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The photochemistry of CH4 adsorbed on Pt(1 1 1) studied by high resolution fast XPS
Surface Science 482±485 (2001) 134±140
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The photochemistry of CH4 adsorbed on Pt(1 1 1) studied by high resolution fast XPS R. Larciprete a,b,*, A. Goldoni a, A. Gro^so a, S. Lizzit a, G. Paolucci a b
a Sincrotrone Trieste, S.S. 14, Km. 163,5, 34012 Basovizza (TS) Italy ENEA, Div. Fisica Applicata, via E. Fermi 45, 00044 Frascati (RM), Italy
Abstract The photochemistry induced at 40 K by synchrotron radiation at 400 eV in CH4 molecules adsorbed on Pt(1 1 1) was studied by high resolution fast XPS. The photoreactions were monitored by acquiring the C1s core level spectrum as a function of time for methane adlayers with coverage ranging from submonolayer to multilayer regime. We have observed molecular desorption of the multilayer and eective dissociation in the layer in contact with the metal. Dierently from UV photons and low energy e-beam induced dissociation of CH4 adsorbed on Pt(1 1 1) ± which only produced adsorbed CH3 and H fragments ± the C1s core level measured on methane photodissociated at 400 eV shows that other species are formed, which start to thermally dissociate at about 100 K, producing CH and possibly H. Probable candidates are CH2 , formed by multiple C±H bond cleavage together with CH3 groups during the initial methane photodissociation, or a C±C containing intermediate, resulting from methyl surface chemistry. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface photochemistry; Alkanes; Platinum; Photoemission (total yield); X-ray photoelectron spectroscopy
1. Introduction On Pt(1 1 1) methane adsorbs molecularly below 50 K and dissociative chemisorption occurs only in the case of hyperthermal molecular beams impacting the metal surface or for high surface temperature. Photolysis is thus an alternative way
* Corresponding author. Fax: +39-040-3758565. Address: ENEA, Div. Fisica Applicata, via E. Fermi 45, 00044 Frascati (RM), Italy. E-mail address: [email protected] (R. Larciprete).
to obtain C±H bond activation which is the initial and most crucial step in methane conversion. Recently it has been reported that methane adsorbed on Pt(1 1 1) ) is dissociated into methyl and hydrogen adsorbates by irradiation with laser photons at 193 nm (6.4 eV) [1±4], although in the gas phase the molecule does not show any appreciable absorption cross-section above 145 nm. Similar results have been reported for Pd(1 1 1) [4] and more recently for Cu(1 1 1) [5]. The occurrence of UV photolysis has been attributed to a strong interaction between molecular electronic excited states and Pt orbitals [2], which reduces the adsorbate symmetry [6] and allows the transition to the red-shifted excited state of the
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R. Larciprete et al. / Surface Science 482±485 (2001) 134±140
adsorbate±substrate complex. Since hybridisation requires a close vicinity between adsorbate and metal atoms only molecules in the layer in contact with Pt are photolysed. In this study we have used high resolution fast XPS with synchrotron radiation to investigate the photochemistry induced by 400 eV photons in CH4 layers adsorbed on Pt(1 1 1). The C1s core level was monitored as a function of time and the evolution of the peak lineshape revealed the photodissociation of methane and the formation of adsorbed photoproducts. In agreement with the results obtained with UV photons we found that the photodissociation yield increases dramatically in the layer in contact with the metal surface. However, in addition to methyl fragments, low temperature photolysis at 400 eV yields also other photoproducts after multiple C±H bond cleavage and/or methyl surface chemistry.
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integrated to obtain the doses in L 1 L 1 106 Torr s 1 . 3. Results and discussion Fig. 1 shows the C1s core level spectra measured at dierent CH4 coverage on Pt(1 1 1). As will be demonstrated below, CH4 adsorbed on Pt(1 1 1) is photolabile at 400 eV. In order to exclude beam eects on the core level lineshape each time a new adsorbed layer was prepared by dosing the clean Pt surface and the spectra displayed in Fig. 1 correspond only to the ®rst scan acquired on the freshly deposited adlayers.
