The role of defects in the dissociative adsorption of CO on Ni(100)

Surface Science 172 (1986) L561-L567 North-Holland, A m s t e r d a m

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S U R F A C E SCIENCE LETTERS T H E R O L E OF D E F E C T S IN T H E DISSOCIATIVE A D S O R P T I O N O F CO O N N i ( l ~ ) H.P. S T E I N R O C K , M.P. D'EVELYN * and R.J. MADIX Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA Received 23 December 1985; accepted for publication 20 March 1986

We present a molecular beam study on the dissociative sticking coefficient for CO on a Ni(100) surface. In the range from 1.6 to 20 k c a l / m o l the observed dissociative sticking coefficient at a surface temperature of 500 K is 0.02 and independent of beam energy. Investigations on a sputter-damaged surface result in a sticking coefficient of 0.40, again independent of beam energy. The observed behavior can be explained by CO dissociation at defect sites.

Dissociative adsorption of CO is an important step in many catalytic reactions such as methanation [1] or Fischer-Tropsch synthesis [1] on transition metal surfaces. It has been shown that there is a trend in the adsorption behavior of carbon monoxide on transition metal surfaces as a function of their position in the periodic table [2]. On transition metals to the left side of a border line drawn from Co to W, dissociation occurs during CO adsorption, whereas on transition metals on the right CO usually adsorbs in molecular form. Nickel is close to this boundary and therefore structural effects on the adsorption behavior may be expected. There is a large variety of studies on the adsorption behavior of CO on nickel surfaces in the literature [2-15], most of them treating molecular chemisorption. At low temperatures ( T < 300 K) CO usually adsorbs in molecular form on all low index nickel planes; heating the surface to 500 K results in CO desorption with a peak maximum around 450 K with no carbon or oxygen remaining on the surface. For polycrystalline samples, stepped surfaces or sputter damaged surfaces several workers have reported CO decomposition upon heating up to 500 K [8-11]. Further investigation showed that exposing nickel surfaces to CO at elevated surface temperatures ( T > 500 K) results in CO decomposition even on low index planes [3,7,12]. Another process that leads to dissociation and decomposition of CO is the interaction of an electron beam (i.e. LEED, Auger) with molecularly adsorbed CO on the surface [3,9,12,13]. * Present address: Department of Chemistry, BG-10, University of Washington, Seattle, Washington 98195, USA.

0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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H.P. Steinri~ck et al. / Role of defects in adsorption of CO on Ni(lO0)

Associative desorption of CO from nickel surfaces occurs in one or both of two peaks in temperature programmed desorption (TPD), depending on the conditions [9,12,14,16]. The first peak appears at temperatures between 600 and 700 K and is explained by recombination of CO dissociated at defect sites on the surface [16] or by recombination of molecularly adsorbed CO dissociated during interaction with an electron beam [9,12]. In a recent paper List and Blakely [16] investigated CO formation on four different nickel surfaces including N i ( l l l ) , stepped N i ( l l l ) , Ni(210) and polycrystalline nickel. They came to the conclusion that the relative height of this first peak (without interaction with an electron beam) is correlated with surface roughness or defect density - the rougher the surface the larger the desorption peak. The second peak appears in the temperature range between 800 and 1000 K and is interpreted in recent papers as to be a result of CO formation from surface graphite [12,14] or carbon diffusing from the bulk to the surface [16] and oxygen adsorbed on the surface. In both cases the height of this second peak is proportional to the amount of oxygen present on the surface. For reaction of surface graphite with adsorbed oxygen the peak maximum does not change its position with oxygen coverage [14], whereas with recombination of carbon from the bulk with oxygen the desorption peak shifts to higher temperatures with increasing oxygen coverage [16]. We have now performed a molecular beam study of dissociative adsorption of CO on Ni(100), where the dissociative sticking coefficient was studied as a function of the energy of the impinging CO molecules. The experimental setup has recently been described in detail [17]. Basically, a three-stage supersonic molecular beam is directed toward the Ni(100) surface in the center of the U H V chamber (base pressure below 1 × 10-10 Torr). The chamber is equipped with LEED, a retarding field Auger spectrometer (AES) for surface analysis and a quadrupole mass spectrometer for TPD. The kinetic energy of the beam was varied between 1.6 and 20 k c a l / m o l by heating the nozzle to 1000 K a n d / o r seeding in He. Beam intensities were of the order of ( 1 - 5 ) × 1013 molecules/cm 2. s. The beam diameter of 1 cm was the size of the Ni(100) crystal. The sample was heated by radiation with a tungsten filament from the back and the temperature was measured with a chromel/alumel thermocouple spotwelded onto the crystal. Typical heating rates were 8 K / s . In a first step we measured the atomic oxygen and carbon coverage as a function of CO exposure in the beam by means of TPD. To do this the clean surface was exposed to the beam at normal incidence for a certain time at a surface temperature of 500 K. This temperature is well above the desorption temperature for molecularly adsorbed CO, and the equilibrium coverage during the beam dose is below 1% of a monolayer. The amount of CO dissociatively adsorbed was then determined by flashing the surface to 900 K and integrating the area under the resulting desorption peak. It is possible that this procedure underestimates the carbon and oxygen coverage somewhat, as

