Activation of Carbon Dioxide on ZnO Nanoparticles Studied by

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J. Phys. Chem. C 2011, 115, 908–914

Activation of Carbon Dioxide on ZnO Nanoparticles Studied by Vibrational Spectroscopy† Heshmat Noei,‡ Christof Wo¨ll,| Martin Muhler,‡ and Yuemin Wang*,‡,§ Laboratory of Industrial Chemistry and Department of Physical Chemistry I, Ruhr-UniVersity Bochum, 44780 Bochum, Germany, and Institute of Functional Interfaces, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany ReceiVed: March 26, 2010; ReVised Manuscript ReceiVed: May 12, 2010

The activation of CO2 on clean and hydroxylated ZnO nanoparticles has been studied by ultrahigh vacuum FTIR spectroscopy (UHV-FTIRS). Exposing the clean ZnO powder samples to CO2 at 300 K leads to the formation of a number of carbonate-related bands. A detailed assignment of these bands was carried out using isotope-substitution experiments with C18O2. On the basis of vibrational and thermal stability data for ZnO single crystal surfaces, a consistent description of the interaction of CO2 with ZnO powder particles can be provided: (1) on the mixed-terminated ZnO(101j0) facets, a tridentate carbonate is formed; (2) on the polar, O-terminated (0001j) facets, a bidentate carbonate species is formed via CO2 activation at oxygen vacancy sites; and (3) additional monodentate or polydentate carbonate species are formed at defect sites such as steps, edges, kinks, and vacancies. The formation of carbonate-related vibrational bands is observed at an exposure temperature as low as 100 K, thus demonstrating the high activity of ZnO nanoparticles with regard to CO2 activation. 1. Introduction The activation of CO2 is a topic of fundamental and applied interest in catalysis, electrochemistry, and environmental chemistry.1–6 Because zinc oxide is used as a catalyst for methanol synthesis from syngas (mixture of H2, CO, and CO2),7,8 it is an obvious candidate for activating the rather inert carbon dioxide molecule. In addition to the interesting chemical properties of ZnO, for example, in connection with other catalytic processes such as the water gas shift reaction,9 this transition-metal oxide presently receives a huge amount of attention because of its unique electrical and optical properties.10–16 In the past, a number of investigations have been reported on CO2 adsorption on polycrystalline ZnO powder surfaces. In particular, adsorption energies were determined using temperature-programmed desorption (TPD),17 adsorption isotherm measurements,18 and microcalorimetry.19,20 So far, only very few infrared spectroscopy studies have been reported for this system.21 Therefore, the identification of the precise nature of CO2-related species present on polycrystalline ZnO nanoparticles has remained a challenge, and for most of the bands present in the complex IR spectra recorded for ZnO powders exposed to CO2, a reliable assignment is missing. Within the so-called “surface science” approach to understanding heterogeneous catalysis, experimental and theoretical studies on well-defined single crystal substrates provide the basis for unraveling the microscopic mechanisms of chemical processes taking place at surfaces of nanoparticles.22 Here this successful approach is applied to the activation of CO2, the most important greenhouse gas, on polycrystalline ZnO nanoparticles. Vibrational spectroscopy is one of the most informative methods when it comes to a thorough characterization of adsorbates on solid surfaces. Recently, we have systematically investigated †

Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. E-mail: [email protected]. ‡ Laboratory of Industrial Chemistry, Ruhr-University. | Karlsruhe Institute of Technology. § Physical Chemistry I, Ruhr-University Bochum.

the adsorption and subsequent reactions of various molecules on metal oxide single crystal surfaces by employing highresolution electron energy loss spectroscopy (HREELS).23–31 By combining the vibrational data with other experimental results as well as with theoretical results, we have obtained detailed information about the interaction of CO2 with differently oriented ZnO surfaces.25,26,31 The corresponding results will be briefly summarized in Section 3.1. To be able to use this information for an understanding of the chemical phenomena occurring at surfaces of ZnO powder particles, we have designed a novel ultrahigh vacuum infrared spectroscopy (UHV-FTIRS) apparatus, which not only allows us to record high-quality IR data at grazing incidence on well-defined oxide single crystal surfaces but also makes it possible to carry out IR experiments in transmission for oxide powder particles.32 The good performance of this UHV-FTIRS apparatus has been demonstrated by investigating the interaction of water with clean, adsorbatefree ZnO powder particles.33 According to the HREELS and thermal stability data obtained for single crystal surfaces, the different bands in the complex OH regime of IR powder data could be assigned in a one-by-one fashion to hydroxyl groups bound to different adsorption sites on the nanoparticle surfaces. In this article, we report on an investigation of the activation of CO2 on clean and hydroxylated ZnO nanoparticles at different temperatures by means of UHV-FTIRS. The interaction of CO2 with ZnO is of complex nature, as evidenced by broad adsorbateinduced IR bands indicating the presence of many different surface species. We will provide a complete assignment of the IR bands, which allows us to gain rather deep insight into the activation of CO2 on ZnO and to derive a consistent picture describing the vibrational, energetical, and structural properties of carbonate species on surfaces of ZnO powders. 2. Experimental Section The sample used in this study was polycrystalline ZnO (NanoTek, provided by Nanophase Technologies; purity: >99%). It was prepared by physical vapor synthesis based on the