2. Experimental The experiment was performed in the high vacuum experimental chamber (base pressure <1 10 10 mbar) of the SUPERESCA beamline [7] at ELETTRA (Trieste). High resolution photoemission spectra were acquired using photons at 400 eV. Photoelectrons were collected by a double pass hemispherical electron energy analyser which mounts a 96-channel detector [8]. The overall energy resolution at 100 eV was 60 meV and the analyser acceptance angle 2°. Measurements were performed with the photon beam impinging at 80° with respect to the sample surface normal and photoelectrons were collected at emission angle of 40°. At 400 eV the typical photon ¯ux at the sample is 1011 photons s 1 . The Pt surface was cleaned by conventional treatment: repeated cycles of Ar sputtering, prolonged heating at 900 K in 1 10 7 mbar of oxygen and annealing to 1100 K. This procedure resulted in a sharp 1 1 LEED pattern and in contaminant concentration lower than the XPS detection limit. The sample was cooled down to 40 K and CH4 was dosed by background exposure via a leak valve. The exposure was evaluated using uncorrected ionisation gauge readings, which were
Fig. 1. C1s core level spectra measured at 40 K on the CH4 / Pt(1 1 1) surface at dierent surface coverage. The number on the left indicates the methane dose.
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At CH4 dose below 1 L the C1s core level spectra show a symmetric lineshape centred at binding energy (BE) of 283.5 eV, corresponding to the adsorption of the ®rst monolayer (ML). This energy is in close agreement with the values obtained by conventional XPS which found a single Gaussian-shaped peak at 283.5 [3] and 283.3 eV [9]. The slight asymmetry evident at 1.5 L on the high BE side indicates the onset of multilayer adsorption. This shoulder develops intensity at higher CH4 doses and becomes a clear peak, which shifts with multilayer thickening. Fig. 1 shows that at 11 L the C1s core level is located at 285.0 eV. The C1s pro®le modi®es with time when the CH4 /Pt(1 1 1) surface is exposed to the synchrotron radiation beam as photodissociation reactions take place. The photochemistry induced at 400 eV in CH4 adlayers of increasing thickness was investigated by dosing with methane the clean metal
surface kept at 40 K and acquiring a series of fast C1s spectra until photoreactions saturated. Fig. 2 shows the results obtained for the Pt(1 1 1) surface dosed with 0.9, 2.3 and 3 L of CH4 . The acquisition of each C1s spectrum lasted 28 s. The C1s lineshape corresponding to the ®rst spectra at the top of the series reported in Fig. 2 exhibits the ®rst layer and the multilayer components at 283.5 and 284.2 eV, with relative intensities depending on the methane coverage. Only the ®rst layer component appears on the surface dosed with 0.9 L (compare Fig. 1) and the sequence of C1s spectra clearly shows that it rapidly disappears leaving two spectral structures centred at 284.0 and 282.9 eV corresponding to surface photoproducts. A similar quick decrease of the ®rst layer component is observed in the C1s sequences taken at higher methane coverages (Fig. 2b and c). Similarly to Fig. 2a, the photoproduct-related
Fig. 2. Top: C1s spectra measured continuously at 40 K on the Pt(1 1 1) surface exposed to (a) 0.9 L, (b) 2.3 L and (c) 3.0 L of methane. Bottom: temporal dependence of the C1s intensity at 284.2 eV (d), 283.5 eV (s) and 282.9 eV (.) measured on the fast XPS spectra for CH4 dose of (d) 0.9 L, (e) 2.3 L and (f) 3.0 L.
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spectral features at 284.0 and 282.9 eV appear with irradiation time, although in the case of the largest methane dose, the structure at 284.0 eV remains buried below the slow decaying multilayer component. The rate of fragment production and the rate of consumption of the molecules adsorbed at the ®rst and at the upper layers can be compared by evaluating how the C1s building blocks (®rst layer, multilayer and photoproduct components) evolve under the action of the photon beam. Immediate, although qualitative information in this sense is obtained by plotting in Fig. 2d±f the temporal dependence of the C1s intensity at BEs representative for the monolayer, multilayer and photoproducts components (see arrows) for the three CH4 doses. In all cases, after an initial decay, in a few minutes the intensity at 283.5 eV reaches a saturation value, which indicates the completion of the photoreactions involving ®rst adlayer molecules. On the contrary, the intensity in correspondence of the multilayer peak at 284.2 eV, both for 2.3 L and for 3 L still exhibits a slow decrease after 15 min. The pro®le at 282.9 eV monitoring the formation of surface photoproducts initially rises and saturates simultaneously with the ®rst adlayer component, attesting negligible photodissociation in correspondence of the slow decrease of the multilayer component, which thus must be mainly related to molecular desorption. This conclusion is con®rmed by the temporal evolution of the C1s integrated intensity reported in Fig. 3 as measured on the Pt surface exposed to CH4 doses ranging from 0.4 to 4 L. In the case of methane coverage larger than the ®rst monolayer (1.5±4 L) the curves reported in Fig. 3a show that the surface carbon atoms are desorbed with rates similar to the decays shown by the multilayer component in Fig. 2b and c. On the contrary, the corresponding curves measured at submonolayer methane coverage (0.4 and 0.6 L), indicate a stable number of C atoms on the surface, demonstrating that the depletion of methane molecules at the ®rst adlayer is entirely due to photodissociation. In the case of the UV photolysis [1±4,6] Pt±CH4 orbital hybridisation and substrate induced reduction of the methane symmetry from Td to C3v seem to be responsible for the fact that photolysis
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Fig. 3. C1s integrated intensity measured on the Pt(1 1 1) surface exposed to dierent methane doses as a function of the irradiation time at 400 eV.
