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Oxygen-induced surface phase transformation of Pd(1 1 1): sticking, adsorption and desorption kinetics
Surface Science 482±485 (2001) 237±242
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Oxygen-induced surface phase transformation of Pd(1 1 1): sticking, adsorption and desorption kinetics Bernhard Kl otzer a,*, Konrad Hayek a, Christoph Konvicka b, Edvin Lundgren b, Peter Varga b a
Institut fur Physikalische Chemie, Universitat Innsbruck, A-6020 Innsbruck, Austria b Institut fur Allgemeine Physik, TU Wien, A-1040 Wien, Austria
Abstract Adsorption and desorption of oxygen on Pd(1 1 1) were studied by high-¯ux molecular beam adsorption, LEED, TDS and scanning tunnelling microscopy (STM) between 300 and 623 K sample temperature for oxygen coverages HO up to 1 ML. While adsorption below HO 0:25 is precursor mediated and proceeds without changes of the Pd substrate, it is activated for HO > 0:25 and induces a massive change of the surface structure. STM reveals the formation of a new surface phase which consists of islands with a local oxygen coverage of 1 ML but less Pd atoms than the bulkterminated (1 1 1) layer. Its formation and decay require activated mass transport of Pd and O atoms over mesoscopic distances. Due to island growth of this phase the oxygen sticking decreases linearly between HO 0:25 and 1 ML. For HO > 0:25 ML the TPD rate maxima are shifted towards higher temperature with increasing initial coverage, indicating autocatalytic desorption kinetics. Desorption occurs preferentially from a dilute chemisorbed phase on Pd(1 1 1) terraces, with the islands of the high oxygen-density phase acting as a reservoir for O. The experimental TPD data can be well described by a simple mathematical model considering phase equilibrium during desorption. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Low index single crystal surfaces; Palladium; Oxygen; Thermal desorption; Models of surface kinetics; Adsorption kinetics; Sticking; Scanning tunneling microscopy
1. Introduction Numerous studies of the Pd(1 1 1)-oxygen system have been performed in wide temperature and pressure ranges [1±16]. At room temperature oxygen adsorption gives rise to a p(2 2) LEED pattern, which corresponds to HO 0:25 ML [1], with O atoms most likely occupying HCP sites [16]. The coverage can be extended further up to * Corresponding author. Tel.: +43-512-507-5071; fax: +43512-507-2925. E-mail address: [email protected] (B. Kl otzer).
>1 ML, using dissociative adsorption of NO2 [10]. A complex, yet hardly understood LEED pattern was reported for high HO [10,11]. Whether this pattern results from chemisorbed oxygen [10] on a reconstructed substrate or a ``pseudo-oxidic'' surface phase [11] is still under scrutiny, and in this work we will refer to the corresponding structure as ``complex''. Recent scanning tunnelling microscope (STM) work and the possibility to react the oxygen and remove this surface phase quantitatively with CO at 423 K provide strong evidence that the oxygen atoms are located within the surface layer [17]. We rather believe that the
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B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
complex phase is similar to the oxide ®lm on Ag(1 1 1) characterized by STM in [18]. In most of the previous work also indications were found that oxygen atoms migrate into the palladium bulk. A recent study gave additional evidence both for the formation of the complex high oxygen-density surface phase in the coverage range 0:25 < HO < 1, and separately for subsurface migration of oxygen at elevated temperatures [15]. In this work we concentrate on the formation and decay of the ``complex'' phase. 2. Experimental The UHV chamber was equipped with a capillary array doser (Galileo Optics, in a set-up similar to [19]) for molecular beam experiments, LEED/ AES, and a dierentially pumped QMS for lineof-sight detection of molecules desorbing from the central part (3 mm diameter spot) of the Pd(1 1 1) face. Calibration of beam ¯uxes was achieved by measurement of the pressure decrease in a dierentially pumped gas dosing system by means of a MKS Baratron absolute pressure transducer [19]. A second QMS monitoring the pressure changes in the main chamber was used to perform King and Wells sticking measurements [20]. Since only about 50% of the gas eusing from the doser hit the sample at the given geometry, the accurate value for this fraction was determined by comparison of the initial sticking probability of CO and O2 with data of from supersonic beam experiments [2,12, 13]. The oxygen uptake was determined by integrating net adsorption rates dHO =dt flux s t over time and by TPD peak integration. In this work beam ¯uxes, rates of adsorption and desorption and coverages always refer to 1 ML as a 1:1 ratio of oxygen atoms to 1:53 1015 Pd surface atoms/cm2 in the (1 1 1) bulk-terminated geometry. The Pd(1 1 1) sample (12 mm diameter 3 mm thick) was oriented within 0.2° of the bulk (1 1 1) plane and cleaned by ¯ashing to 1250 K, sputtering with 700 eV Ar , ¯ashing to 1200 K, exposure to 5 10 7 mbar oxygen during cooling (1200±600 K), and ®nal annealing to 1200 K. Cleanliness was checked by AES and ¯ash desorption of adsorbed oxygen or CO.