10.1021/jp102751t  2011 American Chemical Society Published on Web 05/28/2010

Activation of Carbon Dioxide on ZnO Nanoparticles oxidation of vaporized metallic Zn, followed by condensation of ZnO. After heating at 723 K for 4 h, its specific surface area determined by N2 physisorption applying the BET equation amounted to 14 m2/g. UHV-FTIRS experiments on the ZnO nanoparticles were performed in an UHV apparatus, which combines a state-of-the-art vacuum IR spectrometer (Bruker, VERTEX 80v) with a novel UHV system (PREVAC). (For details, please see ref 32.) The ZnO powder samples were first pressed into a stainless steel grid and then mounted on a sample holder, which was specially designed for the FTIR transmission measurements under UHV conditions. The base pressure in the measurement chamber amounted to 2 × 10-10 mbar. The optical path inside the IR spectrometer and the space between the spectrometer and the UHV chamber were also evacuated to avoid atmospheric moisture adsorption, thus resulting in a high sensitivity and long-term stability. The ZnO nanoparticles were cleaned in the UHV chamber by heating to 850 K to remove all adsorbed species such as carbon-containing contaminants and hydroxyl groups. Prior to each exposure, a spectrum of the clean ZnO nanoparticles was recorded to be used as a background reference. We carried out the exposure of the sample to CO2 and H2O by backfilling the measurement chamber through a leak valve. An additional purification of H2O was achieved by repeated cycles of freezing, pumping, and thawing. The exposures are given in units of Langmuir (L) (1 L ) 1.33 × 106 mbar × s). All UHV-FTIR spectra were collected with 1024 scans at a resolution of 4 cm-1 in transmission mode. 3. Results and Discussion 3.1. Background: Interaction of CO2 with Different ZnO Single Crystal Surfaces. To facilitate the assignment of the CO2-induced vibrational bands, we first briefly summarize the interaction of CO2 with different ZnO single crystal surfaces as obtained by investigations with HREELS, He-atom scattering (HAS), TPD, low-energy electron diffraction (LEED), X-ray photoelectronspectroscopy(XPS),andtheoreticalcalculations.25,26,31 Whereas the chemisorption of CO2 on metal surfaces generally leads to the formation of bent CO2δ- anion species, the activation of CO2 on oxides usually occurs via the formation of surfaces carbonates.1 The mixed-terminated ZnO(101j0) surface is the energetically most favorable surface and exposes Zn-O ion pairs, with both ions being one-fold coordinatively unsaturated. Previous work has reported a reactive adsorption of CO2 on the stoichiometric ZnO(101j0) surface leading to the formation of an unusual tridentate species at temperature as low as 95 K.25 On the basis of theoretical calculations, a detailed structural model was derived, where the carbon atom interacts with a surface oxygen atom and the two oxygen atoms of the CO2 molecule are almost equivalently bound to two different Zn surface atoms. This tridentate carbonate species can form two different stable structures: a saturated, close packed (1 × 1) phase with θ ) 1 monolayer (ML) and a binding energy of 0.47 eV, as well as an open (1 × 2) structure with a coverage of θ ) 1/2 ML and a substantially higher binding energy of 0.70 eV. The latter low-coverage carbonate phase is characterized by vibrations at 839, 994, 1329, and 1609 cm-1, which are assigned to an out-of-plane deformation mode (π(CO3), 839 cm-1) and three C-O stretching modes (ν(C-O), νs(OCO), and νas(OCO)), respectively. Increasing the carbonate coverage leads to a slight blue shift of the νs(OCO) and νas(OCO) modes to higher frequencies. They are observed at 1340 and 1617 cm-1 for the saturated carbonate (1 × 1) structure, whereas the π(CO3) and ν(C-O) modes are not sensitive to the carbonate coverage, and the corresponding frequencies remain unchanged. The presence of two different tridentate carbonate phases is also