occurs selectively in the layer in contact with the metal. The fact that the photodissociation in the ®rst molecular layer is similarly enhanced with 400 eV photons indicates that even in the X-ray region the C±H bond cleavage is favoured by the interaction between the molecular and the metal electronic systems. For UV photons [1±4] and e-beam [9] irradiation of the CH4 /Pt(1 1 1) interface, selective synthesis of methyl radicals and adsorbed hydrogen was observed without any indication for the presence of CHx x < 3 radicals or other species on the surface. The ®nal C1s spectra of Fig. 2 showing complex spectral structures related to photoproducts, indicate the formation of more than one surface species, which is then peculiar of X-ray photon processing of methane adsorbed on platinum. In the case of UV laser radiation at 6.4 eV, direct adsorbate excitation plays a dominant role with respect to substrate mediated excitation channels, since even the lowest states of the CH4 ion leading to dissociative electron attachment are
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not energetically accessible for the system [3]. With high energy photons this limitation is removed and dissociative attachment of hot carriers photogenerated in the metal may promote photochemical processes. Furthermore, the perturbation of the molecular bonding structure caused by core level excitation and subsequent Auger decay, in the case of methane molecules interacting with the catalytic Pt surface may result in bond rupture. Therefore for X-ray photolysis it is conceivable that both direct and substrate mediated excitation play a role and that the articulate fragmentation pattern and the presence of surface species dierent from methyls and hydrogen can be related to the combined action of electron and X-ray photon induced chemistry. Fig. 4a shows the high resolution C1s spectrum measured on the Pt surface dosed with methane after a prolonged exposure to 400 eV photons. The core level shows three main peaks at 282.8, 283.2 and 283.8 eV and two shoulders at 284.2 and 284.6 eV. Since CH3 fragments adsorbed on Pt(1 1 1) were identi®ed at 282.7 [3] and 282.6 [9] eV, the peak we observe at 282.8 eV is reasonably attributed to methyl groups. Concerning the three features at 283.8, 284.2 and 284.6 eV, it is interesting to note that they exhibit a degrading intensity and an energy separation of 400 meV, which is typical for vibrational multiplets of adsorbed CHx groups. Similar spectral characteristics have been observed in the C1s core level measured in the case of methoxy adsorbed on Cu(100) [10] and ethylidyne on Rh(1 1 1) [11,12]. On the basis of these consideration, it seems reasonable to consider the three peaks as belonging to a vibrational multiplet. Additional information for attributing the C1s components is given by the evolution of the core level lineshape with the temperature. A detailed description of the results obtained while performing the thermal desorption of photodissociated methane will be reported elsewhere. Here we only summarise the main results and show in Fig. 4b selected C1s spectra measured at increasing temperature. It is worth noting that the sample considered for this experiment (Fig. 4b) and the one corresponding to Fig. 4a were not the same and the dierence in the initial CH4 coverage and photon dose are responsible for the slightly dif-
Fig. 4. High resolution C1s spectra taken on photodissociated methane. Spectra displayed in the part (a) and (b) of the ®gure were taken on samples diering for initial coverage and photon dose at 400 eV. The bottom panel shows the dependence of the C1s lineshape on the temperature.
ferences in the relative intensity of the C1s components evident in the spectra measured at 40 K. On the CH4 /Pt(1 1 1) surface not exposed to the photon beam, desorption of the multilayer and of molecules in contact with Pt occurs at 50 and 73 K, respectively [1±4,9]. The desorption of intact molecules was demonstrated by the fact that after heating the surface the inspection of sample
R. Larciprete et al. / Surface Science 482±485 (2001) 134±140
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regions which had not been exposed to photons in the presence of methane did not reveal any adsorbate traces. On the contrary, the C1s lineshape measured on photodissociated methane did not modify up to 100 K and above this temperature showed the following changes:
Additional measurements and a more detailed process characterisation are needed for a de®nite fragment identi®cation.
