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Oxidation of CO by molecular oxygen on a Ag(110 ) surface studied by scanning tunneling microscopy
Surface Science 513 (2002) 359–366 www.elsevier.com/locate/susc
Oxidation of CO by molecular oxygen on a Ag(1 1 0) surface studied by scanning tunneling microscopy J.V. Barth a
a,*
, T. Zambelli
a,1
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Received 12 January 2002; accepted for publication 19 April 2002
Abstract The oxidation of CO by O2 molecularly adsorbed on a Ag(1 1 0) surface was investigated by scanning tunneling microscopy in the temperature range between 60 and 110 K. At the lowest temperatures, CO remains weakly bound at Ag(1 1 0). Striped CO superstructures running along the [0 0 1] direction can be observed at intermediate coverages, indicating weak anisotropic interactions between the molecules. Upon exposing the oxygen precovered surface to CO at T K 90 K, O2 molecules either react off or (CO O2 )ad complexes evolve, signaling CO accumulation on the surface and attractive intermolecular interactions. At T J 90 K, adsorbed O2 -molecules are highly mobile and CO molecules do not accumulate on the surface. Oxidation readily takes place upon CO exposure, whereby single oxygen atoms were identified as an intermediate product, which could be further titrated with CO molecules. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Surface chemical reaction; Silver; Carbon monoxide; Oxygen; Low index single crystal surfaces
1. Introduction Temperature controlled scanning tunneling microscopy (STM) has proven to be a versatile tool to elucidate surface chemical reactions [1]. In previous work, we have used this technique to directly characterize adsorption, surface mobility
* Corresponding author. Present address: Institut de Physique des Nanostructures, Ecole Polytechnique Federale de Lausanne, PHB-Eclubens, CH-1015 Lausanne, Switzerland. Fax: +41-21-6933604. E-mail address: johannes.barth@epfl.ch (J.V. Barth). 1 Present address: Centre d’Elaboration de Materiaux et d’Etudes Structurales, 29, rue Jeanne Marvig, BP 4347, F-31055 Toulouse Cedex 4, France.
and lateral interactions of molecular oxygen at Ag(1 1 0) [2,3]. In the following, STM observations on the oxidation of CO by this species will be presented. The system O2 /Ag(1 1 0) is of particular interest with respect to CO oxidation. Whereas on other catalysts such as platinum, where oxygen may similarly be stabilized in a molecular state at low temperatures, CO2 production occurs only following dissociation of O2 , electron energy-loss spectroscopy (EELS) experiments [4] provided evidence that on Ag(1 1 0) CO can be directly oxidized by molecular oxygen leading to the formation of carbon dioxide. In order to obtain more information on the reaction mechanism, recently thermal desorption spectroscopy (TDS) and molecular beam
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(MB) experiments have been performed in the temperature range between 100 and 130 K [5,6], where exclusively molecular oxygen exists on the surface [7]. The production of gaseous CO2 could be demonstrated at all temperatures. It was concluded that molecular oxygen is directly involved in the primary step of CO oxidation and that the reaction is of the Langmuir–Hinshelwood type whereby CO initially accumulates on the surface [6]. Two explanations were proposed in order to rationalize the characteristics of the reaction [6]: (i) An intermediate adsorbed CO O2 complex is formed, i.e., a step ðCOÞad þ ðO2 Þad ¢ ðCO O2 Þad takes place prior to the final oxidation (CO O2 Þad ! ðCO2 Þg þ Oad . Introducing this complex formation in model simulations improved the quality of the fit of the titration curves. (ii) The adsorption energy of CO is enhanced by the presence of oxygen on the entire surface without local effects. Both hypotheses lead to a stabilization of CO molecules, and this effect is actually corroborated by earlier TDS/EELS experiments of Ref. [4], according to which oxygen slightly increases the desorption temperature of CO on Ag(1 1 0). If hypothesis (i) is correct, a change of the STM imaging of the O2 molecules is expected to occur upon dosing CO, indicative of (CO O2 )ad complex formation. If, however, hypothesis (ii) describes what actually happens, no such local arrangements are expected.
2. Experimental The experiments were performed in an ultrahigh-vacuum (UHV) chamber with a base pressure of 5 1011 Torr, equipped with standard facilities for sample preparation and characterization. The home-built STM is of the beetle type and is cooled by liquid He [8]. Higher temperatures are achieved by counter heating the sample with a tungsten filament. The temperature is measured with a NiCr–Ni thermocouple, attached directly to the sample. The temperature scale was checked against the nominal desorption temperature of CO on Ag(1 1 0) at 77 K [9]. The STM data were ob-
tained in the constant-current mode with tunnel currents typically between 0.3 and 1 nA and voltages of 500–1000 mV. The Ag(1 1 0) single crystal sample was prepared by cycles of Arþ sputtering (1 lA/cm2 , 700 eV, 10 min at 300 K) and annealing at 750 K until no contaminations could be detected by Auger electron spectroscopy, and STM resolved large, defect-free atomic terraces. Gas doses are given in Langmuir (1 L ¼ 1 106 Torr s). Coverages are given in monolayers (1 ML corresponds to one adsorbed molecule per substrate atom).