In order to establish complete mass balance between adsorption and desorption, a high temperature oxygen treatment (8000 ML oxygen at 973 K) was required to saturate the subsurface oxygen reservoir of the sample prior to desorption [15].
3. Results and discussion Fig. 1 displays the dissociative O2 sticking probability at sample temperatures Ts of 323 and 623 K, respectively, as a function of HO . In the low coverage range 0 < HO < 0:25 the sticking probability was determined by the King and Wells technique, while in the high coverage/low sticking regime 0:25 < HO < 1 it was obtained from TDS peak integration. At HO < 0:25 the sticking is dominated by precursor kinetics [12,13] and in this coverage range the sticking probability at 623 K is considerably lower than at 323 K (Fig. 1). At HO 0:25 a sharp 2 2 LEED pattern was observed. Beyond 0.25 ML the sticking probability becomes very low (<0.01) and the temperature dependence is reversed, i.e. the sticking increases with increasing sample temperature (Fig. 1, right side). This indicates that the formation of the complex phase involves an activated step. In the range 0:25 < HO < 1 LEED shows a superposition of the simple p 2 2 pattern and the complex pattern referred to in [11], due to formation of islands larger than the coherence length of the LEED electrons. At HO 1:0 0:1 ML saturation was reached and only the complex LEED pattern [11] remained. STM image A in Fig. 2 was obtained after exposing the complex structure ``cp'' (formed by dosing 1600 ML oxygen at 623 K) to 25 L CO at 573 K. After this treatment the cp was completely removed, and a Pd(1 1 1) surface with holes of monatomic depth (Fig. 2A) remained. Most likely this eect is caused by a lowered Pd atom density within the cp. Since 31% of the total area is covered by the holes, we conclude that the Pd atom density within the cp is 1:05 1015 cm 2 , 31% less than that of the (1 1 1) bulk truncated surface. Thus mass transport of Pd atoms over mesoscopic distances must be involved in the build-up of the
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
239
Fig. 1. Sticking probability of oxygen on Pd(1 1 1) as a function of oxygen coverage for sample temperatures 323 and 623 K.
