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Ru surfaces
Surface Science 532–535 (2003) 173–178 www.elsevier.com/locate/susc
Adsorption kinetics of CO on Cr/Ru surfaces R. Denecke *, B. Tr€ ankenschuh, M.P. Engelhardt, H.-P. Steinr€ uck Physikalische Chemie II, Universit€at Erlangen–N€urnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany
Abstract We report on sticking coefficient measurements for CO adsorption at 90 K on differently prepared Cr layers on Ru(0 0 0 1). Using a supersonic molecular beam we study initial and coverage-dependent sticking coefficients, S0 and SðhÞ, respectively, as function of the kinetic energy of the CO molecules. For a pseudomorphic Cr monolayer (prepared at 700 K), S0 decreases from 0.93 for low kinetic energies (0.09 eV) to 0.82 at 0.4 eV, and stays unchanged for higher kinetic energies up to 2.15 eV. In contrast, on a Cr/Ru surface alloy (for Cr deposition at 1000 K) S0 closely follows the behavior of clean Ru(0 0 0 1), decreasing from 0.93 to about 0.6 for 2.15 eV. The coverage-dependent sticking coefficients at low kinetic energies show pronounced precursor-mediated behavior, while for higher kinetic energies a Langmuir-like adsorption kinetics is found, which is most prominent for the Cr monolayer. 2003 Elsevier Science B.V. All rights reserved. Keywords: Adsorption kinetics; Surface chemical reaction; Carbon monoxide; Metallic films; Alloys; Chromium; Ruthenium
1. Introduction Ultrathin layers of metal A on metal B exhibit a chemical reactivity that can be quite different from the clean bulk phases of both constituents [1–5, and references therein]. Of special interest are pseudomorphic monolayers, which allow for an exchange of the metal A element without changing the geometry. So far, mainly systems with moderate reactivity have been studied, e.g., the adsorption of CO on thin Cu layers on Ru(0 0 0 1) [1–3,6]. However, to our knowledge no layered systems have been investigated, where dissociation of CO occurs. In this contribution we present results for ultrathin Cr layers on Ru(0 0 0 1). This * Corresponding author. Tel.: +49-9131-852-7320; fax: +499131-852-8867. E-mail addresses: [email protected], [email protected] (R. Denecke).
material combination appeared interesting to us, since for the (1 1 0) surface of a bulk Cr single crystal CO decomposition has been observed in the literature [7–9]; one thus also might expect a similar behavior for ultrathin Cr layers on Ru(0 0 0 1). The growth of Cr on Ru(0 0 0 1) has been studied previously [10–12]. Also, in a previous study using temperature programmed desorption (TPD), for both a pseudomorphic Cr monolayer on Ru(0 0 0 1) and a Cr/Ru surface alloy we found CO desorption peaks at temperatures above 600 K, indicative of recombinative desorption [12]. It turned out that the pseudomorphic monolayer exhibits a higher reactivity towards CO dissociation than the Cr/Ru surface alloy: 47% and 18% of the saturated CO layer on these two surfaces dissociate upon heating, respectively; note that no dissociation occurs on Ru(0 0 0 1). In order to study the adsorption kinetics of CO on these well defined layers we present results of
0039-6028/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00170-5
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coverage-dependent sticking coefficient measurements as a function of the kinetic energy of the impinging molecules. The results are compared with observations for the clean Ru(0 0 0 1) surface.
