High Resolution Electron Energy Loss Spectroscopy

High Resolution Electron Energy Loss Spectroscopy on Perfect and Defective Oxide Surfaces By Yuemin Wang* Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44801 Bochum, Germany, and Lehrstuhl für Technische Chemie, Ruhr-Universität Bochum, 44801 Bochum, Germany

Dedicated to Prof. Dr. Michael Grunze on the occasion of his 60th birthday (Received October 16, 2007; accepted in revised form December 4, 2007)

Vibrational Spectroscopy . Surface Chemistry . Oxide . Heterogeneous Catalysis . HREELS High resolution electron energy loss spectroscopy (HREELS) is a powerful method for the study of vibrational and electronic excitations at solid surfaces and has been extensively applied to metal single crystal surfaces. As a result of experimental difficulties, unfortunately, much less information is available on adsorbate vibrations at oxide surfaces. This review focuses on recent results showing the successful application of HREELS to study adsorption and reaction of molecules on metal oxide single crystal surfaces. The chemical reactivity of perfect surfaces is first investigated systematically using HREELS combined with thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED). Furthermore, it is demonstrated that the interaction of adsorbates with surface defects (in particular oxygen vacancies) can also be monitored by vibrational spectroscopy.

1. Introduction Metal oxides have recently received enormous attention from both fundamental and technological perspectives because of their extensive applications including heterogeneous catalysis, chemical sensors, solar cells, and electronic materials [1–4]. Numerous experimental and theoretical studies have been reported on oxide surfaces (see e.g., in [5–14]). However, much remains to be understood regarding their properties at the atomic and molecular level. In particular, very

* Corresponding author. E-mail: [email protected]

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Z. Phys. Chem. 222 (2008) 927–966 . DOI 10.1524.zpch.2008.6016 © by Oldenbourg Wissenschaftsverlag, München

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little information is available about the interaction of molecules with well-characterized oxide surfaces, since the poor electric conductivity of many oxides prohibits or at least complicates the use of electron-spectroscopic methods. Recently, it was found that the preparation of thin oxide films by epitaxially growing on metal substrates is an effective method to overcome the charging problems associated with the insulating properties of many oxides [11]. In the case of heterogeneous catalysis, studies on well-controlled model catalysts (usually single crystal surfaces) under ultrahigh vacuum (UHV) conditions allow to elucidate the microscopic reaction mechanisms underlying heterogeneously catalyzed reactions. To correlate these results to the performance of powder catalysts operating under industrial conditions, however, is often difficult, and the bridging of this “material” and “pressure gap” is one of the main challenges in modern surface science. The success of this surface science approach for real catalysis has been demonstrated in a few cases, including, e.g., ammonia synthesis on Fe and Ru surfaces performed by Ertl and coworkers [15,16]. High-resolution electron energy loss spectroscopy (HREELS) is one of the most advanced surface analytical methods [17] and has been widely used to investigate the vibrational spectra of surface and adsorbate species under UHV conditions. By assigning the observed vibrational modes in HREELS data important information can be obtained, involving the chemical bonding and adsorption geometry of adsorbates. More importantly, it is possible to clarify the reaction mechanisms by identifying the intermediates formed. Compared to infrared (IR) spectroscopy, HREELS has a higher surface sensitivity and wider energy window of vibration (normally, from 5 to 500 meV). In addition, HREELS is a powerful method for the study of electronic transitions (i. e., plasmon excitations) at surfaces and can be applied to determine the growth of metal clusters on oxide surfaces [10,18]. This technique has been used extensively to characterize adsorbed species on metal single crystal surfaces. In contrast to metals, its application to the study of adsorbates on metal oxides is scarce. This is – in addition to the electric conductivity problems – attributed to the intense substrate lattice excitations (Kuchs-Kliewer phonons [19]) which dominate most of the energy loss region for vibrations of adsorbed species. In this article recent HREELS studies on metal oxide surfaces will be briefly reviewed. The intention is not to cover the whole literature, but rather to focus on our recent results performed in our laboratories. The selected HREELS works on different RuO2, ZnO and TiO2 surfaces will be discussed to show the strength of this method for exploring adsorption and reaction of adsorbates on oxide surfaces.


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2. Experiment 2.1 The HREEL spectrometer The experiments reported in this review were performed using a HREEL spectrometer of the latest Ibach type (Delta 0.5, SPECS, Germany) which has been described in detail in [20,21]. Briefly, it consists of two electron monochromators to yield a very well-defined electron beam with a desired incident energy. The highly monochromatic electrons are then focused onto the sample and analyzed after scattering using an appropriate electron energy analyzer. The intensity of the detected electrons as a function of their energy loss (related to the elastic peak) can be recorded by the spectrometer. This most advanced instrument employs lens systems and dispersive elements which are not only optimized in the classical sense of electron optics but also take the space charge effects in beams of high electron density into account, thus giving an ultrahigh energy resolution with a record of 0.65 meV observed in the spectra of CO adsorbed on W(110) [21]. This enables nearly all the expected vibrational modes including the low energy frustrated translations to be observed (see e.g. [22–24]).

2.2 Scattering mechanisms There are three main inelastic excitation mechanisms in HREELS: dipole scattering, impact scattering and negative ion resonance (NIR) scattering [17]. The dipole scattering involves surface excitations by adsorption of an energy quantum from the electric field associated with the incoming electrons. These excitations can be phonons, plasmons and adsorbate vibrations. This mechanism is characterized by a long range interaction (of the order of 100 Å) between the electron and the surface. The momentum transfer is very small and thus the angular distribution of the inelastically scattered electrons is close to the specular direction. Due to the existence of induced image dipoles on metal surfaces, only the modes with a component of dynamic dipole moment perpendicular to the surface can be observed in HREELS. This selection rule can help to identify the geometrical structure of an adsorbate-surface complex. The Impact mechanism is based on scattering of the impinging electrons by the atomic potentials of the substrate or adsorbate atoms. The details of impact scattering are complicated and need a complex quantum mechanical description of the interaction between low energy electrons and the surface atoms [25]. This mechanism is characterized by a short-range direct Coulomb interaction (of the order of 1 Å). Due to the large momentum transfer the impact scattering will be observed in a broad angular distribution and is not concentrated in the specular direction. The cross section of impact scattering is about 2 or 3 orders of magnitude lower than that of dipole scattering. Therefore, it is usually detected only when measured in an off-specular direction. In contrast to the dipole scattering, the excitation by impact scattering does not depend on the direction of the inci-


HREELS on Perfect and Defective Oxide Surfaces …

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dent electron beam. Therefore, the impact scattering plays an important role to identify the binding geometry of adsorbates. The other important scattering mechanism is the so-called negative-ion resonance (NIR) in which the incoming electron is temporarily captured in a resonant state derived from an empty orbital above the vacuum level [26]. NIR has the following unique properties: (1) resonance in excitation energy, (2) characteristic angular distribution in emission, and (3) high overtone intensity of the intra- and intermolecular vibrations. This scattering mechanism has been studied in detail for weakly adsorbed molecules at metal surfaces by Jacobi and coworkers [27– 31].

2.3 Oxide surfaces In general, the HREEL spectra of metal oxide surfaces are dominated by the intense optical surface phonons (Fuchs-Kliewer phonons [19]) involving the primary and multiple losses [17,20,32]. These optical phonons provide important information on the stoichiometry and structure of oxide surfaces and have been used to characterize the growth of thin film oxide surfaces by many research groups [33–39]. In this review we focus on studies of the interaction of adsorbates with oxide surfaces. In this case the intense Fuchs-Kliewer phonons obscure the relatively weak modes arising from the vibrations of adsorbed species. To overcome this problem two methods have been proposed in the literature: collection of spectra using high primary energies and off-specular analysis in the impact scattering regime [40] and Fourier deconvolution of combination losses [41]. The latter has been confirmed as a more valid method which can remove the multiphonons completely. As shown in Fig. 1, the raw HREEL spectrum, recorded after C18O2 ¯ 0) at 95 K, is dominated by the intense primary phonon adsorption on ZnO(101 at 69 meV and its multiple excitations at 138, 207 and 274 meV. The vibrational losses of the carbonate species formed via CO2 reaction with the surface [42] are much weaker in intensity and can not be easily seen in the raw data. After Fourier deconvolution the combined surface phonons have been removed completely, and now the carbonate species can be clearly identified by the typical modes at 120, 158 and 196 meV. This process has been applied in our most studies on ZnO and TiO2 surfaces in order to obtain high quality HREELS data. As mentioned in Section 1, the limited electric conductivity of many oxides results in charging problems and makes the application of various modern surface techniques difficult. This problem was resolved in the HREELS studies on RuO2, where a metallic thin RuO2(110) film was prepared by epitaxially growing on the clean Ru(0001) substrate (see Section 3.1). In the case of ZnO and TiO2, single crystal samples were used which, in contrast to other oxides, are both sufficiently conductive so that electron-based methods including HREELS can be used without significant difficulties.


