Vibrational Investigation of Catalyst Surfaces: Change of the

FEATURE ARTICLE pubs.acs.org/JPCC

Vibrational Investigation of Catalyst Surfaces: Change of the Adsorption Site of CO Molecules upon Coadsorption Antonio Politano†,§ and Gennaro Chiarello*,†,‡ † ‡

Dipartimento di Fisica, Universita degli Studi della Calabria, via ponte Bucci, cubo 31/C, Rende 87036, Cosenza, Italy CNISM, Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Via della Vasca Navale 84, 00146 Roma, Italy ABSTRACT: The understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase is one of the foremost goals of surface science. Adsorption on solid surfaces is the first step in catalytic reactions. Therefore, the individuation of adsorption sites for reactive chemical species is essential information to tailor catalytic properties of surfaces and interfaces. As a matter of fact, the change in adsorption site often implies a different reactivity for chemisorbed adsorbates and a selective catalytic activity. In this Feature Article, we evidence how vibrational spectroscopy can be used for individuating adsorption sites in coadsorption systems on catalyst surfaces. In particular, we studied CO coadsorption with oxygen, nitrogen, and hydrogen on the Ni(111) and the Ni(100) surfaces. Our attention was focused on the determination of CO adsorption sites in the various investigated systems. For CO adsorbed alone on the substrate, the preferential adsorption sites are the 3-fold hollow on the (111) face and the atop and bridge for the (100) surface. Striking changes in the CO adsorption site occur whenever CO is coadsorbed with other chemical species. In the CO + O coadsorption system, atop sites are populated by CO on the Ni(111) surface, while bridge and 4-fold hollow sites are occupied on Ni(100). In the CO + N phase on Ni(111), CO molecules occupy the bridge site of the pseudo-(100) reconstructed surface. For a H-precovered Ni(100) surface at 150 K, the CO stretching frequency is near its gas-phase value, thus suggesting the occurrence of a weakly bonded CO phase without changes of adsorption sites.

I. INTRODUCTION Adsorption Sites. Adsorption on catalytic surfaces is one of the fundamental steps in chemical reactions in the hetereogenous phase.118 To focus the attention on the individual steps of the reaction of interest, most experiments have been performed so far under well-controlled, ultrahigh vacuum (UHV) conditions. The use of UHV allows studying surface chemical reactions and adsorbatesubstrate interactions. Moreover, surfaces and interfaces can be easily prepared with excellent reproducibility.1923 The chemical activity of catalysts is strongly influenced by both geometric and electronic structure.24 In particular, the dependence of the catalytic reactions on the geometric surface structure can be assessed by using single crystals surfaces cut along low Miller index planes2529 as model catalysts. The determination of adsorption sites of coadsorbed chemical species on single-crystal surfaces has been investigated by using several experimental techniques and theoretical models.3038 The knowledge of the geometrical arrangement of the reactants on the surface allows shedding the light on the step-by-step sequence of elementary reactions through which surface chemical species transform into other ones.39 Such investigations lead to more efficient industrial processes and to a reduction of production costs and wastes. Moreover, studies in environmental heterogeneous catalysis are motivated by the possibility of using catalysts for removing pollutants from the atmosphere. This could also help to create a more green chemistry.4042 r 2011 American Chemical Society

It has been demonstrated that reactivity should be strongly influenced by changing the adsorption site of adsorbed chemical species.4346 As an example, CO oxidation on Ru(0001) occurs only when O atoms are adsorbed in bridge sites,43,47 while it is hindered if O atoms occupy 3-fold sites. On the other hand, the active site is a fundamental concept in heterogeneous catalysis,48 and its identification is the first step for tailoring catalysts with an improved efficiency and selectivity.48 Furthermore, temperature and coverage strongly influence the occupation of specific adsorption sites and the formation of ordered overlayers.4953 In Figure 1 we report the available adsorption sites for a (111)oriented surface of a face-centered cubic (fcc) sample. It offers the following adsorption sites: atop sites (the adsorbate is coordinated with a single substrate atom); bridge sites (coordination with two substrate atoms); and 3-fold hollow sites (coordination with three substrate atoms). On the other hand, on the fcc(100) surface, atop, bridge, and 4-fold hollow (coordination with four substrate atoms) sites can be occupied by adsorbates. Because of their importance in catalytic reactions, much attention has been dedicated to CO coadsorption systems on transition-metal substrates.5356 Site blocking has been reported Received: March 8, 2011 Revised: May 27, 2011 Published: June 13, 2011 13541

dx.doi.org/10.1021/jp202212a | J. Phys. Chem. C 2011, 115, 13541–13553

The Journal of Physical Chemistry C

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Figure 1. Adsorption sites for a (111)-oriented surface of a fcc sample.