2. Experimental The details of the sample preparation [12] and the experimental setup can be found elsewhere [6]. Cr was evaporated from an electron beam evapo/ rator onto Ru(0 0 0 1) at a deposition rate of 0.4 A min, measured with a quartz crystal balance. We have prepared two different Cr/Ru surfaces: By evaporating at a surface temperature of 700 K we obtain a pseudomorphic Cr monolayer with a . This value correnominal thickness of 2.0 A sponds well both to the layer spacing in Cr bulk ) and in the Ru bulk (2.14 A ). It also agrees (2.03 A with the findings by Albrecht et al. [10,11]. The formation of the pseudomorphic monolayer has been verified in our previous study by a change in slope of Auger intensity measurements as function of Cr coverage and by the observation of a (1 · 1) LEED pattern, identical to that of clean Ru(0 0 0 1) [12]. In addition, at this coverage distinct changes are observed in the TPD spectra of CO. Further increase of the Cr coverage leads to formation of islands which exhibit a bcc(1 1 0)-like surface structure with a very characteristic highly commensurate LEED pattern [12]. One should note that the preparation of a homogeneous pseudomorphic monolayer is only successful in a narrow Cr coverage range; small deviations thus may result in additional small errors in the sticking coefficient measurements. Evaporation of Cr with a nominal thickness of some MLÕs at a substrate temperature of 1000 K results in a Cr/Ru surface alloy, as derived again from Auger and TPD results. The absence of a LEED superstructure besides the (1 · 1) pattern suggests that the alloy is chemically disordered [12]. Its stoichiometry is unknown. For both systems, the deposited Cr could be desorbed completely after the measurements by heating to 1450 K. The adsorption kinetics of CO on these surfaces have been studied using a supersonic molecular beam apparatus [6]. Heating the nozzle (up to
2100 K) and seeding CO in He allows to vary the kinetic energy from 0.09 to 2.15 eV. The kinetic energies have been determined using time-of-flight techniques. The sticking coefficients were measured using the method devised by King and Wells [13]. Here, the change of the CO partial pressure in the chamber as response to adsorption on the sample is recorded by a quadrupole mass spectrometer as a function of time. The starting point of adsorption is defined by removing a flag from the beam path. The CO partial pressure measured with the flag still blocking the molecular beam in front of the sample is used to normalize the pressure change observed upon adsorption. Thus, only differential pressure changes are recorded. The error in determination of the sticking coefficient is about ±0.02. Taking the above mentioned uncertainty in the preparation of the Cr monolayer into account, we estimate a total error of ±0.04 for these results. Using the coverage calibration obtained from TPD experiments the coverage-dependent sticking coefficients are obtained.
3. Results and discussion The coverage-dependent sticking coefficients, SðhÞ, are shown in Fig. 1 for a range of kinetic energies of the incident CO molecules, all measured at a surface temperature of 90 K. The initial sticking coefficients, S0 , derived from these curves for h ¼ 0, are separately shown in Fig. 2, as function of the kinetic energy. In both figures we compare data for clean Ru(0 0 0 1), the pseudomorphic Cr monolayer on Ru(0 0 0 1), and the Cr/ Ru surface alloy. Data for CO adsorption on Cr(1 1 0) are not available. For better comparison, the coverage scale in Fig. 1 is given relative to the saturation coverage. The actual saturation coverage for CO on Ru(0 0 0 1) obtained under the conditions used in this study is 0.55 ± 0.02 ML (given relative to the number of surface atoms) [6]. Derived from that we find saturation coverages of hCO ¼ 0:58 0:02 ML for the Cr monolayer and hCO ¼ 0:60 0:02 ML for the Cr/Ru alloy, from a comparison of the integrated TPD signals. These values are independent of kinetic energy.
R. Denecke et al. / Surface Science 532–535 (2003) 173–178
175
1.0 0.09 eV 0.26 eV 0.37 eV 0.58 eV 1.30 eV 1.75 eV 2.15 eV
0.8
SCO
0.6 0.4 0.2
(a) Ru(0001)
0.0 0.8
SCO
0.6 0.4 0.2
(b) Cr/Ru surface alloy
Fig. 2. Initial sticking coefficient for CO at 90 K as derived from Fig. 1 for h ¼ 0. Lines are guides to the eye. The error bars represent errors of ±0.02 for Ru(0 0 0 1) and the Cr/Ru surface alloy, and ±0.04 for the Cr monolayer to also account for the uncertainty in the monolayer preparation.
0.0 0.8 0.6
SCO
0.4 0.2
(c) Cr monolayer/Ru(0001)
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Θ/Θsat Fig. 1. Coverage-dependent sticking coefficient for CO at 90 K on (a) Ru(0 0 0 1), (b) a Cr/Ru surface alloy, (c) a pseudomorphic Cr monolayer on Ru(0 0 0 1), as function of kinetic energy. Traces for increasing kinetic energies are always appearing from top to bottom.
For low kinetic energies a high sticking coefficient up to near-saturation coverages is observed in Fig. 1 for all three systems; the spectra for Ru(0 0 0 1) (1a) resemble those published previously [6]. 1 An adsorption behavior like this has been termed precursor-mediated [14,15]. This means, molecules can accommodate (i.e., dissipate their kinetic energy) not only on empty sites of the surface but also on occupied sites, from where they have a certain probability to reach an empty site before they desorb (called extrinsic precursor)
1
There is a small difference in the shape close to saturation coverage that is due to difficulties with the correction of the background signal in the earlier study [6].