cp, explaining its very slow formation at 323 K (cf. Figs. 1 and 3A). STM image B in Fig. 2 shows the sharp phase boundary between the cp (left side) and the low-coverage unreconstructed surface area dilute phase \dp" p 2 2, right side after dosing 270 ML (750 L) oxygen at 573 K. The formation of cp takes place within a few atom distances and requires local density ¯uctuations both of oxygenand Pd atoms. In this respect it resembles the basic idea of nonlinear phase growth kinetics e.g. on Ir(1 0 0) [21]. We are currently testing whether adsorption at temperatures above 623 K is accompanied by a ¯ux-dependent sticking probability, which is expected if the local dp coverage is controlled by a dynamic adsorption±desorption equilibrium. In Fig. 2B the size of the cp islands is well above We conclude that the linear decrease of the 100 A. sticking at 623 K between 0:25 < HO < 1 (dashed line in Fig. 1) is due to linear replacement of dp by cp islands on which the oxygen sticking is unmeasurably low. An attempt to increase HO beyond 1 ML failed. Even after dosing 40 000 ML oxygen at 623 K no substantial changes of the subsequent TPD spectra, as compared to the largest TPD peak in Fig. 3B, were observed. In Fig. 3 the TPD spectra starting between 0 and 0.25 ML initial HO reveal second order desorption
kinetics of O2 , both on the basis of the peak shapes and of the temperature shift of the TPD maxima [8]. Above 0.25 ML a second TPD maximum evolves. At 323 K it grows only slowly (Fig. 3A), but at 623 K it develops into a very sharp, intense desorption feature (Fig. 3B). Surprisingly, this new rate maximum is shifted to higher temperatures with increasing initial coverage, resulting in crossover of the leading edges of the peaks. Similar desorption kinetics were observed on other systems [22]; in this work such behaviour was explained by a transformation between coexisting surface phases with high and low local adsorbate coverage. For modelling our experimental TPD spectra the mathematical model presented in [22] was further simpli®ed. The kinetics are based on the following processes: 2O dp ¢ O2 g desorption=adsorption of oxygen on dp O dp ¢ O cp
transformation dp±cp
1 2
The following assumptions were included in the model: 1. Direct desorption from the cp is much slower than from the dp and can therefore be neglected.
240
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
of the dp and cp surface fractions during adsorption and desorption. Within the cp islands the same structure was always observed. Thus we assume that within the cp islands a constant critical oxygen coverage of 1 ML persists. 4. The local dp coverage is a result of simultaneous supply from the cp and loss to the gas phase. If desorption from the dp (Eq. (1)) is rate determining rather than the phase transformation (Eq. (2) ) at the temperature of desorption, the surface can approach internal equilibrium with respect to process (2). As a result the local oxygen coverage in each phase stays constant as long as both phases coexist on the surface. In this ideal case no assumptions and no parameter input concerning the phase transformation kinetics are required. Increasing the temperature from 573 to 623 K did not markedly enhance the adsorption rate; hence the surface is already close to internal quasiequilibrium around 600 K. Thus we believe this assumption to be even better matched at >650 K, where desorption takes place. Once the total coverage is lower than 0.25 ML, the cp is consumed and from thereon simple second order desorption governs the kinetics. On this basis the rate equations and the massbalance equation can be written: Fig. 2. STM image A: Holes of monoatomic depth produced by CO reduction of the complex phase (25 ML CO at 573 K). In addition a step separating two distinct (1 1 1) terraces was observed. STM image B: Phase boundary observed within a single terrace between complex phase (left) and p 2 2-O (right) after exposure of clean Pd(1 1 1) to 250 ML oxygen at 573 K.
2. The cp can only decay via the dp. Thus the decay of the cp acts as a feedstock for oxygen on the dp. This assumption is supported by STM images taken in dierent stages of desorption which showed the gradual removal of cp surface area. 3. During desorption the local coverage within the cp islands is constant. In the coverage range 0:25 < HO < 1:0 STM showed only a change
dH dp 2 m dpe Edes dp=RT H dp dt local desorption rate from dp
3
dH tot dH dp 1 F dt dt total mean desorption rate
4
Z H tot H initial
dH tot dt dt
total mean coverage F
H tot H dpcrit: H cpcrit: H dpcrit:
5 surface fraction of cp 6
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
Fig. 3. TPD spectra of oxygen from Pd(1 1 1). Series A: 2±2500 ML oxygen dosed onto clean Pd(1 1 1) at Ts 323 K. Series B: identical exposures at Ts 623 K.