Figure 3. The mechanism for CO adsorption on metal substrates, which implies a back-donation process101,102 from 5σ to antibonding 2π* orbitals of CO, so as to weaken the intramolecular bond of CO molecules.

Figure 4. Vibrational frequencies for CO adsorbed on a (111)-oriented solid surface.

Figure 2. Set of adsorption sites of generic adsorption of A atoms and AB molecules showing the correlation between each adsorbate structure and expected loss peaks.

for CO coadsorption with oxygen,57 nitrogen,58,59 and hydrogen.37,6063 Even small amounts of adsorbed molecules could already cause site blocking which usually prevents double site population. The occupation of a specific site may be experimentally determined by low-energy electron diffraction (LEED),51,6466 reflection high-energy electron diffraction,67,68 photoelectron diffraction (PD)6971 measurements, or by vibrational techniques. Vibrational Spectroscopy. Vibrational spectroscopy is a powerful tool for identifying chemisorbed species at surfaces and, moreover, the species generated by surface reactions. In principle, any technique that can be used to obtain vibrational data from solid-state or gas-phase samples can be applied to the study of surfaces. This is the case of infrared reflection absorption spectroscopy7274 (IRAS) and Raman spectroscopy.75,76 In addition, other specific techniques such as high-resolution electron energy loss spectroscopy72,77 (HREELS), inelastic helium7881 or neon82,83 atom scattering, and sum-frequency generation84,85 (SFG) have been developed to study the vibrations of molecules at interfaces. In particular, with the advent of a new generation of energy loss spectrometers,8688 HREELS has reached a great diffusion because of its surface sensitivity and its high resolution in both the frequency and momentum domain.72,77,8997 Information obtained from this technique complements data obtained with other surface spectroscopies and offers ease of interpretation for experimentalists. Its main advantage is the wide energy range of the various excited modes.98 By contrast, the spectral region up to 50 meV is difficult to be investigated by optical spectroscopies.

Moreover, since vibrational modes are characteristic for the strength and the type of the bonds,72,77 vibrational spectra of adsorbed species on single-crystal metal surfaces can provide valuable information on surface chemical bonds. The vibrational frequency of an adsorbate represents the main quantity of observation for surface vibrational spectroscopies, and it contains important information regarding adsorbatesubstrate and adsorbateadsorbate interactions. It has been demonstrated that each adsorption site of the vibrating adatom can be identified by a proper vibrational frequency.3,6776 As shown in Figure 2, the vibrational spectrum depends on the adsorption geometry. For adsorption of a generic A atom in high-simmetry adsorption sites (cases a and b of Figure 2), a single adsorbateinduced vibrational feature is recorded in the HREEL spectrum, which is the AM stretch, where M stands for the metal substrate. For the adsorption of a generic diatomic molecule AB perpendicular to the surface (case c), two different vibrations exist: the ABM vibration and, at higher energy, the AB intramolecular stretch. For chemisorbed CO molecules, the energy of the intramolecular stretching vibration decreases by 1550 meV from its gasphase value (266 meV99). This shift is mainly ascribed to its chemical bonding configuration at the surface100 (Figure 3). A charge transfer from 5σ to 2π* orbitals of CO molecules is mediated by the metal substrate. Hence, the intramolecular bond of CO molecules is softened as a consequence of the partial population of antibonding 2π* orbitals. The frequency of the CO internal vibration decreases with increasing bond order.103 Such site specificity easily allows the discrimination between different adsorption sites,104 as schematically shown in Figure 4. Thus, the analysis of the internal vibrational modes of chemisorbed species is not only a fingerprint of the adsorbed chemical species but also provides information on surface chemical bonds and on its local environment.105 13542

