- Email: [email protected]
NEXAFS and TPD studies of molecular adsorption of hydrocarbons on Cu(100): segmental correlations with the heats of adsorption
surface science ELSEVIER
Surface Science 396 (1998) 340-348
NEXAFS and TPD studies of molecular adsorption of hydrocarbons on Cu ( 100): segmental correlations with the heats of adsorption Andrew V. Teplyakov a, Alejandra B. Gurevich a, Michael X. Yang a, Brian E. Bent a, Jingguang G. Chen b,, a Department of Chemistry and Columbia Radiation Laboratory, Columbia University, New York 10027, USA b Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, N J 08801, USA
Received 5 November 1996; accepted for publication 31 July 1997
Abstract Near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monolayer of hydrocarbons on a Cu(100) surface. Fourteen different saturated and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on the following factors: (1) the length of the saturated hydrocarbon linear chain; (2) the presence and location of a double bond; (3) the cyclic versus acyclic nature of the hydrocarbons; and (4) the accessibility of the CH, groups for creating van der Waals interactions with the surface. Similar to previous observations on other surfaces, our results show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 kcal/mol. Our results also indicate that the presence of a double bond in a position where overlap between ~-orbitals of a hydrocarbon and d-orbitals of the metal is significant (double bond parallel to the Cu(100) surface) increases the binding energy of an olefin molecule by 0.75 kcal/mol with respect to that of the corresponding saturated hydrocarbon. © 1998 Published by Elsevier Science B.V. Keywords: Alkanes; Alkenes; Cycloalkanes; Cycloalkenes; Near edge extended X-ray absorption fine structure (NEXAFS);
Temperature programmed desorption
1. Introduction The molecular adsorption of hydrocarbons on m e t a l s u r f a c e s is a k e y step in a m a j o r i t y o f c a t a l y t i c t r a n s f o r m a t i o n s . F o r t h e last 30 years, this p h e n o m e n o n has b e e n s t u d i e d e x t e n s i v e l y b o t h o n single c r y s t a l s u r f a c e s a n d o n a m o r p h o u s c a t a * Corresponding author. Tel: (+ 1) 908 730 2768; fax: (908) 730-3344; e-mail: [email protected]
lysts. S t u d i e s o n single c r y s t a l s u r f a c e s p r e s e n t a u n i q u e o p p o r t u n i t y to d i r e c t l y d e t e r m i n e b i n d i n g e n e r g i e s a n d d e s o r p t i o n rates f o r h y d r o c a r b o n s , allowing general trends and their dependencies on d i f f e r e n t f a c t o r s at t h e m o l e c u l a r level to be determined. The adsorption and bonding of hydrocarbon m o l e c u l e s t o s u r f a c e s h a v e b e e n r e v i e w e d in s e v e r a l r e c e n t articles [ 1 10]. H o w e v e r , m o s t o f t h e e x p e r i m e n t a l studies p r e s e n t e d in t h e l i t e r a t u r e d e a l w i t h
0039-6028/98/$19.00 © 1998 Published by Elsevier Science B.V. All rights reserved. PH S0039-6028 (97) 00688-2
A. V. Teplyakov et at /Surface Science 396 (1998) 340-348
adsorption of hydrocarbons on reactive surfaces, such as Ni [11,12], Pt [13-17], Pd [18], Ru [12,19,20] and Rh [21]. All these studies are extremely important for the development of catalytic systems and mechanistic descriptions of hydrocarbon transformations. They provide information about adsorption sites and the geometry of hydrocarbons on metal surfaces. However, the issue of activation energy of desorption on these reactive surfaces might be complicated by other competing reactions such as isomerization, hydrogenation, and dehydrogenation of the adsorbed hydrocarbon molecules. In the studies presented in this paper, we attempt to concentrate on studying the desorption kinetics of different hydrocarbons from a Cu(100) surface. This surface is an ideal substrate for such studies because copper is not reactive towards the dissociation of C - H , C - C , or C = C bonds in hydrocarbons under ultrahigh vacuum conditions. The absence of a large database for molecular desorption makes it difficult to generalize the trends in the adsorption-desorption processes for hydrocarbons on metal surfaces, but some previous results allow for speculation on certain dependencies. The most explored trend is a change of binding energy for linear hydrocarbons and their derivatives upon increasing the length of the chain. Sexton et al. reported a linear dependence for alcohols on P t ( l l l ) [22,23]. The same group investigated the behavior of alcohols and linear alkanes on Cu(100) [23]. The adsorption of linear chain alcohols on Ag(110) surface was examined by Zhang and GeUman [24]. Finally, similar experiments with different alkyl chlorides were performed by Lin et al. [25]. The results of these experiments are compared with the results obtained in this work in Section 3. Another interesting trend in the binding energies of hydrocarbons is related to the presence, position and orientation of a C = C double bond. For example, the presence of a double bond, in a position that allows significant overlap between the ~c-orbitals of the alkene and d-orbitals of a metal, should increase the binding energy of the alkene with respect to that of a corresponding alkane, which has only a weak van der Waals interaction with the metal surface. To the best of
341
our knowledge, there has been no attempt to systematically study these dependencies, although some interesting features were noted in studies comparing the desorption of C4 alkanes and alkenes from A g ( l l 0 ) [26], and in the investigation of the steric conformational effects in the adsorption and decomposition of cis- and trans2-butene on Si(100)-(2 x 1) [27]. One of the most suitable spectroscopies to study the geometry of a double bond in unsaturated compounds on single crystal surfaces is near-edge X-ray absorption fine structure ( N E X A F S ) . As demonstrated in previous studies [11,28-30], N E X A F S is extremely sensitive to the orientation of the C = C double bond in unsaturated compounds, one of the parameters that we hope to correlate to the activation energy for desorption. N E X A F S studies of hydrocarbons adsorbed on Ni [11] and Pt [28-30] have been reported, but these investigations describe primarily the interaction of unsaturated hydrocarbons with reactive metals, where the interaction of a double bond of a hydrocarbon with a surface completely dominates any other interactions. On the other hand, the available results of N E X A F S studies of aromatic hydrocarbons [31] and alkene molecules [32] on surfaces of chemically inert metals such as Ag and Cu were not aimed at correlating the activation energy of desorption of these compounds with their orientation on a surface. In this paper, we attempt to establish some general trends by studying the desorption of a variety of molecules from a Cu(100) surface. For example, a comparison of the desorption of 1-alkenes with the desorption of alkanes having the same number of carbon atoms quantitatively describes the dependence of hydrocarbon binding energy on the presence of a double bond. The significance of the position of the double bond is analyzed based on the results of studies of alkenes with terminal and internal double bonds. The trends found for acyclic alkanes and alkenes are also compared with those for cyclic hydrocarbons. Finally, bicyclic and branched alkenes are used to establish the dependence of hydrocarbon binding energy on the accessibility to the metal surface of methylene groups and/or 7c-orbitals of the double bond.
