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Mechanism of an adsorbate-induced surface phase transformation: CO on Pt(100)
L553
Surface Science 121 (1982) L553-L560 North-Holland Publishing Company
SURFACE
SCIENCE
LEmERS
MECHANISM OF AN ADSORBATE-INDUCED TRANSFORMATION: CO ON Pt(100) P.A. THIEL *, R.J. BEHM **, P.R. NORTON Institut fiir Physikalische Fed. Rep. of Germany Received
SURFACE
PHASE
*** and G. ERTL
Chemie der Universitiit Miinchen, Sophienstrasse
II, D-8000 Miinchen 2,
10 May 1982
The mechanism of the (5 X 20) - (1 X 1) transition of Pt( 100) during adsorption of CO has been investigated using a fast Video-LEED technique. Analysis of the coverage-dependence of the intensities of several LEED spots on different surfaces leads to a straightforward model. The phase transition occurs by a nucleation-trapping mechanism of the adsorbed CO. The driving force is the difference in stabilities of CO adsorbed on the reconstructed and unreconstructed Pt surfaces.
In this Letter we report new LEED results which elucidate several aspects of the mechanism by which the reconstruction of the Pt(lOO) surface is removed during adsorption of CO. The results indicate that the (5 X 20) + (1 X 1) Pt phase transition occurs by nucleation and trapping of the CO. The role of the heat of adsorption in this mechanism is discussed. Whereas adsorbate-induced phase transitions of the metal surface have been observed for several systems, such as H on W(100) [l] and 0 on Cu(ll0) [2], the mechanism by which adsorption can change the metal surface is not clear. It is not known, for example, whether adsorption and the phase transition occur in one step (which would imply that the energy liberated during adsorption directly drives the transformation) or sequentially. Our data show that, in the case of CO on Pt( loo), adsorption and phase transformation occur in separate steps; furthermore, the heat of adsorption of CO plays an indirect role. It is also not known whether transition occurs via a local process or by sudden change in phase of large surface areas. We propose that the transition occurs on Pt(lOO) by a local nucleation and trapping mechanism which parallels the nucleation phenomena well known in condensed phase transitions [31. * Present address: Sandia National Laboratories, Applied Physics Division 8343, Livermore, California 94550, USA. ** Present address: IBM Research Laboratories K33, 5600 Cottle Road, San Jose, California 95193, USA. *** Present address: Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ lJ0, Canada.
0039-6028/82/0000-0000/$02.75
0 1982 North-Holland
L554
P.A. Thiel et al. / Mechanism
of adsorbate-induced
surface phase trans/ormation
Critical to the experimental results is the ability to simultaneously monitor several ordered phases during the adsorption process. This was achieved using a fast Video-LEED apparatus based on the system of Lang et al. [4], and which is fully described elsewhere [5]. The adsorption and desorption of CO, and the concomitant (5 X 20) ---*(1 x 1) Pt phase transition, have been studied in some detail by other workers. The clean (5 X 20) phase is probably a hexagonal reconstruction with some surface corrugation [6], whereas the (1 X 1) phase is simply an extension of the bulk lattice [7]. The clean (1 X 1) phase reconstructs irreversibly at about 400 K [S], but it is known that the reconstruction is removed at lower temperatures upon adsorption of CO 191 and several other molecules (e.g., ref. [IO]). Norton et al. [8] have studied the CO-induced phase transition using primarify Rutherford backscattering (RBS). They report that this transition is complete at a CO coverage, 8, of 0.5 monolayers. This, together with LEED observations, led them to suggest that the reconstruction is removed during adsorption by areas of CO with a local coverage of 0.5. Crossley and King [l l] have studied the effect of exposing the (5 X 20)-Pt surface to CO using infrared (IR) reflectance-absorption spectroscopy and thermal desorption mass spectrometry (TDS). The IR data showed that the adsorbed CO molecules form islands even at low coverages, which was interpreted as due to attractive CO-CO interactions [ 111. We show directly in this Letter that the adsorbate island formation is a consequence of the Pt phase transition, rather than of lateral adsorbate interactions. Experimental conditions and facilities will be described in detail in a more extensive paper [ 121. In the LEED experiments, the intensities of three kinds of LEED spots were measured: one adsorbate- and two substrate-related spots. Other authors have reported that CO forms a c(2 X 2) pattern during adsorption on the clean (1 X 1) surface [ 131; this pattern is observed also during