dx.doi.org/10.1021/jp202212a |J. Phys. Chem. C 2011, 115, 13541–13553


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On the other hand, in coadsorption systems, a continuous shift of the vibration frequency upon adsorption of another chemical species is a fingerprint of an occurring surface charge transfer between coadsorbates.106109 It is worth remembering that lateral interactions between coadsorbates110,111 have a pivotal role in catalytic surface reactions, as they greatly influence reactivity. Nickel is a suitable substrate for a dedicated investigation as it is extensively used in heterogeneous catalytic processes.8592 A comparative study on CO coadsorption systems on the same single-crystal surfaces could put in evidence the coadsorbateinduced effects on CO molecules (site blocking, charge transfers, etc.). Moreover, studies on carbon monoxide are particularly intriguing in environmental heterogeneous catalysis as CO is a pollutant, and its emission is reduced by automobile catalytic converters. Herein, we report on HREELS measurements on CO coadsorption with O, N, and H on the (111) and (100) faces of nickel. We found that CO molecules change their preferential adsorption site in the presence of other chemisorbed chemical species. In the CO + O coadsorbed phase at 250 K, atop sites are occupied by CO on Ni(111), while bridge and 4-fold hollow sites are populated on the Ni(100) surface. In the CO + N phase on Ni(111) at 170 K, CO molecules occupy the bridge site of the pseudo-(100) reconstructed surface. For an O- and H-precovered Ni(100) surface at 150 K, a CO phase weakly bonded (WB) to the substrate is observed, but no changes in the adsorption site are detected. Hence, the investigation of adsorption sites on the low-index Miller planes is an important tool for understanding the microscopic mechanisms ruling catalysis. On the other hand, such information is helpful in catalysis studies on metal nanoparticles.112114 In fact, such nanoparticles could be synthesized with desired shape and facets.115117 Furthermore, our results could be particularly important for shedding the light on the activity or selectivity of Ni-based catalysts.118121 In fact, as shown in the review by Somorjai,122 the improvement of current understanding of the molecular components of model catalyst nanosystems is mandatory to improve the efficiency and the selectivity of real catalysts.

Figure 5. LEED pattern and corresponding geometrical structure of c(4 2)-CO/Ni(111) and p(2 2)-O/Ni(111).

II. EXPERIMENTAL SECTION Measurements were carried out in a UHV chamber operating at a base pressure of 5 109 Pa, equipped with standard facilities for surface characterizations. The samples were singlecrystal surfaces of Ni(111) and Ni(100) with a purity of 99.999% purchased from MaTecK GmbH. Surfaces were cleaned by repeated cycles of ion sputtering and annealing at 12001300 K. Surface cleanliness and order were checked using Auger electron spectroscopy measurements and LEED, respectively. The Ni(111) and Ni(100) surfaces showed an excellent LEED pattern characterized by sharp spots against a very low background. Carbon monoxide, oxygen, nitrogen, and hydrogen were dosed through a precise leak valve. Their coverage (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) was estimated from the exposure time taking as reference the coverage of well-known LEED patterns of the overlayer (Figure 5). In HREELS experiments, a primary electron beam impinges with energy Ep onto a metal surface. In the inelastic scattering process phonons, single-particle transitions, vibrations of atoms or molecule adsorbed onto the surface or plasmons77,90 could be

excited. Thus, the energy of the backscattered beam is Ep Eloss, where Eloss is the loss energy. In most cases, the inelastic event occurs far from the surface. This process is called dipole scattering as the long-range electric field of primary electrons interacts with the fluctuating dipolar field associated with the induced surface charges. On the other hand, for impact scattering, the loss event occurs in the close vicinities of ion cores, and the scattering intensity is not peaked in the specular direction. Scattering in the specular direction arises mainly from dipole scattering,77,123 and in this case HREEL spectra give basically the same information as IRAS. For the dipole scattering mechanism on metal surfaces, the parallel components of dipole moments are perfectly screened by their image dipoles. Thus, only vibrations bearing a dipole moment perpendicular to the surface could be excited. In the framework of group theory, the surface selection rule therefore states that only the modes belonging to the total symmetric representation A0 (Cs group) and A1 (Cnv groups) (Figure 6) are active in inelastic electron scattering via the dipole scattering or in surface IR-spectroscopy. By contrast, for HREELS in off-specular geometry, electrons may transfer energy to surface species by a short-range impact

Figure 6. Vibrations of diatomic molecules adsorbed in bridge geometry on a solid surface. Only A modes are dipole-active; B modes are not observable in dipole scattering.

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Figure 7. Scattering geometry in HREELS experiments.

Figure 8. HREELS spectrum for CO adsorption on Ni(111). The bottom spectrum is related to c(4 2)-CO/Ni(111), while the top spectrum has been recorded for a Ni(111) surface saturated with CO.

Figure 9. HREEL spectra of CO on Ni(100) at room temperature and for different CO coverages. The peaks at 58 and 251 meV are assigned, respectively, to the CONi and CO vibrations of CO molecules adsorbed at atop sites.

