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Chlorine adsorption on Pt(111) and Pt(110)
surface science ELSEVIER
Surface Science 380 (1997) 9-16
Chlorine adsorption on Pt(111) and Pt(110) R. Schennach, E. Bechtold * Institut ffir Physikalische Chemie, Universittit Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria
Received 22 October 1996; accepted for publication 20 December 1996
Abstract The adsorption of chlorine on Pt(ll 1) and Pt(ll0) is studied in an ultrahigh vacuum system using line-of-sight desorption mass spectrometry and LEED. C12 molecules adsorb dissociatively on both surfaces leading to maximum concentrations of 9.0 x 1014 C1 atoms cm -z on P t ( l l l ) and 9.3 x 1014 Cl atoms cm -2 on Pt(ll0). In the case of Pt(lll), the (3 x 3) LEED pattern is stable in a concentration range between 6.8 x 1014 and 8.5 x 1014 C1 atoms cm -2. On the Pt(ll0), surface the adsorption of chlorine lifts the (1 × 2) reconstruction and further chlorine dosage first leads to the formation of the known (2 x 1) structure and subsequently to c(2 x 2), (4 x 1) and (4 x 2) LEED patterns. During heating, desorption is observable between 600 and 1000 K from P t ( l l l ) and 600 and 1070 K from Pt(110). Atomic and molecular desorption occurs from both faces. A detailed balancing consideration suggests that the magnitude of the Cl-to-C12 ratio corresponds to the gas-phase equilibrium ratio at the surface temperature. From maximum initial-concentration adlayers, about 2/9 of the adsorbed chlorine atoms desorb as C12 molecules in the case of Pt(110), and about 1/2 in the case of Pt(lll). Keywords: Adsorption kinetics; Chemisorption; Chlorine; Low energy electron diffraction; Low index single crystal surfaces; Platinum;
Thermal desorption; Thermal desorption spectroscopy
1. Introduction T h e i n t e r a c t i o n of chlorine with p l a t i n u m has a t t r a c t e d interest for p r a c t i c a l reasons with respect to electrochemistry, h e t e r o g e n e o u s catalysis a n d c o r r o s i o n , as well as for f u n d a m e n t a l aspects. Studies with the m o s t i m p o r t a n t l o w - i n d e x faces, P t ( l l l ) , (110) a n d (100), revealed t h a t C12 m o l e cules a d s o r b dissociatively with high efficiency via a p r e c u r s o r state to form o r d e r e d structures [ 1 - 4 ] . T h e initially r e c o n s t r u c t e d clean P t ( 1 1 0 ) a n d (100) s u b s t r a t e structures are c h a n g e d b y the a d s o r p t i o n of chlorine; the (111) face retains the h e x a g o n a l
* Corresponding author. Fax: +43 512 5072925.
symmetry. T h e r m a l d e s o r p t i o n yields elemental chlorine. D e s o r b i n g C1 a t o m s a n d C12 molecules were o b s e r v e d with P t ( 1 0 0 ) I-3], b u t for P t ( l l l ) a n d (110), only d e s o r p t i o n of CI a t o m s is r e p o r t e d [ 1,2]. A n u m b e r of questions emerge from the c o m p a r i son of the findings r e p o r t e d for the v a r i o u s singlecrystal faces. I n the following, chlorine a d s o r p t i o n is studied with P t ( l l l ) a n d (110) surfaces. T h e results are c o m p a r e d with previous e x p e r i m e n t s with P t ( 1 0 0 ) which were p e r f o r m e d u n d e r identical c o n d i t i o n s [ 3 ] . T h e e m p h a s i s is on the d e t e r m i n a tion of the chlorine surface c o n c e n t r a t i o n s a n d the respective L E E D patterns, o n the c o m p o s i t i o n s of the d e s o r b i n g beams, a n d on the d e s o r p t i o n kinetics.
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V, All rights reserved PII S0039-6028 (96) 01593-2
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R. Schennach, E. Bechtold / Surface Science 380 (1997) 9 16
2. Experimental
The ultrahigh vacuum chamber used for the experiments is equipped with an LEED/AES system and a shielded quadrupole mass spectrometer system which enables line-of-sight detection of species desorbing from the central part of the crystal surface [3]. Base pressures lower than 1 × 10 -1° mbar are obtained after bakeout using a turbomolecular pump in combination with an ion getter pump and two liquid-nitrogen cooled titanium getter pumps. The platinum (110) and (111) crystals were mounted on an L-shaped holder in two different planes of the manipulator in order to move each of them separately to positions for argon-ion bombardment, LEED/AES experiments, chlorine dosing and temperature-programmed desorption. Both crystals could be heated separately using electron bombardment or radiation from a tungsten filament controlled by a programmable power supply using chromel-alumel thermocouples spot-welded to the sides of the crystals [5]. Elemental chlorine was produced by decomposition of AgC1 in a solid-state electrochemical cell. The density of C12 in the beam at the location of the crystal surface corresponded to about 2.5 × 10 -8 mbar under the usual working conditions. Absolute dosages cannot be given since the shape of the dosing beam is known only approximately [3,5]. Both crystals were cleaned by alternating cycles of argon-ion bombardment, heating to 1200 K, cooling to room temperature in an atmosphere of 5 × 10 -7 mbar oxygen and subsequent heating to 1200 K. After this procedure, the crystals were clean as judged by AES. The (111) surface showed the ( 1 × 1) LEED pattern of the clean surface. The (110) surface showed the (1 × 2) LEED pattern of the clean, reconstructed surface [6,7]. The surface chlorine concentrations were derived from the intensities of the AES signal at 181 V and from thermal desorption yields. Calibration of the AES signal is obtained with the complete c(2 × 2) C1 adlayer on Pt(100) with a C1/Pt ratio of 0.5 or 6.5 × 1014 C1 atoms cm -2 [3]. Temperature programmed desorption of low initial-concentration adlayers produce only 35 amu signals, which indicates desorption of chlorine atoms and enables the
calibration of the 35 amu signal. Signals at 35 and 70 amu indicate the desorption of C1 atoms and C12 molecules from adlayers with higher initial concentrations (Section 3.2.). In this case, the 35 amu signal results in part from C12 fragmentation in the ion source of the mass spectrometer. In the low-temperature range of the desorption spectrum, the constant ratio of the 35 and 70 amu signals points to the exclusive desorption of C12 molecules under these conditions. This ratio determines the fragmentation factor of C12 in the ion source, which is used to calculate the contribution of C12 fragmentation to the 35 amu signals at higher temperatures. The fragmentation-corrected 35 amu signals (termed the Cl-atom signal in the following) yields the amount of desorbing chlorine atoms, and in combination with the initial concentration, which is known from the AES signal, the amount of desorbing C12 molecules. In the present arrangement the sensitivity of the quadrupole system depends on the velocity of the particles which are to be detected. In the following, constant mean sensitivities are used which are determined from the desorption yields as described above.
