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CO dissociation and CO2 formation catalysed by Na atoms adsorbed on Ni(1 1 1)
Chemical Physics Letters 398 (2004) 118–122 www.elsevier.com/locate/cplett
CO dissociation and CO2 formation catalysed by Na atoms adsorbed on Ni(1 1 1) A. Cupolillo *, G. Chiarello, F. Veltri, D. Pacile`, M. Papagno, V. Formoso, E. Colavita, L. Papagno Istituto Nazionale di Fisica della Materia and Dipartimento di Fisica, Universita` degli Studi della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy Received 19 July 2004; in final form 7 September 2004 Available online 5 October 2004
Abstract The co-adsorption of CO and Na on Ni(1 1 1) was investigated by High-Resolution-Electron Energy-Loss-Spectroscopy. The measurements were performed both at room temperature and at 160 K and for several Na coverages, from a sub-monolayer to almost two monolayers. At room temperature the CO molecules dissociate above a critical Na coverage. Unexpectedly, the CO dissociation takes place also at 160 K. Low Na coverages induce a direct CO–CO reaction, whose products, CO2 molecules and C atoms, remain adsorbed on the Ni surface at low temperature. 2004 Elsevier B.V. All rights reserved.
The co-adsorption of CO and alkali-metal atoms on single crystal metal surfaces has received much attention over the last 20 years [1–3]. Such interest is motivated by the involvement of alkali atoms in many important catalytic reactions. Unfortunately, such topics as the dissociation and the reaction of CO promoted by alkali-atoms, are still poorly understood. Alkali metal atoms strongly affect the adsorption properties of CO because they cause a decrease of the activation barrier for the CO dissociation and an increase of the CO adsorption energy. As a result, the CO–substrate bond is strengthened while the internal CO bond is weakened, thus giving rise to a shift of the CO stretching vibration to lower frequencies as a function of the alkali and the CO coverages. The weakening of the C–O bond gives rise to several questions in relation to the dissociation and the reaction of CO promoted by alkali metals. For example, it is not yet clear why the adsorption of CO on low index copper surfaces modified by sub-monolayers of alkali metals is *
Corresponding author. Fax: +39 0984 494401. E-mail address: cupolillo@fis.unical.it (A. Cupolillo).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.035
non-dissociative [4–11]; on the contrary, the CO dissociation and the CO2 formation were observed at low temperatures on stepped copper surfaces, i.e. K/Cu (1 1 2) [12], K/Cu (1 1 7) [12] and K/Cu (1 1 5) [13]; moreover, upon heating these surfaces up to 220 K, CO2 reacts with oxygen and forms a surface carbonate. Interestingly, these studies show a strict correlation between the surface step density and the existence of a critical alkali metal coverage for the occurrence of the CO dissociation. It is worth noting that on the basis of earlier studies on Cu and Pd [14,15], the formation of polycarbonyl salts was suggested as a general phenomenon of the K and CO co-adsorption on metal surfaces. More recently, Pratt and King [16] performed a careful investigation on the co-adsorption of CO and K on Pd (1 1 0) at room temperature. Their results convincingly highlight the promoting action of K: the alkali metal promotes the dissociation of CO and the recombination, through a catalytic surface chemical reaction, into a new compound identified as K2CO3. Recently, we performed a HREEL investigation of (K + CO + O) [17] co-adsorbed on Ni(1 1 1) at low and
A. Cupolillo et al. / Chemical Physics Letters 398 (2004) 118–122
Ep=4eV
x20
24
RT O I Na
166
C I Ni
59 x40
High coverage
36
Intensity (arb. units)
room temperatures. At 160 K we found that the addition of potassium on a pre-adsorbed layer of (CO + O) gave rise to new loss features at 80 and 293 meV. These peaks were identified as two vibrational modes of CO2 molecules. The formation of CO2 was interpreted as the result of the CO oxidation promoted by K atoms. Indeed, successive investigations [18] showed that the formation of CO2 molecules was also promoted by the addition of K on a CO-c(4 · 2) layer, while no CO2 molecules were found to be adsorbed on the clean Ni(1 1 1) surface. In order to interpret these findings, we suggest at least two reaction pathways: one involving the dissociation of CO and the other involving the direct CO–CO interaction. In order to shine more light on these processes, we investigated the co-adsorption of CO and Na on the Ni(1 1 1) surface using the HREEL spectroscopy. We show that a partial dissociation of CO occurs at both room and low temperatures, provided that the alkali coverage is sufficiently high. The dissociation and the oxidation of CO seem to proceed independently since we observe the CO2 formation even for