exposure of CO to the (5 X 20) surface at T< 400 K [8]. We have used the integrated intensity of the (l/2, l/2) spot of this c(2 X 2) pattern, at an incident beam energy of 75 eV, as a measure of the CO in this ordered phase. The possibility that different kinds of ~(2 X 2)-CO form on the (5 X 20) and (1 X 1) surfaces, respectively, was excluded, based on the identical intensity versus voltage curves of the (l/2,1/2) spots taken at two coverages of CO, following exposure to an initially clean (5 X 20)-Pt surface. These coverages corresponded to 60% and 100% removal of the Pt reconstruction, based upon LEED intensities of the (5 X 20) spots. Thus, we conclude that the c(2 X 2) pattern which forms during exposure of CO to an initially clean (5 X 20)-Pt surface always results from ordered CO on (1 X I)-Pt surface areas. Similarly, two substrate beam intensities were measured during exposure of the Pt surfaces to CO. The intensity of a doublet centered at the (2/5,0) position has been used as a measure of the fraction of the total surface area existing as the (5 X 20)-Pt phase. The second substrate beam was at the (1,O)
P.A. Thiel et al. / Mechanism of adsorbate-induced
surface phase transformation
L555
position, where there are intensity contributions from both the (5 X 20) and (1 X 1) phases. Experimental data are shown in fig. 1 for exposure of CO to the (1 X I)-Pt surface, prepared by the method of Broden et al. [ 131, at 380 K. In fig. 1, the initial effect of CO adsorption is most evident in the (1,O) spot intensity, which decreases by 65% of its initial value. The intensity of the (l/2, l/2) spot does not increase until the (1,O) beam has decreased by 40%. Thus it is qualitatively apparent that an appreciable amount of CO absorbs onto the (1 X I)-Pt surface before the ordered c(2 X 2)-CO overlayer forms. Similar data are shown in fig. 2 during exposure of CO to (5 X 20)-Pt at 395 K. The intensity of the (2/5,0) doublet decreases immediately and continuously, due to CO adsorption and removal of the reconstruction, reaching zero at an exposure of CO where the (l/2, l/2) beam reaches its maximum intensity. Since a perfectly ordered c(2 X 2)-CO structure would also have maximum intensity at 0.5 monolayers, this supports the RBS results [8], which showed complete removal of the (5 X 20)-Pt phase at B = 0.5. The c(2 X 2) pattern formed in fig. 2 is relatively diffuse, however, indicating that the long-range order is poor. The (1,0) substrate spot, meanwhile, is almost constant in intensity, in sharp contrast to its behavior in fig. 1. This is due to competing and opposite effects: an increase in intensity from the (5 X 20) + (1 X 1) transformation, based upon relative (1,0) intensities for the two clean surfaces, and a simultaneous decrease from CO adsorption on the same (1 X 1) areas [ 121. The qualitative behavior of the (2/5,0) and (1,O) substrate beams of figs. 1 and 2 can be used to interpret similar experimetal data for a “mixed” surface, shown in fig. 3. The mixed surface was prepared by heating a clean (1 x 1) surface briefly to 470 K in vacuum, which caused partial reconstruction. We estimate that this surface was 30 to 40% reconstructed, based upon the (2/5,0) doublet intensity. This surface, with coexisting areas of clean (5 X 20)-Pt and
CO EXPOSURE,
LANGMUIRS
Fig. I. Integrated intensities of LEED spots during exposure of CO to an initially clean (1 X I)-Pt surface. PC0 = I .8 X IO-’ Tom, T = 378 K.
L55h
P.A. Thiel et al. / Mechanism
CO EXPOSURE, Fig. 2. Integrated intensities surface. PC0 = 5X10-‘Torr,
of adsorbate-induced
surface phase transformation
LANGMUIRS
of LEED spots during exposure T=395 K.
of CO to an initially clean (5 x 20)-Pt
(1 X I)-Pt, was then exposed to CO. Up to a 1.4 L exposure, the intensity of the (1,0) beam decreases sharply, consistent with the effect illustrated in fig. 1, but in contrast to fig. 2. Meanwhile, an intensity reduction of only 15% occurs for the (2/j, 0) doublet, also in contrast to fig. 2, where the same CO exposure caused a 40% intensity decrease. At a CO exposure of 1.4 L, the intensity of the (1,0) spot stops changing and, simultaneously, the (2/5,0) intensity decreases abruptly. The two substrate beams then behave as they would also during CO exposure to the pure (5 X 20) surface, fig. 2. The reconstruction is completely removed at about 4 L co. On the mixed surface of fig. 3, we conclude that CO initially adsorbs primarily on the (1 X 1) patches, with small effect on the (5 X 20) areas. Only
I
I
I
1
I
CO EXPOSURE,
I
/
I
I
I
I
LANGMUIRS
Fig. 3. Integrated intensities of LEED spots during exposure of CO to an initially clean surface with areas of coexisting (1 X I)-Pt and (5 X 20)-Pt. PC0 = 3.4 X IO-’ Tom, T = 3 15 K.