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– Fig. 1. HREELS data obtained by exposing the ZnO(1010) surface to 0.5 L 18CO2 at 95 K. (a) raw spectrum; (b) after deconvolution all the multiple surface phonons are removed and the carboate-related losses show up. The spectra were recorded at 95 K in specular direction with an incidence angle of 55° and with a primary energy of 10 eV [42].

3. RuO2(110) While under UHV conditions Ru is practically inactive with regard to CO oxidation, it exhibits high reactivity if this reaction is performed under atmospheric pressures for both supported particles [43] and single crystals [44]. Recently it has been shown [45] that this apparent contradiction has to be attributed to a “material gap” rather than to a “pressure gap”: Ru surfaces are in fact covered by a thin oxide layer in the corresponding reactive environments, in the case of Ru(0001) by an epitaxial RuO2(110) film. The latter exhibits a high catalytic activity for CO oxidation even down to room temperature due to its specific properties in adsorbing both oxygen and CO which has been extensively studied by experimental and theoretical methods [45–62]. Due to its metallic conductivity RuO2(110) is an ideally suited oxide surface for HREELS investigations since the spectra are not disturbed by the intense Fuchs-Kliewer phonons characteristic for surfaces of bulk oxide substrates. Our systematic studies using this technique have demonstrated that RuO2(110) exhibits high catalytic activity not only for CO oxidation but also for many other reactions which will be briefly reviewed in this paper.

3.1 Preparation and structure of the RuO2(110) surfaces Thin RuO2(110) films were grown epitaxially on Ru(0001) by exposing the metal single crystal to about 1!107 L (1 L = 1.33!10–6 mbar s) of O2 at a sample



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Fig. 2. Ball-and-stick-model of the differently modified RuO2(110) surfaces in perspective view: (a) the stoichiometric surface; (b) the O-rich surface; and (c) the reduced surface.

temperature of 700 K employing a glass capillary array doser directed toward the sample [47]. This procedure results in a surface which is covered by a thin single-crystalline layer of RuO2(110) consisting of three different domains, which are rotated relatively to each other by 120° as checked by low energy electron diffraction (LEED). The preparation can be repeated after restoring the original Ru(0001) surface through sputter-annealing cycles. The structures of different modified RuO2(110) surfaces are presented in Fig. 2. The stoichiometric RuO2(110) surface (Fig. 2a) contains two kinds of coordinatively unsaturated (cus) atoms: two-fold coordinated oxygen atoms (Obridge) and five-fold coordinated Ru atoms (Ru-cus) [45]. The characteristic feature of this surface is an intense peak in HREELS located at 69 meV (Fig. 3a)


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Fig. 3. HREEL spectra for (A) the stoichiometric RuO2(110) surface produced by exposing the Ru(0001) surface to 1!107 L of O2 at 700 K; (B) the O-rich RuO2(110) surface obtained after an additional does of 1 L O2 at 300 K showing a loss due to O-cus at 103 eV. All the spectra were recorded at 300 K in specular geometry with a primary electron energy of 3 eV at an incidence angle of 55° [48].

which is attributed to the stretching mode of O-bridge against the Ru atoms underneath. This assignment is in good agreement with the DFT calculations [49]. It is important to note that the stretching mode of O atoms in an O(1!1) monolayer on Ru(0001) at 81 meV [63] was not observed in Fig. 3, confirming the presence of a perfect RuO2(100) thin film instead of the coexistence with a (1!1) O monolayer. By exposure the clean RuO2(110) surface to O2 at 300 K, additional oxygen atoms (called O-cus) can be adsorbed on top of Ru-cus. In the following this surface will be referred to as O-rich RuO2(110) with a maximal coverage of about 80% of the O-cus atoms (see Fig. 2b) [49,64]. The corresponding HREEL spectrum shows a new intense peak at 103 meV (Fig. 3b), which is characteristic for the O-rich surface and originates from the stretching mode of the O-cus atoms. This species is weakly bonded to the Ru-cus atoms and desorbs associatively in the temperature range of 400–500 K [64]. The calculated binding energy of O-cus is only 3.2 eV which is much lower than that of the O-bridge (4.6 eV) [49], indicating that the O-cus species is more active for the interaction with adsorbed molecules than the O-bridge atoms, as will be verified below.



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Fig. 4. HREELS data for RuO2(110) saturated with weakly bonded oxygen O-cus (O-rich RuO2(110)) and after different exposure of CO at 300 K. Parameter as for Fig. 3 [48].

A reduced RuO2(110) surface can be prepared by removing O-cus and Obridge atoms via CO oxidation at room temperature as discussed below in Section 3.2. The structure of this surface is displayed in Fig. 2c. It possesses two types of coordinatively unsaturated Ru atoms: Ru-cus and Ru-bridge. The corresponding HREELS data revealed the disappearance of the O-cus and O-bridge modes and the subsequent adsorption of CO on the free Ru-bridge sites (see Fig. 4).

3.2 CO oxidation on RuO2(110) CO oxidation on RuO2(110) was first studied using HREELS to evaluate the reactivity of this surface. After exposing the stoichiometric RuO2(110) surface to CO at 85 K the C-O stretching mode was observed at 262 meV, indicating that CO is relatively weakly bonded on Ru-cus sites [47]. Upon heating to about 250 K, CO undergoes either desorption or reaction with the neighboring Obridge atoms forming CO2, as evidenced by both HREELS and TDS results


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[47,48]. If the RuO2(110) surface is exposed to CO at room temperature the above-mentioned reaction with O-bridge takes place spontaneously until most O-bridge species are consumed. On the O-rich RuO2(110) surface [48], two CO oxidation channels were identified, one with O-cus and one with O-bridge atoms. As shown in Fig. 4, a continuous decrease of the O-cus peak at 103 meV was first observed with increasing CO exposure at 300 K, demonstrating that CO oxidation takes place mainly on the Ru-cus rows (COad + O-cus / CO2[) as long as O-cus is present. If most O-cus is reacted off, the O-bridge peak at 69 meV begins to decrease in intensity, indicating that the second oxidation channel opens up: COad + Obridge / CO2[. Finally, both surface oxygen species, O-cus and O-bridge, are removed by CO oxidation giving rise to a reduced surface as shown in Fig. 2c. Importantly, the HREELS data further revealed that this oxygen depleted surface can be completely restored by O2 exposure at room temperature, establishing a remarkable surface redox system. At 300 K CO is stabilized only at the Ru-bridge sites, as supported by the observation of two new C-O stretching modes at 235 and 248 meV (see Fig. 4). A detail TDS and HREELS study [51] further identified the presence of three kinds of CO-bridge species on the reduced RuO2(110) surface: double bonded CO-bridge (β1: 520 K, 234 meV), single bonded CO-bridge (β3: 415 K, 248 meV) and single bonded species in the vicinity of O-bridge residues (β2: 465 K, 242 meV). After saturation of CO-bridge sites the adsorption of CO on Ru-cus (CO-cus) was also observed with two desorption maxima at 200 and 320 K [52], corresponding to a surface coverage of 1.0 (α1 state) and 0.5 monolayer (α2 state), respectively. From time-dependent STM and TDS [52] the binding energy of α2-state CO-cus was estimated to be 1.0 eV. Based on the above obtained HREELS and TDS data for the adsorption and reaction of CO on RuO2(110), the steady state kinetics of CO oxidation was studied under UHV conditions, and the reaction rate of CO2 formation was recorded as a function of temperature and partial pressures up to 3!10–6 mbar [50]. The stage of the surface could be monitored by HREELS. The oxidation of CO proceeds via the Langmuir-Hinshelwood mechanism by interaction between chemisorbed O (O-cus and O-bridge) and CO (CO-cus and CO-bridge) species. It was found that the temperature dependence of the rate in a 1:1 mixture of CO and O2 exhibits good agreement with that obtained on small supported RuO2 particles at atmospheric pressure [65]. This was traced back to the fact that under these conditions the reaction essentially proceeds between CO and O species adsorbed at Ru-cus. The total pressure becomes largely insignificant so that for this system the “pressure gap” is bridged. CO oxidation on RuO2(110) under steady state conditions was also studied theoretically using first-principle kinetic Monte Carlo simulations [57–62]. A complete surface phase diagram showing the dependence on gas phase (CO and O2) pressure and temperature was obtained. In particular, the steady state rate of



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Fig. 5. HREEL spectra for (a) the stoichiometric RuO2(110) surface and after exposure of 1 L H2 at 85 K and warming to the indicated temperatures; (b) the O-rich RuO2(110) surface prepared by an exposure of 1 L O2 at 300 K and after an additional exposure of 2.5 L H2 at 85 K and warming to the indicated temperatures [67]. All the spectra were recorded at 85 K in specular direction with an incidence angle of 55° and with a primary energy of 3 eV.

