Langmuir 1993,9, 1290-1298
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Surface Reconstruction and Thermal Dmrption: The Missing-Row Model for CO/Pt( 110) A. V. Myshlyavtsevt and V. P. Zhdanov*JJ Institute of Catalysis, Novosibirsk 63oo9o, Russia, and Department of Applied Physics, Chalmers University of Technology, S-412 96 GBteborg, Sweden Received October 14,1992. In Final Form: January 22,1993 A lattice-gas model with anisotropiclateral interactionsis used to describeformationof the missing-row structure of the clean Pt(ll0) surface and destruction of this structureduring CO adsorption at coverages above a quarter of a monolayer, and to interpret the special features of thermal desorption spectra of the COlPt(ll0) system. 1. Introduction
Rate processes in adsorbed overlayersat finite coverages represent an example of a nontrivialphenomenon in manybody dynamical systems. To simulate these processes, the latticegas models are customarily In the framework of these models, the surface lattice is usually assumed to be rigid and to possess a well-defined twodimensional periodicity closely resembling the atomic ordering in the bulk. In many instances, however, the adsorbate-inducedchanges in the surface are rather strong. In particular, the adsorbate-induced buckling and lateral shifts of substrate atoms are now found even for closepacked surfaces.3 Stronger changes in the arrangement of substrate atom are well knownto accompanyadsorption on many more open surfaces.4~5Thus, it is of interest to analyze the effect of adsorbate-induced changes in the surface on rate processes in adsorbed overlayers. The studies in this field are just beginning. In particular, Inaoka and Yoshimori*have simulatedthermal desorption spectra for the H/W(001) system employing a phenomenological model of surface reconstruction. A similar model has been used to study the effect of surface reconstructionon diffusion of adsorbed particle^.^ Various models have been employed to analyze the influence of adsorbate-inducedchanges in the surface on the apparent Arrhenius parameters for CO desorptionfrom Pt(001) and hydrogen desorption from W(001).8 The effect of adsorbate-induced missing-rowsurface reconstruction on thermaldeaorption of hydrogen from Cu(ll0)and oxygenfrom Ag(ll0) has recently been studied in the framework of the lattice-gas modeleg Ertl et al. have simulated kinetic oscillations and chemical waves in CO oxidation on Pt-
* To whom correspondence should be addressed a t the Chalmers University of Technology. + Institute of Catalysis. t Chalmers University of Technology. (1) Zhdanov, V. P. Elementary Physicochemical Processes on Solid Surfaces; Plenum: New York, 1991. (2) Lombardo, S. J.; Bell, A. T. Surf. Sci. Rep. 1991, 13, 1. (3) Lindros, M.; Pfnur, H.; Held, G.; Menzel, D. Surf. Sci. 1989,222, 451. (4) Estrup, P.
J. In Chemistry and Physics of Solid Surfaces. V; Vanselow, R., Howe, R., Eds.; Springer: Berlin, 1984. Willis, R. F. In Dynamical Phenomenu at Surfaces,Interfacesand Superluttices;Nizzoli, F., Rieder, K. H., Willis, R. F., Eds.; Springer: Berlin, 1985. Somorjai, G. A.; Van Hove, M. A. Prog. Surf. Sci. 1989,30,201. ( 5 ) Wintterlin, J.; Behm, R. J. In Scanning Tunneling Microscopy; Gdntherodt, H. J., Wiesendanger, R., Eds.; Springer: Berlin, 1991; Vol. 1. (6) Inaoka, T.; Yoshimori, A. Surf. Sci. 1985, 149, 241. (7) Zhdanov, V. P. Langmuir 1989, 5, 1044. (8)Zhdanov, V. P. Surf. Sci. Rep. 1991, 12, 183. (9) Zhdanov, V. P. Surf. Sci. 1992, 277, 155.