P. A. Thiel et al. / Mechanism of adsorbate-ivrduced surface phase transformation
L557
whm the ~(2 X 2) phase has filled the (1 X I) patches does the CO “attack” the (5 X 20) areas, remoting the reconstruction. The ratio of the initial sticking coefficients of CQ on (1 X l)-Pt and (5 X 2Q)-Pt is approximately 2.5 [l2]; the relative sticking probability of CO on the two phases, however, clearly cannot account for the discontinuity in the experiment of fig. 3 at 1.4 L CO, since CO adsorbs c~~dinuoz&y on both Pt phases. It must be concluded that the (I X 1) areas act as traps for the adsorbed CO. Furthermore, the results indicate that adsorbed CO is sufficiently mobile on the (5 X 20) areas that migration of adsorbed CO across the (5 X 20) areas, into the (1 X 1) traps, up to c(2 X 2) saturation on the latter areas, occurs faster than the (5 X 20) + (1 X 1) transition To convert the CO exposure of figs. 1 and 2 into absolute coverages, TDS data were used in combination with the known saturation coverages IS]. The intensities of the (l/2,1/2) spot for the two surfaces are shown as functions of CO coverage in fig. 4. During exposure to the (I X 1) surface, it is obvious that no formation of c(2 X 2) islands takes place at Tow coverage, as would occur if next-nearest-neighbor fnnn) attractions were dominant at these temperatures. The sudden development of the c(2 X 2) phase at higher coverage (@- 0.4) is typical of phase formation caused by repulsive nearest-neighbor (nn) interactions, based on comparison with the development of the (fi X 2fi)R45’-2CO phase of CO on Pd(100) [14], In the fatter case nn repulsions are known to exist, and slight nnn repulsions may also occur [14, X5]+Further suppurt comes from the TDS data which shaw development of a low-temperature shoulder on
Fig, 6. Integrated intensity of the j‘iJ2, l/2) LEED spot as a function of CO c~vera&e during exposure to the (i X I)-Pt and (5X20)-Pt surfaces, derived from the data of figs. 1 and 2, respectively. The maximum intensitiltts have been normalized to the same value.
LSSX
P.A. Thiel et al. / Mechanism
of adsorbate-induced
surface phase transformation
the single CO TDS peak at 8 = 0.4, for CO adsorbed on (1 x 1)-Pt under conditions similar to fig. 1 [ 121. This shoulder is explained well by the development of CO-CO repulsion at this coverage [ 161, and is not attributable to the population of a second adsorption site [12]. On the other hand, the almost continuous increase of the c(2 x 2) phase which accompanies exposure to the (5 X 20) surface indicates island formation. Island formation under these conditions was explained by Crossley and King as due to attractive pairwise interactions between adsorbed particles [ 111. However, this is inconsistent with the results of the present study, since we have shown that the c(2 X 2) structure is formed on the (1 X 1) surface only, and that repulsive interactions dominate in this case, at these temperatures. Therefore we shall propose another mechanism. The c(2 X 2) islands form at low coverages, not due to pairwise attractions, but because they are trapped on the patches of (1 X I)-Pt which they create during adsorption. This occurs in spite of the aforementioned repulsions. This is similar to the trapping process evident also in the mixed surface experiment of fig. 3; the only difference is in how the (1 X 1)-Pt patches are initially created. During CO exposure to the (5 X 20) surface, CO clusters are formed either by heterogeneous nucleation at defects or by homogeneous nucleation which could occur via statistical density fluctuations. Both types of nucleation mechanisms are well known for bulk nucleation phenomena [3], and may even occur competitively, depending on experimental conditions which determine diffusion lifetimes and nucleation probabilities. During or after formation of the CO cluster, the underlying Pt transforms from the (5 X 20) to the (1 X 1) phase. When this occurs, our data show that the CO is trapped on the (1 X 1) areas because the total energy of the c(2 X 2)-CO adsorbed on the (1 X I)-Pt is lower than that of CO adsorbed on the (5 X 20)-Pt. This difference in energy levels is closely related to the difference in the heats of adsorption of CO on (5 X 20)-Pt and (1 X 1)-Pt, which we have investigated in detail using equilibrium LEED measurements. A detailed description of these measurements will be reported in a subsequent publication [12]. The results are summarized in fig. 5, however, to illustrate the role of the heats of adsorption on the two Pt surfaces in the mechanism of the removal of the reconstruction. The heats of adsorption are 27.5 kcal/mole on (5 X 20)-Pt at low coverage of CO, and 33 kcal/mole on (1 X I)-Pt at 0 = 0.5 [ 121. The difference in energies which causes the trapping phenomenon equals the difference in the heats of adsorption of CO on the two Pt phases, minus the unknown difference in the heats of formation of the two clean Pt(lOO) phases, as shown in fig. 5. In summary, we propose that CO removes the Pt(lOO) reconstruction by a mechanism in which CO adsorption on the (5 X 20) phase is followed by migration, nucleation, rapid (5 X 20) + (1 X 1) conversion of the local substrate area, and trapping of the CO molecules on the resultant (1 X 1) patches. The trapping phenomenon is driven by the difference in the stability of CO on (1 X 1) and (5 X 20)-Pt [12].