· in agreement with the behaviour typical of the CH3 group, the peak at 282.8 eV disappeared between 200 and 250 K in correspondence of methyl associative recombination with adsorbed hydrogen to give CH4 [1 4,9]; · the intensity of the peak at 283.2 eV started to decrease at around 100 K simultaneously to a net emission increase slightly below 284 eV as it is evident in the spectrum taken at 190 K; · at temperature higher than 250 K the C1s spectrum only showed the peak at 283.7 and the shoulder at 284.1 eV ± likely a vibrational component ± which remain stable on the surface up to 430 K. This spectrum is attributed to CH [13,14]; · above 450 K the main peak shifts towards higher BE as a consequence of dehydrogenation forming adsorbed carbon.
The photolysis of CH4 adsorbed on Pt(1 1 1) induced by the synchrotron beam at 400 eV was investigated by using high resolution fast XPS to monitor in real time the modi®cations occurring in the C1s lineshape. The results obtained on methane adlayers ranging from submonolayer to multilayer coverage showed that molecules in the multilayer structure are mainly desorbed by the photon beam, whereas molecule in contact with the metal surface are photodissociated with negligible desorption. For multilayer coverage, dissociation saturates when the ®rst layer molecule are consumed and photodesorption proceeds with a much lower rate. The C1s core level measured after the photolysis shows the presence of CH3 groups, which leaves the surface between 200 and 250 K via associative recombination with adsorbed hydrogen to produce methane. In addition to CH3 ± which is the only photoproduct besides H identi®ed after UV photolysis ± other species are formed, which start to thermally dissociate at about 100 K, producing CH and possibly H. Probable candidate are CH2 , which would be then produced together with CH3 groups during the initial methane photodissociation, or a C±C containing intermediate, resulting from methyl surface chemistry.
The formation of CH, indicated by the intensity rise at 283.7 eV, manifests a close relation with the decay of the peak at 283.2 eV and with the loss of intensity in correspondence of the vibrational multiplet at high BE. These observations suggest that the multiplet and the peak at 282.8 eV are related with each other and act somehow as CH percursors. A possible interpretation for such behaviour would be the presence of an intermediate resulting from C±C coupling, which around 100 K decomposes to produce CH and hydrogen. However only rarely the thermal chemistry of methyls on the Pt(1 1 1) surface has showed C±C coupling, with formation of C2 or higher hydrocarbons, and however in general after CH3 dosing from the gas phase [15]. Alternatively, the peak at 283.2 eV can be reasonably attributed to CH2 radicals resulting from multiple C±H bond cleavage and the multiplet to an independent species. Indeed formation of CH2 during the photolysis at 193 nm of methane on Cu(1 1 1) has been revealed by C2 H4 desorption [5].
4. Conclusions
Acknowledgements The authors wish to thank M. Barnaba for his indispensable technical assistance.
References [1] Y.A. Grudzkov, K. Watanabe, K. Sawabe, Y. Matsumoto, Chem. Phys. Lett. 227 (1994) 243. [2] K. Watanabe, K. Savabe, Y. Matsumoto, Phys. Rev. Lett. 76 (1996) 1751.
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[3] Y. Matsumoto, Y.A. Grudzkov, K. Watanabe, K. Sawabe, J. Chem. Phys. 105 (1996) 4775. [4] K. Watanabe, Y. Matsumoto, Surf. Sci. 390 (1997) 250. [5] K. Watanabe, Y. Matsumoto, Surf. Sci. 454 (2000) 262. [6] J. Yoshinobu, H. Ogasawara, M. Kawai, Phys. Rev. Lett. 75 (1995) 2176. [7] A. Baraldi, M. Barnaba, B. Brena, D. Cocco, G. Comelli, S. Lizzit, G. Paolucci, R. Rosei, J. Electr. Rel. Phenom. 76 (1995) 145. [8] L. Gori, R. Tommasini, G. Cautero, D. Giuressi, M. Barnaba, A. Accardo, S. Carrabo, G. Paolucci, Nucl. Instr. Meth. Phys. Res. A 431 (1999) 338. [9] D. J. Alberas-Sloan, J. M. White, Surf. Sci. 365 (1996) 212.