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bulk dissolution of both carbon [18] and oxygen [19] have been reported on Ni(100) above 500-600 K. In fig. 1 a set of thermal desorption spectra for different exposures of a CO beam at 5.8 kcal/mol kinetic energy is presented. The shape of the desorption peak is nearly symmetric; the peak maximum is about 700 K and shifts to lower temperatures with increasing coverage. This is a typical result for second order desorption [20] expected for recombination of O and C atomically adsorbed on the surface. Surface analysis by AES during the experiments showed carbon and oxygen built up after exposing the surface to the beam. The AES analysis was usually done after the largest exposure. To make sure that the AES electron beam did not influence the results, we did the TPD experiment twice; first immediately after exposing the surface to CO and secondly after taking an AES after CO exposure. Within the margin of error (+5%) no difference between these two measurements could be observed. After the surface was flashed to 900 K, no carbon or oxygen remained on the surface. Determination of the area under the several desorption spectra gave the results shown in fig. 2, where surface coverage is plotted versus exposure for different beam energies. Obviously, the beam energy does not change the adsorption behaviour of CO on Ni(100) significantly. In addition to the investigations with CO in the beam we also measured the dissociation probability for CO background adsorption from an ambient gas at 1.5 × 10 -8 Torr pressure. These data are added in fig. 2 and the behavior matches that obtained with the beam.

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H.P. Steinrftck et al. / Role of defects in adsorption of CO on Ni(lO0) I

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Evaluation of the initial slope in fig. 2 for different beam energies supposing second order adsorption leads to the initial sticking coefficients plotted in fig. 3. Independent of beam energy, the sticking coefficient for CO on this Ni(100) surface is 0.02 + 0.01. This result leads us to the conclusion that the observed dissociative CO adsorption is not activated. This result is at odds with the result obtained by Rosei et al. [21] for dissociative adsorption of CO on Ni(ll0). They indicate a dissociation probability in the order of 1 0 - 4 with an activation energy of 23 kcal/mol. Furthermore, the results suggest that surface defects are responsible for the relatively high sticking coefficient observed in our experiments. To verify this conclusion we measured the initial sticking probability for a sputter-damaged surface, presumed to have a higher defect concentration. The technique to measure the initial sticking coefficient was somewhat different, since T P D could not be used because the surface damage was annealed by flashing the surface up to 900 K. The technique applied was introduced by King and Wells in 1974 [22] and allows the determination of the initial sticking probability directly. After sputtering the surface at 300 K with argon ions (voltage 300 V, ion current 10 /~A) the surface temperature was held at 500 K, and the sticking coefficient was determined by the method mentioned above. Sputtering of the surface was done prior to each experiment

