17772
2008, 112, 17772–17774 Published on Web 10/23/2008
Mechanisms Leading to Alkali Oxidation on Metal Surfaces A. Politano,† V. Formoso,†,‡ and G. Chiarello*,†,§ UniVersita` degli Studi della Calabria, Dipartimento di Fisica 87036 Rende, Cosenza, Italy, Laboratorio Regionale LICRYL (Liquid Crystal Laboratory) INFM-CNR, and Consorzio Nazionale InteruniVersitario per le Scienze Fisiche della Materia (CNISM) ReceiVed: September 3, 2008; ReVised Manuscript ReceiVed: October 2, 2008
The coadsorption of alkalis with CO and O on Cu(111) and Ni(111) was studied by vibrational measurements. Unexpectedly, we found an enhanced alkali oxidation rate for alkali + CO compared with alkali + O. Such finding was ascribed to the short-range alkali-CO interaction which lowers the barrier for CO dissociation and favors a giant alkali oxidation. Surprisingly, no alkali-O bond was formed for O/alkalis. The metal-oxygen bond plays an important role in many biological and synthetic reactions.1 Moreover, oxidation processes of metal and semiconductor surfaces are of primary importance in electronic-device technology. On the other hand, alkali metals are widely used in the chemical industry as promoters of chemical reactions at surfaces. They catalyze processes such as manganation,2,3 zincation,4-7 ammonia synthesis,8 Fischer-Tropsch,8 and epoxidation processes.9-11 At the fundamental level, alkalis are employed as model systems for understanding their promotional role and the nature of the chemical bond with the substrate and with other coadsorbed species. Using high-resolution electron energy loss spectroscopy (HREELS) measurements of alkali (Na, K) coadsorption with CO and O on Cu(111) and Ni(111), we provide direct evidence that the oxidation of alkali atoms is more efficient if promoted by the dissociation of CO molecules than by direct exposures of alkalis to O2. It is well-known that, when coadsorbed with CO, alkalis cause a weakening of the intramolecular C-O bond8,12 and a strengthening of the metal-CO bond.8,12 In some cases, depending on substrate, temperature, and alkali coverage, the dissociation of CO molecules has been observed. On the other hand, alkalis were demonstrated to weaken the O-substrate bond.13,14 At room temperature, Na on Cu(111) forms a disordered phase up to the formation of the (3/2 × 3/2) structure15 (0.44 ML) with Na atoms occupying 3-fold hollow sites.16 On the contrary, on Ni(111) a (3 × 3)R30°-Na structure has also been recently revealed for intermediate coverages17 (0.33 ML). Instead, on both surfaces potassium forms a p(2 × 2) structure15 (0.25 ML) with atoms in atop sites. On Ni(111) oxygen molecules adsorb dissociatively forming p(2 × 2) and (3 × 3)R30° ordered structures15 at room temperature. * To whom correspondence should be addressed. E-mail: chiarello@ fis.unical.it. † Università degli Studi della Calabria. ‡ Laboratorio Regionale LICRYL (Liquid Crystal Laboratory) INFMCNR. § Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia (CNISM).
10.1021/jp8078277 CCC: $40.75
Experiments were performed by using an electron energy loss spectrometer (Delta 0.5, SPECS). The samples were cleaned by repeated cycles of ion sputtering and annealing at 900-1100 K. Surface cleanliness and order were checked using Auger electron spectroscopy (AES) measurements and low-energy electron diffraction (LEED), respectively. Na and K were deposited onto the substrate by evaporating from well-outgassed getter sources. Carbon monoxide and oxygen were admitted through precise leak valves. Alkali, CO, and O coverages (the coverage of one monolayer, ML, is defined as the ratio between the number of the atoms of the adsorbate and that of the topmost layer of the substrate) were estimated from the exposure time taking as reference the coverage of well-known LEED structures, that is: (3/2 × 3/2)-Na, p(2 × 2)-K, c(4 × 2)-CO, and p(2 × 2)-O. A constant sticking coefficient was assumed to obtain other desired coverage. Similar results were achieved by calibrating using AES. Loss spectra were taken in specular geometry (dipole scattering) with an incident angle of 55° with respect to the surface normal. A primary electron beam energy of 3 eV was used. The use of low-energy impinging electrons ensures of the absence of irradiation-induced CO dissociation. The energy resolution of the spectrometer ranged from 2 to 3 meV. All deposition and measurements were carried out at room temperature. A particular care was dedicated to avoid any contamination of the alkali adlayer. Figure 1 shows HREEL spectra acquired for (3/2 × 3/2)-Na (top panel) and p(2 × 2)-K (bottom panel) deposited on Cu(111) and Ni(111) exposed to CO (0.4 L, 1 L ) 10-6 torr · s) at room temperature. Loss spectra show several features giving unambiguous information on the chemical reactions occurred at the investigated surfaces. In particular, we observe peaks at 19-24 meV for Na (top panel) and 15 meV for K (bottom panel), assigned to the alkali vibration against the substrate.17 Loss features recorded in the range 169-197 meV were assigned to the intramolecular C-O stretching vibration. The internal C-O frequency is red-shifted with respect to its value recorded on the clean surfaces8 as a consequence of the alkaliCO interaction. Interestingly, we observed features at 36 meV and at 24-29 meV. These losses are due to O-Na18 and O-K18 vibrations, respectively. This finding is a unambiguous and direct evidence of the dissociation of a part of CO molecules. Carbon 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17773
Figure 1. HREEL spectra acquired at 300 K (at a fixed CO exposure of 0.4 L) for: (top panel) (3/2 × 3/2)-Na/Cu(111) and (3/2 × 3/2)-Na/ Ni(111); (bottom panel) p(2 × 2)-K/Cu(111) and p(2 × 2)-K/Ni(111). All spectra were normalized to the intensity of the elastic peak and multiplied by the same factor.