[10] M. Wiklund, A. Jaworowski, F. Strisland, A. Beutler, A. Sandell, R. Nyholm, S.L. Sorensen, J.N. Andersen, Surf. Sci. 418 (1998) 210. [11] J.N. Andersen, A. Beutler, S.L. Sorensen, R. Nyholm, B. Setlik, D. Heskett, Chem. Phys. Lett. 269 (1997) 371. [12] M. Wiklund, A. Beutler, R. Nyholm, J.N. Andersen, Surf. Sci. 461 (2000) 107. [13] M.E. Pansoy-Hjelvik, R. Xu, Q. Gao, K. Weller, F. Feher, J.C. Hemminger, Surf. Sci. 312 (1994) 97. [14] M.B. Hugenschmidt, M.E. Domalaga, C.T. Campbell, Surf. Sci. 285 (1992) 121. [15] D.H. Fairbrother, X.D. Peng, R. Viswanathan, P.C. Star, M. Trenary, J. Fan, Surf. Sci. Lett. 285 (1993) L455.
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The photochemistry of CH4 adsorbed on Pt(1 1 1) studied by high resolution fast XPS R. Larciprete a,b,*, A. Goldoni a, A. Gro^so a, S. Lizzit a, G. Paolucci a b
a Sincrotrone Trieste, S.S. 14, Km. 163,5, 34012 Basovizza (TS) Italy ENEA, Div. Fisica Applicata, via E. Fermi 45, 00044 Frascati (RM), Italy
Abstract The photochemistry induced at 40 K by synchrotron radiation at 400 eV in CH4 molecules adsorbed on Pt(1 1 1) was studied by high resolution fast XPS. The photoreactions were monitored by acquiring the C1s core level spectrum as a function of time for methane adlayers with coverage ranging from submonolayer to multilayer regime. We have observed molecular desorption of the multilayer and eective dissociation in the layer in contact with the metal. Dierently from UV photons and low energy e-beam induced dissociation of CH4 adsorbed on Pt(1 1 1) ± which only produced adsorbed CH3 and H fragments ± the C1s core level measured on methane photodissociated at 400 eV shows that other species are formed, which start to thermally dissociate at about 100 K, producing CH and possibly H. Probable candidates are CH2 , formed by multiple C±H bond cleavage together with CH3 groups during the initial methane photodissociation, or a C±C containing intermediate, resulting from methyl surface chemistry. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface photochemistry; Alkanes; Platinum; Photoemission (total yield); X-ray photoelectron spectroscopy
1. Introduction On Pt(1 1 1) methane adsorbs molecularly below 50 K and dissociative chemisorption occurs only in the case of hyperthermal molecular beams impacting the metal surface or for high surface temperature. Photolysis is thus an alternative way
* Corresponding author. Fax: +39-040-3758565. Address: ENEA, Div. Fisica Applicata, via E. Fermi 45, 00044 Frascati (RM), Italy. E-mail address: [email protected] (R. Larciprete).
to obtain C±H bond activation which is the initial and most crucial step in methane conversion. Recently it has been reported that methane adsorbed on Pt(1 1 1) ) is dissociated into methyl and hydrogen adsorbates by irradiation with laser photons at 193 nm (6.4 eV) [1±4], although in the gas phase the molecule does not show any appreciable absorption cross-section above 145 nm. Similar results have been reported for Pd(1 1 1) [4] and more recently for Cu(1 1 1) [5]. The occurrence of UV photolysis has been attributed to a strong interaction between molecular electronic excited states and Pt orbitals [2], which reduces the adsorbate symmetry [6] and allows the transition to the red-shifted excited state of the
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 0 7 5 8 - 0
R. Larciprete et al. / Surface Science 482±485 (2001) 134±140
adsorbate±substrate complex. Since hybridisation requires a close vicinity between adsorbate and metal atoms only molecules in the layer in contact with Pt are photolysed. In this study we have used high resolution fast XPS with synchrotron radiation to investigate the photochemistry induced by 400 eV photons in CH4 layers adsorbed on Pt(1 1 1). The C1s core level was monitored as a function of time and the evolution of the peak lineshape revealed the photodissociation of methane and the formation of adsorbed photoproducts. In agreement with the results obtained with UV photons we found that the photodissociation yield increases dramatically in the layer in contact with the metal surface. However, in addition to methyl fragments, low temperature photolysis at 400 eV yields also other photoproducts after multiple C±H bond cleavage and/or methyl surface chemistry.