low Na coverages, for which the CO dissociation does not occur. The present experiments were performed by using an electron energy loss spectrometer (Delta 0.5 by SPECS) operating routinely at a base pressure of 8 · 10 9 Pa. Loss Spectra were recorded in specular geometry with an incident angle of 55 with respect to the surface normal, using an electron beam energy of 4 eV and an energy resolution of 3–4 meV. The Ni(1 1 1) crystal was cleaned in a preparation chamber (base pressure of 3 · 10 8 Pa) through repeated cycles of ion sputtering and annealing at 900 K. Surface cleanliness and order were checked by the same LEED–Auger apparatus. Sodium was evaporated from a well out gassed SAES getter source. Na coverages were deduced from the evaporation time using as a reference the time needed to get the (3/2 · 3/2) LEED pattern which is assumed to occur for the completion of the first Na layer, i.e. 0.45 ML. In order to minimise the contamination arising from the interaction of the residual CO gas with a surface whose reactivity is greatly increased by the presence of adsorbed Na atoms, we precovered the Ni(1 1 1) surface with a c(4 · 2)-CO before the Na deposition. HREELS measurements were performed on a (Na + CO) overlayer, both at room and low temperatures. Fig. 1 shows the HREEL spectra taken on the COc(4 · 2)-Ni(1 1 1) surface and on the same system exposed to increasing Na doses at room temperature. The adsorption of a selected amount of CO molecules on the clean Ni(1 1 1) surface gives rise to a double peak at 230 and 250 meV and to a broad structure at about 50 meV. The losses at 230 and 250 meV correspond to the two internal stretching vibrations of CO molecules adsorbed at threefold and on top sites respectively, while
119
173
Θ(Na)=0.80ML
Θ(Na)=0.45ML Na I Ni O I C
Θ(Na)=0.32ML 230 c(4x2)-CO clean
0
50
100
150
200
250
300
Energy loss (meV) Fig. 1. HREEL spectra obtained by adsorbing sodium at room temperature on a c(4 · 2)-CO/Ni(1 1 1) surface.
the broad feature around 50 meV is due to the overlapping CO–Ni vibrations occurring at the two adsorption sites [19]. After the deposition of 0.32 ML of Na, a new feature at 20 meV assigned to the Na–Ni vibration [20] rises up while a single CO stretching vibration at 204 meV is observed. Further Na exposures give rise to a shift of the Na–Ni vibration from 20 to 24 meV while, the internal bonding of CO molecules becomes weaker and weaker as it can be deduced by the shift towards lower energies of their stretching energies. Actually, it also becomes broad, very likely because of the coexistence on the surface of different atomic species coming from the dissociation process. Two new losses at 36 and 59 meV, interpreted as due to O–Na [21] and C–Ni [22] vibrations, begin to appear for a sodium coverage of 0.45 ML. Their presence unambiguously demonstrates that a partial CO dissociation in atomic oxygen and carbon species occurs. By further increasing the Na coverage, the intensity of the Na–Ni vibration decreases down to its complete disappearance while the oxygen–sodium vibration (36 meV) shows a huge growth and the CO stretching vibration continues to move to lower energy. It is well known that for increasing alkali-metal depositions on single crystal surfaces, the alkali-nickel vibration intensity gets lower because of the growing surface metallization. On the other hand CO molecules or oxygen atoms co-adsorbed on the same surface inhi-
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bit such metallization [23], so the observed attenuation of the Na–Ni vibration intensity must have a more complex interpretation. Very likely, in presence of a partial CO dissociation sodium atoms strongly interact with highly electronegative oxygen atoms rather than with nickel substrate. HREEL experiments performed at low temperature are shown in Fig. 2. In this case the spectra were obtained by adsorbing sodium on the c(4 · 2)-CO surface at 160 K. At low temperature the CO molecules arranged in the c(4 · 2)-CO structure adsorb only at threefold sites, as confirmed by the presence of one loss peak at 235 meV. The deposition of only 0.15 ML of Na causes a substantial change in the spectrum because a new intense feature at 81 meV is observed. The Na–Ni loss at 19 meV is highly dependent on the sodium coverage according to the analogous feature measured at room temperature. Other minor features around 158, 170 and 290 meV are also present. The losses at 81 and 290 meV are representative of the formation of CO2 molecules on the surface. Actually, this interpretation is based on the fact that their energies [24] correspond to the bending vibration mode and to the asymmetric stretching vibration of a linear O–C–O molecule of the CO2 gas phase, respectively.
x20
Ep=4eV
Intensity (arb. units)
T=160K
T= 220 K
CO2 (ν
)
2
x500 Na I Ni
CO2 (asy. str.)