[14,15]. The curves in Fig. 1 approach a Langmuirtype characteristic (only direct adsorption on an empty site) for higher kinetic energies, since accommodation in the weakly bound precursor state is less likely. This transition occurs for the Cr monolayer (Fig. 1c) at lower kinetic energies (above 0.4 eV) and much more pronounced (one observes an almost ideal linear decrease of the sticking coefficient with increasing coverage) as compared to both the Cr/Ru surface alloy and the clean Ru(0 0 0 1) surface. Since this trend follows the line of decreasing reactivity towards CO dissociation, this could be a hint towards different CO adsorption geometries on the different substrates. Note that for Cr(1 1 0) a CO species with its molecular axis parallel to the surface has been proposed for coverages below 0.25 ML [7–9]. The coverage-dependent sticking coefficient for CO on Ru(0 0 0 1) shows a shallow minimum around hCO =hCO;sat ¼ 0:6 for energies below 0.58 eV, as was reported before [6]. For p this surface, an p ordered CO structure showing a ð 3 3ÞR30 LEED pattern has been observed, with a coverage of 0.33 ML (giving hCO =hCO;sat ¼ 0:6). The completion of this adsorbate structure could be the cause of the local reduction in SðhÞ [6]. For the Cr covered surfaces no ordered CO surface structures could be obtained.
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Although the reactivity towards CO dissociation differs significantly between clean Ru(0 0 0 1), the Cr/Ru surface alloy and the pseudomorphic Cr monolayer on Ru(0 0 0 1) (0%, 18% and 47% dissociation upon heating, respectively [12]), the initial sticking coefficients for low kinetic energies, as seen in Fig. 2, are almost identical, at a high value of about 0.93. This could imply that the observed dissociation for small CO doses on the Cr monolayer does not directly occur upon adsorption on the surface at 90 K, as has already been concluded before from the coverage-dependent TPD spectra [12]. For both the Ru(0 0 0 1) surface and the Cr/Ru surface alloy the initial sticking coefficient continuously decreases with increasing kinetic energy. Although the surface alloy shows an increased reactivity towards CO dissociation as compared to clean Ru(0 0 0 1) [12], this similarity could indicate that Ru is the majority element in the surface layer. The decrease of S0 with increasing kinetic energy signals a nonactivated adsorption, as would be the case, if no activation barrier between the accommodated state and the adsorbed state is involved. Therefore, direct molecular adsorption takes place which is less probable for higher kinetic energies [6]. For the very reactive pseudomorphic Cr monolayer, S0 decreases only for kinetic energies below 0.4 eV and stays almost constant at a value of about 0.8 for higher energies. This behavior is possibly due to the enhanced reactivity or a stronger bond of CO as compared to the alloy. One could also imagine that for an adsorption geometry with the molecular axis parallel to the surface as for Cr(1 1 0) [7–9], energy dissipation should be quite different. So far adsorption of CO was studied for surface temperatures of 90 K. Fig. 3 shows the coveragedependent sticking coefficients at a low (0.09 eV) and a high kinetic energy (1.75 eV) for both the pseudomorphic monolayer (Fig. 3a) and the surface alloy (Fig. 3b) as function of adsorption temperature. The curves for adsorption at 90 K are the same as in Fig. 1. For adsorption at 600 K, a CO saturation coverage of 0.27 ± 0.02 ML for the Cr monolayer (Fig. 3a) and of 0.21 ± 0.02 ML for the Cr/Ru surface alloy (Fig. 3b) is observed, again independent of kinetic energy. The fact that mo-
lecularly adsorbed CO already desorbs below 600 K [12] implies dissociative adsorption at this temperature. Now two adsorption sites are needed to accommodate the CO molecule, resulting in a reduced saturation coverage. Interestingly, the initial sticking coefficient at h ¼ 0 in Fig. 3a and b is the same as for the low surface temperatures. This indicates that the first step in the adsorption process is the same at low and at high temperature, namely that the molecule is trapped on the surface. From this state it is transferred into a molecularly adsorbed state at low temperatures (90 and 180 K), and into a dissociated state at high temperatures (600 K). This latter transition is thermally activated. The difference in coverage-dependent sticking coefficient at 90 and 180 K is attributed to a higher desorption rate from the weakly bound precursor state at higher temperatures, keeping in mind that desorption from the chemisorbed state starts only at around 220 K for the Cr monolayer and at 200 K for the surface alloy [12]. In line with this argument, the adsorption of high kinetic energy CO molecules on the pseudomorphic Cr monolayer does not show this temperature dependence (Fig. 3a) because the adsorption is always direct following Langmuir behavior. The difference between the sticking coefficient traces for 90 and 180 K at high kinetic energies for the surface alloy (Fig. 3b) is probably caused by slightly different sample preparations for the two temperatures.