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integrated to obtain the doses in L 1 L 1 106 Torr s 1 . 3. Results and discussion Fig. 1 shows the C1s core level spectra measured at dierent CH4 coverage on Pt(1 1 1). As will be demonstrated below, CH4 adsorbed on Pt(1 1 1) is photolabile at 400 eV. In order to exclude beam eects on the core level lineshape each time a new adsorbed layer was prepared by dosing the clean Pt surface and the spectra displayed in Fig. 1 correspond only to the ®rst scan acquired on the freshly deposited adlayers.
2. Experimental The experiment was performed in the high vacuum experimental chamber (base pressure <1 10 10 mbar) of the SUPERESCA beamline [7] at ELETTRA (Trieste). High resolution photoemission spectra were acquired using photons at 400 eV. Photoelectrons were collected by a double pass hemispherical electron energy analyser which mounts a 96-channel detector [8]. The overall energy resolution at 100 eV was 60 meV and the analyser acceptance angle 2°. Measurements were performed with the photon beam impinging at 80° with respect to the sample surface normal and photoelectrons were collected at emission angle of 40°. At 400 eV the typical photon ¯ux at the sample is 1011 photons s 1 . The Pt surface was cleaned by conventional treatment: repeated cycles of Ar sputtering, prolonged heating at 900 K in 1 10 7 mbar of oxygen and annealing to 1100 K. This procedure resulted in a sharp 1 1 LEED pattern and in contaminant concentration lower than the XPS detection limit. The sample was cooled down to 40 K and CH4 was dosed by background exposure via a leak valve. The exposure was evaluated using uncorrected ionisation gauge readings, which were
Fig. 1. C1s core level spectra measured at 40 K on the CH4 / Pt(1 1 1) surface at dierent surface coverage. The number on the left indicates the methane dose.
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At CH4 dose below 1 L the C1s core level spectra show a symmetric lineshape centred at binding energy (BE) of 283.5 eV, corresponding to the adsorption of the ®rst monolayer (ML). This energy is in close agreement with the values obtained by conventional XPS which found a single Gaussian-shaped peak at 283.5 [3] and 283.3 eV [9]. The slight asymmetry evident at 1.5 L on the high BE side indicates the onset of multilayer adsorption. This shoulder develops intensity at higher CH4 doses and becomes a clear peak, which shifts with multilayer thickening. Fig. 1 shows that at 11 L the C1s core level is located at 285.0 eV. The C1s pro®le modi®es with time when the CH4 /Pt(1 1 1) surface is exposed to the synchrotron radiation beam as photodissociation reactions take place. The photochemistry induced at 400 eV in CH4 adlayers of increasing thickness was investigated by dosing with methane the clean metal
surface kept at 40 K and acquiring a series of fast C1s spectra until photoreactions saturated. Fig. 2 shows the results obtained for the Pt(1 1 1) surface dosed with 0.9, 2.3 and 3 L of CH4 . The acquisition of each C1s spectrum lasted 28 s. The C1s lineshape corresponding to the ®rst spectra at the top of the series reported in Fig. 2 exhibits the ®rst layer and the multilayer components at 283.5 and 284.2 eV, with relative intensities depending on the methane coverage. Only the ®rst layer component appears on the surface dosed with 0.9 L (compare Fig. 1) and the sequence of C1s spectra clearly shows that it rapidly disappears leaving two spectral structures centred at 284.0 and 282.9 eV corresponding to surface photoproducts. A similar quick decrease of the ®rst layer component is observed in the C1s sequences taken at higher methane coverages (Fig. 2b and c). Similarly to Fig. 2a, the photoproduct-related
Fig. 2. Top: C1s spectra measured continuously at 40 K on the Pt(1 1 1) surface exposed to (a) 0.9 L, (b) 2.3 L and (c) 3.0 L of methane. Bottom: temporal dependence of the C1s intensity at 284.2 eV (d), 283.5 eV (s) and 282.9 eV (.) measured on the fast XPS spectra for CH4 dose of (d) 0.9 L, (e) 2.3 L and (f) 3.0 L.