342
A.V. Teplyakov et al. / Surface Science 396 (1998) 340-348
2. Experimental The experiments were performed in two U H V systems at Columbia University and Brookhaven National Laboratory. Details of the Columbia U H V chamber and of the experimental protocol for T P D studies are given in Ref. [33]. The Cu(100) crystal, purchased from Monocrystals Inc. as a circular disc (diameter 1 cm and thickness 2 ram), was attached to a molybdenum button heater mounted on a manipulator with capabilities for heating the sample resistively and cooling it with liquid nitrogen. The sample was cleaned by Ar ÷ sputtering [34] to free the surface of carbon, oxygen and sulfur, as confirmed by Auger electron spectroscopy. The clean sample was annealed in vacuum at 9 8 0 K until a sharp (1 x 1) L E E D pattern was obtained. Reactants were adsorbed onto the crystal by backfilling t h e chamber. All liquid hydrocarbons used in the studies presented here were purchased from Aldrich and had a purity of not less than 98%. They were further purified by several freeze-pump-thaw cycles with liquid nitrogen prior to their introduction into the vacuum chamber. Their purities were confirmed in situ by mass spectrometry. All exposures are reported in Langmuirs [1 Langmuir ( L ) = 1 x 1 0 - 6 Torr/s] and are uncorrected for differing ion gage sensitivities. The quadrupole mass spectrometer (QMS) is installed behind a differentially pumped shield containing a 2 mm diameter aperture. In T P D studies, the sample was positioned ~ 2 mm from the aperture so that only molecules evolved from the central portion of the 1 cm diameter crystal contribute to the detected signal. The heating rate in all T P D experiments was 3 K/s. The N E X A F S measurements were conducted on the U1A beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory: A detailed description of the experimental end station apparatus appears in Ref. [35],. The two-stage U H V chamber is equipped with an ion sputtering gun, a quadrupole mass spectrometer and an Auger electron spectrometer, which allowed confirmation that the desired hydrocarbon coverages were reproduced in this chamber. Control experiments on clean and hydrocarbon pre-covered Cu(100) surfaces revealed that carbon
monoxide and water adsorption from background gases ( P < 1 x 10 .9 Torr) had no detectable effect on the N E X A F S spectra reported here. All spectra were recorded with a partial electron yield detector having a retarding voltage of - 2 0 0 eV. The resolution of the synchrotron monochromator was set at 0.4 eV at the C K-edge. All N E X A F S spectra reported here have been divided by the spectra of a clean surface taken at the same incidence angle and by the ratio of the signals from a reference grid which measures the incident beam intensity simultaneously with the N E X A F S spectra. The validity of such a treatment has been discussed in detail previously [36].
3. Results and discussion We begin our analysis with the T P D studies of hydrocarbons on a Cu(100) surface. T P D spectra of monolayer coverages of alkenes are presented in Fig. 1. The first observation is that for the terminal alkenes (top four T P D spectra) there is a linear dependence between the desorption temperature and the length of the alkyl chain. An increase in the alkyl group length by one methylene group increases the desorption temperature of an alkene by 23 _+ 1 K. This result is in excellent agreement with the data reported by Lin et al. [25], where the addition of a methylene group increased the desorption temperature of alkyl chlorides from Cu(100) by 21 K. A very similar temperature increment was also reported by Sexton et al. [23] for the desorption of linear alcohols ( ~ 2 0 K for 5 K/s heating rate) and alkanes ( ~ 2 5 K for 5 K/s heating rate) on Cu(100). A similar dependence is observed for the linear chain alkanes studied here, as shown in Fig. 2. For example, the difference in t h e desorption temperatures for hexane and pentane is 24 K. Based on the Redhead method [37], the 23 K temperature increment corresponds to a difference of 1.5kcal/mol in the binding energy, assuming a first-order desorption kinetics with a pre-exponential factor of 1013 s-1. The next simple dependence is for the effect of a terminal double bond in a linear hydrocarbon molecule. By comparing thermal desorption data for 1-hexene and 1-pentene from Fig. 1 with those
343
A. V. Teplyakov et al. / Surface Science 396 (1998) 340 348
196 208
_/
I
1-Hexene, m/e=8
/
i
1-Pentene, m/e=7.2_I
_/',,._
Propene, m/e=42
_
r6 n"
f
,,,
218 I
-n ~4 rr
Pentane, m/e=72
<
20s"X, Z LU F.Z Z 0
172
1-Butene, m/e=56
~o I--
Hexane, m/e=86
C y c l o h ~ t e n e , m/e=96
Cyclohexene,rn/e=82 f
Jl
Cyclopentene, rn/e=68 ,//
>181
z uJ
z - -
I
fi
Gyclohexane, m/e=84
9
192
Norbornylene, m/e=94 J 2-Pentene, m/e=70 , , f
openine m/e=70
3,3-Dirnethylbutene, m/e=84 f I
200
I
I
300 400 TEMPEI~IATURE (K)
500
Fig. 1. Temperature-programmed desorption studies of monolayer desorption of different alkenes on a Cu(100) surface.
for hexane and pentane in Fig. 2, this difference is estimated to be 12 4- 1 K. Thus, the presence of a double bond in the terminal position increases the desorption temperature of a hydrocarbon by 12 K (or 0.75 kcal/mol [37]) with respect to a normal alkane having the same number of carbon atoms in the alkyl chain. For cyclic hydrocarbons, the temperature increment corresponding to one additional CH2 group in the alkyl chain is different from that for linear hydrocarbons. A comparison of the desorption temperatures of cycloalkenes of Fig. 1 and cycloalkanes of Fig. 2 suggest this increment to be 16_+ 1 K (or 1.0 kcal/mol [37]), which is smaller than the value of 23__ 1 K (or 1.5 kcal/mol [37]) for linear hydrocarbons. Furthermore, the T P D results in Figs. 1 and 2 also reveal that the presence of a double bond in a cyclic hydrocarbon increases the desorption temperature by 20-}-1 K (or 1.25 kcal/mol [37]) with respect to the correspond-
l
150
1
T - - T
200 250 300 TEMPERATURE (K)
F
350
400
Fig. 2. Temperature-programmed desorption studies of monolayer desorption of different alkanes on a Cu(100) surface.
ing cycloalkane. This value is larger than the difference of 12_+1K (or 0.75kcal/mol [37]) observed for 1-alkenes versus linear alkanes. These differences are most probably related to the restrictions of cyclic structures, which in turn change the strength of the interaction of the C = C bonds and CH2 groups of hydrocarbons with the Cu(100) surface. More detailed studies are needed to understand differences between cyclic and linear hydrocarbons, particularly their different conformational changes as the the number of methylene group increases. One of the questions which may arise after analyzing the data presented in Figs. 1 and 2 is the accuracy in applying the Redhead analysis to the desorption of alkenes and alkanes from a copper surface. To make it clear that the desorption processes presented here can be best described by the first-order kinetics, and that the temperature
A.V. Teplyakov et aL / Surface Science 396 (1998) 340 348
344
of the desorption peak does not depend significantly on the hydrocarbon coverage, Fig. 3 compares the coverage dependent T P D studies of normal and cyclic hexanes and hexenes. For saturated hydrocarbons, the temperature of the desorption peak increases only slightly with the coverage, whereas for unsaturated compounds the desorption temperature decreases upon increasing the surface coverage up to a monolayer saturation. In either case, the variation in temperature does not exceed 4 K for increasing the coverage from 25 to 100% of a monolayer, suggesting that the desroption process can be described by the first-order approximation. It should be noted that the activation energy of desorption can be determined by Redhead method only with about 2 kcal/mol accuracy, but for the studies presented here, we are interested in the difference in the activation energy of desorption for a class of structurally similar compounds. Therefore, even though the absolute values of activation energy of desorption may be slightly different from those determined by more accurate methods, the error of this type of measurements
135
should be very similar for all the compounds studied here, and the qualitative trends in the activation energy of desorption should hold true. Furthermore, it should also be noted that despite the rather small variation in the desorption temperatures from submonolayer to monolayer coverages, the overall shape of some of the TPD traces in Fig. 3 might also be described by a zero-order process. However, the difference between the values of desorption energy derived from zero order or first order was rather small in the current study. For example, the desorption energy of cyclohexane, derived by a zero order estimation using the leading edge of the T P D spectra, was within 1 kcal/mol of that derived from the first-order approximation. Because the main purpose of this paper is to compare the general trend in, not the absolute values of, the desorption energy of different hydrocarbons, it is important to use a consistent first-order analysis throughout the manuscript. The left panel of Fig. 4 summarizes the activation energies for the desorption of various hydrocarbon molecules from Cu(100) as a function of
n-Hexane/Cu(100)
m/e=86
i.-
202
141
Cyclohexene/Cu(100) m/e=82
7L
W
4L
_o
W i_
z
1L
Z
f 200
3~o
4;0
A
208
1-Hexene/Cu(100)
m/e=84
i
i
500
150
TEMPERATURE(K)
Z
5L 4L 3L 2L 1L
>. I--
5L
',
13.s
i
i
300 200 250 TEMPERATURE(~
Cyclohexane/Cu(100)
181
1
m/e=84 5L
a5
4L
5L 4L
.
