- Email: [email protected]
Reflection absorption infrared spectroscopy and kinetic studies of the reactivity of ethylene on Pt(111) surfaces
surface science Surface Science 368 (1996) 371-376
ELSEVIER
Reflection absorption infrared spectroscopy and kinetic studies of the reactivity of ethylene on Pt(111) surfaces Francisco Zaera *, Ton V.W. Janssens, Helmut Ofner Department of Chemistry, University of California, Riverside, CA 92521, USA Received 1 May 1996; accepted for publication 1 August 1996
Abstract
The chemistry of ethylene on P t ( l l l ) single-crystal surfaces has proven quite complex because it involves the simultaneous occurrence of several reactions, namely molecular desorption, dehydrogenation to ethylidyne, H-D exchange within the adsorbed molecules, and hydrogenation to ethane. Reflection absorption infrared spectroscopy (RAIRS) has been used here in conjunction with isothermal kinetic measurements to identify the possible intermediates involved in each of those reactions, and to follow their thermal chemistry on the platinum surface. All vinyl, ethyl and ethylidene moieties were prepared by thermal decomposition of their corresponding iodides and characterized by RAIRS. The experimental data available to date favors the formation of ethylidene as an intermediate in the conversion of ethylene to ethylidyne, but the complexity of the kinetics of that reaction, which changes significantly with changing surface coverages, makes the final proof of this mechanism quite difficult. In addition, a side ethyleneethyl equifibrium which starts at temperatures below those required for the formation of ethylidyne is responsible for H-D exchange in ethylene. Finally, the hydrogenation of ethylene to ethane also involves an ethyl intermediate, but only occurs at the ethylene high coverages needed for the transition of the di-tr strongly bonded species to a weak it configuration. The relevance of the reactions seen under vacuum to the high-pressure catalytic hydrogenation of ethylene is briefly discussed.
Keywords: Adsorption kinetics; Catalysis; Ethylene; Isotopic exchange; Platinum; Reflection absorption infrared spectroscopy; Single crystal surfaces; Surface chemical reactions; Thermal desorption spectroscopy
1. Introduction Since its discovery by Sabatier and Senderens in 1897, the hydrogenation of olefins over metal catalysts has been one of the most studied chemical processes [ 1,2]. As a representative case of those reactions, the chemistry of ethylene on P t ( l l l ) surfaces has received particular attention from the surface science community [ 3-5 ]. The study of this seemingly simple system is complicated by the fact that there appears to be at least two types of molecular adsorption states for ethylene, namely, a * Corresponding author.
strong di-a bonding and a weaker n-bonded configuration [6,7]. Furthermore, heating chemisorbed ethylene to around room temperature triggers several competitive reaction pathways, including hydrogenation to ethane, H - D exchange and conversion to ethylidyne [5]. Here we report some recent studies carried out in our laboratory designed to address each of those reactions in an individual manner.
2. Experimental Most experiments reported here were performed in an ultra-high vacuum (UHV) chamber evacu-
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 01078-3
372
F. Zaera et aL / Surface Science 368 (1996) 371-376
ated to a base pressure below 1 x 10 -1° Torr and equipped with sample cleaning and characterization facilities, as well as with instrumentation for temperature-programmed desorption (TPD) and reflection absorption infrared (RAIRS) spectroscopies [8]. The RAIRS spectra reported here were taken using a mercury--cadmium telluride (MCT) detector, by averaging 1000 scans taken with 4 cm -1 resolution and ratioing that against spectra for the clean surface recorded immediately before dosing. The kinetic measurements were performed in a second UHV system [9] specifically designed to perform dynamic kinetic measurements using a variation of the molecular-beam method initially developed by King and Wells [10]. The kinetic data reported here were taken isothermally by exposing the clean platinum surface to a directional collimated beam composed of mixtures of ethylene and hydrogen gases [ 11 ].
bonds) [21]. In that case vinylidene disappears around 170 K while acetylene subsists to 200 K, but above that temperature both appear to convert to chemisorbed ethylene, and only after heating to about 300 K is ethylidyne formation detected. At the present time ethylidene seems to be the most likely intermediate in the conversion of ethylene to ethylidyne. Indeed, 1,1-diiodoethane, used as a precursor for ethylidene, converts readily to ethylidyne [22]. This is illustrated by Fig. 1, which displays infrared spectra for 5.0 L (dose to saturation coverage, which amounts to about 0.10 ML) of 1,1-diiodoethane adsorbed on P t ( l l l ) after annealing the surface to different temperatures. The initial molecular adsorption below 100 K is apparent by the signals at 1197 and 1371 cm -1, which correspond to the ~-C-H and symmetric (umbrella) methyl deformation modes respectively.
CH3CHI2/Pt(111) RAIRS 3. Formation of ethylidyne The reaction which perhaps has received the most attention in relation to the thermal reactivity of ethylene chemisorbed on transition metal surfaces is its conversion to ethylidyne. This has turned out to be a particularly puzzling process, and the extensive studies performed on this system have not yet led to the development of a clear idea of the mechanism for the reaction. It has been argued that ethyl [12], vinyl [ 13], and/or ethylidene [14,15] moieties are the most likely surface intermediates in this conversion. We have taken advantage of the fact that halide derivatives are good precursors for hydrocarbon moieties [ 16,17] to prepare and characterize the chemistry of those three species by RAIRS. Ethyl moieties, prepared by thermal decomposition of chemisorbed ethyl iodide, were shown to convert to ethylene via a/3hydride elimination step at temperatures much lower than those required for the formation of ethylidyne [8,18-20]. Vinyl species, also made via thermal activation of its iodide, display a much more complex surface chemistry which involves the initial formation of a mixture of/~3-~/2-vinylidene and p3-~/2-acetylene (both moieties coordinated to several metal atoms via both a and
8 -~ EthylldyneYlel~ [ ~0
.0002
~' ~4
100L " ~
'°~,,.o~, T, iP°
5.0 L Exposure Tads= 100 K 1339i Apnealina 1197 II 1371 T/K
A !