3. Results and discussion
3.1. Adsorption and LEED patterns Fig. 1 shows the uptake of chlorine on the platinum (110) and (111) surfaces at room temperature. For both surfaces, the adsorption starts with high and nearly identical sticking coefficients. The wide concentration range up to 7.5 × 1014 C1 atoms cm-2, where the adsorption rate stays constant, points to precursor kinetics [1-3]. On further exposure, the sticking probabilities gradually decline to small values until the uptake curves reach a plateau at concentrations of 9.3× 1014 Clatomscm -2 on the (110) face and 8.5 × 1014 C1 atoms cm -2 on the (111) face. On the (110)face, the maximum concentration of 9.3 × 1014 C1 atoms cm -2 is reached with the platinum crystal at room temperature. In case of the (111) face, higher concentrations than the room-temperature maximum of 8.5x
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
a: chlorine uptake on Pt(1 10) 9 8 7 ~ a t o m s + molecules [] C/atoms + CI m o l e c u l e s
6 5 4
~,3 1
• b: chlorine uptake on Pt(1 1 1)
~ 9 o
~
8
~
7
/
~ e ~5 ~
4
~
3
f
~o
/
Cl atoms + m o l e c u l e s • maximum concentration ~ CI a t o m s
2 1 O~ 0
i
i
i
i
i
5
i
i
J
I
1o exposure
[au]
Fig, 1. Chlorine uptake at room temperature as a function of relative exposures on (a) P t ( l l 0 ) and (h) P t ( l l l ) . The circles correspond to the chlorine concentration derived from the sum of the C1 atom (squares) and the C12 molecule (crosses) desorption yields. The maximum concentration on P t ( l l l ) , indicated by a cross in a circle, is only reached at elevated temperatures (see text for details).
1014 C1 atoms cm -2 are obtained at elevated temperatures. The maximum concentration of 9.0 × 1014 C1 atoms cm -2 (indicated in Fig. 1 by a single point) is obtained at 420 K, or by heating to 510 K and chlorine dosing during cooling to room temperature. The clean (111) face shows the known L E E D pattern of the (1 × 1) structure. With increasing chlorine concentration the background intensity increases until the sharp (3 × 3) structure [2] develops at a concentration of 6.8 x 1014 C1 atoms c m - 2 . During chlorine dosing at 300 K, the structure persists up to the finally attainable concentration of 8.5 x 1014 C1 atoms cm -2 (Fig. 2a). The (3 x 3)
11
structure disappears on heating to 510K, and during cooling the (3 x 3) structure is built up again at about 420 K. Heating and further dosing destroys the (3 × 3) order, leads to an increase in the background intensity (Fig. 2b) and results in a concentration of 9.0 x 1014 Cl atoms cm -2. With the maximum concentration at room temperature, weak (1/3, 1/3) spots appear on a brighter background than at lower concentrations (Fig. 2c). In a similar manner, on Pt(100) the maximum surface concentration of 8.7 x 1014 Cl atoms cm -2 was obtained only at elevated temperatures [3]. The LEED features are tentatively interpreted by chlorine chemisorbed on the largely undisturbed (111) substrate structure. At 8.5 × 1014 C1 atoms cm-2, which is the highest chlorine concentration attainable at 300 K, the (3 x 3) unit cell contains five chlorine atoms. The transformation of the ordered (3 x 3) arrangement to a disordered adlayer in the temperature range near 500 K suggests that the adsorption is not very site-specific. Elevated substrate temperatures enable further chlorine uptake beyond this concentration, apparently because of the increased mobility and density fluctuations in the adlayer. The maximum concentration of 9 × 10 a4 C1 atoms cm -2, which is attainable under the present dosing conditions corresponds to that of a densely packed out-ofregistry layer of C1 atoms with van der Waals radii of 1.8 x 10 -s cm [-8] (8.9 x 1014 C1 atoms cm-2). The clean (110) face shows the (1 x 2) pattern of the reconstructed surface which was interpreted as a missing-row structure [6,7]. With adsorbed chlorine up to a concentration of 2.9 × 1014 C1 atoms cm -2 only the (1 x 2) reconstruction of the substrate can be seen between room temperature and 600 K, where significant chlorine desorption begins. In the concentration range between 2.9 x 1014 and 3.4 x 1014 C1 atoms cm -2, the halforder spots decrease in intensity and the background intensity increases, indicating that the original reconstruction is lifted. With chlorine concentrations between 3.4 × 10 TM and 4.8 x 1014 C1 atoms cm -2, streaky half-order features of a (2 x 1) pattern appear in addition to the integral-order spots (Fig. 2d). At temperatures above 500 K the half-order streaks vanish, and only integral-order spots are observable on a bright
12
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
0
O
d
.....
~
i~¸
Fig. 2. LEED patterns from surfaces with different chlorine concentrations. (a) Pt(lll), (3 x 3), 8.5 x 1014 C1 atoms cm z, 55 eV, 300 K; (b) Pt(lll), (1 x 1), 9.0 x 1014 CI atoms cm z, 55 eV, 400 K; (c) Pt(lll), faint (3 x 3), 9.0 x 10TM C1 atoms cm -2, 55 eV, 300 K; (d) Pt(ll0), streaky (2x 1), 4.0x1014 Clatomscm 2, 75eV, 300K; (e) Pt(ll0), ( l x l ) with diffuse additional intensities, 4.0 x 1014 C1 atoms cm -2, 73 eV, 500 K; (f) Pt(ll0), (2 x 1) and faint c(2 x 2), 6.5 x 1014 Cl atoms cm -2, 75 eV, 300 K; (g) Pt(ll0), streaky c(2 x 2), 6.5 x 1014 C1 atoms cm -2, 75 eV, 450 K; (h) Pt(ll0), (4 x 1), 9.3 x 1014 Cl atoms cm -2, 73 eV, 300 K; (i) Pt(ll0), faint (4 x 2), 9.3 x 10TMCI atoms cm -2, 75 eV, 300 K.
