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Reactive phase of oxygen on Cu(100) at 100 K studied by HREELS and TPD
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Applied Surface Science 121/122 (1997) 562-566
Reactive phase of oxygen on Cu(100) at 100 K studied by HREELS and TPD Tsuyoshi Sueyoshi, Takehiko Sasaki, Yasuhiro Iwasawa * Department of Chemistry, Graduate School of Science, The Unit'ersi O"(2f Tol~vo, Hongo, Bunk) o-ku, Tok3o 113, Japan Received 29 October 1996; accepted 15 February 1997
Abstract The reactivity of oxygen adatoms formed by dissociative adsorption of oxygen at 100 K on Cu(100) for CO oxidation was investigated by means of HREELS and TPD in comparison with the reactivity of adsorbed oxygen in thermally stable phases. On Cu(100) covered with oxygen adatoms formed at 100 K, designated as oxygen-as-exposed surface, post-dosed CO was oxidized at 125 K in TPD. On annealing the as-exposed surface to 300 K, a change in the loss feature of u(Cu-O) was observed, indicating the formation of mixed phases comprised of pseudo c(2 × 2)-0 with O(a) in fourfold hollow sites and ( ~ - × 2~-)R45°-O with - C u - O - chains along the [001] direction. On these two phases, CO 2 formation in TPD was reduced to one-ninth as compared to that on the as-exposed surface. These results may provide a new insight into the adsorbed phase of surface oxygen active for oxidation reaction and a way to develop a promising catalytic material with Cu metal. © 1997 Elsevier Science B.V. PACS: 82.65.- i; 82.65.My Kevwords." Electron energy loss spectroscopy; Surface chemical reaction; Low index single crystal surfaces; Copper; Oxygen: Carbon monoxide
1. Introduction Reactivity of adsorbed oxygen on metal surfaces has received considerable interest in relation to heterogeneous catalysis. In particular, the reactivity of adsorbed oxygen for CO oxidation on Pt, Pd and Rh has been extensively studied because of the importance in the fundamental and practical aspects. However, the nature of oxygen species on the metallic Cu surface has not been characterized well since C u -
Corresponding author. Fax: +81-3-38142627; e-mail: [email protected].
oxide phases are formed easily under oxygen exposure. In recent studies, CO oxidation was found to proceed on metallic Cu films and unreconstructed Cu(l 10) with lower activation energies than those on Pt and Pd [1-3], suggesting that it is valuable to examine the reactivity of adsorbed oxygen on Cu metal surfaces. Oxygen adsorption on Cu(100) has been investigated using various methods [4]. Most of studies concentrated on adsorption above room temperature and revealed that only the structure with long-range order is the reconstructed (~/~ X 2 ~ - ) R 4 5 ° - O structure with the saturation coverage of 0.5 ML. In this
0169-4332/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved. PH S0 169-4332(97)00367-X
563
T. Sueyoshi et al./ Applied Surface Science 121 / 122 (1997) 562-566
reconstructed phase, paired - C u - O - chains grow along the [001] direction and every fourth row of Cu atoms along the [001] direction is removed [5]. In addition, existence of a different phase without long-range order, designated as pseudo c(2 x 2 ) - 0 phase, was verified below 0.34 M L [6-8]. A recent S E X A F S study concluded that atomic oxygen adsorbs in lburfold hollow sites in the pseudo c(2 N 2)O phase [7]. The (~/2 X 2f2-)R45°-O phase and the pseudo c(2 × 2 ) - 0 phase were characterized by different frequencies of ~,(Cu-O) at 290 and 340 c m - 1, respectively, by means of HREELS [6,9]. However, the oxygen adlayer formed at low temperatures has not been investigated with HREELS. Furthermore, the reactivity of oxygen species formed at 100 K has not been clarified well. The purpose of the present study is to characterize oxygen adlayers formed by exposure at 100 K, designated as as-exposed surfaces, by HREELS and to investigate difference in the reactivity of atomic oxygen among the as-exposed adlayer, the pseudo c(2 X 2 ) - 0 phase and the (v~- × 2 ~ - ) R 4 5 ° - O phase. Oxygen atoms in the asexposed surface were found to be more reactive for CO oxidation by a factor of 9 than those in the pseudo c(2 X 2 ) - 0 and ( ~ - × 2 ~ - ) R 4 5 ° - O phases.
