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Effect of defects and pre-adsorbates on adsorption probabilities: a Monte Carlo simulation point of view
Surface Science 507–510 (2002) 736–741 www.elsevier.com/locate/susc
Effect of defects and pre-adsorbates on adsorption probabilities: a Monte Carlo simulation point of view J. Stephan a, U. Burghaus
b,*
a
b
Institute of Physics, University of Potsdam, 14415 Potsdam, Germany Physical Chemistry I, Department of Chemistry, Ruhr-University of Bochum, Universit€atstrasse, 44801 Bochum, Germany
Abstract The possible effect of defects and the influence of pre-adsorbates on the coverage dependence of adsorption probabilities will be discussed by the proposed Monte Carlo simulation scheme. In particular, the quenching of the adsorbate-assisted adsorption with increasing defect density—observed for CO/Cu(1 1 0)—and the scaling of the adsorption probability curves with increasing coverage of pre-adsorbates—observed for CO–O/Ir(1 1 0)—can qualitatively be reproduced. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Monte Carlo simulations; Sputtering; Sticking; Chemisorption; Copper; Iridium; Carbon monoxide; Single crystal surfaces
1. Introduction A large number of studies are devoted to gaining an understanding of the correlation between the surface structure with the corresponding reactivity of the system. The effect of special adsorption sites has therefore frequently been discussed. However, no strict rules could so far be established which would correlate e.g. the density of defects, C, with the chemical activity of the catalyst. For example an enhancement of the CO adsorption rate [1] and an increased reaction rate for CO oxidation [2] has been observed on step-
*
Corresponding author. Fax: +49-23432-14182. E-mail address: [email protected] Burghaus). URL: http://www.uweburghaus.de
(U.
edges for Pd surfaces. On the other hand, the CO2 formation rate on Pt(1 1 2) [3] was reduced by surface defects. Furthermore, for smooth NiO(1 0 0) surfaces, no adsorption of hydroxyl groups could be detected but rough NiO(1 0 0) films adsorbed OH effectively [4]. Regarding especially the effect of defects on adsorption probabilities, an increase in the initial adsorption probability, S0 , with decreasing C has been observed for CO/O–ZnO but the Zn–ZnO surface showed no effect [5]. Additionally, a decrease in S0 with C has been observed for O2 /Ag(0 0 1) [6], whereas the general shape of S vs. surface coverage, H, curves did not change significantly with C. By contrast, the so-called adsorbate-assisted adsorption (i.e., an increase in S with increasing H) was reduced for CO adsorption on Cu(1 1 0) [7] with increasing C. Thus, for special systems not only can S0 be strongly altered by defects, but the
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 3 4 5 - 6
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
whole shapes of SðHÞ can be modified as well. In close relation with the influence of defects should be site blocking effects which are caused by preadsorbates. For example, for the non-reactive adsorption of CO on O–Ir(1 1 0) [8] SðHÞ changed significantly with HOxygen . In this conference proceedings paper an effort is made to contribute to the discussion by proposing a simple Monte Carlo simulation, MCS, scheme which accounts for the possible effects of defects and pre-adsorbates on SðHÞ. The results are qualitatively discussed with regard to the molecular adsorption of CO on a sputtered Cu(1 1 0) [7] and on an oxygen pre-covered Ir(1 1 0) surface [8], respectively. Although analytical models such as the Langmuirian, the Kisliuk, and modifications of the Kisliuk model [9] lead to a perfect parameterization of SðHÞ curves (which is pertinent for a microkinetical modeling) MCS in particular are perfectly suited to discuss the influence of structural modifications of surfaces [10].