b a c k g r o u n d (Fig. 2e). W h e n the system is c o o l e d to b e l o w 500 K, the p a t t e r n s of the o r d e r e d structures r e a p p e a r . Between about 4.8 x 1014 and 7.2 x 1014 C l a t o m s c m - z (2 x 1) a n d faint c ( 2 x 2 ) p a t t e r n s can be seen s i m u l t a n e o u s l y on the surface (Fig. 2f). A t elevated t e m p e r a t u r e s ( a r o u n d 4 5 0 + 2 0 K ) a n d in a limited c o n c e n t r a t i o n range
a r o u n d 6.5 x 1014 ___0.5 x 1014 C1 a t o m s ~T1-2 o n l y faint (1/2, 1/2) spots of the s t r e a k y c(2 x 2) p a t t e r n (Fig. 2g) are present. A b o v e 500 K, all e x t r a spots d i s a p p e a r a n d only the (1 x 1) p a t t e r n remains. W i t h chlorine c o n c e n t r a t i o n s between 7.2 x 10 TM a n d 9 . 3 x 1 0 1 4 C l a t o m s c m -2, either a ( 4 x l ) p a t t e r n (Fig. 2h) o r a faint ( 4 x 2) p a t t e r n on a b r i g h t b a c k g r o u n d (Fig. 2i) are the d o m i n a t i n g
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R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
structures. Whether (4 x 1) or (4 × 2) structures are formed could not be controlled by experimentally adjustable parameters. Only the (1 x 1) pattern remains at temperatures above 450 K. The LEED observations indicate that rearrangement of the substrate is involved in the formation of the structures at higher chlorine concentrations. Up to chlorine concentrations near to 3 x 1014 Clatomscm -2, the dominating (1 x2) periodicity of the clean, reconstructed surface persists. With higher chlorine concentrations, the LEED pattern changes to the (1 x 1) periodicity. STM investigations on the adsorption of bromine on Pt(ll0) showed ordered, mixed Pt/Br adlayer structures [9] which produced similar LEED patterns to those observed with chlorine. The maximum chlorine concentration obtained on Pt(ll0) (9.3 x 1014 C1 atoms cm -2) is only slightly higher than that on P t ( l l l ) , in spite of the more complex structural aspects.
b: chlorine desorption f r o m P t ( 1 1 0 )
8 7 6
5 td
~ a2
e
/
~ 0 a: c h l o r i n e d e s o r p t i o n f r o m P t ( 1 1 1 ) .. 7
/I ] \ / \
__ __
5
35 ainu signal CI a t o m s C/molecules
/
4
e
;"" '
3
//
3.2. Temperature-programmed desorption Thermal desorption yields C1 atoms and C12 molecules. Desorbing platinum chlorides were not found. Fig. 3 shows TPD spectra of C1 atoms and C12 molecules from the (110) and (111) faces, respectively, for various initial concentrations. The Cl-atom signals in Fig. 3 are corrected for the 35 amu fragment caused by fragmentation of the C12 molecules in the ion source of the quadrupole mass filter. For comparison, one uncorrected 35amu desorption trace is shown in Fig. 3a (dashed line) for the case of desorption from Pt(111).
3.2.1. Desorption from P t ( l l l ) The shapes of the Cl-atom peaks are typical for first-order kinetics, but the temperature at the peak maximum shifts to lower temperatures with increasing initial coverage. For a more detailed consideration, desorption rates for constant temperatures and the corresponding actual surface concentrations were extracted from a series of temperature-programmed peaks which were obtained with different initial concentrations. The resulting isothermal rates are plotted as a function of the surface concentration for 800, 850 and 900 K
O[ 600
700
800
900
1000 T/K
Fig. 3. Thermal desorption peaks of chlorine atoms and chlorine molecules from adlayers with different initial concentrations. The heating rate was 20 K s-1. Chlorine was dosed at room temperature. The Cl-atom peaks are corrected with respect to the contribution from the fragmentation of the C12 molecules in the ion source. The letters indicate the Cl-atom peaks and the C12 molecule peaks obtained from adlayers with identical initial concentrations. (a) P t ( l l l ) . The dashed peak shows the uncorrected 35 amu signal. (b) Pt(ll0).
in Fig. 4b. For simple first-order kinetics, the desorption rates should increase linearly with the concentration. The stronger rate increase points to a decreasing desorption energy with increasing concentration. A similar coverage dependence of the desorption energy was found for chlorine desorption from Pt(100) [-3], and was interpreted in terms of repulsive interactions in the adlayer. From the temperature of the maximum of low initial-concentration peaks, a desorption energy of 55 kcalmo1-1 is derived using the Redhead formula [10] (based on a pre-exponential factor of 1 x 1013 s-l).
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R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
4
a: i s o t h e r m a l data on Pt(1 10) o 800 K D 850 K + 900 K
2
E
2
maximum temperature as those from P t ( l l l ) . Isothermal data obtained from temperatureprogrammed desorption peaks are shown in Fig. 4a. As in the case of the (111) face, on the (110) face the plots also deviate strongly from linearity. The Redhead formula [10] applied to low initial-concentration peaks yields a desorption energy of 58 kcal mol-1. D e s o r b i n g C12 molecules are observable in the same temperature range as on the P t ( l l l ) face. With increasing concentration, no extra desorption feature appears on the leading edge, in contrast to the P t ( l l l ) face. The initially saturated adlayer during desorption with decreasing chlorine concentration passes through a variety of structures, which includes rearrangements in the substrate. These structural changes and interactions between the adsorbed C1 atoms are expected to influence the desorption kinetics.
1
O, 0
1
2 3 4 c o n c e n t r a t i o n [1074 Cl a t o m s / c r n 2 ]
Fig. 4. Isothermal desorption rates derived from temperatureprogrammed experiments with different initial concentrations. (a) Pt(ll0) and (b) P t ( l l l ) at 800, 850 and 900 K.
During temperature-programmed experiments, the desorption of C12 molecules becomes measurable with initial concentrations above 3 x 1014 C1 atoms cm -2 (Fig. 1). With the maximum initial concentration, molecular desorption is observable between 600 and 900 K. The small C12 peaks exhibit the nearly symmetric shape typical for second-order kinetics. With higher concentrations, a shoulder on the leading edge of the peak evolves and leads with further increasing initial coverage to a sharp, narrow peak. This shape seems to result from the repulsive interaction between the adsorbed chlorine atoms in the more densely packed adlayer.