3. Results and discussion 3.1. Separate adsorption of CO and O2 CO is only weakly bound to the Ag(1 1 0) surface ( 0.2 eV binding energy [6,10,11]). Ultraviolet photoemission spectroscopy results led to the suggestion of a physisorption state [12]. However, based on later experiments with direct core and valence photoemission, X-ray absorption and autoionization of core excited states, chemisorption of CO on the surface was concluded [13]. EELS observations following the specular peak intensity revealed that it initially remains low, but increases abruptly with appreciable coverages. It was inferred that the CO molecules initially lie parallel to the surface and then reorientate such that the molecular axis is oriented normal to the surface [9]. With our setup even at the lowest possible temperature ( 55 K), single CO molecules are expected to be too mobile to be directly observed with STM [14]. However, it turned out to be possible to follow the ordering of CO at appreciable coverages. This is demonstrated by the STM data reproduced in Fig. 1, where the ordering of CO was followed in situ, i.e., the data were obtained while exposing the surface to CO. Initially, only white dashes and spikes appear in single scanlines, which are associated with isolated CO molecules which cannot be imaged due to their high mobility. For doses exceeding 5 L (Fig. 1a), one-dimensional ordering of CO molecules occurs. Stripes oriented along the [0 0 1] direction with lengths
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Fig. 1. (a) STM image of Ag(1 1 0) after exposure to 10 L CO at 55 K. CO molecules order in stripes running over hundreds of ngstr€ A oms along [0 0 1]. (b) STM contour line along one of the periodicity on the stripes; CO stripes in (a) revealing a 4 A every protrusion is associated with a CO molecule. A fraction of . (c) Same area the stripes forms ribbons with a distance of 9 A as in (a): after exposing to 600 L patches of a regular (3 1) superstructure can be discerned in a mobile adlayer (460 220 2 , Ut ¼ 0:4 V, It ¼ 0:3 nA). A
ngstr€ up to several hundred A oms evolve. With increasing coverage, these stripes cluster into ribbons , correwhereby the distance in [1 1 0] is 9 A sponding to 3a (a ¼ 2:885 A is the lattice constant along the [1 1 0]-direction). Segments of some stripes can be displaced by 1a, signaling that also the stripes are mobile at this temperature, though much less than single molecules. Accordingly chains
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may occur in very different positions in successive STM frames. In Fig. 1b a representative STM contour line along the ridge of a stripe is reproduced. It reveals a regular corrugation of with a periodicity of 4 A , corresponding
0.4 A to the value of the lattice constant along [0 0 1] . Each protrusion is associated with b ¼ 4:08 A a CO molecule, i.e., the stripes are understood as densely packed CO rows with the molecules in upright positions. Since the substrate atomic structure could not be resolved simultaneously, the adsorption site for the CO molecules remains to be determined. The STM image in Fig. 1c was recorded after dosing of 600 L CO. The CO stripes are now arranged in regular domains, where they are separated by 3a. Between these domains, CO stripes separated by 3a are imaged in a ragged way, indicating appreciable mobility in the CO layer. CO molecules at 55 K thus form domains with a (3 1) structure. The existence of any CO superstructure on Ag(1 1 0) was not reported in the literature so far. However, similar arrangements have been observed with low-energy electron diffraction on Cu(1 1 0), where a (2 1) pattern upon CO adsorption was reported [15]. In agreement, 1dim ordering of CO on Cu(1 1 0) along [0 0 1] was recently found with STM experiments at temperatures below 50 K [16]. Since CO molecules on Ag(1 1 0) are mobile at the temperature employed for the experiments, the formation of the stripes indicates attractive interactions along the [0 0 1] direction. As the chains are mobile as well, their preferred separation of 3a in the domains reflects a long-range attractive interaction between the chains along [1 1 0]. A chemisorbed molecular oxygen species can be stabilized on Ag(1 1 0) in the 40–150 K temperature range [7,17–21]. In contrast to CO, adsorbed O2 molecules are imaged as depressions. Due to the higher binding energy and diffusion barrier isolated O2 can be imaged for temperatures up to 80 K [2]. A corresponding STM image is reproduced in Fig. 2 (recorded after cooling down a layer adsorbed at 100 K). It demonstrates the formation of molecular strings oriented along the close-packed atomic [1 1 0] rows of the substrate, which are similarly resolved in the image. The
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low O–O vibrational stretching mode observed by EELS [4], presumably accounts for the proceeding of the catalytic oxidation of CO in the low-temperature range. 3.2. Coadsorption and reaction of CO and O2
Fig. 2. Ag(1 1 0) with 0.03 ML O2 at 60 K (following exposure to 50 L at 100 K). Oxygen molecules are resolved as depressions and frequently arranged as strings running along the close-packed [1 1 0] direction. The intermolecular equilib2 ). rium distance in this direction is 2a (image size 210 200 A
distance of neighboring molecules in the strings is always 2a. Note that recent experiments demonstrate two distinct molecular species coexisting upon oxygen adsorption at lower temperatures (<80 K) [22,23]. In the course of the present investigations it was similarly noted that upon adsorption of oxygen molecules at T 60 K species evolve at the surface with distinct imaging characteristics. These observations support earlier ab initio calculations, where two adsorbed states with the molecular axis either oriented along [1 1 0] or [0 0 1] were found to be stable [24]. The latter configuration is energetically slightly preferred (by 0.04 eV with the Ag surface relaxed). Near-edge X-ray adsorption fine structure investigations indicate that the O2 molecules with the axis parallel to [1 1 0] exhibit only a r resonance , i.e., substanand have a bond length of 1.47 A tially stretched with respect to that of the free ) [19]. It was thus concluded that molecule (1.21 A the molecules are bound as a single-bonded peroxo species (O2 2 ), in accordance with theoretical investigations, suggesting a peroxolike chemisorbed state [24–28]. The corresponding weakened O–O bond, which is similarly reflected in an unusually
The reaction experiments were carried out in the following way: following Ref. [6], oxygen was dosed onto the clean surface at 100 K in order to prevent premature adsorption of CO, which is unavoidably present in the chamber during the O2 exposure. Then the sample was brought to the desired temperature in the range between 60 and 110 K. Subsequently CO was dosed and the evolution of the reaction was followed either during the STM measurement or with the tunneling tip retracted. We found that the results can be divided into two categories according to the temperature employed: T K 90 K and T J 90 K, respectively. In the following exemplaric observations are presented. The STM data presented in Fig. 3 were obtained during an in situ experiment at 85 K. Prior to cool down the surface was exposed to 140 L O2 (corresponding to a precoverage of 0.05 ML) and subsequently CO was dosed while continuously recording data at a partial pressure of 2 108 Torr. The surface in Fig. 3a was scanned from the bottom to the top; the start of the CO admission coincides with the acquisition of the first scanlines. In the lower image area the O2 molecules are visible with their typical imaging and ordering characteristics. A, B and C are reference points to facilitate the comparison between Fig. 3a and b. After 1 L CO some O2 molecules are surrounded by white clouds, which become more and more numerous with increasing CO dose until all molecules are decorated at 1.5 L (the actual exposure at the scanned area is reduced due to the shielding of the STM tip). The STM image shown in Fig. 3b demonstrates that the decoration persists at higher CO exposures. The resolution of the halo depends on the tip state and does not allow to clarify whether reaction has taken place. In Fig. 3a, the gradual development of the halo around the O2 molecules as function of the CO dose assures that the clouds do not reflect a tip change. Hence, we interpret them as CO decorating molecular O2 .