2. Algorithm To reproduce not only the traditional Langmuirian (SðHÞ ¼ S0 ð1 HÞ) and Kisliuk (SðHÞ constant, for large lifetimes of the precursor state) adsorption dynamics but also adsorbate-assisted adsorption (which has frequently been observed, see e.g. [11–13]), an MC version of the modified Kisliuk model has recently been proposed [14]. By distinguishing the desorption probabilities from clean, pclean ¼ 1 S0 , and occupied adsorption sites, poccu , the enhancement of adsorption with H could be modeled for both polar surfaces of ZnO [5]. In distinguishing pclean from poccu the variation in the mass-mismatch with increasing H can be accounted for, which is one possible way of explaining the observed increase in S with H. In the course of the adsorption process H approaches saturation (e.g. H ¼ 1 ML). Therefore the mass-mismatch changes from, c ¼ mðgas-phase speciesÞ=mðsurface atomsÞ for H ¼ 0, to c ¼ 1 for H ¼ 1 ML. The better c the more efficient the energy transfer processes should be, i.e., the larger the reactivity of the surface and the larger S. Thus, S should increase with H. Another view of the scenario is to compare
737
particles which have been trapped and thermalized in an extrinsic precursor state with particles encountering a direct adsorption event on clean surface sites. Regarding c the desorption probability will in the latter case be larger than in the former. Assuming poccu < pclean in the MCS leads indeed to the effect of the adsorbate-assisted adsorption [14]. Furthermore, for small poccu and large lifetimes of the precursor state, the MCS is consistent with the Kisliuk model and for poccu ¼ 1 (i.e., no precursor state) Langmuirian adsorption dynamics are obeyed [14]. Thus, the MC version of the modified Kisliuk model includes two parameters (poccu , pclean ), which allows us to simulate consistently the traditional shapes of SðHÞ and additionally the adsorbate-assisted adsorption phenomenon. To account for the additional experimental parameters, the prior presented MCS [14] had to be enlarged (and slightly modified). Briefly, prior to modeling the adsorption process, defect or preadsorbate sites are chosen at random (on an nx by ny square lattice), until the desired coverage Hdef or Hpre has been reached; afterwards the MCS cycle starts. A molecule that hits a regular and bare lattice site desorbs with a probability of pclean , and an impact on a defect site leads to desorption with clean pdef . If a filled site has been chosen, the molecule will be trapped in an extrinsic precursor with a occu occu probability of 1 poccu , 1 pdef , or 1 ppre depending on whether an adsorption site blocked by the adsorbate on a regular site, by an adsorbate anchored by a defect site or an adsorption site blocked by a pre-adsorbate has been found. The trapped particle performs a random walk on the surface by a maximum number of mext hops (with mext ¼ 40 ¼ nx for all simulations). If an empty site has been found within mext hops, the particle adsorbs or otherwise it desorbs. Thus, the lifetime of the particle is defined by the number of hops [14]. Although the heat of adsorption, Ed , and the nearest neighbor interaction energies, Eij , are parameters, as common for MCS, they are not free parameters. Commonly a set of SðHÞ curves will be simulated in agreement with a single experimentally obtained EðHÞ curve [14]. However, for the results shown, Ed on regular and modified sites as well as the corresponding Eij have not been distinguished. As discussed in detail in [14], only
738
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
large ratios of Ed =Eij change the shape of SðHÞ significantly. The discussion will therefore be restricted to dynamic effects; i.e., Ed < 0 and Eij ¼ 0 have been chosen. Finally, SðHÞ is calculated by counting the number of desorption, ndes , and adsorption, nads , events, i.e., SðHÞ ¼ nads =ðnads þ ndes Þ. In summarizing the algorithm, to account for the effect of defects, two dynamic parameters clean occu (pdef , pdef ) have been included and just one paoccu rameter, (ppre ), considering pre-adsorbates. For direct adsorption on a clean defect site the desorption probability should differ from pclean . Moreover also the precursor dynamics might be modified by defects. Therefore the parameters clean occu pdef and pdef have been introduced. Regarding the differences in c, in the case of pre-adsorbates, pclean should not be altered, but the desorption probability of the gas-phase species targeting a pre-adsorbate should differ from poccu . Thus, it is occu plausible to include the parameter ppre . 3. Discussion The parameters have been varied systematically to discuss the possible effect of defects or preadsorbates on SðHÞ as modeled by the MCS; the results are summarized in Figs. 1 and 2; the bold lines refer to the pristine surface case. 3.1. Defects In Fig. 1A and B the initial precursor dynamics correspond to adsorbate-assisted adsorption and for the MCS shown in Fig. 1C and D the traditional Kisliuk and ‘‘Langmuirian like’’ adsorption dynamics, respectively, have been used as the starting point. Fig. 1A demonstrates that for desorption probabilities from defect sites which are smaller than the corresponding probability from pristine clean sites (pdef < pclean ), a quenching of the adsorbateassisted adsorption is generated; i.e., the slope of S clean vs. H curves decreases with decreasing pdef (atype defects). This effect which is already very pronounced for rather small C (i.e., small Hdef ) increases further with Hdef (see upper part of Fig. clean 1A). On the other hand for pdef > pclean an
Fig. 1. Effect of defects for (A, B) adsorbate-assisted, (C) Kisliuk, and (D) ‘‘Langmuirian like’’ adsorption dynamics.