19
158 170
81
290 222
Θ(Na)=0.15ML CO I Ni
235 O I C
49
0
50
100
150
200
c(4x2)-CO
250
300
Energy Loss (meV) Fig. 2. HREEL spectra obtained by adsorbing 0.15 ML of sodium at 160 K on a c(4 · 2)-CO/Ni(1 1 1) surface. The uppermost spectrum is taken after an annealing of the sample at 220 K.
The losses in the energy range 158–170 meV are due to a Fermi resonance which occurs when the energies of two vibrational levels are nearly degenerate. Also this resonance was assigned by detailed studies of CO2 molecules in the gas phase and in fact, the overtone of the bending mode m2 was found very close to the fundamental line of the symmetric vibration mode m1. Thus, they can be considered as another sign of the existence of CO2 species on the Ni(1 1 1) surface [25]. In our experiment, these losses systematically disappear after a mild annealing of the surface at 220 K and this supports the occurrence of a weak bond between CO2 molecules and the surface as for physisorbed molecules. The present results are similar to those obtained for CO2 adsorbed at 100 K on the Fe(1 1 1) surface [26] and for CO2 obtained at 160 K on the Ni(1 1 1) surface [17]. In order to exclude any spurious contribution due to the alkali exposure, in some experiments, the c(4 · 2)CO overlayer was exposed to sodium above the CO2 desorption temperature. Only when the sample reached 160 K, at a pressure of 8 · 10 9 Pa, we observed a rapid growth of the CO2 features. It is also worth noting that the measured background pressure of CO2 in the UHV chamber was negligible, thus allowing us to affirm that the observed molecules are the product of a surface reaction and not a contamination coming from the background gas. So far, we have a clear evidence that the CO oxidation occurs either at low or at room temperature, although the product of the reaction in the form of CO2 species is detected only at low temperatures because they can remain adsorbed on the surface (Fig. 2). Since the HREEL technique is sensitive to the component of the dipole moment perpendicular to the surface, slight changes in the adsorbed geometry may cause a different intensity of the loss features. The relative intensity of the feature at about 81 and 290 meV can thus be used to gain some information about the geometrical orientation of CO2 molecules with respect to the surface. In the present experimental results, the weak intensity of the loss at 290 meV, as compared to the bending mode (m2) at 81 meV, strongly suggests that CO2 molecules adsorb in a nearly linear configuration with their molecular axis nearly parallel to the surface [27]. Measurement reported in Fig. 3 were carried out at 160 K by exposing to a different amount of sodium the c(4 · 2)-CO/Ni(1 1 1) surface. At the alkali coverage of 0.30 ML, two intense peaks at 22 and 204 meV, associated to the Na–Ni and CO stretching vibration modes, respectively, are present together with two minor losses at 59 and 81 meV, already interpreted as the C–Ni vibration and the m2 mode of CO2. At higher sodium coverages important changes occur in the spectra: the CO stretching feature attenuates, it becomes broader and
A. Cupolillo et al. / Chemical Physics Letters 398 (2004) 118–122
36
Ep=4eV O I Na
x20
T=160K
C I Ni
180 High coverage
59
Θ(Na)=0.80ML
Intensity (arb. units)
22 Na I Ni
CO2 (ν 2)
204
81
Θ(Na)=0.30ML
236 50
0
50
c(4x2)-CO
100
150
200
250
300
Energy loss (meV) Fig. 3. HREEL spectra obtained at 160 K by adsorbing a different amount of sodium starting, for each experiment, from the c(4 · 2)-CO/ Ni(1 1 1) surface.