)

scattering mechanism. In this case, all vibration modes are in principle allowed. The inelastic interaction could be treated as a classical energy loss of a charged particle reflected from a surface within the framework of the dielectric theory of inelastic electron scattering.77 The system is represented by its complex dielectric functions ε(ω) and its complex dynamic polarizabilities R(ω), respectively. The loss probability P(q , ω) is proportional to )

Pðq , ωÞ µ

ε2 ðωÞ 1 2 ¼ Im εðωÞ + 1 jεðωÞ + 1j

ð1Þ

)

where q is the parallel momentum transfer. Conservation of energy leads to Eloss ¼ Ep ES

ð2Þ

where Eloss is the energy lost by electrons, Ep the primary electron beam energy, and ES the energy of the scattered electron beam. Present HREELS experiments were performed by using an electron energy loss spectrometer (Delta 0.5, SPECS). Loss spectra were taken in specular geometry (dipole scattering) with an incident angle of 55° with respect to the surface normal (Figure 7). The primary energy used in present investigations is 4 eV. The energy resolution of the spectrometer ranged from 2 to 4 meV.

III. RESULTS AND DISCUSSION CO Adsorption on Ni Single Crystals. First, it is worth discussing the vibrational spectra of CO adsorbed alone on the Ni(111) and Ni(100) surfaces to have well-established reference data. For c(4 2)-CO/Ni(111) (see Figure 5) the CO coverage is 0.5 ML, and CO molecules occupy only 3-fold hollow sites52,93 with the CONi vibration at 50 meV and the CO stretching at

235 meV9,124 (bottom spectrum of Figure 8). Interestingly, for a Ni(111) surface saturated with CO (coverage above 0.5 ML) both the CO stretching and the CONi vibrations split into two spectral components, with new features at 59 and 250 meV (top spectrum of Figure 8). This indicates the existence of another possible adsorption site for the molecule, that is, the atop Ni site.30,125 Hence, a difference in the CONi vibrational energy of about 9 meV for the two different adsorption sites (3-fold and atop) of CO on Ni(111) exists. On the other hand, CO molecules on Ni(100) are known to preferentially occupy atop sites.126 However, at a saturated layer, bridge sites are also populated by CO.79,97 At room temperature, regardless of CO coverage, stretching modes have been recorded127 at 238242 and 251 meV and assigned to CO at bridge and atop sites, respectively (Figure 9). However, at 150 K and for small CO coverages (Figure 10), the loss spectra showed a single stretching peak at 240 meV, which is assigned to the intramolecular vibration of CO in bridge sites. For higher CO coverage, an intense peak at 260 meV appeared in HREEL spectra.127 Because of its unusual high stretching frequency, it has been ascribed to the stretching vibration of CO molecules which are WB to the Ni surface (an electronic effect arising from a reduced back-donation of electrons into the empty 2π* antibonding orbitals of CO102). Other authors128 suggested that the feature at 260 meV in CO/Ni(100) could be indicative of a multilayer CO adsorption, as found for measurements at 2335 K in CO/Cu(100).129 However, we 13544



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Figure 11. HREEL spectra acquired after having deposited 0.10 ML of CO on different O-precovered Ni(111) surfaces. Exposures and measurements have been performed at 250 K.

Figure 10. HREEL spectra of CO on Ni(100) at 150 K and for different CO coverages. The peaks at 50 and 240 meV are assigned, respectively, to the CONi and CO vibrations of CO molecules adsorbed at bridge sites.

Table 1. Values of the CO Stretching Energy (meV)a

a

atop

bridge

3-fold

Ni(111)

257

243

235

Ni(100)

251

236

4-fold

212

Its value for the gas phase is 266 meV.

want to point out that its existence up to 200 K prevents us from its assignment to a CO mode in CO multilayers. The CO stretching energies obtained for the various adsorption sites of the (111) and (100) surfaces of nickel are reported in Table 1. CO Coadsorption with O. The study of the coadsorption of CO and O allows following step-by-step the reaction pathways for the formation of carbonates130132 or carbon dioxide.133135 At the sample temperature used for the comparative investigation on Ni(111) and Ni(100); that is, 250 K, oxygen molecules adsorb dissociatively on 3-fold hollow sites on the Ni(111) surface by forming p(2 2) (the LEED pattern has been

