12656
J. Phys. Chem. 1993,97, 12656-12659
Site Blocking Effects in Ethylidyne Decomposition Kinetics on Ru(001): ImSitu Study with Infrared Reflection Absorption Spectroscopy at Elevated Pressure Charles A. Mims,'J Mark D. Weisel,* Friedrich M. Hoffmann,'**John H. Sinfelt,$ and John M. Whites Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S lA4; Exxon Corporate Research, Annandale, New Jersey 08801; and Department of Chemistry, University of Texas, Austin, Texas 78712 Received: July 20, 1993; In Final Form: September 27, 19938
Utilizing in-situ FT-IRAS, we have investigated the decomposition kinetics of ethylidyne in the presence of coadsorbates on Ru(001). The results demonstrate that the stabilizing effect of coadsorbed CO and hydrogen is significantly enhanced a t elevated pressure, supporting a model wherein an empty neighboring metal site is required before decomposition can proceed. We identify a weakly adsorbed CO species in 3-fold hollow sites in close proximity to the ethylidyne, whose coverage is controlled by the CO pressure at elevated temperatures. The results demonstrate the importance of weakly adsorbed species at high coverage in controlling surface reactions a t elevated pressures and temperatures.
The reaction rates and pathways of surface species not only depend on the surface site on which they reside but also may depend on the population of neighboring sites. The presence of coadsorbates may alter the reactivity by blocking neighboring sites, by alteration of the electronic structure of the adsorbent, or by more direct complex formation. The "site" concept itself (intentionally) hides the complexity of surface reactions which involve rearrangementsof an ensemble of adsorbent and adsorbate atoms. A sufficiently high partial pressure can maintain a high equilibrium surface coverage by weakly bound adsorbates at elevated temperaturesand greatly influence the progress of surface reactions. These conditions are not availablein ultrahigh vacuum (UHV), since readsorption from the gas phase is not significant. Understanding these details is, of course, important for a deeper understanding of catalysis. Ethylidyne ('C-CH3) is a persistent hydrocarbon fragment which occupies 3-fold hollow sites on many close-packed metal surfaces. On Ru(OOl), as on other surfaces, ethylene decomposition leads to ethylidyne formation (refs 1-6 and references therein). Other pathways such as CH2 recombination7 and acetylene hydrogenations also yield ethylidyne. Extensivesinglecrystal studies in UHV have revealed many structural and kinetic aspectsof ethylidyne surface chemistry. In addition to annealing experiments3.496 to determine surface reaction pathways, kinetic parametersfor ethylidyne formationand decomposition have been determined by TPD-MS,3*4.9time-dependent SSIMS,6-9and IRAS.10 NMR has been used to follow the analogous chemistry on supported catalysts.11 Coadsorbates, particularly CO, have been shown to influence the surface chemistryof ethylidyne. On Ru(001), CO influences the ethylidyne formation pathway516and, on Rh( 11l), induces orderingin the adlayer.12 Ethylidynedecomposition on Ru(001) is influenced more dramatically by the presence of a saturation coverage of coadsorbed C0.5.6J3 Ethylidyne is stabilized until the onset of CO desorption (20-40 K higher than without CO) and has been interpreted as a site-blocking effect, wherein decomposition requires an empty neighboring metal site which only arises when the CO desorb^.^.^ Decomposition selectivity is also altered with retention of more C-C bonds in the products. Similar strong influences of coadsorbates have been reported in t
University of Toronto. Exxon Corporate Research.
iUniversity of Texas.
Abstract published in Advance ACS Abstracts, December 1, 1993.