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spectral features at 284.0 and 282.9 eV appear with irradiation time, although in the case of the largest methane dose, the structure at 284.0 eV remains buried below the slow decaying multilayer component. The rate of fragment production and the rate of consumption of the molecules adsorbed at the ®rst and at the upper layers can be compared by evaluating how the C1s building blocks (®rst layer, multilayer and photoproduct components) evolve under the action of the photon beam. Immediate, although qualitative information in this sense is obtained by plotting in Fig. 2d±f the temporal dependence of the C1s intensity at BEs representative for the monolayer, multilayer and photoproducts components (see arrows) for the three CH4 doses. In all cases, after an initial decay, in a few minutes the intensity at 283.5 eV reaches a saturation value, which indicates the completion of the photoreactions involving ®rst adlayer molecules. On the contrary, the intensity in correspondence of the multilayer peak at 284.2 eV, both for 2.3 L and for 3 L still exhibits a slow decrease after 15 min. The pro®le at 282.9 eV monitoring the formation of surface photoproducts initially rises and saturates simultaneously with the ®rst adlayer component, attesting negligible photodissociation in correspondence of the slow decrease of the multilayer component, which thus must be mainly related to molecular desorption. This conclusion is con®rmed by the temporal evolution of the C1s integrated intensity reported in Fig. 3 as measured on the Pt surface exposed to CH4 doses ranging from 0.4 to 4 L. In the case of methane coverage larger than the ®rst monolayer (1.5±4 L) the curves reported in Fig. 3a show that the surface carbon atoms are desorbed with rates similar to the decays shown by the multilayer component in Fig. 2b and c. On the contrary, the corresponding curves measured at submonolayer methane coverage (0.4 and 0.6 L), indicate a stable number of C atoms on the surface, demonstrating that the depletion of methane molecules at the ®rst adlayer is entirely due to photodissociation. In the case of the UV photolysis [1±4,6] Pt±CH4 orbital hybridisation and substrate induced reduction of the methane symmetry from Td to C3v seem to be responsible for the fact that photolysis
137
Fig. 3. C1s integrated intensity measured on the Pt(1 1 1) surface exposed to dierent methane doses as a function of the irradiation time at 400 eV.
occurs selectively in the layer in contact with the metal. The fact that the photodissociation in the ®rst molecular layer is similarly enhanced with 400 eV photons indicates that even in the X-ray region the C±H bond cleavage is favoured by the interaction between the molecular and the metal electronic systems. For UV photons [1±4] and e-beam [9] irradiation of the CH4 /Pt(1 1 1) interface, selective synthesis of methyl radicals and adsorbed hydrogen was observed without any indication for the presence of CHx x < 3 radicals or other species on the surface. The ®nal C1s spectra of Fig. 2 showing complex spectral structures related to photoproducts, indicate the formation of more than one surface species, which is then peculiar of X-ray photon processing of methane adsorbed on platinum. In the case of UV laser radiation at 6.4 eV, direct adsorbate excitation plays a dominant role with respect to substrate mediated excitation channels, since even the lowest states of the CH4 ion leading to dissociative electron attachment are
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not energetically accessible for the system [3]. With high energy photons this limitation is removed and dissociative attachment of hot carriers photogenerated in the metal may promote photochemical processes. Furthermore, the perturbation of the molecular bonding structure caused by core level excitation and subsequent Auger decay, in the case of methane molecules interacting with the catalytic Pt surface may result in bond rupture. Therefore for X-ray photolysis it is conceivable that both direct and substrate mediated excitation play a role and that the articulate fragmentation pattern and the presence of surface species dierent from methyls and hydrogen can be related to the combined action of electron and X-ray photon induced chemistry. Fig. 4a shows the high resolution C1s spectrum measured on the Pt surface dosed with methane after a prolonged exposure to 400 eV photons. The core level shows three main peaks at 282.8, 283.2 and 283.8 eV and two shoulders at 284.2 and 284.6 eV. Since CH3 fragments adsorbed on Pt(1 1 1) were identi®ed at 282.7 [3] and 282.6 [9] eV, the peak we observe at 282.8 eV is reasonably attributed to methyl groups. Concerning the three features at 283.8, 284.2 and 284.6 eV, it is interesting to note that they exhibit a degrading intensity and an energy separation of 400 meV, which is typical for vibrational multiplets of adsorbed CHx groups. Similar spectral characteristics have been observed in the C1s core level measured in the case of methoxy adsorbed on Cu(100) [10] and ethylidyne on Rh(1 1 1) [11,12]. On the basis of these consideration, it seems reasonable to consider the three peaks as belonging to a vibrational multiplet. Additional information for attributing the C1s components is given by the evolution of the core level lineshape with the temperature. A detailed description of the results obtained while performing the thermal desorption of photodissociated methane will be reported elsewhere. Here we only summarise the main results and show in Fig. 4b selected C1s spectra measured at increasing temperature. It is worth noting that the sample considered for this experiment (Fig. 4b) and the one corresponding to Fig. 4a were not the same and the dierence in the initial CH4 coverage and photon dose are responsible for the slightly dif-
Fig. 4. High resolution C1s spectra taken on photodissociated methane. Spectra displayed in the part (a) and (b) of the ®gure were taken on samples diering for initial coverage and photon dose at 400 eV. The bottom panel shows the dependence of the C1s lineshape on the temperature.
