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Co dissociation and recombination reactions on Ni(100)
Surface Science 220 (1989) 322-332 North-Holland, Amsterdam
322
CO DISSOCIATION
AND RECOMBINATION
REACTIONS
ON Ni(100)
C. ASTALDI ‘, A. SANTONI 234,F. DELLA VALLE 133and R. ROSE1 334 ’ L.&oratorio Tecnologie Avanrate Superfici e Catalisi de1 Consorzio INFM, Area Padriciano 99, I-34012 Trieste, Italy 2 ENEA, Dipartimento TIB, Divisione Ingegneria dei Materiali, CRE Casaccia, I-001 00 Roma A. D., Ita& ’ Dipartimento di Fish, Universitci di Trieste, Via A. Valerio 2, I-34127 Trieste, Ita@ 4 Sincrotrone Trieste, Padriciano 99, I-34012 Trieste, Italy
di Ricerca,
Received 30 January 1989; accepted for publication 15 May 1989
The kinetics of atomic carbon and oxygen buildup on a Ni(1OO) surface exposed to carbon monoxide at high temperatures have been investigated by Auger electron spectroscopy. The experimental data, taken at different sample temperatures (453 5 T I 573 K) and at different CO partial pressures (3 x lo-’ < PC0 5 3 X 10F6 mbar) allowed the identification of the CO dissociation mechanism. By fitting the experimental data with a set of rate equations describing CO dissociation, CO reduction of surface oxygen, and C and 0 recombination, we have been able to determine the pre-exponential factors and the activation energies of these processes.
1. Introduction According to the most recent literature, carbon monoxide dissociation on transition metal catalysts is the first step of the methanation reaction [l-4]. CO dissocation should lead to adsorbed “carbidic carbon” which is subsequently hydrogenated to methane. Although CO on Ni is one of the most investigated systems in surface science, most of the work on the interaction of carbon monoxide with Ni single crystal surfaces has been carried out at room temperature and very little is known about CO interaction with a Ni surface in the temperature and pressure regime where CO dissociation starts to take place. Many fundamental questions on the mechanism of dissociation and on its associated energetics are still to be answered. In the following we report on an experiment which unambiguously elucidates the CO dissociation mechanism on Ni(lOO) in the low CO pressure regime and determines the pre-exponential factor and the activation energy of the process. It was also possible to extract from the experimental data the kinetic parameters for the inverse reaction (C and 0 recombination) and for the surface oxygen reduction by CO. A preliminary account of this experiment has already appeared [5]. 0039-6028/89/$03.50
(North-Holland
0 Elsevier Science Publishers B.V. Physics Publishing Division)
C. Astaldi et al. / CO dissociation and recombination on Ni(100)
323
2. Experimental A Ni single crystal was cut to expose the (100) surface within 1 O, polished and mounted in a Leybold LH-10 ESCA-Auger system. The sample was cleaned in situ by repeated ion-etching cycles at room temperatures and by oxygen exposures at about 800 K. LEED observations were carried out by means of a rear view LEED apparatus (Omicron GmbH) mounted in the sample preparation chamber. The sharp diffraction spots and low background indicated a good surface quality and a low density of steps. The experiment was performed by exposing the sample, heated at a fixed temperature, to a determined CO partial pressure for a given time. After an exposure period, the CO admission valve was closed and CO was pumped away while the sample temperature was kept constant. The pressure in the chamber dropped rapidly in the lOPro mbar range while all residual molecular carbon monoxide desorbed from the surface. (Dynamical photoemission experiments on nickel surfaces show, for instance, that at T = 449 K complete CO desorption is observed in about 250 ms [6].) At this point, only atomic carbon and oxygen species originated from CO thermal dissociation were present on the surface and an Auger spectrum was taken to assess their coverage. This procedure has been repeated for new exposure periods at the same CO pressure and at the same sample temperature. During the exposures the electron gun was always switched off, in order to avoid electron beam induced effects [7]. By measuring the carbon and oxygen accumulation after each exposure, the kinetic process could be followed until an approximately stationary coverage of both species was reached. The overall exposure time required to follow a complete kinetic process was about one hour. Repeated measurements of the Auger spectra did not show any decrease of the carbon and oxygen peaks excluding the possibility of artifacts due to electron stimulated desorption. After each kinetic run we checked by AES that only carbon and oxygen were present on the surface and no further contaminants. Therefore we can rule out the presence of carbonyls in the gas feed. Many kinetic runs have been measured with the same procedure for different CO pressures and/or different sample temperatures, filling up a grid of different pressures (in the range 3 x lo-’ I PC0 I 3 X lop6 mbar) and of different temperatures (in the range 453 I T I 573 K) for a total of 11 complete kinetic runs. 3. Experimental
results
The kinetics of a typical run of CO interaction with the Ni(lOO) surface in the lower region of CO pressure and sample temperature of our measurements
324
C. Astaldi et al. / CO dissociation and recombination
I
.6
/
on Ni(100)
I
I
/
m Carbon
.o' 0
I
I
I
T = 453 K , I
2 &ne
(S,6
(x10+
10
SI )
12
Fig. 1. Carbon (squares) and oxygen (circles) coverages versus CO exposure time. The lines are drawn as a guide for the eye.
is drawn in fig. 1. The carbon Auger spectrum always showed the characteristic “carbidic” [l] lineshape (fig. 2), and no “graphitic” [l] carbon was observed even of the highest temperatures of our experiment.