121
A possible explanation for the existence of a critical Na coverage for dissociation can be given assuming an important role for lateral asymmetric Na–CO interactions that weaken the CO stretching bond up to dissociation. Very likely, several Na atoms are involved in the process leading to the CO dissociation. At low monolayer coverages the Na atoms are expected to be uniformly spread over the surface because of the repulsion between atoms due to their large dipole moments; at higher coverages, each Na atom is obliged to get closer to each other, so that each CO molecule can interact with more than one alkali metal atom. Within this microscopical mechanism it is straightforward to attach an important role to the lateral distances between adsorbates because they will define the above critical Na coverage for CO dissociation. The present results may be explained with two different mechanisms: the first would suggest that the CO dissociation is an essential condition preceding the CO oxidation according to the following two reactions: (a) 2CO = C + O + CO (b) C + O + CO = C + CO2 The second mechanism would, instead, give rise directly to the CO2 formation according to the Boudouard process [12,13]: (c) CO + CO = CO2 + C
moves to lower energies; the Na–Ni vibration intensity decreases; a loss develops at 36 meV and the feature at 59 meV is better resolved. Interestingly, the peak at 81 meV is no longer observed when the Ni surface is completely covered by the alkali metal. Possibly the CO2 formation is hindered by a thick Na layer which blocks any diffusion along the surface. This effect was already observed in a previous work on the CO oxidation promoted by potassium on the same surface [17]. The losses at 36 and 59 meV show that the CO dissociation occurs even at low temperatures, confirming the result obtained already at room temperature, but only above a critical sodium coverage. At room temperature that critical value is equal to 0.45 ML while with our experimental data we cannot give a similar precise coverage for the low temperature experiment, we only can estimate a critical coverage between 0.30 and 0.80 ML. To our knowledge, this is the first experimental evidence of the CO dissociation induced by Na atoms on a flat surface both at room and low temperatures. Our experimental results suggest that two important reactions involving the CO molecules take place on the Ni(1 1 1) surface. The presence of sodium is necessary to induce both the dissociation and the oxidation of CO, nevertheless, these two phenomena seem to be unrelated since they occur for different alkali coverages.
In the latter case the CO dissociation would not at all be implied. Our experiment demonstrates that the CO adsorption on a Na pre-covered Ni(1 1 1) flat surface is partially dissociative, provided that the coverage is higher than a critical Na coverage at low as well as at room temperatures. Therefore, the CO2 production observed at low Na coverages and at low temperatures, leads us to reject the first two reactions. Actually, we cannot completely exclude some partial CO dissociation and a complete reaction of the oxygen produced, but this seems unlikely to occur since at equilibrium some oxygen released by the (a) and (b) chemical reaction is expected to be present. We suggest therefore that, at least for low alkalimetal coverages, the only possible mechanism leading to the formation of CO2 is the Boudouard process. At high sodium coverage, the absence of CO2 molecules on the low temperature surface clearly demonstrates that CO2 production is inhibited possibly because, almost all nickel sites are occupied and the mobility of chemical species along the surface is strongly reduced. In conclusion, we show loss measurements which support the evidence for the Na induced CO–CO chemical reaction on a flat c(4 · 2)-CO/Ni(1 1 1) surface. The Na coverage is critical because it discriminates between two different mechanisms: at low Na coverages and
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low temperatures, the Boudouard process takes place, while for coverages higher than a critical Na coverage, the CO dissociation occurs, both at low and at room temperature. Acknowledgement This work was funded by the MIUR-Programmi di rilevante interesse nazionale (COFIN 2001).
References [1] H.P. Bonzel, G. Pirug, in: D.A. King, D.P. Woodruff (Eds.), The Chemical Physics of Solid Surfaces, Elsevier, Amsterdam, 1993. [2] M.P. Kiskinova, in: Poisoning and Promotion in Catalysis Based on Surface Science Concepts and ExperimentsStudies in Surface Science and Catalysis, vol. 70, Elsevier, Amsterdam, 1992. [3] H.P. Bonzel, A.M. Bradshaw, G. Ertl (Eds.), Physics and Chemistry of Alkali Metal Adsorption, Materials Science Monograph, vol. 57, Elsevier, Amsterdam, 1989. [4] D. Lackey, M. Surman, S. Jacobs, D. Grider, D.A. King, Surf. Sci. 152/153 (1985) 513. [5] D. Heskett, I. Strathy, E.W. Plummer, R.A. de Paola, Phys. Rev. B 32 (1985) 6222. [6] M. Surman, K.C. Prince, L. Sorba, A.M. Bradshaw, Surf. Sci. 206 (1988) L864. [7] W. Eberhardt, F.M. Hoffmann, R. de Paola, D. Heskett, I. Strathy, E.W. Plummer, R.H. Moser, Phys. Rev. Lett. 54 (1985) 1856. [8] L.H. Dubois, B.R. Zegarski, H.S. Luftman, J. Chem. Phys. 87 (1987) 11367. [9] G. Paolucci, M. Surman, K.C. Prince, L. Sorba, A.M. Bradshaw, C.F. McConville, D.P. Woodruff, Phys. Rev. B 34 (1986) 1340. [10] A.F. Carley, M.W. Roberts, A.J. Strutt, J. Phys. Chem. 98 (1994) 9175.