3. Results and discussion Fig. 1 shows HREEL spectra for oxygen adlayers on Cu(100). The results for oxygen adlayers prepared above 300 K are shown in spectra (i)-(ii). At 0.11 ML (i), a peak at 340 cm -1 is assigned to ~,(Cu-O) of O(a) in the pseudo c(2 × 2 ) - 0 phase where oxygen atoms adsorbed at fourfold hollow sites [6-8]. At 0.40 M L (ii), u ( C u - O ) appeared at 290 cm l, which is characteristic of the ( ~ - X 2 ~ - ) R 4 5 ° - O phase [6]. These results are in agreement with the previous studies [6,9]. In a HREEL spectrum for an oxygen-as-exposed
c'-
e 2. Experimental
C 09
E All experiments were performed in an UHV chamber equipped with rear-view L E E D / A E S optics, a quadrupole mass spectrometer and a home built HREELS spectrometer as described elsewhere [10]. The mass spectrometer was enclosed in a glass cap with a hole of 6 m m in diameter. The contribution except from a sample surface could be suppressed when the sample was placed in front of the hole at a distance of 2 cm [2,10]. The amount of desorbed CO 2 was determined by estimating the relative mass intensity ratio of CO 2 to CO in consideration with the relative sensitivity of CO 2 to CO for a B - A gauge in a similar way to our previous studies [2,10]. The saturation coverage of c(2 × 2)CO phase was assumed to be 0.5 M L [11,12]. The oxygen coverage on Cu(100) was estimated by means of AES based on the saturation coverage of the (~/2- X 2 ~ - ) R 4 5 ° - O structure (0.5 ML) [5-7].
0
400
800
W a v e n u m b e r / cm -~
Fig. 1. HREEL spectra measured at 100 K for Cu(100) with oxygen. Spectra (i)-(ii) were measured after Cu(100) was exposed to 50 L of oxygen at 300 K (i) and to 300 L of oxygen at 500 K (ii). Spectrum (iii) was measured after exposing to 50 L of oxygen at 100 K. Spectrum (iv) was measured alter the surface (iii) was annealed to 300 K. Spectrum (v) was measured after exposing to 100 L of oxygen at 100 K. Spectra (vi)-(vii) were measured after the surface (v) was annealed to 200 K (iv) and 300 K (v). Oxygen coverage: 0.11 ML (i), 0.40 ML (ii), 0.13 ML (iii)-(iv), 0.24 ML (v)-(vii).
564
T. Sueyoshi et al. / Applied Surface Science 121 / 122 (1997) 562-566
surface prepared by exposing to 50 L of oxygen at 100 K (0 o = 0.13 ML), a peak was observed at 340 cm 1 with a shoulder at 380 cm i (iii). The 340 cm-1 peak is ascribed to O(a) at fourfold hollow sites as in (i). The 380 cm 1 peak is assigned to ~,(Cu-O) of O(a) at lower coordination sites such as bridge sites and atop sites in consideration with the frequency. A shoulder at 440 cm-J is assigned to ~,(Cu-OH) of OH(a) produced by dissociative adsorption of water originating from the residual gas. The intensity of this peak was enhanced by subsequent exposure to water. After annealing to 300 K (iv), the 340 cm-J peak remained and the shoulder at 380 cm -~ disappeared, indicating that oxygen atoms were accommodated in stable fourfold hollow sites to produce the pseudo c(2 × 2) phase. For the as-exposed surface with 0.24 ML of oxygen (v), the p(Cu-O) mode was observed at 370 cm -I, indicating the increase of oxygen atoms on lower coordination sites. A shoulder at 440 cm-~ is assigned to J,(Cu-OH) as mentioned above. After annealing the as-exposed surface to 200 K (vi), the peak of v ( C u O) moved downward to 340 cm 1. Further annealing to 300 K caused downward shift of v(Cu-O) to 320 cm 1 (vii). Since the 320 cm i peak is attributed to superposition of the u(Cu-O) modes of O(a) in the pseudo c(2 X 2)-0 and ( ~ - X 2~/2-)R45°-O phases according to Wuttig et al. [6], the downward shift of v(Cu-O) in the spectra (v)-(vii) on annealing to 300 K is attributed to the growth of the pseudo c(2 X 2)-0 and ( ~ - X 2V~-)R45°-O phases. The change in the reactivity of oxygen atoms on annealing the as-exposed surface to 300 K was investigated by means of TPD. The results are shown in Fig. 2a,b,c,d. Fig. 2a shows TPD spectra measured after the as-exposed surface (0 o = 0.24 ML) was exposed to CO at 100 K. Fig. 2b,c show the results for the as-exposed surfaces pre-annealed before exposure to CO at 100 K. Fig. 2d shows a TPD spectrum for a pure CO adlayer for comparison. In the case of the oxygen-as-exposed surface without pre-annealing before CO adsorption (Fig. 2a), the CO 2 desorption peak was observed at 125 K and the desorbed CO 2 amounted to 0.051 ML (Fig. 2a). By pre-annealing to 200 K (Fig. 2b) and 300 K (Fig. 2c) before CO adsorption, the amount of desorbed CO 2 decreased to 0.011 ML (Fig. 2b) and 0.006 ML (Fig.