3.2.2. Desorption from Pt ( l l O) The Cl-atom peaks from the (110) surface exhibit similar overall concentration dependences of the
3.2.3. Simultaneous desorption of Cl2 molecules and Cl atoms Fig. 1 shows the chlorine molecule and the chlorine atom desorption yields as a function of chlorine exposure. From the (110) face, about 2/9 of the maximum concentration desorb as molecules and from the (111) face about 1/2. On the (110) face the first C12 molecule signal is detected with an initial concentration of about 5.5x 1014Clatomscm -2, and on the (111) face at an initial concentration of about 3x 1014 C1 atoms cm -2. In order to interpret the relevance of desorption of C1 atoms and C12 molecules, desorption from both surfaces is considered for conditions where both rates can be measured simultaneously. In case of the (110) face this is in a temperature range between 650 and 880K and concentrations between 7.9 x 1014 and 5.6 × 10x4 C1 atoms cm -2. For the (111) face, simultaneous desorption is observable between 700 and 880K and concentrations between 5.9 X 1014 and 1.6 × 10TM C1 atoms cm -2. The desorption rates of C1 and C12 are assumed to follow the equations
exp( )
R. Schennach, E. BechtoM / Surface Science 380 (1997) 9-16
and
15
36[ a: Platinum (110)
I
respectively, where n is the actual surface concentration of adsorbed chlorine atoms, and EI~.) and Ezra) are the respective desorption activation energies. Within the error limit of the data
ln(r~ \r2/
=ln (~
RT
(1)
depends linearly on 1/r, indicating negligible dependences of ln(f2tz,~)/f2~r,.)) and 2Elt~)- E2t~)/R on the surface concentration and the temperature. A plot of ln(r2/r2) versus 1/T is shown for both surfaces in Fig. 5 for a temperature range between 670 and 900 K. For the (110) face, the linear fit is not as good as for the (111) face, probably because the C12 molecule rate is much lower than on P t ( l l l ) . If fltT,.)=Vl.n and f2{T,~)=2V2n2, then flt~)/f2¢,)=Vl/2V2. 2 2 The experimental rate data yield for Pt(111) V2/VE=3X 1029-+1 S - 1 particles cm -2 and for Pt(ll0) v2/v2=2xlO28-+ls -1 particles cm -2, which can be rationalized with preexponentials in the ranges of Vl = 1013 S- 1 and v2 = 10 -3 s -1 c m 2 particles -1. The slope of the plot yields 2E1-E2=59_+2kcal for Pt(111) and 2E1 - E 2 = 53 -[-4 kcal for Pt(110). If the activation energies of adsorption for chlorine atoms and chlorine molecules can be neglected, then 2E1- E2 represents the binding energy in gaseous C12 (57.1 kcal [-11]). For further discussion, the desorption rates are tentatively equated to the exchange rates between the chemisorbed adlayer and the gas phase under the condition of established adsorption equilibrium. With the equilibrium pressures at the surface temperature Pcl and Pc12, the rates are rl--
PclSI(n,T)
and 2pcI2S2(n,T)
r2=
X/2g2mokT"
26
o
36 b: Platinum (111) 34
30 28 .26 0 . 0' 0 1 2
0 . '0 0 1 3
O.'00 14 1/7"[I/K1
Fig. 5. Data according to Eq. (1) obtained from temperatureprogrammed experiments in the range of simultaneous C1 atom and C12 molecule desorption (see text for details). (a) Pt(110) and (b) Pt(111) with two different experiments (squares and circles) in both cases.
which results in the ratio s2(.,r)
r2 =2 -
S2(n,T)
r2
1 V
makr=-_ 1%
for the sticking coefficients. The gas-phase dissociation c o n s t a n t K p of C12 molecules, which is introduced for P~za/Po2, is calculated with standard statistical thermodynamical methods [,12] from the atomic and molecular parameters [,8,13] (CI: electronic ground s t a t e : 2P3/2, excited s t a t e : 2p1/2, 0.109eV; C12: r = 1 . 9 8 8 x 1 0 - a c m , ~=561cm-1; Ddiss = 57.1 kcal). In the range of temperatures and surface concentrations covered by the experiments, the ratio is s2(~,r)/s2(n,r)=3+_l for P t ( l l l ) and S2(.,T)/S2(.,T)= 20_ 8 for Pt(110). Under the conditions of desorption, the individual sticking coeffi-
16
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
cients of C1 and C12 are not known. Near 300 K the adsorption of C12 occurs via a precursor, and the sticking probability remains near unity up to high surface concentrations (see Section 3.1.). At the temperatures of the desorption a lower sticking probability is expected, in particular for CI2, in view of the precursor kinetics. Taken together, these considerations suggest that chlorine atoms and molecules desorb approximately according to their equilibrium ratio at the surface temperature, with desorption energies determined largely by the energy difference of adsorbed and gaseous species.
4. Summary In the following, we summarize the new aspects found during the present work on the chlorine adsorption on Pt(111) and Pt(110). M a x i m u m surface concentrations of 9.3 × 1014 C l a t o m s c m -2 for P t ( l l 0 ) and 9.0x1014C1 atoms cm -2 for P t ( l l l ) are determined with a calibration based on 6.5 × 1014 C1 atoms cm -2 for the c ( 2 × 2 ) structure on Pt(100). On Pt(100), the m a x i m u m concentration is 8.7 x 1014 C1 atoms cm -2 [3]. On the (111) surface, the ( 3 × 3 ) structure is formed at concentrations above 6.8×1014 C1 atoms cm-2. The structure is stable over a wide concentration range and up to a temperature of 420 K. The m a x i m u m concentration is reached only at temperatures above 420 K. During cooling the (3 x 3) structure reappears, in the case of the high-concentration adlayer, but with only a low degree of order. On the (110) surface, the initial (1 x 2) reconstruction is lifted at a concentration above 2.9 x 1014 C1 atoms cm -2, and the pre-
viously described [ 1 ] (2 × 1) structure is formed at a concentration of 3.4 × 1014 C1 atoms cm -2. A number of new ordered structures (c(2 x 2), (4 × 1) and ( 4 × 2 ) ) are observed in the concentration range from 4.8x 1014 C l a t o m s c m -2 up to saturation. Atomic and molecular desorption of chlorine occurs from both surfaces, similar to Pt(100) [3]. The order of magnitude of the C1 atom to C12 molecule ratio in the desorbing beam corresponds to the gas-phase equilibrium at the surface temperature. The different desorption temperatures on the two surfaces lead to different fractions of atomic and molecular desorption: from the maximumconcentration adlayers, about 2/9 of the adsorbed chlorine atoms are desorbed as C12 molecules in the case of P t ( l l 0 ) , and about 1/2 in the case of P t ( l l l ) .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
W. Erley, Surf. Sci. 114 (1981) 47. W. Erley, Surf. Sci. 94 (1980) 281. H. Gutleben and E. Bechtold, Surf. Sci. 236 (1990) 313. H.H. Farell, Surf. Sci. 100 (1980) 613. R. Schennach, PhD thesis, Innsbruck, 1995. K. Korte and G. Meyer-Ehmsen,Surf. Sci. 271 (1992) 616. T. Gritsch, D. Coulman, R.J. Behm and G. Ertl, Surf. Sci. 257 (1991) 55. CRC Handbook of Chemistry and Physics, 67th ed., Ed. R.C. Weast (CRC, Boca Raton, 1986/87). E. Bertel, private communications. P. Hanesch, PhD thesis, Bayreuth, 1996; P. Sandl, PhD thesis, Bayreuth, 1993. P.A. Redhead, Vacuum 12 (1962) 203. I. Barin, Thermodynamical Data of Pure Substances (VCH, Weinheim, 1993). See, e.g.N. Davidson, Statistical Mechanics (McGrawHill, New York, 1962). L. Brrnstein, Numerical Data and Functional Relationships in Science and Technology, New Series (Springer, Berlin).