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Figure 1. UHV-FTIR spectra obtained after exposing the clean ZnO nanoparticles to various amounts of CO2 at room temperature in an UHV chamber. Prior to CO2 adsorption, the clean ZnO sample was prepared via heating in the UHV chamber to 850 K to remove all adsorbed species: (A) clean surface, (B) 5 × 10-7 mbar, (C) 1 × 10-6 mbar, (D) 5 × 10-6 mbar, (E) 1 × 10-5 mbar, (F) 5 × 10-5 mbar, and (G) 1 × 10-4 mbar CO.

evidenced by the TPD results showing two dominant peaks centered at 200 K (β state) and 325 K (R state) for CO2 desorption. Heating to 400 K removes all adsorbate species from the surface. In addition to the chemisorption of CO2-yielding carbonate species, physisorbed linear CO2 species are observed after exposing ZnO(101j0) to CO2 at lower temperature, as evidenced by the characteristic νas(OCO) mode at 2355 cm-1 and a desorption maximum in TPD at 125 K.25 In contrast with the nonpolar ZnO(101j0) surface, the polar oxygen-terminated ZnO(0001j) (O-ZnO) surface is electrostatically unstable because of uncompensated surface charges. It has been proposed that the clean adsorbate-free O-ZnO surface exhibits a (1 × 3) structure, where one-third of the surface oxygen atoms are missing.10 This (1 × 3) O-ZnO(0001j) surface is very active with regards to CO2 activation, as directly evidenced by HREELS results.31 After CO2 adsorption at 120 K, two intense vibrational bands were detected at 1293 and 1615 cm-1, which originate from bidentate carbonate species formed through CO2 activation on O vacancy sites, and are assigned to the ν(CdO) and νas(OCO) modes, respectively. The thermal dissociation of these carbonate species is characterized by a very broad CO2 desorption maximum centered at ∼350 K. An additional weak and broad desorption feature centered at ∼490 K was present on the high-temperature side. No desorption signal was detected for temperatures >600 K. In the lowtemperature regime, a desorption peak of CO2 was observed at 150 K. This peak remained nearly unchanged when the (1 × 3) O-ZnO surface was first modified by water adsorption to block all oxygen vacancies. Accordingly, this TPD signal was assigned to a weakly bonded carbonate-like species via interactions of the carbon atom with a single surface oxygen ion. Finally, the HREELS and TPD data obtained after CO2 adsorption at 120 K did not show any indications of a physisorbed CO2 adsorbate species on O-ZnO surfaces.

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Figure 2. UHV-FTIR spectra obtained after (A) exposing the clean ZnO nanoparticles to 1 × 10-4 mbar CO2 at room temperature and (B) storing in UHV at room temperature for 24 h.

3.2. Interaction of CO2 with Clean ZnO Nanoparticles. Figure 1 presents UHV-FTIR spectra recorded after CO2 adsorption on the clean ZnO nanoparticles at 300 K. Prior to the exposure to CO2, the sample was heated to 850 K and then cooled to 300 K in a UHV chamber, in which the readsorption of CO2 or other species from the residual gas in the course of experiments is negligible. As a result, no IR bands are observed in the corresponding spectrum (Figure 1A), revealing the presence of the clean, adsorbate-free ZnO particle surfaces. Exposing the clean ZnO samples to CO2 at 300 K leads to the appearance of several IR bands in the frequency range 1800-800 cm-1. The intensities of these bands increase with increasing CO2 pressure. When a pressure of 1 × 10-4 mbar of CO2 is maintained, the IR spectrum shows four strong bands located at 1592, 1334, 1000, and 850 cm-1 (Figure 1G). In addition, weak IR bands (shoulders) are detected at 1643, 1543, 1292, 973, and 871 cm-1. After the CO2-treated sample in the UHV chamber (10-10 mbar) is stored at 300 K for 24 h, the following changes are found in the IR spectrum (Figure 2B): (1) The main absorption bands at 1592, 1334, 1000, and 850 cm-1 strongly decrease in intensity. In addition, the bands at 1592 and 1334 cm-1 shift slightly to lower frequencies at 1581 and 1329 cm-1. (2) The intensities of the IR bands at 1543, 1292, 974, and 871 cm-1 remain unchanged. (3) The weak feature at 1643 cm-1 is shifted to 1621 cm-1, whereas an additional shoulder is resolved at 1313 cm-1. To get more insight into the interaction of CO2 with the ZnO nanoparticles, IR spectra were recorded after the sample was heated to higher temperatures. The corresponding data are displayed in Figure 3. We first focus on the frequency regime

Noei et al.