√ √ reported in Figure 5) and ( 3 3)-R30° overstructures, depending on coverage.136 On the other hand, O adatoms on Ni(100) occupy 4-fold hollow sites, and they form p(2 2) and c(2 2)103105 patterns. CO molecules adsorbed at 250 K on the O-precovered Ni(111) surface have two distinct internal vibrations at 224234 and 257259 meV (Figure 11). Such modes are assigned to CO molecules adsorbed in 3-fold hollow sites and atop sites, respectively. Moreover, we found that for 0.25 ML of O precoverage, in correspondence of a p(2 2)-O structure, a site change of CO molecules from 3-fold hollow to atop occurs. On a 0.35 ML O/Ni(111) surface CO adsorption is allowed only on atop sites (vibration at 259 meV). The same results were obtained with PD measurements.137 By contrast, both oxygen and carbon monoxide, when adsorbed alone on Ni(111), occupy the same 3-fold-hollow sites.138 Thus, O and CO are in competition for the occupation of the same site. In this case, the blocking of 3-fold hollow sites by preadsorbed O atoms force CO molecules to adsorb in atop sites. The peak at 31 meV in Figure 11 is assigned to a surface phonon of the Ni(111) surface (the longitudinal mode, polarized along the surface plane139). The excitation of this mode is activated by O adsorption,136 in agreement with previous calculations139 and experiments.136 Interestingly, the ONi stretching vibration was recorded at 70 meV,108,136,140,141 and its frequency is not affected by the CO adsorption. Instead, for the oxidation of CO on the Rh(111)142 and Pt(111)143 surfaces a CO-promoted softening of the Osubstrate bond was reported. On the basis of results in Figure 11, we suggest that on Ni surfaces, CO molecules do not promote any softening of the O-substrate bond. As regards the shift of the CO stretching in 3-fold hollow sites from 224 to 234 meV, it is worth mentioning that dipoledipole interactions may induce strong changes in the vibration frequency.144 Such effect occurs also for increasing CO coverage. As a matter of fact, the energy of the CO mode continuously shifts from 224 to 235 meV, that is, the value recorded for 0.5 ML of CO,9,15,145,146 as a function of coverage. This remarkable frequency shift could not be interpreted in terms of changes in CO adsorption site, as unambiguously shown by X-ray PD studies of this system.147,148 The back-donated charge (from Ni d electrons to 2π* antibonding CO orbitals) for each CO molecule diminish with increasing coverage. This 13545



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Figure 12. HREEL spectra acquired after the deposition of 0.10 ML of CO on different O-precovered Ni(100) surfaces: (a) 0.08 ML O/Ni(100); (b) 0.16 ML O/Ni(100); (c) 0.20 ML O/Ni(100); (d) 0.30 ML O/Ni(100). Exposures and measurements have been performed at 250 K.

strengthens the CO bond and causes an upward shift of the CO frequency, as predicted by the theoretical model presented in a previous work.149 Such blue-shift is observed also for CO coadsorbed with other chemical species141,150 because of local electrostatic surface fields causing an upward shift and depletion of the CO 2π* orbital.150 For CO dosed onto the oxygenated Ni(111) surface, the vibration of the whole CO molecule against the substrate has been recorded at 55 and 50 meV for CO molecules in atop and 3-fold hollow sites, respectively. As for the CO stretching mode, also the frequency of the CO-substrate vibration increases for decreasing bond order. By contrast, on the Ni(100) surface oxygen atoms preferentially adsorb in 4-fold hollow sites.151 For CO adsorption (0.10 ML) onto an O-precovered Ni(100) surface (0.08 ML, spectrum a of Figure 12), CO molecules occupy atop sites (CO stretching at 251 meV). By increasing O precoverage, also bridge sites could be populated (feature at 236 meV in spectra b and c), so as to suggest a lowering of the adsorption energy of CO molecules in bridge sites. For an O precoverage of 0.30 ML (spectrum d), the appearance of a mode at 212 meV upon CO exposure indicates that also 4-fold hollow sites are occupied. Such behavior indicates that part of O adatoms change their site from 4-fold hollow to bridge and atop, so as to leave free some 4-fold hollow sites for CO adsorption. However, the occurrence of CONi and ONi vibrations at the same energy does not allow separating their respective spectral contributions from the broad feature at 5257 meV. Unfortunately, this precludes the possibility of unambiguously assign the adsorption sites in this case by HREELS measurements. For experiments performed at low temperature, that is, 150 K, an additional band at 260 meV because of CO stretching was observed (Figure 13). As shown in Figure 10 for CO/Ni(100),127 the WB CO phase is unambiguously individuated by the CONi mode and the CO stretching at 40 and 260 meV, respectively.

Figure 13. HREEL spectra of CO adsorbed at 150 K on the Ni(100) surface precovered with 0.5 ML of oxygen. The LEED pattern was a c(2 2)-O. The top spectrum was recorded after an annealing at 300 K. Note that the symbol “θ” stands for coverage. The continuous line separates regions with different magnification factor. The right part of loss spectra of O+CO/Ni(100) has been multiplied by a factor 2 to more efficiently put in evidence the vibrational band of CO stretching.

Figure 14. HREEL spectra for 0.10 ML CO/Ni(111) successively exposed to (a) 10 L N2 at 230 K; (b) 10 L after N2 at 170 K; (c) 100 L N2 at 170 K. Inset: 10 L N2 deposited on a p(2 2)-O + 0.10 ML CO/ Ni(111) surface at 170 K (similar results have been obtained at 230 K).