1200
1300
2~K
1400
1500
Frequency / ¢m "1 Fig. 1. Reflection absorption infrared (RAIRS) spectra from 5.0L of 1,1-diiodoethane on P t ( l l l ) , immediately following dosing at 100 K and after subsequent annealing to 150, 200, 250 and 300 K. The signals at 1197 and 1371 cm -1 correspond to C - H deformation modes in the molecular diiodoethane, while that at 1339 cm -1 is assigned of the symmetric deformation of methyl groups in ethylidyne. The inset displays the changes in the amount of ethyfidyne produced as a function of annealing temperature for low (2.0 L) and high (10.0 L) initial coverages of 1,1-diiodoethane. Two reactivity regimes are clearly distinguished here, one around 150 K favored at low coverages, and a second around room temperature which most probably involves the previous formation of ethylene, and which is preferred on saturated surfaces.
F. Zaera et al. / Surface Science 368 (1996) 371-376
The interesting observation from these experiments is the fact that the peak at 1339 cm -1 associated with ethylidyne starts to grow around 150 K, a temperature well below that required to make ethylidyne from ethylene, ethyl or vinyl species. The chemistry of ethylidene is nevertheless complicated by the fact that variations in surface coverages induce significant changes in its kinetic behavior: after exposures above 2.0L (0= 0.05 ML), additional ethylidyne is made around 300 K, and no low-temperature conversion at all is seen after a 10.0 L dose (Fig. 1, inset). These results highlight the fact that the availability of empty surface sites plays a key role in the kinetic of ethylidyne formation. The isothermal kinetics of the conversion of ethylene to ethylidyne was initially thought to follow a simple first-order behavior 1-23-26], but was later proven to display at least two different rate constants as the reaction progresses [27]. Furthermore, a scanning tunneling microscopy in situ characterization of this system has highlighted the fact that the reaction occurs preferentially at the edges of ethylene domains, indicating that special surface ensembles are needed for the conversion to proceed 1,28]. Finally, results from TPD experiments with the isotopically labelled CHD=CD2 ethylene argue for the need of an ethylene--ethylidene pre-equilibrium to precede the subsequent conversion to ethylidyne at high coverages [13]. All this is consistent with a mechanism in which ethylene always isomerizes to ethylidene prior to its conversion to ethylidyne, and where that first step is rate-limiting at low coverages but leads to an ethylene-ethylidene pre-equilibrium at coverages close to saturation because of the unavailability of empty sites, which poisons the following ethylidene-to-ethylidyne conversion 1,223.
4. H-D exchange in chemisorbed ethylene In the presence of surface deuterium, the conversion of ethylene to ethylidyne on Pt(111) is accompanied by exchange of hydrogen for deuterium atoms. Indeed, infrared spectra obtained after coadsorbing C2H4 and C2D4 at 330 K reveal the
373
formation of partially deuterated ethylidyne [29]. However, extensive isotope scrambling is also observed when coadsorbing C2D, with H2, indicating that the exchange most likely involves surface hydrogen. Fig. 2 shows the key RAIRS spectra that corroborate those observations. Additional TPD evidence also points to the idea that the detection of the observed H-D-exchanged ethylidyne is not necessarily connected with the reactions leading to its formation, but due to previous exchange on chemisorbed ethylene instead. The kinetics of H-D exchange in chemisorbed ethylene has been independently studied by laser-induced desorption isothermal kinetic experiments [30]. Those experiments have shown that there is significant H-D exchange reactivity at temperatures as low as 215 K, much lower than those required for ethylidyne formation, and that the reaction displays a complex kinetic behavior most probably associated with a limited diffusivity of hydrogen (deuterium) atoms on metal surfaces covered with hydrocarbon moieties.
H-D Exchange in Ethylene/Pt(111) RAIRS of Ethylidyne from 5.0 L Ethylene dosed at 330 K I0.0005
o O r¢=
0 Io
5s(CHa) 1339
"(C-CX3) : J~ : Jl ~
.
,
.
,
.
.
8s(CDHz) i P(CD2H) :
i
.
,
.
.
iC2H4+C D
.
,
.
.
.
,
.
.
.
,
.
.
1000 1100 1200 1300 1400 Frequency / c m "1
Fig. 2. RAIRS spectra for the ethylidyne produced by adsorption of 5.0 L of pure C2H+ (top), pure C2D4 (second from top), and a mixture of C2H+ and C2D+ (ratio C2H+: C~D4= 1:2.5) (third from top) on P t ( l l l ) under vacuum, and by adsorption of 5.0L of C2D4 in the presence of 5 x l 0 - a T o r r of Hz (bottom). All exposures were performed at 330 K. The peak at 1247cm -1 seen in the two last spectra is due to partially deuterated cthylidyne, and is indicative of the occurrence of H-D exchange reactions.
374
F Zaera et al. / Surface Science 368 (1996) 371-376
The most likely mechanism for the H - D exchange in ethylene is one involving the formation of an ethyl intermediate [31,32]. This idea is quite reasonable, given that a similar pathway has been demonstrated in organometallic systems [17,33], and because ethyl, once formed, does undergo a clean and facile #-hydride elimination step back to ethylene on most surfaces studied to date [17,18,34,35]. H - D exchange between ethylidyne and surface hydrogen (or deuterium) does occur too, but at a much lower rate than ethylidyne formation [29,36]; the mechanism for that reaction has not yet been elucidated.
