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Pt(100)
Surface Science 181 (1987) L163-L166 North-Holland, Amsterdam
L163
S U R F A C E S C I E N C E LETTERS E F F E C T O F T H E A D S O R B A T E - I N D U C E D SURFACE R E C O N S T R U C T I O N O N T H E R M A L D E S O R P T I O N SPECTRA: H 2 / Pt(100) V.A. S O B Y A N I N and V.P. Z H D A N O V Institute of Catalysis. No~,osibirsk 630090, USSR Received 15 August 1986; accepted for publication 14 November 1986
The high temperature (T > 300 K) thermal desorption spectra for hydrogen from Pt(100) are characterized by a shift of the peak maximum to higher temperatures with increasing initial coverage and by narrow peak widths. These features are explained by the adsorbate-induced surface reconstruction.
Adsorbate-induced reconstruction of the metal surface has been observed for several systems. In particular, Norton and co-workers [1] have reported results of an extensive study of the (5 x 1) to (1 x 1) reconstruction that occurs when a clean Pt(100) surface is exposed to hydrogen, CO or NO. This reconstruction can be regarded as a first-order phase transition [1]. We have recently presented a simple model for such reconstruction [2]. In this Letter, we consider the effect of the induced reconstruction on thermal desorption spectra. In particular, associative desorption of hydrogen from the Pt(100) surface is discussed. Experimental data. There have been several studies of thermal desorption spectra for hydrogen adsorbed on Pt(100) [3,4]. Various results are not in good agreement. For this reason, we have investigated the desorption of hydrogen from Pt(100) again. The experiments were conducted in an U H V all-glass apparatus with a base pressure below 10-8 Pa and with a mass-spectrometric analysis of gases. The experimental system and recording procedure for thermal desorption spectra have been described in more detail previously [5]. The Pt(100) sample (99.999% purity) was oriented to within 62' and then polished on both sides by standard techniques. Other characteristics of the sample and surface cleaning procedures are described in refs. [5,6]. Our inspection [6] of the clean surface by LEED showed a hexagonal structure (this structure is frequently denoted by (5 x 1) [7] or (5 x 20) [1]). Adsorption of hydrogen at low temperatures (T_< 230 K) leads to the formation of the (1 x 1) structure [6]. On heating in hydrogen (PH2 = 10-5 Pa), the 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L164
V.A. SolD,anin, V.P. Zhdanot~ /Adsorbate-inducedsurface reconstruction
EXPERIMENT
THEORY
EXP.
= 150 L
8 ° = 1.00
6 L
0.75
3 L
8.50
1.2 L
0.25
380
408
380
400
T, E
Fig. 1. High temperature thermal desorption spectra for hydrogen adsorbed on Pt(100). Heating rate is 3 K/s, Tad s ~ 1 0 0 K , exposures are in Langmuir units. (1 × 1) ~ (5 × 1) transition occurs at 380 < T_< 430 K [6]. These temperatures are somewhat higher than those reported by Norton and co-workers [1] (according to them the hydrogen-produced (1 × 1) surface reconstructs at T > 350 K). Thus, the critical temperature, Tc, for the induced reconstruction is higher than 350 K. U n f o r t u n a t e l y , the value of the critical temperature is not known at present. In particular, we are not sure that Tc is in the range 380 < Tc < 430 K. Indeed, the hydrogen coverage decreases quickly on heating, which raises the question of whether the observed transition temperature reflects the hydrogen desorption rather than the reconstruction itself. Nevertheless, we have assumed in our calculation (see below) that T~ = 400 K. The measured high temperature thermal desorption spectra are shown in fig. 1. Our data for high (T > 300 K) and low (T < 300 K) temperatures are in agreement with ~he results obtained by Barteau and co-workers [4]. Unfortunately, they have not studied in detail the variation of the peak shape with increasing initial coverage. The observed thermal desorption spectra are characterized by a shift of the peak maximum to higher temperatures with increasing initial coverage and by narrow peak widths. The observed full width at half of the maximum amplitude is 23 K. These spectra are not adequately described by simple second-order desorption kinetics, which predicts an opposite shift of the peak maximum with increasing initial coverage and broader peak widths. The full width at half of the maximum amplitude expected for simple second-order desorption with a pre-exponential factor of 1013 s - I and an activation energy of 25 kcal/mol is 42 K.