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
Fig. 2. Effect of pre-adsorbates for (A, B) Kisliuk, (C) ‘‘Langmuirian like’’, and (D) adsorbate-assisted adsorption dynamics.
739
enhancement of the adsorbate-assisted adsorption has been simulated (b-type defects). Thus, a variclean ation in pdef for otherwise constant parameters changes the whole shape of SðHÞ, including variations in S0 . This result is in contrast to a possible modification of the extrinsic precursor dynamics occu by defects, as shown in Fig. 1B. If only pdef is occu varied, for pdef < poccu an enhancement (b-type) occu and for pdef > poccu a significant reduction (atype) of the adsorbate-assisted adsorption, respectively, is simulated. However, S0 remains constant. Both effects obtained by the last MCS (constant S0 , a-type defects, Fig. 1B) have indeed recently been observed for CO adsorption on Cu(1 1 0) (see Fig. 7B in [7]). The experimentally determined S0 values remained unaffected within 0.05 for the sputtered surface as compared to the smooth surface. However, within the same Arþ exposure range, a cross-over from adsorbateassisted adsorption to ‘‘Kisliuk like’’ adsorption dynamics has been observed by inducing defects at very low Ts of 50 K [7]. This procedure leads to heavily damaged surfaces as shown in [7] by He atom scattering. Fig. 1C and D demonstrate that also for Langmuirian and Kisliuk adsorption dynamics, respectively, the shape of the original SðHÞ curves depends very much on Hdef and the corresponding precursor dynamics. In view of the MCS the effect of defects can be explained in terms of the mass-mismatch approach, as outlined above. By introducing defects, the surface is partitioned into two distinguishable kinds of patches which will certainly be characterized by differences in their dynamic parameters. The adsorbate-assisted adsorption is obtained by the MCS, in cases where the energy transfer processes for trapped particles are more efficient than the direct energy transfer from the gas-phase species to bare surface sites. This is simply the case for clean poccu < pclean [14]. Assuming now pdef < pclean assigns to the clean defect sites a behavior which is similar to the one for occupied sites of the pristine surface. Therefore the effect leading to the adsorbate-assisted adsorption will be weakened (adefects) since the difference between the overall poccu and pclean decreases. For a quenching of the adsorbate-assisted adsorption by defects we have
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J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
thus to assume that the efficiency of the energy transfer increases by the interaction of the particles clean with defects (i.e., pdef < pclean ). This conclusion appears plausible since defects might lead to a mixing of the parallel and perpendicular components of the momentum of the particles consistent with the enhancement of the adsorbate-assisted adsorption for normal angles of incidence [7]. The results shown in Fig. 1B–D can be explained by following the same reasoning; since S0 clean occu is determined by pclean and pdef , a variation in pdef modifies only the shape of SðHÞ and not S0 (Fig. occu 1B, C). That pdef differs from poccu and that the adsorbate assisted-adsorption is quenched for occu pdef > poccu (Fig. 1B) might be related to the stiffness of the gas–surface interaction potential. For example, for oxygen adsorption on a sputtered Ag(0 0 1) surface a shift of the Ag–O vibration to larger loss energies with increasing C has been observed [6]. Thus, the interaction potential becomes stiffer above occupied defect sites, i.e., a less efficient energy transfer should take place. 3.2. Pre-adsorbates Recently a distinct influence of pre-adsorbed oxygen on SðHÞ of CO has been observed experimentally for Ir(1 1 0) [8]. The co-adsorption has been studied below the onset of CO2 formation. For large impact energies S0 decreased significantly with increasing Hpre thereby conserving the general shape of SðHÞ. Again the proposed MCS is well suited to simulate this effect since defects and preadsorbates are conceptionally similar. The only difference with respect to the MCS is that preadsorbates really block at least one adsorption site, thereby leading to a reduction of the saturation coverage, Hsat , of the adsorbate, whereas the overall rules of the precursor dynamics are identical. The results as obtained for Kisliuk, Langmuirian, and adsorbate-assisted adsorption dynamics, respectively, are summarized in Fig. 2A–D. The general shape and trends simulated agree perfectly with the experimental results obtained for CO/O–Ir(1 1 0) (cf. Fig. 3A in [8] with Fig. 2A, B and Fig. 3C in [8] with Fig. 2C). For Kisliuk shapes and in the case of a precursor dynamics which is identical for the adsor-