Surface Science 380 (1997) 9-16
Chlorine adsorption on Pt(111) and Pt(110) R. Schennach, E. Bechtold * Institut ffir Physikalische Chemie, Universittit Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria
Received 22 October 1996; accepted for publication 20 December 1996
Abstract The adsorption of chlorine on Pt(ll 1) and Pt(ll0) is studied in an ultrahigh vacuum system using line-of-sight desorption mass spectrometry and LEED. C12 molecules adsorb dissociatively on both surfaces leading to maximum concentrations of 9.0 x 1014 C1 atoms cm -z on P t ( l l l ) and 9.3 x 1014 Cl atoms cm -2 on Pt(ll0). In the case of Pt(lll), the (3 x 3) LEED pattern is stable in a concentration range between 6.8 x 1014 and 8.5 x 1014 C1 atoms cm -2. On the Pt(ll0), surface the adsorption of chlorine lifts the (1 × 2) reconstruction and further chlorine dosage first leads to the formation of the known (2 x 1) structure and subsequently to c(2 x 2), (4 x 1) and (4 x 2) LEED patterns. During heating, desorption is observable between 600 and 1000 K from P t ( l l l ) and 600 and 1070 K from Pt(110). Atomic and molecular desorption occurs from both faces. A detailed balancing consideration suggests that the magnitude of the Cl-to-C12 ratio corresponds to the gas-phase equilibrium ratio at the surface temperature. From maximum initial-concentration adlayers, about 2/9 of the adsorbed chlorine atoms desorb as C12 molecules in the case of Pt(110), and about 1/2 in the case of Pt(lll). Keywords: Adsorption kinetics; Chemisorption; Chlorine; Low energy electron diffraction; Low index single crystal surfaces; Platinum;
Thermal desorption; Thermal desorption spectroscopy
1. Introduction T h e i n t e r a c t i o n of chlorine with p l a t i n u m has a t t r a c t e d interest for p r a c t i c a l reasons with respect to electrochemistry, h e t e r o g e n e o u s catalysis a n d c o r r o s i o n , as well as for f u n d a m e n t a l aspects. Studies with the m o s t i m p o r t a n t l o w - i n d e x faces, P t ( l l l ) , (110) a n d (100), revealed t h a t C12 m o l e cules a d s o r b dissociatively with high efficiency via a p r e c u r s o r state to form o r d e r e d structures [ 1 - 4 ] . T h e initially r e c o n s t r u c t e d clean P t ( 1 1 0 ) a n d (100) s u b s t r a t e structures are c h a n g e d b y the a d s o r p t i o n of chlorine; the (111) face retains the h e x a g o n a l
* Corresponding author. Fax: +43 512 5072925.
symmetry. T h e r m a l d e s o r p t i o n yields elemental chlorine. D e s o r b i n g C1 a t o m s a n d C12 molecules were o b s e r v e d with P t ( 1 0 0 ) I-3], b u t for P t ( l l l ) a n d (110), only d e s o r p t i o n of CI a t o m s is r e p o r t e d [ 1,2]. A n u m b e r of questions emerge from the c o m p a r i son of the findings r e p o r t e d for the v a r i o u s singlecrystal faces. I n the following, chlorine a d s o r p t i o n is studied with P t ( l l l ) a n d (110) surfaces. T h e results are c o m p a r e d with previous e x p e r i m e n t s with P t ( 1 0 0 ) which were p e r f o r m e d u n d e r identical c o n d i t i o n s [ 3 ] . T h e e m p h a s i s is on the d e t e r m i n a tion of the chlorine surface c o n c e n t r a t i o n s a n d the respective L E E D patterns, o n the c o m p o s i t i o n s of the d e s o r b i n g beams, a n d on the d e s o r p t i o n kinetics.
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V, All rights reserved PII S0039-6028 (96) 01593-2
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R. Schennach, E. Bechtold / Surface Science 380 (1997) 9 16
2. Experimental
The ultrahigh vacuum chamber used for the experiments is equipped with an LEED/AES system and a shielded quadrupole mass spectrometer system which enables line-of-sight detection of species desorbing from the central part of the crystal surface [3]. Base pressures lower than 1 × 10 -1° mbar are obtained after bakeout using a turbomolecular pump in combination with an ion getter pump and two liquid-nitrogen cooled titanium getter pumps. The platinum (110) and (111) crystals were mounted on an L-shaped holder in two different planes of the manipulator in order to move each of them separately to positions for argon-ion bombardment, LEED/AES experiments, chlorine dosing and temperature-programmed desorption. Both crystals could be heated separately using electron bombardment or radiation from a tungsten filament controlled by a programmable power supply using chromel-alumel thermocouples spot-welded to the sides of the crystals [5]. Elemental chlorine was produced by decomposition of AgC1 in a solid-state electrochemical cell. The density of C12 in the beam at the location of the crystal surface corresponded to about 2.5 × 10 -8 mbar under the usual working conditions. Absolute dosages cannot be given since the shape of the dosing beam is known only approximately [3,5]. Both crystals were cleaned by alternating cycles of argon-ion bombardment, heating to 1200 K, cooling to room temperature in an atmosphere of 5 × 10 -7 mbar oxygen and subsequent heating to 1200 K. After this procedure, the crystals were clean as judged by AES. The (111) surface showed the ( 1 × 1) LEED pattern of the clean surface. The (110) surface showed the (1 × 2) LEED pattern of the clean, reconstructed surface [6,7]. The surface chlorine concentrations were derived from the intensities of the AES signal at 181 V and from thermal desorption yields. Calibration of the AES signal is obtained with the complete c(2 × 2) C1 adlayer on Pt(100) with a C1/Pt ratio of 0.5 or 6.5 × 1014 C1 atoms cm -2 [3]. Temperature programmed desorption of low initial-concentration adlayers produce only 35 amu signals, which indicates desorption of chlorine atoms and enables the
calibration of the 35 amu signal. Signals at 35 and 70 amu indicate the desorption of C1 atoms and C12 molecules from adlayers with higher initial concentrations (Section 3.2.). In this case, the 35 amu signal results in part from C12 fragmentation in the ion source of the mass spectrometer. In the low-temperature range of the desorption spectrum, the constant ratio of the 35 and 70 amu signals points to the exclusive desorption of C12 molecules under these conditions. This ratio determines the fragmentation factor of C12 in the ion source, which is used to calculate the contribution of C12 fragmentation to the 35 amu signals at higher temperatures. The fragmentation-corrected 35 amu signals (termed the Cl-atom signal in the following) yields the amount of desorbing chlorine atoms, and in combination with the initial concentration, which is known from the AES signal, the amount of desorbing C12 molecules. In the present arrangement the sensitivity of the quadrupole system depends on the velocity of the particles which are to be detected. In the following, constant mean sensitivities are used which are determined from the desorption yields as described above.