0
200
400 600 I$tiic energy C&l
800
lOO(
Fig. 2. Complete Auger spectrum taken after a kinetic run at T= 473 K and PC0 = 3 x lo-” mbar. The inset shows enlarged the typical “carbidic” carbon lineshape as well as the oxygen and Ni signal as recorded by the data acquisition system.
C. Asialdi et al. / CO dissociation and recombination
on Ni(100)
325
.6
a
----___
=
f-4631(
.o I
0
I
1
2ooo
Nme (a)
1 -
0
2000
WO’
3*10-’
T = 613
4w6
I 6000
0
2000
613 K
4000 Time
p,
mbr
4606 Tim* (m)
.O
mbar
K
6000
(8)
= 3*X)-’
mbor
T = 513 K
6060
2000
Time !I!
4000
600@
Fig. 3. Panels (a) to (d): Carbon (squares) and oxygen (circles) deposition kinetics as a function of the CO exposure for different operating pressures and temperatures.
The carbon and oxygen coverages (0, and 0,) were estimated from Auger data obtained on a CO saturated Ni(lOO) surface. The rates of carbon and oxygen accumulation have both an approximately exponential behavior and both coverages saturate at about 0.5 ML, which is the known saturation coverage of carbon and oxygen on the Ni(lOO) surface [7,8]. Fig. 3 shows a limited representative set of measurement runs. The apparent CO dissociation rate does not change appreciably by varying the temperature. CO pressure, on the other hand, has a much stronger influence, and the rate of CO dissociation increases markedly when the operating pressure is increased. In particular we have found that the initial rates of carbon and oxygen accumulation at a given temperature are proportional to CO pressure. Another remarkable effect due to the pressure is the lowering of the oxygen saturation level on increasing the CO operating pressure (panels b, c, and d of fig. 3). We explain this with the oxygen depletion reaction
o,,, + co --,co2.
0)
In the upper range of operating temperatures of our experiment, both carbon and oxygen saturation levels decrease by increasing the temperature
326
C. Astaldi et al. / CO disswiation
and recambination
on Ni(li?O)
(panels a, b, and c of fig. 3). We interpret this effect as due to the onset of carbon and oxygen recombination reaction. Carbon and oxygen in principle may also diffuse into the bulk. However, this happens at temperatures considerably higher than the ones used in our experiment [9,10]. Besides we have found that (even at 573 K, the highest of our operating temperature) they decreased with the same rate (i.e. in the exact stoichiometric ratio of the recombination reaction). 4. Discussion
The dissection of carbon monoxide is a key step in the fo~ation of hydrocarbons on nickel catalysts. It is therefore of strong interest to find the exact mechanism of this simple reaction. This would allow a meaningful ~omp~son between the expe~ment~y derived energetics of the process and the results of theoretical calculations. It has long been known that by exposing a Ni single crystal surface at high temperature to a relatively high CO pressure (10-6-10-5 Torr), carbon deposition occurs. Carbon deposition may be the result of the disproportionation reaction 2GO~c,,,+CO,,
(2)
which, in principle, may proceed either through the Lanker-Hinshelwood (LH) or through the Eley-Rideal (ER) mechanism. Working at low pressures on a Ni(llO) single crystal surface, Rosei and coworkers [ll] have discarded these possibilities on the ground of the behavior of the carbon a~umulation rate versus CO pressure. This rate was found to be strictly linear with pressure while the assumption that reaction (2) proceeds through the LH or ER mechanism would imply a quadratic dependence (in this low pressure regime). Our present results on Ni(lOO) also show a linear dependence of the carbon accumulation rate on the CO pressure. Moreover we find both carbon and atomic oxygen as a result of CO interaction with the surface. Hence we conclude that in our working temperature and pressure regime, CO dissociation is a unimolecular reaction also on Ni(100). Therefore we are led to the proposition that CO is first molec~~ly adsorbed and thermalized on the surface and then it dissociates into its atomic constituents. It is also apparent that atomic oxygen forms a noticeably stronger bond on a Ni(100) surface than on Niflll) or Ni(l10) where oxygen was never observed as a result of high temperature exposure to CO. At high temperatures the CO desorption rate is substantial so that a CO ~~~b~urn coverage is quickly reached in presence of a CO partial pressure. The kinetic equation describing this process can be written as d%o/dt
= ~~{e~o)p~o
- K% exp(
-&J~T)@co,
(3)
C. Astaldi et al. / CO dissociation and recombination
on Ni(lO0)
327
where the coefficient K,(e) conglobates the coverage dependence of the CO molecular sticking coefficient. K!!, and Edes are the pre-exponential factor and activation energy for desorption, respectively. From eq. (3) a pseudo-regime CO coverage versus temperature and pressure can be obtained. By comparing our operating pressures and temperatures with CO isosteres on Ni(100) [12] we deduce that our CO coverage during the experiments is always less than 0.2 ML. At sufficiently high temperature a fraction of the adsorbed molecules can overcome the energy barrier for dissociation which starts then to compete with the thermal desorption process. This process can be described by the equation d&/dt
= K: exp( - E,,/RT)ecoev,
(4)
where Ki and Edis are the pre-exponential factor and activation energy for dissociation respectively and &, is the “coverage” of the empty sites. The factor 8, takes into account that an adsorbed molecule needs an empty site in order to be able to dissociate. The equation describing the accumulation rate for oxygen can be written as de,/dt = Ki exp( - E&RT)
f3,,0,
- Kt
exp( - E++,/RT)
e,e,, .