CO oxidation was simulated and found to be in surprisingly good agreement with the experimental data.

3.3 Interaction of hydrogen with RuO2(110) As manifested in Section 3.2, the O-cus species is more active for CO oxidation than the O-bridge species. The activity difference between the two surface oxygen species on RuO2(110) was also demonstrated in hydrogen oxidation. It was found [66,67] that hydrogen interacts in a complicated manner with the stoichiometric RuO2(110) surface. At 90 K, a H2 molecule, instead of dissociating at Ru-cus, weakly adsorbs as dihydrogen on top of the Ru-cus atom giving a H-H stretching mode at 367 meV. At the same time, hydrogen interacts dissociatively with O-bridge to form a metastable dihydride (H2O-bridge) complex, as evidenced by the HREELS data (Fig. 5a) which exhibit the translational (27, 58.5 meV), librational (75, 108 meV), scissor (225.231 meV) and OH stretching (436 meV) modes. Upon heating to 350 K, the H2O-bridge species is transformed completely into monohydride (OH-bridge) by release of hydrogen. As shown in Fig. 5a, all features arising from the H2O -bridge species disappear, and the two intense vibrations at 55.5 and 447 meV are typical for the hydroxyl groups. This conclusion was further supported by the DFT calculations [66,68].


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On the O-rich surface, the incoming hydrogen interacts preferentially with O-cus forming H2O-cus [67]. The HREELS data (Fig. 5b) show new losses which are characteristic for the H2O-cus species [69] and attributed to the frustrated translational (15, 27, 61 meV), librational (97, 109, 128 meV), scissor (196 meV) and OH stretching (446 meV) modes, respectively. The fingerprint peak for H2O-bridge at about 230 meV was not observed, demonstrating that no interaction of hydrogen with O-bridge takes place. After heating to higher temperatures, H2O-cus undergoes molecular desorption (see Fig. 5b) to give a sharp peak located at 400 K in TDS. Importantly, if all Ru-cus sites are blocked by CO, hydrogen can neither interact with O-cus nor with O-bridge [67], indicating that hydrogen needs activation at Ru-cus prior to reaction with surface oxygen species. An Eley-Rideal type reaction between gas-phase hydrogen and Obridge can be excluded here.

3.4 Adsorption and reaction of CO2 on RuO2(110) The activation of CO2, an important greenhouse gas, is one of the most important topics in heterogeneous catalysis. Our recent combined HREELS and TDS studies [64,70] revealed that the RuO2(110) surface also exhibits high catalytic activity towards CO2. The interaction of CO2 with the stoichiometric RuO2(110) surface is of complex nature and leads to the formation of many different surface species [70]. Despite the rather complex nature of the HREEL spectra recorded for this system, all vibrational modes could be assigned in a consistent manner (see Table 1). Based on these data a rather through insight into the reaction processes could be gained. At 85 K, CO2 was found to adsorb only on the Table 1. Vibrational energies [meV] and mode assignments for physisorbed CO2, chemisorbed CO2δ–, dimerized CO2•CO2δ–, and reacted CO3δ–carbonate, all adsorbed on the stoichiometric RuO2(110) surface a [70]. modes Tz δ ω– ω+ δ(2) νs ν(C–O) ν(C=O) νas

CO2 phys. CO2δ–(I) CO2•CO2δ–(I) 50.5 51 81.8 107.5 81.8 106 159.6 160 169.5 169 163.6b 165.5b 152 152.5 210 214 291.5 291

CO2δ–(II) CO2•CO2δ–(I) CO3δ–(I) CO3δ–(II) 50.5 51.5 107.5 82 84 108.6 160 169 152

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197 207 151.5 173 290.7 a The vibrational modes are denominated as: T, translational mode; δ in-plane bending mode; νs and νas, symmetric and asymmetric stretching modes; ω– and ω+, Fermi dyad due to resonance between νs and δ(2) (overtone of δ); δ(C–O), C–O stretching mode; δ(C=O), C=O stretching mode. b Calculated values. 204


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Fig. 6. HREEL spectra for an exposure of 1 L CO2 at 85 K on an O-rich RuO2(110) surface prepared by exposing a stoichiometric RuO2(110) to 0.2 L O2 at room temperature. The spectrum of the O-rich RuO2(110) surface is given together with the spectra obtained directly after dosing 1 L CO2, and after subsequent annealing to 120, 186, 250 and 370 K [64]. Parameter as for Fig. 5.

Ru-cus sites giving rise to three different types of species: physisorbed CO2, chemisorbed CO2δ–, and CO2•CO2δ– dimers. In addition, two kinds of chemisorbed CO2δ– could be discriminated, which further interact with CO2 forming CO2•CO2δ– dimers upon heating. At 175 K, a bidenate carbonate is also formed via reaction of CO2δ– with O-bridge. This species undergoes a thermally activated conversion to a monodenate CO3δ– species when heated to 260 K. The latter is stable up to about room temperature and then decomposes into CO2 and Oad species. The fact that CO2 activation occurs only on the Ru-cus sites provides further evidence for the key role played by the latter in the reactivity of the RuO2(110) surface. On the O-rich RuO2(110) surface the main reaction channel was confirmed to be CO2 oxidation yielding carbonate species [64]. Through interaction of


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CO2δ– with O-cus, both chemisorbed on neighboring Ru-cus sites, a bidentate transient state and finally a monodenate carbonate is formed. Based on isotope substitution experiments the CO3δ–-related losses were unambiguously identified (see Fig. 6): the in-plane deformations at 84.4 and 87.5 meV, the symmetric CO stretching mode at 150.8 meV, and the asymmetric one at 174.9 meV. The relatively low-lying stretching modes as well as their small splitting (24 meV) further reveal the formation of a monodentate CO3δ– species. Whereas the outof-plane bending mode was not observed (dipole forbidden), the spectrum is dominated by the intense νas(OCO) mode (dipole active). These findings strongly suggest a geometry in which CO3δ– is bonded to Ru-cus with its molecular plane perpendicular to the surface and with a tilted RuO-CO2 axis. The optimum Ocus pre-coverage was found to be 0.2 L giving rise to a maximal carbonate coverage of about 25%.

3.5 Interaction of NO with RuO2(110) The interaction of NO with solid surfaces is of considerable interest because of the importance of the catalytic reduction of NO in the control of air pollution. However, in contrast to metal surfaces, information about the adsorption of NO on well-defined oxide surfaces is still rather scarce [71,72]. We have also performed HREELS studies of NO on RuO2 to evaluate the catalytic activity of this surface for NO reduction. It was found [73] that the interaction of NO with the stoichiometric RuO2(110) surface depends on both coverage and temperature. Four NO states were observed in TDS and could be identified based on the HREELS data: (1) The initial exposure (below 0.5 L) of NO at 85 K leads to molecular adsorption of NO-cus, as evidenced by the typical NO stretching mode at 230 meV. This species is located on top of Ru-cus with its molecular axis orientated perpendicular to the surface. A substantial tilt can be ruled out since in this case the NO bending mode should have been detected. Interestingly, this mode was observed at 90 meV when the final third of the NOcus monolayer is adsorbed, indicating that the molecular axis of NO is now slightly bent out of the surface normal. Given the large separation of the Ru-cus sites (3.18 Å), the NO bending at higher coverage most likely is due to an electronic effect rather than being of steric origin. (2) After saturation of NOcus, NO reacts with O-bridge yielding an NO2 species which is characterized by the translational (74 meV), bending (108 meV) and NO stretching (204 meV) modes. HREELS data provided further information about the adsorption geometry. This species decomposes upon heating to 250 K again yielding NO and Oad. (3) At higher exposure, a weak N-N stretching mode at 283 meV is indicative of the presence of a small amount of N2O-cus formed presumably via an (NO)2 dimer intermediate. It desorbs at about 190 K, giving rise to NO and N2 as cracking products. (4) Interestingly, the formation of N2 was observed in TDS, revealing that NO reduction occurs on the RuO2 surface. It was speculated that N2 is formed via (NO)2-dimer and N2O formation at Ru-cus sites via the following pathway: NO + NO / (NO)2 / N2O + Oad / N2[ + 2Oad.