5. Hydrogenation to ethane Finally, a small amount of ethane formation from self-hydrogenation is also seen in TPD experiments with ethylene on P t ( l l l ) under vacuum [32,37]. Isotope-labelling experiments have shown that this occurs in a stepwise fashion, via the slow formation of an ethyl intermediate [5,32], but there has been some discussion on the relevance of this mechanism to the catalytic ethylene hydrogenation process which occurs at higher pressures, since under those conditions the surface is covered almost completely with other hydrocarbon moieties [38]. In the initial surface science study that identified the presence of ethylidyne during the catalytic process, it was suggested that the surface moieties could act as hydrogen-transfer agents between the metal and ethylene molecules weakly adsorbed on a second layer [38], but later work has led to an alternative model in which ethylidyne acts just as a site blocker, and where ethylene hydrogenation still occurs via ethyl surface intermediates [39]. Distinguishing between these two ideas is particularly hard not only because it has not been possible to emulate the catalytic nature of the hydrogenation under vacuum, but also because the reaction probabilities per impinging molecule under most practical conditions are extremely low, on the order of 10 - 4 o r less, and this suggests a correspondingly small steady-state concentration of surface intermediates. We have recently performed some isothermal vacuum kinetic measurements on this system [11,40] which indicate that: (i) ethylidyne may
indeed only poison the hydrogenation reaction. In fact, the rate for ethylene hydrogenation may be limited by the rate of ethylidyne removal, since that reaction would be required to open surface sites for the adsorption of both hydrogen and ethylene. (ii) Hydrogen adsorption is strongly inhibited by the presence of ethylene in the gas phase. The competitive adsorption of hydrogen does not occur under vacuum, even when gas mixtures with hydrogen-to-ethylene ratios as high as 1000:1 are used. This may explain the firstorder dependence on hydrogen and the negative order on ethylene obtained for the rate law of ethylene hydrogenation in most catalytic cases [2,38]. (iii) Only a weakly, probably n-bonded, ethylene species (for which there is in fact some spectroscopic evidence of its presence during the catalytic process [41,42]) is active in ethylene hydrogenation. This is illustrated by the kinetic results displayed in Fig. 3, which displays the time evolution of the partial pressures of both ethylene and ethane during King and Wells-type experi-
Ethylene Hydrogenation/Pt(111) Isothermal Kinetic M e a s u r e m e n t s at 2 5 0 K Using a C o l l i m a t e d Directional G a s B e a m
I:
ol
:3 ¢o ¢0
///
' C~H.
~ ._.
a. 1~:1 b. 7s:1 ~ 300:1
o marion
, \ .
,'~'~,z,~,,x~
#/
0,. B ol
f#
1::
/hr
D.
.
,
-10
.
.
.
.
,
-5
.
.
.
.
,
0
.
.
.
.
,
.
.
.
.
5
,
10
.
.
.
.
,
15
.
.
.
.
,
.
20
Time / s
Fig. 3. Isothermal kinetic data for the hydrogenation of ethylene at 250 K, as obtained by using a variation of the so-called King and Wells method where collimated beams of hydrogen+ethylene gas mixtures are impinged onto a clean P t ( l l l ) surface. Both the uptake of ethylene (C2H4) on the surface and the evolution of ethane (C2H6, the hydrogenation product) are shown here as a function of time for three different gas mixtures. The figure shows that ethane formation starts only after reaching ethylene coverages high enough to induce the transition from di-a to ~ bonding, and that an increase in hydrogen content in the impinging beam mixture exerts two main effects on the kinetics of the reaction, namely, an increase in both the hydrogenation rate and its yield, and an inhibition of the uptake of ethylene on the surface.
375
F. Zaera et aL / Surface Science 368 (1996) 371-376
ments [ 10]. Collimated beams of mixtures of ethylene and hydrogen (where the flux of ethylene was fixed at F(C2H4)=0.05ML s -1 and that of hydrogen was varied between F(H2)=0.8 and 15 M L s -1) were turned on around t = - 1 0 s, but were kept away from the crystal until t = 0 , at which point the intercepting flag was removed and the gases were allowed to impinge directly onto the platinum surface. The drop in ethylene signal seen from that point on is proportional to the ethylene sticking coefficient on P t ( l l l ) (which initially is of the order of 0.8), while the increase in ethane partial pressure is proportional to the rate of ethylene hydrogenation. One fact that becomes immediately apparent from the data in Fig. 3 is that there is a delay between the beginning of the ethylene uptake on the surface and the start of ethane formation. In fact, the threshold for ethane production is seen at the point where the weakly adsorbed ethylene state starts to be populated. Two other effects are highlighted by these results, namely: (i) that the rate for ethane formation is higher and continues for a longer time when the hydrogen content in the beam is increased, and (ii) that the uptake of ethylene on the surface is inhibited by coadsorbed hydrogen as well. Finally, additional isotope-labelling experiments indicate that the transition from the strongly bonded di-tr state to the weaker rc interaction for ethylene is fast and reversible, occurs once a given surface coverage is reached, and may either be collective or take place via a fast displacement mechanism
[41]. 6. Conclusions The present paper summarizes the results from some of our most recent studies on the chemistry of ethylene on platinum surfaces. It is apparent from the brief review provided here that even though some progress has been made on the understanding of this chemistry at a molecular level, most of the key questions still remain unanswered. At the present time we would suggest that the catalytic hydrogenation of ethylene most probably occurs via the stepwise incorporation of hydrogen atoms into weakly adsorbed ethylene, to form initially ethyl moieties and ultimately ethane
CHaCH3(g) Ethane Ethylidene ~+H Hydrogenation CH~ H-D Exchange (~H2 +H H~TCH24. ~ ' ~ ' ~ Ethyl
~-Ethylene
di-o-Ethylene
Ethylidyne/ ~+H~HI~CH2 Vinyl
Reaction Energetics AHfO/kcal/mol 15.