[11] M. Christiansen, E.V. Thomes, J. Onsgaard, Surf. Sci. 261 (1992) 179. [12] L. Bech, J. Onsgaard, S.V. Hoffmann, P.J. Godowski, Surf. Sci. 482–485 (2001) 243. [13] J. Onsgaard, S.V. Hoffmann, P.J. Godowski, P. Moller, J.B. Wagner, A. Groso, A. Baraldi, G. Comelli, G. Paolucci, Chem. Phys. Lett. 322 (2000) 247. [14] D. Lackey, M. Surman, S. Jacobs, D. Grider, D.A. King, Surf. Sci. 152/153 (1985) 513. [15] D. Lackey, D.A. King, J. Chem. Soc., Faraday Trans. I 83 (1987) 2001. [16] S.J. Pratt, D.A. King, Surf. Sci. 540 (2003) 185. [17] A. Cupolillo, G. Chiarello, V. Formoso, D. Pacile`, M. Papagno, F. Veltri, E. Colavita, L. Papagno, Phys. Rev. B 66 (2002) 233407. [18] Preliminary not yet published results by the authors of the present work. [19] G. Chiarello, A. Cupolillo, C. Giallombardo, R.G. Agostino, V. Formoso, D. Pacile`, L. Papagno, E. Colavita, Surf. Sci. 536 (2003) 33. [20] C. Astaldi, P. Rudolf, S. Modesti, Solid State Commun. 75 (11) (1990) 847. [21] G. Chiarello, P. Lucozzi, A. Cupolillo, L.S. Caputi, L. Papagno, E. Colavita, Surf. Rev. Lett. 5 (5) (1998) 1029. [22] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, 1982. [23] K. Jacobi, H. Shi, M. Gruyters, G. Ertl, Phys. Rev. B 49 (1994) 5733. [24] G. Herzberg, Infrared and Raman Spectra, Wiley, New York, 1945. [25] H.-J. Freund, M.W. Roberts, Surf. Sci. Rep. 25 (1996) 225. [26] G. Hess, H. Froitzheim, Ch. Baumgartner, Surf. Sci. 331 (1995) 138. [27] It is worth noting that the present interpretation is analogous to that suggested for the potassium-promoted oxidation of CO on Ni(1 1 1) [17], although the relative intensity of the features at 81 and 290 meV is different. This result, however, should support the suggested dependence of the intensity of these losses on their environment.
CO dissociation and CO2 formation catalysed by Na atoms adsorbed on Ni(1 1 1) A. Cupolillo *, G. Chiarello, F. Veltri, D. Pacile`, M. Papagno, V. Formoso, E. Colavita, L. Papagno Istituto Nazionale di Fisica della Materia and Dipartimento di Fisica, Universita` degli Studi della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy Received 19 July 2004; in final form 7 September 2004 Available online 5 October 2004
Abstract The co-adsorption of CO and Na on Ni(1 1 1) was investigated by High-Resolution-Electron Energy-Loss-Spectroscopy. The measurements were performed both at room temperature and at 160 K and for several Na coverages, from a sub-monolayer to almost two monolayers. At room temperature the CO molecules dissociate above a critical Na coverage. Unexpectedly, the CO dissociation takes place also at 160 K. Low Na coverages induce a direct CO–CO reaction, whose products, CO2 molecules and C atoms, remain adsorbed on the Ni surface at low temperature. 2004 Elsevier B.V. All rights reserved.
The co-adsorption of CO and alkali-metal atoms on single crystal metal surfaces has received much attention over the last 20 years [1–3]. Such interest is motivated by the involvement of alkali atoms in many important catalytic reactions. Unfortunately, such topics as the dissociation and the reaction of CO promoted by alkali-atoms, are still poorly understood. Alkali metal atoms strongly affect the adsorption properties of CO because they cause a decrease of the activation barrier for the CO dissociation and an increase of the CO adsorption energy. As a result, the CO–substrate bond is strengthened while the internal CO bond is weakened, thus giving rise to a shift of the CO stretching vibration to lower frequencies as a function of the alkali and the CO coverages. The weakening of the C–O bond gives rise to several questions in relation to the dissociation and the reaction of CO promoted by alkali metals. For example, it is not yet clear why the adsorption of CO on low index copper surfaces modified by sub-monolayers of alkali metals is *
Corresponding author. Fax: +39 0984 494401. E-mail address: cupolillo@fis.unical.it (A. Cupolillo).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.09.035