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3.6 NH3 oxidation on RuO2(110) In addition to catalytic activity, selectivity is also a very important issue in heterogenous catalysis [74]. The knowledge about the various factors affecting selectivity, however, is still rather limited. Recently, using HREELS and TDS we have studied the selective oxidation of ammonia to either N2 or NO on RuO2(110) as a model system, and rather detailed information on both, the reaction mechanism and the catalytic selectivity, could be gained [75]. The interaction of ammonia with the stochiometric RuO2(110) surface was first investigated. In the absence of O-cus, ammonia chemisorbs molecularly on Ru-cus and desorbs completely again below 500 K without any chemical modification. Compared to the Ru single-crystal surfaces [24,76,77], NH3 is more strongly bound on RuO2(110) due to the enhanced electron donation from the N lone pair of NH3 to the surface Ru-cus atoms. A relatively high activation energy of 43 kJ.mol was detected for the desorption of second-layer NH3 and could be attributed to an additional hydrogen bond with the surface O-bridge. After exposing the O-rich RuO2(110) surface to ammonia at 90 K, the TDS data showed desorption of N2 and NO, whereas no O2 signal from O-cus recombination was observed. These findings confirmed that a selective oxidation of ammonia has taken place on this surface. Detailed TDS studies revealed that for a given NH3 exposure the selectivity depends on the O-cus coverage: At low Ocus coverage NH3 oxidation is dominated by N2 formation, whereas for high Ocus coverage NO is preferentially formed. Ammonia oxidation occurs via the following overall reaction scheme:

HREELS data also provided further insight into the reaction mechanism. As shown in Fig. 7, after exposing the O-saturated RuO2(110) surface (with an Ocus coverage of about 80%) to 0.2 L NH3 at 90 K, additional modes at 28, 40, 58, 151, 199, 207, 404 and 420 meV were observed, which are indicative of the formation of ammonia bilayers, in which the two NH3-layers can be distinguished by the δas(NH3) modes at 199 meV (first layer NH3) and 207 meV (second-layer NH3). Importantly, a NH2 reaction intermediate was clearly identified by the observation of the corresponding characteristic rocking (172 meV) and scissoring (186 meV) modes [23]. These observations revealed that the NH3 oxidation starts to occur at 90 K as further supported by the significant reduce of the O-cus peak intensity. With heating to higher temperatures this reaction proceeds and gives rise to the following changes in the HREELS data: (1) At 200 K a weak loss at 52 meV is resolved and assigned to an OH group at Rucus, another intermediate formed in the ammonia oxidation. (2) At the same temperature new losses at 61, 110, 127, 192 and 446 meV are observed which are characteristic for the formation of H2O-cus. The intensities of the vibrational modes assigned to this species increase with temperature, until H2O is released


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Fig. 7. HREEL spectra recorded after exposing the O-cus saturated RuO2(110) surface to 0.2 L NH3 at 90 K and subsequent annealing to the indicated temperatures [75]. Parameter as for Fig. 5.

from the surface at about 400 K. (3) The O-cus and NH3-related losses disappear nearly completely at 320 K, indicating that chemisorbed NH3 has been reacted off. (4) The formation of NO at Ru-cus is also clearly identified by the ν(NO) mode at 231 (225) meV appearing at 250 K. This mode becomes the most intense one after H2O-cus has desorbed completely at 400 K. The preferential oxidation of NH3 to N2 at low O-cus coverage was also demonstrated by the HREELS data, in which case the NO formation is negligible. Based on the isotope substitution experiments on the 18O-rich RuO2(110) surface, the interaction of ammonia with O-bridge could be definitively excluded. The Ru-cus atoms were clearly identified as active sites for both ammonia and oxygen adsorption. The concentration of O-cus determines both the reactivity and selectivity. In summary, we can provide the following overall reaction schema for selective oxidation of ammonia on the O-rich RuO2(110) surface:



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The reactivity and selectivity of ammonia oxidation on RuO2(110) were further studied under steady-state conditions. The reaction rate (turnover frequency) was measured as function of temperature and partial pressures of NH3 and oxygen. Fig. 8 shows the kinetic data obtained at 500 K with p(NH3) = 1!10–7 mbar as a function of p(O2). For p(O2) = 0, i.e., in the absence of O-cus, no formation of NO and N2 was detected, thus further corroborating our conclusions that NH3 and O2 have to be adsorbed on Ru-cus to undergo the catalytic reaction. With increasing p(O2) first rN2 rises, passes through a maximum and then decreases again, while rNO has a sigmoidal shape and rises continuously. The selectivity for N2 is larger at relatively low p(O2), whereas that for NO production reaches a maximum value of about 0.8 for p(NH3) = 1.5!10–6 mbar. In addition, a reasonable kinetic modeling based on the proposed reaction mechanism could be obtained (see Fig. 8). Almost 100% selectivity for NO formation is reached at 530 K, much lower than the temperatures applied in the technical Ostwald process with Pt-based catalysts (>1100 K) [78]. This offers interesting prospects for future applications.

3.7 Interaction of ethylene with RuO2(110) The partial oxidation (epoxidation) of ethylene to ethylene oxide using silver as the main catalyst [79] is of great interest in chemical industry. Due to the low reaction probability and the consequent requirement of high C2H4 pressure, a direct investigation of this reaction under UHV conditions is rather difficult. The interaction of ethylene with RuO2(110) surfaces was recently investigated by vibrational spectroscopy [80,81]. Whereas the stoichiometric surface is inactive, on the O-rich surface ethylene oxidation is really observed. The HREELS data revealed that ethylene is adsorbed molecularly on Ru-cus sites of the stoichiometric RuO2(110) surface with a π-bonded configuration [80]. Upon heating to 260 K a transition from π-bonded to σ-bonded ethylene occurs, as evidenced by the shift of the C-C stretching mode from 194 meV (double C=C bond) to 144 meV (single C-C bond). The chemisorbed ethylene was observed to desorb at about 320 K, no evidence for a reaction with O-bridge atoms was detected. In contrast, ethylene adsorbed on the O-rich surface is completely oxidized through interaction with O-cus and O-bridge upon heating to 500 K [81]. In


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Fig. 8. (a) Steady-state rates of N2 and NO formation (expressed as turnover frequencies) for a constant NH3 partial pressure, p(NH3) = 1!10–7 mbar, as a function of O2 partial pressure p(O2) at T = 500 K. (b) The steady-state selectivities for N2 and NO formation resulting from the data of the Fig. 8a. The experimental data (marked by stars) were compared with a fit to the theoretical model with suitable parameters [75].

Fig. 9 we present a set of HREEL spectra obtained following stepwise annealing of the ethylene adlayer on the O-saturated RuO2(110) surface. In order to give a clear assignment of the observed vibrational modes experiments were also carried out for the surface saturated with 18O2 instead of 16O2 and then exposing to C2H4 or C2D4. Compared with the HREEL spectrum at 85 K, the most important changes after heating can be summarized as follows: (1) The ν(C=C) mode at 194 meV disappears, and a new ν(C-C) mode characteristic for C-C single bonds shows up at 144 meV. (2) The characteristic O-cus vibration at 103 meV is strongly reduced in intensity, indicating that ethylene oxidation takes place. (3) Two new losses appear at 211 and 164 meV, which are attributed to the ν(C=O) and ν(C-O) stretching modes, respectively. (4) The vibrations at 55, 95, 109, 128, and 446 meV are indicative of the formation of H2O on Ru-cus sites (H2Ocus), since the fingerprint peak for H2O-bridge at ~230 meV is missing. Based on the above HREELS data a model for the first steps of ethylene oxidation on this surface has been proposed and is sketched in Fig. 10.