(5)
The last term in eq. (5) describes the reduction reaction with carbon monoxide which leads to oxygen depletion and CO, formation. Our experimental data cannot distinguish between the ER or the LH mechanism for this reaction. In a preliminary account of the experiment [5] we have evaluated the kinetic parameter assuming the ER recombination mechanism. Since the LH mechanism is commonly regarded as responsible for the depletion reaction on most transition metals [13], in the present account of our experiment, we have chosen this model for our data analysis. Eqs. (3)-(5) can fit nicely our experimental results at low temperature (up to about 500 K), but they cannot account for the higher temperatures behavior of our kinetic data. In order to fit the data above 500 K we must introduce the term K!,
exp( - E,,/RT)
O,t’,
(6)
in eqs. (3)-(5) to account for the recombination process between carbon and oxygen. K!?, and E,, are the pre-exponential factor and activation energy barrier for recombination respectively. The complete set of equations used to fit our experimental data is therefore the following: d&,/d
t = K, PcotJv - K- #,-,
de,-.dt
= Kzecoev - ~_,e,e,,
d e,/d t = K,ecoev - K_ ,e,e,
- K,8,,8,
+ K_ &t?,
,
(7) (8)
- K3eoeco.
(9)
C. Astaldi et al. / CO dissociation and recombination
328
- 3
.
u .l .o
l
0
l
on Ni(100)
IO* mbar
pm - 3
l
1Oa mbar
t=4WK
t-4WK
I 1 1 1 I I 600 1wO1sOOmm2M)o30003600
a ’ 0
I 1 1 I I 1 600 10001M)o2000260030003600
Time (I)
Time (I)
Fig. 4. Kinetics of deposition for carbon (a) and oxygen (b). The solid lines are a fit obtained by solving eqs. (7)-(9).
The 0, recombination reaction has not been taken into account by this model because it takes place at a much higher temperature [9]. The results of the fit are shown in fig. 4. We have assumed K, = (27r~zkT)-~/~ (which implies a unitary molecular sticking coefficient) while K!, and Edes were obtained from the literature [14,15]. From the fit we obtain values for K,, K_, and K, at each of our operating temperatures. Figs. 5a, 5b, and 5c show logarithmic plots of K,, K_, and K3 obtained in this way, versus 1000/T. All three sets of data fit nicely an Arrhenius plot giving Kt = 5.4 x lO’(monolayer Kt = 2 X lO”(monolayer K!,
. s)-‘, *s)-*,
= 1.2 x 10’5(monolayer.
s)-‘,
Edis = 23.4 kcal/mol, Edep, = 29.8 kcal/mol, E,, = 42.7 kcal/mol.
The errors on the activation energies have been estimated to be about + 15% and the ones on the pre-exponential factors + 2 in the exponent. The values for Erec and K!, are in good agreement with those obtained by Benzinger et al. on the Ni(ll1) surface [9]. Edis is about the same of a previous similar experiment on Ni(ll0) [ll] and is in excellent agreement with a number of theoretical evaluations [16,17]. Fig. 6 shows a schematic potential energy diagram along a “reaction coordinate” which accounts for the adsorption-desorption and dissociationrecombination processes of CO on Ni(lOO). In our calculations the bottom of the adsorption potential well (point A in fig. 6) is the zero of the activation energy barrier for dissociation (as in ref. [ll]). If instead we take the zero of energy as defined for a molecule infinitely far from the surface (as customary), the apparent activation energy for dissociation is Edis = - Edes + Edis = - 3 kcal/mol.
C. Astaldi et al. / CO dissociation and recombination on Ni(100)
329
a
-7
’
I
1.7
1.3
1.0
2.0
2.1
2.2
2.3
1.7
1.8
-10
I 1.7
1.2
Fig. 5. Arrhenius
1.2
2.0
1.9
2.0
1O’JW
1000/T 00
2.1
2.2
2.1
2.2
2.3
WI
I 2.3
plots of the rate constants for CO dissociation and oxygen depletion (c).
(a), oxygen recombination
(b),
In this respect we note that our present results (and the results of ref. [ll]) for the activation energy for dissociation are not in contradiction with those reported by Steinrtick et al. [Ml. These authors have carried out a molecular beam study of CO on a Ni(lOO) surface at 500 K, and have found the dissociative sticking coefficient to be 0.02, independent of the CO beam energy from 1.6 to 20 kcaI/mol. From this finding they concluded that the
B Fig. 6. Schematic potential energy diagram along the “reaction coordinate”.