occu ), a bates and the pre-adsorbates (poccu ¼ ppre simple site blocking is simulated (Fig. 2A), as expected. However, for a precursor dynamics leading due to the pre-adsorbates to a less efficient energy occu transfer (ppre > poccu , Fig. 2B) a decrease in S0 is obtained, whereas the general shape of SðHÞ is conserved, as observed [8]. Again this result is consistent with the different impact scenarios and the related differences in the mass-mismatch. (Note: for CO–O/ Ir(1 1 0), c ¼ mðCOÞ=mðCOÞ > c ¼ mðCOÞ=mðOÞ is obeyed.) In cases where Langmuirian like dynamics (Fig. 2C) or adsorbate-assisted adsorption dynamics (Fig. 2D) has been chosen as the starting point of occu the MCS, even for identical (poccu ¼ ppre ) precursor dynamics, SðHÞ decreases or increases, respectively, in a scaled manner (as compared to SðH; Hpre ¼ 0Þ) with increasing Hpre (as observed [8]). This, at first glance unexpected, result can be explained as discussed above for the effect of defects. For Langmuirian like dynamics (pclean < poccu ) a decrease in S0 is certainly induced by the pre-adsorbates since the fractional part of the surface where the particles can directly adsorb with a probability of 1 pclean is simply reduced and trapping in the precursor state is not that efoccu ficient (i.e., pclean < poccu ¼ ppre ). Additionally, for small differences in the corresponding precursor occu dynamics, poccu ppre , the general shape of SðHÞ will be conserved and the saturation coverage, sat Hsat , reduced according to Hsat Hpre . pre ¼ H occu For adsorbate-assisted adsorption and ppre ¼ occu p (Fig. 2D) the effect of pre-adsorbates on S0 is opposite to the Langmuirian dynamics case (Fig. 2C) since the starting condition is also contrary (i.e., poccu < pclean ). Furthermore, the effect which leads to an increase in S with H, i.e., the condition occu poccu < pclean , will be enhanced for ppre < poccu and occu occu reduced for ppre > p (not shown) similarly with the effect of defects (see above).
4. Summary A simple MCS scheme has been presented which explains qualitatively the experimentally observed defect induced quenching of the adsor-
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
bate-assisted adsorption and the effect of preadsorbates on SðHÞ for CO/Cu(1 1 0) and CO/O– Ir(1 1 0).
Acknowledgements Discussions with L. Brehmer and Ch. W€ oll are gratefully acknowledged as well as a critical reading of the paper by B. Hawley and R. Treiling.
References [1] J.T. Yates, J. Vac. Sci. Technol. A 13 (1995) 1359. [2] R. Castell, S. Reiff, W. Drachsel, J.H. Block, Surf. Sci. 377–379 (1997) 770. [3] A. Szabo, M.A. Henderson, J.T. Yates, J. Chem. Phys. 96 (1992) 6191.
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[4] D. Cappus, C. Xu, D. Ehrlich, B. Dillmann, C.A. Ventrice, K.A. Shamery, H. Kuhlenbeck, H.J. Freund, Chem. Phys. 177 (1993) 533. [5] T. Becker, M. Kunat, C. Boas, U. Burghaus, C. W€ oll, J. Chem. Phys. 113 (2000) 6334. [6] G. Costantini, F.B.d. Mongeot, S. Rusponi, C. Boragno, U. Valbusa, L. Vattuone, U. Burghaus, L. Savio, M. Rocca, J. Chem. Phys. 112 (2000) 6840. [7] M. Kunat, C. Boas, T. Becker, U. Burghaus, C. W€ oll, Surf. Sci. 474 (2001) 114. [8] U. Burghaus, J. Ding, W.H. Weinberg, Surf. Sci. 384 (1997) L869. [9] J.F. Weaver, M. Ikai, A. Carlsson, R.J. Madix, Surf. Sci. 470 (2001) 226. [10] J. Stephan, L. Brehmer, Synth. Met. 109 (2000) 333. [11] P. Zeppenfeld, J. George, M. B€ uchel, R. David, G. Comsa, Surf. Sci. 318 (1994) L1187. [12] L.Q. Jiang, B.E. Koel, J.L. Falconer, Surf. Sci. 273 (1992) 273. [13] M. Bowker, D.A. King, J. Chem. Soc. Faraday Trans. 75 (1979) 2100. [14] U. Burghaus, Surf. Rev. Lett. 8 (2001) 353.