(001)lO and Pt(1lO)lltaking into account surface reconstruction. In the latter case,loJ1surface reconstruction however has not been analyzed in detail. For example, CO oxidation on the Pt(ll0) surface is accompanied by the (1x2) * (1x1) phase transition. To describe this phenomenon, Ertl et al." introduce the parameter w defined as the fraction of the surface covered by the (1x1) phase. The clean surface is in the (1x2)missing-rowstate (w = 0). CO adsorption is known5 to lift a missing-row reconstruction. The model proposed by Ertl et al.ll just postulated that at equilibrium the Pt(ll0) surface is in the (1x2) state at Bco < 0.2 (Le., w = 0 in this region), in the (1x1)state (w = 1)at Bco > 0.5, and in the mixed state (0 < w < 1)at 0.2 < Bco < 0.5. Then, the time dependence of the parameter w is describedby the first-order equation containingthe rate constant with the Arrheniusparameters vP = 102 s-l and E p = 7 kcal/mol. In the present paper, we considerthe effect of adsorbateinduced changes in the surface on thermal desorption spectra for the CO/Pt(llO) system. This system is of particular interest because as we have already pointed out above CO adsorption lifts a missing-rowreconstruction of the Pt(ll0) surface, and it is rather instructive to discuss how the latter phenomenon manifests itself in thermal desorption. In our study, we attempt to construct a detailed statistical model of the CO/Pt(llO) system, employingthe lattice-gas approximation(section2). This model is used to calculate temperature-programmed desorption (TPD) spectra for the system under consideration (section 3). Our main conclusions are presented in section 4. 2. Model 2.1. Clean Pt(ll0) Surface. The (110) face of facecentered cubic (fcc) metals is well known to have a strong tendency to form the missing-row (1x2)structure (Figure 1). This type of surface reconstruction is spontaneously realized on Pt, Au, and Ir.495 In particular, the missingrow structure of the Pt(ll0) surface has been confirmed by direct observation employinghigh-resolution scanning tunneling micriscopy5J2and also by detailed X-ray diffractionstudies.13J4The clean (110)surfaceof other metals (10) Imbihl, R.; Cox, M. P.; Ertl, G.; Muller, H.; Brenig, W. J. Chem. Phys. 1985, 83, 1578. Mijller, P.; Wetzel, K.; Eiswirth, M.; Ertl, G. J. Chem. Phys. 1985,85,5328. Andrade, R. F. S.; Dewel, G.; Borckmans, P.J. Chem. Phys. 1989, 91,2675. (11) Ertl, G. Science 1991,254,1750. Krischer, K.; Eiswirth, M.; Ertl, G . Surf. Sci. 1991,251/252,900; J. Chem. Phys. 1992,96,9161. Bar, M.; Falcke, M.; Zulicke, C.; Engel, H.; Eiswirth, M.; Ertl, G. Surf. Sci. 1992, 269/270,471. Falcke, M.: Bar,M.; Engel, H.; Eiswirth, M. J. Chem.Phys. 1992,97,4555. (12) Gritsch, T.; Coulman, D.; Behm, R. J.; Ertl, G. Phys. Reu. Lett. 1989, 63, 1086.
0743-746319312409-1290$04.00/0 0 1993 American Chemical Society
Mbeing-Row Model for C O P t ( l l 0 )
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Figure 1. Missing-row model of the fcc ( l l O ) - ( l X Z ) structure. The open circles represent the first layer of metal atoms; the shaded circles show the second and third layers.
00 00 00 00 0 0 0 0 0 0
o 0 0 8 q l ~
ooo 0
00 0 Y t0 0 0 0 0 0
0
0
Figure 2. Arrangement of metal atoms on the Pt(ll0) surface. Large circlesrepresent metal atoms in the topmost layer. Middlesized and small circles show the second and third metal layers. The solid and dashed lines indicate the nearest-neighbor metalmetal interactions ehMand &, respectively.