while 1 > F > 0 : H dp H dpcrit:
phase transition
7
F 0: H dp H tot second order desorption solely from dp
8
The modelled TPD spectra in Fig. 4 were obtained by stepwise numerical integration (based on the experimental temperature ramp of 4.5 K/s). From Eq. (4) it is clear that the kinetics during the phase transformation are autocatalytic factor 1 F , since the dp area needs at ®rst to be increased in order to increase the total desorption rate. In the meantime the temperature of the sample increases, too. If desorption is started from a situation where almost no dp area is initially available, the autocatalytic decrease of cp area F is, at the same temperature (time), much slower than if the starting situation involves e.g. 30% dp
241
Fig. 4. Series of TPD spectra calculated on the basis of the phase-equilibrium model described in the text. Initial oxygen coverages were varied from 0.05 up to 0.9 ML.
area. This means that the same dp area will become available at a markedly higher temperature, yielding ``explosive'' desorption [22]. The calculated TPD series reproduces the salient features of the experimental data (Fig. 2B), most importantly the peak shift to higher temperature for 0:25 < HO < 1, the crossover of the leading edges of the peaks, and the second order desorption behaviour for 0 < HO < 0:25. We note that no ®tting entered into the calculations: the critical concentrations on dp and cp are taken from our experiments as 0.25 and 1 ML, respectively. The second order preexponential and the coverage-dependent desorption energy on the dp was taken from [8] as m dp 3 1013 s 1 (0.02 cm2 s 1 if the rate is expressed as a change of surface concentration (O atoms cm 2 s 1 )) and Edes dp 53 0:7 H(dp) kcal/mol. The initial HO was varied between 0.05 and 0.9 ML. 4. Summary Both adsorption and desorption kinetics of oxygen on Pd(1 1 1) in the coverage range
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0:25 < HO < 1 are governed by the phase transformation between a low oxygen-coverage unreconstructed surface dilute phase ``dp'' p 2 2 and a complex high oxygen-density phase (``cp''). The formation and decay of this phase involves a massive rearrangement of Pd surface atoms and a change of the surface geometry. Acknowledgements This work was supported by the Joint Research Programme ``Gas±Surface Interactions'' (S8105, S8103) of the Austrian Science Foundation. References [1] H. Conrad, G. Ertl, J. K uppers, E.E. Latta, Surf. Sci. 65 (1977) 245. [2] T. Engel, J. Chem. Phys. 69 (1978) 373. [3] D.L. Weissman, M.L. Shek, W.E. Spicer, Surf. Sci. 92 (1980) L59. [4] P. Legare, L. Hilaire, G. Maire, G. Krill, A. Amamou, Surf. Sci. 107 (1981) 533. [5] D.L. Weissman-Wenocur, M.L. Shek, P.M. Stefan, I. Lindau, W.E. Spicer, Surf. Sci. 127 (1983) 513.