P.A. Thiel et al. / Mechanism c\ I/ \ : f I
:
: Clean
(5x20)-Pt
of adsorbate-induced
4,\,
Clean
surface phase transformation
LS59
( 1x 1I-Pt b
1’
A
27.5
33 kcal/mole
kcal/mole
8=0.5 r CO adsorbed
on (5x20)-Pt,
I \ ‘, CO adsorbed
on (1x11-Pt
Fig. 5. Energy diagram for the system CO and Pt(lOO), where the values of the heats of adsorption are derived from ref. [ 121. Measured values of E, are 21.0 kcal/mole [S] and 25.4 kcal/mole [ 171. The details of E’, are not known.
This work was supported by the Deutsche Forschungsgemeinschaft via SFB 128. Two of us (P.A.T. and P.R.N.) acknowledge the support of Alexander von Humboldt Foundation Fellowships.
References [II D.A. King and G. Thomas, Surface Sci. 92 (1980) 210. L-4 R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 112 (1981) 133. The Theory of Transformations in Metals and Alloys (Pergamon, Oxford, [31 J.W. Christiansen, 1975). 141 E. Lang, P. Heilmann, G. Hanke, H. Heinz and K. Mtiller, Appl. Phys. 19 (1979) 287. [51 R.J. Behm and G. Ertl, in preparation. [61 M.A. Van Hove, R.J. Koestner, P.C. Stair, J.P. Biberian, L.L. Kesmodel, I. BartoS and G.A. Somorjai, Surface Sci. 103 (1981) 218. [71 J.A. Davies, T.E. Jackman, D.P. Jackson and P.R. Norton, Surface Sci. 109 (1981) 20. PI P.R. Norton, J.A. Davies, D.K. Creber, C.W. Sitter and T.E. Jackman, Surface Sci. 108 (1981) 205. [91 A.E. Morgan and G.A. Somorjai, J. Chem. Phys. 51 (1969) 3309; Surface Sci. 12 (1968) 405. [lOI M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99; H.P. Bonzel and G. Pirug, Surface Sci. 62 (1977) 45; T.E. Felter and A.T. Hubbard, J. Electroanal. Chem. 100 (1979) 473. 1111 A. Crossley and D.A. King, Surface Sci. 95 (1980) 13 1. 1121 R.J. Behm, P.A. Thiel, P.R. Norton and G. Ertl, in preparation [ 131 G. Brodtn, G. Pirug and H.P. Bonzel, Surface Sci. 72 (1978) 45. [ 141 R.J. Behm, K. Christmann, G. Ertl and M.A. Van Hove, J. Chem. Phys. 73 (1980) 2984,
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P.A. Thiel et al. / Mechanism
of adsorbare-induced
surface phase transformation
[ 151 J.C. Tracy and P.W. Palmberg, J. Chem. Phys. 5 1 (1969) 4852; A.M. Bradshaw and F.M. Hoffmann, Surface Sci. 72 (1978) 513. [16] D.A. King, Surface Sci. 47 (1975) 384; and references therein. [ 171 K. Strauss, E. Lang, H. Heinz and K. Miiller, Verhandlungen der Deutschen Gesellschaft, Miinster (1981).