"
.
.
3L
.
2L 1L
Z
_ o ~
35O
i
i
L
200
300
400
TEMPERATURE (K)
i
500
150
i
300 200 250 TEMPERATURE (K)
350
Fig. 3. Coverage dependent TPD studies of n-hexane (re~e= 86) and 1-hexene (m/e = 84), cyclohexane (m/e = 84) and cyclohexene (m/e = 82).
A. V. Teplyakov et aL / Surface Science 396 (1998) 340-348 a
Alkanes
o Cycloalkanes
o
Atkenes Cycloalkenes
13-
12-
10
2! 4
5
6
7
NUMBER OF CARBON A T O M S
Fig. 4. Comparison of activation energies of desorption of different hydrocarbons from Cu(100) as a function of the number of carbon atoms in a hydrocarbon. The heats of vaporization are also presented in the right panel. the n u m b e r o f c a r b o n atoms. F o r ease o f c o m p a r i son, the h y d r o c a r b o n s are classified in the categories o f linear alkane, linear 1-alkene, cyclic a l k a n e a n d cyclic alkene. I n a d d i t i o n , the h e a t o f v a p o r i z a t i o n o f these molecules [38,39] is also c o m p a r e d in the right p a n e l o f Fig. 4. These values are also listed in Table 1. O n e general o b s e r v a t i o n
345
in Fig. 4 is that, for a n y h y d r o c a r b o n molecule, the a c t i v a t i o n energy for d e s o r p t i o n f r o m C u ( 1 0 0 ) resulting f r o m its i n t e r a c t i o n with the m e t a l surface is always 4 - 6 k c a l / m o l h i g h e r t h a n the h e a t o f v a p o r i z a t i o n for the liquid h y d r o c a r b o n . A n o t h e r i m p o r t a n t o b s e r v a t i o n in Fig. 4 is t h a t the presence o f a C = C b o n d does n o t c h a n g e the h e a t o f v a p o r i z a t i o n . H o w e v e r , the a c t i v a t i o n energy for d e s o r p t i o n increases b y a b o u t 0.75 k c a l / m o l f r o m a l k a n e s to 1-alkenes (or b y 1.0 k c a l / m o l f r o m c y c l o a l k a n e s to cycloalkenes) w h e n they are a d s o r b e d on C u ( 1 0 0 ) . This increase is related to the i n t e r a c t i o n o f n - o r b i t a l s o f the C = C b o n d w i t h the d - o r b i t a l s o f the C u ( 1 0 0 ) substrate, as will be discussed below. T h e T P D results s u m m a r i z e d in Fig. 1 a n d Table 1 also indicate t h a t for u n s a t u r a t e d h y d r o c a r b o n s with a n identical n u m b e r o f c a r b o n a t o m s , the d e s o r p t i o n t e m p e r a t u r e d e p e n d s on the location o f the C = C b o n d s . F o r example, the d e s o r p tion t e m p e r a t u r e s for 1-pentene a n d 2-pentene are 185 a n d 175 K , respectively. A n even m o r e o b v i o u s difference are the d e s o r p t i o n t e m p e r a t u r e s for 1-hexene a n d 3,3-dimethylbutene, which are 208 a n d 175 K, respectively. We a t t e m p t to explain these differences b y s t u d y i n g the angles o f the C = C b o n d with respect to the surface. T h e i m p o r -
Table 1 Comparison of desorption temperatures of hydrocarbons from Cu(100) Hydrocarbon
Tdes (K)
E~ (kcal/mol)a
AH~ (kcal/mol)b
1-Hexene 1 -Pentene 1 -Butene Propene Cycloheptene Cyclohexene Cyclopentene Norbornylene 2-Pentene 3,3-Dimethylbutene Hexane Pentane Cyclohexane Cyclopentane
208 185 163 139 218 202 185 192 175 175 196 172 181 166
12.6 11.2 9.8 8.3 13.2 12.2 11.2 11.6 10.6 t 0.6 11.9 10.4 10.9 10.0
7.4 6.1 4.8 3.4 8.0 6.7 8.9 6.4 6.4 7.6 6.4 7.9 6.8
a Activation energy of desorption determined by Redhead method assuming prefactor of 1013 S - 1 for the first-order reaction. b The values are taken from Refs [38,39]. ° The angle between the plane of the double bond in alkenes and the (100) plane of the copper surface.
/_ (o)c 24
32 20 15 30 26 ~ 55
346
A.V. Teplyakov et aL / Surface Science 396 (1998) 340-348
tance of the geometric arrangement of the C = C b o n d on a metal surface can be understood by recalling that the strength of interaction between a C = C bond ~z-orbital and the d-orbitals of a metal depends on their overlap, which will be maximized when the double bond is parallel to the surface. This hypothesis is substantiated by N E X A F S studies, where the angle between the double bond of an adsorbed unsaturated hydrocarbon and the plane of the metal surface can be easily determined [40]. Figs. 5 and 6 present the results of N E X A F S studies of monolayer coverages of cyclohexene and 3,3-dimethylbutene on Cu(100). In Fig. 5, the ~* transition in adsorbed cyclohexene is extremely small when the incident radiation is directed perpendicular to the surface (90°), whereas the intensity of this transition increases with decreasing incidence angles reaching a maximum at a glancing incidence (20 ° was the smallest angle studied). This means that the re* transition is directed nearly perpendicular to the Cu(100) surface, which, in turn, leads to the conclusion that the double bond
Cyclohexene/Cu(100)
~o %:. ^ O9
rr
<.% >-
O3 Z UJ }--
z
I
280
I
I
I
290 300 310 ENERGY (EV)
I
320
Fig. 5. NEXAFS studies of the dependence of the transition intensities on the incident angle: cyclohexeneon Cu(100).