330
C. Astaldi et al. / CO dissociation and recombination
on Ni(100)
value of the apparent activation energy for the process should not be larger than zero. The results of this work and the interpretation of these authors are therefore perfectly coherent with our conclusions. From our data we may also obtain an equivalent dissociation sticking coefficient Sdis (defined as the ratio between the number of dissociated molecules per unit time and the number of CO molecules impinging per unit time on the surface). At T = 513 K we obtain Sais= 0.01, also in good agreement with the value quoted by Steinriick et al. [18]. These authors [18] also conclude from their experiments that surface defects are responsible for the dissociation phenomena they have observed. We believe that the dissociation properties we have determined are intrinsic of the (100) surface (terrace sites), although we cannot completely exclude a small role of surface defects (especially in the initial part of the kinetics). In fact, we have found that the atomic carbon and oxygen coveragea approach 0.5 ML while a rough estimate of the surface defects concentration shows that they cannot exceed about 0.02 ML. On the other hand the assumption that after dissociation of CO at defect sites, the C and 0 atoms migrate away to allow further dissociation, is in contradiction to the accepted view that adsorbates are more strongly bound to defect sites. A simple calculation using our kinetic parameters for recombination shows that they would lead to a thermal desorption peak (in a TDS experiment) around 600 K. Erley and Wagner [19] showed very convincingly that the recombination of terrace carbon and oxygen atoms on nickel leads to a TDS peak at 620 K (in excellent agreement with our finding) while the associative desorption of carbon and oxygen atoms from step/kink sites gives a peak at a much higher temperature (820 K). Erley and Wagner concluded that the recombination of Cads and Oads on the terraces requires a smaller activation energy than at step sites. Furthermore, they concluded that the appearance of the higher temperature peak on the stepped surface implies no appreciable diffusion of C and 0 from step to terrace sites up to 820 K and that this fact may be correlated with a higher binding energy at step sites. For these reasons we are convinced that the dissociation mechanism we have investigated is related mainly to terrace sites. We have hints that the carbon and oxygen accumulation rates in the very initial part of the kinetic runs (up to about 0.02 ML coverages) are somewhat higher than predicted by the fit. This very small effect, if confirmed by more accurate experiments, could be due to surface defects. However, we emphasize that it would not change appreciably the kinetic parameters we have obtained in this work. By performing the same experiment on Ni(ll1) and on Ni(ll0) surfaces in similar pressure and temperatures intervals, surface oxygen coming from the CO molecule dissociation was never observed. On these surfaces, therefore, the oxygen depletion reaction (third term in eq. (9)) proceeds at a substantially faster rate than on Ni(lOO). The behavior of nickel is therefore noticeably
C. Astaldi et al. / CO dissociation and recombination on Ni(100)
331
different, in this respect, from the behavior of the 4d and 5d metals where the CO oxidation reaction has been found to be largely structure insensitive. On the other hand, oxygen is known to form a stronger bond on Ni(lOO), than on Ni(ll1) and Ni(ll0) [20], and this behavior may explain why substantially higher temperatures and/or CO pressures are needed to deplete oxygen from this surface. The value we obtain for the activation energy of the CO + 0 reaction (29.8 kcal/mol) is also much higher than the value found on most platinum-group metals and this result may again be ascribed to the much stronger bond formed by oxygen on nickel.
5. Conclusions The measurements we present in this paper allow a comprehensive description of the adsorption-desorption and of the dissociation-recombination reactions of CO on a Ni(100) surface in the low CO coverage regime. In particular we have been able to measure the kinetic parameters (pre-exponential factors and activation energy barriers) for CO dissociation and CO recombination on this surface. We have also established that the CO dissociation is a unimolecular reaction in which molecular CO adsorption is most likely an intermediate step. As a byproduct we have also obtained, from the data of our experiment, the kinetic parameters of the side-reaction of CO oxidation by surface oxygen.
Acknowledgment The authors thank the Area per la Ricerca Trieste for the partial support of this work.
Scientifica
a Tecnologia
of
References [l] 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). [2] D.W. Goodman, R.D. Kelley, T.E. Madey and J.M. White, J. Catalysis 64 (1980) 479. [3] H.P. Bonzel and H.J. Krebs, Surface Sci. 91 (1980) 499. [4] D.W. Goodn& R.D. Kelley, T.E. Madey and J.T. Yates, Jr., J. Catalysis 63 (1980) 226. [5] A. Santoni, C. Astaldi, F. Della Valle and R. Rosei, in: Structures and Reactivity of Surfaces, Proc. of the European Conference, Trieste, Italy, September 13-16, 1988, Eds. A. Zecchina, G. Costa and C. Morterra (Elsevier, Amsterdam), to be published. [6] G.W. Rubloff, Surface Sci. 89 (1979) 566. [7] F. Labohm, C.WR. Engelen, O.L.J. Gijzeman, J.W. Geus and G.A. Bootsma, Surface Sci. 126 (1983) 429. [S] J.H. Gnuferko, D.P. Woodruff and B.W. Holland, Surface Sci. 87 (1979) 357.
332
C. Astaldi et al. / CO dissociation and recombinaiion
on Ni(100)
[9] J.B. Benzinger and R.E. Preston, Surface Sci. 141 (1984) 567. [lo] F.C. Schouten, E.T. Brake, O.L.J. Gijzeman and G.A. Bootsma, Surface Sci. 74 (1978) 1. [ll] R. Rosei, F. Ciccacci, R. Memeo, C. Mariani, L.S. Caputi and L. Papagno, J. Catalysis 83 (1983) 19. [12] J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. [13] T. Engel and G. ErtJ, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. Ring and D.P. Woodruff (Elsevier, Amsterdam, 1982) and references therein. [14] J.T. Yates, Jr. and D.W. Goodman, J. Chem. Phys. 73 (1980) 5371. [15] S. Johnson and R.J. Madix, Surface Sci. 108 (1981) 77. [16] E. Miyazaki, J. Catalysis 65 (1980) 84. [17] T.H. Upton, Lectures of the International Summer School on Surface Science and Catalysis of the Methanation Reaction, Sangineto Lido, July l-10, 1982, unpublished. [18] H.P. Steimiick, M.P. d’Evelyn and R.J. Madix, Surface Sci. 172 (1986) L561. [19] W. Erley and H. Wagner, Surface Sci. 74 (1978) 333. [20] F. Labohm, O.L.J. Gijzeman and J.W. Geus, Surface Sci. 135 (1983) 409.