Physikalischen
Surface Science 121 (1982) L553-L560 North-Holland Publishing Company
SURFACE
SCIENCE
LEmERS
MECHANISM OF AN ADSORBATE-INDUCED TRANSFORMATION: CO ON Pt(100) P.A. THIEL *, R.J. BEHM **, P.R. NORTON Institut fiir Physikalische Fed. Rep. of Germany Received
SURFACE
PHASE
*** and G. ERTL
Chemie der Universitiit Miinchen, Sophienstrasse
II, D-8000 Miinchen 2,
10 May 1982
The mechanism of the (5 X 20) - (1 X 1) transition of Pt( 100) during adsorption of CO has been investigated using a fast Video-LEED technique. Analysis of the coverage-dependence of the intensities of several LEED spots on different surfaces leads to a straightforward model. The phase transition occurs by a nucleation-trapping mechanism of the adsorbed CO. The driving force is the difference in stabilities of CO adsorbed on the reconstructed and unreconstructed Pt surfaces.
In this Letter we report new LEED results which elucidate several aspects of the mechanism by which the reconstruction of the Pt(lOO) surface is removed during adsorption of CO. The results indicate that the (5 X 20) + (1 X 1) Pt phase transition occurs by nucleation and trapping of the CO. The role of the heat of adsorption in this mechanism is discussed. Whereas adsorbate-induced phase transitions of the metal surface have been observed for several systems, such as H on W(100) [l] and 0 on Cu(ll0) [2], the mechanism by which adsorption can change the metal surface is not clear. It is not known, for example, whether adsorption and the phase transition occur in one step (which would imply that the energy liberated during adsorption directly drives the transformation) or sequentially. Our data show that, in the case of CO on Pt( loo), adsorption and phase transformation occur in separate steps; furthermore, the heat of adsorption of CO plays an indirect role. It is also not known whether transition occurs via a local process or by sudden change in phase of large surface areas. We propose that the transition occurs on Pt(lOO) by a local nucleation and trapping mechanism which parallels the nucleation phenomena well known in condensed phase transitions [31. * Present address: Sandia National Laboratories, Applied Physics Division 8343, Livermore, California 94550, USA. ** Present address: IBM Research Laboratories K33, 5600 Cottle Road, San Jose, California 95193, USA. *** Present address: Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ lJ0, Canada.
0039-6028/82/0000-0000/$02.75
0 1982 North-Holland
L554
P.A. Thiel et al. / Mechanism
of adsorbate-induced
surface phase trans/ormation
Critical to the experimental results is the ability to simultaneously monitor several ordered phases during the adsorption process. This was achieved using a fast Video-LEED apparatus based on the system of Lang et al. [4], and which is fully described elsewhere [5]. The adsorption and desorption of CO, and the concomitant (5 X 20) ---*(1 x 1) Pt phase transition, have been studied in some detail by other workers. The clean (5 X 20) phase is probably a hexagonal reconstruction with some surface corrugation [6], whereas the (1 X 1) phase is simply an extension of the bulk lattice [7]. The clean (1 X 1) phase reconstructs irreversibly at about 400 K [S], but it is known that the reconstruction is removed at lower temperatures upon adsorption of CO 191 and several other molecules (e.g., ref. [IO]). Norton et al. [8] have studied the CO-induced phase transition using primarify Rutherford backscattering (RBS). They report that this transition is complete at a CO coverage, 8, of 0.5 monolayers. This, together with LEED observations, led them to suggest that the reconstruction is removed during adsorption by areas of CO with a local coverage of 0.5. Crossley and King [l l] have studied the effect of exposing the (5 X 20)-Pt surface to CO using infrared (IR) reflectance-absorption spectroscopy and thermal desorption mass spectrometry (TDS). The IR data showed that the adsorbed CO molecules form islands even at low coverages, which was interpreted as due to attractive CO-CO interactions [ 111. We show directly in this Letter that the adsorbate island formation is a consequence of the Pt phase transition, rather than of lateral adsorbate interactions. Experimental conditions and facilities will be described in detail in a more extensive paper [ 121. In the LEED experiments, the intensities of three kinds of LEED spots were measured: one adsorbate- and two substrate-related spots. Other authors have reported that CO forms a c(2 X 2) pattern during adsorption on the clean (1 X 1) surface [ 131; this pattern is observed also during exposure of CO to the (5 X 20) surface at T< 400 K [8]. We have used the integrated intensity of the (l/2, l/2) spot