ferences in the relative intensity of the C1s components evident in the spectra measured at 40 K. On the CH4 /Pt(1 1 1) surface not exposed to the photon beam, desorption of the multilayer and of molecules in contact with Pt occurs at 50 and 73 K, respectively [1±4,9]. The desorption of intact molecules was demonstrated by the fact that after heating the surface the inspection of sample
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regions which had not been exposed to photons in the presence of methane did not reveal any adsorbate traces. On the contrary, the C1s lineshape measured on photodissociated methane did not modify up to 100 K and above this temperature showed the following changes:
Additional measurements and a more detailed process characterisation are needed for a de®nite fragment identi®cation.
· in agreement with the behaviour typical of the CH3 group, the peak at 282.8 eV disappeared between 200 and 250 K in correspondence of methyl associative recombination with adsorbed hydrogen to give CH4 [1 4,9]; · the intensity of the peak at 283.2 eV started to decrease at around 100 K simultaneously to a net emission increase slightly below 284 eV as it is evident in the spectrum taken at 190 K; · at temperature higher than 250 K the C1s spectrum only showed the peak at 283.7 and the shoulder at 284.1 eV ± likely a vibrational component ± which remain stable on the surface up to 430 K. This spectrum is attributed to CH [13,14]; · above 450 K the main peak shifts towards higher BE as a consequence of dehydrogenation forming adsorbed carbon.
The photolysis of CH4 adsorbed on Pt(1 1 1) induced by the synchrotron beam at 400 eV was investigated by using high resolution fast XPS to monitor in real time the modi®cations occurring in the C1s lineshape. The results obtained on methane adlayers ranging from submonolayer to multilayer coverage showed that molecules in the multilayer structure are mainly desorbed by the photon beam, whereas molecule in contact with the metal surface are photodissociated with negligible desorption. For multilayer coverage, dissociation saturates when the ®rst layer molecule are consumed and photodesorption proceeds with a much lower rate. The C1s core level measured after the photolysis shows the presence of CH3 groups, which leaves the surface between 200 and 250 K via associative recombination with adsorbed hydrogen to produce methane. In addition to CH3 ± which is the only photoproduct besides H identi®ed after UV photolysis ± other species are formed, which start to thermally dissociate at about 100 K, producing CH and possibly H. Probable candidate are CH2 , which would be then produced together with CH3 groups during the initial methane photodissociation, or a C±C containing intermediate, resulting from methyl surface chemistry.
The formation of CH, indicated by the intensity rise at 283.7 eV, manifests a close relation with the decay of the peak at 283.2 eV and with the loss of intensity in correspondence of the vibrational multiplet at high BE. These observations suggest that the multiplet and the peak at 282.8 eV are related with each other and act somehow as CH percursors. A possible interpretation for such behaviour would be the presence of an intermediate resulting from C±C coupling, which around 100 K decomposes to produce CH and hydrogen. However only rarely the thermal chemistry of methyls on the Pt(1 1 1) surface has showed C±C coupling, with formation of C2 or higher hydrocarbons, and however in general after CH3 dosing from the gas phase [15]. Alternatively, the peak at 283.2 eV can be reasonably attributed to CH2 radicals resulting from multiple C±H bond cleavage and the multiplet to an independent species. Indeed formation of CH2 during the photolysis at 193 nm of methane on Cu(1 1 1) has been revealed by C2 H4 desorption [5].
4. Conclusions
Acknowledgements The authors wish to thank M. Barnaba for his indispensable technical assistance.
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