STM and DFT Study of Chlorine Adsorption on the Ag(111)-p(4x4)-O

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

STM and DFT Study of Chlorine Adsorption on the Ag(111)-p(4x4)-O Surface Boris V. Andryushechkin, Vladimir M. Shevlyuga, Tatiana V. Pavlova, Georgy Mikhailovich Zhidomirov, and Konstantin N. Eltsov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10443 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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STM and DFT Study of Chlorine Adsorption on the Ag(111)-p(4×4)-O Surface Boris V. Andryushechkin,∗,† Vladimir M. Shevlyuga,† Tatiana V. Pavlova,† Georgy M. Zhidomirov,†,‡ and Konstantin N. Eltsov†,¶ †Prokhorov General Physics Institute of the Russian Academy of Sciences, Vavilov str. 38, 119991 Moscow, Russia ‡Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Lavrentieva ave. 5, 630090 Novosibirsk, Russia ¶National Research University Higher School of Economics, Myasnitskaya str. 20, 101000 Moscow, Russia E-mail: [email protected] Phone: +7 499 5038784. Fax: +7 499 5038769

Abstract

Introduction

Coadsorption of chlorine and oxygen on the Ag(111) surface has been studied with lowtemperature scanning tunneling microscopy (LT-STM) in a combination with density functional theory (DFT) calculations. Room temperature adsorption of chlorine onto the Ag(111)-p(4×4)-O surface leads to the appearance of new bright objects located between protrusions of the 4×4 reconstruction. As chlorine adsorbs, objects form ”rosettes” around corner holes. This configuration coincides with the configuration of the chlorine atoms in the Ag(111)-(3×3)-Cl reconstruction structure. We conclude that the adsorption of chlorine on the Ag(111)-p(4×4)-O surface occurs dissociatively, with chlorine atoms displacing oxygen atoms from the fourfold positions. Adsorption of chlorine at 77 K results in the formation of the mixed Cl–O species on the Ag6 triangles of the p(4×4) reconstruction. Both scenarios of chlorine adsorption are unexpected and cannot be explained within a commonly accepted Ag6 model of the p(4×4) reconstruction.

Adsorption of chlorine and oxygen on silver surfaces has been studied since the beginning of the 1970s. 1,2 The research works in this field are related with the industrial reaction of ethylene epoxidation, 3,4 in which chlorinecontaining species play role of promoters. 4 The noticeable progress have been achieved in separate studying of O/Ag(111) and Cl/Ag(111). On the first stage of research, the structure of chlorinated silver (111) surface has been studied by low-energy electron diffraction (LEED), 2,5–10 room-temperature scanning tunneling microscopy (RT-STM) 11 and extended xray adsorption fine structure spectroscopy (EXAFS). 10,12 The room temperature chlorine adsorption on Ag(111) gives rise to the appearance of a complex LEED pattern interpreted as distorted (3×3), 2,6 (10×10), 7,10 (17×17), 11 double diffraction from epitaxial AgCl layer. 5,8 The application of low-temperature scanning tunneling microscopy in a combination with DFT calculations makes possible recognition of all atomic structures formed by chlorine on Ag(111). 13–16 In particular, it has been shown that complex patterns observed by many authors may be explained by the formation and


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have been reported in the literature. In this paper, we present STM-DFT study of the early stages of adsorption of chlorine onto Ag(111)-p(4×4) surface. We have shown that depending on the temperature of adsorption, the scenarios of adsorption are different.