Figure 3. UHV-FTIR spectra obtained after exposing the clean ZnO nanoparticles to 1 × 10-4 mbar CO2 at room temperature in a UHV chamber and then heating to: (A) 300, (B) 325, (C) 350, (D) 375, (E) 400, (F) 425, (G) 450, (H) 475, (I) 500, (J) 525, (K) 550, and (L) 600 K.

between 1100 and 2000 cm-1 containing the fingerprint IR bands for chemisorbed species resulting from CO2 adsorption, in particular, carbonates. With increasing temperatures, the main bands at 1581 and 1329 cm-1 decrease in intensity until they disappear completely at 450 K; simultaneously, the relatively weak bands (shoulders) at 1621 and 1292 cm-1 decrease and are removed completely at 500 K. The IR features at 1543 and 1313 cm-1 first remain unchanged upon heating but disappear after reaching temperatures >600 K. Furthermore, with heating to higher temperatures, the 1543 cm-1 band shifts slightly to 1531 cm-1, whereas the 1313 cm-1 band shows a small blue shift to 1325 cm-1. (See Figure 3.) To corroborate unambiguously the assignment of the observed IR bands to CO2-related adsorbate species, the measurements presented above were repeated with the isotopomer C18O2. The corresponding UHV-FTIR spectra are displayed in Figure 4. All observed IR bands shift to lower frequencies with respect to the corresponding data obtained after C16O2 adsorption; the amount of the shift is consistent with the expected isotopic shifts for C18O2-related bands. On the basis of the vibrational and thermal stability data reported in previous work for the ZnO single crystal surfaces, we can provide a consistent assignment of the IR bands observed on the ZnO nanoparticles. (See Tables 1 and 2.) The most intense bands at 1592, 1334, 1000, and 850 cm-1 are characteristic for carbonate species and are assigned to three C-O stretching modes and the π(CO3) (850 cm-1) mode, respectively. These frequencies are in excellent agreement with the HREELS results reported for carbonate species on the mixed-terminated

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TABLE 1: Assignment of Majority Carbonate Species Formed after CO2 Adsorption on ZnO Powder Surfacesa vibrational modes νas(OCO) νs(OCO) ν(CdO) νas(OCO)

IR data [cm-1]

HREELS data [cm-1]

1617/1592 1343/1334 1621 1292

b

c

1617 /1609 1340b/1323c 1615d 1293d

assignment tridentate carbonate on ZnO(101j0) bidentate carbonate on O-ZnO(0001j)

a For comparison, the experimental vibrational frequencies (inverse centimeters) of carbonate species observed on different ZnO surfaces are also given. b 1.0 monolayer (ML) carbonate on ZnO(101j0). c 0.5 ML carbonate on ZnO(101j0). d Carbonate on O-ZnO(0001j).

ZnO(101j0) surface and can be further identified as an unusual tridentate carbonate formed via activation of CO2 on the perfect ZnO(101j0) surface (Table 1). The present IR data reveal that the interaction of CO2 with ZnO nanoparticles leads to the formation of a majority carbonate species on the ZnO(101j0) facets. This can be explained by the fact that the surfaces exposed by polycrystalline ZnO nanoparticles mainly consist of ZnO(101j0) facets, as demonstrated by recent transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements.34 Upon evacuation to 10-10 mbar, the bands at 1592 and 1534 cm-1 shift slightly to lower wavenumbers, which is explained by the decrease in the carbonate coverage on the ZnO(101j0) surface, in perfect agreement with the HREELS results on single crystal surfaces.25 The assignment of the majority species to a tridentate carbonate on ZnO(101j0) is further confirmed by the heating experiments, where the corresponding IR bands disappear completely upon heating to 450 K, in line with the TPD data reported for the ZnO(101j0) surface.25