0022-3654/93/2097-12656$04.00/0
studiesof other systems.14 Under steady-stateCO hydrogenation conditions, the surfaces of supported Fischer-Tropsch catalysts15 as well as a model Ru(OO1) catalyst16 are covered by CO. Therefore, the stabilizing effects of coadsorbed CO noted above could allow ethylidyne to participate in catalytic reaction channels at temperatures well above the UHV decomposition temperature. The investigation of these effects is important for ethylidyne chemistry specifically as well as for the general understanding of surface reactions in the presence of weakly held coadsorbates. Therefore, we have used reflection infrared absorption spectroscopy (IRAS) to study in situ the decomposition kinetics of ethylidyne on Ru(001) under elevated pressures of CO and/or
H2. The experimentsreported here were performed in a combined UHV/high-pressure reactor system which has been previously described.16J7 The high-pressure reactor can be isolated from the UHV chamber and the gas pressure controlled from 10-lo Torr to 1 atm. However, in the experiments described here, where cryogenic cooling of the sample was required, the total pressure was kept at less than 1 Torr for practical considerations. Infrared spectra were obtained in singlereflection mode at 4-cm-1 resolution in the 4000-600-~m-~ range by averaging from 16 to 500 s ~ 8 n s . l ~ The Ru(001) single crystal was cleaned and characterized according to procedures described previously.18 Ethylidyne layers were prepared by saturation with ethylene at 85 K followed by annealing to 240 K. This resulted in ethylidyne layers of reproducible surface coverage (e 0.15). In order to maintain a reproducible ethylidyne coverage throughoutthese experiments, the coadsorbates (CO, H2) were postadsorbed since CO affects the decomposition pathway of e t h ~ l e n e . ~ Vibrational spectra of ethylidyne adsorbed on Ru(OO1) with differing amounts of coadsorbed CO are presented in Figure 1. Spectrum a of ethylidyne alone shows the CH3 deformation "umbrella" mode, b,(CH3), at 1340 cm-I and the symmetric CH stretch, v,(CH), at 2877 cm-1. A third mode observed at 2803 cm-l is barely visible but was observed with increased signal averaging (not shown here). This mode has been previously assigned by Malik et a1.19to a Fermi resonance between the C-H stretch and the overtone of the asymmetric CH3 bend, 6,(CH3). A fourth mode (not shown here) at 1120 cm-I, assigned to the C-C stretch, is also very weak and only observed with increased signal averaging. Coadsorption of CO with ethylidyne is shown in spectra b-k in Figure 1. Spectra b-f were taken in a time-resolved mode
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Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12657
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WAVENUMBER (cm-‘) IRASofethylidyneonRu(001): (a)CCH,preparedbyheating a saturation layer of ethylene (100 K) to 240 K (b-f) addition of CO at 3 X lo” Torr; (g-k) have been equilibrated under CO pressures of 3 X IO-’, 3 X l P , 3 X 10-4,3X 1W2, and 2 X 10-l Torr, respectively.
A
during exposure at 3 X 10-8 Torr of CO. The remaining spectra (g-k) were obtained under equilibrium pressures ranging from 3 X l e 7 to 0.2 Torr. Initial adsorption of CO results in the appearance of a broad band at 1946 cm-’, which gradually shifts to 1955 cm-l with increasing exposure. Further increase in CO coverage results in the appearance of a second CO band at 1776 cm-1 (8) while the main band sharpens and shifts to 2001 cm-1. Spectra j and k contain also contributions from the unresolved bands of gas-phase CO (centered at 2143 cm-l) in the highpressure reactor. By analogy with structural and spectroscopic studies on Rh(l1 1),l2 there can be little doubt that the second band, which has been also observed by Henderson et a1.,6 is due to adsorption in a 3-fold hollow site. CO adsorbed on clean Ru(001) in the absence of coadsorbates exhibits only a single band which shifts with CO coverage from 1984 to 2060 cm-’ and which is attributed to CO adsorbed on the on-top site.’* The presence of a coadsorbate, however, can result in the occupation of bridge or hollow sites as a result of steric crowding or lateral interactions as shown for the system CO + O/Ru(001).20 The order of filling indicates a weaker chemisorption bond for this CO than for the on-top sites. The lower frequency (1946 cm-l (b)) of the initial CO could also be due to CO adsorbed in 2-fold bridge sites or result from a substantial downshift of linearly adsorbed CO. Such a shift is conceivable in the light of similar upward shifts of 40-80 cm-l observed for the coadsorption of oxygen.21 In any case, adsorbed CO is substantially perturbed by the presence of ethylidyne. A detaileddiscussion of the frequency shifts observed here will be given in a subsequent publication. The vibrational modes of ethylidynealso indicate a significant interaction with CO. Although the ethylidyne modes are affected only marginally by a low coverage of coadsorbed CO, large effects are observed at high CO coverage, coincident with the appearance of the CO band at 1776 cm-l. The intensity of the C-H stretch isdrastically reduced, and the 6,(CHp) mode shows a pronounced increase in intensity’alongwith a shift from 1346 to 1356 cm-I. Similar intensity and frequency effects have been previously observed for the coadsorption of CO with ethylidyneon Rh( 11 1) by Blackman et a1.l2and for CO with CDpOD, D20, and Xe on
lsbo
WAVENUMBER (cm.‘)
-1.