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Fig. 3. Ag(1 1 0) precovered with 0.05 ML O2 (following exposure to 140 L at 100 K): Images recorded during exposure of CO (pCO ¼ 2 108 Torr) at 85 K (a) after doses exceeding
1 L, the oxygen molecules and islands are surrounded by a white halo, indicating CO decoration. (b) Situation at higher exposures. A, B and C mark identical positions in both images 2 , Ut ¼ 0:5 V, It ¼ 0:3 to facilitate comparison (230 230 A nA).
Although the temperature of the experiment was slightly above the nominal desorption temperature of CO, the CO molecules impinging onto the surface remain trapped for a certain time prior to desorption. The fact that CO molecules rest in the vicinity of the O2 molecules signals an attractive interaction between these two species. The STM data reproduced in Fig. 4 have been selected from a further in situ series (CO pressure: 1 108 Torr, 0.04 ML O2 precoverage) performed at 70 K. In Fig. 4a (2 L dose), the number and arrangement of the O2 molecules re-
Fig. 4. Images recorded during exposure of Ag(1 1 0) precovered with 0.04 ML O2 to CO (pCO ¼ 1 108 Torr) at 70 K: (a) situation after 200 s with oxygen molecules resolved; (b) same area after 800 s: some molecules have reacted off (cf. the upper left of the image in (a)), while others are decorated by two protrusions such that the resulting complex is oriented in [1 1 0]; (c) for exposures exceeding 10 L (i.e., after t J 1000 s), most molecules have reacted off. Furthermore, a sudden change in the imaging characteristics appears, and the complexes in (b) are imaged differently as a species decorated by two protrusions oriented in [0 0 1]. No further changes take place at higher CO 2 , Ut ¼ 0:4 V, It ¼ 0:1 nA). doses (150 150 A
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mains as determined initially. In Fig. 4b (8 L dose), first changes become observable. Firstly, some O2 molecules have disappeared (cf. the upper left in Fig. 4a), which is associated with oxidation of CO and concomitant desorption of CO2 . The fact that no atomic oxygen could be observed as reaction intermediate is due to CO accumulating on the surface, which can be readily oxidized by an oxygen atom being far more reactive than the molecular species [29]. Secondly, some molecules are now decorated with two protrusions in [1 1 0] separated by 2a from the respective O2 forming a complex. The decoration remains unaffected during increasing the CO exposure, until for a dose of 10 L an abrupt transformation occurs at the surface, as revealed in Fig. 4c. Obviously a substantial fraction of the O2 molecules are now reacted off. However, the complexes are imaged differently as species decorated with the two bright spots in [0 0 1]. The overall unstable imaging characteristics reflect the presence of a layer of adsorbed CO. In contrast to the findings upon CO adsorption on pristine Ag(1 1 0), no formation of superstructures could be observed in any coadsorption/reaction experiment, which might result from the slightly higher temperatures employed for the coadsorption/reaction experiments. The configuration shown in Fig. 4c could be reproducibly generated and remains stable. No changes are induced at the surface by further increasing the CO dose. However, upon flashing to temperatures exceeding 90 K all CO molecules are desorbed, as deduced from the changed imaging characteristics of the Ag surface and the absence of any decoration effects, which persists after subsequent cooling down. The flash moreover drives the transformation of the complexes into a new structural arrangement, which is resolved as a pair of depressions. Since they have particular imaging characteristics and are immobile at temperatures around 100 K, where isolated O2 molecules readily diffuse [2] (cf. Fig. 5a), it can be definitively concluded that the flash does not simply account for CO desorption and restoration of a state with adsorbed molecular O2 . The pair formation rather strongly indicates that dissociation of O2 molecules is involved. However, this species
Fig. 5. (a) Ag(1 1 0) with 0.04 ML O2 imaged at 100 K (following exposure to 100 L O2 at 100 K). The mobility in the oxygen layer is appreciable at this temperature. While isolated molecules appear as dashes in individual scanlines; strings of molecules oriented in [1 1 0] can be imaged entirely due to their higher stability. (b) The same surface following exposure to 2 L CO at 100 K with the STM tip retracted: the molecules have completely reacted off and two distinct immobile features are resolved (imaged as grey (A) and black (B) depressions); (c) after an additional dose of 8 L CO at 100 K only black depressions prevail, revealing that the species imaged grey 2 , Ut ¼ 0:5 V, It ¼ is reactive atomic oxygen (480 250 A 0:3 nA).
must be different from the highly reactive atomic oxygen resulting from thermal dissociation, since it is unreactive towards coadsorbed CO, in strong contrast to the former species [29].