[6] L. Surnev, G. Bliznakov, M. Kiskinova, Surf. Sci. 140 (1984) 249. [7] T. Matsushima, Surf. Sci. 157 (1985) 297. [8] X.C. Guo, A. Homan, J.T. Yates Jr., J. Chem. Phys. 90 (10) (1989) 5787. [9] A. Homan, X.C. Guo, J.T. Yates Jr., J. Chem. Phys. 90 (10) (1989) 5793. [10] B.A. Banse, B.E. Koel, Surf. Sci. 232 (1990) 275. [11] E.H. Voogt, A.J.M. Mens, O.L.J. Gijzeman, J.W. Geus, Surf. Sci. 373 (1997) 210. [12] P. Sj ovall, P. Uvdal, Chem. Phys. Lett. 282 (1998) 355. [13] P. Sj ovall, P. Uvdal, J. Vac. Sci. Technol. A16 (1998) 943. [14] R. Imbihl, J.E. Demuth, Surf. Sci. 173 (1986) 395. [15] F.P. Leisenberger, G. Koller, M. Sock, S. Surnev, M.G. Ramsey, F.P. Netzer, B. Kl otzer, K. Hayek, Surf. Sci. 445 (1999) 380. [16] A. Steltenpohl, N. Memmel, Surf. Sci. 443 (1999) 13. [17] C. Konvicka, E. Lundgren, P. Varga, B.Kl otzer, K. Hayek, in preparation. [18] C.I. Carlisle, D.A. King, M.L. Bocquet, J. Cerda, P. Sautet, Phys. Rev. Lett. 84 (17) (2000) 3899. [19] M.J. Bozack, L. Muehlho, J.N. Russell Jr., W.J. Choyke, J.T. Yates Jr., J. Vac. Sci. Technol. A5 (1987) 1. [20] D.A. King, M.G. Wells, Surf. Sci. 29 (1972) 245. [21] T. Ali, B. Kl otzer, A.V. Walker, D.A. King, J. Chem. Phys. 109 (1998) 10996. [22] J.F. Skelly, T. Bertrams, A.W. Munz, M.J. Murphy, A. Hodgson, Surf. Sci. 415 (1998) 48.
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Oxygen-induced surface phase transformation of Pd(1 1 1): sticking, adsorption and desorption kinetics Bernhard Kl otzer a,*, Konrad Hayek a, Christoph Konvicka b, Edvin Lundgren b, Peter Varga b a
Institut fur Physikalische Chemie, Universitat Innsbruck, A-6020 Innsbruck, Austria b Institut fur Allgemeine Physik, TU Wien, A-1040 Wien, Austria
Abstract Adsorption and desorption of oxygen on Pd(1 1 1) were studied by high-¯ux molecular beam adsorption, LEED, TDS and scanning tunnelling microscopy (STM) between 300 and 623 K sample temperature for oxygen coverages HO up to 1 ML. While adsorption below HO 0:25 is precursor mediated and proceeds without changes of the Pd substrate, it is activated for HO > 0:25 and induces a massive change of the surface structure. STM reveals the formation of a new surface phase which consists of islands with a local oxygen coverage of 1 ML but less Pd atoms than the bulkterminated (1 1 1) layer. Its formation and decay require activated mass transport of Pd and O atoms over mesoscopic distances. Due to island growth of this phase the oxygen sticking decreases linearly between HO 0:25 and 1 ML. For HO > 0:25 ML the TPD rate maxima are shifted towards higher temperature with increasing initial coverage, indicating autocatalytic desorption kinetics. Desorption occurs preferentially from a dilute chemisorbed phase on Pd(1 1 1) terraces, with the islands of the high oxygen-density phase acting as a reservoir for O. The experimental TPD data can be well described by a simple mathematical model considering phase equilibrium during desorption. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Low index single crystal surfaces; Palladium; Oxygen; Thermal desorption; Models of surface kinetics; Adsorption kinetics; Sticking; Scanning tunneling microscopy
1. Introduction Numerous studies of the Pd(1 1 1)-oxygen system have been performed in wide temperature and pressure ranges [1±16]. At room temperature oxygen adsorption gives rise to a p(2 2) LEED pattern, which corresponds to HO 0:25 ML [1], with O atoms most likely occupying HCP sites [16]. The coverage can be extended further up to * Corresponding author. Tel.: +43-512-507-5071; fax: +43512-507-2925. E-mail address: [email protected] (B. Kl otzer).