3,3-Dimethylbutene/Cu(100)
<.5. >}-
N z
uJ
z
Z80
Z;0
3()0 340
320
ENERGY (EV) Fig. 6. NEXAFS studies of the dependence of the transition intensities on the incident angle: 3,3-dimethylbutene on Cu(lO0). in adsorbed cyclohexene is nearly parallel to the Cu(100) surface. From the angular dependence of the rc* transition, the angle between the C = C bond in cyclohexene and the Cu(100) surface can be calculated to be 24 ° [40] by using a polarization factor of 85% for the incident beam. N E X A F S studies of cyclohexene performed at different surface coverages showed no significant differences for the coverages between 25 and 100% of the monolayer. On the other hand, when the same angular dependence is studied by N E X A F S for 3,3-dimethylbutene (Fig. 6), the intensity of the ~z* transition does not depend on the incident angle of radiation and the overall spectrum does not show any significant angular dependence (only a slight change in the C~c_n transition is observed). Again, from the angular dependence of the 7c* transition, the angle between the C = C bond in 3,3-dimethylbutene and the Cu(100) surface can be calculated to be ~ 5 5 ° [40]. We have also studied a number of other unsaturated hydrocarbons by N E X A F S and the angles between the
A. V. Teplyakov et al. /Surface Science 396 (1998) 340-348
C = C bonds and the Cu(100) surface are summarized in Table 1. The large difference in the angles between C = C bonds and the Cu(100) surface should contribute to the significant differences in the thermal desorption temperatures of 1-hexene and 3,3-dimethylbutene (208 and 175 K, respectively). As shown in Table 1, the angles between the C = C bonds for these two molecules and the Cu(100) surface are 24 ° and ~ 5 5 °, respectively. The large angle in 3,3-dimethylbutene should reduce the degree of d-re interaction, resulting in a lower desorption temperature for this molecule. Finally, another interesting comparison in Fig, 1 and Table 1 is the desorption temperatures for cycloheptene (218 K ) and norbornylene (bicyclo [2,2,1 ] hept-2-ene) ( 192 K). The N E X A F S results suggest that they have similar C = C bond angles with respect to the Cu(100) surface, 32 ° and 30 °, respectively. One possible way to explain the difference is the effectiveness of the van der Waals interaction between the CHn groups and the Cu(100) surface. Because of the bicyclic structure, at least one of the C H , groups in norbornylene would be far away from the surface, preventing an effective interaction with the surface. In fact, the desorption temperature of norbornylene (192 K ) is between that of cyclopentene (185 K ) and cyclohexene (202 K). One can thus conclude that even though norbornylene has an overall formula C7Hlo, the effective interaction of the C H , groups with the metal surface in this compound is equivalent to the C5 or C6 instead of the C7 cyclic molecules.
4. Summary The following general trends are observed in the combined N E X A F S and T P D investigation for hydrocarbon molecules on Cu(100): (1) Similar to previous observations, the addition of a methylene group increases the activation energy of desorption for linear hydrocarbons by 1.5 kcat/mol and cyclic hydrocarbons by 1.0 kcal/mol. (2) The presence of a terminal C = C bond increases the activation energy for desorption
347
of linear alkanes by 0.75 kcal/mol, while the presence of a C = C bond increases the activation energy for desorption of cyclic hydrocarbons by 1.25 kcal/mol. (3) For alkenes with an identical number of carbon atoms, the geometrical arrangement of the C - - C bond with respect to the surface plays an important role in determining the desorption temperatures. (4) The accessibility of the CHn groups to the metal surface, which can affect van der Waals interactions, also alters the desorption temperature. This is particularly important for bicyclic and branched hydrocarbons.
Acknowledgements Financial support from the National Science Foundation (grant number CHE-93-18625) and from the Joint Services Electronics Program through the Columbia Radiation L a b o r a t o r y (contract D A A H 04-94-G-0057) is gratefully acknowledged.
References [1] F.P. Netzer, M.G. Ramsey, Crit. Rev. Solid State Mater. Sci. i7 (1992) 397. [2] N. Sheppard, A. Rev. Phys. Chem. 39 (1988) 589. [3] F. Zaera, J. Mol. Catal. 86 (1994) 221. [4] F. Zaera, Chem. Rev. 95 (1995) 2651. [5] J.M. White, Langmuir 10 (1994) 3946. [6] E.L. Muetterties, Chem. Soc. Rev. 11 (1982)283. [7] E.L. Canning, R.J. Madix, J. Phys. Chem. 88 (1984) 2437. [8] B.E. Bent, G.A. Somorjai, Adv. Colloid Interface Sci. 29 (1989) 223. [9] C.M. Friend, J.T. Roberts, Acc. Chem. Res. 21 (1988) 394. [10] C.M. Friend, X. Xu, A. Rev. Phys. Chem. 42 (1991) 251. [11] F. Zaera, D.A. Fisher, R.G. Carr, J.L. Gland, J. Chem. Phys. 89 (1988) 5335. [12] F.M. Hoffman, T.H. Upton, J. Phys. Chem. 88 (1984) 6209. [13] N.R. Avery, Surf. Sci. 146 (1984) 363. [14] N.R. Avery, Surf. Sci. 163 (1985) 357. [15] N.R. Avery, J. Electron. Spectrosc. Relat. Phenom. 39 (1986) 1. [16] M.A. Chesters, P. Gardner, Spectrochim. Acta 46A (1990) 1011. [17] C. Xu, Y.-L. Tsai, B.E. Koel, J. Phys. Chem. 98 (1994) 585.
348
A. V. Teplyakov et al. / Surface Science 396 (1998) 340-348
[18] R. Raval, M.E. Pemble, M.A. Chesters, Surf. Sci. 210 (1989) 187. [19] F.M. Hoffman, E.V. O'Brien, J. Hrbek, R.A. dePaola, J. Electron. Spectrosc. Relat. Phenom. 29 (1983) 301. [20] D. Wang, Y. Li, H. Liu, R. Zhai, Catal. Lett. 2 (1989) 9. [21] B.E. Bent, C.M. Mate, J.E. Crowell, B.E. Koel, G.A. Somorjai, J. Phys. Chem. 91 (1987) 1493. [22] B.A. Sexton, K.D. Pendulic, A.E. Hughes, Surf. Sci. 121 (1982) 191. [23] B.A. Sexton, A.E. Hughes, Surf. Sci. 140 (1984) 227. [24] R. Zhang, A. Gellman, J. Phys. Chem. 95 (1991) 7433. [25] J.-L. Lin, B.E. Bent, J. Phys. Chem. 96 (1992) 8529. [26] J. Pawela-Crew, R.J. Madix, Surf. Sci. 339 (1995) 8. [27] M. Kiskinova, J.J.T. Yates, Surf. Sci. 325 (1995) 1. [28] J. St6hr, F. Sette, A.L. Johnson, Phys. Rev. Lett. 53 (1984) 1684. [29] A.P. Hitchcock, D.C. Newbury, I. Ishii, J. St6hr, J.A. Horsley, R.D. Redwing, A . L Johnson, F. Sette, J. Chem. Phys. 85 (1986) 4849.
[30] A.P. Hitchcock, J. Phys. 47 (1986) C8. [31] P. Yannoulis, R. Dudde, K.H. Frank, E.E. Koch, Surf. Sci. 189/190 (1987) 519. [32] J.L. Solomon, R.J. Madix, J. St6hr, J. Chem. Phys. 93 (1990) 8379. [33] M. Xi, B.E. Bent, Surf. Sci. 278 (1992) 19. [34] R.G. Musket, W. Mclean, G.A. Colmenares, D.M. Mackowiecki, W.J. Siekhaus, Appl. Surf. Sci. 10 (1982) 143. [35] D.A. Fisher, J. Colbert, J.L. Gland, Rev. Sci. Instrum. 60 (1989) 1596. [36] D.A. Outka, J. St6hr, J. Chem. Phys. 86 (1988) 3539. [37] P.A. Redhead, Vacuum 12 (1962) 203. [38] CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1990. [39] J.B. Pedley, R.D. Naylor, S.P. Kirby, Thermodynamical Data of Organic Compounds, 2rid ed., Chapman and Hall, London, 1986, p. 792. [40] J. St6hr, Springer Series in Surface Sciences, Springer, New York, 1992, p. 403.