V.A. Sol)vanin, V.P. Zhdanot, /Adsorbate-inducedsurface reconstruction
L165
Theoretical analysis. A real surface r e c o n s t r u c t i o n i n d u c e d by a d s o r p t i o n is a very c o m p l e x p h e n o m e n o n . T o symplify the consideration, we have used the following a s s u m p t i o n s [2]. A n y surface a t o m is located in p o s i t i o n I or II. F o r a clean surface, p o s i t i o n I is stable a n d p o s i t i o n II is metastable. A d s o r b e d particles are d e s c r i b e d in the f r a m e w o r k of a lattice-gas model. The m e t a s t a b l e p h a s e is a s s u m e d to be stabilized b y the interaction between surface a t o m s a n d a d s o r b e d particles. In the mean-field a p p r o x i m a t i o n , the d e p e n d e n c e of the m e a n energy of an a d s o r b e d particle on coverage is represented as [2] ~ H ( O ) = ,-cO- z ~ ,
(1)
where ~ is the energy of the lateral interaction of a d s o r b e d particles (positive for repulsion), K is the coverage of surface a t o m s located in p o s i t i o n II, c~ is the interaction energy p a r a m e t e r , z is the n u m b e r of the n e a r e s t - n e i g h b o u r sites. Strictly speaking, the n u m b e r z changes during reconstruction. F o r simplicity, this n u m b e r is assumed to be c o n s t a n t in our model. The coverage of surface atoms, l o c a t e d in position II, can be written as (see eq. (2) in ref. [2]) K= 1/(1 + exp[(AE-
z a O ) / T ] },
(2)
where ~ E is the energy difference between p o s i t i o n s II and I at a clean surface. S u b s t i t u t i n g eq. (2) into eq. (1), we have
AH(O) = zeO- za/{1 + exp[(AE-
zeeO)/T] ).
(3)
A s s u m i n g that r e c o n s t r u c t i o n is r a p i d in c o m p a r i s o n with d e s o r p t i o n a n d that the m e a n energy (3) makes a p r e d o m i n a n t c o n t r i b u t i o n to the d e p e n d e n c e of the d e s o r p t i o n activation energy on coverage, we derive the following kinetic equation for associative d e s o r p t i o n
dO~dr = - vO 2 exp[ - ( E a
-
-
2 AH(O))/T],
(4)
where v is a p r e - e x p o n e n t i a l factor, E a is the activation energy at low coverages. Eq. (4) is correct at T >~ T~, where Tc is the critical t e m p e r a t u r e . C a l c u l a t e d d e s o r p t i o n spectra for h y d r o g e n a d s o r b e d on Pt(100) are shown in fig. 1. T h e p a r a m e t e r s , used to fit the e x p e r i m e n t a l spectra, were E a = 23 k c a l / m o l , v = 1013 s -1, z~ = 3 k c a l / m o l , zc~ = 5 k c a l / m o l , A E = 2 k c a l / m o l . T h e described p a r a m e t e r s were selected as follows: the " n o r m a l " p r e - e x p o n e n tial factor was p o s t u l a t e d , the activation energy was chosen to r e p r o d u c e the p e a k temperature, the value of the repulsive lateral interaction was selected as the typical m i n i m a l value of the interaction of c h e m i s o r b e d particles, the p a r a m e t e r s zc~ a n d z l E were chosen to give T ~ - - 4 0 0 K. The q u a n t i t a t i v e theoretical results are sensitive to the parameters. A t present, we d o not c o n s i d e r that the selected p a r a m e t e r s p r o v i d e the best possible fit to the e x p e r i m e n t a l data. W e are sure only in that a v a r i a t i o n of the p a r a m e t e r s does not change the theoretical p r e d i c t i o n s qualitatively.