2c).
i
I
i
.~
co
.
~ .~
i
co
002 x 2.5
...............................
CO 2
x
2.5
..............................
(a~.-....
co
..~
C02 x 2.5
I
100
,'
150
200
1-
250
300
Temperature / K Fig. 2. TPD spectra for m / e = 4 4 (CO 2) and m / e = 2 8 (CO) from O-CO coadlayers on Cu(100). Spectra in (a) were measured alter Cu(100) was exposed to 100 L of oxygen at 100 K and subsequently exposed to 6.0 L of CO at 100 K. The precovered oxygen coverage was 0.24 ML. Spectra in (b) and (c) were measured after Cu(100) was exposed to 100 L of oxygen at 100 K, annealed to 200 K (b) and 300 K (c) and cooled to 100 K, followed by exposure to 6.0 L of CO. (d) shows a TPD spectrum measured after exposing Cu(100) to 1.8 L of CO at 100 K. The CO coverage was 0.5 ML.
As for CO desorption, a desorption peak was observed at 180 K from pure CO adlayer as shown in Fig. 2d. In the spectrum for the as-exposed surface without pre-annealing (Fig. 2a), two peaks were observed at 135 and 160 K. The 160 K peak has a tail on the high temperature side. From the surfaces pre-annealed before CO adsorption (Fig. 2b,c), the 135 K peak also appears. In addition, a peak was observed at 170 K. With regard to the amount of adsorbed CO, the effect of pre-annealing was relatively small in contrast to the large decrease of CO 2 production. The CO coverages were 0.41 ML for the surface without pre-annealing (Fig. 2a), 0.35 ML after pre-annealing to 200 K (Fig. 2b) and 0.37 ML after pre-annealing to 300 K (Fig. 2c). Thus, these results in Fig. 2a,b,c indicate that the high reactivity of oxygen atoms in the as-exposed surfaces for CO oxidation decreases on annealing to 200-300 K. The decrease in the reactivity corresponds to the growth of the pseudo c(2 X 2)-0 and ( v ~ - × 2v~)R45°-O phases as indicated in the vibrational spectra (v)-(vii) in Fig. I, which suggests that the
72 Sueyoshi et al. /Applied Surface Science 121 / 122 (1997) 562-566
reactive species for CO oxidation on Cu(100) are metastable oxygen atoms in the as-exposed surfaces before getting accommodated in thermally stable states. The high reactivity is ascribed to the fact that metastable oxygen atoms in the disordered as-exposed surfaces require a smaller activation energy for CO oxidation than oxygen atoms in the pseudo c(2 X 2)-0 and ( ~ - X 2~-)R45 ° phases. It should be noted that CO oxidation by oxygen atoms formed above 300 K was reported at surface temperatures 473-623 K under exposure to (2.6 X 10 4)-(1.3 X 10 ~) Pa of CO [13,14]. The high reactivity of oxygen adatoms prepared at low temperatures was also reported on P t ( l l l ) and Cu(110). On Pt(111), the reactivity of oxygen atoms in a disordered phase, which was prepared by exposing to oxygen below 100 K and annealing to 150 K, is higher than that in p(2 X 2)-0 phase produced by further annealing to 350 K for oxidation of coadsorbed CO [15]. The oxygen atoms in the disordered phase are also reactive for oxygen-exchange reaction between NO(a) and O(a), while those in the p(2 X 2) phase are not reactive [ 16]. Recently we reported that oxygen atoms on unreconstructed Cu(ll0) formed below 230 K show high activity for oxidation of coadsorbed CO and for catalytic CO oxidation, and the reactivity remarkably decreases by annealing above 250 K where the surface is reconstructed to the (2 X 1)-O