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Since the complexes occur while dosing CO, the two bright spots are associated with CO molecules being stabilized by O2 . The fact that either undecorated O2 molecules or O2 molecules with two bright spots, but only rarely O2 molecules with one bright spot signals higher stability of the complex decorated at both sides. With regard to the modified imaging characteristics at appreciable coverages (upon exceeding a dose of 9 L CO during in situ experiments and 4 L for experiments where the tip was retracted while CO exposure taking place, respectively), it is proposed that it is related to a reorientation of adsorbed CO, similar to the one observed by EELS [9]. The alternate possibility: modified imaging characteristics due to pickup of a CO molecule by the STM tip seems unlikely, since the transition took place reproducibly at a specific exposure. The different chemical behaviour of the O2 molecules at T K 90 K reveals that there is no unique reaction channel. The molecules either react with CO and disappear or they form complexes. It is feasible that the existence of different species of molecular oxygen at Ag(1 1 0) is at the origin of the complicated scenario. The oxidation reaction might depend on the orientation of the molecular axis, which again may be influenced by adsorbed CO. Nevertheless, the decoration of the adsorbed oxygen molecules is in agreement with the earlier suggested complex formation as reaction intermediate. However, no unique (CO O2 )ad species could be identified. For temperatures exceeding 90 K the reaction runs much faster than the time scale of data acquisition (cf. Fig. 5). With the surface covered with 0.04 ML O2 at 110 K (following exposure to 100 L of O2 ), a mobile adlayer is present at the surface (cf. Fig. 5a). Since the hopping rate of individual O2 molecules at this temperature is much greater than the frequency of the x-scan, they are imaged as dashes. On the other hand, the O2 molecules arranged in strings along [1 1 0] are less mobile due to space restrictions and the anisotropic attractive intermolecular interactions effectively enhancing the diffusion barrier [2]. While the surface was in this state, the STM tip was withdrawn and the surface was exposed to 5 L of CO. Upon reap-
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proaching the STM tip into the tunneling regime, the surface appeared as shown in Fig. 5b. Clearly the dashes and the strings observed in the previous image have disappeared, which is understood as a result of the reaction with CO. Furthermore two distinct species of adsorbates imaged as grey (A) and black (B) depressions can be identified. Both of them are immobile at this temperature and thus cannot be O2 molecules. The grey species is highly reactive and readily reacts upon further CO exposure (cf. Fig. 5c). It is thus understood as adsorbed atomic oxygen representing a reaction intermediate in the CO oxidation. In contrast to the situation encountered at T K 90 K, CO does not accumulate at the surfaces, so that isolated oxygen atoms can remain on the surface when the CO exposure is stopped. The black depressions are unreactive towards CO. Note that the initial step of the oxidation might be identical over the entire temperature range, since no indication on the interaction of the reactants, including a possible complex formation, can be obtained in view of rapid product formation. The nature of the unreactive features resolved in Fig. 5 cannot be conclusively unraveled on the basis of the present observations. Similar features were observed when dissociation of adsorbed molecular oxygen at the same surface was induced by ultraviolet radiation [30]. It is excluded that they result due to interaction with silver adatoms, which strongly influence the surface chemistry at slightly higher temperatures, where dissociation of molecular oxygen takes place [30,31]. Carbonate can be similarly excluded due to its exclusive formation at elevated temperatures [32–35]. Moreover, these unreactive features cannot be associated with the two distinct reactive species suggested in an MB study on the oxidation of CO by atomic oxygen [11]. On the one hand, the holes might evolve under the influence of coadsorbed water, which is inevitably present on the surface under the applied conditions. Evidence for OH formation under reaction conditions was generally provided by HREELS [35]. On the other hand, they could represent a subsurface oxygen species, which has been shown to exist, e.g., for the Ag(1 1 1) surface [36].
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4. Conclusions In conclusion, we have investigated the oxidation of CO by O2 molecules on Ag(1 1 0) using STM in the temperature range between 60 and 110 K. At T K 90 K, CO can accumulate on the surface, and there is no unique reaction path. The O2 molecules either react off or form (CO O2 )ad complexes indicative of attractive intermolecular interactions. By contrast at T J 90 K, where the reaction rate is much higher, no CO accumulation seems to occur. As a result, single adsorbed oxygen atoms representing a reaction intermediate can be observed at the surface. The observations are in overall agreement with an earlier suggested reaction scenario comprising the steps ðCOÞad þðO2 Þad ¢ðCO O2 Þad and ðCO O2 Þad ! ðCO2 Þg þOad . Acknowledgements Stimulating discussions with J. Wintterlin, H. Conrad, M. Borbach and U. Burghaus are gratefully acknowledged. The work of T.Z. was supported by the Deutscher Akademischer Austauschdienst (DAAD). References [1] J. Wintterlin, Adv. Catal. 45 (2000) 131. [2] J.V. Barth, T. Zambelli, J. Wintterlin, R. Schuster, G. Ertl, Phys. Rev. B 55 (1997) 12902. [3] J.V. Barth, T. Zambelli, J. Wintterlin, G. Ertl, Chem. Phys. Lett. 270 (1997) 152. [4] A.J. Capote, J.T. Roberts, R.J. Madix, Surf. Sci. Lett. 209 (1988) L151. [5] U. Burghaus, H. Conrad, Surf. Sci. 352–354 (1996) 253. [6] U. Burghaus, H. Conrad, Surf. Sci. 364 (1996) 109. [7] C. Backx, C.P.M.d. Droot, P. Biloen, Surf. Sci. 104 (1981) 300.