J
(ads)
Ethane(g) Hlydrogenatlon -2H(ads) H-D Exchange -5
•di-o-Ethylene (ads)
Dohydrogendon
Ethylidyne +H(ads)
Reaction Coordinate
Fig. 4. Top: reaction scheme for the surface chemistry of C2 moieties on Pt(lll). Bottom: energy diagram for the relevant surfacereactionsinvolvedin the thermalconversionof ethylene. Mechanisms are proposed here for both the conversion of ethylene to ethylidynevia an ethylideneintermediate and for H-D exchange and hydrogenation reactions via a common ethyl moiety. (the same as under vacuum). This reaction is inhibited considerably by the competitive formation of ethylidyne, since this last species is particularly stable and hard to rehydrogenate. The conversion of ethylene to ethylidyne is most likely a complex reaction, but it probably does not involve an ethyl intermediate, as the hydrogenation and H - D exchange processes, but an ethylidene moiety instead. All these ideas are illustrated schematically in Fig. 4. The other important observation to keep in mind from this work is the fact that the kinetics of the relevant surface reactions are greatly affected by both the coverages of the different surface species and the availability of surface free sites.
Acknowledgement Financial support for this research was provided by a grant from the National Science Foundation (CHE-9530191).
376
F. Zaera et al./Surface Science 368 (1996) 371-376
References [1] G.C. Bond, Catalysis by Metals (Academic Press, London, 1962). I-2] J. Horiuti and K. Miyahara, Hydrogenation of Ethylene on Metallic Catalysts, National Bureau of Standards, Report NSRDS-NBC No. 13, 1968. [3] S.J. Thomson, in: Catalysis: A Specialist Periodical Report, Vol. 3, Eds. C. Kemball and D.A. Dowen (The Chemical Society, London, 1980) pp. 1-38. [4] F. Zaera and G.A. Somorjai, in: Hydrogen Effects in Catalysis: Fundamentals and Practical Applications, Eds. Z. Pail and P.G. Menon (Marcel Dekker, New York, 1988) pp. 425--447. [5] F. Zaera, Langmuir 12 (1996) 88. [6] N. Sheppard, Annu. Rev. Phys. Chem. 39 (1988) 589. [7] M.B. Hugenschmidt, P. Dolle, J. JupiUe and A. Cassuto, J. Vac. Sci. Technol. A 7 (1989) 3312. [8] H. Hoffmann, P.R. Grifliths and F. Zaera, Surf. Sci. 262 (1992) 141. [9] J. Liu, M. Xu, T. Nordmeyer and F. Zaera, J. Phys. Chem. 99 (1995) 6167. [10] D.A. King and M.G. Wells, Surf. Sci. 29 (1972) 454. [11] H. Ofner and F. Zaera, Proc. 211th ACS National Meeting, New Orleans, March 1996. [12] G.A. Somorjal, M.A. van Hove and B.E. Bent, J. Phys. Chem. 92 (1988) 973. [13] F. Zaera, J. Am. Chem. Soc. I l i (1989) 4240. [14] X.-L Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. 193 (1988) 387. [15] P. Cremer, C. Stanners, J.W. Niemantsverdriet, Y.R. Shen and G. Somorjai, Surf. Sci. 328 (1995) 111. [16] F. Zaera, Acc. Chem. Res. 25 (1992) 260. [17] F. Zaera, Chem. Rev. 95 (1995) 2651. [18] F. Zaera, Surf. Sci. 219 (1989) 453. [19] F. Zaera, J. Am. Chem. Soc. 111 (1989) 8744. [20] F. Zaera, H. Hoffmann and P.R. Grit~ths, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 705. [21] F. Zaera and N. Bernstein, J. Am. Chem. Soc. 116 (1994) 4881. [22] T.V.W. Janssens and F. Zaera, J. Phys. Chem. 100 (1996) 14118.
[23] F. Zaera, D.A. Fischer, R.G. Carr, E.B. Kollin and J.L. Gland, in: Electrochemical Surface Science: Molecular Phenomena at Electrode Surfaces, Vol. 378 of ACS Symposium Series, Ed. M.P. Soriaga (ACS Books, Washington, 1988) pp. 131-140. 1,24] J.L. Gland, F. Zaera, D.A. Fischer, R.G. Carr and E.B. Kollin, Chem. Phys. Lett. 151 (1988) 227. [25] K.M. Ogle, J.R. Creighton, S. Akhter and J.M. White, Surf. Sci. 169 (1986) 246. [26] S.B. Moshin, M. Trenary and H.J. Robota, Chem. Phys. Lett. 154 (1989) 511. [27] W. Erley, Y. Li, D.P. Land and J.C. Hemminger, Surf. Sci. 103 (1994} 177. [28] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger and G. Comsa, J. Chem. Phys. 97 (1992) 6774. 129] T.V.W. Janssens and F. Zaera, Surf. Sci. 344 (1995) 77. [30] T.V.W. Janssens, F. Zaera, D. Stone and J.C. Hemminger, to be published. [31] D. Godbey, F. Zaera, R. Yates and G.A. Somorjai, Surf. Sci. 167 (1986) 150. [32] F. Zaera, J. Phys. Chem. 94 (1990) 5090. [33] J.P. Collman, L.S. Hegedus, J.R. Norton and R.G. Finke, Principles and Applications of Organotransition Metal Chemistry (University Science Books, Mill Valley, 1987). [34] F. Zaera, J. Phys. Chem. 94 (1990) 8350. [35] F. Zaera, J. Mol. Catal. 86 (1994) 221. [36] A. Wieckowski, S.D. Rosasco, G.N. Salaita, A. Hubbard, B.E. Bent, F. Zaera, D. Godbey and G.A. Somorjai, J. Am. Chem. Soc. 107 (1985) 5910. 137] M. Salmer6n and G.A. Somorjai, J. Phys. Chem. 86 (1982) 341. [38] F. Zaera and G.A. Somorjai, J. Am. Chem. Soc. 106 (1984) 2288. 1,39] T.P. Beebe, Jr. and J.T. Yates, Jr., J. Am. Chem. Soc. 108 (1986) 663. [40] H. Ofner and F. Zaera, J. Phys. Chem., submitted. [41] J. Kubota, S. Ichihara, J.N. Kondo, K. Domen and C. Hirose, Langmuir 12 (1996) 1926. [42] P.S. Cremer, X. Su, Y.R. Shen and G.A. Somorjai, J. Am. Chem. Soc. 118 (1996) 2942.