>1 ML, using dissociative adsorption of NO2 [10]. A complex, yet hardly understood LEED pattern was reported for high HO [10,11]. Whether this pattern results from chemisorbed oxygen [10] on a reconstructed substrate or a ``pseudo-oxidic'' surface phase [11] is still under scrutiny, and in this work we will refer to the corresponding structure as ``complex''. Recent scanning tunnelling microscope (STM) work and the possibility to react the oxygen and remove this surface phase quantitatively with CO at 423 K provide strong evidence that the oxygen atoms are located within the surface layer [17]. We rather believe that the
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 7 5 0 - 6
238
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
complex phase is similar to the oxide ®lm on Ag(1 1 1) characterized by STM in [18]. In most of the previous work also indications were found that oxygen atoms migrate into the palladium bulk. A recent study gave additional evidence both for the formation of the complex high oxygen-density surface phase in the coverage range 0:25 < HO < 1, and separately for subsurface migration of oxygen at elevated temperatures [15]. In this work we concentrate on the formation and decay of the ``complex'' phase. 2. Experimental The UHV chamber was equipped with a capillary array doser (Galileo Optics, in a set-up similar to [19]) for molecular beam experiments, LEED/ AES, and a dierentially pumped QMS for lineof-sight detection of molecules desorbing from the central part (3 mm diameter spot) of the Pd(1 1 1) face. Calibration of beam ¯uxes was achieved by measurement of the pressure decrease in a dierentially pumped gas dosing system by means of a MKS Baratron absolute pressure transducer [19]. A second QMS monitoring the pressure changes in the main chamber was used to perform King and Wells sticking measurements [20]. Since only about 50% of the gas eusing from the doser hit the sample at the given geometry, the accurate value for this fraction was determined by comparison of the initial sticking probability of CO and O2 with data of from supersonic beam experiments [2,12, 13]. The oxygen uptake was determined by integrating net adsorption rates dHO =dt flux s t over time and by TPD peak integration. In this work beam ¯uxes, rates of adsorption and desorption and coverages always refer to 1 ML as a 1:1 ratio of oxygen atoms to 1:53 1015 Pd surface atoms/cm2 in the (1 1 1) bulk-terminated geometry. The Pd(1 1 1) sample (12 mm diameter 3 mm thick) was oriented within 0.2° of the bulk (1 1 1) plane and cleaned by ¯ashing to 1250 K, sputtering with 700 eV Ar , ¯ashing to 1200 K, exposure to 5 10 7 mbar oxygen during cooling (1200±600 K), and ®nal annealing to 1200 K. Cleanliness was checked by AES and ¯ash desorption of adsorbed oxygen or CO.
In order to establish complete mass balance between adsorption and desorption, a high temperature oxygen treatment (8000 ML oxygen at 973 K) was required to saturate the subsurface oxygen reservoir of the sample prior to desorption [15].
3. Results and discussion Fig. 1 displays the dissociative O2 sticking probability at sample temperatures Ts of 323 and 623 K, respectively, as a function of HO . In the low coverage range 0 < HO < 0:25 the sticking probability was determined by the King and Wells technique, while in the high coverage/low sticking regime 0:25 < HO < 1 it was obtained from TDS peak integration. At HO < 0:25 the sticking is dominated by precursor kinetics [12,13] and in this coverage range the sticking probability at 623 K is considerably lower than at 323 K (Fig. 1). At HO 0:25 a sharp 2 2 LEED pattern was observed. Beyond 0.25 ML the sticking probability becomes very low (<0.01) and the temperature dependence is reversed, i.e. the sticking increases with increasing sample temperature (Fig. 1, right side). This indicates that the formation of the complex phase involves an activated step. In the range 0:25 < HO < 1 LEED shows a superposition of the simple p 2 2 pattern and the complex pattern referred to in [11], due to formation of islands larger than the coherence length of the LEED electrons. At HO 1:0 0:1 ML saturation was reached and only the complex LEED pattern [11] remained. STM image A in Fig. 2 was obtained after exposing the complex structure ``cp'' (formed by dosing 1600 ML oxygen at 623 K) to 25 L CO at 573 K. After this treatment the cp was completely removed, and a Pd(1 1 1) surface with holes of monatomic depth (Fig. 2A) remained. Most likely this eect is caused by a lowered Pd atom density within the cp. Since 31% of the total area is covered by the holes, we conclude that the Pd atom density within the cp is 1:05 1015 cm 2 , 31% less than that of the (1 1 1) bulk truncated surface. Thus mass transport of Pd atoms over mesoscopic distances must be involved in the build-up of the
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
239
Fig. 1. Sticking probability of oxygen on Pd(1 1 1) as a function of oxygen coverage for sample temperatures 323 and 623 K.