Effect of defects and pre-adsorbates on adsorption probabilities: a Monte Carlo simulation point of view J. Stephan a, U. Burghaus
b,*
a
b
Institute of Physics, University of Potsdam, 14415 Potsdam, Germany Physical Chemistry I, Department of Chemistry, Ruhr-University of Bochum, Universit€atstrasse, 44801 Bochum, Germany
Abstract The possible effect of defects and the influence of pre-adsorbates on the coverage dependence of adsorption probabilities will be discussed by the proposed Monte Carlo simulation scheme. In particular, the quenching of the adsorbate-assisted adsorption with increasing defect density—observed for CO/Cu(1 1 0)—and the scaling of the adsorption probability curves with increasing coverage of pre-adsorbates—observed for CO–O/Ir(1 1 0)—can qualitatively be reproduced. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Monte Carlo simulations; Sputtering; Sticking; Chemisorption; Copper; Iridium; Carbon monoxide; Single crystal surfaces
1. Introduction A large number of studies are devoted to gaining an understanding of the correlation between the surface structure with the corresponding reactivity of the system. The effect of special adsorption sites has therefore frequently been discussed. However, no strict rules could so far be established which would correlate e.g. the density of defects, C, with the chemical activity of the catalyst. For example an enhancement of the CO adsorption rate [1] and an increased reaction rate for CO oxidation [2] has been observed on step-
*
Corresponding author. Fax: +49-23432-14182. E-mail address: [email protected] Burghaus). URL: http://www.uweburghaus.de
(U.
edges for Pd surfaces. On the other hand, the CO2 formation rate on Pt(1 1 2) [3] was reduced by surface defects. Furthermore, for smooth NiO(1 0 0) surfaces, no adsorption of hydroxyl groups could be detected but rough NiO(1 0 0) films adsorbed OH effectively [4]. Regarding especially the effect of defects on adsorption probabilities, an increase in the initial adsorption probability, S0 , with decreasing C has been observed for CO/O–ZnO but the Zn–ZnO surface showed no effect [5]. Additionally, a decrease in S0 with C has been observed for O2 /Ag(0 0 1) [6], whereas the general shape of S vs. surface coverage, H, curves did not change significantly with C. By contrast, the so-called adsorbate-assisted adsorption (i.e., an increase in S with increasing H) was reduced for CO adsorption on Cu(1 1 0) [7] with increasing C. Thus, for special systems not only can S0 be strongly altered by defects, but the
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 3 4 5 - 6
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
whole shapes of SðHÞ can be modified as well. In close relation with the influence of defects should be site blocking effects which are caused by preadsorbates. For example, for the non-reactive adsorption of CO on O–Ir(1 1 0) [8] SðHÞ changed significantly with HOxygen . In this conference proceedings paper an effort is made to contribute to the discussion by proposing a simple Monte Carlo simulation, MCS, scheme which accounts for the possible effects of defects and pre-adsorbates on SðHÞ. The results are qualitatively discussed with regard to the molecular adsorption of CO on a sputtered Cu(1 1 0) [7] and on an oxygen pre-covered Ir(1 1 0) surface [8], respectively. Although analytical models such as the Langmuirian, the Kisliuk, and modifications of the Kisliuk model [9] lead to a perfect parameterization of SðHÞ curves (which is pertinent for a microkinetical modeling) MCS in particular are perfectly suited to discuss the influence of structural modifications of surfaces [10].