Figure 2. Time-evolved TP-IUS spectra of the decomposition of an Ethylidyne layer on Ru(001) in the presence of 4 X l t 3Torr of CO + H2 (1:l). Heating rate = 0.5 K/s.
Pt( 111) by Ehlers et a1.22and discussed in terms of dielectric screening arising from the electronic polarizability of the coadsorbate. As shown experimentallyand theoretically for Xe/ CO on Ru(001) by Hoffmann et al.,*3 the presence of Xe atom induces a significant frequency downshift and intensity decrease of the C-O stretch due to the electrostatic field of the coadsorbed Xe atom. Since the strength of such an interaction depends strongly on the CO-ethylidyne distance, this effect can account for the observed strong reduction (disappearance)in the presence case and, more importantly, indicates a close proximity of the two species. Figure 2 shows the time-resolved vibrational spectra obtained during the thermal decomposition of an ethylidyne layer in the presence of CO Hz(1:l) at a total pressure of 4 X 10-3 Torr. Ethylidyne decomposition occurs between 360 and 390 K as evidenced by the disappearance of the 8,(CH3) mode at 1354 cm-1. The decomposition of ethylidyne coincides with the disappearance of the CO band at about 1780 cm-l, as previously noted by Henderson et a1.6 The disappearanceof the latter band also affects the main CO band at 2002 cm-l, which shifts to 2020 cm-l and increases slightly in intensity. A series of plots of the 6,(CH3) intensity as a function of temperature are shown in Figure 3 and illustrate the effect of CO pressure as well as the presence of other coadsorbates on ethylidyne decomposition. Ethylidyne adsorbed on the clean Ru(001) surface is the least stable (curve a). CO coadsorbed under UHV conditions (curve b) delays ethylidyne decomposition by approximately 20 K, in agreement with previousUHV results. Experiments at elevated CO pressures reveal a much more dramatic effect on ethylidynestability,shifting the decomposition temperature by 100 K at 0.02 Torr of CO (curve e). A somewhat lower pressure PCO = 0.002 Torr (curve d) also stabilizes the ethylidyne, but not as effectively as 0.02 Torr. In both cases the decomposition of ethylidyne coincides with the disappearance of CO adsorbed in the 3-fold hollow sites (YCO = 1780 cm-1). Causality cannot be implied from this coincidence, since the sites are continuously repopulated from the gas phase and these CO sites no longer exist after the ethylidyne decomposes. However, proximity to the ethylidyne and site
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12658 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
equilibrium with gas-phase CO at our conditions ka
CO(g)
+ +_+CO+(ads), Wc0 = k 2 / k 2 )
(2)
k-2
The ethylidyne decomposition rate, k’,is then given by k’=-d[CCH3]/[CCH3]dt = IC,[+] = kl/(l
+ KcoPco) (3)
which reduces to
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,
,
,
,
,
,
,
300
,
350
,
,
,
(
,
,
,
,
r‘
400
T(K) Fi-3. IRAS intensity of the 6,(CHj) (1 340 cm-I) mode of ethylidyne as a function of temperature taken during temperature-programmed experiments. The solid lines, calculated assuming first-order kinetics (k = Y exp{-E,/1,98n), are primarily to guide the eye. (a, A) ethylidyne on clean Ru(001), Y = 2 X 109, & = 15 OOO;(b, A) 7 langmuirs of CO postadsorbedat3 X lVTorr,u = 2.5X lO9,E.= 16 OoO,(c,D)ethylidyne adsorbed on a carbon-contaminated surface, Y = 5 X 106, E. = 13 000; (d, 0 ) ethylidyne in 0.002 Torr each of CO and Hz,Y = 6 X lo1*,Ea = E. = 25 OOO. 25 OOO,(e, 0 ) in 0.02Torr of CO,Y = 1.5 X