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to obtain similar surface conditions. In fig. 3 the results of these measurements are presented. The initial dissociation coefficient increased an order of magnitude in comparison to the annealed surface, but again the obtained value (0.40 + 0.05) was independent of beam energy. This result is strong evidence that dissociation of CO on this Ni(100) surface is due to the interaction with defects on the Ni(100) surface. The large value of 0.40 allows the further conclusion that the dissociation probability at defect sites is very large, perhaps near unity. The fact that both the sticking coefficient on the annealed surface and the sticking coefficient on the sputter damaged surface are independent of b e a m energy is an indication that the same mechanism, namely dissociation at defect sites, is responsible for the observed behavior on the annealed surface. There are two possible paths that could lead to dissociation. The first route arises fromparticles dissociating directly at defect sites upon collision, whereas particles hitting the surface at a defect-free area do not. The second path arises if particles are at first molecularly adsorbed on the surface with a very high probability. Due to the strong bond the lifetime in the molecularly chemisorbed state at 500 K is long enough to allow CO molecules to migrate to

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H.P. Steinrfick et al. / Role of defects in adsorption of CO on Ni(lO0)

defect sites and to dissociate there. As shown previously [23], the initial sticking coefficient for molecularly adsorbed CO is near unity and only weakly dependent on beam energy. Thus molecular adsorption followed by surface migration to defect sites could explain the observed energy independence for the dissociative sticking probability. However, we cannot resolve these two possibilities. We attempted to investigate the dependence of the dissociation probability on the surface temperature, but the measurements were limited to a very small temperature range and the results were inconclusive since at temperatures below 500 K molecular CO adsorption could influence the results, and at temperatures above 580 K the associative d e s o r p t i o n of CO is no longer negligible. Further investigation of the behavior for dissociative adsorption shows (fig. 2) that coverages reach values larger than 10%. Since this value exceeds the initial sticking coefficient of about 2% it appears that after dissociation of CO at defect sites, the C and O atoms migrate away to allow further dissociation. As mentioned above, List and Blakely [16] related the peak height of the first desorption peak with surface defects. Our investigation leads to the conclusion that the reverse process to CO formation, CO dissociation, is dominated by surface defects, as would be expected by microscopic reversibility. A comparison of our data with the results reported by Rosei et al. [21] for the Ni(110) surface, on the other hand, shows that although the (110) surface is rougher than the (100) surface, the values for the sticking probability are much smaller. This contradiction could be explained assuming that our annealed Ni(100) crystal possessed surface defects of another local symmetry which results in the higher reaction probability observed. This interpretation is in good agreement with EELS results by Erley et al. for a stepped Ni[5(111) x (110)] surface [15]. They observe a low frequency C - O stretching vibration of 1520 cm -1 for CO adsorbed at step sites, indicative of a lowering of the CO bond strength. In T P D measurements on the same surface they report that adsorbed CO molecules decompose upon heating the crystal to 430 K, leaving carbon and oxygen on the surface. Further heating results in a high temperature CO recombination/desorption peak at 720 K, similar to the one observed here. On a perfect surface neither the low CO stretching frequency nor CO decomposition has been observed in this kind of experiment. Therefore they explain CO decomposition at step sites (defects) by a lower CO bond strength at these sites, as determined by the lower CO stretching frequency. Finally, we note an inconsistency in the work by Rosei et al. [21]. From their data (they plot C-coverage versus exposure) an initial dissociation probability SO in the order of 10 - 4 is obtained. On the other hand they calculate an activation energy of 23 k c a l / m o l for CO dissociation based on the rate of carbon build up which would lead to much smaller values for SO. G o o d m a n et al. also investigated CO dissociation as one step in the methanation reaction on Ni(100) [24,25]. They report an apparent activation energy of

H.P. Steinr~ck et al. / Role of defects in adsorption of CO on Ni(lO0)