4. Conclusions Coverage-dependent sticking coefficient measurements are reported for CO adsorption on reactive Cr layers prepared on Ru(0 0 0 1). Using a supersonic molecular beam the influence of the kinetic energy of the impinging molecules on the adsorption kinetics is being studied at surface temperatures of 90, 180 and 600 K. The Cr/Ru surface alloy, prepared by deposition of Cr at a surface temperature of 1000 K, still shows some similarities to Ru(0 0 0 1). The initial sticking coefficient decreases from 0.93 at low kinetic energy (0.09 eV) to about 0.6 for high kinetic energies (2.15 eV), a behavior typical for direct non-acti-
R. Denecke et al. / Surface Science 532–535 (2003) 173–178
(a) Cr monolayer/Ru(0001)
(b) Cr/Ru surface alloy
1.0
1.0 E= 0.09 eV
0.9
0.8
90 K
0.7
90 K
0.7
0.6
0.6
180 K
0.5 0.4 0.3
600 K
0.2 0.1 0.0 E= 1.75 eV
0.9 0.8
Sticking coefficientS (Θ)
Sticking coefficientS (Θ)
E = 0.09 eV
0.9
0.8
0.5
180 K
0.4 0.3
600 K
0.2 0.1 0.0 E = 1.75 eV
0.9 0.8
0.7
0.7
0.6
0.6 0.5
0.5 90 K
0.4
90 K
0.4 0.3
0.3 600 K
0.2
0.1
0.2
180 K
0.2
180 K
0.1 0.0 0.0
177
0.3
0.4
0.5
600 K
0.1
0.6
0.7
0.0 0.0
0.1
0.2
ΘCO [ML]
0.3
0.4
0.5
0.6
0.7
ΘCO [ML]
Fig. 3. Coverage-dependent sticking coefficients for CO adsorption as a function of substrate temperature, for selected kinetic energies of impinging molecules: (a) adsorption on the pseudomorphic Cr monolayer on Ru(0 0 0 1) and (b) adsorption on a Cr/Ru surface alloy.
vated adsorption. The coverage-dependent sticking coefficients at 90 K stay close to the initial value, even for coverages approaching saturation, which is a typical sign for precursor-mediated adsorption. Increasing the kinetic energy makes adsorption in the weakly bound precursor less likely which changes the adsorption kinetics to the direct Langmuir case. The more reactive pseudomorphic Cr monolayer exhibits a similar transition from precursor-mediated to Langmuir adsorption kinetics. In contrast to the alloy and Ru(0 0 0 1), above 0.4 eV the initial adsorption is almost independent of the kinetic energy of the incident CO molecules. A different adsorption geometry, i.e. with the molecular axis parallel to the surface, could be the reason for this. For a more detailed understanding, spectroscopic investigations for these systems are called for.
Acknowledgements This work was supported by ‘‘Fonds der chemischen Industrie’’ and ‘‘Max-Buchner-Stiftung’’.
References [1] [2] [3] [4] [5] [6] [7] [8]
J.A. Rodriguez, Surf. Sci. Rep. 24 (1996) 223. D.W. Goodman, Surf. Sci. 299/300 (1994) 837. C.T. Campbell, Ann. Rev. Phys. Chem. 41 (1990) 775. B. Hammer, Y. Morikawa, J.K. Nørskov, Phys. Rev. Lett. 76 (1996) 2141. J.H. Larsen, I. Chorkendorff, Surf. Sci. Rep. 35 (1999) 163. S. Kneitz, J. Gemeinhardt, H.-P. Steinr€ uck, Surf. Sci. 440 (1999) 307. N.D. Shinn, T.E. Madey, J. Vac. Sci. Technol. A 3 (1985) 1673. N.D. Shinn, Phys. Rev. B 38 (1988) 12234.