structure [2,3,10,17]. The thermal behavior of O - C O coadlayers on Cu(100) was investigated by HREELS. Fig. 3 shows HREEL spectra measured after the as-exposed surface with 0.13 ML of oxygen was exposed to 3.0 L of CO at 100 K, followed by annealing to given temperatures. Spectrum (i), shown for comparison, was measured after exposing to 1.8 L of CO at 100 K. Two peaks were observed at 350 and 2080 c m in Fig. 3(i), which are assigned to u(Cu-CO) and u(C-O) [11,12]. In spectrum (ii) for the O - C O coadlayer prepared at 100 K, two peaks were observed at 320 and 2080 cm '. These two peaks are assigned to u(Cu-CO) and u(C-O), respectively. The downward shift of the z,(Cu-CO) peak from 350 to 320 cm-~ is due to influence of coadsorbed O(a). The u(Cu-O) mode seems to be obscured by the u(Cu-CO) mode in the spectrum (ii). On annealing the coadlayer, the 320 cm-~ peak moved to 340 cm -j at 125 K (Fig. 3(iii)) and to 350 cm -~ at 150
565 I
I
x 200 35O x loo 20,0o
)
~
~
(iv) 1 5 0 K . ~ . ~
C
°
-
'~
/
(ii) 100 K
I 0
(i) pure CO adlay 1000 Wavenumber / cm 1
2000
Fig. 3. HREEL spectra measured at 100 K for O-CO coadlayers and a pure CO layer on Cu(100). Spectrum (i) was measured after Cu(100) was exposed to 1.8 L of CO at 100 K. Spectrum (ii) was measured after exposing Cu(100) to 50 L of oxygen and 3.0 L of CO at 100 K. Spectra (iii)-(iv) were measured after annealing the surface (ii) to 125 K (iii) and 150 K (iv).
K (iv). The decrease of the component at 320 c m - l on annealing up to 150 K suggests that CO(a) influenced by coadsorbed oxygen desorbed or migrated to clean patches. In the TPD result for the O - C O coadlayer prepared at 100 K (Fig. 2a), there are two CO desorption peaks at 135 and 170 K. It is, therefore, suggested that the coadsorbed oxygen works as an electronegative modifier and reduces the electron density near the Fermi level of Cu(100) surface, leading to weakening of the CO adsorption. In spite of the downward shift of the v(Cu-CO) peak, the v ( C - O ) peak remained unchanged at 2080 cm-~ in the spectra (i)-(iv), suggesting a weak interaction between the 2"rr orbital of CO(a) and the Cu substrate as indicated in Ref. [18]. There was no longlived intermediate detected with HREELS in CO oxidation on Cu(100). In summary, the reactivity of oxygen atoms in the as-exposed surfaces formed at 100 K for CO oxidation was found to be much higher than that of oxygen atoms in the pseudo c(2 X 2)-0 phase and the ( ~ - X 2v/2-)R45°-O phase. The growth of the pseudo c(2 X 2)-0 and (V~- × 2~/2-)R45°-O phases by annealing the as-exposed surface was indicated from the feature of v(Cu-O) in HREEL spectra.
566
7~ Sueyoshi et al. /Applied Surface Science 121 / 122 (1997) 562 566
References [1] G.G. Jernigan, J. Somorjai, J. Catal. 147 (1994) 567. [2] T. Sueyoshi, T. Sasaki, Y. Iwasawa, Chem. Phys. Lett. 241 (1995) 189. [3] T. Sueyoshi, T. Sasaki, Y. Iwasawa, J. Phys. Chem. 100 (1996) 1048. [4] D.P. Woodruff, in: D.A. King, D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces, vol. 7, Elsevier, Amsterdam, 1994, p. 465. [5] F. Jensen, F. Besenbacher, E. L~egsgaard, I. Stensgaard, Phys. Rev. B 42 (1990) 9206. [6] M. Wattig, G. Franchy, H. Ibach, Surf. Sci. 213 (1989) 103. [7] T. Lederer, D. Arvanitis, G. Comelli, L. Tr~Sger, K. Baberschke, Phys. Rev. B 48 (1993) 15390.