considerations for ethylidyne decomposition make it probable that the 1780-cm-1 CO is the stabilizing species. Of course, both physical site-blocking and electronic effects can contribute to the stability of the ensemble. Results obtained with a static pressure of 0.04 Torr of H2 in the absence of CO (not shown here) show that hydrogen also stabilizes ethylidyne. In this case a curve similar to (d) is obtained, demonstrating that hydrogen is less effective than CO at the same pressure, in keeping with the smaller relative values of the H2 adsorption equilibrium constant.24 Finally, ethylidyne decomposition is also retarded by its own decomposition products. Curve c in Figure 3 shows the decomposition of ethylidyne prepared in the presence of a carbonaceous residue from the decomposition of a previous ethylidyne layer. In this case, the decomposition of ethylidyne occurs over a broader temperature range than with either Hz or CO. This probably results from a distribution of chemical environmentsfor ethylidyneon thissurface. This effect obviously complicatesthe kinetic interpretation of ethylidynedecomposition in UHV. Coadsorbate effects have obvious implications for the reactivity of ethylidyne and other surface intermediates in high-pressure environments. During ethylene hydrogenation on Pt(l1 l), ethylidyne forms a stable layer in which carbon is sequestered, although it appears to be involved in hydrogen transfer.25 That ethylidyne would have decomposed after an equivalent time in UHV at the reaction temperatures indicates that hydrogen stabilizes ethylidyne either by site blocking or by shifting the ethylidyne dehydrogenation/ hydrogenation equlibrium. Alkylidynes have not been identified as such among the large residue of surface hydrocarbon species during the Fischer-Tropsch synthesis. However, a rough estimate of the ethylidyne lifetime on Ru(O0 1) under these relatively severe reaction conditions can be obtained using the following very simple site-blocking model. We assume that ethylidyne requires a single unoccupied adjacent (ensemble) site (heredesignated_+)in order todecompose kl
CCH,(ads) + 5- H,(g),C,H,(ads) (1) and that these adjacent sites are in Langmuir adsorption
k’= k,/~coPco (4) when the adjacent sites are near saturation. An increase in pressure can therefore offset the increase in kl/Kco which results from an increase in temperature. With further assumptions, Le., constant heat of CO adsorption (A) on the adjacent sites and a constant activation energy ( E , ) for reaction 1, the following relation is obtained d(l/T)/d(lnPco)lp = - R / ( E , + A) (5) Thus, an exponential increase in CO pressure produces a linear decrease in the value of 1/T at which a given decomposition rate is observed. Note that the adjacent sites remain almost fully covered during the ethylidyne decomposition but their number decreases as the ethylidyne disappears-in keeping with the spectroscopicresults and distinct from UHV experiments where no readsorption occurs. From our results at the two different CO pressures, a value of 110 f 30 kl/mol is obtained for El + A. This, in turn, results in calculated lifetimes of 1-10 s when used to extrapolate k’to Fischer-Tropsch synthesis conditions (Pco = 1 atm, 500 K). A distinct C2 intermediate with a lifetime of approximately 20 s has been kinetically identified on an operating supported ruthenium catalyst.26 Although, ethylidyne has not been specifically identified on this or other operating Fischer-Tropsch catalysts, stabilization by CO could allow other ethylidyne reaction pathways to compete effectively with decomposition under these relatively severe conditions. In any case, these data serve to emphasize the need to establish the site requirements for any surface reaction before extrapolatinglow-pressure data tocatalytic conditions. Acknowledgment. Support of this research by the Natural Sciences and Engineering Research Council of Canada (C.A.M.), Exxon Research and Engineering (C.A.M.), andthe US.Department of Energy, Chemical Sciences Program (J.M.W.), is gratefully acknowledged. References and Notes (1) Kesmodel, L. L.;Dubois, 1979, 70, 2180.