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[9] S.P. Mehandru, A.B. Anderson, Surf. Sci. Lett. 169 (1986) L281. [10] M. Albrecht, J. Pohl, H. Wider, E.U. Malang, J. K€ ohler, K. Friemelt, E. Bucher, Surf. Sci. 397 (1998) 354. [11] M. Albrecht, J. K€ ohler, K. Friemelt, E. Bucher, Surf. Sci. 415 (1998) 170.
[12] M.P. Engelhardt, T. Fuhrmann, G. Held, R. Denecke, H.P. Steinr€ uck, Surf. Sci. 512 (2002) 107. [13] D.A. King, M.G. Wells, Proc. R. Soc. A 339 (1974) 245. [14] P.J. Kisliuk, J. Phys. Chem. Solids 3 (1957) 95. [15] P.J. Kisliuk, J. Phys. Chem. Solids 5 (1958) 78.
Adsorption kinetics of CO on Cr/Ru surfaces R. Denecke *, B. Tr€ ankenschuh, M.P. Engelhardt, H.-P. Steinr€ uck Physikalische Chemie II, Universit€at Erlangen–N€urnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany
Abstract We report on sticking coefficient measurements for CO adsorption at 90 K on differently prepared Cr layers on Ru(0 0 0 1). Using a supersonic molecular beam we study initial and coverage-dependent sticking coefficients, S0 and SðhÞ, respectively, as function of the kinetic energy of the CO molecules. For a pseudomorphic Cr monolayer (prepared at 700 K), S0 decreases from 0.93 for low kinetic energies (0.09 eV) to 0.82 at 0.4 eV, and stays unchanged for higher kinetic energies up to 2.15 eV. In contrast, on a Cr/Ru surface alloy (for Cr deposition at 1000 K) S0 closely follows the behavior of clean Ru(0 0 0 1), decreasing from 0.93 to about 0.6 for 2.15 eV. The coverage-dependent sticking coefficients at low kinetic energies show pronounced precursor-mediated behavior, while for higher kinetic energies a Langmuir-like adsorption kinetics is found, which is most prominent for the Cr monolayer. 2003 Elsevier Science B.V. All rights reserved. Keywords: Adsorption kinetics; Surface chemical reaction; Carbon monoxide; Metallic films; Alloys; Chromium; Ruthenium
1. Introduction Ultrathin layers of metal A on metal B exhibit a chemical reactivity that can be quite different from the clean bulk phases of both constituents [1–5, and references therein]. Of special interest are pseudomorphic monolayers, which allow for an exchange of the metal A element without changing the geometry. So far, mainly systems with moderate reactivity have been studied, e.g., the adsorption of CO on thin Cu layers on Ru(0 0 0 1) [1–3,6]. However, to our knowledge no layered systems have been investigated, where dissociation of CO occurs. In this contribution we present results for ultrathin Cr layers on Ru(0 0 0 1). This * Corresponding author. Tel.: +49-9131-852-7320; fax: +499131-852-8867. E-mail addresses: [email protected], [email protected] (R. Denecke).
material combination appeared interesting to us, since for the (1 1 0) surface of a bulk Cr single crystal CO decomposition has been observed in the literature [7–9]; one thus also might expect a similar behavior for ultrathin Cr layers on Ru(0 0 0 1). The growth of Cr on Ru(0 0 0 1) has been studied previously [10–12]. Also, in a previous study using temperature programmed desorption (TPD), for both a pseudomorphic Cr monolayer on Ru(0 0 0 1) and a Cr/Ru surface alloy we found CO desorption peaks at temperatures above 600 K, indicative of recombinative desorption [12]. It turned out that the pseudomorphic monolayer exhibits a higher reactivity towards CO dissociation than the Cr/Ru surface alloy: 47% and 18% of the saturated CO layer on these two surfaces dissociate upon heating, respectively; note that no dissociation occurs on Ru(0 0 0 1). In order to study the adsorption kinetics of CO on these well defined layers we present results of
0039-6028/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00170-5
174
R. Denecke et al. / Surface Science 532–535 (2003) 173–178
coverage-dependent sticking coefficient measurements as a function of the kinetic energy of the impinging molecules. The results are compared with observations for the clean Ru(0 0 0 1) surface.