ELSEVIER
Reflection absorption infrared spectroscopy and kinetic studies of the reactivity of ethylene on Pt(111) surfaces Francisco Zaera *, Ton V.W. Janssens, Helmut Ofner Department of Chemistry, University of California, Riverside, CA 92521, USA Received 1 May 1996; accepted for publication 1 August 1996
Abstract
The chemistry of ethylene on P t ( l l l ) single-crystal surfaces has proven quite complex because it involves the simultaneous occurrence of several reactions, namely molecular desorption, dehydrogenation to ethylidyne, H-D exchange within the adsorbed molecules, and hydrogenation to ethane. Reflection absorption infrared spectroscopy (RAIRS) has been used here in conjunction with isothermal kinetic measurements to identify the possible intermediates involved in each of those reactions, and to follow their thermal chemistry on the platinum surface. All vinyl, ethyl and ethylidene moieties were prepared by thermal decomposition of their corresponding iodides and characterized by RAIRS. The experimental data available to date favors the formation of ethylidene as an intermediate in the conversion of ethylene to ethylidyne, but the complexity of the kinetics of that reaction, which changes significantly with changing surface coverages, makes the final proof of this mechanism quite difficult. In addition, a side ethyleneethyl equifibrium which starts at temperatures below those required for the formation of ethylidyne is responsible for H-D exchange in ethylene. Finally, the hydrogenation of ethylene to ethane also involves an ethyl intermediate, but only occurs at the ethylene high coverages needed for the transition of the di-tr strongly bonded species to a weak it configuration. The relevance of the reactions seen under vacuum to the high-pressure catalytic hydrogenation of ethylene is briefly discussed.
Keywords: Adsorption kinetics; Catalysis; Ethylene; Isotopic exchange; Platinum; Reflection absorption infrared spectroscopy; Single crystal surfaces; Surface chemical reactions; Thermal desorption spectroscopy
1. Introduction Since its discovery by Sabatier and Senderens in 1897, the hydrogenation of olefins over metal catalysts has been one of the most studied chemical processes [ 1,2]. As a representative case of those reactions, the chemistry of ethylene on P t ( l l l ) surfaces has received particular attention from the surface science community [ 3-5 ]. The study of this seemingly simple system is complicated by the fact that there appears to be at least two types of molecular adsorption states for ethylene, namely, a * Corresponding author.
strong di-a bonding and a weaker n-bonded configuration [6,7]. Furthermore, heating chemisorbed ethylene to around room temperature triggers several competitive reaction pathways, including hydrogenation to ethane, H - D exchange and conversion to ethylidyne [5]. Here we report some recent studies carried out in our laboratory designed to address each of those reactions in an individual manner.
2. Experimental Most experiments reported here were performed in an ultra-high vacuum (UHV) chamber evacu-
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 01078-3
372
F. Zaera et aL / Surface Science 368 (1996) 371-376
ated to a base pressure below 1 x 10 -1° Torr and equipped with sample cleaning and characterization facilities, as well as with instrumentation for temperature-programmed desorption (TPD) and reflection absorption infrared (RAIRS) spectroscopies [8]. The RAIRS spectra reported here were taken using a mercury--cadmium telluride (MCT) detector, by averaging 1000 scans taken with 4 cm -1 resolution and ratioing that against spectra for the clean surface recorded immediately before dosing. The kinetic measurements were performed in a second UHV system [9] specifically designed to perform dynamic kinetic measurements using a variation of the molecular-beam method initially developed by King and Wells [10]. The kinetic data reported here were taken isothermally by exposing the clean platinum surface to a directional collimated beam composed of mixtures of ethylene and hydrogen gases [ 11 ].
bonds) [21]. In that case vinylidene disappears around 170 K while acetylene subsists to 200 K, but above that temperature both appear to convert to chemisorbed ethylene, and only after heating to about 300 K is ethylidyne formation detected. At the present time ethylidene seems to be the most likely intermediate in the conversion of ethylene to ethylidyne. Indeed, 1,1-diiodoethane, used as a precursor for ethylidene, converts readily to ethylidyne [22]. This is illustrated by Fig. 1, which displays infrared spectra for 5.0 L (dose to saturation coverage, which amounts to about 0.10 ML) of 1,1-diiodoethane adsorbed on P t ( l l l ) after annealing the surface to different temperatures. The initial molecular adsorption below 100 K is apparent by the signals at 1197 and 1371 cm -1, which correspond to the ~-C-H and symmetric (umbrella) methyl deformation modes respectively.
CH3CHI2/Pt(111) RAIRS 3. Formation of ethylidyne The reaction which perhaps has received the most attention in relation to the thermal reactivity of ethylene chemisorbed on transition metal surfaces is its conversion to ethylidyne. This has turned out to be a particularly puzzling process, and the extensive studies performed on this system have not yet led to the development of a clear idea of the mechanism for the reaction. It has been argued that ethyl [12], vinyl [ 13], and/or ethylidene [14,15] moieties are the most likely surface intermediates in this conversion. We have taken advantage of the fact that halide derivatives are good precursors for hydrocarbon moieties [ 16,17] to prepare and characterize the chemistry of those three species by RAIRS. Ethyl moieties, prepared by thermal decomposition of chemisorbed ethyl iodide, were shown to convert to ethylene via a/3hydride elimination step at temperatures much lower than those required for the formation of ethylidyne [8,18-20]. Vinyl species, also made via thermal activation of its iodide, display a much more complex surface chemistry which involves the initial formation of a mixture of/~3-~/2-vinylidene and p3-~/2-acetylene (both moieties coordinated to several metal atoms via both a and
8 -~ EthylldyneYlel~ [ ~0
.0002
~' ~4
100L " ~
'°~,,.o~, T, iP°
5.0 L Exposure Tads= 100 K 1339i Apnealina 1197 II 1371 T/K
A !