L166
V.A. Sobyanm, V.P. Zhdanov /Adsorbate-inducedsurface
reconstruction
The results of our calculations are in good agreement with the experimental data. In particular, the theory reproduces a shift of the peak maximum to higher temperatures with increasing initial coverage and narrow peak widths. M o r e r e f i n e d f e a t u r e s o f e x p e r i m e n t a l s p e c t r a (e.g. t h e w e a k left s h o u l d e r a t high initial coverages) are not reproduced. Our calculations demonstrate that the main features of thermal desorption spectra for hydrogen adsorbed on Pt(100) can be explained by the adsorbateinduced surface reconstruction. T h e a u t h o r s t h a n k t h e r e f e r e e for u s e f u l c o m m e n t s .
References [1] P.R. Norton, J.A. Davies, D.P. Jackson and N. MatsunamL Surface Sci. 85 (1979) 269: P.R. Norton, J.A. Davies, D.C. Creber, C.W. Sitter and T.E. Jackman, Surface Sci. 108 (1981) 205: R.J. Behm, P.A. Thiel, P.R. Norton and G Ertl, J. Chem. Phys. 78 (1983) 7437. [2] V.P. Zhdanov, Surface Sci. 164 (1985) L807. [3] K.E. Lu and R.R. Rye, Surface Sci. 45 (1974) 677; F.P. Netzer and G. Kneringer, Surface Sci. 51 (1975) 526; R.W. McCabe and L.D. Schmidt, in: Proc. 7th Intern. Vacuum Congr,, Vol. 2 (1977) p. 120. [4] M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99. [5] V.A. Sobyanin, G K . Boreskov and A.R. Cholach, Dokl. Akad. Nauk SSSR 278 (1984) 1422; 279 (1984) 1410 (in Russian). [6] V.A. Sobyanin, G.K. Boreskov and A.V. Kalinkin, Dokl. Akad. Nauk SSSR 283 (1985) 922 (in Russian). [7] K. Heinz, E. Lang, K. Strauss and K. Miiller. Appl. Surface Sci. 11/12 (1982) 611.
L163
S U R F A C E S C I E N C E LETTERS E F F E C T O F T H E A D S O R B A T E - I N D U C E D SURFACE R E C O N S T R U C T I O N O N T H E R M A L D E S O R P T I O N SPECTRA: H 2 / Pt(100) V.A. S O B Y A N I N and V.P. Z H D A N O V Institute of Catalysis. No~,osibirsk 630090, USSR Received 15 August 1986; accepted for publication 14 November 1986
The high temperature (T > 300 K) thermal desorption spectra for hydrogen from Pt(100) are characterized by a shift of the peak maximum to higher temperatures with increasing initial coverage and by narrow peak widths. These features are explained by the adsorbate-induced surface reconstruction.
Adsorbate-induced reconstruction of the metal surface has been observed for several systems. In particular, Norton and co-workers [1] have reported results of an extensive study of the (5 x 1) to (1 x 1) reconstruction that occurs when a clean Pt(100) surface is exposed to hydrogen, CO or NO. This reconstruction can be regarded as a first-order phase transition [1]. We have recently presented a simple model for such reconstruction [2]. In this Letter, we consider the effect of the induced reconstruction on thermal desorption spectra. In particular, associative desorption of hydrogen from the Pt(100) surface is discussed. Experimental data. There have been several studies of thermal desorption spectra for hydrogen adsorbed on Pt(100) [3,4]. Various results are not in good agreement. For this reason, we have investigated the desorption of hydrogen from Pt(100) again. The experiments were conducted in an U H V all-glass apparatus with a base pressure below 10-8 Pa and with a mass-spectrometric analysis of gases. The experimental system and recording procedure for thermal desorption spectra have been described in more detail previously [5]. The Pt(100) sample (99.999% purity) was oriented to within 62' and then polished on both sides by standard techniques. Other characteristics of the sample and surface cleaning procedures are described in refs. [5,6]. Our inspection [6] of the clean surface by LEED showed a hexagonal structure (this structure is frequently denoted by (5 x 1) [7] or (5 x 20) [1]). Adsorption of hydrogen at low temperatures (T_< 230 K) leads to the formation of the (1 x 1) structure [6]. On heating in hydrogen (PH2 = 10-5 Pa), the 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L164
V.A. SolD,anin, V.P. Zhdanot~ /Adsorbate-inducedsurface reconstruction
EXPERIMENT
THEORY
EXP.