evolution of the domain walls in the chemisorbed chlorine layer, 15 by the surface reconstruction (3×3) 13 and by the appearance of surface chloride clusters Ag3 Cl7 at saturation. 14,16 As for the oxygen adsorption on Ag(111), Rovida et al. 1 in 1972 reported formation of the (4×4) structure in LEED after the prolonged dosing of molecular oxygen on Ag(111) at 200 ◦ C. On the first stage of the O/Ag(111) system research, oxide-like structural models of the p(4×4) phase have been suggested by Rovida et al. 2 and Campbell et al. 17 The significant progress in studying the oxygen structures in ultra-high vacuum conditions was achieved after the invention by Bare et al. 18 the recipe of the production of the oxygen coverage on silver surfaces by adsorption of NO2 at ≈ 500 K. In addition, the oxidation of Ag(111) was performed using adsorption of atomic oxygen. 19–22 In the 2000s, scanning tunneling microscopy has been applied to the O/Ag(111) system. 23,24 As a result, an Ag1.83 O oxide-like model appeared for the p(4×4) phase. 23 In 2006, a new structural model for the p(4×4) phase containing two triangles of six silver atoms within the unit cell (Ag6 ) has been suggested. 19,25 Apart from the p(4×4) phase, a low coverage disordered oxygen phase has been reported by several authors. 20,24,26,27 Andryushechkin et al. 27 demonstrated that disordered phase (commonly looking in STM as an array of black spots) can be described as an array of local oxide-like rings of six oxygen atoms surrounding the vacancy in the upper Ag(111) layer. 27 Recently, Andryushechkin et al. 28 have shown that the maximum oxygen coverage corresponding to the disordered phase is equal to ≈0.66 ML. Moreover, the authors in Ref. 28 argue that since the p(4×4) phase is formed at larger oxygen doses than the disordered phase, its total coverage (including subsurface region) should be at least not less than 0.66 ML rather than 0.375 ML predicted by a single layer Ag6 model, i.e. the real structure of the p(4×4) phase is likely more complex than it was considered before. Coadsorption of chlorine and oxygen remains poorly studied for so far. 29–34 In particular, no experimental real-space studies of the structures formed by Cl and O on silver (111) surface

Experimental and computational details x

x 2

x

x 1

1

4x4 (a)

2

x4x4

(b)

x

Figure 1: Results of DFT calculations for chlorine adsorption on the perfect (a) and defective (oxygen deficient)(b) Ag(111)-p(4×4)-O surface. White crosses indicate oxygen vacancies. The structures corresponding to local minima are designated in both cases as ”1” and ”2”. Configuration ”1” in both cases corresponds to lower adsorption energy than configuration ”2”. Silver atoms from the upper reconstructed layer are shown in grey, while other silver atoms - in dark grey. Oxygen and chlorine are shown in red and green, respectively. All experiments were carried out in an UHV setup containing LT-STM (GPI CRYO, Sigma Scan) operating at 5-77 K, a cylindrical mirror analyzer and LEED optics. A high-pressure reactor attached to the main setup allowed introduction of molecular oxygen up to 10 Torr keeping the sample temperature in the range of 300–600 K. The silver (111) sample was prepared by repetitive circles of Ar+ bombardment (600 eV) and annealing up to 800 K. Oxygen introduction on Ag(111) was done at 433 K by the backfilling of the chamber by O2 with pressure 5×10−2 Torr. Chlorine introduction on Ag(111) was done at 77 K directly in STM and at 300


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temperatures the probability of adsorption into the fourfold hollow sites should grow up together with the probability of the formation of the oxygen vacancies.

K in the main chamber using fine leak piezovalves. All DFT calculations have been performed by using VASP package 35,36 with PBE exchangecorrelation functional. 37 A (4×4) Ag(111) surface unit cell was used with 4-layer slab with the top two layers as well as the oxygen and chlorine atoms allowed to relax and the bottom two layers fixed in the bulk positions. We used 17 ˚ A vacuum region between the slabs. STM simulations were performed within the Tersoff-Hamann approximation 38 using Hive program. 39

Chlorine adsorption at 77 K Figure 2a shows initial stage of Cl2 adsorption on the Ag(111)-p(4×4)-O surface performed at 77 K in situ in STM. According to the STM