of this c(2 X 2) pattern, at an incident beam energy of 75 eV, as a measure of the CO in this ordered phase. The possibility that different kinds of ~(2 X 2)-CO form on the (5 X 20) and (1 X 1) surfaces, respectively, was excluded, based on the identical intensity versus voltage curves of the (l/2,1/2) spots taken at two coverages of CO, following exposure to an initially clean (5 X 20)-Pt surface. These coverages corresponded to 60% and 100% removal of the Pt reconstruction, based upon LEED intensities of the (5 X 20) spots. Thus, we conclude that the c(2 X 2) pattern which forms during exposure of CO to an initially clean (5 X 20)-Pt surface always results from ordered CO on (1 X I)-Pt surface areas. Similarly, two substrate beam intensities were measured during exposure of the Pt surfaces to CO. The intensity of a doublet centered at the (2/5,0) position has been used as a measure of the fraction of the total surface area existing as the (5 X 20)-Pt phase. The second substrate beam was at the (1,O)
P.A. Thiel et al. / Mechanism of adsorbate-induced
surface phase transformation
L555
position, where there are intensity contributions from both the (5 X 20) and (1 X 1) phases. Experimental data are shown in fig. 1 for exposure of CO to the (1 X I)-Pt surface, prepared by the method of Broden et al. [ 131, at 380 K. In fig. 1, the initial effect of CO adsorption is most evident in the (1,O) spot intensity, which decreases by 65% of its initial value. The intensity of the (l/2, l/2) spot does not increase until the (1,O) beam has decreased by 40%. Thus it is qualitatively apparent that an appreciable amount of CO absorbs onto the (1 X I)-Pt surface before the ordered c(2 X 2)-CO overlayer forms. Similar data are shown in fig. 2 during exposure of CO to (5 X 20)-Pt at 395 K. The intensity of the (2/5,0) doublet decreases immediately and continuously, due to CO adsorption and removal of the reconstruction, reaching zero at an exposure of CO where the (l/2, l/2) beam reaches its maximum intensity. Since a perfectly ordered c(2 X 2)-CO structure would also have maximum intensity at 0.5 monolayers, this supports the RBS results [8], which showed complete removal of the (5 X 20)-Pt phase at B = 0.5. The c(2 X 2) pattern formed in fig. 2 is relatively diffuse, however, indicating that the long-range order is poor. The (1,0) substrate spot, meanwhile, is almost constant in intensity, in sharp contrast to its behavior in fig. 1. This is due to competing and opposite effects: an increase in intensity from the (5 X 20) + (1 X 1) transformation, based upon relative (1,0) intensities for the two clean surfaces, and a simultaneous decrease from CO adsorption on the same (1 X 1) areas [ 121. The qualitative behavior of the (2/5,0) and (1,O) substrate beams of figs. 1 and 2 can be used to interpret similar experimetal data for a “mixed” surface, shown in fig. 3. The mixed surface was prepared by heating a clean (1 x 1) surface briefly to 470 K in vacuum, which caused partial reconstruction. We estimate that this surface was 30 to 40% reconstructed, based upon the (2/5,0) doublet intensity. This surface, with coexisting areas of clean (5 X 20)-Pt and
CO EXPOSURE,
LANGMUIRS
Fig. I. Integrated intensities of LEED spots during exposure of CO to an initially clean (1 X I)-Pt surface. PC0 = I .8 X IO-’ Tom, T = 378 K.
L55h
P.A. Thiel et al. / Mechanism
CO EXPOSURE, Fig. 2. Integrated intensities surface. PC0 = 5X10-‘Torr,
of adsorbate-induced
surface phase transformation
LANGMUIRS
of LEED spots during exposure T=395 K.
of CO to an initially clean (5 x 20)-Pt
(1 X I)-Pt, was then exposed to CO. Up to a 1.4 L exposure, the intensity of the (1,0) beam decreases sharply, consistent with the effect illustrated in fig. 1, but in contrast to fig. 2. Meanwhile, an intensity reduction of only 15% occurs for the (2/j, 0) doublet, also in contrast to fig. 2, where the same CO exposure caused a 40% intensity decrease. At a CO exposure of 1.4 L, the intensity of the (1,0) spot stops changing and, simultaneously, the (2/5,0) intensity decreases abruptly. The two substrate beams then behave as they would also during CO exposure to the pure (5 X 20) surface, fig. 2. The reconstruction is completely removed at about 4 L co. On the mixed surface of fig. 3, we conclude that CO initially adsorbs primarily on the (1 X 1) patches, with small effect on the (5 X 20) areas. Only
I
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1
I
CO EXPOSURE,
I
/
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LANGMUIRS
Fig. 3. Integrated intensities of LEED spots during exposure of CO to an initially clean surface with areas of coexisting (1 X I)-Pt and (5 X 20)-Pt. PC0 = 3.4 X IO-’ Tom, T = 3 15 K.