non-dissociative [4–11]; on the contrary, the CO dissociation and the CO2 formation were observed at low temperatures on stepped copper surfaces, i.e. K/Cu (1 1 2) [12], K/Cu (1 1 7) [12] and K/Cu (1 1 5) [13]; moreover, upon heating these surfaces up to 220 K, CO2 reacts with oxygen and forms a surface carbonate. Interestingly, these studies show a strict correlation between the surface step density and the existence of a critical alkali metal coverage for the occurrence of the CO dissociation. It is worth noting that on the basis of earlier studies on Cu and Pd [14,15], the formation of polycarbonyl salts was suggested as a general phenomenon of the K and CO co-adsorption on metal surfaces. More recently, Pratt and King [16] performed a careful investigation on the co-adsorption of CO and K on Pd (1 1 0) at room temperature. Their results convincingly highlight the promoting action of K: the alkali metal promotes the dissociation of CO and the recombination, through a catalytic surface chemical reaction, into a new compound identified as K2CO3. Recently, we performed a HREEL investigation of (K + CO + O) [17] co-adsorbed on Ni(1 1 1) at low and
A. Cupolillo et al. / Chemical Physics Letters 398 (2004) 118–122
Ep=4eV
x20
24
RT O I Na
166
C I Ni
59 x40
High coverage
36
Intensity (arb. units)
room temperatures. At 160 K we found that the addition of potassium on a pre-adsorbed layer of (CO + O) gave rise to new loss features at 80 and 293 meV. These peaks were identified as two vibrational modes of CO2 molecules. The formation of CO2 was interpreted as the result of the CO oxidation promoted by K atoms. Indeed, successive investigations [18] showed that the formation of CO2 molecules was also promoted by the addition of K on a CO-c(4 · 2) layer, while no CO2 molecules were found to be adsorbed on the clean Ni(1 1 1) surface. In order to interpret these findings, we suggest at least two reaction pathways: one involving the dissociation of CO and the other involving the direct CO–CO interaction. In order to shine more light on these processes, we investigated the co-adsorption of CO and Na on the Ni(1 1 1) surface using the HREEL spectroscopy. We show that a partial dissociation of CO occurs at both room and low temperatures, provided that the alkali coverage is sufficiently high. The dissociation and the oxidation of CO seem to proceed independently since we observe the CO2 formation even for low Na coverages, for which the CO dissociation does not occur. The present experiments were performed by using an electron energy loss spectrometer (Delta 0.5 by SPECS) operating routinely at a base pressure of 8 · 10 9 Pa. Loss Spectra were recorded in specular geometry with an incident angle of 55 with respect to the surface normal, using an electron beam energy of 4 eV and an energy resolution of 3–4 meV. The Ni(1 1 1) crystal was cleaned in a preparation chamber (base pressure of 3 · 10 8 Pa) through repeated cycles of ion sputtering and annealing at 900 K. Surface cleanliness and order were checked by the same LEED–Auger apparatus. Sodium was evaporated from a well out gassed SAES getter source. Na coverages were deduced from the evaporation time using as a reference the time needed to get the (3/2 · 3/2) LEED pattern which is assumed to occur for the completion of the first Na layer, i.e. 0.45 ML. In order to minimise the contamination arising from the interaction of the residual CO gas with a surface whose reactivity is greatly increased by the presence of adsorbed Na atoms, we precovered the Ni(1 1 1) surface with a c(4 · 2)-CO before the Na deposition. HREELS measurements were performed on a (Na + CO) overlayer, both at room and low temperatures. Fig. 1 shows the HREEL spectra taken on the COc(4 · 2)-Ni(1 1 1) surface and on the same system exposed to increasing Na doses at room temperature. The adsorption of a selected amount of CO molecules on the clean Ni(1 1 1) surface gives rise to a double peak at 230 and 250 meV and to a broad structure at about 50 meV. The losses at 230 and 250 meV correspond to the two internal stretching vibrations of CO molecules adsorbed at threefold and on top sites respectively, while
119
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Θ(Na)=0.80ML
Θ(Na)=0.45ML Na I Ni O I C
Θ(Na)=0.32ML 230 c(4x2)-CO clean
0
50
100
150
200
250
300
Energy loss (meV) Fig. 1. HREEL spectra obtained by adsorbing sodium at room temperature on a c(4 · 2)-CO/Ni(1 1 1) surface.