Results and discussion DFT calculations First, we examined the adsorption of the Cl2 molecule onto the Ag(111)-p(4×4)-O surface with DFT. In our calculations, the surface was described by the DFT optimized Ag6 model. We have found that the Cl2 molecule dissociates on the triangles of six silver atoms without any barrier. The most favorable sites of chlorine atoms adsorption on the p(4×4) phase correspond to the threefold hollow positions in the centers of the Ag6 -triangles (position ”1” in Fig. 1a). The position of the chlorine atom in a fourfold site above oxygen atom does not correspond to a local minimum. Instead, the chlorine atom moves into the corner hole (position ”2” in Fig. 1a) after the optimization of coordinates. The position ”2” corresponds to the local minimum and energetically is less favorable than position ”1” by 0.11 eV. Figure 1b presents a scenario that can be realized on the defective Ag(111)-p(4×4)-O surface, when one oxygen atom is removed from the unit cell. Such configuration can be realized at finite temperature, when the diffusion of atoms becomes noticeable. In this case, the fourfold position of chlorine atom appears to be more favorable by 0.19 eV than the threefold position in the center of the Ag6 -triangle. Thus, our DFT data shows that at low temperature the adsorption of chlorine atoms in the centers of triangles is favorable, while at higher

(a)

(b)

(c)

Figure 2: STM images (250×250 ˚ A2 ; It =0.5 nA; Us = 800 mV, T = 77 K) of the Ag(111)p(4×4)-O surface acquired in the course of the step-by-step in situ adsorption of Cl2 at 77 K. Chlorine exposure: (a) 1 min; (b) 4 min; (c) 9 min. The pressure in the STM chamber during adsorption was kept at 5×10−9 Torr.


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3

3 3

3

Clads 2

2

1

3

(a)

3

3 3*

3* 1

(b)

Clads 2

2

(c)

˚2 ; It =0.4 nA; Us = 93 mV, T = 77 K) of the oxidized Ag(111) Figure 3: (a) STM image (87×98 A surface chlorinated at 77 K acquired in the vicinity of the atomic step. The small area close to the step contains chlorine atoms adsorbed on the unreconstructed Ag(111) surface. (b) The fragment of the STM image from (a) highlighted by a white dashed line. (c) The same fragment as in (b) acquired after atomic manipulation. The (4×4) lattice is shown in (b) and (c) by white lines. Four types of chlorine-containing objects are designated: ”1”, ”2”,”3”, and ”3*”. image, chlorine dosing gives rise to the appearance of the well defined pairs of bright objects located mainly in the centers of protrusions of the p(4×4) reconstruction. Further chlorine dosing leads to the occupation of all protrusions of the p(4×4) reconstruction by bright objects, as seen in Fig. 2b,c. A straightforward interpretation of observed features is the dissociation of chlorine molecules on the oxidized Ag(111) surface and adsorption of single Cl atoms in the threefold positions in the centers of the Ag6 triangles. However, additional STM measurements demonstrated that this is not the case. Figure 3a shows a high-resolution STM image of the oxidized Ag(111) surface after the interaction with molecular chlorine acquired in the vicinity of the atomic step. This surface area is of a particular interest due to the presence of a little piece of the unreconstructed surface (see Fig. 3a). Such small areas without reconstruction located near the step edges are often seen in STM images even after prolonged molecular oxygen dosing on Ag(111) at 433 K. In this area, chlorine atoms form a compressed quasi-hexagonal chemisorbed layer with an atomic arrangement similar to the case of the pure Cl/Ag(111) system. 15 Figure 3b shows the fragment of the STM image from Fig. 3a given with a higher magnification. Three types of the

objects can be found on the STM image. Object ”1” corresponds to the weak protrusion in between the triangles of the reconstruction and may be assigned to chlorine atom adsorbed in the fourfold position. Object ”2” is a chemisorbed chlorine atom on the piece of the unreconstructed silver surface near the step edge. However, chlorine adsorption at 77 K leads mainly to the appearance of objects of type ”3” ( dominant bright protrusions) occupying positions in the centers of the Ag6 triangles. Table 1: Relative heights of possible objects on the chlorinated p(4×4) surface derived from theoretical STM images (no bias dependence was found)