322
CO DISSOCIATION
AND RECOMBINATION
REACTIONS
ON Ni(100)
C. ASTALDI ‘, A. SANTONI 234,F. DELLA VALLE 133and R. ROSE1 334 ’ L.&oratorio Tecnologie Avanrate Superfici e Catalisi de1 Consorzio INFM, Area Padriciano 99, I-34012 Trieste, Italy 2 ENEA, Dipartimento TIB, Divisione Ingegneria dei Materiali, CRE Casaccia, I-001 00 Roma A. D., Ita& ’ Dipartimento di Fish, Universitci di Trieste, Via A. Valerio 2, I-34127 Trieste, Ita@ 4 Sincrotrone Trieste, Padriciano 99, I-34012 Trieste, Italy
di Ricerca,
Received 30 January 1989; accepted for publication 15 May 1989
The kinetics of atomic carbon and oxygen buildup on a Ni(1OO) surface exposed to carbon monoxide at high temperatures have been investigated by Auger electron spectroscopy. The experimental data, taken at different sample temperatures (453 5 T I 573 K) and at different CO partial pressures (3 x lo-’ < PC0 5 3 X 10F6 mbar) allowed the identification of the CO dissociation mechanism. By fitting the experimental data with a set of rate equations describing CO dissociation, CO reduction of surface oxygen, and C and 0 recombination, we have been able to determine the pre-exponential factors and the activation energies of these processes.
1. Introduction According to the most recent literature, carbon monoxide dissociation on transition metal catalysts is the first step of the methanation reaction [l-4]. CO dissocation should lead to adsorbed “carbidic carbon” which is subsequently hydrogenated to methane. Although CO on Ni is one of the most investigated systems in surface science, most of the work on the interaction of carbon monoxide with Ni single crystal surfaces has been carried out at room temperature and very little is known about CO interaction with a Ni surface in the temperature and pressure regime where CO dissociation starts to take place. Many fundamental questions on the mechanism of dissociation and on its associated energetics are still to be answered. In the following we report on an experiment which unambiguously elucidates the CO dissociation mechanism on Ni(lOO) in the low CO pressure regime and determines the pre-exponential factor and the activation energy of the process. It was also possible to extract from the experimental data the kinetic parameters for the inverse reaction (C and 0 recombination) and for the surface oxygen reduction by CO. A preliminary account of this experiment has already appeared [5]. 0039-6028/89/$03.50
(North-Holland
0 Elsevier Science Publishers B.V. Physics Publishing Division)
C. Astaldi et al. / CO dissociation and recombination on Ni(100)
323
2. Experimental A Ni single crystal was cut to expose the (100) surface within 1 O, polished and mounted in a Leybold LH-10 ESCA-Auger system. The sample was cleaned in situ by repeated ion-etching cycles at room temperatures and by oxygen exposures at about 800 K. LEED observations were carried out by means of a rear view LEED apparatus (Omicron GmbH) mounted in the sample preparation chamber. The sharp diffraction spots and low background indicated a good surface quality and a low density of steps. The experiment was performed by exposing the sample, heated at a fixed temperature, to a determined CO partial pressure for a given time. After an exposure period, the CO admission valve was closed and CO was pumped away while the sample temperature was kept constant. The pressure in the chamber dropped rapidly in the lOPro mbar range while all residual molecular carbon monoxide desorbed from the surface. (Dynamical photoemission experiments on nickel surfaces show, for instance, that at T = 449 K complete CO desorption is observed in about 250 ms [6].) At this point, only atomic carbon and oxygen species originated from CO thermal dissociation were present on the surface and an Auger spectrum was taken to assess their coverage. This procedure has been repeated for new exposure periods at the same CO pressure and at the same sample temperature. During the exposures the electron gun was always switched off, in order to avoid electron beam induced effects [7]. By measuring the carbon and oxygen accumulation after each exposure, the kinetic process could be followed until an approximately stationary coverage of both species was reached. The overall exposure time required to follow a complete kinetic process was about one hour. Repeated measurements of the Auger spectra did not show any decrease of the carbon and oxygen peaks excluding the possibility of artifacts due to electron stimulated desorption. After each kinetic run we checked by AES that only carbon and oxygen were present on the surface and no further contaminants. Therefore we can rule out the presence of carbonyls in the gas feed. Many kinetic runs have been measured with the same procedure for different CO pressures and/or different sample temperatures, filling up a grid of different pressures (in the range 3 x lo-’ I PC0 I 3 X lop6 mbar) and of different temperatures (in the range 453 I T I 573 K) for a total of 11 complete kinetic runs. 3. Experimental
results
The kinetics of a typical run of CO interaction with the Ni(lOO) surface in the lower region of CO pressure and sample temperature of our measurements
324
C. Astaldi et al. / CO dissociation and recombination
I
.6
/
on Ni(100)
I
I
/
m Carbon
.o' 0
I
I
I
T = 453 K , I
2 &ne
(S,6
(x10+
10
SI )
12
Fig. 1. Carbon (squares) and oxygen (circles) coverages versus CO exposure time. The lines are drawn as a guide for the eye.
is drawn in fig. 1. The carbon Auger spectrum always showed the characteristic “carbidic” [l] lineshape (fig. 2), and no “graphitic” [l] carbon was observed even of the highest temperatures of our experiment.