[8] D. Arvanitis, G. Comelli, T. Lederer, H. Rbus, K. Baberschke, Chem. Phys. Lett, 211 (1993) 53. [9] M,H. Mohamed, L.L. Kesmodel, Surf. Sci. 185 (1987) L467. [10] T. Sueyoshi, T. Sasaki, Y. Iwasawa, Surf. Sci. 343 (1995) 1. [11] S. Andersson, Surf. Sci. 89 (1979) 477. [12] R. Ryberg, Surf. Sci. 114 (1982) 627. [13] F.H.P.M. Habraken, C.M.A.M. Mesters, G.A. Bootsma, Surf. Sci. 97 (1980) 264. [14] G. Ertl, Surf. Sci. 7 (1967) 309. [15] J. Yoshinobu, M. Kawai, J. Chem. Phys. 103 (1995) 3220. [16] K. Sawabe, Y. Matsumoto, J. Yoshinobu, M. Kawai, J. Chem. Phys. 103 (1995) 4757. [17] T. Sasaki, T. Sueyoshi, Y. Iwasawa, Surf. Sci. 316 (1994) L1081. [18] B. Gumhalter, K. Wandelt, Ph. Avouris, Phys. Rev. B 37 (1988) 8048.
Applied Surface Science 121/122 (1997) 562-566
Reactive phase of oxygen on Cu(100) at 100 K studied by HREELS and TPD Tsuyoshi Sueyoshi, Takehiko Sasaki, Yasuhiro Iwasawa * Department of Chemistry, Graduate School of Science, The Unit'ersi O"(2f Tol~vo, Hongo, Bunk) o-ku, Tok3o 113, Japan Received 29 October 1996; accepted 15 February 1997
Abstract The reactivity of oxygen adatoms formed by dissociative adsorption of oxygen at 100 K on Cu(100) for CO oxidation was investigated by means of HREELS and TPD in comparison with the reactivity of adsorbed oxygen in thermally stable phases. On Cu(100) covered with oxygen adatoms formed at 100 K, designated as oxygen-as-exposed surface, post-dosed CO was oxidized at 125 K in TPD. On annealing the as-exposed surface to 300 K, a change in the loss feature of u(Cu-O) was observed, indicating the formation of mixed phases comprised of pseudo c(2 × 2)-0 with O(a) in fourfold hollow sites and ( ~ - × 2~-)R45°-O with - C u - O - chains along the [001] direction. On these two phases, CO 2 formation in TPD was reduced to one-ninth as compared to that on the as-exposed surface. These results may provide a new insight into the adsorbed phase of surface oxygen active for oxidation reaction and a way to develop a promising catalytic material with Cu metal. © 1997 Elsevier Science B.V. PACS: 82.65.- i; 82.65.My Kevwords." Electron energy loss spectroscopy; Surface chemical reaction; Low index single crystal surfaces; Copper; Oxygen: Carbon monoxide
1. Introduction Reactivity of adsorbed oxygen on metal surfaces has received considerable interest in relation to heterogeneous catalysis. In particular, the reactivity of adsorbed oxygen for CO oxidation on Pt, Pd and Rh has been extensively studied because of the importance in the fundamental and practical aspects. However, the nature of oxygen species on the metallic Cu surface has not been characterized well since C u -
Corresponding author. Fax: +81-3-38142627; e-mail: [email protected].
oxide phases are formed easily under oxygen exposure. In recent studies, CO oxidation was found to proceed on metallic Cu films and unreconstructed Cu(l 10) with lower activation energies than those on Pt and Pd [1-3], suggesting that it is valuable to examine the reactivity of adsorbed oxygen on Cu metal surfaces. Oxygen adsorption on Cu(100) has been investigated using various methods [4]. Most of studies concentrated on adsorption above room temperature and revealed that only the structure with long-range order is the reconstructed (~/~ X 2 ~ - ) R 4 5 ° - O structure with the saturation coverage of 0.5 ML. In this
0169-4332/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved. PH S0 169-4332(97)00367-X
563
T. Sueyoshi et al./ Applied Surface Science 121 / 122 (1997) 562-566
reconstructed phase, paired - C u - O - chains grow along the [001] direction and every fourth row of Cu atoms along the [001] direction is removed [5]. In addition, existence of a different phase without long-range order, designated as pseudo c(2 x 2 ) - 0 phase, was verified below 0.34 M L [6-8]. A recent S E X A F S study concluded that atomic oxygen adsorbs in lburfold hollow sites in the pseudo c(2 N 2)O phase [7]. The (~/2 X 2f2-)R45°-O phase and the pseudo c(2 × 2 ) - 0 phase were characterized by different frequencies of ~,(Cu-O) at 290 and 340 c m - 1, respectively, by means of HREELS [6,9]. However, the oxygen adlayer formed at low temperatures has not been investigated with HREELS. Furthermore, the reactivity of oxygen species formed at 100 K has not been clarified well. The purpose of the present study is to characterize oxygen adlayers formed by exposure at 100 K, designated as as-exposed surfaces, by HREELS and to investigate difference in the reactivity of atomic oxygen among the as-exposed adlayer, the pseudo c(2 X 2 ) - 0 phase and the (v~- × 2 ~ - ) R 4 5 ° - O phase. Oxygen atoms in the asexposed surface were found to be more reactive for CO oxidation by a factor of 9 than those in the pseudo c(2 X 2 ) - 0 and ( ~ - × 2 ~ - ) R 4 5 ° - O phases.