1200
1300
2~K
1400
1500
Frequency / ¢m "1 Fig. 1. Reflection absorption infrared (RAIRS) spectra from 5.0L of 1,1-diiodoethane on P t ( l l l ) , immediately following dosing at 100 K and after subsequent annealing to 150, 200, 250 and 300 K. The signals at 1197 and 1371 cm -1 correspond to C - H deformation modes in the molecular diiodoethane, while that at 1339 cm -1 is assigned of the symmetric deformation of methyl groups in ethylidyne. The inset displays the changes in the amount of ethyfidyne produced as a function of annealing temperature for low (2.0 L) and high (10.0 L) initial coverages of 1,1-diiodoethane. Two reactivity regimes are clearly distinguished here, one around 150 K favored at low coverages, and a second around room temperature which most probably involves the previous formation of ethylene, and which is preferred on saturated surfaces.
F. Zaera et al. / Surface Science 368 (1996) 371-376
The interesting observation from these experiments is the fact that the peak at 1339 cm -1 associated with ethylidyne starts to grow around 150 K, a temperature well below that required to make ethylidyne from ethylene, ethyl or vinyl species. The chemistry of ethylidene is nevertheless complicated by the fact that variations in surface coverages induce significant changes in its kinetic behavior: after exposures above 2.0L (0= 0.05 ML), additional ethylidyne is made around 300 K, and no low-temperature conversion at all is seen after a 10.0 L dose (Fig. 1, inset). These results highlight the fact that the availability of empty surface sites plays a key role in the kinetic of ethylidyne formation. The isothermal kinetics of the conversion of ethylene to ethylidyne was initially thought to follow a simple first-order behavior 1-23-26], but was later proven to display at least two different rate constants as the reaction progresses [27]. Furthermore, a scanning tunneling microscopy in situ characterization of this system has highlighted the fact that the reaction occurs preferentially at the edges of ethylene domains, indicating that special surface ensembles are needed for the conversion to proceed 1,28]. Finally, results from TPD experiments with the isotopically labelled CHD=CD2 ethylene argue for the need of an ethylene--ethylidene pre-equilibrium to precede the subsequent conversion to ethylidyne at high coverages [13]. All this is consistent with a mechanism in which ethylene always isomerizes to ethylidene prior to its conversion to ethylidyne, and where that first step is rate-limiting at low coverages but leads to an ethylene-ethylidene pre-equilibrium at coverages close to saturation because of the unavailability of empty sites, which poisons the following ethylidene-to-ethylidyne conversion 1,223.
4. H-D exchange in chemisorbed ethylene In the presence of surface deuterium, the conversion of ethylene to ethylidyne on Pt(111) is accompanied by exchange of hydrogen for deuterium atoms. Indeed, infrared spectra obtained after coadsorbing C2H4 and C2D4 at 330 K reveal the
373
formation of partially deuterated ethylidyne [29]. However, extensive isotope scrambling is also observed when coadsorbing C2D, with H2, indicating that the exchange most likely involves surface hydrogen. Fig. 2 shows the key RAIRS spectra that corroborate those observations. Additional TPD evidence also points to the idea that the detection of the observed H-D-exchanged ethylidyne is not necessarily connected with the reactions leading to its formation, but due to previous exchange on chemisorbed ethylene instead. The kinetics of H-D exchange in chemisorbed ethylene has been independently studied by laser-induced desorption isothermal kinetic experiments [30]. Those experiments have shown that there is significant H-D exchange reactivity at temperatures as low as 215 K, much lower than those required for ethylidyne formation, and that the reaction displays a complex kinetic behavior most probably associated with a limited diffusivity of hydrogen (deuterium) atoms on metal surfaces covered with hydrocarbon moieties.
H-D Exchange in Ethylene/Pt(111) RAIRS of Ethylidyne from 5.0 L Ethylene dosed at 330 K I0.0005
o O r¢=
0 Io
5s(CHa) 1339
"(C-CX3) : J~ : Jl ~
.
,
.
,
.
.
8s(CDHz) i P(CD2H) :
i
.
,
.
.
iC2H4+C D
.
,
.
.
.
,
.
.
.
,
.
.
1000 1100 1200 1300 1400 Frequency / c m "1
Fig. 2. RAIRS spectra for the ethylidyne produced by adsorption of 5.0 L of pure C2H+ (top), pure C2D4 (second from top), and a mixture of C2H+ and C2D+ (ratio C2H+: C~D4= 1:2.5) (third from top) on P t ( l l l ) under vacuum, and by adsorption of 5.0L of C2D4 in the presence of 5 x l 0 - a T o r r of Hz (bottom). All exposures were performed at 330 K. The peak at 1247cm -1 seen in the two last spectra is due to partially deuterated cthylidyne, and is indicative of the occurrence of H-D exchange reactions.
374
F Zaera et al. / Surface Science 368 (1996) 371-376
The most likely mechanism for the H - D exchange in ethylene is one involving the formation of an ethyl intermediate [31,32]. This idea is quite reasonable, given that a similar pathway has been demonstrated in organometallic systems [17,33], and because ethyl, once formed, does undergo a clean and facile #-hydride elimination step back to ethylene on most surfaces studied to date [17,18,34,35]. H - D exchange between ethylidyne and surface hydrogen (or deuterium) does occur too, but at a much lower rate than ethylidyne formation [29,36]; the mechanism for that reaction has not yet been elucidated.