2. Experimental The details of the sample preparation [12] and the experimental setup can be found elsewhere [6]. Cr was evaporated from an electron beam evapo/ rator onto Ru(0 0 0 1) at a deposition rate of 0.4 A min, measured with a quartz crystal balance. We have prepared two different Cr/Ru surfaces: By evaporating at a surface temperature of 700 K we obtain a pseudomorphic Cr monolayer with a . This value correnominal thickness of 2.0 A sponds well both to the layer spacing in Cr bulk ) and in the Ru bulk (2.14 A ). It also agrees (2.03 A with the findings by Albrecht et al. [10,11]. The formation of the pseudomorphic monolayer has been verified in our previous study by a change in slope of Auger intensity measurements as function of Cr coverage and by the observation of a (1 · 1) LEED pattern, identical to that of clean Ru(0 0 0 1) [12]. In addition, at this coverage distinct changes are observed in the TPD spectra of CO. Further increase of the Cr coverage leads to formation of islands which exhibit a bcc(1 1 0)-like surface structure with a very characteristic highly commensurate LEED pattern [12]. One should note that the preparation of a homogeneous pseudomorphic monolayer is only successful in a narrow Cr coverage range; small deviations thus may result in additional small errors in the sticking coefficient measurements. Evaporation of Cr with a nominal thickness of some MLÕs at a substrate temperature of 1000 K results in a Cr/Ru surface alloy, as derived again from Auger and TPD results. The absence of a LEED superstructure besides the (1 · 1) pattern suggests that the alloy is chemically disordered [12]. Its stoichiometry is unknown. For both systems, the deposited Cr could be desorbed completely after the measurements by heating to 1450 K. The adsorption kinetics of CO on these surfaces have been studied using a supersonic molecular beam apparatus [6]. Heating the nozzle (up to
2100 K) and seeding CO in He allows to vary the kinetic energy from 0.09 to 2.15 eV. The kinetic energies have been determined using time-of-flight techniques. The sticking coefficients were measured using the method devised by King and Wells [13]. Here, the change of the CO partial pressure in the chamber as response to adsorption on the sample is recorded by a quadrupole mass spectrometer as a function of time. The starting point of adsorption is defined by removing a flag from the beam path. The CO partial pressure measured with the flag still blocking the molecular beam in front of the sample is used to normalize the pressure change observed upon adsorption. Thus, only differential pressure changes are recorded. The error in determination of the sticking coefficient is about ±0.02. Taking the above mentioned uncertainty in the preparation of the Cr monolayer into account, we estimate a total error of ±0.04 for these results. Using the coverage calibration obtained from TPD experiments the coverage-dependent sticking coefficients are obtained.
3. Results and discussion The coverage-dependent sticking coefficients, SðhÞ, are shown in Fig. 1 for a range of kinetic energies of the incident CO molecules, all measured at a surface temperature of 90 K. The initial sticking coefficients, S0 , derived from these curves for h ¼ 0, are separately shown in Fig. 2, as function of the kinetic energy. In both figures we compare data for clean Ru(0 0 0 1), the pseudomorphic Cr monolayer on Ru(0 0 0 1), and the Cr/ Ru surface alloy. Data for CO adsorption on Cr(1 1 0) are not available. For better comparison, the coverage scale in Fig. 1 is given relative to the saturation coverage. The actual saturation coverage for CO on Ru(0 0 0 1) obtained under the conditions used in this study is 0.55 ± 0.02 ML (given relative to the number of surface atoms) [6]. Derived from that we find saturation coverages of hCO ¼ 0:58 0:02 ML for the Cr monolayer and hCO ¼ 0:60 0:02 ML for the Cr/Ru alloy, from a comparison of the integrated TPD signals. These values are independent of kinetic energy.
R. Denecke et al. / Surface Science 532–535 (2003) 173–178
175
1.0 0.09 eV 0.26 eV 0.37 eV 0.58 eV 1.30 eV 1.75 eV 2.15 eV
0.8
SCO
0.6 0.4 0.2
(a) Ru(0001)
0.0 0.8
SCO
0.6 0.4 0.2
(b) Cr/Ru surface alloy
Fig. 2. Initial sticking coefficient for CO at 90 K as derived from Fig. 1 for h ¼ 0. Lines are guides to the eye. The error bars represent errors of ±0.02 for Ru(0 0 0 1) and the Cr/Ru surface alloy, and ±0.04 for the Cr monolayer to also account for the uncertainty in the monolayer preparation.