[8] J. Wintterlin, R. Schuster, G. Ertl, Phys. Rev. Lett. 77 (1996) 123. [9] L.D. Peterson, S.D. Kevan, J. Chem. Phys. 95 (1991) 8592. [10] G. McElhiney, H. Papp, J. Pritchard, Surf. Sci. (1976) 617. [11] U. Burghaus, H. Conrad, Surf. Sci. 338 (1995) L869. [12] S. Krause, C. Mariani, K.C. Prince, K. Horn, Surf. Sci. 138 (1984) 305. [13] A. Sandell, P. Bennich, A. Nilsson, B. Hernn€as, O. Bj€ orneholm, N. M artensson, Surf. Sci. 310 (1994) 16. [14] J.V. Barth, Surf. Sci. Rep. 40 (2000) 75. [15] C. Harendt, J. Goschnick, W. Hirschwald, Surf. Sci. 152/ 153 (1985) 453. [16] B. Briner, M. Doering, H.-P. Rust, A.M. Bradshaw, Science 278 (1997) 257. [17] B.A. Sexton, R.J. Madix, Chem. Phys. Lett. 76 (1980) 294. [18] M.A. Barteau, R.J. Madix, Surf. Sci. 97 (1980) 101. [19] D.A. Outka, J. St€ ohr, W. Jark, P. Stevens, J. Solomon, R.J. Madix, Phys. Rev. B 35 (1987) 4119. [20] F. Besenbacher, J.K. Nørskov, Prog. Surf. Sci. 4 (1993) 5. [21] A. Raukema, D.A. Butler, A.W. Kleyn, J. Phys. C: CM 8 (1996) 2247. [22] F. Bartolucci, R. Franchy, J.C. Barnard, R.E. Palmer, Phys. Rev. Lett. 80 (1998) 5224. [23] J.R. Hahn, H.J. Lee, W. Ho, Phys. Rev. Lett. 85 (2000) 1914. [24] P.A. Gravil, D.M. Bird, J.A. White, Phys. Rev. Lett. 77 (1996) 3933. [25] T.H. Upton, P. Stevens, R.J. Madix, J. Chem. Phys. 88 (1988) 3988. [26] P.J. v.d. Hoek, E.J. Baerends, Surf. Sci. 221 (1989) L791. [27] H. Nakatsuji, K. Nakai, J. Chem. Phys. 98 (1993) 2423. [28] P.A. Gravil, D.M. Bird, J.A. White, Surf. Sci. 352–354 (1996) 248. [29] U. Burghaus, H. Conrad, Surf. Sci. 370 (1997) 17. [30] T. Zambelli, J.V. Barth, J. Wintterlin, J. Phys. Cond. Matt. 14 (2002) 4241. [31] T. Zambelli, J.V. Barth, J. Wintterlin, Phys. Rev. B 58 (1998) 12663. [32] I. Stensgaard, E. Lægsgaard, F. Besenbacher, J. Chem. Phys. 103 (1995) 9825. [33] Y. Okawa, K. Tanaka, Surf. Sci. 344 (1995) L1207. [34] X.-C. Guo, R.J. Madix, J. Phys. Chem. B 105 (2001) 3878. [35] M. Borbach, Ph.D. Thesis, TU Berlin, 1997. [36] X. Bao, J.V. Barth, G. Lehmpfuhl, R. Schuster, Y. Uchida, R. Schl€ ogl, G. Ertl, Surf. Sci. 284 (1993) 14.
Oxidation of CO by molecular oxygen on a Ag(1 1 0) surface studied by scanning tunneling microscopy J.V. Barth a
a,*
, T. Zambelli
a,1
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Received 12 January 2002; accepted for publication 19 April 2002
Abstract The oxidation of CO by O2 molecularly adsorbed on a Ag(1 1 0) surface was investigated by scanning tunneling microscopy in the temperature range between 60 and 110 K. At the lowest temperatures, CO remains weakly bound at Ag(1 1 0). Striped CO superstructures running along the [0 0 1] direction can be observed at intermediate coverages, indicating weak anisotropic interactions between the molecules. Upon exposing the oxygen precovered surface to CO at T K 90 K, O2 molecules either react off or (CO O2 )ad complexes evolve, signaling CO accumulation on the surface and attractive intermolecular interactions. At T J 90 K, adsorbed O2 -molecules are highly mobile and CO molecules do not accumulate on the surface. Oxidation readily takes place upon CO exposure, whereby single oxygen atoms were identified as an intermediate product, which could be further titrated with CO molecules. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Surface chemical reaction; Silver; Carbon monoxide; Oxygen; Low index single crystal surfaces
1. Introduction Temperature controlled scanning tunneling microscopy (STM) has proven to be a versatile tool to elucidate surface chemical reactions [1]. In previous work, we have used this technique to directly characterize adsorption, surface mobility
* Corresponding author. Present address: Institut de Physique des Nanostructures, Ecole Polytechnique Federale de Lausanne, PHB-Eclubens, CH-1015 Lausanne, Switzerland. Fax: +41-21-6933604. E-mail address: johannes.barth@epfl.ch (J.V. Barth). 1 Present address: Centre d’Elaboration de Materiaux et d’Etudes Structurales, 29, rue Jeanne Marvig, BP 4347, F-31055 Toulouse Cedex 4, France.
and lateral interactions of molecular oxygen at Ag(1 1 0) [2,3]. In the following, STM observations on the oxidation of CO by this species will be presented. The system O2 /Ag(1 1 0) is of particular interest with respect to CO oxidation. Whereas on other catalysts such as platinum, where oxygen may similarly be stabilized in a molecular state at low temperatures, CO2 production occurs only following dissociation of O2 , electron energy-loss spectroscopy (EELS) experiments [4] provided evidence that on Ag(1 1 0) CO can be directly oxidized by molecular oxygen leading to the formation of carbon dioxide. In order to obtain more information on the reaction mechanism, recently thermal desorption spectroscopy (TDS) and molecular beam
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(MB) experiments have been performed in the temperature range between 100 and 130 K [5,6], where exclusively molecular oxygen exists on the surface [7]. The production of gaseous CO2 could be demonstrated at all temperatures. It was concluded that molecular oxygen is directly involved in the primary step of CO oxidation and that the reaction is of the Langmuir–Hinshelwood type whereby CO initially accumulates on the surface [6]. Two explanations were proposed in order to rationalize the characteristics of the reaction [6]: (i) An intermediate adsorbed CO O2 complex is formed, i.e., a step ðCOÞad þ ðO2 Þad ¢ ðCO O2 Þad takes place prior to the final oxidation (CO O2 Þad ! ðCO2 Þg þ Oad . Introducing this complex formation in model simulations improved the quality of the fit of the titration curves. (ii) The adsorption energy of CO is enhanced by the presence of oxygen on the entire surface without local effects. Both hypotheses lead to a stabilization of CO molecules, and this effect is actually corroborated by earlier TDS/EELS experiments of Ref. [4], according to which oxygen slightly increases the desorption temperature of CO on Ag(1 1 0). If hypothesis (i) is correct, a change of the STM imaging of the O2 molecules is expected to occur upon dosing CO, indicative of (CO O2 )ad complex formation. If, however, hypothesis (ii) describes what actually happens, no such local arrangements are expected.