3. Results and discussion Fig. 1 shows HREEL spectra for oxygen adlayers on Cu(100). The results for oxygen adlayers prepared above 300 K are shown in spectra (i)-(ii). At 0.11 ML (i), a peak at 340 cm -1 is assigned to ~,(Cu-O) of O(a) in the pseudo c(2 × 2 ) - 0 phase where oxygen atoms adsorbed at fourfold hollow sites [6-8]. At 0.40 M L (ii), u ( C u - O ) appeared at 290 cm l, which is characteristic of the ( ~ - X 2 ~ - ) R 4 5 ° - O phase [6]. These results are in agreement with the previous studies [6,9]. In a HREEL spectrum for an oxygen-as-exposed
c'-
e 2. Experimental
C 09
E All experiments were performed in an UHV chamber equipped with rear-view L E E D / A E S optics, a quadrupole mass spectrometer and a home built HREELS spectrometer as described elsewhere [10]. The mass spectrometer was enclosed in a glass cap with a hole of 6 m m in diameter. The contribution except from a sample surface could be suppressed when the sample was placed in front of the hole at a distance of 2 cm [2,10]. The amount of desorbed CO 2 was determined by estimating the relative mass intensity ratio of CO 2 to CO in consideration with the relative sensitivity of CO 2 to CO for a B - A gauge in a similar way to our previous studies [2,10]. The saturation coverage of c(2 × 2)CO phase was assumed to be 0.5 M L [11,12]. The oxygen coverage on Cu(100) was estimated by means of AES based on the saturation coverage of the (~/2- X 2 ~ - ) R 4 5 ° - O structure (0.5 ML) [5-7].
0
400
800
W a v e n u m b e r / cm -~
Fig. 1. HREEL spectra measured at 100 K for Cu(100) with oxygen. Spectra (i)-(ii) were measured after Cu(100) was exposed to 50 L of oxygen at 300 K (i) and to 300 L of oxygen at 500 K (ii). Spectrum (iii) was measured after exposing to 50 L of oxygen at 100 K. Spectrum (iv) was measured alter the surface (iii) was annealed to 300 K. Spectrum (v) was measured after exposing to 100 L of oxygen at 100 K. Spectra (vi)-(vii) were measured after the surface (v) was annealed to 200 K (iv) and 300 K (v). Oxygen coverage: 0.11 ML (i), 0.40 ML (ii), 0.13 ML (iii)-(iv), 0.24 ML (v)-(vii).