3. Results and discussion
3.1. Adsorption and LEED patterns Fig. 1 shows the uptake of chlorine on the platinum (110) and (111) surfaces at room temperature. For both surfaces, the adsorption starts with high and nearly identical sticking coefficients. The wide concentration range up to 7.5 × 1014 C1 atoms cm-2, where the adsorption rate stays constant, points to precursor kinetics [1-3]. On further exposure, the sticking probabilities gradually decline to small values until the uptake curves reach a plateau at concentrations of 9.3× 1014 Clatomscm -2 on the (110) face and 8.5 × 1014 C1 atoms cm -2 on the (111) face. On the (110)face, the maximum concentration of 9.3 × 1014 C1 atoms cm -2 is reached with the platinum crystal at room temperature. In case of the (111) face, higher concentrations than the room-temperature maximum of 8.5x
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
a: chlorine uptake on Pt(1 10) 9 8 7 ~ a t o m s + molecules [] C/atoms + CI m o l e c u l e s
6 5 4
~,3 1
• b: chlorine uptake on Pt(1 1 1)
~ 9 o
~
8
~
7
/
~ e ~5 ~
4
~
3
f
~o
/
Cl atoms + m o l e c u l e s • maximum concentration ~ CI a t o m s
2 1 O~ 0
i
i
i
i
i
5
i
i
J
I
1o exposure
[au]
Fig, 1. Chlorine uptake at room temperature as a function of relative exposures on (a) P t ( l l 0 ) and (h) P t ( l l l ) . The circles correspond to the chlorine concentration derived from the sum of the C1 atom (squares) and the C12 molecule (crosses) desorption yields. The maximum concentration on P t ( l l l ) , indicated by a cross in a circle, is only reached at elevated temperatures (see text for details).
1014 C1 atoms cm -2 are obtained at elevated temperatures. The maximum concentration of 9.0 × 1014 C1 atoms cm -2 (indicated in Fig. 1 by a single point) is obtained at 420 K, or by heating to 510 K and chlorine dosing during cooling to room temperature. The clean (111) face shows the known L E E D pattern of the (1 × 1) structure. With increasing chlorine concentration the background intensity increases until the sharp (3 × 3) structure [2] develops at a concentration of 6.8 x 1014 C1 atoms c m - 2 . During chlorine dosing at 300 K, the structure persists up to the finally attainable concentration of 8.5 x 1014 C1 atoms cm -2 (Fig. 2a). The (3 x 3)
11
structure disappears on heating to 510K, and during cooling the (3 x 3) structure is built up again at about 420 K. Heating and further dosing destroys the (3 × 3) order, leads to an increase in the background intensity (Fig. 2b) and results in a concentration of 9.0 x 1014 Cl atoms cm -2. With the maximum concentration at room temperature, weak (1/3, 1/3) spots appear on a brighter background than at lower concentrations (Fig. 2c). In a similar manner, on Pt(100) the maximum surface concentration of 8.7 x 1014 Cl atoms cm -2 was obtained only at elevated temperatures [3]. The LEED features are tentatively interpreted by chlorine chemisorbed on the largely undisturbed (111) substrate structure. At 8.5 × 1014 C1 atoms cm-2, which is the highest chlorine concentration attainable at 300 K, the (3 x 3) unit cell contains five chlorine atoms. The transformation of the ordered (3 x 3) arrangement to a disordered adlayer in the temperature range near 500 K suggests that the adsorption is not very site-specific. Elevated substrate temperatures enable further chlorine uptake beyond this concentration, apparently because of the increased mobility and density fluctuations in the adlayer. The maximum concentration of 9 × 10 a4 C1 atoms cm -2, which is attainable under the present dosing conditions corresponds to that of a densely packed out-ofregistry layer of C1 atoms with van der Waals radii of 1.8 x 10 -s cm [-8] (8.9 x 1014 C1 atoms cm-2). The clean (110) face shows the (1 x 2) pattern of the reconstructed surface which was interpreted as a missing-row structure [6,7]. With adsorbed chlorine up to a concentration of 2.9 × 1014 C1 atoms cm -2 only the (1 x 2) reconstruction of the substrate can be seen between room temperature and 600 K, where significant chlorine desorption begins. In the concentration range between 2.9 x 1014 and 3.4 x 1014 C1 atoms cm -2, the halforder spots decrease in intensity and the background intensity increases, indicating that the original reconstruction is lifted. With chlorine concentrations between 3.4 × 10 TM and 4.8 x 1014 C1 atoms cm -2, streaky half-order features of a (2 x 1) pattern appear in addition to the integral-order spots (Fig. 2d). At temperatures above 500 K the half-order streaks vanish, and only integral-order spots are observable on a bright
12
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
0
O
d
.....
~
i~¸
Fig. 2. LEED patterns from surfaces with different chlorine concentrations. (a) Pt(lll), (3 x 3), 8.5 x 1014 C1 atoms cm z, 55 eV, 300 K; (b) Pt(lll), (1 x 1), 9.0 x 1014 CI atoms cm z, 55 eV, 400 K; (c) Pt(lll), faint (3 x 3), 9.0 x 10TM C1 atoms cm -2, 55 eV, 300 K; (d) Pt(ll0), streaky (2x 1), 4.0x1014 Clatomscm 2, 75eV, 300K; (e) Pt(ll0), ( l x l ) with diffuse additional intensities, 4.0 x 1014 C1 atoms cm -2, 73 eV, 500 K; (f) Pt(ll0), (2 x 1) and faint c(2 x 2), 6.5 x 1014 Cl atoms cm -2, 75 eV, 300 K; (g) Pt(ll0), streaky c(2 x 2), 6.5 x 1014 C1 atoms cm -2, 75 eV, 450 K; (h) Pt(ll0), (4 x 1), 9.3 x 1014 Cl atoms cm -2, 73 eV, 300 K; (i) Pt(ll0), faint (4 x 2), 9.3 x 10TMCI atoms cm -2, 75 eV, 300 K.