2. Experimental The experiments were performed in an ultrahigh-vacuum (UHV) chamber with a base pressure of 5 1011 Torr, equipped with standard facilities for sample preparation and characterization. The home-built STM is of the beetle type and is cooled by liquid He [8]. Higher temperatures are achieved by counter heating the sample with a tungsten filament. The temperature is measured with a NiCr–Ni thermocouple, attached directly to the sample. The temperature scale was checked against the nominal desorption temperature of CO on Ag(1 1 0) at 77 K [9]. The STM data were ob-
tained in the constant-current mode with tunnel currents typically between 0.3 and 1 nA and voltages of 500–1000 mV. The Ag(1 1 0) single crystal sample was prepared by cycles of Arþ sputtering (1 lA/cm2 , 700 eV, 10 min at 300 K) and annealing at 750 K until no contaminations could be detected by Auger electron spectroscopy, and STM resolved large, defect-free atomic terraces. Gas doses are given in Langmuir (1 L ¼ 1 106 Torr s). Coverages are given in monolayers (1 ML corresponds to one adsorbed molecule per substrate atom).
3. Results and discussion 3.1. Separate adsorption of CO and O2 CO is only weakly bound to the Ag(1 1 0) surface ( 0.2 eV binding energy [6,10,11]). Ultraviolet photoemission spectroscopy results led to the suggestion of a physisorption state [12]. However, based on later experiments with direct core and valence photoemission, X-ray absorption and autoionization of core excited states, chemisorption of CO on the surface was concluded [13]. EELS observations following the specular peak intensity revealed that it initially remains low, but increases abruptly with appreciable coverages. It was inferred that the CO molecules initially lie parallel to the surface and then reorientate such that the molecular axis is oriented normal to the surface [9]. With our setup even at the lowest possible temperature ( 55 K), single CO molecules are expected to be too mobile to be directly observed with STM [14]. However, it turned out to be possible to follow the ordering of CO at appreciable coverages. This is demonstrated by the STM data reproduced in Fig. 1, where the ordering of CO was followed in situ, i.e., the data were obtained while exposing the surface to CO. Initially, only white dashes and spikes appear in single scanlines, which are associated with isolated CO molecules which cannot be imaged due to their high mobility. For doses exceeding 5 L (Fig. 1a), one-dimensional ordering of CO molecules occurs. Stripes oriented along the [0 0 1] direction with lengths
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Fig. 1. (a) STM image of Ag(1 1 0) after exposure to 10 L CO at 55 K. CO molecules order in stripes running over hundreds of ngstr€ A oms along [0 0 1]. (b) STM contour line along one of the periodicity on the stripes; CO stripes in (a) revealing a 4 A every protrusion is associated with a CO molecule. A fraction of . (c) Same area the stripes forms ribbons with a distance of 9 A as in (a): after exposing to 600 L patches of a regular (3 1) superstructure can be discerned in a mobile adlayer (460 220 2 , Ut ¼ 0:4 V, It ¼ 0:3 nA). A
ngstr€ up to several hundred A oms evolve. With increasing coverage, these stripes cluster into ribbons , correwhereby the distance in [1 1 0] is 9 A sponding to 3a (a ¼ 2:885 A is the lattice constant along the [1 1 0]-direction). Segments of some stripes can be displaced by 1a, signaling that also the stripes are mobile at this temperature, though much less than single molecules. Accordingly chains
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may occur in very different positions in successive STM frames. In Fig. 1b a representative STM contour line along the ridge of a stripe is reproduced. It reveals a regular corrugation of with a periodicity of 4 A , corresponding
0.4 A to the value of the lattice constant along [0 0 1] . Each protrusion is associated with b ¼ 4:08 A a CO molecule, i.e., the stripes are understood as densely packed CO rows with the molecules in upright positions. Since the substrate atomic structure could not be resolved simultaneously, the adsorption site for the CO molecules remains to be determined. The STM image in Fig. 1c was recorded after dosing of 600 L CO. The CO stripes are now arranged in regular domains, where they are separated by 3a. Between these domains, CO stripes separated by 3a are imaged in a ragged way, indicating appreciable mobility in the CO layer. CO molecules at 55 K thus form domains with a (3 1) structure. The existence of any CO superstructure on Ag(1 1 0) was not reported in the literature so far. However, similar arrangements have been observed with low-energy electron diffraction on Cu(1 1 0), where a (2 1) pattern upon CO adsorption was reported [15]. In agreement, 1dim ordering of CO on Cu(1 1 0) along [0 0 1] was recently found with STM experiments at temperatures below 50 K [16]. Since CO molecules on Ag(1 1 0) are mobile at the temperature employed for the experiments, the formation of the stripes indicates attractive interactions along the [0 0 1] direction. As the chains are mobile as well, their preferred separation of 3a in the domains reflects a long-range attractive interaction between the chains along [1 1 0]. A chemisorbed molecular oxygen species can be stabilized on Ag(1 1 0) in the 40–150 K temperature range [7,17–21]. In contrast to CO, adsorbed O2 molecules are imaged as depressions. Due to the higher binding energy and diffusion barrier isolated O2 can be imaged for temperatures up to 80 K [2]. A corresponding STM image is reproduced in Fig. 2 (recorded after cooling down a layer adsorbed at 100 K). It demonstrates the formation of molecular strings oriented along the close-packed atomic [1 1 0] rows of the substrate, which are similarly resolved in the image. The
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low O–O vibrational stretching mode observed by EELS [4], presumably accounts for the proceeding of the catalytic oxidation of CO in the low-temperature range. 3.2. Coadsorption and reaction of CO and O2
Fig. 2. Ag(1 1 0) with 0.03 ML O2 at 60 K (following exposure to 50 L at 100 K). Oxygen molecules are resolved as depressions and frequently arranged as strings running along the close-packed [1 1 0] direction. The intermolecular equilib2 ). rium distance in this direction is 2a (image size 210 200 A
distance of neighboring molecules in the strings is always 2a. Note that recent experiments demonstrate two distinct molecular species coexisting upon oxygen adsorption at lower temperatures (<80 K) [22,23]. In the course of the present investigations it was similarly noted that upon adsorption of oxygen molecules at T 60 K species evolve at the surface with distinct imaging characteristics. These observations support earlier ab initio calculations, where two adsorbed states with the molecular axis either oriented along [1 1 0] or [0 0 1] were found to be stable [24]. The latter configuration is energetically slightly preferred (by 0.04 eV with the Ag surface relaxed). Near-edge X-ray adsorption fine structure investigations indicate that the O2 molecules with the axis parallel to [1 1 0] exhibit only a r resonance , i.e., substanand have a bond length of 1.47 A tially stretched with respect to that of the free ) [19]. It was thus concluded that molecule (1.21 A the molecules are bound as a single-bonded peroxo species (O2 2 ), in accordance with theoretical investigations, suggesting a peroxolike chemisorbed state [24–28]. The corresponding weakened O–O bond, which is similarly reflected in an unusually