does not reconstruct. In the latter case, however, the missing-row structure may be often induced by chemisorbed atoms or molecules (see ref 9 and references therein). The formation of the missing-rowstructure on the (110) surface is directly connected with strong anisotropy of lateral interactions between metal atoms located in the top layer. In particular, the simplestorder-disorder model of the spontaneous(1X2) surfacereconstruction,proposed by Campuzano et al.15in order to simulate the low-energy electron desorption (LEED) data for the (110) face of Pt and Au, takes into account attractive lateral interactions (GM< 0) in they direction (along the rows) and repulsive interactions (chM> 0) in the x direction (Figure 2); Le., the Hamiltonian of the surface is represented as
where nij is the occupationnumber of site i,j . The critical temperature for this model is given by the well-known Onsager equation (we set Izg = 1) The energetics of the missing-rowreconstruction of the (110) surface of Ni, Pd, Pt, Cu, Au, and Al has recently been investigated by Chen and Voter16using the embedded atom method. Employingthe surface energiesbefore and after (1x2) reconstruction, u;ri and ulX2 llo !obtained in ref 16, and taking into account that formation of the (110) surface is accompanied by the breaking of six nearestneighbor bonds per site, it is possible to estimate the interactions chMand GMas f~llows:~ 1x1
& 'M
2(u110
- J XllO)sl10 2
(3)
and ~~~
~
(13)Robinson,I. K.;Vlieg,E.;Kern,K.Phys.Reu.Lett. 1989,63,2578. (14)Vlieg, E.;Robinson, I. K.; Kern, K. Surf. Sci. 1990,233,248. (15)Campuzano, J. C.; Lahee, A.M.; Jennings, G. Surf. Sci. 1986,152, 68. (16)Chen, S.P.;Voter, A. F. Surf. Sci. 1991,244,L107.
where S110= a 2 / d 2 is the area per site, and a the cube side. For Pt(llO), we respectively have cLM = 1.5 kcal/ mol and GM= -8.8 kcal/mol, and accordingly T,= 1200 K (eq 2). The latter magnitude of the critical temperature is in good agreement with the experiment13where T,= 1080 f 50 K. The initial process of formation of the missing-row structure on the clean Pt(ll0) surface seems to be connected with steps.15 As a crystal is cut, it has a certain step density, i.e., terraces of the (1x1)surface which are not reconstructed. As the crystal is heated for the first time, the atoms in the terraces spread over the underlying surface, creating the (1x2) structure. The Ising-like model described above should of course be considered only as a first step in simulations because it does not take into account some particular aspects of the (1x2) reconstruction. For example, the X-ray diffraction study13of the Pt(ll0)phase transition shows that above T,steps are created spontaneously and their density diverges with temperature. Because step proliferation has been found to be the central component of the disorder above T,, the (1x2) reconstruction must therefore be classified as a roughening transition rather than a simple two-state 2D Ising transition. In addition, the model does not take into account multiatom interactions which may be of significance on the (110) surface of fcc metals.l7 Despite the comments presented, we will use the model described above in order to study the effect of the missingrow surface reconstruction on TPD spectra. This is justified by several reasons. First, the formation of steps13 seems to be negligible at temperatures where CO desorbs (T 0.2. Then, the formation of the (1x1) islands at 8 < 0.2 is completely ignored in their simulations' of CO oxidation on Pt(ll0). We may present two arguments supporting our statement that the (1x1) islands are not formed at 8 C 0.25. First, the statistics of the system under consideration is rather complex at 0.2 5 8 5 0.5. For this reason, the conclusion23that in the latter region the LEED data indicate formation of the (1x1)islands is not too reliable, and it is not reasonable to extrapolate this conclusion to the low-coverageregion (8 < 0.2). Of course,this argument is not strong and in principle may be ignored. The second (and stronger) argument is connected with thermal desorption data. If the (1x1)islands coexist with the (1x2) phase at 8 C0.25, this should be manifested in the TPD spectra. Using the phenomenologicalapproach, Kreuser and Pay1140 have shown that within the coexistenceregion the zero-order desorption should occur if the sticking coefficients for adsorption to the diluted and condensed phases are equal to each other. In the case of CO adsorption on Pt(llO), the sticking coefficient