0.0 0.8 0.6
SCO
0.4 0.2
(c) Cr monolayer/Ru(0001)
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Θ/Θsat Fig. 1. Coverage-dependent sticking coefficient for CO at 90 K on (a) Ru(0 0 0 1), (b) a Cr/Ru surface alloy, (c) a pseudomorphic Cr monolayer on Ru(0 0 0 1), as function of kinetic energy. Traces for increasing kinetic energies are always appearing from top to bottom.
For low kinetic energies a high sticking coefficient up to near-saturation coverages is observed in Fig. 1 for all three systems; the spectra for Ru(0 0 0 1) (1a) resemble those published previously [6]. 1 An adsorption behavior like this has been termed precursor-mediated [14,15]. This means, molecules can accommodate (i.e., dissipate their kinetic energy) not only on empty sites of the surface but also on occupied sites, from where they have a certain probability to reach an empty site before they desorb (called extrinsic precursor)
1
There is a small difference in the shape close to saturation coverage that is due to difficulties with the correction of the background signal in the earlier study [6].
[14,15]. The curves in Fig. 1 approach a Langmuirtype characteristic (only direct adsorption on an empty site) for higher kinetic energies, since accommodation in the weakly bound precursor state is less likely. This transition occurs for the Cr monolayer (Fig. 1c) at lower kinetic energies (above 0.4 eV) and much more pronounced (one observes an almost ideal linear decrease of the sticking coefficient with increasing coverage) as compared to both the Cr/Ru surface alloy and the clean Ru(0 0 0 1) surface. Since this trend follows the line of decreasing reactivity towards CO dissociation, this could be a hint towards different CO adsorption geometries on the different substrates. Note that for Cr(1 1 0) a CO species with its molecular axis parallel to the surface has been proposed for coverages below 0.25 ML [7–9]. The coverage-dependent sticking coefficient for CO on Ru(0 0 0 1) shows a shallow minimum around hCO =hCO;sat ¼ 0:6 for energies below 0.58 eV, as was reported before [6]. For p this surface, an p ordered CO structure showing a ð 3 3ÞR30 LEED pattern has been observed, with a coverage of 0.33 ML (giving hCO =hCO;sat ¼ 0:6). The completion of this adsorbate structure could be the cause of the local reduction in SðhÞ [6]. For the Cr covered surfaces no ordered CO surface structures could be obtained.
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Although the reactivity towards CO dissociation differs significantly between clean Ru(0 0 0 1), the Cr/Ru surface alloy and the pseudomorphic Cr monolayer on Ru(0 0 0 1) (0%, 18% and 47% dissociation upon heating, respectively [12]), the initial sticking coefficients for low kinetic energies, as seen in Fig. 2, are almost identical, at a high value of about 0.93. This could imply that the observed dissociation for small CO doses on the Cr monolayer does not directly occur upon adsorption on the surface at 90 K, as has already been concluded before from the coverage-dependent TPD spectra [12]. For both the Ru(0 0 0 1) surface and the Cr/Ru surface alloy the initial sticking coefficient continuously decreases with increasing kinetic energy. Although the surface alloy shows an increased reactivity towards CO dissociation as compared to clean Ru(0 0 0 1) [12], this similarity could indicate that Ru is the majority element in the surface layer. The decrease of S0 with increasing kinetic energy signals a nonactivated adsorption, as would be the case, if no activation barrier between the accommodated state and the adsorbed state is involved. Therefore, direct molecular adsorption takes place which is less probable for higher kinetic energies [6]. For the very reactive pseudomorphic Cr monolayer, S0 decreases only for kinetic energies below 0.4 eV and stays almost constant at a value of about 0.8 for higher energies. This behavior is possibly due to the enhanced reactivity or a stronger bond of CO as compared to the alloy. One could also imagine that for an adsorption geometry with the molecular axis parallel to the surface as for Cr(1 1 0) [7–9], energy dissipation should be quite different. So far adsorption of CO was studied for surface temperatures of 90 K. Fig. 3 shows the coveragedependent sticking coefficients at a low (0.09 eV) and a high kinetic energy (1.75 eV) for both the pseudomorphic monolayer (Fig. 3a) and the surface alloy (Fig. 3b) as function of adsorption temperature. The curves for adsorption at 90 K are the same as in Fig. 1. For adsorption at 600 K, a CO saturation coverage of 0.27 ± 0.02 ML for the Cr monolayer (Fig. 3a) and of 0.21 ± 0.02 ML for the Cr/Ru surface alloy (Fig. 3b) is observed, again independent of kinetic energy. The fact that mo-