monoxide adsorption is partially dissociative for all above systems, except for Na/Cu(111), for which it is fully dissociative as indicated by the absence of C-O stretching losses. Moreover, carbon-derived modes at 55-56 meV (arising from CO dissociation) were observed for alkalis/nickel but not for alkalis/ copper. Hence, loss measurements indicate that Na and K oxidation is promoted by CO dissociation. Moreover, it is worth noticing that for K/Cu(111) part of the oxygen arising from CO dissociation migrates underneath the surface, as suggested by the occurrence of the feature at 63 meV19 (atomic oxygen in subsurface sites). On the contrary, no oversurface O was present as deduced by the absence of its vibration against Cu(111) expected at 46 meV.20 The same alkali-precovered surfaces of Figure 1 were exposed to O2. In these spectra (Figure 2), besides alkali-substrate vibrations (21-25 meV for Na13 and 16-17 meV for K17), there are additional peaks assigned to oxygen-substrate stretching vibrations. In particular, the feature at 66 meV is due to the O-Ni vibration (red-shifted by the presence of alkalis12,14). On alkali-modified Cu(111), we observe peaks due to oxygen adatoms in oversurface (45-46 meV feature20) and in subsurface sites (62 meV,19 only for K).
Figure 2. HREEL spectra (at a fixed O2 exposure of 5.0 L) for (top panel) (3/2 × 3/2)-Na/Cu(111) and (3/2 × 3/2)-Na/Ni(111); (bottom panel) p(2 × 2)-K/Cu(111) and p(2 × 2)-K/Ni(111).
Unexpectedly, the direct exposure of the alkali-doped surface to O2 does not lead to alkali oxidation (Figure 2), as indicated by the absence in loss spectra of vibrational modes assignable to O-Na (36 meV18) and O-K (24-29 meV18), respectively. We suggest that the local and short-range interaction between alkali atoms and CO molecules enhances the CO dissociation rate and thus the efficiency of alkali oxidation (alkali-O bond). For CO adsorbed in sites directly adjacent to alkali adatoms, the lowered electrostatic potential stabilize the electronegative adsorbate and activate a charge transfer from the surface to the 2 π* antibonding orbital, so as to facilitate CO dissociation.14 In agreement with present findings, experiments performed for K+CO/Co(101j0)21 revealed the existence of a direct K-O bond. On the other hand, short-range interactions have been unambiguously demonstrated to be predominant in alkali coadsorption with CO.13,14,22 The alkali oxidation rate is enhanced for Cu(111), as suggested by spectra in Figure 1. In fact, the ratio between the intensity of the vibrational mode of the alkali-O bond with respect to that of molecular CO (C-O intramolecular stretching) is higher for alkali/Cu(111) than for alkali/Ni(111). This finding
17774 J. Phys. Chem. C, Vol. 112, No. 46, 2008
Figure 3. HREEL spectrum acquired for 0.10 ML K deposited onto a 0.10 ML O-modified Ni(111) surface (spectrum a). Reversing the order of adsorption, the alkali-O bond is formed. Similar results were obtained by substituting K with Na (spectrum b).