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Fig. 9. HREEL spectra recorded at 85 K after preparing an oxygen-rich RuO2(110) surface with 1 L of O2 at 300 K, and exposing 0.6 L of C2H4 at 85 K and heating to the indicated temperatures [81]. Parameter as for Fig. 5.

When the temperature is increased to 360 K, further dehydrogenation of ethylene occurs accompanied by C-C bond rupture and interaction of the CHx fragments with O-cus and O-bridge (see Fig. 9). At 500 K, the dehydrogenation is complete and the CO intermediate reacts with a surface O atom to form CO2, which desorbs immediately accompanied by a release of H2O and H2. The maximum reaction rate is reached for C2H4 adsorbed at Ru-cus with O-cus neighbors on each side. Under no experimental conditions did we observe the formation of ethylene oxide.

4. ZnO surfaces Zinc oxide is an important material with a wide range of technological applications in catalysis, solar cells, as gas sensor and in semiconductor devices [5]. In catalysis, Cu.ZnO is the standard heterogeneous catalyst for the industrial methanol synthesis [82]. In the past years a number of experimental and theoretical studies have been reported on this catalyst system using ZnO single crystals as model supports [5,6,10,83,84]. However, so far it is not fully understood how this catalyst works. Particularly important is the fact that the information concerning


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Fig. 10. Schematic overview of different surface species possible as intermediates upon interaction of ethylene with Ru-cus and O-cus [81]: (a) π-bonded C2H4; (b) σ-bonded C2H4 with one C=O double bond (left-hand side), one C-O single bond (right-hand side), and one HO-cus; (c) σ-bonded C2H4 with one C=O double bond (left-hand side), one C-O single bond (right-hand side), and one H2O-cus. Geometric interatomic distances in the drawing correspond approximately to bond lengths.

molecular vibrations from HREELS is very scarce. For example, the OH-species, which is of utmost importance for the chemical properties of metal oxide surfaces, has not been identified directly on single crystal surfaces by vibrational spectroscopy. The only case where the HREELS method had been applied in previous works was studies on the adsorption of formic acid on ZnO surfaces [85–87]. Here we will briefly review our recent HREELS studies on the chemical ¯ ) surface) properties of clean ZnO surfaces (in particular the polar O-ZnO(0001 as well as their interaction with different adsorbates.



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– Fig. 11. Ball-and-stick model of the ZnO(1010) surface in side view (O: red, Zn: gray). which are formed by threefold coordinated Zn and O atoms. In the bulk, the Zn and O atoms are fourfold coordinated. At the surface one of these nearest neighbour bonds is broken.

4.1 Interaction of hydrogen with ZnO(101¯ 0) Whereas the interaction of hydrogen with ZnO polycrystalline powders has been extensively studied by FTIR [88–94], no vibrational data on H atoms adsorbed on ZnO single crystal surfaces were available. We have first carried out a HREELS study of hydrogen adsorption on the mixed-terminated ZnO ¯ 0) surface [95]. This orientation is the energetically most favorable one; it (101 also exhibits no electrostatic instability like the two polar surfaces of ZnO ¯ 0) surface consists of rows of ZnO dimers (see Fig. 11), [96,97]. The ZnO(101 ¯ 0) surface is dominated by the The HREEL spectrum of the clean ZnO(101 intense primary Fuchs-Kliewer phonon mode at 69 meV and the corresponding overtones [32] (Fig. 12). Exposing the surface to 2000 L atomic hydrogen at 200 K, performed by dissociating H2 on a hot tungsten filament in line-of-sight from the substrate, led to the appearance of two new bands at 455 and 200 meV which are assigned to the OH and ZnH stretching modes, respectively. These ¯ 0) surface containing observations indicate the presence of a 2H(1!1) ZnO(101 two H atoms per unit cell (see Fig. 13). At room temperature the HREEL spectrum revealed only the presence of an OH species on the surface, no ZnH vibration could be detected, thus indicating the formation of a new phase. This hypothesis is confirmed by a structural analysis using He-atom scattering (HAS) and LEED. All together, the experimental results reveal a transition from the low-temperature 2H(1!1) phase to a 1H(1!1) phase with one hydroxyl species per unit cell (Fig. 13). The absence of Zn-H species could be confirmed by post¯ 0) surface to CO at 70 K. It is known that CO exposing the 1H(1!1) ZnO(101 chemisorbs on Zn atoms but not on ZnH species and only physisorbs on OH groups [97–99]. As shown in Fig. 12, two new bands appear at 28 and 269 meV which are characteristic for CO adsorbed on Zn sites, directly confirming that at room temperature the Zn sites are available for CO adsorption. For the 2H(1!1) ¯ 0) surface, where all surface Zn-atoms are hydrogenated, no CO-related ZnO(101 vibrations could be detected after exposure to CO.


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In addition, the HREELS data revealed that the ν(C-O) stretching mode is only slightly red-shifted from 272 meV for the clean surface [100,101] to 269 meV, whereas DFT calculations predicted a much larger red-shift to 246.5 meV [95]. This surprising discrepancy between experiment and theory might indicate that the extra charge added to the surface by hydroxylating the surface O atoms is mainly transferred to bulk defect states rather than to the surface Zn atoms. Most interestingly, a significant broadening of the incoherent elastic peak in ¯ 0) surface (see the HREELS data was detected for the 1H(1!1) ZnO(101 Fig. 12). The full width at half maximum (FWHM) increases from 5.7 meV for the clean surface to about 20 meV at maximum H coverage, whereas at 200 K for the 2H(1!1) structure the FWHM of the elastic peak does not change upon H adsorption. The unexpected broadening of the incoherent elastic peak has been attributed to low-lying electronic excitations corresponding to electron-hole (eh) pair creations. This implies the presence of a metallic surface formed upon H adsorption, since for a semiconductor such low-lying electronic excitations do not exist because to the large band gap (3.2 eV for ZnO). Indeed, the band structure of the 1H(1!1) ZnO(1!1) surface as determined by DFT calculations revealed a partially filled electronic state which renders the surface metallic [95]. For the 2H(1!1) structure with two H atoms per unit cell the new electronic state is completely filled by two electrons, giving rise to a similar band gap as for the clean surface (see Fig. 13) so that no metallization could be detected. In ¯ 0) was also observed in the addition, hydrogen induced metallization on ZnO(101 corresponding STM data [95,102], which exhibited an approximately ohmic I(V) curve, typical for a metallic behavior [83]. ¯ 0) 4.2 Partial dissociative adsorption of H2O on ZnO(101

Studies of the interaction of H2O with oxide surfaces are of fundamental and ¯ 0) was first studied by applied interest [103]. The adsorption of H2O on ZnO(101 Jacobi and coworkers [104,105] using TDS and UPS. They found that H2O molecules chemisorb at the coordinative unsaturated Zn sites, and no indication of dissociation was observed. More recently, a combined experimental (HAS, STM, and LEED) and theoretical study [106] revealed that half of the water ¯ 0) surface self-dissociate resulting in a well molecules on the perfect ZnO(101 defined (2!1) superstructure. Direct evidence for this coexistence of dissociated and intact water molecules within an ordered adlayer was provided by vibrational spectroscopy using HREELS [107]. ¯ 0) substrate to water molecules at room Following exposure of the ZnO(101 temperature three new losses in the HREELS data (see Fig. 14) were observed at 396, 456 and 460 meV which are assigned to different types of O-H stretching modes. The broad loss at 396 meV is characteristic for the ν(O-Hbr) vibration of a water molecule where one H atom takes part in a hydrogen bond [22,108]. After annealing to 380 K water molecules desorb completely as shown by TDS,



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– Fig. 12. (a) HREELS data recorded for the clean ZnO(1010) surface (curve A), after exposure to hydrogen at 200 K (curve B), after H saturation at room temperature (curve C), and after exposing the room temperature hydrogenated surface to 45 L CO at 70 K (curve D). The spectra were taken at 300 K (curves A and C) and 70 K (curves B and D), respectively. The prominent losses at 70, 140 and 209 meV correspond to Fuchs-Kliewer phonons. (b) FWHM of the quasielastic peak in the HREELS spectra as a function of H atom exposure at room temperature, – and (c) as a function of temperature after exposing the ZnO(1010) surface to 2000 L Hydrogen at 200 K [95]. The HREEL spectra were recorded in a 10° off-specular direction with a high primary electron energy of 10 eV in order to suppress the multiphonon losses.

and at the same time the vibrational modes at 396 and 460 meV disappear. The observations allow to also relate the mode at 460 meV to chemisorbed water, it is assigned to the O-H stretch vibration of a non-hydrogen bonded OH group. The mode at 456 meV is still present after annealing, thus strongly indicating that this band has to be attributed to a surface hydroxyl species. All the above mentioned assignments are consistent with the outcome of experiments carried out using deuterated water, D2O [107]. Interestingly, the ν(O-Hbr) mode at 396 meV is substantially red shifted in comparison to that observed for H-bonded water at metal surfaces (about 420 meV) [22,108]. This finding reveals the presence of an unusually strong


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– Fig. 13. Atomic and electronic structures of the ZnO(1010) surfaces with (a) two and (b) one adsorbed H atom per unit cell as obtained from DFT calculations. The energy of the highest occupied level is set to zero [95].

