 10 L O2 at 90 K directly without surface 15

treatment. During the third cycle, the yield of low-temperature CO2 product is nearly the same with that in the first cycle, and the yield of high temperature CO2 product gradually decreases. However, the yield of high temperature CO2 product in the third cycle is nearly the same with that in the second cycle, demonstrating that the Co3O4-like film on Co(0001) is very stable for low-temperature CO oxidation.

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Fig. 7 (a) Series of CO2 (m/z = 44) TPD spectra as a function of predosed O2 on Co(0001) with 1 L CO at 90 K. (b) Cycles of CO2 (m/z = 44) TPD spectra from the oxidized Co (0001) surfaces with 1 L CO at 90 K. During each cycle, no surface treatment was performed, and the oxidized Co (0001) surfaces were prepared by exposing the surfaces to 10 L O 2 at 90 K.

4. Conclusions In summary, using TPD method, the low-temperature CO oxidation process on oxidized Co (0001) surfaces has been investigated. We have shown that no CO2 is formed when the Co(0001) surface is covered with a chemisorbed (2 × 2) O adlayer. However, when the clean surface is oxidized to the Co3O4-like oxide surface with loosely bound oxygen species, large amount of CO2 is produced at 90 K, thus, we 16

suggest that cobalt oxides play a central role in catalyzing CO oxidation at cryogenic temperatures.

Acknowledgements This work was supported by the Chinese Academy of Sciences, National Science Foundation of China (21673235, 21403224), the Chinese Ministry of Science and Technology (2013CB834605), the Youth Innovation Promotion Association CAS, and the Key Research Program of the Chinese Academy of Sciences.

Notes The authors declare no competing financial interest.

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Highlights Low-temperature oxidation of CO, perhaps the most extensively studied reaction in the history of heterogeneous catalysis, is becoming increasingly important in the context of cleaning air and lowering automotive emissions. However, the elementary processes of low temperature CO oxidation on Co-based photocatalysts still remain unclear. In this submitted manuscript, we have carried out systematic experiments on low-temperature CO oxidation on the Co(0001) surface using TPD method. Our results demonstrate that low temperature oxidation of CO occurs efficiently on the Co3O4-like oxide surface at 90 K, leading to the formation of CO2. Whereas, the formation of carbonate species via the CO2 product and weakly bound oxygen species will reduce the yield of low temperature CO2. Thus, how to block the formation of carbonate plays a key role in enhancing the low-temperature CO2 production on the Co3O4-like oxide surfaces. We believe this is a very important result that will generate tremendous interests in the scientific community, and will very likely push the field of fundamental study of low temperature CO oxidation a significant step forward.