is well described within the framework of a local and shortrange interaction between alkali and CO. As a matter of fact, on clean copper surfaces the sticking coefficient for CO molecules at 300 K is extremely reduced with respect to transition-metal catalysts (Ni, Pt, Ru). The saturation coverage for CO molecules on clean copper was found to be about zero for temperatures higher than 200 K,23 while for Ni(111) the surface was found to be fully covered by CO molecules at 300 K even for small CO exposures.24 Accordingly, it is quite expected that CO adsorption on alkali-modified copper substrates might occur only in the close vicinity of alkali adatoms. We expect that the distances between alkali-metal atoms and reactants are quite short, i.e., about 3 Å,14 so as to allow the orbital overlap and the formation of a direct bond.14 Hence, in these conditions oxygen atoms arising from CO dissociation interact essentially with alkali atoms and less with the substrate. As regards results reported in Figure 2 (direct O2 exposures), it is quite surprising that the two coadsorbed species (alkali adatoms and atomic oxygen) do not interact with each other but directly with the substrate, even for higher oxygen exposures than 5.0 L. Very likely, in the alkali + O coadsorbed phase on Ni(111) and Cu(111), the distance between O and the alkali adatom should always be higher than 3 Å.14 Weak electrostatic interactions between alkalis and O already dominate (without the occurrence of alkali oxidation) for distances of 4 Å.14 Measurements were also performed by reversing the order of the adsorption, i.e. oxygen first of alkalis. In these conditions, alkali oxidation is readily observed, as suggested by the appearance of the O-K (28 meV18) and O-Na (34 meV18) in spectra (Figure 3). The direct bond of atomic oxygen with the substrate, which modifies the electronic properties of the system, should be responsible for this behavior. Our results highlight the general features of the mechanisms of alkali oxidation. For alkali-precovered surfaces, a giant alkali
Letters oxidation is achieved upon CO adsorption and dissociation. This is ascribed to the short-range interaction between alkalis and CO, which lowers the CO dissociation barrier. On the contrary, no alkali oxidation occurs for oxygen exposures on alkaliprecovered Ni(111) and Cu(111) surfaces. However, by changing the adsorption order (oxygen first of alkalis), the alkali-O bond was formed. We argue that this is a consequence of the modification of the surface reactivity through the adsorption of an electronegative species. Present results open new effective pathways for alkali oxidation and should provide a decisive advancement of the understanding of the heterogeneous catalysis at metal surfaces. References and Notes (1) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste, F. D. J. Am. Chem. Soc. 2003, 125, 4056. (2) Carrella, L. M.; Clegg, W.; Graham, D. V.; Hogg, L. M.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Rentschler, E.; Russo, L. Angew. Chem., Int. Ed. 2007, 46, 4662. ´ lvarez, J.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Angew. (3) Garcia-A Chem., Int. Ed. 2007, 46, 1105. (4) Clegg, W.; Dale, S. H.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.; Mulvey, R. E.; O’Hara, C. T. Angew. Chem., Int. Ed. 2006, 45, 6548. Angew. Chem., Int. Ed. 2006, 45, 2370. (5) Armstrong, D. R.; Clegg, W.; Dale, S. H.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.; Mulvey, R. E. Angew. Chem., Int. Ed. 2006, 45, 3775. (6) Clegg, W.; Dale, S. H.; Harrington, R. W.; Hevia, E.; Honeyman, G. W.; Mulvey, R. E. Angew. Chem., Int. Ed. 2006, 45, 2374. (7) Barley, H. R. L.; Clegg, W.; Dale, S. H.; Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E. Angew. Chem., Int. Ed. 2005, 44, 6018. (8) Physics and Chemistry of Alkali Metal Adsorption; Bonzel, H. P., Bradshaw, A. G., Ertl, G., Eds; Elsevier: Amsterdam, 1989. (9) Kokalj, A.; Gava, P.; De Gironcoli, S.; Baroni, S. J. Catal. 2008, 254, 304. (10) Linic, S.; Barteau, M. A. J. Am. Chem. Soc. 2004, 126, 8086. (11) Wang, Y.; Chu, H.; Zhu, W.; Zhang, Q. Catal. Today 2008, 131, 496. (12) Politano, A.; Agostino, R. G.; Colavita, E.; Formoso, V.; Chiarello, G. Phys. ReV. B 2007, 76, 233403. Politano, A.; Formoso, V.; Chiarello, G. App. Surf. Sci. 2008, 254, 6854. (13) Politano, A.; Agostino, R. G.; Formoso, V.; Chiarello, G. ChemPhysChem 2008, 9, 1189. (14) Liu, Z.-P.; Hu, P. J. Am. Chem. Soc. 2001, 123, 12596. (15) Diehl, R. D.; McGrath, R. Surf. Sci. Rep. 1996, 23, 43. (16) Zhang, R.; Makarenko, B.; Bahrim, B.; Rabalais, J. W. Phys. ReV. B 2007, 76, 113407. (17) Politano, A.; Agostino, R. G.; Colavita, E.; Formoso, V.; Tenuta, L.; Chiarello, G. J. Phys. Chem. C 2008, 112, 6977. (18) Politano, A.; Formoso, V.; Chiarello, G. Surf. Sci. 2008, 602, 2096. (19) Savio, L.; Gerbi, A.; Vattuone, L.; Pushpa, R.; Bonini, N.; De Gironcoli, S.; Rocca, M. J. Phys. Chem. C 2007, 111, 10923. Savio, L.; Vattuone, L.; Rocca, M. Appl. Phys. A: Mater. Sci. Process. 2007, 87, 399. (20) Sueyoshi, T.; Sasaki, T.; Iwasawa, Y. Surf. Sci. 1996, 365, 310. (21) Toomes, R. L.; King, D. A. Surf. Sci. 1996, 349, 19. (22) Zhang, A. H.; Zhu, J.; Duan, W. H. Phys. ReV. B 2006, 74, 045425. (23) Kunat, M.; Boas, Ch.; Becker, Th.; Burghaus, U.; Wo¨ll, Ch. Surf. Sci. 2000, 474, 114. (24) Elliott, J. A. W.; Ward, C. A. J. Chem. Phys. 1997, 106, 5667.
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