– Fig. 14. HREEL spectra recorded for the clean ZnO(1010) surface and after exposure to various amounts of water at room temperature. The spectra were obtained at 300 K in specular geometry with an incidence angle of 55° with respect to the surface normal and with a primary energy of 10 eV. The spectrum shown in the inset was recorded in 5° off-specular direction [107].

¯ 0) to both, neighboring H O hydrogen bonding of H2O adsorbed on ZnO(101 2 molecules and the surface O atoms. The HREELS data thus fully support the mechanism suggested by DFT calculations [106]: Isolated H2O molecules remain undissociated, while placing a second water molecule at a neighboring site is



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– Fig. 15. HREELS data recorded after adsorption of CO2 on ZnO(1010) at 95 K [42]. Curve a is the raw spectrum which is dominated by the intense Fuchs-Kliewer phonons; the Fourierdeconvoluted spectra are shown in curves b–d. In curve d the phonon at 1113 cm–1 is not completely removed by the Fourier-deconvolution. The spectra were recorded at 95 K in specular direction with an incidence angle of 55° and with a primary energy of 10 eV.

already sufficient to trigger the dissociation due to the strong hydrogen bonding interaction between the two water molecules. Finally, it should be noted that a combined STM and DFT theory study [109,110] demonstrated that the (2!1) configuration coexists with a minority (1!1) structure containing intact H2O molecules.

4.3 Interaction of CO2 with ZnO(101¯ 0) CO2 activation on ZnO is an interesting topic since for methanol synthesis from syngas (CO.CO2.H2) using Cu.ZnO catalysts CO2 is the carbon source for the product molecule, methanol [82], whereas in the catalytic conversion over ZnO (without Cu) CO is the carbon source [111]. The interaction of CO2 with ZnO ¯ 0) has been first studied using NEXAFS [112]. In this study, it was proposed (101 that CO2 adsorption leads to the formation of a carbonate species with a bidentate configuration. More recently, we have reinvestigated this system by employing HREELS together with other experimental methods (TDS, HAS, LEED and XPS) and accurate theoretical calculations [42]. ¯ 0) surface to The HREELS data recorded after exposing the clean ZnO(101 CO2 at 95 K are presented in Fig. 15. The intense Fuchs-Kliewer phonons could almost completely be removed by Fourier-deconvolution of the raw data and the HREEL spectra exhibit distinct, CO2-induced bands at 104, 123, 166 and


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– Fig. 16. (a), (b) Side view of the atomic structure of an isolated carbonate ion on ZnO(1010) upon CO2 adsorption; (c) relative thermodynamic stability of the half and full monolayer CO2 coverage [42].

200 meV. These frequencies are characteristic for carbonate species and attributed to the π(CO3) mode (104 meV) and three C-O stretching modes, respectively. The DFT slab calculations revealed [42] that the most stable CO2 binding geometry is not that of a bidentate carbonate as proposed previously [112], but corresponds to an unusual tridentate configuration in which the C atom binds to a surface O atom, and both O atoms of the CO2 molecule form almost equivalent bonds to the neighboring Zn atoms (see Fig. 16). ¯ 0) was determined by HAS The phase diagram of CO2 adlayers on ZnO(101 and LEED [42]. Two well-ordered carbonate phases were observed: a close packed (1!1) structure at saturation and an open (2!1) phase at lower coverage and higher temperatures. The corresponding TDS data show also two main desorption peaks of CO2 at 325 K (β state) and 200 K (α state), where the desorption of the β state is accompanied by the transformation of the LEED pattern ¯ 0) was from a (1!1) to a (2!1) phase. The phase diagram of CO2 on ZnO(101 further demonstrated by the theoretical calculations [42] which found the presence of two stable carbonate phases: a full (1!1) monolayer at saturation with a binding energy of 0.47 eV and a half monolayer with (2!1) structure and a binding energy of 0.70 eV, in excellent agreement with the experimental observations. ¯ 0) 4.4 Coadsorption of CO and CO2 on ZnO(101 ¯ 0) leads to the formation of As mentioned above, CO2 adsorption on ZnO(101 two ordered carbonate structures [42]. In addition to a densely packed (1!1)



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– Fig. 17. HREELS data recorded after adsorption of 2 L CO at 95 K on a, b) clean ZnO(1010) – surface; c) CO2-modified ZnO( 1010) surface prepared by exposing first to 2 L CO2 at 90 K and then heating to 220 K. Curve a is the raw spectrum which is dominated by the intense Fuchs-Kliewer phonons; the Fourier-deconvoluted spectra are shown in curves b and c. The spectra were recorded at 95 K in specular direction with an incidence angle of 55° and with a primary energy of 10 eV [101].

phase an open (2!1) phase was observed above 200 K, where only every other surface Zn-ion is coordinated to the O-atom of a surface carbonate. It is very interesting to further investigate whether reactions can be induced between the carbonate species formed by CO2 adsorption and the molecule adsorbed on the free Zn sites of the open (2!1) adlayer. We have first tested the interaction between the carbonate and other adsorbates for the case of CO [101]. The TDS data revealed that CO is only weakly bonded to the Zn2+ ions of ¯ 0) surface, the binding energy amounts to 30.5 kJ.mol. When the clean ZnO(101 the surface was modified by exposure to CO2 at 100 K, the interaction of CO ¯ 0) becomes much stronger. HREELS data recorded for this coadwith ZnO(101 sorption system provided a deeper understanding of this unexpected effect. On ¯ 0) surface the ν(C-O) mode for CO adsorbed on the Zn site the clean ZnO(101 ¯ 0) amounts to 272 meV (Fig. 17). After exposure of the CO2 pretreated ZnO(101 surface to 2 L CO at 100 K, new vibrations at 125, 165.5 and 197 meV show up which are attributed to the presence of tridentate carbonate species (see discussion above) [42]. In addition to these modes, the loss observed at 273 meV is indicative of the post-adsorbed CO on the free Zn sites within the (2!1) CO2 ¯ 0). adlayer, confirming the coexistence of CO and CO2 on ZnO(101