b a c k g r o u n d (Fig. 2e). W h e n the system is c o o l e d to b e l o w 500 K, the p a t t e r n s of the o r d e r e d structures r e a p p e a r . Between about 4.8 x 1014 and 7.2 x 1014 C l a t o m s c m - z (2 x 1) a n d faint c ( 2 x 2 ) p a t t e r n s can be seen s i m u l t a n e o u s l y on the surface (Fig. 2f). A t elevated t e m p e r a t u r e s ( a r o u n d 4 5 0 + 2 0 K ) a n d in a limited c o n c e n t r a t i o n range
a r o u n d 6.5 x 1014 ___0.5 x 1014 C1 a t o m s ~T1-2 o n l y faint (1/2, 1/2) spots of the s t r e a k y c(2 x 2) p a t t e r n (Fig. 2g) are present. A b o v e 500 K, all e x t r a spots d i s a p p e a r a n d only the (1 x 1) p a t t e r n remains. W i t h chlorine c o n c e n t r a t i o n s between 7.2 x 10 TM a n d 9 . 3 x 1 0 1 4 C l a t o m s c m -2, either a ( 4 x l ) p a t t e r n (Fig. 2h) o r a faint ( 4 x 2) p a t t e r n on a b r i g h t b a c k g r o u n d (Fig. 2i) are the d o m i n a t i n g
13
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
structures. Whether (4 x 1) or (4 × 2) structures are formed could not be controlled by experimentally adjustable parameters. Only the (1 x 1) pattern remains at temperatures above 450 K. The LEED observations indicate that rearrangement of the substrate is involved in the formation of the structures at higher chlorine concentrations. Up to chlorine concentrations near to 3 x 1014 Clatomscm -2, the dominating (1 x2) periodicity of the clean, reconstructed surface persists. With higher chlorine concentrations, the LEED pattern changes to the (1 x 1) periodicity. STM investigations on the adsorption of bromine on Pt(ll0) showed ordered, mixed Pt/Br adlayer structures [9] which produced similar LEED patterns to those observed with chlorine. The maximum chlorine concentration obtained on Pt(ll0) (9.3 x 1014 C1 atoms cm -2) is only slightly higher than that on P t ( l l l ) , in spite of the more complex structural aspects.
b: chlorine desorption f r o m P t ( 1 1 0 )
8 7 6
5 td
~ a2
e
/
~ 0 a: c h l o r i n e d e s o r p t i o n f r o m P t ( 1 1 1 ) .. 7
/I ] \ / \
__ __
5
35 ainu signal CI a t o m s C/molecules
/
4
e
;"" '
3
//
3.2. Temperature-programmed desorption Thermal desorption yields C1 atoms and C12 molecules. Desorbing platinum chlorides were not found. Fig. 3 shows TPD spectra of C1 atoms and C12 molecules from the (110) and (111) faces, respectively, for various initial concentrations. The Cl-atom signals in Fig. 3 are corrected for the 35 amu fragment caused by fragmentation of the C12 molecules in the ion source of the quadrupole mass filter. For comparison, one uncorrected 35amu desorption trace is shown in Fig. 3a (dashed line) for the case of desorption from Pt(111).
3.2.1. Desorption from P t ( l l l ) The shapes of the Cl-atom peaks are typical for first-order kinetics, but the temperature at the peak maximum shifts to lower temperatures with increasing initial coverage. For a more detailed consideration, desorption rates for constant temperatures and the corresponding actual surface concentrations were extracted from a series of temperature-programmed peaks which were obtained with different initial concentrations. The resulting isothermal rates are plotted as a function of the surface concentration for 800, 850 and 900 K
O[ 600
700
800
900
1000 T/K
Fig. 3. Thermal desorption peaks of chlorine atoms and chlorine molecules from adlayers with different initial concentrations. The heating rate was 20 K s-1. Chlorine was dosed at room temperature. The Cl-atom peaks are corrected with respect to the contribution from the fragmentation of the C12 molecules in the ion source. The letters indicate the Cl-atom peaks and the C12 molecule peaks obtained from adlayers with identical initial concentrations. (a) P t ( l l l ) . The dashed peak shows the uncorrected 35 amu signal. (b) Pt(ll0).
in Fig. 4b. For simple first-order kinetics, the desorption rates should increase linearly with the concentration. The stronger rate increase points to a decreasing desorption energy with increasing concentration. A similar coverage dependence of the desorption energy was found for chlorine desorption from Pt(100) [-3], and was interpreted in terms of repulsive interactions in the adlayer. From the temperature of the maximum of low initial-concentration peaks, a desorption energy of 55 kcalmo1-1 is derived using the Redhead formula [10] (based on a pre-exponential factor of 1 x 1013 s-l).
14
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
4
a: i s o t h e r m a l data on Pt(1 10) o 800 K D 850 K + 900 K
2
E
2
maximum temperature as those from P t ( l l l ) . Isothermal data obtained from temperatureprogrammed desorption peaks are shown in Fig. 4a. As in the case of the (111) face, on the (110) face the plots also deviate strongly from linearity. The Redhead formula [10] applied to low initial-concentration peaks yields a desorption energy of 58 kcal mol-1. D e s o r b i n g C12 molecules are observable in the same temperature range as on the P t ( l l l ) face. With increasing concentration, no extra desorption feature appears on the leading edge, in contrast to the P t ( l l l ) face. The initially saturated adlayer during desorption with decreasing chlorine concentration passes through a variety of structures, which includes rearrangements in the substrate. These structural changes and interactions between the adsorbed C1 atoms are expected to influence the desorption kinetics.
1
O, 0
1
2 3 4 c o n c e n t r a t i o n [1074 Cl a t o m s / c r n 2 ]
Fig. 4. Isothermal desorption rates derived from temperatureprogrammed experiments with different initial concentrations. (a) Pt(ll0) and (b) P t ( l l l ) at 800, 850 and 900 K.
During temperature-programmed experiments, the desorption of C12 molecules becomes measurable with initial concentrations above 3 x 1014 C1 atoms cm -2 (Fig. 1). With the maximum initial concentration, molecular desorption is observable between 600 and 900 K. The small C12 peaks exhibit the nearly symmetric shape typical for second-order kinetics. With higher concentrations, a shoulder on the leading edge of the peak evolves and leads with further increasing initial coverage to a sharp, narrow peak. This shape seems to result from the repulsive interaction between the adsorbed chlorine atoms in the more densely packed adlayer.