The reaction experiments were carried out in the following way: following Ref. [6], oxygen was dosed onto the clean surface at 100 K in order to prevent premature adsorption of CO, which is unavoidably present in the chamber during the O2 exposure. Then the sample was brought to the desired temperature in the range between 60 and 110 K. Subsequently CO was dosed and the evolution of the reaction was followed either during the STM measurement or with the tunneling tip retracted. We found that the results can be divided into two categories according to the temperature employed: T K 90 K and T J 90 K, respectively. In the following exemplaric observations are presented. The STM data presented in Fig. 3 were obtained during an in situ experiment at 85 K. Prior to cool down the surface was exposed to 140 L O2 (corresponding to a precoverage of 0.05 ML) and subsequently CO was dosed while continuously recording data at a partial pressure of 2 108 Torr. The surface in Fig. 3a was scanned from the bottom to the top; the start of the CO admission coincides with the acquisition of the first scanlines. In the lower image area the O2 molecules are visible with their typical imaging and ordering characteristics. A, B and C are reference points to facilitate the comparison between Fig. 3a and b. After 1 L CO some O2 molecules are surrounded by white clouds, which become more and more numerous with increasing CO dose until all molecules are decorated at 1.5 L (the actual exposure at the scanned area is reduced due to the shielding of the STM tip). The STM image shown in Fig. 3b demonstrates that the decoration persists at higher CO exposures. The resolution of the halo depends on the tip state and does not allow to clarify whether reaction has taken place. In Fig. 3a, the gradual development of the halo around the O2 molecules as function of the CO dose assures that the clouds do not reflect a tip change. Hence, we interpret them as CO decorating molecular O2 .
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Fig. 3. Ag(1 1 0) precovered with 0.05 ML O2 (following exposure to 140 L at 100 K): Images recorded during exposure of CO (pCO ¼ 2 108 Torr) at 85 K (a) after doses exceeding
1 L, the oxygen molecules and islands are surrounded by a white halo, indicating CO decoration. (b) Situation at higher exposures. A, B and C mark identical positions in both images 2 , Ut ¼ 0:5 V, It ¼ 0:3 to facilitate comparison (230 230 A nA).
Although the temperature of the experiment was slightly above the nominal desorption temperature of CO, the CO molecules impinging onto the surface remain trapped for a certain time prior to desorption. The fact that CO molecules rest in the vicinity of the O2 molecules signals an attractive interaction between these two species. The STM data reproduced in Fig. 4 have been selected from a further in situ series (CO pressure: 1 108 Torr, 0.04 ML O2 precoverage) performed at 70 K. In Fig. 4a (2 L dose), the number and arrangement of the O2 molecules re-
Fig. 4. Images recorded during exposure of Ag(1 1 0) precovered with 0.04 ML O2 to CO (pCO ¼ 1 108 Torr) at 70 K: (a) situation after 200 s with oxygen molecules resolved; (b) same area after 800 s: some molecules have reacted off (cf. the upper left of the image in (a)), while others are decorated by two protrusions such that the resulting complex is oriented in [1 1 0]; (c) for exposures exceeding 10 L (i.e., after t J 1000 s), most molecules have reacted off. Furthermore, a sudden change in the imaging characteristics appears, and the complexes in (b) are imaged differently as a species decorated by two protrusions oriented in [0 0 1]. No further changes take place at higher CO 2 , Ut ¼ 0:4 V, It ¼ 0:1 nA). doses (150 150 A
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mains as determined initially. In Fig. 4b (8 L dose), first changes become observable. Firstly, some O2 molecules have disappeared (cf. the upper left in Fig. 4a), which is associated with oxidation of CO and concomitant desorption of CO2 . The fact that no atomic oxygen could be observed as reaction intermediate is due to CO accumulating on the surface, which can be readily oxidized by an oxygen atom being far more reactive than the molecular species [29]. Secondly, some molecules are now decorated with two protrusions in [1 1 0] separated by 2a from the respective O2 forming a complex. The decoration remains unaffected during increasing the CO exposure, until for a dose of 10 L an abrupt transformation occurs at the surface, as revealed in Fig. 4c. Obviously a substantial fraction of the O2 molecules are now reacted off. However, the complexes are imaged differently as species decorated with the two bright spots in [0 0 1]. The overall unstable imaging characteristics reflect the presence of a layer of adsorbed CO. In contrast to the findings upon CO adsorption on pristine Ag(1 1 0), no formation of superstructures could be observed in any coadsorption/reaction experiment, which might result from the slightly higher temperatures employed for the coadsorption/reaction experiments. The configuration shown in Fig. 4c could be reproducibly generated and remains stable. No changes are induced at the surface by further increasing the CO dose. However, upon flashing to temperatures exceeding 90 K all CO molecules are desorbed, as deduced from the changed imaging characteristics of the Ag surface and the absence of any decoration effects, which persists after subsequent cooling down. The flash moreover drives the transformation of the complexes into a new structural arrangement, which is resolved as a pair of depressions. Since they have particular imaging characteristics and are immobile at temperatures around 100 K, where isolated O2 molecules readily diffuse [2] (cf. Fig. 5a), it can be definitively concluded that the flash does not simply account for CO desorption and restoration of a state with adsorbed molecular O2 . The pair formation rather strongly indicates that dissociation of O2 molecules is involved. However, this species
Fig. 5. (a) Ag(1 1 0) with 0.04 ML O2 imaged at 100 K (following exposure to 100 L O2 at 100 K). The mobility in the oxygen layer is appreciable at this temperature. While isolated molecules appear as dashes in individual scanlines; strings of molecules oriented in [1 1 0] can be imaged entirely due to their higher stability. (b) The same surface following exposure to 2 L CO at 100 K with the STM tip retracted: the molecules have completely reacted off and two distinct immobile features are resolved (imaged as grey (A) and black (B) depressions); (c) after an additional dose of 8 L CO at 100 K only black depressions prevail, revealing that the species imaged grey 2 , Ut ¼ 0:5 V, It ¼ is reactive atomic oxygen (480 250 A 0:3 nA).
must be different from the highly reactive atomic oxygen resulting from thermal dissociation, since it is unreactive towards coadsorbed CO, in strong contrast to the former species [29].