= 150 L
8 ° = 1.00
6 L
0.75
3 L
8.50
1.2 L
0.25
380
408
380
400
T, E
Fig. 1. High temperature thermal desorption spectra for hydrogen adsorbed on Pt(100). Heating rate is 3 K/s, Tad s ~ 1 0 0 K , exposures are in Langmuir units. (1 × 1) ~ (5 × 1) transition occurs at 380 < T_< 430 K [6]. These temperatures are somewhat higher than those reported by Norton and co-workers [1] (according to them the hydrogen-produced (1 × 1) surface reconstructs at T > 350 K). Thus, the critical temperature, Tc, for the induced reconstruction is higher than 350 K. U n f o r t u n a t e l y , the value of the critical temperature is not known at present. In particular, we are not sure that Tc is in the range 380 < Tc < 430 K. Indeed, the hydrogen coverage decreases quickly on heating, which raises the question of whether the observed transition temperature reflects the hydrogen desorption rather than the reconstruction itself. Nevertheless, we have assumed in our calculation (see below) that T~ = 400 K. The measured high temperature thermal desorption spectra are shown in fig. 1. Our data for high (T > 300 K) and low (T < 300 K) temperatures are in agreement with ~he results obtained by Barteau and co-workers [4]. Unfortunately, they have not studied in detail the variation of the peak shape with increasing initial coverage. The observed thermal desorption spectra are characterized by a shift of the peak maximum to higher temperatures with increasing initial coverage and by narrow peak widths. The observed full width at half of the maximum amplitude is 23 K. These spectra are not adequately described by simple second-order desorption kinetics, which predicts an opposite shift of the peak maximum with increasing initial coverage and broader peak widths. The full width at half of the maximum amplitude expected for simple second-order desorption with a pre-exponential factor of 1013 s - I and an activation energy of 25 kcal/mol is 42 K.
V.A. Sol)vanin, V.P. Zhdanot, /Adsorbate-inducedsurface reconstruction
L165
Theoretical analysis. A real surface r e c o n s t r u c t i o n i n d u c e d by a d s o r p t i o n is a very c o m p l e x p h e n o m e n o n . T o symplify the consideration, we have used the following a s s u m p t i o n s [2]. A n y surface a t o m is located in p o s i t i o n I or II. F o r a clean surface, p o s i t i o n I is stable a n d p o s i t i o n II is metastable. A d s o r b e d particles are d e s c r i b e d in the f r a m e w o r k of a lattice-gas model. The m e t a s t a b l e p h a s e is a s s u m e d to be stabilized b y the interaction between surface a t o m s a n d a d s o r b e d particles. In the mean-field a p p r o x i m a t i o n , the d e p e n d e n c e of the m e a n energy of an a d s o r b e d particle on coverage is represented as [2] ~ H ( O ) = ,-cO- z ~ ,
(1)
where ~ is the energy of the lateral interaction of a d s o r b e d particles (positive for repulsion), K is the coverage of surface a t o m s located in p o s i t i o n II, c~ is the interaction energy p a r a m e t e r , z is the n u m b e r of the n e a r e s t - n e i g h b o u r sites. Strictly speaking, the n u m b e r z changes during reconstruction. F o r simplicity, this n u m b e r is assumed to be c o n s t a n t in our model. The coverage of surface atoms, l o c a t e d in position II, can be written as (see eq. (2) in ref. [2]) K= 1/(1 + exp[(AE-
z a O ) / T ] },
(2)
where ~ E is the energy difference between p o s i t i o n s II and I at a clean surface. S u b s t i t u t i n g eq. (2) into eq. (1), we have
AH(O) = zeO- za/{1 + exp[(AE-
zeeO)/T] ).