lecularly adsorbed CO already desorbs below 600 K [12] implies dissociative adsorption at this temperature. Now two adsorption sites are needed to accommodate the CO molecule, resulting in a reduced saturation coverage. Interestingly, the initial sticking coefficient at h ¼ 0 in Fig. 3a and b is the same as for the low surface temperatures. This indicates that the first step in the adsorption process is the same at low and at high temperature, namely that the molecule is trapped on the surface. From this state it is transferred into a molecularly adsorbed state at low temperatures (90 and 180 K), and into a dissociated state at high temperatures (600 K). This latter transition is thermally activated. The difference in coverage-dependent sticking coefficient at 90 and 180 K is attributed to a higher desorption rate from the weakly bound precursor state at higher temperatures, keeping in mind that desorption from the chemisorbed state starts only at around 220 K for the Cr monolayer and at 200 K for the surface alloy [12]. In line with this argument, the adsorption of high kinetic energy CO molecules on the pseudomorphic Cr monolayer does not show this temperature dependence (Fig. 3a) because the adsorption is always direct following Langmuir behavior. The difference between the sticking coefficient traces for 90 and 180 K at high kinetic energies for the surface alloy (Fig. 3b) is probably caused by slightly different sample preparations for the two temperatures.
4. Conclusions Coverage-dependent sticking coefficient measurements are reported for CO adsorption on reactive Cr layers prepared on Ru(0 0 0 1). Using a supersonic molecular beam the influence of the kinetic energy of the impinging molecules on the adsorption kinetics is being studied at surface temperatures of 90, 180 and 600 K. The Cr/Ru surface alloy, prepared by deposition of Cr at a surface temperature of 1000 K, still shows some similarities to Ru(0 0 0 1). The initial sticking coefficient decreases from 0.93 at low kinetic energy (0.09 eV) to about 0.6 for high kinetic energies (2.15 eV), a behavior typical for direct non-acti-
R. Denecke et al. / Surface Science 532–535 (2003) 173–178
(a) Cr monolayer/Ru(0001)
(b) Cr/Ru surface alloy
1.0
1.0 E= 0.09 eV
0.9
0.8
90 K
0.7
90 K
0.7
0.6
0.6
180 K
0.5 0.4 0.3
600 K
0.2 0.1 0.0 E= 1.75 eV
0.9 0.8
Sticking coefficientS (Θ)
Sticking coefficientS (Θ)
E = 0.09 eV
0.9
0.8
0.5
180 K
0.4 0.3
600 K
0.2 0.1 0.0 E = 1.75 eV
0.9 0.8
0.7
0.7
0.6
0.6 0.5
0.5 90 K
0.4
90 K
0.4 0.3
0.3 600 K
0.2
0.1
0.2
180 K
0.2
180 K
0.1 0.0 0.0
177
0.3
0.4
0.5
600 K
0.1
0.6
0.7
0.0 0.0
0.1
0.2
ΘCO [ML]
0.3
0.4
0.5
0.6
0.7
ΘCO [ML]
Fig. 3. Coverage-dependent sticking coefficients for CO adsorption as a function of substrate temperature, for selected kinetic energies of impinging molecules: (a) adsorption on the pseudomorphic Cr monolayer on Ru(0 0 0 1) and (b) adsorption on a Cr/Ru surface alloy.
vated adsorption. The coverage-dependent sticking coefficients at 90 K stay close to the initial value, even for coverages approaching saturation, which is a typical sign for precursor-mediated adsorption. Increasing the kinetic energy makes adsorption in the weakly bound precursor less likely which changes the adsorption kinetics to the direct Langmuir case. The more reactive pseudomorphic Cr monolayer exhibits a similar transition from precursor-mediated to Langmuir adsorption kinetics. In contrast to the alloy and Ru(0 0 0 1), above 0.4 eV the initial adsorption is almost independent of the kinetic energy of the incident CO molecules. A different adsorption geometry, i.e. with the molecular axis parallel to the surface, could be the reason for this. For a more detailed understanding, spectroscopic investigations for these systems are called for.
Acknowledgements This work was supported by ‘‘Fonds der chemischen Industrie’’ and ‘‘Max-Buchner-Stiftung’’.
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