0
200
400 600 I$tiic energy C&l
800
lOO(
Fig. 2. Complete Auger spectrum taken after a kinetic run at T= 473 K and PC0 = 3 x lo-” mbar. The inset shows enlarged the typical “carbidic” carbon lineshape as well as the oxygen and Ni signal as recorded by the data acquisition system.
C. Asialdi et al. / CO dissociation and recombination
on Ni(100)
325
.6
a
----___
=
f-4631(
.o I
0
I
1
2ooo
Nme (a)
1 -
0
2000
WO’
3*10-’
T = 613
4w6
I 6000
0
2000
613 K
4000 Time
p,
mbr
4606 Tim* (m)
.O
mbar
K
6000
(8)
= 3*X)-’
mbor
T = 513 K
6060
2000
Time !I!
4000
600@
Fig. 3. Panels (a) to (d): Carbon (squares) and oxygen (circles) deposition kinetics as a function of the CO exposure for different operating pressures and temperatures.
The carbon and oxygen coverages (0, and 0,) were estimated from Auger data obtained on a CO saturated Ni(lOO) surface. The rates of carbon and oxygen accumulation have both an approximately exponential behavior and both coverages saturate at about 0.5 ML, which is the known saturation coverage of carbon and oxygen on the Ni(lOO) surface [7,8]. Fig. 3 shows a limited representative set of measurement runs. The apparent CO dissociation rate does not change appreciably by varying the temperature. CO pressure, on the other hand, has a much stronger influence, and the rate of CO dissociation increases markedly when the operating pressure is increased. In particular we have found that the initial rates of carbon and oxygen accumulation at a given temperature are proportional to CO pressure. Another remarkable effect due to the pressure is the lowering of the oxygen saturation level on increasing the CO operating pressure (panels b, c, and d of fig. 3). We explain this with the oxygen depletion reaction
o,,, + co --,co2.
0)
In the upper range of operating temperatures of our experiment, both carbon and oxygen saturation levels decrease by increasing the temperature
326
C. Astaldi et al. / CO disswiation
and recambination
on Ni(li?O)
(panels a, b, and c of fig. 3). We interpret this effect as due to the onset of carbon and oxygen recombination reaction. Carbon and oxygen in principle may also diffuse into the bulk. However, this happens at temperatures considerably higher than the ones used in our experiment [9,10]. Besides we have found that (even at 573 K, the highest of our operating temperature) they decreased with the same rate (i.e. in the exact stoichiometric ratio of the recombination reaction). 4. Discussion
The dissection of carbon monoxide is a key step in the fo~ation of hydrocarbons on nickel catalysts. It is therefore of strong interest to find the exact mechanism of this simple reaction. This would allow a meaningful ~omp~son between the expe~ment~y derived energetics of the process and the results of theoretical calculations. It has long been known that by exposing a Ni single crystal surface at high temperature to a relatively high CO pressure (10-6-10-5 Torr), carbon deposition occurs. Carbon deposition may be the result of the disproportionation reaction 2GO~c,,,+CO,,
(2)
which, in principle, may proceed either through the Lanker-Hinshelwood (LH) or through the Eley-Rideal (ER) mechanism. Working at low pressures on a Ni(llO) single crystal surface, Rosei and coworkers [ll] have discarded these possibilities on the ground of the behavior of the carbon a~umulation rate versus CO pressure. This rate was found to be strictly linear with pressure while the assumption that reaction (2) proceeds through the LH or ER mechanism would imply a quadratic dependence (in this low pressure regime). Our present results on Ni(lOO) also show a linear dependence of the carbon accumulation rate on the CO pressure. Moreover we find both carbon and atomic oxygen as a result of CO interaction with the surface. Hence we conclude that in our working temperature and pressure regime, CO dissociation is a unimolecular reaction also on Ni(100). Therefore we are led to the proposition that CO is first molec~~ly adsorbed and thermalized on the surface and then it dissociates into its atomic constituents. It is also apparent that atomic oxygen forms a noticeably stronger bond on a Ni(100) surface than on Niflll) or Ni(l10) where oxygen was never observed as a result of high temperature exposure to CO. At high temperatures the CO desorption rate is substantial so that a CO ~~~b~urn coverage is quickly reached in presence of a CO partial pressure. The kinetic equation describing this process can be written as d%o/dt
= ~~{e~o)p~o
- K% exp(
-&J~T)@co,
(3)
C. Astaldi et al. / CO dissociation and recombination
on Ni(lO0)
327
where the coefficient K,(e) conglobates the coverage dependence of the CO molecular sticking coefficient. K!!, and Edes are the pre-exponential factor and activation energy for desorption, respectively. From eq. (3) a pseudo-regime CO coverage versus temperature and pressure can be obtained. By comparing our operating pressures and temperatures with CO isosteres on Ni(100) [12] we deduce that our CO coverage during the experiments is always less than 0.2 ML. At sufficiently high temperature a fraction of the adsorbed molecules can overcome the energy barrier for dissociation which starts then to compete with the thermal desorption process. This process can be described by the equation d&/dt
= K: exp( - E,,/RT)ecoev,
(4)
where Ki and Edis are the pre-exponential factor and activation energy for dissociation respectively and &, is the “coverage” of the empty sites. The factor 8, takes into account that an adsorbed molecule needs an empty site in order to be able to dissociate. The equation describing the accumulation rate for oxygen can be written as de,/dt = Ki exp( - E&RT)
f3,,0,
- Kt
exp( - E++,/RT)
e,e,, .