For the sake of completeness, it is worth noticing that the vibrational band at 260 meV has been observed also for H + CO coadsorption. 13546



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√ Figure 15. Schematization of the Ni(111)-c(5 3 9)rect-N struc165 ture. A reconstruction of the first Ni atomic layer occurs with the N atoms adsorbed on 4-fold sites in a (2 2) phase.

CO Coadsorption with N. Investigations on nitrogen dissociation on transition-metal catalysts are motivated by ammonia synthesis, whose rate-limiting step is the dissociation of chemisorbed nitrogen molecules.152154 Many efforts have been carried out for replacing iron with novel catalysts.155,156 Mortensen et al. using density functional theory (DFT) have demonstrated that the energetic barrier for N2 dissociation is quite high (above 1 eV) for different surfaces.157159 In our experiments, atomic nitrogen was produced by hot-filament dissociation of N2 molecules. The HREEL spectrum recorded for N adsorbed at 230 K on a CO-precovered Ni(111) sample (with 0.10 ML CO) presents an intense peak at 50 meV which has spectral components from both the NNi160,161 and the CONi9,15 stretching modes (spectrum a of Figure 14). The peak at 21 meV could be in principle ascribed to the N2Ni stretch. However, N2 adsorbs on Ni(111) only for sample temperature below 90 K,162 and it is characterized by NN stretch, which is recorded at 256 meV on Ni(111)163 and at 276289 meV on other surfaces.164 Thus, we assign this peak to a phonon loss because of surface reconstruction induced √by N. As a matter of fact, nitrogen induces the formation of a c(5 3 9)rect-N structure on Ni(111), as evidenced by quantitative LEED experiments.165 The occurrence of such pattern is explained165 by the formation of a (100)-oriented nickel monolayer, with nitrogen atoms in the 4-fold hollow sites in a (2 2) phase (depicted in Figure 15). Instead, a peak at similar frequencies has been observed in both theoretical160 and experimental161 investigations for the N/ Ni(100) surface and assigned to a surface phonon loss because of the N-induced p4g reconstruction. CO molecules stay on 3-fold hollow sites, and the intramolecular vibration of CO is recorded at 235 meV, as for c(4 2)CO/Ni(111).9,15,145 For N2 exposure at low sample temperature (170 K, spectra b and c of Figure 14), a second component of the CO peak was observed at 243 meV and assigned to CO adsorbed at bridge sites. The population of bridge sites is directly related to the formation of a pseudo-Ni(100)-(2 2) monolayer on top of the Ni(111) surface. Further increasing N coverage (spectrum c), the main spectral component of the intramolecular CO vibration becomes that one at 243 meV. Once the saturation of available bridge sites has occurred, atop sites are also occupied (shoulder at 257 meV).

Figure 16. HREEL spectra of CO adsorbed on the Ni(100) surface precovered with 0.10 ML of hydrogen dosed at 150 K. The weak feature at 80 meV is the HNi stretching vibration.

To attain further confirmation to our suggestions, we have also investigated N adsorption on the coadsorbed phase p(2 2)O + 0.10 ML CO/Ni(111). In this case 3-fold sites are saturated by O atoms, and CO molecules move to atop sites (peak at 259 meV in the inset of Figure 14), as shown in Figure 11. Upon N adsorption, bridge sites are also populated (feature at 242 meV in the same inset). CO Coadsorption with H. Coadsorption of hydrogen and CO is an important initial step for catalytic hydrogenation.61,166,167 Hydrogen molecules readily dissociate when adsorbed on Ni(111)168170 in 3-fold hollow sites.171,172 A (2 2) unit cell with respect to the substrate unit cell is formed on Ni(111)169,173,174 with p(2 2)-2H (0.5 ML of H) and p(1 1)-1H (1.0 ML of H) structures. On the other hand, hydrogen adsorbs in 4-fold hollow sites175,176 on Ni(100). A significant local restructuring of the H/Ni(100) surface upon CO adsorption, as shown by PD and DFT.126 After hydrogen adsorption on Ni(100), the intensities of Bragg beams decrease, whereas the background intensity increases, as a consequence of the formation of a disordered adlayer.177 H can also occupy a subsurface interstitial site located beneath a surface Ni atom, but such a configuration is not stable,178 and it has a high potential energy barrier of 0.67 eV.175 Moreover, exposures at high pressure are required to populate subsurface sites.179 Figure 16 shows the HREEL spectrum of 0.10 ML H coadsorbed with 0.10 ML CO on Ni(100) at 150 K. The preferential site for CO on this surface is the bridge (peaks at 48 and 235 meV). The weak feature at 80 meV is the only loss 13547



Figure 17. HREEL spectra acquired after the annealing at 270 K of the Ni(100) surface saturated with hydrogen and CO (top spectrum of Figure 18).