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Interestingly, the band of post-adsorbed CO is slightly blue shifted with re¯ 0) surface. This observation strongly suggests gard to that on the clean ZnO(101 that the Lewis acidity at the surface has changed upon CO2 adsorption as further demonstrated by the DFT calculations. Enhanced Lewis acidity of the Zn cations is expected to increase the 5σ donation from the CO molecule to the Zn surface ion. Given that the 5σ orbital is weakly antibonding, enhanced electron donation strengthens the C-O bond and leads to a blue-shift of the ν(C-O) band. Simultaneously, this effect results in a stronger interaction of CO with Zn cations, causing an increase in the CO adsorption energy. The calculated results are in excellent agreement with the experimental data. Importantly, the same enhancement of CO binding energies by CO2 co-adsorption was also observed for polycrystalline ZnO powders studied by adsorption microcalorimetry [101]. Because the polycrystalline ZnO nanoparticles are ¯ 0) surface as determined by a combined transmisin fact dominated by the (101 sion electron microscopy (TEM) and x-ray diffraction (XRD) study [113], it was concluded that the increase of the interaction strength of CO with ZnO powders results from the formation of tridentate carbonate species on the mixed-termi¯ 0) surface, which increases the Lewis acidity of Zn2+ cations. nated (101 ¯ ) surface 4.5 Structure of the polar O-ZnO(0001 ¯ ) surface is shown in Fig. 18a. This polar The structure of the ideal O-ZnO(0001 surface exhibits an electrostatic instability due to the uncompensated surface charges. In general, this instability could be removed through reconstruction of the clean surface or adsorption of charged adsorbates, and most polar oxide surfaces undergo a major structural rearrangement [7]. Surprisingly, numerous previous studies [114–116] on the polar O-ZnO surface have failed to observe any reconstruction. Recently, a combined HAS and LEED study [117] revealed ¯ ) surface, prepared under strict UHV-conditions (bethat the clean O-ZnO(0001 –10 mbar), exhibits a clear (1!3) reconstruction. A corresponding low 1!10 structural model has been proposed, in which every third oxygen atom in the [10 ¯ 10] direction is missing (see Fig. 18b). ¯ ) surface was The presence of such a (1!3) reconstruction on the O-ZnO(0001 clearly confirmed by HREELS [118]. First, a clean, hydrogen-free O-ZnO surface was prepared, which can be identified by the absence of any OH band in vibrational spectroscopy (Fig. 19). Furthermore, no desorption of hydrogen or H2O was observed in TDS for a H-free surface. For such a clean O-ZnO surface a (1!3) periodicity was seen in the LEED data, demonstrating that this polar surface is reconstructed forming a (1!3) structure, and a clean, stable and H-free (1!1) surface does not exist. When this clean surface was exposed to water at room temperature, the LEED data revealed a transformation from a (1!3) to a (1!1) structure. Simultaneously, one OH band at 449 meV shows up in Fig. 19, which is attributed to the OH species formed via dissociative adsorption of water molecules on the Ovacancies [119]. A stable hydroxylated (1!1) O-ZnO surface (see Fig. 18c) could



Y. Wang

– Fig. 18. (a) Structural models of differently modified oxygen-terminated polar ZnO(0001) surfaces [5]. (a) the ideal surface with a (1!1) periodicity; (b) the (1!3) reconstructed surface. This model consists of an ordered array of O-vacanies which renders the surface a very high reactivity. (c) the H(1!1) O-ZnO surface with all surface oxygen atoms forming hydroxyl (OH) species. This surface is experimentally found to be very stable.

be also prepared by exposure to atomic hydrogen, as evidenced in the HREELS data (Fig. 19). It is rather likely that most of the previous measurements on O-ZnO( ¯ ) were actually performed for H(1!1) O-ZnO(0001 ¯ ), because freshly pre0001 ¯ ) surfaces quickly undergo hydroxylation even under pared clean O-ZnO(0001 UHV conditions (see discussion in Ref. [5]). Recently, a detailed theoretical study on the relative stability of various structural models of the O-ZnO surface has been carried out by Meyer [120]. It revealed that a clean and H-free (1!1) O-ZnO surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, a theoretical confirmation of the stability of a (1!3) structure with an ordered array of O-vacancy is still lacking. Very recently, HREELS data revealed that exposing the clean O-ZnO surface to H atoms leads to a significant broadening of the incoherent elastic peak, indicating again that a surface metallization can be induced by hydrogen adsorp¯ 0) surface (see discussion above) [95]. tion, similar to the ZnO(101

5. TiO2(110) In recent years the rutile TiO2(110) surface has become the prototypic metal oxide model system for surface science research. A large number of experimental


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– Fig. 19. HREELS data of differently modified oxygen-terminated polar ZnO(0001) surfaces. (a) the clean, hydrogen-free (1!3) reconstructed surface;. (b) the H(1!1) O-ZnO surface prepared by exposure of 2 L H2O at 300 K. The formation of OH species is characterized by the mode at 449 meV; (C) the H(1!1) O-ZnO surface prepared by exposure of 1000 L atomic hydrogen at 300 K. Note that the significant broadening of the quasielastic peak in curve C is attributed to the hydrogen induced metallization of the surface [118]. The spectra were recorded at 300 K in specular direction with an incidence angle of 55° and with a primary energy of 10 eV.

and theoretical studies on TiO2(110) have been reported [13], including the HREELS investigations of water, formic acid and methanol adsorbed on this surface [121–125]. However, although the presence of H adatoms has pronounced effects on the chemical activity of TiO2(110) surfaces (e.g., for photoactivation of TiO2 [3]), there is very little information available about the precise nature of hydroxyl species on TiO2 substrates. In particular, it has been well documented that the oxygen vacancies formed by thermal annealing or sputtering play a crucial role in the surface chemistry of TiO2(110) and have been proposed to be active sites for many chemical processes occurring on this surface [13,126]. However, vibrational spectroscopic studies on these defect sites are still very scarce. Here, we focus on our recent combined HREELS and TDS studies on TiO2(110) in these two important aspects.

5.1 Interaction of hydrogen with TiO2(110) The (110) surface is the most thermodynamically stable termination of rutile TiO2 and has been studied in great detail using in particular STM [13]. The structure of this surface is shown in Fig. 20, the characteristic features are alternating rows of five-fold coordinated Ti atoms (Ti5f) and two-fold coordinated O



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Fig. 20. Ball and stick model of the bulk-terminated TiO2(110) surface in perspective view.

atoms (O-bridge) which are both coordinatively unsaturated with one dangling bond perpendicular to the surface. Upon exposure to hydrogen, one would expect that the two dangling bonds are saturated by H adsorption giving rise to both OH and TiH species at the surface [127,128]. To understand the nature of hydrogen adsorption on TiO2(110) a detailed HREELS study has been carried out [129]. As shown in Fig. 21, the raw spectrum of the clean surface is dominated by the intense primary Fuchs-Kliewer phonon losses at 45, 55 and 94 meV as well as their combination and multiple excitations. Exposure of TiO2(110) to atomic hydrogen were carried out by backfilling the chamber with hydrogen and by dissociating hydrogen on a hot tungsten filament (~2000 °C) which was placed in line-of-sight from the substrate surface (distance ~10 cm). After exposing the surface to 2000 L atomic hydrogen at room temperature a new band at 456 meV was observed, which is assigned to the stretching mode of the OH species formed on the O-bridge sites (OHbridge), The presence of OH-bridge species was also clearly confirmed by STM studies, which showed the formation of an ordered OH adlayers with a (1!1) periodicity, in good agreement with the HAS data [127]. However, this (1!1) structure exhibits a significant amount of H vacancies, and a maximum OHbridge coverage of 0.7 ML could not be exceeded. After exposing the TiO2(110) surface to atomic hydrogen at room temperature no Ti-H stretching mode could be observed in the HREELS data (Fig. 21). The absence of any difference between the Ti-atoms on the clean and the Hatom exposed surface was demonstrated by a CO adsorption experiment: When the hydroxylated TiO2(110) surface was subsequently exposed to CO at 95 K, CO desorption at 129 K was visible in TDS, the same temperature as seen for the clean surface. This observation revealed that CO is adsorbed on Ti5f sites [130], thus confirming that the latter are not occupied by hydrogen and are available for CO adsorption. Most interestingly, the HREELS data revealed that the ν(O-H) mode disappears upon heating to high temperatures. However, in TDS almost no H2O (or H2) molecules were found to desorb from the surface, as commonly observed for hydrogen adsorption on metals and on oxide surfaces. This unexpected finding indicates that upon heating the hydrogen atoms adsorbed on O-bridge un-


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Fig. 21. HREELS spectra recorded for the clean TiO2(110) surface (curves A and B) and after exposure to 2000 L atomic hydrogen at room temperature and subsequent annealing to the indicated temperatures (curves C–E). Curve A displays the raw spectrum, and the Fourierdeconvoluted spectra are shown in curves B–E [129]. Parameter as for Fig. 19.

dergo, instead of desorption, diffusion into the bulk. This explanation was supported by ab-initio calculations [129]: The activation energy for H-atoms migrating into the bulk, 1.11 eV, is significantly smaller than that for recombinative desorption of H2, 2.64 eV). These findings have important consequences for chemical processes involving H atoms occurring at TiO2 surfaces.