(5)
The last term in eq. (5) describes the reduction reaction with carbon monoxide which leads to oxygen depletion and CO, formation. Our experimental data cannot distinguish between the ER or the LH mechanism for this reaction. In a preliminary account of the experiment [5] we have evaluated the kinetic parameter assuming the ER recombination mechanism. Since the LH mechanism is commonly regarded as responsible for the depletion reaction on most transition metals [13], in the present account of our experiment, we have chosen this model for our data analysis. Eqs. (3)-(5) can fit nicely our experimental results at low temperature (up to about 500 K), but they cannot account for the higher temperatures behavior of our kinetic data. In order to fit the data above 500 K we must introduce the term K!,
exp( - E,,/RT)
O,t’,
(6)
in eqs. (3)-(5) to account for the recombination process between carbon and oxygen. K!?, and E,, are the pre-exponential factor and activation energy barrier for recombination respectively. The complete set of equations used to fit our experimental data is therefore the following: d&,/d
t = K, PcotJv - K- #,-,
de,-.dt
= Kzecoev - ~_,e,e,,
d e,/d t = K,ecoev - K_ ,e,e,
- K,8,,8,
+ K_ &t?,
,
(7) (8)
- K3eoeco.
(9)
C. Astaldi et al. / CO dissociation and recombination
328
- 3
.
u .l .o
l
0
l
on Ni(100)
IO* mbar
pm - 3
l
1Oa mbar
t=4WK
t-4WK
I 1 1 1 I I 600 1wO1sOOmm2M)o30003600
a ’ 0
I 1 1 I I 1 600 10001M)o2000260030003600
Time (I)
Time (I)
Fig. 4. Kinetics of deposition for carbon (a) and oxygen (b). The solid lines are a fit obtained by solving eqs. (7)-(9).
The 0, recombination reaction has not been taken into account by this model because it takes place at a much higher temperature [9]. The results of the fit are shown in fig. 4. We have assumed K, = (27r~zkT)-~/~ (which implies a unitary molecular sticking coefficient) while K!, and Edes were obtained from the literature [14,15]. From the fit we obtain values for K,, K_, and K, at each of our operating temperatures. Figs. 5a, 5b, and 5c show logarithmic plots of K,, K_, and K3 obtained in this way, versus 1000/T. All three sets of data fit nicely an Arrhenius plot giving Kt = 5.4 x lO’(monolayer Kt = 2 X lO”(monolayer K!,
. s)-‘, *s)-*,
= 1.2 x 10’5(monolayer.
s)-‘,
Edis = 23.4 kcal/mol, Edep, = 29.8 kcal/mol, E,, = 42.7 kcal/mol.
The errors on the activation energies have been estimated to be about + 15% and the ones on the pre-exponential factors + 2 in the exponent. The values for Erec and K!, are in good agreement with those obtained by Benzinger et al. on the Ni(ll1) surface [9]. Edis is about the same of a previous similar experiment on Ni(ll0) [ll] and is in excellent agreement with a number of theoretical evaluations [16,17]. Fig. 6 shows a schematic potential energy diagram along a “reaction coordinate” which accounts for the adsorption-desorption and dissociationrecombination processes of CO on Ni(lOO). In our calculations the bottom of the adsorption potential well (point A in fig. 6) is the zero of the activation energy barrier for dissociation (as in ref. [ll]). If instead we take the zero of energy as defined for a molecule infinitely far from the surface (as customary), the apparent activation energy for dissociation is Edis = - Edes + Edis = - 3 kcal/mol.
C. Astaldi et al. / CO dissociation and recombination on Ni(100)
329
a
-7
’
I
1.7
1.3
1.0
2.0
2.1
2.2
2.3
1.7
1.8
-10
I 1.7
1.2
Fig. 5. Arrhenius
1.2
2.0
1.9
2.0
1O’JW
1000/T 00
2.1
2.2
2.1
2.2
2.3
WI
I 2.3
plots of the rate constants for CO dissociation and oxygen depletion (c).
(a), oxygen recombination
(b),
In this respect we note that our present results (and the results of ref. [ll]) for the activation energy for dissociation are not in contradiction with those reported by Steinrtick et al. [Ml. These authors have carried out a molecular beam study of CO on a Ni(lOO) surface at 500 K, and have found the dissociative sticking coefficient to be 0.02, independent of the CO beam energy from 1.6 to 20 kcaI/mol. From this finding they concluded that the
B Fig. 6. Schematic potential energy diagram along the “reaction coordinate”.