3.2.2. Desorption from Pt ( l l O) The Cl-atom peaks from the (110) surface exhibit similar overall concentration dependences of the
3.2.3. Simultaneous desorption of Cl2 molecules and Cl atoms Fig. 1 shows the chlorine molecule and the chlorine atom desorption yields as a function of chlorine exposure. From the (110) face, about 2/9 of the maximum concentration desorb as molecules and from the (111) face about 1/2. On the (110) face the first C12 molecule signal is detected with an initial concentration of about 5.5x 1014Clatomscm -2, and on the (111) face at an initial concentration of about 3x 1014 C1 atoms cm -2. In order to interpret the relevance of desorption of C1 atoms and C12 molecules, desorption from both surfaces is considered for conditions where both rates can be measured simultaneously. In case of the (110) face this is in a temperature range between 650 and 880K and concentrations between 7.9 x 1014 and 5.6 × 10x4 C1 atoms cm -2. For the (111) face, simultaneous desorption is observable between 700 and 880K and concentrations between 5.9 X 1014 and 1.6 × 10TM C1 atoms cm -2. The desorption rates of C1 and C12 are assumed to follow the equations
exp( )
R. Schennach, E. BechtoM / Surface Science 380 (1997) 9-16
and
15
36[ a: Platinum (110)
I
respectively, where n is the actual surface concentration of adsorbed chlorine atoms, and EI~.) and Ezra) are the respective desorption activation energies. Within the error limit of the data
ln(r~ \r2/
=ln (~
RT
(1)
depends linearly on 1/r, indicating negligible dependences of ln(f2tz,~)/f2~r,.)) and 2Elt~)- E2t~)/R on the surface concentration and the temperature. A plot of ln(r2/r2) versus 1/T is shown for both surfaces in Fig. 5 for a temperature range between 670 and 900 K. For the (110) face, the linear fit is not as good as for the (111) face, probably because the C12 molecule rate is much lower than on P t ( l l l ) . If fltT,.)=Vl.n and f2{T,~)=2V2n2, then flt~)/f2¢,)=Vl/2V2. 2 2 The experimental rate data yield for Pt(111) V2/VE=3X 1029-+1 S - 1 particles cm -2 and for Pt(ll0) v2/v2=2xlO28-+ls -1 particles cm -2, which can be rationalized with preexponentials in the ranges of Vl = 1013 S- 1 and v2 = 10 -3 s -1 c m 2 particles -1. The slope of the plot yields 2E1-E2=59_+2kcal for Pt(111) and 2E1 - E 2 = 53 -[-4 kcal for Pt(110). If the activation energies of adsorption for chlorine atoms and chlorine molecules can be neglected, then 2E1- E2 represents the binding energy in gaseous C12 (57.1 kcal [-11]). For further discussion, the desorption rates are tentatively equated to the exchange rates between the chemisorbed adlayer and the gas phase under the condition of established adsorption equilibrium. With the equilibrium pressures at the surface temperature Pcl and Pc12, the rates are rl--
PclSI(n,T)
and 2pcI2S2(n,T)
r2=
X/2g2mokT"
26
o
36 b: Platinum (111) 34
30 28 .26 0 . 0' 0 1 2
0 . '0 0 1 3
O.'00 14 1/7"[I/K1
Fig. 5. Data according to Eq. (1) obtained from temperatureprogrammed experiments in the range of simultaneous C1 atom and C12 molecule desorption (see text for details). (a) Pt(110) and (b) Pt(111) with two different experiments (squares and circles) in both cases.
which results in the ratio s2(.,r)
r2 =2 -
S2(n,T)
r2
1 V
makr=-_ 1%
for the sticking coefficients. The gas-phase dissociation c o n s t a n t K p of C12 molecules, which is introduced for P~za/Po2, is calculated with standard statistical thermodynamical methods [,12] from the atomic and molecular parameters [,8,13] (CI: electronic ground s t a t e : 2P3/2, excited s t a t e : 2p1/2, 0.109eV; C12: r = 1 . 9 8 8 x 1 0 - a c m , ~=561cm-1; Ddiss = 57.1 kcal). In the range of temperatures and surface concentrations covered by the experiments, the ratio is s2(~,r)/s2(n,r)=3+_l for P t ( l l l ) and S2(.,T)/S2(.,T)= 20_ 8 for Pt(110). Under the conditions of desorption, the individual sticking coeffi-
16
R. Schennach, E. Bechtold / Surface Science 380 (1997) 9-16
cients of C1 and C12 are not known. Near 300 K the adsorption of C12 occurs via a precursor, and the sticking probability remains near unity up to high surface concentrations (see Section 3.1.). At the temperatures of the desorption a lower sticking probability is expected, in particular for CI2, in view of the precursor kinetics. Taken together, these considerations suggest that chlorine atoms and molecules desorb approximately according to their equilibrium ratio at the surface temperature, with desorption energies determined largely by the energy difference of adsorbed and gaseous species.
4. Summary In the following, we summarize the new aspects found during the present work on the chlorine adsorption on Pt(111) and Pt(110). M a x i m u m surface concentrations of 9.3 × 1014 C l a t o m s c m -2 for P t ( l l 0 ) and 9.0x1014C1 atoms cm -2 for P t ( l l l ) are determined with a calibration based on 6.5 × 1014 C1 atoms cm -2 for the c ( 2 × 2 ) structure on Pt(100). On Pt(100), the m a x i m u m concentration is 8.7 x 1014 C1 atoms cm -2 [3]. On the (111) surface, the ( 3 × 3 ) structure is formed at concentrations above 6.8×1014 C1 atoms cm-2. The structure is stable over a wide concentration range and up to a temperature of 420 K. The m a x i m u m concentration is reached only at temperatures above 420 K. During cooling the (3 x 3) structure reappears, in the case of the high-concentration adlayer, but with only a low degree of order. On the (110) surface, the initial (1 x 2) reconstruction is lifted at a concentration above 2.9 x 1014 C1 atoms cm -2, and the pre-
viously described [ 1 ] (2 × 1) structure is formed at a concentration of 3.4 × 1014 C1 atoms cm -2. A number of new ordered structures (c(2 x 2), (4 × 1) and ( 4 × 2 ) ) are observed in the concentration range from 4.8x 1014 C l a t o m s c m -2 up to saturation. Atomic and molecular desorption of chlorine occurs from both surfaces, similar to Pt(100) [3]. The order of magnitude of the C1 atom to C12 molecule ratio in the desorbing beam corresponds to the gas-phase equilibrium at the surface temperature. The different desorption temperatures on the two surfaces lead to different fractions of atomic and molecular desorption: from the maximumconcentration adlayers, about 2/9 of the adsorbed chlorine atoms are desorbed as C12 molecules in the case of P t ( l l 0 ) , and about 1/2 in the case of P t ( l l l ) .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
W. Erley, Surf. Sci. 114 (1981) 47. W. Erley, Surf. Sci. 94 (1980) 281. H. Gutleben and E. Bechtold, Surf. Sci. 236 (1990) 313. H.H. Farell, Surf. Sci. 100 (1980) 613. R. Schennach, PhD thesis, Innsbruck, 1995. K. Korte and G. Meyer-Ehmsen,Surf. Sci. 271 (1992) 616. T. Gritsch, D. Coulman, R.J. Behm and G. Ertl, Surf. Sci. 257 (1991) 55. CRC Handbook of Chemistry and Physics, 67th ed., Ed. R.C. Weast (CRC, Boca Raton, 1986/87). E. Bertel, private communications. P. Hanesch, PhD thesis, Bayreuth, 1996; P. Sandl, PhD thesis, Bayreuth, 1993. P.A. Redhead, Vacuum 12 (1962) 203. I. Barin, Thermodynamical Data of Pure Substances (VCH, Weinheim, 1993). See, e.g.N. Davidson, Statistical Mechanics (McGrawHill, New York, 1962). L. Brrnstein, Numerical Data and Functional Relationships in Science and Technology, New Series (Springer, Berlin).