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Since the complexes occur while dosing CO, the two bright spots are associated with CO molecules being stabilized by O2 . The fact that either undecorated O2 molecules or O2 molecules with two bright spots, but only rarely O2 molecules with one bright spot signals higher stability of the complex decorated at both sides. With regard to the modified imaging characteristics at appreciable coverages (upon exceeding a dose of 9 L CO during in situ experiments and 4 L for experiments where the tip was retracted while CO exposure taking place, respectively), it is proposed that it is related to a reorientation of adsorbed CO, similar to the one observed by EELS [9]. The alternate possibility: modified imaging characteristics due to pickup of a CO molecule by the STM tip seems unlikely, since the transition took place reproducibly at a specific exposure. The different chemical behaviour of the O2 molecules at T K 90 K reveals that there is no unique reaction channel. The molecules either react with CO and disappear or they form complexes. It is feasible that the existence of different species of molecular oxygen at Ag(1 1 0) is at the origin of the complicated scenario. The oxidation reaction might depend on the orientation of the molecular axis, which again may be influenced by adsorbed CO. Nevertheless, the decoration of the adsorbed oxygen molecules is in agreement with the earlier suggested complex formation as reaction intermediate. However, no unique (CO O2 )ad species could be identified. For temperatures exceeding 90 K the reaction runs much faster than the time scale of data acquisition (cf. Fig. 5). With the surface covered with 0.04 ML O2 at 110 K (following exposure to 100 L of O2 ), a mobile adlayer is present at the surface (cf. Fig. 5a). Since the hopping rate of individual O2 molecules at this temperature is much greater than the frequency of the x-scan, they are imaged as dashes. On the other hand, the O2 molecules arranged in strings along [1 1 0] are less mobile due to space restrictions and the anisotropic attractive intermolecular interactions effectively enhancing the diffusion barrier [2]. While the surface was in this state, the STM tip was withdrawn and the surface was exposed to 5 L of CO. Upon reap-
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proaching the STM tip into the tunneling regime, the surface appeared as shown in Fig. 5b. Clearly the dashes and the strings observed in the previous image have disappeared, which is understood as a result of the reaction with CO. Furthermore two distinct species of adsorbates imaged as grey (A) and black (B) depressions can be identified. Both of them are immobile at this temperature and thus cannot be O2 molecules. The grey species is highly reactive and readily reacts upon further CO exposure (cf. Fig. 5c). It is thus understood as adsorbed atomic oxygen representing a reaction intermediate in the CO oxidation. In contrast to the situation encountered at T K 90 K, CO does not accumulate at the surfaces, so that isolated oxygen atoms can remain on the surface when the CO exposure is stopped. The black depressions are unreactive towards CO. Note that the initial step of the oxidation might be identical over the entire temperature range, since no indication on the interaction of the reactants, including a possible complex formation, can be obtained in view of rapid product formation. The nature of the unreactive features resolved in Fig. 5 cannot be conclusively unraveled on the basis of the present observations. Similar features were observed when dissociation of adsorbed molecular oxygen at the same surface was induced by ultraviolet radiation [30]. It is excluded that they result due to interaction with silver adatoms, which strongly influence the surface chemistry at slightly higher temperatures, where dissociation of molecular oxygen takes place [30,31]. Carbonate can be similarly excluded due to its exclusive formation at elevated temperatures [32–35]. Moreover, these unreactive features cannot be associated with the two distinct reactive species suggested in an MB study on the oxidation of CO by atomic oxygen [11]. On the one hand, the holes might evolve under the influence of coadsorbed water, which is inevitably present on the surface under the applied conditions. Evidence for OH formation under reaction conditions was generally provided by HREELS [35]. On the other hand, they could represent a subsurface oxygen species, which has been shown to exist, e.g., for the Ag(1 1 1) surface [36].
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4. Conclusions In conclusion, we have investigated the oxidation of CO by O2 molecules on Ag(1 1 0) using STM in the temperature range between 60 and 110 K. At T K 90 K, CO can accumulate on the surface, and there is no unique reaction path. The O2 molecules either react off or form (CO O2 )ad complexes indicative of attractive intermolecular interactions. By contrast at T J 90 K, where the reaction rate is much higher, no CO accumulation seems to occur. As a result, single adsorbed oxygen atoms representing a reaction intermediate can be observed at the surface. The observations are in overall agreement with an earlier suggested reaction scenario comprising the steps ðCOÞad þðO2 Þad ¢ðCO O2 Þad and ðCO O2 Þad ! ðCO2 Þg þOad . Acknowledgements Stimulating discussions with J. Wintterlin, H. Conrad, M. Borbach and U. Burghaus are gratefully acknowledged. The work of T.Z. was supported by the Deutscher Akademischer Austauschdienst (DAAD). References [1] J. Wintterlin, Adv. Catal. 45 (2000) 131. [2] J.V. Barth, T. Zambelli, J. Wintterlin, R. Schuster, G. Ertl, Phys. Rev. B 55 (1997) 12902. [3] J.V. Barth, T. Zambelli, J. Wintterlin, G. Ertl, Chem. Phys. Lett. 270 (1997) 152. [4] A.J. Capote, J.T. Roberts, R.J. Madix, Surf. Sci. Lett. 209 (1988) L151. [5] U. Burghaus, H. Conrad, Surf. Sci. 352–354 (1996) 253. [6] U. Burghaus, H. Conrad, Surf. Sci. 364 (1996) 109. [7] C. Backx, C.P.M.d. Droot, P. Biloen, Surf. Sci. 104 (1981) 300.
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