330
C. Astaldi et al. / CO dissociation and recombination
on Ni(100)
value of the apparent activation energy for the process should not be larger than zero. The results of this work and the interpretation of these authors are therefore perfectly coherent with our conclusions. From our data we may also obtain an equivalent dissociation sticking coefficient Sdis (defined as the ratio between the number of dissociated molecules per unit time and the number of CO molecules impinging per unit time on the surface). At T = 513 K we obtain Sais= 0.01, also in good agreement with the value quoted by Steinriick et al. [18]. These authors [18] also conclude from their experiments that surface defects are responsible for the dissociation phenomena they have observed. We believe that the dissociation properties we have determined are intrinsic of the (100) surface (terrace sites), although we cannot completely exclude a small role of surface defects (especially in the initial part of the kinetics). In fact, we have found that the atomic carbon and oxygen coveragea approach 0.5 ML while a rough estimate of the surface defects concentration shows that they cannot exceed about 0.02 ML. On the other hand the assumption that after dissociation of CO at defect sites, the C and 0 atoms migrate away to allow further dissociation, is in contradiction to the accepted view that adsorbates are more strongly bound to defect sites. A simple calculation using our kinetic parameters for recombination shows that they would lead to a thermal desorption peak (in a TDS experiment) around 600 K. Erley and Wagner [19] showed very convincingly that the recombination of terrace carbon and oxygen atoms on nickel leads to a TDS peak at 620 K (in excellent agreement with our finding) while the associative desorption of carbon and oxygen atoms from step/kink sites gives a peak at a much higher temperature (820 K). Erley and Wagner concluded that the recombination of Cads and Oads on the terraces requires a smaller activation energy than at step sites. Furthermore, they concluded that the appearance of the higher temperature peak on the stepped surface implies no appreciable diffusion of C and 0 from step to terrace sites up to 820 K and that this fact may be correlated with a higher binding energy at step sites. For these reasons we are convinced that the dissociation mechanism we have investigated is related mainly to terrace sites. We have hints that the carbon and oxygen accumulation rates in the very initial part of the kinetic runs (up to about 0.02 ML coverages) are somewhat higher than predicted by the fit. This very small effect, if confirmed by more accurate experiments, could be due to surface defects. However, we emphasize that it would not change appreciably the kinetic parameters we have obtained in this work. By performing the same experiment on Ni(ll1) and on Ni(ll0) surfaces in similar pressure and temperatures intervals, surface oxygen coming from the CO molecule dissociation was never observed. On these surfaces, therefore, the oxygen depletion reaction (third term in eq. (9)) proceeds at a substantially faster rate than on Ni(lOO). The behavior of nickel is therefore noticeably
C. Astaldi et al. / CO dissociation and recombination on Ni(100)
331
different, in this respect, from the behavior of the 4d and 5d metals where the CO oxidation reaction has been found to be largely structure insensitive. On the other hand, oxygen is known to form a stronger bond on Ni(lOO), than on Ni(ll1) and Ni(ll0) [20], and this behavior may explain why substantially higher temperatures and/or CO pressures are needed to deplete oxygen from this surface. The value we obtain for the activation energy of the CO + 0 reaction (29.8 kcal/mol) is also much higher than the value found on most platinum-group metals and this result may again be ascribed to the much stronger bond formed by oxygen on nickel.
5. Conclusions The measurements we present in this paper allow a comprehensive description of the adsorption-desorption and of the dissociation-recombination reactions of CO on a Ni(100) surface in the low CO coverage regime. In particular we have been able to measure the kinetic parameters (pre-exponential factors and activation energy barriers) for CO dissociation and CO recombination on this surface. We have also established that the CO dissociation is a unimolecular reaction in which molecular CO adsorption is most likely an intermediate step. As a byproduct we have also obtained, from the data of our experiment, the kinetic parameters of the side-reaction of CO oxidation by surface oxygen.
Acknowledgment The authors thank the Area per la Ricerca Trieste for the partial support of this work.
Scientifica
a Tecnologia
of
References [l] 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). [2] D.W. Goodman, R.D. Kelley, T.E. Madey and J.M. White, J. Catalysis 64 (1980) 479. [3] H.P. Bonzel and H.J. Krebs, Surface Sci. 91 (1980) 499. [4] D.W. Goodn& R.D. Kelley, T.E. Madey and J.T. Yates, Jr., J. Catalysis 63 (1980) 226. [5] A. Santoni, C. Astaldi, F. Della Valle and R. Rosei, in: Structures and Reactivity of Surfaces, Proc. of the European Conference, Trieste, Italy, September 13-16, 1988, Eds. A. Zecchina, G. Costa and C. Morterra (Elsevier, Amsterdam), to be published. [6] G.W. Rubloff, Surface Sci. 89 (1979) 566. [7] F. Labohm, C.WR. Engelen, O.L.J. Gijzeman, J.W. Geus and G.A. Bootsma, Surface Sci. 126 (1983) 429. [S] J.H. Gnuferko, D.P. Woodruff and B.W. Holland, Surface Sci. 87 (1979) 357.
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C. Astaldi et al. / CO dissociation and recombinaiion
on Ni(100)
[9] J.B. Benzinger and R.E. Preston, Surface Sci. 141 (1984) 567. [lo] F.C. Schouten, E.T. Brake, O.L.J. Gijzeman and G.A. Bootsma, Surface Sci. 74 (1978) 1. [ll] R. Rosei, F. Ciccacci, R. Memeo, C. Mariani, L.S. Caputi and L. Papagno, J. Catalysis 83 (1983) 19. [12] J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. [13] T. Engel and G. ErtJ, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. Ring and D.P. Woodruff (Elsevier, Amsterdam, 1982) and references therein. [14] J.T. Yates, Jr. and D.W. Goodman, J. Chem. Phys. 73 (1980) 5371. [15] S. Johnson and R.J. Madix, Surface Sci. 108 (1981) 77. [16] E. Miyazaki, J. Catalysis 65 (1980) 84. [17] T.H. Upton, Lectures of the International Summer School on Surface Science and Catalysis of the Methanation Reaction, Sangineto Lido, July l-10, 1982, unpublished. [18] H.P. Steimiick, M.P. d’Evelyn and R.J. Madix, Surface Sci. 172 (1986) L561. [19] W. Erley and H. Wagner, Surface Sci. 74 (1978) 333. [20] F. Labohm, O.L.J. Gijzeman and J.W. Geus, Surface Sci. 135 (1983) 409.