L567

2 2 - 2 5 k c a l / m o l at a s t e a d y state c a r b o n c o v e r a g e of a b o u t 0.1 M L (1 M L = 1.62 × 1015 a t o m s c m - 2 ) , a t t r i b u t i n g this b a r r i e r to d i s s o c i a t i o n o n (100) terraces w i t h the defects b l o c k e d b y a d s o r b e d c a r b o n . I n s u m m a r y , t h e d i s s o c i a t i v e s t i c k i n g c o e f f i c i e n t for C O o n N i ( 1 0 0 ) has b e e n e x a m i n e d u s i n g m o l e c u l a r b e a m s . T h e o b s e r v e d v a l u e of 0.02 o n the a n n e a l e d N i ( 1 0 0 ) s u r f a c e is i n d e p e n d e n t of k i n e t i c e n e r g y of t h e C O m o l e cules. I n v e s t i g a t i o n s o n a s p u t t e r - d a m a g e d N i ( 1 0 0 ) s u r f a c e result i n a dissociative s t i c k i n g c o e f f i c i e n t larger b y a n o r d e r of m a g n i t u d e (0.40), a g a i n i n d e p e n d e n t of k i n e t i c e n e r g y . T h e r e is s t r o n g e v i d e n c e t h a t defect sites are r e s p o n s i b l e for dissociative C O a d s o r p t i o n . A t a s u r f a c e t e m p e r a t u r e of 500 K the d i s s o c i a t i o n p r o b a b i l i t y at these defect sites is n e a r u n i t y a n d is i n d e p e n d e n t of t r a n s l a t i o n a l energy. W e g r a t e f u l l y a c k n o w l e d g e the D O E office of Basic E n e r g y Sciences, D i v i s i o n o f C h e m i c a l Science ( D E A T 0 3 - 7 9 E R - 1 0 4 - 9 0 ) , for s u p p o r t of this work.

References [1] R.D. Kelley and D.W. Goodman, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1982) p. 427ff. [2] G. Brod6n, T.N. Rhodin, C. Brucker, R. Benbow and Z. Hurych, Surface Sci. 59 (1976) 593. [3] J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. [4] J.T. Yates, Jr. and D.W. Goodman, J. Chem. Phys. 73 (1980) 5371. [5] K. Klier, A.C. Zettlemoyer and H. Leidheiser, Jr., J. Chem. Phys. 52 (1970) 589. [6] S.L. Tang, M.B. Lee, J.D. Beckerle, M.A. Hines and S.T. Ceyer, J. Chem. Phys. 82 (1985) 2826. [7] K. Christmann, O. Schober and G. Ertl, J. Chem. Phys. 60 (1974) 4719. [8] R.W. Joyner and M.W. Roberts, J. Chem. Soc. Faraday I, 10 (1974) 1819. [9] W. Erley and H. Wagner, Surface Sci. 74 (1978) 333. [10] Z. Murayama, I. Kojima, E. Miyazaki and I. Yasumori, Surface Sci. 118 (1982) L281. [11] D.E. Eastman, J.E. Demuth and J.M. Baker, J. Vacuum Sci. Technol. 11 (1974) 273. [12] H.H. Madden and G. Ertl, Surface Sci. 35 (1973) 211. [13] E.G. Keim, F. Labohm, O.L.J. Gijzeman, G.A. Bootsma and J.W. Geus, Surface Sci. 112 (1982) 52. [14] J.B. Benzinger and R.E. Preston, Surface Sci. 141 (1984) 567. [15] W. Erley, H. Ibach, S. Lehwald and H. Wagner, Surface Sci. 83 (1979) 585. [16] F.A. List and J.M. Blakely, Surface Sci. 152/153 (1985) 463. [17] M.P. D'Evelyn, A.V. Hamza, G.E. Gdowski and R.J. Madix, Surface Sci. 167 (1986) 451. [18] For example: E.I. Ko and R.J. Madix, Appl. Surface Sci. 3 (1979) 236. [19] D.E. Taylor and R.L. Park, Surface Sci. 125 (1983) L73. [20] P.A. Redhead, Vacuum 12 (1962) 203. [21] R. Rosei, F. Ciccacci, R. Memeo, C. Mariani, L.S. Caputi and L. Papagno, J. Catalysis 83 (1983) 19. [22] D.A. King and M.G. Wells, Proc. Roy. Soc. (London) A339 (1974) 245. [23] M.P. D'Evelyn, H.P. Steinri~ck and R.J. Madix, to be published. [24] D.W. Goodman, J. Vacuum Sci. Technol. 20 (1982) 522. [25] D.W. Goodman, D.R. Kelley, T.E. Madey and J.M. White, J. Catalysis 64 (1980) 479.