Surface Science 396 (1998) 340-348
NEXAFS and TPD studies of molecular adsorption of hydrocarbons on Cu ( 100): segmental correlations with the heats of adsorption Andrew V. Teplyakov a, Alejandra B. Gurevich a, Michael X. Yang a, Brian E. Bent a, Jingguang G. Chen b,, a Department of Chemistry and Columbia Radiation Laboratory, Columbia University, New York 10027, USA b Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, N J 08801, USA
Received 5 November 1996; accepted for publication 31 July 1997
Abstract Near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed desorption (TPD) studies have been performed to establish the relationship between adsorbate structure and binding energy in a monolayer of hydrocarbons on a Cu(100) surface. Fourteen different saturated and monounsaturated hydrocarbons were studied. The activation energy for desorption of these compounds has been found to be dependent on the following factors: (1) the length of the saturated hydrocarbon linear chain; (2) the presence and location of a double bond; (3) the cyclic versus acyclic nature of the hydrocarbons; and (4) the accessibility of the CH, groups for creating van der Waals interactions with the surface. Similar to previous observations on other surfaces, our results show that an increase of the linear hydrocarbon chain length by one methylene group increases the binding energy of a hydrocarbon by 1.5 kcal/mol. Our results also indicate that the presence of a double bond in a position where overlap between ~-orbitals of a hydrocarbon and d-orbitals of the metal is significant (double bond parallel to the Cu(100) surface) increases the binding energy of an olefin molecule by 0.75 kcal/mol with respect to that of the corresponding saturated hydrocarbon. © 1998 Published by Elsevier Science B.V. Keywords: Alkanes; Alkenes; Cycloalkanes; Cycloalkenes; Near edge extended X-ray absorption fine structure (NEXAFS);
Temperature programmed desorption
1. Introduction The molecular adsorption of hydrocarbons on m e t a l s u r f a c e s is a k e y step in a m a j o r i t y o f c a t a l y t i c t r a n s f o r m a t i o n s . F o r t h e last 30 years, this p h e n o m e n o n has b e e n s t u d i e d e x t e n s i v e l y b o t h o n single c r y s t a l s u r f a c e s a n d o n a m o r p h o u s c a t a * Corresponding author. Tel: (+ 1) 908 730 2768; fax: (908) 730-3344; e-mail: [email protected]
lysts. S t u d i e s o n single c r y s t a l s u r f a c e s p r e s e n t a u n i q u e o p p o r t u n i t y to d i r e c t l y d e t e r m i n e b i n d i n g e n e r g i e s a n d d e s o r p t i o n rates f o r h y d r o c a r b o n s , allowing general trends and their dependencies on d i f f e r e n t f a c t o r s at t h e m o l e c u l a r level to be determined. The adsorption and bonding of hydrocarbon m o l e c u l e s t o s u r f a c e s h a v e b e e n r e v i e w e d in s e v e r a l r e c e n t articles [ 1 10]. H o w e v e r , m o s t o f t h e e x p e r i m e n t a l studies p r e s e n t e d in t h e l i t e r a t u r e d e a l w i t h
0039-6028/98/$19.00 © 1998 Published by Elsevier Science B.V. All rights reserved. PH S0039-6028 (97) 00688-2
A. V. Teplyakov et at /Surface Science 396 (1998) 340-348
adsorption of hydrocarbons on reactive surfaces, such as Ni [11,12], Pt [13-17], Pd [18], Ru [12,19,20] and Rh [21]. All these studies are extremely important for the development of catalytic systems and mechanistic descriptions of hydrocarbon transformations. They provide information about adsorption sites and the geometry of hydrocarbons on metal surfaces. However, the issue of activation energy of desorption on these reactive surfaces might be complicated by other competing reactions such as isomerization, hydrogenation, and dehydrogenation of the adsorbed hydrocarbon molecules. In the studies presented in this paper, we attempt to concentrate on studying the desorption kinetics of different hydrocarbons from a Cu(100) surface. This surface is an ideal substrate for such studies because copper is not reactive towards the dissociation of C - H , C - C , or C = C bonds in hydrocarbons under ultrahigh vacuum conditions. The absence of a large database for molecular desorption makes it difficult to generalize the trends in the adsorption-desorption processes for hydrocarbons on metal surfaces, but some previous results allow for speculation on certain dependencies. The most explored trend is a change of binding energy for linear hydrocarbons and their derivatives upon increasing the length of the chain. Sexton et al. reported a linear dependence for alcohols on P t ( l l l ) [22,23]. The same group investigated the behavior of alcohols and linear alkanes on Cu(100) [23]. The adsorption of linear chain alcohols on Ag(110) surface was examined by Zhang and GeUman [24]. Finally, similar experiments with different alkyl chlorides were performed by Lin et al. [25]. The results of these experiments are compared with the results obtained in this work in Section 3. Another interesting trend in the binding energies of hydrocarbons is related to the presence, position and orientation of a C = C double bond. For example, the presence of a double bond, in a position that allows significant overlap between the ~c-orbitals of the alkene and d-orbitals of a metal, should increase the binding energy of the alkene with respect to that of a corresponding alkane, which has only a weak van der Waals interaction with the metal surface. To the best of
341
our knowledge, there has been no attempt to systematically study these dependencies, although some interesting features were noted in studies comparing the desorption of C4 alkanes and alkenes from A g ( l l 0 ) [26], and in the investigation of the steric conformational effects in the adsorption and decomposition of cis- and trans2-butene on Si(100)-(2 x 1) [27]. One of the most suitable spectroscopies to study the geometry of a double bond in unsaturated compounds on single crystal surfaces is near-edge X-ray absorption fine structure ( N E X A F S ) . As demonstrated in previous studies [11,28-30], N E X A F S is extremely sensitive to the orientation of the C = C double bond in unsaturated compounds, one of the parameters that we hope to correlate to the activation energy for desorption. N E X A F S studies of hydrocarbons adsorbed on Ni [11] and Pt [28-30] have been reported, but these investigations describe primarily the interaction of unsaturated hydrocarbons with reactive metals, where the interaction of a double bond of a hydrocarbon with a surface completely dominates any other interactions. On the other hand, the available results of N E X A F S studies of aromatic hydrocarbons [31] and alkene molecules [32] on surfaces of chemically inert metals such as Ag and Cu were not aimed at correlating the activation energy of desorption of these compounds with their orientation on a surface. In this paper, we attempt to establish some general trends by studying the desorption of a variety of molecules from a Cu(100) surface. For example, a comparison of the desorption of 1-alkenes with the desorption of alkanes having the same number of carbon atoms quantitatively describes the dependence of hydrocarbon binding energy on the presence of a double bond. The significance of the position of the double bond is analyzed based on the results of studies of alkenes with terminal and internal double bonds. The trends found for acyclic alkanes and alkenes are also compared with those for cyclic hydrocarbons. Finally, bicyclic and branched alkenes are used to establish the dependence of hydrocarbon binding energy on the accessibility to the metal surface of methylene groups and/or 7c-orbitals of the double bond.