564
T. Sueyoshi et al. / Applied Surface Science 121 / 122 (1997) 562-566
surface prepared by exposing to 50 L of oxygen at 100 K (0 o = 0.13 ML), a peak was observed at 340 cm 1 with a shoulder at 380 cm i (iii). The 340 cm-1 peak is ascribed to O(a) at fourfold hollow sites as in (i). The 380 cm 1 peak is assigned to ~,(Cu-O) of O(a) at lower coordination sites such as bridge sites and atop sites in consideration with the frequency. A shoulder at 440 cm-J is assigned to ~,(Cu-OH) of OH(a) produced by dissociative adsorption of water originating from the residual gas. The intensity of this peak was enhanced by subsequent exposure to water. After annealing to 300 K (iv), the 340 cm-J peak remained and the shoulder at 380 cm -~ disappeared, indicating that oxygen atoms were accommodated in stable fourfold hollow sites to produce the pseudo c(2 × 2) phase. For the as-exposed surface with 0.24 ML of oxygen (v), the p(Cu-O) mode was observed at 370 cm -I, indicating the increase of oxygen atoms on lower coordination sites. A shoulder at 440 cm-~ is assigned to J,(Cu-OH) as mentioned above. After annealing the as-exposed surface to 200 K (vi), the peak of v ( C u O) moved downward to 340 cm 1. Further annealing to 300 K caused downward shift of v(Cu-O) to 320 cm 1 (vii). Since the 320 cm i peak is attributed to superposition of the u(Cu-O) modes of O(a) in the pseudo c(2 X 2)-0 and ( ~ - X 2~/2-)R45°-O phases according to Wuttig et al. [6], the downward shift of v(Cu-O) in the spectra (v)-(vii) on annealing to 300 K is attributed to the growth of the pseudo c(2 X 2)-0 and ( ~ - X 2V~-)R45°-O phases. The change in the reactivity of oxygen atoms on annealing the as-exposed surface to 300 K was investigated by means of TPD. The results are shown in Fig. 2a,b,c,d. Fig. 2a shows TPD spectra measured after the as-exposed surface (0 o = 0.24 ML) was exposed to CO at 100 K. Fig. 2b,c show the results for the as-exposed surfaces pre-annealed before exposure to CO at 100 K. Fig. 2d shows a TPD spectrum for a pure CO adlayer for comparison. In the case of the oxygen-as-exposed surface without pre-annealing before CO adsorption (Fig. 2a), the CO 2 desorption peak was observed at 125 K and the desorbed CO 2 amounted to 0.051 ML (Fig. 2a). By pre-annealing to 200 K (Fig. 2b) and 300 K (Fig. 2c) before CO adsorption, the amount of desorbed CO 2 decreased to 0.011 ML (Fig. 2b) and 0.006 ML (Fig.
2c).
i
I
i
.~
co
.
~ .~
i
co
002 x 2.5
...............................
CO 2
x
2.5
..............................
(a~.-....
co
..~
C02 x 2.5
I
100
,'
150
200
1-
250
300
Temperature / K Fig. 2. TPD spectra for m / e = 4 4 (CO 2) and m / e = 2 8 (CO) from O-CO coadlayers on Cu(100). Spectra in (a) were measured alter Cu(100) was exposed to 100 L of oxygen at 100 K and subsequently exposed to 6.0 L of CO at 100 K. The precovered oxygen coverage was 0.24 ML. Spectra in (b) and (c) were measured after Cu(100) was exposed to 100 L of oxygen at 100 K, annealed to 200 K (b) and 300 K (c) and cooled to 100 K, followed by exposure to 6.0 L of CO. (d) shows a TPD spectrum measured after exposing Cu(100) to 1.8 L of CO at 100 K. The CO coverage was 0.5 ML.
As for CO desorption, a desorption peak was observed at 180 K from pure CO adlayer as shown in Fig. 2d. In the spectrum for the as-exposed surface without pre-annealing (Fig. 2a), two peaks were observed at 135 and 160 K. The 160 K peak has a tail on the high temperature side. From the surfaces pre-annealed before CO adsorption (Fig. 2b,c), the 135 K peak also appears. In addition, a peak was observed at 170 K. With regard to the amount of adsorbed CO, the effect of pre-annealing was relatively small in contrast to the large decrease of CO 2 production. The CO coverages were 0.41 ML for the surface without pre-annealing (Fig. 2a), 0.35 ML after pre-annealing to 200 K (Fig. 2b) and 0.37 ML after pre-annealing to 300 K (Fig. 2c). Thus, these results in Fig. 2a,b,c indicate that the high reactivity of oxygen atoms in the as-exposed surfaces for CO oxidation decreases on annealing to 200-300 K. The decrease in the reactivity corresponds to the growth of the pseudo c(2 X 2)-0 and ( v ~ - × 2v~)R45°-O phases as indicated in the vibrational spectra (v)-(vii) in Fig. I, which suggests that the