P. A. Thiel et al. / Mechanism of adsorbate-ivrduced surface phase transformation
L557
whm the ~(2 X 2) phase has filled the (1 X I) patches does the CO “attack” the (5 X 20) areas, remoting the reconstruction. The ratio of the initial sticking coefficients of CQ on (1 X l)-Pt and (5 X 2Q)-Pt is approximately 2.5 [l2]; the relative sticking probability of CO on the two phases, however, clearly cannot account for the discontinuity in the experiment of fig. 3 at 1.4 L CO, since CO adsorbs c~~dinuoz&y on both Pt phases. It must be concluded that the (I X 1) areas act as traps for the adsorbed CO. Furthermore, the results indicate that adsorbed CO is sufficiently mobile on the (5 X 20) areas that migration of adsorbed CO across the (5 X 20) areas, into the (1 X 1) traps, up to c(2 X 2) saturation on the latter areas, occurs faster than the (5 X 20) + (1 X 1) transition To convert the CO exposure of figs. 1 and 2 into absolute coverages, TDS data were used in combination with the known saturation coverages IS]. The intensities of the (l/2,1/2) spot for the two surfaces are shown as functions of CO coverage in fig. 4. During exposure to the (I X 1) surface, it is obvious that no formation of c(2 X 2) islands takes place at Tow coverage, as would occur if next-nearest-neighbor fnnn) attractions were dominant at these temperatures. The sudden development of the c(2 X 2) phase at higher coverage (@- 0.4) is typical of phase formation caused by repulsive nearest-neighbor (nn) interactions, based on comparison with the development of the (fi X 2fi)R45’-2CO phase of CO on Pd(100) [14], In the fatter case nn repulsions are known to exist, and slight nnn repulsions may also occur [14, X5]+Further suppurt comes from the TDS data which shaw development of a low-temperature shoulder on
Fig, 6. Integrated intensity of the j‘iJ2, l/2) LEED spot as a function of CO c~vera&e during exposure to the (i X I)-Pt and (5X20)-Pt surfaces, derived from the data of figs. 1 and 2, respectively. The maximum intensitiltts have been normalized to the same value.
LSSX
P.A. Thiel et al. / Mechanism
of adsorbate-induced
surface phase transformation
the single CO TDS peak at 8 = 0.4, for CO adsorbed on (1 x 1)-Pt under conditions similar to fig. 1 [ 121. This shoulder is explained well by the development of CO-CO repulsion at this coverage [ 161, and is not attributable to the population of a second adsorption site [12]. On the other hand, the almost continuous increase of the c(2 x 2) phase which accompanies exposure to the (5 X 20) surface indicates island formation. Island formation under these conditions was explained by Crossley and King as due to attractive pairwise interactions between adsorbed particles [ 111. However, this is inconsistent with the results of the present study, since we have shown that the c(2 X 2) structure is formed on the (1 X 1) surface only, and that repulsive interactions dominate in this case, at these temperatures. Therefore we shall propose another mechanism. The c(2 X 2) islands form at low coverages, not due to pairwise attractions, but because they are trapped on the patches of (1 X I)-Pt which they create during adsorption. This occurs in spite of the aforementioned repulsions. This is similar to the trapping process evident also in the mixed surface experiment of fig. 3; the only difference is in how the (1 X 1)-Pt patches are initially created. During CO exposure to the (5 X 20) surface, CO clusters are formed either by heterogeneous nucleation at defects or by homogeneous nucleation which could occur via statistical density fluctuations. Both types of nucleation mechanisms are well known for bulk nucleation phenomena [3], and may even occur competitively, depending on experimental conditions which determine diffusion lifetimes and nucleation probabilities. During or after formation of the CO cluster, the underlying Pt transforms from the (5 X 20) to the (1 X 1) phase. When this occurs, our data show that the CO is trapped on the (1 X 1) areas because the total energy of the c(2 X 2)-CO adsorbed on the (1 X I)-Pt is lower than that of CO adsorbed on the (5 X 20)-Pt. This difference in energy levels is closely related to the difference in the heats of adsorption of CO on (5 X 20)-Pt and (1 X 1)-Pt, which we have investigated in detail using equilibrium LEED measurements. A detailed description of these measurements will be reported in a subsequent publication [12]. The results are summarized in fig. 5, however, to illustrate the role of the heats of adsorption on the two Pt surfaces in the mechanism of the removal of the reconstruction. The heats of adsorption are 27.5 kcal/mole on (5 X 20)-Pt at low coverage of CO, and 33 kcal/mole on (1 X I)-Pt at 0 = 0.5 [ 121. The difference in energies which causes the trapping phenomenon equals the difference in the heats of adsorption of CO on the two Pt phases, minus the unknown difference in the heats of formation of the two clean Pt(lOO) phases, as shown in fig. 5. In summary, we propose that CO removes the Pt(lOO) reconstruction by a mechanism in which CO adsorption on the (5 X 20) phase is followed by migration, nucleation, rapid (5 X 20) + (1 X 1) conversion of the local substrate area, and trapping of the CO molecules on the resultant (1 X 1) patches. The trapping phenomenon is driven by the difference in the stability of CO on (1 X 1) and (5 X 20)-Pt [12].