is in fact independent of coverage at 8 5 0.5. Thus, if the (1x1) islandstake place at 8 C 0.2, the order of desorption should be zero. This conclusion is rather general because it is based on the phenomenological analysis and does not depend on the details of island formation. If one assumes that during adsorptionthe stickingcoefficientis not equal to zero only for the dilute phase, the desorption kinetics should be described by an equation of the following type (see eqs 4.5.6 in ref 1): d8/dt 1- 8 In the latter case, the desorption order is in fact negative. Meanwhile, the real TPD spectra do not show zero or negative order of desorption at 8 C 0.2. In the experiment, the desorption order is certainlypositive (Figure 13).Thus, the TPD data exclude formation of the (1x1) islands at low coverages. The TPD spectra calculated in the framework of our model are in reasonable agreement with the experiment. Comparingthe results of simulations and the experimental data, one should take into account that we have employed only one adjustable parameter (the next-nearest-neighbor adsorbate-adsorbate interaction). But even this parameter in fact has not been varied. We have just used E& = -1 kcal/mol. In addition, one should note that the various lateral interactions employed in the calculationsare rather high compared to temperature, and the results of simulations are sensitive to these interactions. To improve the agreement between the theory and experiment, one N
(40)Kreuzer, H.J.; P a p , S.H.Surf. Sci. 1988,200, L433;1988,205, 153.
Myshlyavtsev and Zhdanov should choose the values of introduced lateral interactions more carefully or take into account effective three-body adsorbateadsorbate interactions (the latter problem is discussed in ref 29). 4. Conclusions In the present work, we have formulated a lattice-gas model of CO adsorption on the Pt(ll0) surface. Our main conclusions are as follows. (1)Formation of the missing-row structure of the clean Pt(ll0) surface is connected with attractive lateral interactions between metal atoms in the [I101 direction and repulsive interactions in the [Ooll direction. The most important qualitative feature of these interactions is their strong anisotropy, Iq&I >> (2) The nearest-neighbor CO-CO repulsion is very strong. At low coverages up to 8 = 0.25, CO molecules can be adsorbed on the topmost metal atoms of the missingrow structure so that the nearest-neighbor adsorbateadsorbate pairs are absent. With increasing coverage, it is more favorable to adsorb part of the CO molecules on metal atoms located in the second and third layers of the missing-row structure or to destroy this structure and recover the (1x1) arrangement of metal atoms. Accordingly, with increasing coverage, the adsorbate/substrate system is in the disordered state containing the (1x2) regions up to 8 0.5. At 8 R 0.5, the (1x1) phase is dominant. The latter phase is not well ordered near 8 = 0.5. The well-ordered state is expected at 8 2 0.75. (3)At 8 5 0.5, the activation energy for CO desorption from Pt(ll0) is high. At 8 0.5, a considerable part of the CO molecules is adsorbed on metal atoms located in the second layer of the substrate. The activation energy for desorption of these molecules is lower than that at low coverages. With increasingcoverage,the nearest-neighbor CO-CO repulsion results in the further strong decrease in the activation energy for desorption at 8 R 0.6. The latter factor yields a dominant contribution to the appearance of the low-temperaturepeak in thermal desorptionspectra of the CO/Pt(llO) system. (4) Our model does not predict formation of the (1x1) islands at 8 5 0.25, and we have presented arguments in favor of this conclusion. Finally, it is reasonable to note that the CO/Pt(110) system is rather complex, and we do not claim that our model is completely correct and should be accepted. In fact, this paper is just an attempt to understand the system under consideration. The results presented reflect our personal vision of the situation in this field. Almost all the problems analyzed in our paper are open for further discussion. Acknowledgment. We thank I. V. Yudanov who took part in the initial stage of this project. The final stage of the project waa done by V.P.Zh. during a visit at Chalmers University of Technology. It is a pleasure to thank the hospitality of Dr. B. Hellsing and Prof. B. Kasemo. Financial support from the Swedish Natural Science Research Council for this visit is also gratefully acknowledged.