cp, explaining its very slow formation at 323 K (cf. Figs. 1 and 3A). STM image B in Fig. 2 shows the sharp phase boundary between the cp (left side) and the low-coverage unreconstructed surface area dilute phase \dp" p 2 2, right side after dosing 270 ML (750 L) oxygen at 573 K. The formation of cp takes place within a few atom distances and requires local density ¯uctuations both of oxygenand Pd atoms. In this respect it resembles the basic idea of nonlinear phase growth kinetics e.g. on Ir(1 0 0) [21]. We are currently testing whether adsorption at temperatures above 623 K is accompanied by a ¯ux-dependent sticking probability, which is expected if the local dp coverage is controlled by a dynamic adsorption±desorption equilibrium. In Fig. 2B the size of the cp islands is well above We conclude that the linear decrease of the 100 A. sticking at 623 K between 0:25 < HO < 1 (dashed line in Fig. 1) is due to linear replacement of dp by cp islands on which the oxygen sticking is unmeasurably low. An attempt to increase HO beyond 1 ML failed. Even after dosing 40 000 ML oxygen at 623 K no substantial changes of the subsequent TPD spectra, as compared to the largest TPD peak in Fig. 3B, were observed. In Fig. 3 the TPD spectra starting between 0 and 0.25 ML initial HO reveal second order desorption
kinetics of O2 , both on the basis of the peak shapes and of the temperature shift of the TPD maxima [8]. Above 0.25 ML a second TPD maximum evolves. At 323 K it grows only slowly (Fig. 3A), but at 623 K it develops into a very sharp, intense desorption feature (Fig. 3B). Surprisingly, this new rate maximum is shifted to higher temperatures with increasing initial coverage, resulting in crossover of the leading edges of the peaks. Similar desorption kinetics were observed on other systems [22]; in this work such behaviour was explained by a transformation between coexisting surface phases with high and low local adsorbate coverage. For modelling our experimental TPD spectra the mathematical model presented in [22] was further simpli®ed. The kinetics are based on the following processes: 2O dp ¢ O2 g desorption=adsorption of oxygen on dp O dp ¢ O cp
transformation dp±cp
1 2
The following assumptions were included in the model: 1. Direct desorption from the cp is much slower than from the dp and can therefore be neglected.
240
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
of the dp and cp surface fractions during adsorption and desorption. Within the cp islands the same structure was always observed. Thus we assume that within the cp islands a constant critical oxygen coverage of 1 ML persists. 4. The local dp coverage is a result of simultaneous supply from the cp and loss to the gas phase. If desorption from the dp (Eq. (1)) is rate determining rather than the phase transformation (Eq. (2) ) at the temperature of desorption, the surface can approach internal equilibrium with respect to process (2). As a result the local oxygen coverage in each phase stays constant as long as both phases coexist on the surface. In this ideal case no assumptions and no parameter input concerning the phase transformation kinetics are required. Increasing the temperature from 573 to 623 K did not markedly enhance the adsorption rate; hence the surface is already close to internal quasiequilibrium around 600 K. Thus we believe this assumption to be even better matched at >650 K, where desorption takes place. Once the total coverage is lower than 0.25 ML, the cp is consumed and from thereon simple second order desorption governs the kinetics. On this basis the rate equations and the massbalance equation can be written: Fig. 2. STM image A: Holes of monoatomic depth produced by CO reduction of the complex phase (25 ML CO at 573 K). In addition a step separating two distinct (1 1 1) terraces was observed. STM image B: Phase boundary observed within a single terrace between complex phase (left) and p 2 2-O (right) after exposure of clean Pd(1 1 1) to 250 ML oxygen at 573 K.