342
A.V. Teplyakov et al. / Surface Science 396 (1998) 340-348
2. Experimental The experiments were performed in two U H V systems at Columbia University and Brookhaven National Laboratory. Details of the Columbia U H V chamber and of the experimental protocol for T P D studies are given in Ref. [33]. The Cu(100) crystal, purchased from Monocrystals Inc. as a circular disc (diameter 1 cm and thickness 2 ram), was attached to a molybdenum button heater mounted on a manipulator with capabilities for heating the sample resistively and cooling it with liquid nitrogen. The sample was cleaned by Ar ÷ sputtering [34] to free the surface of carbon, oxygen and sulfur, as confirmed by Auger electron spectroscopy. The clean sample was annealed in vacuum at 9 8 0 K until a sharp (1 x 1) L E E D pattern was obtained. Reactants were adsorbed onto the crystal by backfilling t h e chamber. All liquid hydrocarbons used in the studies presented here were purchased from Aldrich and had a purity of not less than 98%. They were further purified by several freeze-pump-thaw cycles with liquid nitrogen prior to their introduction into the vacuum chamber. Their purities were confirmed in situ by mass spectrometry. All exposures are reported in Langmuirs [1 Langmuir ( L ) = 1 x 1 0 - 6 Torr/s] and are uncorrected for differing ion gage sensitivities. The quadrupole mass spectrometer (QMS) is installed behind a differentially pumped shield containing a 2 mm diameter aperture. In T P D studies, the sample was positioned ~ 2 mm from the aperture so that only molecules evolved from the central portion of the 1 cm diameter crystal contribute to the detected signal. The heating rate in all T P D experiments was 3 K/s. The N E X A F S measurements were conducted on the U1A beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory: A detailed description of the experimental end station apparatus appears in Ref. [35],. The two-stage U H V chamber is equipped with an ion sputtering gun, a quadrupole mass spectrometer and an Auger electron spectrometer, which allowed confirmation that the desired hydrocarbon coverages were reproduced in this chamber. Control experiments on clean and hydrocarbon pre-covered Cu(100) surfaces revealed that carbon
monoxide and water adsorption from background gases ( P < 1 x 10 .9 Torr) had no detectable effect on the N E X A F S spectra reported here. All spectra were recorded with a partial electron yield detector having a retarding voltage of - 2 0 0 eV. The resolution of the synchrotron monochromator was set at 0.4 eV at the C K-edge. All N E X A F S spectra reported here have been divided by the spectra of a clean surface taken at the same incidence angle and by the ratio of the signals from a reference grid which measures the incident beam intensity simultaneously with the N E X A F S spectra. The validity of such a treatment has been discussed in detail previously [36].
3. Results and discussion We begin our analysis with the T P D studies of hydrocarbons on a Cu(100) surface. T P D spectra of monolayer coverages of alkenes are presented in Fig. 1. The first observation is that for the terminal alkenes (top four T P D spectra) there is a linear dependence between the desorption temperature and the length of the alkyl chain. An increase in the alkyl group length by one methylene group increases the desorption temperature of an alkene by 23 _+ 1 K. This result is in excellent agreement with the data reported by Lin et al. [25], where the addition of a methylene group increased the desorption temperature of alkyl chlorides from Cu(100) by 21 K. A very similar temperature increment was also reported by Sexton et al. [23] for the desorption of linear alcohols ( ~ 2 0 K for 5 K/s heating rate) and alkanes ( ~ 2 5 K for 5 K/s heating rate) on Cu(100). A similar dependence is observed for the linear chain alkanes studied here, as shown in Fig. 2. For example, the difference in t h e desorption temperatures for hexane and pentane is 24 K. Based on the Redhead method [37], the 23 K temperature increment corresponds to a difference of 1.5kcal/mol in the binding energy, assuming a first-order desorption kinetics with a pre-exponential factor of 1013 s-1. The next simple dependence is for the effect of a terminal double bond in a linear hydrocarbon molecule. By comparing thermal desorption data for 1-hexene and 1-pentene from Fig. 1 with those
343
A. V. Teplyakov et al. / Surface Science 396 (1998) 340 348
196 208
_/
I
1-Hexene, m/e=8
/
i
1-Pentene, m/e=7.2_I
_/',,._
Propene, m/e=42
_
r6 n"
f
,,,
218 I
-n ~4 rr
Pentane, m/e=72
<
20s"X, Z LU F.Z Z 0
172
1-Butene, m/e=56
~o I--
Hexane, m/e=86
C y c l o h ~ t e n e , m/e=96
Cyclohexene,rn/e=82 f
Jl
Cyclopentene, rn/e=68 ,//
>181
z uJ
z - -
I
fi
Gyclohexane, m/e=84
9
192
Norbornylene, m/e=94 J 2-Pentene, m/e=70 , , f
openine m/e=70
3,3-Dirnethylbutene, m/e=84 f I
200
I
I
300 400 TEMPEI~IATURE (K)
500
Fig. 1. Temperature-programmed desorption studies of monolayer desorption of different alkenes on a Cu(100) surface.
for hexane and pentane in Fig. 2, this difference is estimated to be 12 4- 1 K. Thus, the presence of a double bond in the terminal position increases the desorption temperature of a hydrocarbon by 12 K (or 0.75 kcal/mol [37]) with respect to a normal alkane having the same number of carbon atoms in the alkyl chain. For cyclic hydrocarbons, the temperature increment corresponding to one additional CH2 group in the alkyl chain is different from that for linear hydrocarbons. A comparison of the desorption temperatures of cycloalkenes of Fig. 1 and cycloalkanes of Fig. 2 suggest this increment to be 16_+ 1 K (or 1.0 kcal/mol [37]), which is smaller than the value of 23__ 1 K (or 1.5 kcal/mol [37]) for linear hydrocarbons. Furthermore, the T P D results in Figs. 1 and 2 also reveal that the presence of a double bond in a cyclic hydrocarbon increases the desorption temperature by 20-}-1 K (or 1.25 kcal/mol [37]) with respect to the correspond-
l
150
1
T - - T
200 250 300 TEMPERATURE (K)
F
350
400
Fig. 2. Temperature-programmed desorption studies of monolayer desorption of different alkanes on a Cu(100) surface.
ing cycloalkane. This value is larger than the difference of 12_+1K (or 0.75kcal/mol [37]) observed for 1-alkenes versus linear alkanes. These differences are most probably related to the restrictions of cyclic structures, which in turn change the strength of the interaction of the C = C bonds and CH2 groups of hydrocarbons with the Cu(100) surface. More detailed studies are needed to understand differences between cyclic and linear hydrocarbons, particularly their different conformational changes as the the number of methylene group increases. One of the questions which may arise after analyzing the data presented in Figs. 1 and 2 is the accuracy in applying the Redhead analysis to the desorption of alkenes and alkanes from a copper surface. To make it clear that the desorption processes presented here can be best described by the first-order kinetics, and that the temperature
A.V. Teplyakov et aL / Surface Science 396 (1998) 340 348
344
of the desorption peak does not depend significantly on the hydrocarbon coverage, Fig. 3 compares the coverage dependent T P D studies of normal and cyclic hexanes and hexenes. For saturated hydrocarbons, the temperature of the desorption peak increases only slightly with the coverage, whereas for unsaturated compounds the desorption temperature decreases upon increasing the surface coverage up to a monolayer saturation. In either case, the variation in temperature does not exceed 4 K for increasing the coverage from 25 to 100% of a monolayer, suggesting that the desroption process can be described by the first-order approximation. It should be noted that the activation energy of desorption can be determined by Redhead method only with about 2 kcal/mol accuracy, but for the studies presented here, we are interested in the difference in the activation energy of desorption for a class of structurally similar compounds. Therefore, even though the absolute values of activation energy of desorption may be slightly different from those determined by more accurate methods, the error of this type of measurements
135
should be very similar for all the compounds studied here, and the qualitative trends in the activation energy of desorption should hold true. Furthermore, it should also be noted that despite the rather small variation in the desorption temperatures from submonolayer to monolayer coverages, the overall shape of some of the TPD traces in Fig. 3 might also be described by a zero-order process. However, the difference between the values of desorption energy derived from zero order or first order was rather small in the current study. For example, the desorption energy of cyclohexane, derived by a zero order estimation using the leading edge of the T P D spectra, was within 1 kcal/mol of that derived from the first-order approximation. Because the main purpose of this paper is to compare the general trend in, not the absolute values of, the desorption energy of different hydrocarbons, it is important to use a consistent first-order analysis throughout the manuscript. The left panel of Fig. 4 summarizes the activation energies for the desorption of various hydrocarbon molecules from Cu(100) as a function of
n-Hexane/Cu(100)
m/e=86
i.-
202
141
Cyclohexene/Cu(100) m/e=82
7L
W
4L
_o
W i_
z
1L
Z
f 200
3~o
4;0
A
208
1-Hexene/Cu(100)
m/e=84
i
i
500
150
TEMPERATURE(K)
Z
5L 4L 3L 2L 1L
>. I--
5L
',
13.s
i
i
300 200 250 TEMPERATURE(~
Cyclohexane/Cu(100)
181
1
m/e=84 5L
a5
4L
5L 4L
.