5. Hydrogenation to ethane Finally, a small amount of ethane formation from self-hydrogenation is also seen in TPD experiments with ethylene on P t ( l l l ) under vacuum [32,37]. Isotope-labelling experiments have shown that this occurs in a stepwise fashion, via the slow formation of an ethyl intermediate [5,32], but there has been some discussion on the relevance of this mechanism to the catalytic ethylene hydrogenation process which occurs at higher pressures, since under those conditions the surface is covered almost completely with other hydrocarbon moieties [38]. In the initial surface science study that identified the presence of ethylidyne during the catalytic process, it was suggested that the surface moieties could act as hydrogen-transfer agents between the metal and ethylene molecules weakly adsorbed on a second layer [38], but later work has led to an alternative model in which ethylidyne acts just as a site blocker, and where ethylene hydrogenation still occurs via ethyl surface intermediates [39]. Distinguishing between these two ideas is particularly hard not only because it has not been possible to emulate the catalytic nature of the hydrogenation under vacuum, but also because the reaction probabilities per impinging molecule under most practical conditions are extremely low, on the order of 10 - 4 o r less, and this suggests a correspondingly small steady-state concentration of surface intermediates. We have recently performed some isothermal vacuum kinetic measurements on this system [11,40] which indicate that: (i) ethylidyne may
indeed only poison the hydrogenation reaction. In fact, the rate for ethylene hydrogenation may be limited by the rate of ethylidyne removal, since that reaction would be required to open surface sites for the adsorption of both hydrogen and ethylene. (ii) Hydrogen adsorption is strongly inhibited by the presence of ethylene in the gas phase. The competitive adsorption of hydrogen does not occur under vacuum, even when gas mixtures with hydrogen-to-ethylene ratios as high as 1000:1 are used. This may explain the firstorder dependence on hydrogen and the negative order on ethylene obtained for the rate law of ethylene hydrogenation in most catalytic cases [2,38]. (iii) Only a weakly, probably n-bonded, ethylene species (for which there is in fact some spectroscopic evidence of its presence during the catalytic process [41,42]) is active in ethylene hydrogenation. This is illustrated by the kinetic results displayed in Fig. 3, which displays the time evolution of the partial pressures of both ethylene and ethane during King and Wells-type experi-
Ethylene Hydrogenation/Pt(111) Isothermal Kinetic M e a s u r e m e n t s at 2 5 0 K Using a C o l l i m a t e d Directional G a s B e a m
I:
ol
:3 ¢o ¢0
///
' C~H.
~ ._.
a. 1~:1 b. 7s:1 ~ 300:1
o marion
, \ .
,'~'~,z,~,,x~
#/
0,. B ol
f#
1::
/hr
D.
.
,
-10
.
.
.
.
,
-5
.
.
.
.
,
0
.
.
.
.
,
.
.
.
.
5
,
10
.
.
.
.
,
15
.
.
.
.
,
.
20
Time / s
Fig. 3. Isothermal kinetic data for the hydrogenation of ethylene at 250 K, as obtained by using a variation of the so-called King and Wells method where collimated beams of hydrogen+ethylene gas mixtures are impinged onto a clean P t ( l l l ) surface. Both the uptake of ethylene (C2H4) on the surface and the evolution of ethane (C2H6, the hydrogenation product) are shown here as a function of time for three different gas mixtures. The figure shows that ethane formation starts only after reaching ethylene coverages high enough to induce the transition from di-a to ~ bonding, and that an increase in hydrogen content in the impinging beam mixture exerts two main effects on the kinetics of the reaction, namely, an increase in both the hydrogenation rate and its yield, and an inhibition of the uptake of ethylene on the surface.
375
F. Zaera et aL / Surface Science 368 (1996) 371-376
ments [ 10]. Collimated beams of mixtures of ethylene and hydrogen (where the flux of ethylene was fixed at F(C2H4)=0.05ML s -1 and that of hydrogen was varied between F(H2)=0.8 and 15 M L s -1) were turned on around t = - 1 0 s, but were kept away from the crystal until t = 0 , at which point the intercepting flag was removed and the gases were allowed to impinge directly onto the platinum surface. The drop in ethylene signal seen from that point on is proportional to the ethylene sticking coefficient on P t ( l l l ) (which initially is of the order of 0.8), while the increase in ethane partial pressure is proportional to the rate of ethylene hydrogenation. One fact that becomes immediately apparent from the data in Fig. 3 is that there is a delay between the beginning of the ethylene uptake on the surface and the start of ethane formation. In fact, the threshold for ethane production is seen at the point where the weakly adsorbed ethylene state starts to be populated. Two other effects are highlighted by these results, namely: (i) that the rate for ethane formation is higher and continues for a longer time when the hydrogen content in the beam is increased, and (ii) that the uptake of ethylene on the surface is inhibited by coadsorbed hydrogen as well. Finally, additional isotope-labelling experiments indicate that the transition from the strongly bonded di-tr state to the weaker rc interaction for ethylene is fast and reversible, occurs once a given surface coverage is reached, and may either be collective or take place via a fast displacement mechanism
[41]. 6. Conclusions The present paper summarizes the results from some of our most recent studies on the chemistry of ethylene on platinum surfaces. It is apparent from the brief review provided here that even though some progress has been made on the understanding of this chemistry at a molecular level, most of the key questions still remain unanswered. At the present time we would suggest that the catalytic hydrogenation of ethylene most probably occurs via the stepwise incorporation of hydrogen atoms into weakly adsorbed ethylene, to form initially ethyl moieties and ultimately ethane
CHaCH3(g) Ethane Ethylidene ~+H Hydrogenation CH~ H-D Exchange (~H2 +H H~TCH24. ~ ' ~ ' ~ Ethyl
~-Ethylene
di-o-Ethylene
Ethylidyne/ ~+H~HI~CH2 Vinyl
Reaction Energetics AHfO/kcal/mol 15.
J
(ads)
Ethane(g) Hlydrogenatlon -2H(ads) H-D Exchange -5
•di-o-Ethylene (ads)
Dohydrogendon
Ethylidyne +H(ads)
Reaction Coordinate
Fig. 4. Top: reaction scheme for the surface chemistry of C2 moieties on Pt(lll). Bottom: energy diagram for the relevant surfacereactionsinvolvedin the thermalconversionof ethylene. Mechanisms are proposed here for both the conversion of ethylene to ethylidynevia an ethylideneintermediate and for H-D exchange and hydrogenation reactions via a common ethyl moiety. (the same as under vacuum). This reaction is inhibited considerably by the competitive formation of ethylidyne, since this last species is particularly stable and hard to rehydrogenate. The conversion of ethylene to ethylidyne is most likely a complex reaction, but it probably does not involve an ethyl intermediate, as the hydrogenation and H - D exchange processes, but an ethylidene moiety instead. All these ideas are illustrated schematically in Fig. 4. The other important observation to keep in mind from this work is the fact that the kinetics of the relevant surface reactions are greatly affected by both the coverages of the different surface species and the availability of surface free sites.