L. H.; Somorjai, G. A. J. Chcm. Phys.
(2) Stuve, E. M.; Madix, R. J. J. Phys. Chcm. 1985, 89, 105, 3183. (3) Hills, M. M.; Parmeter, J. E.;Mullins, C. B.; Weinberg, W. H. J . Am. Chem. Soc. 1986,108,3554. (4) Barteau, M. A.; Broughton, J. Q.; Menzel, D. Appl. Surf. Sei. 1984, 19. 92. ( 5 ) Hills, M. M.; Parmeter, J. E.;Weinberg, W. H. J . Am. Chrm. Soc. 1986, 108, 7215. (6) Henderson, M. A.; Mitchell, G. E.;White, J. M. SurJScl. 1988,203,
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(7) Henderson, M. A.; Radloff, P. L.;White, J. M.; Mims, C . A. J. Phys. Chcm. 1988,92, 41 11. (8) Parmeter, J. E.;Hills, M. M.; Weinberg, W. H. J. Am. Chrm. Soc. 1987, 109, 72.
(9) Grcenlief, C. M.; Radloff, P. L.;Zhou, X.L.;White, J. M. SurJSci. 1987,191, 93. (10) Mohsin, S. B.; Trenary, M.;Robota, H. J. Chem. Phys. Lctr. 1989,
154, 511. (1 1) Zax, D. B.; Mug, C. A.; Slichter,C. P.; Sinfelt,J. H. 1.Phys. Chrm. 1986, 93, 5009. Klug, C. A.; Slichter, C. P.; Sinfelt, J. H. J. Phys. Chrm. 1991, 95, 2119. Klug,C. A,; Slichter, C. P.;Sinfelt, J. H. J. Phys. Chrm. 1991, 95, 7033.
Letters (12) Blackman, G.S.;Kao, C. T.; Bent, B. E.; Mate, C. M.; Van Hove, M.: Somoriai. G. A. Surf Sci. 1988. 207. 66. (13) Eiawa, C.; NaiG, S.; T a m & K.J. Chem. Soc., Faraday Trans. 1, 1986,82,3197. (14) Akhter,S.; White, J. M.Surf.Sci. 1987,180,19.Akhter,S.; White, J. M. J. Yac. Sci. Technol. 1988., Ab.. 864. Akhtcr. S.;White, J. M. CRC Rev. 1988, 14, 131. (15) Biloen, P.; Helle, J. N.; van den Berg, F. G.A.; Sachtler, W. M. J. Caral. 1983,81,450.Furusawa, T.;Suzuki, M.; Smith, J. M. Carol. Reu. 1W6,23,42. Cant, N. W.; Bell, A. T. J. Caral. 1982,73,257. (16) Hoffman, F.M.; Robbins, J. L. J. ElecrronSpecrrosc.Relar. Phcnom. 1987,45,421.Proc.Inr. Cong.onCaralysis, 9rh, Calgav;CIC PES: Ottawa, 1988;p 1144. (17) Hoffmann, F.M.J. Chem. Phys. 1989,90,2816. (18) Pfnilr, H.; Hoffmann, F. M.; Ortega, A.; Menzel, D.; Bradshaw, A. M. Surf. Sci. 1980, 93,431. DePaola, R. A.; Hrbek, J.; Hoffmann, F. M. J. Chcm. Phys. 1985,82,2484.
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12659 (19) Malik, I. J.; Brubaker, M. E.; Mohsin, S.B.; Trcnary, M. J. Chem. Phys. 1907,87,5554. (20) Kostov, K.L.;Jakob, P.; Rauscher, H.; Menzel, D.J. Phys. Chem.
1991,95,7785. (21) Hoffmann, F. M.; Weisel, M.D.;Pedcn, C. H. F. Surf. Sci. 1991, 253,59. (22) Ehlers, D.H.; Esser,A. P.;Spitzer, A.; LUth, H. Surf. Sci. 1987,191, 466. (23) Hoffmann, F. M.; Lang, N.D.; Nerskov, J. K.Surf. Scf. 1990,226, L48. (24) Shimizu, H.; Chiristman, K.;Ertl, G. J. Carol. 1980, 61,412. (25) Godbey, D.;Zacra, F.; Ycates, R.; Somorjai, G. A. Surf. Sci. 1986, 150, 150. (26) Mims, C.A.; McCandlish, L.E.;Melchior, M. T. Catal. Lcrr. 1988, 1, 121. Mims, C. A.; McCandlish, L.E.; Melchior, M. T. Proc. Inr. Cong. on Catalysis, 9rh, Calgary; CIC Press: Ottawa, 1988;p 1992.