"
.
.
3L
.
2L 1L
Z
_ o ~
35O
i
i
L
200
300
400
TEMPERATURE (K)
i
500
150
i
300 200 250 TEMPERATURE (K)
350
Fig. 3. Coverage dependent TPD studies of n-hexane (re~e= 86) and 1-hexene (m/e = 84), cyclohexane (m/e = 84) and cyclohexene (m/e = 82).
A. V. Teplyakov et aL / Surface Science 396 (1998) 340-348 a
Alkanes
o Cycloalkanes
o
Atkenes Cycloalkenes
13-
12-
10
2! 4
5
6
7
NUMBER OF CARBON A T O M S
Fig. 4. Comparison of activation energies of desorption of different hydrocarbons from Cu(100) as a function of the number of carbon atoms in a hydrocarbon. The heats of vaporization are also presented in the right panel. the n u m b e r o f c a r b o n atoms. F o r ease o f c o m p a r i son, the h y d r o c a r b o n s are classified in the categories o f linear alkane, linear 1-alkene, cyclic a l k a n e a n d cyclic alkene. I n a d d i t i o n , the h e a t o f v a p o r i z a t i o n o f these molecules [38,39] is also c o m p a r e d in the right p a n e l o f Fig. 4. These values are also listed in Table 1. O n e general o b s e r v a t i o n
345
in Fig. 4 is that, for a n y h y d r o c a r b o n molecule, the a c t i v a t i o n energy for d e s o r p t i o n f r o m C u ( 1 0 0 ) resulting f r o m its i n t e r a c t i o n with the m e t a l surface is always 4 - 6 k c a l / m o l h i g h e r t h a n the h e a t o f v a p o r i z a t i o n for the liquid h y d r o c a r b o n . A n o t h e r i m p o r t a n t o b s e r v a t i o n in Fig. 4 is t h a t the presence o f a C = C b o n d does n o t c h a n g e the h e a t o f v a p o r i z a t i o n . H o w e v e r , the a c t i v a t i o n energy for d e s o r p t i o n increases b y a b o u t 0.75 k c a l / m o l f r o m a l k a n e s to 1-alkenes (or b y 1.0 k c a l / m o l f r o m c y c l o a l k a n e s to cycloalkenes) w h e n they are a d s o r b e d on C u ( 1 0 0 ) . This increase is related to the i n t e r a c t i o n o f n - o r b i t a l s o f the C = C b o n d w i t h the d - o r b i t a l s o f the C u ( 1 0 0 ) substrate, as will be discussed below. T h e T P D results s u m m a r i z e d in Fig. 1 a n d Table 1 also indicate t h a t for u n s a t u r a t e d h y d r o c a r b o n s with a n identical n u m b e r o f c a r b o n a t o m s , the d e s o r p t i o n t e m p e r a t u r e d e p e n d s on the location o f the C = C b o n d s . F o r example, the d e s o r p tion t e m p e r a t u r e s for 1-pentene a n d 2-pentene are 185 a n d 175 K , respectively. A n even m o r e o b v i o u s difference are the d e s o r p t i o n t e m p e r a t u r e s for 1-hexene a n d 3,3-dimethylbutene, which are 208 a n d 175 K, respectively. We a t t e m p t to explain these differences b y s t u d y i n g the angles o f the C = C b o n d with respect to the surface. T h e i m p o r -
Table 1 Comparison of desorption temperatures of hydrocarbons from Cu(100) Hydrocarbon
Tdes (K)
E~ (kcal/mol)a
AH~ (kcal/mol)b
1-Hexene 1 -Pentene 1 -Butene Propene Cycloheptene Cyclohexene Cyclopentene Norbornylene 2-Pentene 3,3-Dimethylbutene Hexane Pentane Cyclohexane Cyclopentane
208 185 163 139 218 202 185 192 175 175 196 172 181 166
12.6 11.2 9.8 8.3 13.2 12.2 11.2 11.6 10.6 t 0.6 11.9 10.4 10.9 10.0
7.4 6.1 4.8 3.4 8.0 6.7 8.9 6.4 6.4 7.6 6.4 7.9 6.8
a Activation energy of desorption determined by Redhead method assuming prefactor of 1013 S - 1 for the first-order reaction. b The values are taken from Refs [38,39]. ° The angle between the plane of the double bond in alkenes and the (100) plane of the copper surface.
/_ (o)c 24
32 20 15 30 26 ~ 55
346
A.V. Teplyakov et aL / Surface Science 396 (1998) 340-348
tance of the geometric arrangement of the C = C b o n d on a metal surface can be understood by recalling that the strength of interaction between a C = C bond ~z-orbital and the d-orbitals of a metal depends on their overlap, which will be maximized when the double bond is parallel to the surface. This hypothesis is substantiated by N E X A F S studies, where the angle between the double bond of an adsorbed unsaturated hydrocarbon and the plane of the metal surface can be easily determined [40]. Figs. 5 and 6 present the results of N E X A F S studies of monolayer coverages of cyclohexene and 3,3-dimethylbutene on Cu(100). In Fig. 5, the ~* transition in adsorbed cyclohexene is extremely small when the incident radiation is directed perpendicular to the surface (90°), whereas the intensity of this transition increases with decreasing incidence angles reaching a maximum at a glancing incidence (20 ° was the smallest angle studied). This means that the re* transition is directed nearly perpendicular to the Cu(100) surface, which, in turn, leads to the conclusion that the double bond
Cyclohexene/Cu(100)
~o %:. ^ O9
rr
<.% >-
O3 Z UJ }--
z
I
280
I
I
I
290 300 310 ENERGY (EV)
I
320
Fig. 5. NEXAFS studies of the dependence of the transition intensities on the incident angle: cyclohexeneon Cu(100).