2. The cp can only decay via the dp. Thus the decay of the cp acts as a feedstock for oxygen on the dp. This assumption is supported by STM images taken in dierent stages of desorption which showed the gradual removal of cp surface area. 3. During desorption the local coverage within the cp islands is constant. In the coverage range 0:25 < HO < 1:0 STM showed only a change
dH dp 2 m dpe Edes dp=RT H dp dt local desorption rate from dp
3
dH tot dH dp 1 F dt dt total mean desorption rate
4
Z H tot H initial
dH tot dt dt
total mean coverage F
H tot H dpcrit: H cpcrit: H dpcrit:
5 surface fraction of cp 6
B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
Fig. 3. TPD spectra of oxygen from Pd(1 1 1). Series A: 2±2500 ML oxygen dosed onto clean Pd(1 1 1) at Ts 323 K. Series B: identical exposures at Ts 623 K.
while 1 > F > 0 : H dp H dpcrit:
phase transition
7
F 0: H dp H tot second order desorption solely from dp
8
The modelled TPD spectra in Fig. 4 were obtained by stepwise numerical integration (based on the experimental temperature ramp of 4.5 K/s). From Eq. (4) it is clear that the kinetics during the phase transformation are autocatalytic factor 1 F , since the dp area needs at ®rst to be increased in order to increase the total desorption rate. In the meantime the temperature of the sample increases, too. If desorption is started from a situation where almost no dp area is initially available, the autocatalytic decrease of cp area F is, at the same temperature (time), much slower than if the starting situation involves e.g. 30% dp
241
Fig. 4. Series of TPD spectra calculated on the basis of the phase-equilibrium model described in the text. Initial oxygen coverages were varied from 0.05 up to 0.9 ML.
area. This means that the same dp area will become available at a markedly higher temperature, yielding ``explosive'' desorption [22]. The calculated TPD series reproduces the salient features of the experimental data (Fig. 2B), most importantly the peak shift to higher temperature for 0:25 < HO < 1, the crossover of the leading edges of the peaks, and the second order desorption behaviour for 0 < HO < 0:25. We note that no ®tting entered into the calculations: the critical concentrations on dp and cp are taken from our experiments as 0.25 and 1 ML, respectively. The second order preexponential and the coverage-dependent desorption energy on the dp was taken from [8] as m dp 3 1013 s 1 (0.02 cm2 s 1 if the rate is expressed as a change of surface concentration (O atoms cm 2 s 1 )) and Edes dp 53 0:7 H(dp) kcal/mol. The initial HO was varied between 0.05 and 0.9 ML. 4. Summary Both adsorption and desorption kinetics of oxygen on Pd(1 1 1) in the coverage range
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B. Klotzer et al. / Surface Science 482±485 (2001) 237±242
0:25 < HO < 1 are governed by the phase transformation between a low oxygen-coverage unreconstructed surface dilute phase ``dp'' p 2 2 and a complex high oxygen-density phase (``cp''). The formation and decay of this phase involves a massive rearrangement of Pd surface atoms and a change of the surface geometry. Acknowledgements This work was supported by the Joint Research Programme ``Gas±Surface Interactions'' (S8105, S8103) of the Austrian Science Foundation. References [1] H. Conrad, G. Ertl, J. K uppers, E.E. Latta, Surf. Sci. 65 (1977) 245. [2] T. Engel, J. Chem. Phys. 69 (1978) 373. [3] D.L. Weissman, M.L. Shek, W.E. Spicer, Surf. Sci. 92 (1980) L59. [4] P. Legare, L. Hilaire, G. Maire, G. Krill, A. Amamou, Surf. Sci. 107 (1981) 533. [5] D.L. Weissman-Wenocur, M.L. Shek, P.M. Stefan, I. Lindau, W.E. Spicer, Surf. Sci. 127 (1983) 513.
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