P.A. Thiel et al. / Mechanism c\ I/ \ : f I
:
: Clean
(5x20)-Pt
of adsorbate-induced
4,\,
Clean
surface phase transformation
LS59
( 1x 1I-Pt b
1’
A
27.5
33 kcal/mole
kcal/mole
8=0.5 r CO adsorbed
on (5x20)-Pt,
I \ ‘, CO adsorbed
on (1x11-Pt
Fig. 5. Energy diagram for the system CO and Pt(lOO), where the values of the heats of adsorption are derived from ref. [ 121. Measured values of E, are 21.0 kcal/mole [S] and 25.4 kcal/mole [ 171. The details of E’, are not known.
This work was supported by the Deutsche Forschungsgemeinschaft via SFB 128. Two of us (P.A.T. and P.R.N.) acknowledge the support of Alexander von Humboldt Foundation Fellowships.
References [II D.A. King and G. Thomas, Surface Sci. 92 (1980) 210. L-4 R.P.N. Bronckers and A.G.J. de Wit, Surface Sci. 112 (1981) 133. The Theory of Transformations in Metals and Alloys (Pergamon, Oxford, [31 J.W. Christiansen, 1975). 141 E. Lang, P. Heilmann, G. Hanke, H. Heinz and K. Mtiller, Appl. Phys. 19 (1979) 287. [51 R.J. Behm and G. Ertl, in preparation. [61 M.A. Van Hove, R.J. Koestner, P.C. Stair, J.P. Biberian, L.L. Kesmodel, I. BartoS and G.A. Somorjai, Surface Sci. 103 (1981) 218. [71 J.A. Davies, T.E. Jackman, D.P. Jackson and P.R. Norton, Surface Sci. 109 (1981) 20. PI P.R. Norton, J.A. Davies, D.K. Creber, C.W. Sitter and T.E. Jackman, Surface Sci. 108 (1981) 205. [91 A.E. Morgan and G.A. Somorjai, J. Chem. Phys. 51 (1969) 3309; Surface Sci. 12 (1968) 405. [lOI M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99; H.P. Bonzel and G. Pirug, Surface Sci. 62 (1977) 45; T.E. Felter and A.T. Hubbard, J. Electroanal. Chem. 100 (1979) 473. 1111 A. Crossley and D.A. King, Surface Sci. 95 (1980) 13 1. 1121 R.J. Behm, P.A. Thiel, P.R. Norton and G. Ertl, in preparation [ 131 G. Brodtn, G. Pirug and H.P. Bonzel, Surface Sci. 72 (1978) 45. [ 141 R.J. Behm, K. Christmann, G. Ertl and M.A. Van Hove, J. Chem. Phys. 73 (1980) 2984,
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P.A. Thiel et al. / Mechanism
of adsorbare-induced
surface phase transformation
[ 151 J.C. Tracy and P.W. Palmberg, J. Chem. Phys. 5 1 (1969) 4852; A.M. Bradshaw and F.M. Hoffmann, Surface Sci. 72 (1978) 513. [16] D.A. King, Surface Sci. 47 (1975) 384; and references therein. [ 171 K. Strauss, E. Lang, H. Heinz and K. Miiller, Verhandlungen der Deutschen Gesellschaft, Miinster (1981).
Physikalischen