3,3-Dimethylbutene/Cu(100)
<.5. >}-
N z
uJ
z
Z80
Z;0
3()0 340
320
ENERGY (EV) Fig. 6. NEXAFS studies of the dependence of the transition intensities on the incident angle: 3,3-dimethylbutene on Cu(lO0). in adsorbed cyclohexene is nearly parallel to the Cu(100) surface. From the angular dependence of the rc* transition, the angle between the C = C bond in cyclohexene and the Cu(100) surface can be calculated to be 24 ° [40] by using a polarization factor of 85% for the incident beam. N E X A F S studies of cyclohexene performed at different surface coverages showed no significant differences for the coverages between 25 and 100% of the monolayer. On the other hand, when the same angular dependence is studied by N E X A F S for 3,3-dimethylbutene (Fig. 6), the intensity of the ~z* transition does not depend on the incident angle of radiation and the overall spectrum does not show any significant angular dependence (only a slight change in the C~c_n transition is observed). Again, from the angular dependence of the 7c* transition, the angle between the C = C bond in 3,3-dimethylbutene and the Cu(100) surface can be calculated to be ~ 5 5 ° [40]. We have also studied a number of other unsaturated hydrocarbons by N E X A F S and the angles between the
A. V. Teplyakov et al. /Surface Science 396 (1998) 340-348
C = C bonds and the Cu(100) surface are summarized in Table 1. The large difference in the angles between C = C bonds and the Cu(100) surface should contribute to the significant differences in the thermal desorption temperatures of 1-hexene and 3,3-dimethylbutene (208 and 175 K, respectively). As shown in Table 1, the angles between the C = C bonds for these two molecules and the Cu(100) surface are 24 ° and ~ 5 5 °, respectively. The large angle in 3,3-dimethylbutene should reduce the degree of d-re interaction, resulting in a lower desorption temperature for this molecule. Finally, another interesting comparison in Fig, 1 and Table 1 is the desorption temperatures for cycloheptene (218 K ) and norbornylene (bicyclo [2,2,1 ] hept-2-ene) ( 192 K). The N E X A F S results suggest that they have similar C = C bond angles with respect to the Cu(100) surface, 32 ° and 30 °, respectively. One possible way to explain the difference is the effectiveness of the van der Waals interaction between the CHn groups and the Cu(100) surface. Because of the bicyclic structure, at least one of the C H , groups in norbornylene would be far away from the surface, preventing an effective interaction with the surface. In fact, the desorption temperature of norbornylene (192 K ) is between that of cyclopentene (185 K ) and cyclohexene (202 K). One can thus conclude that even though norbornylene has an overall formula C7Hlo, the effective interaction of the C H , groups with the metal surface in this compound is equivalent to the C5 or C6 instead of the C7 cyclic molecules.
4. Summary The following general trends are observed in the combined N E X A F S and T P D investigation for hydrocarbon molecules on Cu(100): (1) Similar to previous observations, the addition of a methylene group increases the activation energy of desorption for linear hydrocarbons by 1.5 kcat/mol and cyclic hydrocarbons by 1.0 kcal/mol. (2) The presence of a terminal C = C bond increases the activation energy for desorption
347
of linear alkanes by 0.75 kcal/mol, while the presence of a C = C bond increases the activation energy for desorption of cyclic hydrocarbons by 1.25 kcal/mol. (3) For alkenes with an identical number of carbon atoms, the geometrical arrangement of the C - - C bond with respect to the surface plays an important role in determining the desorption temperatures. (4) The accessibility of the CHn groups to the metal surface, which can affect van der Waals interactions, also alters the desorption temperature. This is particularly important for bicyclic and branched hydrocarbons.
Acknowledgements Financial support from the National Science Foundation (grant number CHE-93-18625) and from the Joint Services Electronics Program through the Columbia Radiation L a b o r a t o r y (contract D A A H 04-94-G-0057) is gratefully acknowledged.
References [1] F.P. Netzer, M.G. Ramsey, Crit. Rev. Solid State Mater. Sci. i7 (1992) 397. [2] N. Sheppard, A. Rev. Phys. Chem. 39 (1988) 589. [3] F. Zaera, J. Mol. Catal. 86 (1994) 221. [4] F. Zaera, Chem. Rev. 95 (1995) 2651. [5] J.M. White, Langmuir 10 (1994) 3946. [6] E.L. Muetterties, Chem. Soc. Rev. 11 (1982)283. [7] E.L. Canning, R.J. Madix, J. Phys. Chem. 88 (1984) 2437. [8] B.E. Bent, G.A. Somorjai, Adv. Colloid Interface Sci. 29 (1989) 223. [9] C.M. Friend, J.T. Roberts, Acc. Chem. Res. 21 (1988) 394. [10] C.M. Friend, X. Xu, A. Rev. Phys. Chem. 42 (1991) 251. [11] F. Zaera, D.A. Fisher, R.G. Carr, J.L. Gland, J. Chem. Phys. 89 (1988) 5335. [12] F.M. Hoffman, T.H. Upton, J. Phys. Chem. 88 (1984) 6209. [13] N.R. Avery, Surf. Sci. 146 (1984) 363. [14] N.R. Avery, Surf. Sci. 163 (1985) 357. [15] N.R. Avery, J. Electron. Spectrosc. Relat. Phenom. 39 (1986) 1. [16] M.A. Chesters, P. Gardner, Spectrochim. Acta 46A (1990) 1011. [17] C. Xu, Y.-L. Tsai, B.E. Koel, J. Phys. Chem. 98 (1994) 585.
348
A. V. Teplyakov et al. / Surface Science 396 (1998) 340-348
[18] R. Raval, M.E. Pemble, M.A. Chesters, Surf. Sci. 210 (1989) 187. [19] F.M. Hoffman, E.V. O'Brien, J. Hrbek, R.A. dePaola, J. Electron. Spectrosc. Relat. Phenom. 29 (1983) 301. [20] D. Wang, Y. Li, H. Liu, R. Zhai, Catal. Lett. 2 (1989) 9. [21] B.E. Bent, C.M. Mate, J.E. Crowell, B.E. Koel, G.A. Somorjai, J. Phys. Chem. 91 (1987) 1493. [22] B.A. Sexton, K.D. Pendulic, A.E. Hughes, Surf. Sci. 121 (1982) 191. [23] B.A. Sexton, A.E. Hughes, Surf. Sci. 140 (1984) 227. [24] R. Zhang, A. Gellman, J. Phys. Chem. 95 (1991) 7433. [25] J.-L. Lin, B.E. Bent, J. Phys. Chem. 96 (1992) 8529. [26] J. Pawela-Crew, R.J. Madix, Surf. Sci. 339 (1995) 8. [27] M. Kiskinova, J.J.T. Yates, Surf. Sci. 325 (1995) 1. [28] J. St6hr, F. Sette, A.L. Johnson, Phys. Rev. Lett. 53 (1984) 1684. [29] A.P. Hitchcock, D.C. Newbury, I. Ishii, J. St6hr, J.A. Horsley, R.D. Redwing, A . L Johnson, F. Sette, J. Chem. Phys. 85 (1986) 4849.
[30] A.P. Hitchcock, J. Phys. 47 (1986) C8. [31] P. Yannoulis, R. Dudde, K.H. Frank, E.E. Koch, Surf. Sci. 189/190 (1987) 519. [32] J.L. Solomon, R.J. Madix, J. St6hr, J. Chem. Phys. 93 (1990) 8379. [33] M. Xi, B.E. Bent, Surf. Sci. 278 (1992) 19. [34] R.G. Musket, W. Mclean, G.A. Colmenares, D.M. Mackowiecki, W.J. Siekhaus, Appl. Surf. Sci. 10 (1982) 143. [35] D.A. Fisher, J. Colbert, J.L. Gland, Rev. Sci. Instrum. 60 (1989) 1596. [36] D.A. Outka, J. St6hr, J. Chem. Phys. 86 (1988) 3539. [37] P.A. Redhead, Vacuum 12 (1962) 203. [38] CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1990. [39] J.B. Pedley, R.D. Naylor, S.P. Kirby, Thermodynamical Data of Organic Compounds, 2rid ed., Chapman and Hall, London, 1986, p. 792. [40] J. St6hr, Springer Series in Surface Sciences, Springer, New York, 1992, p. 403.