72 Sueyoshi et al. /Applied Surface Science 121 / 122 (1997) 562-566
reactive species for CO oxidation on Cu(100) are metastable oxygen atoms in the as-exposed surfaces before getting accommodated in thermally stable states. The high reactivity is ascribed to the fact that metastable oxygen atoms in the disordered as-exposed surfaces require a smaller activation energy for CO oxidation than oxygen atoms in the pseudo c(2 X 2)-0 and ( ~ - X 2~-)R45 ° phases. It should be noted that CO oxidation by oxygen atoms formed above 300 K was reported at surface temperatures 473-623 K under exposure to (2.6 X 10 4)-(1.3 X 10 ~) Pa of CO [13,14]. The high reactivity of oxygen adatoms prepared at low temperatures was also reported on P t ( l l l ) and Cu(110). On Pt(111), the reactivity of oxygen atoms in a disordered phase, which was prepared by exposing to oxygen below 100 K and annealing to 150 K, is higher than that in p(2 X 2)-0 phase produced by further annealing to 350 K for oxidation of coadsorbed CO [15]. The oxygen atoms in the disordered phase are also reactive for oxygen-exchange reaction between NO(a) and O(a), while those in the p(2 X 2) phase are not reactive [ 16]. Recently we reported that oxygen atoms on unreconstructed Cu(ll0) formed below 230 K show high activity for oxidation of coadsorbed CO and for catalytic CO oxidation, and the reactivity remarkably decreases by annealing above 250 K where the surface is reconstructed to the (2 X 1)-O structure [2,3,10,17]. The thermal behavior of O - C O coadlayers on Cu(100) was investigated by HREELS. Fig. 3 shows HREEL spectra measured after the as-exposed surface with 0.13 ML of oxygen was exposed to 3.0 L of CO at 100 K, followed by annealing to given temperatures. Spectrum (i), shown for comparison, was measured after exposing to 1.8 L of CO at 100 K. Two peaks were observed at 350 and 2080 c m in Fig. 3(i), which are assigned to u(Cu-CO) and u(C-O) [11,12]. In spectrum (ii) for the O - C O coadlayer prepared at 100 K, two peaks were observed at 320 and 2080 cm '. These two peaks are assigned to u(Cu-CO) and u(C-O), respectively. The downward shift of the z,(Cu-CO) peak from 350 to 320 cm-~ is due to influence of coadsorbed O(a). The u(Cu-O) mode seems to be obscured by the u(Cu-CO) mode in the spectrum (ii). On annealing the coadlayer, the 320 cm-~ peak moved to 340 cm -j at 125 K (Fig. 3(iii)) and to 350 cm -~ at 150
565 I
I
x 200 35O x loo 20,0o
)
~
~
(iv) 1 5 0 K . ~ . ~
C
°
-
'~
/
(ii) 100 K
I 0
(i) pure CO adlay 1000 Wavenumber / cm 1
2000
Fig. 3. HREEL spectra measured at 100 K for O-CO coadlayers and a pure CO layer on Cu(100). Spectrum (i) was measured after Cu(100) was exposed to 1.8 L of CO at 100 K. Spectrum (ii) was measured after exposing Cu(100) to 50 L of oxygen and 3.0 L of CO at 100 K. Spectra (iii)-(iv) were measured after annealing the surface (ii) to 125 K (iii) and 150 K (iv).
K (iv). The decrease of the component at 320 c m - l on annealing up to 150 K suggests that CO(a) influenced by coadsorbed oxygen desorbed or migrated to clean patches. In the TPD result for the O - C O coadlayer prepared at 100 K (Fig. 2a), there are two CO desorption peaks at 135 and 170 K. It is, therefore, suggested that the coadsorbed oxygen works as an electronegative modifier and reduces the electron density near the Fermi level of Cu(100) surface, leading to weakening of the CO adsorption. In spite of the downward shift of the v(Cu-CO) peak, the v ( C - O ) peak remained unchanged at 2080 cm-~ in the spectra (i)-(iv), suggesting a weak interaction between the 2"rr orbital of CO(a) and the Cu substrate as indicated in Ref. [18]. There was no longlived intermediate detected with HREELS in CO oxidation on Cu(100). In summary, the reactivity of oxygen atoms in the as-exposed surfaces formed at 100 K for CO oxidation was found to be much higher than that of oxygen atoms in the pseudo c(2 X 2)-0 phase and the ( ~ - X 2v/2-)R45°-O phase. The growth of the pseudo c(2 X 2)-0 and (V~- × 2~/2-)R45°-O phases by annealing the as-exposed surface was indicated from the feature of v(Cu-O) in HREEL spectra.
566
7~ Sueyoshi et al. /Applied Surface Science 121 / 122 (1997) 562 566
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