Acknowledgement Financial support for this research was provided by a grant from the National Science Foundation (CHE-9530191).
376
F. Zaera et al./Surface Science 368 (1996) 371-376
References [1] G.C. Bond, Catalysis by Metals (Academic Press, London, 1962). I-2] J. Horiuti and K. Miyahara, Hydrogenation of Ethylene on Metallic Catalysts, National Bureau of Standards, Report NSRDS-NBC No. 13, 1968. [3] S.J. Thomson, in: Catalysis: A Specialist Periodical Report, Vol. 3, Eds. C. Kemball and D.A. Dowen (The Chemical Society, London, 1980) pp. 1-38. [4] F. Zaera and G.A. Somorjai, in: Hydrogen Effects in Catalysis: Fundamentals and Practical Applications, Eds. Z. Pail and P.G. Menon (Marcel Dekker, New York, 1988) pp. 425--447. [5] F. Zaera, Langmuir 12 (1996) 88. [6] N. Sheppard, Annu. Rev. Phys. Chem. 39 (1988) 589. [7] M.B. Hugenschmidt, P. Dolle, J. JupiUe and A. Cassuto, J. Vac. Sci. Technol. A 7 (1989) 3312. [8] H. Hoffmann, P.R. Grifliths and F. Zaera, Surf. Sci. 262 (1992) 141. [9] J. Liu, M. Xu, T. Nordmeyer and F. Zaera, J. Phys. Chem. 99 (1995) 6167. [10] D.A. King and M.G. Wells, Surf. Sci. 29 (1972) 454. [11] H. Ofner and F. Zaera, Proc. 211th ACS National Meeting, New Orleans, March 1996. [12] G.A. Somorjal, M.A. van Hove and B.E. Bent, J. Phys. Chem. 92 (1988) 973. [13] F. Zaera, J. Am. Chem. Soc. I l i (1989) 4240. [14] X.-L Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. 193 (1988) 387. [15] P. Cremer, C. Stanners, J.W. Niemantsverdriet, Y.R. Shen and G. Somorjai, Surf. Sci. 328 (1995) 111. [16] F. Zaera, Acc. Chem. Res. 25 (1992) 260. [17] F. Zaera, Chem. Rev. 95 (1995) 2651. [18] F. Zaera, Surf. Sci. 219 (1989) 453. [19] F. Zaera, J. Am. Chem. Soc. 111 (1989) 8744. [20] F. Zaera, H. Hoffmann and P.R. Grit~ths, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 705. [21] F. Zaera and N. Bernstein, J. Am. Chem. Soc. 116 (1994) 4881. [22] T.V.W. Janssens and F. Zaera, J. Phys. Chem. 100 (1996) 14118.
[23] F. Zaera, D.A. Fischer, R.G. Carr, E.B. Kollin and J.L. Gland, in: Electrochemical Surface Science: Molecular Phenomena at Electrode Surfaces, Vol. 378 of ACS Symposium Series, Ed. M.P. Soriaga (ACS Books, Washington, 1988) pp. 131-140. 1,24] J.L. Gland, F. Zaera, D.A. Fischer, R.G. Carr and E.B. Kollin, Chem. Phys. Lett. 151 (1988) 227. [25] K.M. Ogle, J.R. Creighton, S. Akhter and J.M. White, Surf. Sci. 169 (1986) 246. [26] S.B. Moshin, M. Trenary and H.J. Robota, Chem. Phys. Lett. 154 (1989) 511. [27] W. Erley, Y. Li, D.P. Land and J.C. Hemminger, Surf. Sci. 103 (1994} 177. [28] T.A. Land, T. Michely, R.J. Behm, J.C. Hemminger and G. Comsa, J. Chem. Phys. 97 (1992) 6774. 129] T.V.W. Janssens and F. Zaera, Surf. Sci. 344 (1995) 77. [30] T.V.W. Janssens, F. Zaera, D. Stone and J.C. Hemminger, to be published. [31] D. Godbey, F. Zaera, R. Yates and G.A. Somorjai, Surf. Sci. 167 (1986) 150. [32] F. Zaera, J. Phys. Chem. 94 (1990) 5090. [33] J.P. Collman, L.S. Hegedus, J.R. Norton and R.G. Finke, Principles and Applications of Organotransition Metal Chemistry (University Science Books, Mill Valley, 1987). [34] F. Zaera, J. Phys. Chem. 94 (1990) 8350. [35] F. Zaera, J. Mol. Catal. 86 (1994) 221. [36] A. Wieckowski, S.D. Rosasco, G.N. Salaita, A. Hubbard, B.E. Bent, F. Zaera, D. Godbey and G.A. Somorjai, J. Am. Chem. Soc. 107 (1985) 5910. 137] M. Salmer6n and G.A. Somorjai, J. Phys. Chem. 86 (1982) 341. [38] F. Zaera and G.A. Somorjai, J. Am. Chem. Soc. 106 (1984) 2288. 1,39] T.P. Beebe, Jr. and J.T. Yates, Jr., J. Am. Chem. Soc. 108 (1986) 663. [40] H. Ofner and F. Zaera, J. Phys. Chem., submitted. [41] J. Kubota, S. Ichihara, J.N. Kondo, K. Domen and C. Hirose, Langmuir 12 (1996) 1926. [42] P.S. Cremer, X. Su, Y.R. Shen and G.A. Somorjai, J. Am. Chem. Soc. 118 (1996) 2942.