2. Algorithm To reproduce not only the traditional Langmuirian (SðHÞ ¼ S0 ð1 HÞ) and Kisliuk (SðHÞ constant, for large lifetimes of the precursor state) adsorption dynamics but also adsorbate-assisted adsorption (which has frequently been observed, see e.g. [11–13]), an MC version of the modified Kisliuk model has recently been proposed [14]. By distinguishing the desorption probabilities from clean, pclean ¼ 1 S0 , and occupied adsorption sites, poccu , the enhancement of adsorption with H could be modeled for both polar surfaces of ZnO [5]. In distinguishing pclean from poccu the variation in the mass-mismatch with increasing H can be accounted for, which is one possible way of explaining the observed increase in S with H. In the course of the adsorption process H approaches saturation (e.g. H ¼ 1 ML). Therefore the mass-mismatch changes from, c ¼ mðgas-phase speciesÞ=mðsurface atomsÞ for H ¼ 0, to c ¼ 1 for H ¼ 1 ML. The better c the more efficient the energy transfer processes should be, i.e., the larger the reactivity of the surface and the larger S. Thus, S should increase with H. Another view of the scenario is to compare
737
particles which have been trapped and thermalized in an extrinsic precursor state with particles encountering a direct adsorption event on clean surface sites. Regarding c the desorption probability will in the latter case be larger than in the former. Assuming poccu < pclean in the MCS leads indeed to the effect of the adsorbate-assisted adsorption [14]. Furthermore, for small poccu and large lifetimes of the precursor state, the MCS is consistent with the Kisliuk model and for poccu ¼ 1 (i.e., no precursor state) Langmuirian adsorption dynamics are obeyed [14]. Thus, the MC version of the modified Kisliuk model includes two parameters (poccu , pclean ), which allows us to simulate consistently the traditional shapes of SðHÞ and additionally the adsorbate-assisted adsorption phenomenon. To account for the additional experimental parameters, the prior presented MCS [14] had to be enlarged (and slightly modified). Briefly, prior to modeling the adsorption process, defect or preadsorbate sites are chosen at random (on an nx by ny square lattice), until the desired coverage Hdef or Hpre has been reached; afterwards the MCS cycle starts. A molecule that hits a regular and bare lattice site desorbs with a probability of pclean , and an impact on a defect site leads to desorption with clean pdef . If a filled site has been chosen, the molecule will be trapped in an extrinsic precursor with a occu occu probability of 1 poccu , 1 pdef , or 1 ppre depending on whether an adsorption site blocked by the adsorbate on a regular site, by an adsorbate anchored by a defect site or an adsorption site blocked by a pre-adsorbate has been found. The trapped particle performs a random walk on the surface by a maximum number of mext hops (with mext ¼ 40 ¼ nx for all simulations). If an empty site has been found within mext hops, the particle adsorbs or otherwise it desorbs. Thus, the lifetime of the particle is defined by the number of hops [14]. Although the heat of adsorption, Ed , and the nearest neighbor interaction energies, Eij , are parameters, as common for MCS, they are not free parameters. Commonly a set of SðHÞ curves will be simulated in agreement with a single experimentally obtained EðHÞ curve [14]. However, for the results shown, Ed on regular and modified sites as well as the corresponding Eij have not been distinguished. As discussed in detail in [14], only
738
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
large ratios of Ed =Eij change the shape of SðHÞ significantly. The discussion will therefore be restricted to dynamic effects; i.e., Ed < 0 and Eij ¼ 0 have been chosen. Finally, SðHÞ is calculated by counting the number of desorption, ndes , and adsorption, nads , events, i.e., SðHÞ ¼ nads =ðnads þ ndes Þ. In summarizing the algorithm, to account for the effect of defects, two dynamic parameters clean occu (pdef , pdef ) have been included and just one paoccu rameter, (ppre ), considering pre-adsorbates. For direct adsorption on a clean defect site the desorption probability should differ from pclean . Moreover also the precursor dynamics might be modified by defects. Therefore the parameters clean occu pdef and pdef have been introduced. Regarding the differences in c, in the case of pre-adsorbates, pclean should not be altered, but the desorption probability of the gas-phase species targeting a pre-adsorbate should differ from poccu . Thus, it is occu plausible to include the parameter ppre . 3. Discussion The parameters have been varied systematically to discuss the possible effect of defects or preadsorbates on SðHÞ as modeled by the MCS; the results are summarized in Figs. 1 and 2; the bold lines refer to the pristine surface case. 3.1. Defects In Fig. 1A and B the initial precursor dynamics correspond to adsorbate-assisted adsorption and for the MCS shown in Fig. 1C and D the traditional Kisliuk and ‘‘Langmuirian like’’ adsorption dynamics, respectively, have been used as the starting point. Fig. 1A demonstrates that for desorption probabilities from defect sites which are smaller than the corresponding probability from pristine clean sites (pdef < pclean ), a quenching of the adsorbateassisted adsorption is generated; i.e., the slope of S clean vs. H curves decreases with decreasing pdef (atype defects). This effect which is already very pronounced for rather small C (i.e., small Hdef ) increases further with Hdef (see upper part of Fig. clean 1A). On the other hand for pdef > pclean an
Fig. 1. Effect of defects for (A, B) adsorbate-assisted, (C) Kisliuk, and (D) ‘‘Langmuirian like’’ adsorption dynamics.