the broad feature around 50 meV is due to the overlapping CO–Ni vibrations occurring at the two adsorption sites [19]. After the deposition of 0.32 ML of Na, a new feature at 20 meV assigned to the Na–Ni vibration [20] rises up while a single CO stretching vibration at 204 meV is observed. Further Na exposures give rise to a shift of the Na–Ni vibration from 20 to 24 meV while, the internal bonding of CO molecules becomes weaker and weaker as it can be deduced by the shift towards lower energies of their stretching energies. Actually, it also becomes broad, very likely because of the coexistence on the surface of different atomic species coming from the dissociation process. Two new losses at 36 and 59 meV, interpreted as due to O–Na [21] and C–Ni [22] vibrations, begin to appear for a sodium coverage of 0.45 ML. Their presence unambiguously demonstrates that a partial CO dissociation in atomic oxygen and carbon species occurs. By further increasing the Na coverage, the intensity of the Na–Ni vibration decreases down to its complete disappearance while the oxygen–sodium vibration (36 meV) shows a huge growth and the CO stretching vibration continues to move to lower energy. It is well known that for increasing alkali-metal depositions on single crystal surfaces, the alkali-nickel vibration intensity gets lower because of the growing surface metallization. On the other hand CO molecules or oxygen atoms co-adsorbed on the same surface inhi-
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bit such metallization [23], so the observed attenuation of the Na–Ni vibration intensity must have a more complex interpretation. Very likely, in presence of a partial CO dissociation sodium atoms strongly interact with highly electronegative oxygen atoms rather than with nickel substrate. HREEL experiments performed at low temperature are shown in Fig. 2. In this case the spectra were obtained by adsorbing sodium on the c(4 · 2)-CO surface at 160 K. At low temperature the CO molecules arranged in the c(4 · 2)-CO structure adsorb only at threefold sites, as confirmed by the presence of one loss peak at 235 meV. The deposition of only 0.15 ML of Na causes a substantial change in the spectrum because a new intense feature at 81 meV is observed. The Na–Ni loss at 19 meV is highly dependent on the sodium coverage according to the analogous feature measured at room temperature. Other minor features around 158, 170 and 290 meV are also present. The losses at 81 and 290 meV are representative of the formation of CO2 molecules on the surface. Actually, this interpretation is based on the fact that their energies [24] correspond to the bending vibration mode and to the asymmetric stretching vibration of a linear O–C–O molecule of the CO2 gas phase, respectively.
x20
Ep=4eV
Intensity (arb. units)
T=160K
T= 220 K
CO2 (ν
)
2
x500 Na I Ni
CO2 (asy. str.)
19
158 170
81
290 222
Θ(Na)=0.15ML CO I Ni
235 O I C
49
0
50
100
150
200
c(4x2)-CO
250
300
Energy Loss (meV) Fig. 2. HREEL spectra obtained by adsorbing 0.15 ML of sodium at 160 K on a c(4 · 2)-CO/Ni(1 1 1) surface. The uppermost spectrum is taken after an annealing of the sample at 220 K.
The losses in the energy range 158–170 meV are due to a Fermi resonance which occurs when the energies of two vibrational levels are nearly degenerate. Also this resonance was assigned by detailed studies of CO2 molecules in the gas phase and in fact, the overtone of the bending mode m2 was found very close to the fundamental line of the symmetric vibration mode m1. Thus, they can be considered as another sign of the existence of CO2 species on the Ni(1 1 1) surface [25]. In our experiment, these losses systematically disappear after a mild annealing of the surface at 220 K and this supports the occurrence of a weak bond between CO2 molecules and the surface as for physisorbed molecules. The present results are similar to those obtained for CO2 adsorbed at 100 K on the Fe(1 1 1) surface [26] and for CO2 obtained at 160 K on the Ni(1 1 1) surface [17]. In order to exclude any spurious contribution due to the alkali exposure, in some experiments, the c(4 · 2)CO overlayer was exposed to sodium above the CO2 desorption temperature. Only when the sample reached 160 K, at a pressure of 8 · 10 9 Pa, we observed a rapid growth of the CO2 features. It is also worth noting that the measured background pressure of CO2 in the UHV chamber was negligible, thus allowing us to affirm that the observed molecules are the product of a surface reaction and not a contamination coming from the background gas. So far, we have a clear evidence that the CO oxidation occurs either at low or at room temperature, although the product of the reaction in the form of CO2 species is detected only at low temperatures because they can remain adsorbed on the surface (Fig. 2). Since the HREEL technique is sensitive to the component of the dipole moment perpendicular to the surface, slight changes in the adsorbed geometry may cause a different intensity of the loss features. The relative intensity of the feature at about 81 and 290 meV can thus be used to gain some information about the geometrical orientation of CO2 molecules with respect to the surface. In the present experimental results, the weak intensity of the loss at 290 meV, as compared to the bending mode (m2) at 81 meV, strongly suggests that CO2 molecules adsorb in a nearly linear configuration with their molecular axis nearly parallel to the surface [27]. Measurement reported in Fig. 3 were carried out at 160 K by exposing to a different amount of sodium the c(4 · 2)-CO/Ni(1 1 1) surface. At the alkali coverage of 0.30 ML, two intense peaks at 22 and 204 meV, associated to the Na–Ni and CO stretching vibration modes, respectively, are present together with two minor losses at 59 and 81 meV, already interpreted as the C–Ni vibration and the m2 mode of CO2. At higher sodium coverages important changes occur in the spectra: the CO stretching feature attenuates, it becomes broader and
A. Cupolillo et al. / Chemical Physics Letters 398 (2004) 118–122
36
Ep=4eV O I Na
x20
T=160K
C I Ni
180 High coverage
59
Θ(Na)=0.80ML
Intensity (arb. units)
22 Na I Ni
CO2 (ν 2)
204
81
Θ(Na)=0.30ML
236 50
0
50
c(4x2)-CO
100
150
200
250
300
Energy loss (meV) Fig. 3. HREEL spectra obtained at 160 K by adsorbing a different amount of sodium starting, for each experiment, from the c(4 · 2)-CO/ Ni(1 1 1) surface.