Cl Ag3 Cl Ag3 O Cl2 ClO ClO2 ClO3

Height 0˚ A +2.5 ˚ A ˚ +1.5 A +2.0 ˚ A +0.5 ˚ A +0.5 ˚ A +0.5 ˚ A

A simple comparison of the bright features ”3” on the p(4×4) reconstruction and chlorine


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Model

atoms in the unreconstructed area allows to conclude that they are not identical: the protrusion on the p(4×4) reconstruction (”3”) is wider and higher (by ≈0.5 ˚ A) than that on the unreconstructed surface (”2”) (see Fig. 3b). Note that this difference is not bias dependent (at least in the range of -1.5 V – +1.5 V). We performed DFT simulation of the STM image containing chlorine atoms adsorbed in the centers of the Ag6 triangles and in the fcc site on the unreconstructed Ag(111) surface and found that they should be looking as identical protrusions. In this connection, objects ”3” cannot be associated with single chlorine atoms. Further insights into the atomic structure of chlorine containing species on the oxidized silver surface come from atomic manipulations. Figure 3c shows the same surface area as (b) acquired after the scanning with the tunnel current enhanced to 6 nA. One can see that most of the features in STM-image remain on their places, but some of them were modified. In particular, some objects of type ”3” were transformed into more weak features ”3*”. According to the STM image, the width and height parameters of objects ”3*” appear to be very similar to those of objects ”2”. Therefore, objects ”3*” most likely represent individual chlorine atoms adsorbed on Ag6 triangles and looking similar to chemisorbed chlorine atoms near the step edge. Summarizing our observations, we conclude that original objects ”3” are more complex and can consist of chlorine, oxygen, and possibly silver atoms. Table 1 shows calculated relative heights of different possible species on the chlorinated p(4×4) surface. The height parameter of most of the proposed species does not correspond to the experiment. A good correspondence with the experimental height demonstrate ClO, ClO2 and ClO3 species. However, the optimization of the ClO2 model leads to the protrusions nonsymmetrical with respect to Ag6 triangles. In the case of the ClO3 model, the protrusion has a well-defined triangular shape. In this connection, the ClO quasi-molecule demonstrates a best correspondence with the experiment (see Fig. 4) and seems to be a suitable candidate for object ”3”. It is likely that ClO (object ”3”) can

(a) STM-DFT

0Å +0.5 Å

(b)

Figure 4: Model (a) and simulated STM image (U=+100 mV) (b) of the Cl atom and ClO quasi-molecule adsorbed on the Ag(111)p(4×4)-O surface. The relative heights of protrusions are indicated. Oxygen atoms in fourfold positions and silver atoms are shown in red and grey, respectively. Chlorine atoms are shown by semitransparent green circles. Oxygen atom underneath of chlorine atom is shown in dark yellow. dissociate under the high tunnel current leaving on the triangle single chlorine atom (objects ”3*”). Figure 4 shows the optimized structural model and theoretical STM image of individual Cl atom and ClO quasi-molecule adsorbed on the Ag(111)-p(4×4)-O surface. A good correspondence of the theoretical STM image with the experimental one from Fig. 3 indicates that the proposed model is highly probable. However, we cannot exclude completely the possibility of the formation of the ClO2 and ClO3 species. It is also noteworthy that warming up the Cl/Ag(111)-p(4×4)-O system to 300 K leads to a decrease in the number of Cl-O objects and an increase in the number of chlorine atoms adsorbed in fourfold positions (objects ”1” in Fig. 4). Therefore, the complete decomposition of Cl-O objects does not occur after heating up to 300 K.