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
Fig. 2. Effect of pre-adsorbates for (A, B) Kisliuk, (C) ‘‘Langmuirian like’’, and (D) adsorbate-assisted adsorption dynamics.
739
enhancement of the adsorbate-assisted adsorption has been simulated (b-type defects). Thus, a variclean ation in pdef for otherwise constant parameters changes the whole shape of SðHÞ, including variations in S0 . This result is in contrast to a possible modification of the extrinsic precursor dynamics occu by defects, as shown in Fig. 1B. If only pdef is occu varied, for pdef < poccu an enhancement (b-type) occu and for pdef > poccu a significant reduction (atype) of the adsorbate-assisted adsorption, respectively, is simulated. However, S0 remains constant. Both effects obtained by the last MCS (constant S0 , a-type defects, Fig. 1B) have indeed recently been observed for CO adsorption on Cu(1 1 0) (see Fig. 7B in [7]). The experimentally determined S0 values remained unaffected within 0.05 for the sputtered surface as compared to the smooth surface. However, within the same Arþ exposure range, a cross-over from adsorbateassisted adsorption to ‘‘Kisliuk like’’ adsorption dynamics has been observed by inducing defects at very low Ts of 50 K [7]. This procedure leads to heavily damaged surfaces as shown in [7] by He atom scattering. Fig. 1C and D demonstrate that also for Langmuirian and Kisliuk adsorption dynamics, respectively, the shape of the original SðHÞ curves depends very much on Hdef and the corresponding precursor dynamics. In view of the MCS the effect of defects can be explained in terms of the mass-mismatch approach, as outlined above. By introducing defects, the surface is partitioned into two distinguishable kinds of patches which will certainly be characterized by differences in their dynamic parameters. The adsorbate-assisted adsorption is obtained by the MCS, in cases where the energy transfer processes for trapped particles are more efficient than the direct energy transfer from the gas-phase species to bare surface sites. This is simply the case for clean poccu < pclean [14]. Assuming now pdef < pclean assigns to the clean defect sites a behavior which is similar to the one for occupied sites of the pristine surface. Therefore the effect leading to the adsorbate-assisted adsorption will be weakened (adefects) since the difference between the overall poccu and pclean decreases. For a quenching of the adsorbate-assisted adsorption by defects we have
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J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
thus to assume that the efficiency of the energy transfer increases by the interaction of the particles clean with defects (i.e., pdef < pclean ). This conclusion appears plausible since defects might lead to a mixing of the parallel and perpendicular components of the momentum of the particles consistent with the enhancement of the adsorbate-assisted adsorption for normal angles of incidence [7]. The results shown in Fig. 1B–D can be explained by following the same reasoning; since S0 clean occu is determined by pclean and pdef , a variation in pdef modifies only the shape of SðHÞ and not S0 (Fig. occu 1B, C). That pdef differs from poccu and that the adsorbate assisted-adsorption is quenched for occu pdef > poccu (Fig. 1B) might be related to the stiffness of the gas–surface interaction potential. For example, for oxygen adsorption on a sputtered Ag(0 0 1) surface a shift of the Ag–O vibration to larger loss energies with increasing C has been observed [6]. Thus, the interaction potential becomes stiffer above occupied defect sites, i.e., a less efficient energy transfer should take place. 3.2. Pre-adsorbates Recently a distinct influence of pre-adsorbed oxygen on SðHÞ of CO has been observed experimentally for Ir(1 1 0) [8]. The co-adsorption has been studied below the onset of CO2 formation. For large impact energies S0 decreased significantly with increasing Hpre thereby conserving the general shape of SðHÞ. Again the proposed MCS is well suited to simulate this effect since defects and preadsorbates are conceptionally similar. The only difference with respect to the MCS is that preadsorbates really block at least one adsorption site, thereby leading to a reduction of the saturation coverage, Hsat , of the adsorbate, whereas the overall rules of the precursor dynamics are identical. The results as obtained for Kisliuk, Langmuirian, and adsorbate-assisted adsorption dynamics, respectively, are summarized in Fig. 2A–D. The general shape and trends simulated agree perfectly with the experimental results obtained for CO/O–Ir(1 1 0) (cf. Fig. 3A in [8] with Fig. 2A, B and Fig. 3C in [8] with Fig. 2C). For Kisliuk shapes and in the case of a precursor dynamics which is identical for the adsor-