(3)
A s s u m i n g that r e c o n s t r u c t i o n is r a p i d in c o m p a r i s o n with d e s o r p t i o n a n d that the m e a n energy (3) makes a p r e d o m i n a n t c o n t r i b u t i o n to the d e p e n d e n c e of the d e s o r p t i o n activation energy on coverage, we derive the following kinetic equation for associative d e s o r p t i o n
dO~dr = - vO 2 exp[ - ( E a
-
-
2 AH(O))/T],
(4)
where v is a p r e - e x p o n e n t i a l factor, E a is the activation energy at low coverages. Eq. (4) is correct at T >~ T~, where Tc is the critical t e m p e r a t u r e . C a l c u l a t e d d e s o r p t i o n spectra for h y d r o g e n a d s o r b e d on Pt(100) are shown in fig. 1. T h e p a r a m e t e r s , used to fit the e x p e r i m e n t a l spectra, were E a = 23 k c a l / m o l , v = 1013 s -1, z~ = 3 k c a l / m o l , zc~ = 5 k c a l / m o l , A E = 2 k c a l / m o l . T h e described p a r a m e t e r s were selected as follows: the " n o r m a l " p r e - e x p o n e n tial factor was p o s t u l a t e d , the activation energy was chosen to r e p r o d u c e the p e a k temperature, the value of the repulsive lateral interaction was selected as the typical m i n i m a l value of the interaction of c h e m i s o r b e d particles, the p a r a m e t e r s zc~ a n d z l E were chosen to give T ~ - - 4 0 0 K. The q u a n t i t a t i v e theoretical results are sensitive to the parameters. A t present, we d o not c o n s i d e r that the selected p a r a m e t e r s p r o v i d e the best possible fit to the e x p e r i m e n t a l data. W e are sure only in that a v a r i a t i o n of the p a r a m e t e r s does not change the theoretical p r e d i c t i o n s qualitatively.
L166
V.A. Sobyanm, V.P. Zhdanov /Adsorbate-inducedsurface
reconstruction
The results of our calculations are in good agreement with the experimental data. In particular, the theory reproduces a shift of the peak maximum to higher temperatures with increasing initial coverage and narrow peak widths. M o r e r e f i n e d f e a t u r e s o f e x p e r i m e n t a l s p e c t r a (e.g. t h e w e a k left s h o u l d e r a t high initial coverages) are not reproduced. Our calculations demonstrate that the main features of thermal desorption spectra for hydrogen adsorbed on Pt(100) can be explained by the adsorbateinduced surface reconstruction. T h e a u t h o r s t h a n k t h e r e f e r e e for u s e f u l c o m m e n t s .
References [1] P.R. Norton, J.A. Davies, D.P. Jackson and N. MatsunamL Surface Sci. 85 (1979) 269: P.R. Norton, J.A. Davies, D.C. Creber, C.W. Sitter and T.E. Jackman, Surface Sci. 108 (1981) 205: R.J. Behm, P.A. Thiel, P.R. Norton and G Ertl, J. Chem. Phys. 78 (1983) 7437. [2] V.P. Zhdanov, Surface Sci. 164 (1985) L807. [3] K.E. Lu and R.R. Rye, Surface Sci. 45 (1974) 677; F.P. Netzer and G. Kneringer, Surface Sci. 51 (1975) 526; R.W. McCabe and L.D. Schmidt, in: Proc. 7th Intern. Vacuum Congr,, Vol. 2 (1977) p. 120. [4] M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99. [5] V.A. Sobyanin, G K . Boreskov and A.R. Cholach, Dokl. Akad. Nauk SSSR 278 (1984) 1422; 279 (1984) 1410 (in Russian). [6] V.A. Sobyanin, G.K. Boreskov and A.V. Kalinkin, Dokl. Akad. Nauk SSSR 283 (1985) 922 (in Russian). [7] K. Heinz, E. Lang, K. Strauss and K. Miiller. Appl. Surface Sci. 11/12 (1982) 611.