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Chlorine adsorption at 300 K

of bright features in between of the p(4×4) protrusions (Fig. 5a). As the coverage increases, bright features form rosettes around corner holes of the (4×4) reconstruction, as seen in Fig. 5b,c. The geometry of rosettes and nearest neighbor distances appear to be very similar to those in the Ag(111)-(3×3)-Cl reconstruction observed on the chlorinated Ag(111) surface 13 (see insert to Fig. 5c ). In this connection, it reasonable to associate these features with adsorbed chlorine atoms. Thus, we have reason to believe that room temperature adsorption of molecular chlorine on the Ag(111)-p(4×4)-O surface occurs dissociatively, and the chlorine atoms tend to be adsorbed in fourfold positions in the ditches replacing oxygen atoms. Figure 6 shows the STM image of the chlorinated surface Ag(111)-p(4×4)-O (corresponding to the same exposure as in Fig. 5c) with the superimposed 4×4 grid. Small blue circles indicate the positions of the silver atoms forming the triangles. A detailed analysis of the STM image shows that there are three types of objects on the surface. Object ”1” is a weak spot located between four atoms of silver, an object of type ”2” is located similar to the object ”1”, but is higher by 0.2–0.3 ˚ A. Object ”3” ˚ has a height of 0.5 A larger than object ”1” and is located in the center of triangles of silver atoms. On the basis of DFT modeling it was established that the objects ”1” and ”2” can be attributed to individual chlorine atoms adsorbed in positions between four silver atoms. However, in the case of the object ”1”, there is an oxygen atom located between the layers of silver underneath of the chlorine atom, while in the case of the object ”2” it is absent. According to DFT, the height difference in STM images between objects ”1” and ”2” is equal to ≈0.3 ˚ A irrespective of the bias voltage. Thus, in both cases, the oxygen atom is displaced from the fourfold position. The height and width parameters of object ”3” in Fig. 6 correspond to those of the quasi-molecule ClO (see Fig. 4b) formed at low temperature chlorine adsorption.

Figure 5 shows STM images acquired in the course of the step-by-step adsorption of chlorine onto the Ag(111)-p(4×4)-O surface at 300 K. Exposure of chlorine leads to the appearance

Cl

Cl Cl

4x4

(a)

Cl6-ring

Cl Cl

4x4

(b) Cl/Ag(111)

Cl6-ring

4x4

(c) Figure 5: STM images (92× 82˚ A2 ; It =1 nA; Us = 780 mV, T = 77 K) of the Ag(111)-p(4×4)O surface acquired for different exposures of Cl2 at 300 K: (a) 5×10−11 Torr, 10 s; (b) 5×10−11 Torr, 20 s; (c) 1×10−10 Torr, 30 s. The inset to (c) demonstrates an STM image of the chlorinated Ag(111) surface showing the island of the (3×3) reconstruction surrounded by chemisorbed chlorine. 13


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Figure 6: The STM image of the chlorinated Ag(111)-p (4×4)-O surface with a superimposed 4×4 grid. Small blue circles indicate the positions of the silver atoms in the upper layer. Three types of chlorine-containing objects are designated: ”1”, ”2” and ”3”.

Chlorine adsorption and Ag6 model

atoms in the centers of the Ag6 triangles corresponds to the local minimum, and its theoretical STM-image is in an agreement with the experiment. The DFT analysis also shows that the ClO quasi-molecule can be easily formed in the case of the adsorption of the chlorine atom on the oxygen atom situated on the Ag6 -triangle. In the frames of the modified model, the oxygen coverage corresponding to the p(4×4) phase should be equal to 0.5 ML. This value is not in line with commonly accepted Ag6 model of the p(4×4) reconstruction with a coverage of 0.375 ML, 19,25 but is in a good agreement with a value of 0.51 ML measured by Bare et al. 18 using Nuclear Reaction Analysis (NRA) technique. Recently, Andryushechkin et al. 28 reported that the total coverage for the p(4×4) phase should not be less than 0.66 ML. Although, this value seems to be too high, however, it correlates with our new findings if assume that some of oxygen atoms can occupy positions below the surface. Thus, we demonstrated that the interaction of chlorine with the oxidized Ag(111) surface allows to reveal unusual reactivity of the p(4×4) phase that points to the necessity to improve the Ag6 model of the p(4×4) reconstruction. In particular, we concluded that each Ag6 triangle should contain at least one chemisorbed oxygen atom.