5.2 Adsorption of formaldehyde on the perfect TiO2(110) surface The chemical reactivity of the TiO2(110) surface was further examined using formaldehyde as a probe molecule [131]. In previous works based on TDS [132] it was proposed that formaldehyde reacts with surface O vacancies giving ethylene via a reductive coupling. This kind of carbon-carbon bond formation is one of the most fundamental and essential reaction in nature and has also been reported on O-defected TiO2(001) single crystal surfaces [133–137]. The production of ethylene from formaldehyde, although the simplest of all reductive coupling reactions, is complex and the reaction mechanism remains to be elucidated. Our TDS data revealed that on the perfect rutile TiO2(110) surface formaldehyde desorbs molecularly upon heating and three desorption states at 290, 268 and 128 K were observed. The HREELS data provided detailed information about the different adsorbate species present on the substrate. After exposing this surface to 5 L CH2O at 100 K new losses at 137, 143, 156, 177, 210 and 363 meV were observed. Physisorbed CH2O can be first identified by the typical



Y. Wang

Fig. 22. Structural model of the bulk-terminated TiO2(110) surface in perspective view together with surface oxygen vacancies and reaction intermediate, diolate species formed after formaldehyde activation on two adjacent bridge O-vacancies [131].

ν(C=O) mode at 210 meV together with the ω(CH2) mode at 143 meV. These modes vanished after heating the sample above 200 K, revealing that the TDSpeak at 128 K is attributed to the desorption of physisorbed CH2O. The chemisorption of formaldehyde on a solid substrate can lead to the formation of monomer or paraformaldehyde species. The latter has been often reported to occur on metal [138] or on metal oxide surfaces [139,140]. A formaldehyde monomer is typically bonded to the oxide surface in an oxygen end-on η1(O) configuration [139,140] giving a typical ν(C=O) mode at around 205 meV due to the slightly reduced C=O bond. A mode with this frequency was not observed in the HREELS data and we can therefore rule out the presence of such a CH2O monomer on Ti2O(110). The losses at 137 and 177 meV are characteristic for the formation of paraformaldehyde and are attributed to the ν(C-O) and δ(CH2) modes, respectively. Based on TDS experiments performed after CO coadsorption it was concluded that the polymerization of formaldehyde to yield paraformaldehyde occurs on Ti5f sites [131]. It was proposed that the terminal oxygen atoms of the oligomer chain are singly bonded to the Ti5f atoms, while many of the interior oxygen could be bonded to the surface via electrostatic interaction due to the mismatch caused by the large Ti-Ti separation in TiO2. The different desorption energies associated with different lengths of adsorbed paraformaldehyde chains might be behind the difference of TDS phases (268 and 290 K) and the broadening of the TDS peak.

5.3 Interaction of formaldehyde with the defective TiO2(110) surface The structure model of a reduced (defective) TiO2(110) surface is shown in Fig. 22. There exist three different methods to create such a substrate: thermal annealing, Ar+ sputtering and electron irradiation [13]. The O vacancies serve as donors of electrons and have a profound influence on the chemical and electrical behavior of metal oxides. Despite extensive studies, the interaction of these defective surfaces with adsorbed species is not yet completely understood.


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A reduced TiO2(110) surface was first prepared by annealing the sample to 900 K to produce bridge O vacancies. Upon CH2O adsorption at 100 K, in addition to the molecular desorption of CH2O at 285 K a weak desorption state at 620 K was detected for masses 27 and 26, indicating the formation of ethylene by reductive coupling of two formaldehyde molecules on O vacancy sites. This finding is in good agreement with the previous TDS results [132]. By comparison of the integrated peak areas the ratio of ethylene to formaldehyde was found equal to 3.7%. Assuming full saturation of formaldehyde and that the formation of ethylene deposits two oxygen atoms, the O vacancy concentration was estimated to be 7.4%, in very good agreement with the value reported in the literature [121,141]. In addition to thermal annealing we have also used Ar+ ion sputtering to create a defective TiO2(110) surface. It was found that the amount of ethylene desorption increased significantly after surface modification by Ar+ sputtering, reaching a maximum value of 19%. This finding indicates that Ar+ ion sputtering is a more effective method to create O vacancies giving a maximum density of 0.38 monolayer. The reaction of formaldehyde with O vacancies was further demonstrated by the coadsorption experiments with H2O: According to previous work [127], exposing the defective TiO2(110) surface to water at room temperature leads to an inhibition of all the O vacancies by H2O dissociative adsorption, yielding two hydroxyl groups per O vacancy. When this modified surface was subsequently exposed to formaldehyde, no C2H4 desorption was observed in the TDS data as expected, confirming that the reduced sites, surface oxygen vacancies, are directly involved in the formation of ethylene. A more detailed insight into the mechanism of formaldehyde activation on O vacancies was obtained by HREELS. The defective TiO2(110) surface, prepared by Ar+ sputtering, was first exposed to 5 L C2H4 at 300 K and then heated to 400 K to remove all the paraformaldehyde species adsorbed at Ti5f sites (see Section 5.2). The corresponding HREEL spectrum is shown in Fig. 23, where it is compared to the spectrum recorded for the perfect surface. At 400 K formaldehyde adsorbed at Ti5f sites on the perfect surface is not stable and desorbs completely from the surface, as also evidenced from the corresponding HREEL spectrum. However, for the defective surface five vibrational losses were clearly observed which must originate from the adsorbate species bonded on O vacancies. The HREELS data strongly indicate the presence of a diolate type species (-OCH2CH2O-, see Fig. 22), and the losses observed at 138, 143, 174, 180 and 363 meV are assigned to the νas(C-O), νs(C-O), ν(C-C)+ω(CH2), δ(CH2) and ν(CH2) modes, respectively. This conclusion was fully supported by the DFT calculations [131]. The diolate species can be formed via activation of two formaldehyde molecules adsorbed at adjacent O vacancies (Fig. 22). Upon heating this reaction intermediate undergoes deoxygenation leading to the formation of ethylene as final product. Two O atoms of the diolate species are left on the surface to fill up two O vacancies.



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Fig. 23. Fourier deconvoluted HREELS data taken after exposing the (a) perfect and (b) sputtered TiO2(110) surfaces to 5 L CH2O at room temperature and subsequent heating to 400 K. Both spectra were recorded at 300 K. Parameter as for Fig. 19 [131].

6. Conclusions and outlook In summary, it has been demonstrated that HREELS is a powerful method to study in detail the adsorption and reaction of molecules on metal oxide surfaces. Based on HREELS data very useful information about vibrational, geometric and electronic properties of surfaces and adsorbates can be obtained. Furthermore, this technique enables to unambiguously identify the reaction intermediates and products, so that the reaction mechanisms taking place at oxide surfaces can be elucidated. In addition to the studies on perfect oxide surfaces, it is possible to use HREELS to characterize the activation of adsorbed species on defect sites (in particular for O vacancies), which play a crucial role in the surface chemistry of oxides. Despite the experimental difficulties encountered when applying this method to defective oxide surfaces, the results presented here demonstrate that this method can be successfully applied to identify the reaction intermediate formed on oxygen vacancies of the TiO2(110) surface. In case of zinc ¯ ) surface has been demonstrated to be oxide the oxygen terminated polar (0001 the most active surface for methanol synthesis from syngas on ZnO powders ¯ ) surface is (1!3) reconstructed and exposes 33% [111]. The clean O-ZnO(0001 O vacancies. The importance of these defect sites for the high chemical activity of this surface should be explored in future work by vibrational spectroscopy. Finally, the study of small metal particles deposited on oxide surfaces has recently received considerable attention due to their importance in catalysis [10,83,84,142]. It is expected that HREELS will provide more detailed information about the growth, the chemical reactivity and the electronic structure (e.g. plasmon excitations) of metal deposits on oxides.


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Acknowledgement The work presented in this review was supported in part by the German Science Foundation (DFG) within the collaborative research center SFB 558 “MetalSubstrate Interaction in Heterogeneous Catalysis”. The author benefited from helpful discussions with Prof. Ch. Wöll, Prof. M. Muhler, Prof. H. Idriss and many colleagues at Ruhr-University Bochum. The research on RuO2 was performed at the Fritz Haber Institute of the Max Planck Society in Berlin. The author would like in particular to thank Prof. K. Jacobi and Prof. G. Ertl for many fruitful discussions over the years. I thank also Prof. Ch. Wöll and Prof. M. Muhler for a careful reading of the manuscript.

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