occu ), a bates and the pre-adsorbates (poccu ¼ ppre simple site blocking is simulated (Fig. 2A), as expected. However, for a precursor dynamics leading due to the pre-adsorbates to a less efficient energy occu transfer (ppre > poccu , Fig. 2B) a decrease in S0 is obtained, whereas the general shape of SðHÞ is conserved, as observed [8]. Again this result is consistent with the different impact scenarios and the related differences in the mass-mismatch. (Note: for CO–O/ Ir(1 1 0), c ¼ mðCOÞ=mðCOÞ > c ¼ mðCOÞ=mðOÞ is obeyed.) In cases where Langmuirian like dynamics (Fig. 2C) or adsorbate-assisted adsorption dynamics (Fig. 2D) has been chosen as the starting point of occu the MCS, even for identical (poccu ¼ ppre ) precursor dynamics, SðHÞ decreases or increases, respectively, in a scaled manner (as compared to SðH; Hpre ¼ 0Þ) with increasing Hpre (as observed [8]). This, at first glance unexpected, result can be explained as discussed above for the effect of defects. For Langmuirian like dynamics (pclean < poccu ) a decrease in S0 is certainly induced by the pre-adsorbates since the fractional part of the surface where the particles can directly adsorb with a probability of 1 pclean is simply reduced and trapping in the precursor state is not that efoccu ficient (i.e., pclean < poccu ¼ ppre ). Additionally, for small differences in the corresponding precursor occu dynamics, poccu ppre , the general shape of SðHÞ will be conserved and the saturation coverage, sat Hsat , reduced according to Hsat Hpre . pre ¼ H occu For adsorbate-assisted adsorption and ppre ¼ occu p (Fig. 2D) the effect of pre-adsorbates on S0 is opposite to the Langmuirian dynamics case (Fig. 2C) since the starting condition is also contrary (i.e., poccu < pclean ). Furthermore, the effect which leads to an increase in S with H, i.e., the condition occu poccu < pclean , will be enhanced for ppre < poccu and occu occu reduced for ppre > p (not shown) similarly with the effect of defects (see above).
4. Summary A simple MCS scheme has been presented which explains qualitatively the experimentally observed defect induced quenching of the adsor-
J. Stephan, U. Burghaus / Surface Science 507–510 (2002) 736–741
bate-assisted adsorption and the effect of preadsorbates on SðHÞ for CO/Cu(1 1 0) and CO/O– Ir(1 1 0).
Acknowledgements Discussions with L. Brehmer and Ch. W€ oll are gratefully acknowledged as well as a critical reading of the paper by B. Hawley and R. Treiling.
References [1] J.T. Yates, J. Vac. Sci. Technol. A 13 (1995) 1359. [2] R. Castell, S. Reiff, W. Drachsel, J.H. Block, Surf. Sci. 377–379 (1997) 770. [3] A. Szabo, M.A. Henderson, J.T. Yates, J. Chem. Phys. 96 (1992) 6191.
741
[4] D. Cappus, C. Xu, D. Ehrlich, B. Dillmann, C.A. Ventrice, K.A. Shamery, H. Kuhlenbeck, H.J. Freund, Chem. Phys. 177 (1993) 533. [5] T. Becker, M. Kunat, C. Boas, U. Burghaus, C. W€ oll, J. Chem. Phys. 113 (2000) 6334. [6] G. Costantini, F.B.d. Mongeot, S. Rusponi, C. Boragno, U. Valbusa, L. Vattuone, U. Burghaus, L. Savio, M. Rocca, J. Chem. Phys. 112 (2000) 6840. [7] M. Kunat, C. Boas, T. Becker, U. Burghaus, C. W€ oll, Surf. Sci. 474 (2001) 114. [8] U. Burghaus, J. Ding, W.H. Weinberg, Surf. Sci. 384 (1997) L869. [9] J.F. Weaver, M. Ikai, A. Carlsson, R.J. Madix, Surf. Sci. 470 (2001) 226. [10] J. Stephan, L. Brehmer, Synth. Met. 109 (2000) 333. [11] P. Zeppenfeld, J. George, M. B€ uchel, R. David, G. Comsa, Surf. Sci. 318 (1994) L1187. [12] L.Q. Jiang, B.E. Koel, J.L. Falconer, Surf. Sci. 273 (1992) 273. [13] M. Bowker, D.A. King, J. Chem. Soc. Faraday Trans. 75 (1979) 2100. [14] U. Burghaus, Surf. Rev. Lett. 8 (2001) 353.