In this section, we discuss our results on chlorine adsorption on the oxidized Ag(111) surface and demonstrate that the Ag6 model of the p(4×4) phase needs to be improved. Indeed, we see that the system behavior does not follow the simple scheme that can be derived from DFT calculations from Fig.1. According to the theory, the best adsorption sites for chlorine atoms are the centers of the Ag6 triangles. However, both room and low temperature experiments point to different scenarios. At room temperature, chlorine atoms replace oxygen atoms from fourfold positions between triangles. It is likely that room temperature is enough for oxygen atoms to be pushed down in between silver layers or moved to other sites. From the viewpoint of the Ag6 model, it is already not clear why chlorine atoms do not adsorb directly into the centers of Ag6 triangles. In the case of the low temperature adsorption, the diffusion is low, and chlorine cannot replace oxygen from the fourfold positions between triangles. According to our results presented in the previous sections, chlorine adsorption at 77 K leads to the formation of the ClO species. Therefore, oxygen atoms should be already located on the Ag6 triangles. DFT calculations show that the configuration constructed from the usual Ag6 model as a result of the adding of two oxygen


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Conclusions

misorption on the Silver (111) Surface. Jpn. J. Appl. Phys. 1974, 13, 117–120.

Thus, we have found that molecular chlorine dissociates on the Ag(111)-p(4×4)-O surface at both 300 K and 77 K preserving the unchanged period of the reconstruction. At 300 K, chlorine atoms tend to replace oxygen atoms from the ditches between the Ag6 triangles. At 77 K, chlorine atoms adsorb mainly in the centers of the Ag6 triangles forming likely the ClO structures. A comparative analysis of the scenarios of chlorine adsorption at 300 K and 77 K indicate that the coverage of the p(4×4) phase should be not less than 0.5 ML, rather than 0.375 ML predicted by the ordinary Ag6 model. 19,25 The indication of the formation of the species containing Cl–O bonds is of the particular interest for the recognition of the role of chlorine in alkene epoxidation reactions. Indeed, it is known that sodium chlorite can be used for the production of styrene epoxide without any catalyst. 40–42 Therefore, the formation of the Cl-O species in our case can open an additional channel of the epoxidation. It is also noteworthy that this derivation is in line with recent publication by Jones et al., 43 in which authors postulated that the epoxidation can occur on the specific SO4 complexes adsorbed on the silver surface.

(3) Serafin, J.; Liu, A.; Seyedmonir, S. Surface Science and the Silver-Catalyzed Epoxidation of Ethylene: An Industrial Perspective. J. Mol. Catal. A: Chem. 1998, 131, 157–168. (4) Chen, C.-J.; Harris, J. W.; Bhan, A. Kinetics of Ethylene Epoxidation on a Promoted Ag/α-Al2O3 Catalyst - The Effects of Product and Chloride Co-Feeds on Rates and Selectivity. Chem. Eur. J 2018, 24, 12405–12415. (5) Bowker, M.; Waugh, K. C. The Adsorption of Chlorine and Chloridation of Ag(111). Surf. Sci. 1983, 134, 639–664. (6) Rovida, G.; Pratesi, F. Chlorine Monolayers on the Low-Index Faces of Silver. Surf. Sci. 1975, 51, 270–282. (7) Goddard, P. J.; Lambert, R. M. Adsorption-desorption Properties and Surface Structural Chemistry of Chlorine on Cu(111) and Ag(111). Surf. Sci. 1977, 67, 180–194. (8) Tu, Y. Y.; Blakely, J. M. Chlorine Adsorption on Silver Surfaces. J. Vac. Sci. Technol. 1978, 15, 563–567.

Acknowledgments

(9) Wu, K.; Wang, D.; Deng, J.; Wei, X.; Cao, Y.; Zei, M.; Zhai, R.; Guo, X. Chlorine on Ag(111): The Intermediate Coverage Case. Surf. Sci. 1992, 264, 249– 259.

This work was supported by Grant of the Russian Science Foundation #16-12-10546. We are grateful to the Joint Supercomputer Center of RAS for the possibility of using their computational resources for our calculations.

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STM and DFT Study of Chlorine Adsorption on the Ag(111)-p(4x4)-O

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