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A possible explanation for the existence of a critical Na coverage for dissociation can be given assuming an important role for lateral asymmetric Na–CO interactions that weaken the CO stretching bond up to dissociation. Very likely, several Na atoms are involved in the process leading to the CO dissociation. At low monolayer coverages the Na atoms are expected to be uniformly spread over the surface because of the repulsion between atoms due to their large dipole moments; at higher coverages, each Na atom is obliged to get closer to each other, so that each CO molecule can interact with more than one alkali metal atom. Within this microscopical mechanism it is straightforward to attach an important role to the lateral distances between adsorbates because they will define the above critical Na coverage for CO dissociation. The present results may be explained with two different mechanisms: the first would suggest that the CO dissociation is an essential condition preceding the CO oxidation according to the following two reactions: (a) 2CO = C + O + CO (b) C + O + CO = C + CO2 The second mechanism would, instead, give rise directly to the CO2 formation according to the Boudouard process [12,13]: (c) CO + CO = CO2 + C
moves to lower energies; the Na–Ni vibration intensity decreases; a loss develops at 36 meV and the feature at 59 meV is better resolved. Interestingly, the peak at 81 meV is no longer observed when the Ni surface is completely covered by the alkali metal. Possibly the CO2 formation is hindered by a thick Na layer which blocks any diffusion along the surface. This effect was already observed in a previous work on the CO oxidation promoted by potassium on the same surface [17]. The losses at 36 and 59 meV show that the CO dissociation occurs even at low temperatures, confirming the result obtained already at room temperature, but only above a critical sodium coverage. At room temperature that critical value is equal to 0.45 ML while with our experimental data we cannot give a similar precise coverage for the low temperature experiment, we only can estimate a critical coverage between 0.30 and 0.80 ML. To our knowledge, this is the first experimental evidence of the CO dissociation induced by Na atoms on a flat surface both at room and low temperatures. Our experimental results suggest that two important reactions involving the CO molecules take place on the Ni(1 1 1) surface. The presence of sodium is necessary to induce both the dissociation and the oxidation of CO, nevertheless, these two phenomena seem to be unrelated since they occur for different alkali coverages.
In the latter case the CO dissociation would not at all be implied. Our experiment demonstrates that the CO adsorption on a Na pre-covered Ni(1 1 1) flat surface is partially dissociative, provided that the coverage is higher than a critical Na coverage at low as well as at room temperatures. Therefore, the CO2 production observed at low Na coverages and at low temperatures, leads us to reject the first two reactions. Actually, we cannot completely exclude some partial CO dissociation and a complete reaction of the oxygen produced, but this seems unlikely to occur since at equilibrium some oxygen released by the (a) and (b) chemical reaction is expected to be present. We suggest therefore that, at least for low alkalimetal coverages, the only possible mechanism leading to the formation of CO2 is the Boudouard process. At high sodium coverage, the absence of CO2 molecules on the low temperature surface clearly demonstrates that CO2 production is inhibited possibly because, almost all nickel sites are occupied and the mobility of chemical species along the surface is strongly reduced. In conclusion, we show loss measurements which support the evidence for the Na induced CO–CO chemical reaction on a flat c(4 · 2)-CO/Ni(1 1 1) surface. The Na coverage is critical because it discriminates between two different mechanisms: at low Na coverages and
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low temperatures, the Boudouard process takes place, while for coverages higher than a critical Na coverage, the CO dissociation occurs, both at low and at room temperature. Acknowledgement This work was funded by the MIUR-Programmi di rilevante interesse nazionale (COFIN 2001).
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