Figure 18. HREEL spectra of the Ni(100) and Ni(111) surfaces saturated with hydrogen and CO. Exposures and measurements have been carried out at 150 K.

related to hydrogen; it corresponds to the HNi vibration.180 Its low intensity is because of the weak H-dipole. Upon exposure to CO, two new features at 41 and 260 meV arose in HREEL spectrum, which are assigned to the vibration of the whole CO molecule against the surface (41 meV) and to the CO stretching vibration (260 meV) in the WB CO phase.127 The WB CO phase is not stable at higher sample temperature. In fact, by annealing the Ni(100) surface saturated with H and CO at 270 K, the WB CO phase disappears and CO molecules populate the atop site (Figure 17). For the sake of completeness, it is worth mentioning that the peak at 260 meV in CO + H/Ni(100) was first observed by Westerlund et al.181 Although loss measurements in Figure 16 are very similar to those reported in ref 181, our interpretation is quite different. The authors of ref 181 assigned the vibrational band centered at 260 meV to a shift by about 9 meV of the stretching vibration of CO at atop sites, induced by the presence of H atoms. However, the peak at 260 meV is not peculiar of H + CO coadsorption. It is present also in HREELS measurements at

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150 K in CO/Ni(100) (Figure 10) and for CO/O/Ni(100) (Figure 13). Moreover, our results on CO/H/Ni(100) do not confirm the conclusions by Westerlund et al.181 In fact, they suggested that at low temperature CO molecules should occupy both bridge and atop sites while the annealing at 270 K causes the occupation of only bridge sites. Instead, we clearly observe that heating the phase prepared at low temperature causes the occupation of both bridge and atop sites (Figure 17). Finally, on the basis of our HREEL spectra we exclude a recombination of H atoms at 150 K to form chemisorbed H2, in contrast with conclusions reported in refs 180 and 182. In Figure 18, we report the vibrational spectra recorded after the saturation of the Ni(100) and Ni(111) surfaces with both hydrogen and carbon monoxide at 150 K. For Ni(100), the broad band of the HNi vibration moved to 90 meV and a new feature at 30 meV arose. The peak at 30 meV is a COH mode,89 which for higher temperatures, that is, 500 K, acts as an intermediate for the hydrogen-promoted CO dissociation occurring on nickel.183 This opens up the possibility that CO reacts through a COH intermediate formed from adsorbed CO and H. But at 150 K, only COH formation is observed without CO dissociation. With regard to Ni(111), a scarce interaction between H and CO has been observed. CO molecules remain in 3-fold hollow sites, as indicated by the CO stretching at 231 meV. The very small shoulder around 242 meV suggests that only a reduced part of CO molecules moves to bridge sites.

IV. SUMMARY AND OUTLOOK Present vibrational investigations have shown that CO molecules on Ni(111) and Ni(100) change their preferential adsorption site whenever they are coadsorbed with other chemical species. In particular, the atop site is populated by CO in oxygenated Ni(111). In CO + O/Ni(100) at 150 K, CO molecules move from atop to bridge and 4-fold hollow sites. On the other hand, nitrogen induces a pseudo-(100) reconstruction on Ni(111). Carbon monoxide adsorbs in the bridge site of the reconstructed surface. As concerns the H- and O-modified Ni(100) surfaces at 150 K, a WB-CO phase is observed without changes in the adsorption site. As CO occupies different adsorption sites with and without coadsorbed chemical species, DFT calculations are needed to analyze in details dissimilarities in the heat of adsorption of CO on different sites. Moreover, on the basis of findings reported previously,4346 a different chemical reactivity is expected upon changing the adsorption site. The aim of this kind of investigation is to improve our knowledge of surface chemical bonds and of mechanisms ruling surface catalytic reactions. UHV surface science techniques using single crystals provide detailed information about chemisorbed species and the structure of model catalyst systems. However, such investigations in principle have two major limitations which hinder their direct application for the tailoring of more effective catalysts: the structure and the pressure gap. The structure gap is related to the inability of single crystal surfaces to reproduce the structural complexity of real catalysts. In the systems prepared by the customary methods, the catalyst is hidden in the pores of the high-surface-area supports and is inaccessible to many of the probes of surface science. Little is known about the structure of this highly inhomogeneous system and it is not uncommon that catalysts with the same chemical 13548


The Journal of Physical Chemistry C composition may have dissimilar catalytic activities because of differences in their structure or even porosity. An alternative, practiced successfully by researchers in surface science, has been to prepare flat model catalysts fabricated in UHV and accessible to a large number of surface science tools. Regarding the understanding to what extent they are representative of practical catalysts, it is worth mentioning that Stair examined in his review paper184 the opportunity of fabricating homogeneous catalysts with elevated surface area with atomic control, by atomic layer deposition (ALD). Moreover, because of the approximately 13 orders of magnitude pressure difference between UHV measurements and typical catalyst operating conditions, UHV experiments in principle may not capture the chemical nature of the catalytically active phases. This is known as the pressure gap.22,185187 It has often been mentioned as a limitation of surface science; however, surprisingly, there are systems for which such a gap is not observed and for which rate constants determined in UHV were used to fit the high-pressure kinetics.122,188 Surface science has overcome this limitation by using high-pressure cells that allow the transfer of samples from the UHV chamber and back without contamination. Moreover, as evidenced in the review by Christensen and Nørskov,48 the combination of calculations, experiments on model systems, and moreover, the synthesis and the in situ characterization of nanostructured catalysts leads to the tailoring of new more efficient catalysts and to the improvement of existing ones. In the future, it will be mandatory to explore new approaches to correlate the chemical nature of adsorbates on complex model catalysts with catalyst structure under elevated pressure conditions. This is expected to provide a more detailed understanding of model catalysts near operating conditions. Concerning vibrational spectroscopies, it would be worth using more complex model systems to reduce the structure gap, such as kinked and stepped surfaces.189 These systems deviate from a perfect flat crystal, while still maintaining well-defined surfaces. Likewise, the study of adsorption on self-nanostructured (as periodically rippled graphene190192) or nanopatterned (by ion beam erosion193,194) surfaces would be particularly intriguing. With the combined use of recently developed surfacesensitive experimental techniques at elevated pressure, such as polarization-modulation infrared reflection absorption spectroscopy195197 or SFG,195,198200 it would be possible to bridge the pressure and structure gaps so as to facilitate a more effective development of real catalysts. Another interesting issue is to obtain local HREEL spectra with a spatial resolution of 110 nm. The introduction of novel scanning probe energy loss spectrometers (SPELS)201204 allows measuring the local energy loss spectrum (in the range 150 eV) of a surface, which is irradiated by electrons from the tip of a scanning tunnelling microscope operating in field emission mode. This ensures spatial resolution but hitherto the energy resolution of SPELS is not appropriated for vibrational studies. A long-term research target will be a reduction of the energy resolution into the meV regime to enable single-molecule vibrational spectroscopy. A local vibrational investigation would make possible the study of the influence of shape and size of nanoparticles on their catalytic activity.113,205,206 Finally, time-resolved HREELS could provide a method to directly measure fundamental surface rate processes in real time.49,207209 Such experiments could be the key to improve our knowledge of surface kinetics at the molecular level. They

FEATURE ARTICLE

would also allow studying adsorption site occupations versus coverage and temperature. Time-resolved, femtosecond EELS has been demonstrated to be able to map electronic structural changes in the course of nuclear motions210212 and to investigate chemical bonding dynamics.213,214 With the achievement of femtosecond-resolved HREELS and the extension to the attosecond domain, there is the promising perspective of providing real-time electronic and structural information215 as hitherto attained through the use of free-electron lasers.216

’ AUTHOR INFORMATION Corresponding Author

*Phone: +39-984-496157. Fax: +39-984-494401. E-mail: gennaro. chiarello@fis.unical.it. Present Addresses

Departamento de Fisica de la Materia Condensada, Universidad Autonoma de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain.

§

’ BIOGRAPHIES

Antonio Politano is a postdoc fellow at Universidad Autonoma de Madrid. He obtained his B.S. at the University of Calabria in 2005 and his Ph.D. in 2008 at the same University. His graduate work dealt with the study of electronic and vibrational properties of thin metal films deposited on metallic substrates by high-resolution electron energy loss spectroscopy. In the postdoc experience at the Laboratory of Surface Science of the Universidad Autonoma de Madrid, he investigated graphene adsorption on transition-metal surfaces by helium atom scattering and scanning tunneling microscopy.

13549


The Journal of Physical Chemistry C Gennaro Chiarello is a Professor of Applied Physics in the Department of Physics of the University of Calabria. He received his B.S. in 1980 and his Ph.D. in 1984. He was a researcher at CISE Laboratories in Milan. Since 2001, he has been Professor at the University of Calabria. He has been the supervisor of Ph.D. activities in Physics. He is associated with the Consorzio Nazionale Interuniversitario di Scienze Fisiche della Materia (CNISM). His research interests include vibrational, chemical, and electronic properties of thin alkali films, carbon nanostructures, and bimetallic surfaces.

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Vibrational Investigation of Catalyst Surfaces: Change of the

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