A second group of vibrational bands appears at 1621, 1292, 974, and 871 cm-1. These frequencies are in good agreement with those observed for carbonate species formed on polar O-terminated ZnO(0001j) surfaces and are assigned to the ν(CdO), νas(OCO), νs(OCO), and π(CO3) modes of a bidentate carbonate, respectively (Table 1). These bands exhibit a slightly higher thermal stability and are removed completely only after heating to 500 K, further supporting the assignment to a bidentate carbonate species on O-ZnO(0001j) surfaces. According to the structural model for the clean (3 × 1) reconstructed O-ZnO, this surface contains a high density (33%) of O vacancies. These O vacancies exhibit a high reactivity for reactive adsorption (e.g., for H2O35,36). The formation of carbonate species resulting from the interaction of CO2 with O-vacancies on the O-ZnO surface is thus highly plausible. Finally, we discuss the two weak bands located at 1543 (1531) and 1313 (1325) cm-1. Although these vibrations are also characteristic for carbonate species, a straightforward assignment by comparing to available experimental results for single crystal substrates is not possible. The relatively small splitting of these two bands suggests an assignment to a monodentate carbonate species (Table 2).37 On the basis of the fact that this pair of bands was not observed on the well-defined ZnO single crystal surfaces, we attribute these vibrations to carbonate species located at defect sites such as steps, kinks, and edges (separating differently oriented facets). The rather small intensities and broad peak shapes of these bands would be consistent with this assignment, as is the fact that these carbonate species exhibit a fairly large thermal stability and disappear only after heating to temperatures >600 K. It should be noted that on the basis of a recent STM study, the polar Zn-terminated ZnO(0001) surface is stabilized by a reconstruction forming triangular-shaped islands with a large number of steps.38 This experimental finding has been corroborated by density functional density (DFT) calculations.38–40 The corresponding step edges are likely sites for CO2 activation to yield carbonate species. Further studies, for example, theoretical calculations for cluster models simulating defects on ZnO surfaces, are required to unravel the precise nature of this minority carbonate species. Although so far no vibrational data have been reported for the Zn-ZnO surface, we consider it highly unlikely that CO2 can react with the perfect ZnO(0001) surface to form carbonate species. The assignment of the carbonate-related bands observed on the ZnO nanoparticles is further supported by a comparison of the IR data obtained during the exposure of the clean ZnO sample to 1 × 10-4 mbar of CO2 at different temperatures. (See Figure 5.) At 350 and 450 K, the corresponding IR spectra TABLE 2: Vibrational Frequencies (inverse centimeters) and Assignment of Minority Carbonate Species Formed after CO2 Adsorption on ZnO Powder Surfaces

Figure 4. UHV-FTIR spectra obtained after exposing the clean ZnO nanoparticles to 5 × 10-5 mbar C18O2 at room temperature and then heating to: (A) 300, (B) 325, (C) 350, (D) 400, (E) 425, (F) 450, (G) 500, (H) 550, and (I) 600 K.

vibrational modes

monodentate carbonate

polydentate carbonate

νas(OCO) νs(OCO)

1543/1520 1313/1323

1466 1386

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Figure 5. UHV-FTIR spectra obtained after exposing the clean ZnO nanoparticles (A) to 1 × 10-4 mbar CO2 at different temperatures: (B) 350, (C) 450, and (D) 550 K.

(curves B and C) reveal the presence of all the three kinds of carbonate species: a tridentate carbonate on ZnO(101j0) surfaces (1580 and 1328 cm-1), a bidentate carbonate on O-ZnO(0001j) surfaces (1619 and 1292 cm-1), and a monodentate carbonate on defect sites (1543 and 1313 cm-1). Compared with the other two kinds of carbonates, the relative intensity of the bands at 1580 and 1328 cm-1 is substantially decreased with increasing temperature. This pair of bands is not observed in the IR spectrum recorded at 550 K, indicating again that the tridentate carbonate on ZnO(101j0) is less stable, in line with the TPD results on ZnO single crystal surfaces. When the exposure temperatureis increased, the second group of bands (1619 and 1292 cm-1) is stable up to 450 K, revealing a higher thermal stability of this bidentate carbonate on the O-ZnO surfaces than of the tridentate carbonate species on ZnO(101j0). The IR data recorded at 550 K only show broad bands at 1543 and 1313 cm-1. These bands are thus assigned to the most stable carbonates resulting from CO2 adsorption on defect sites of ZnO nanoparticles. To obtain information on the activation energy governing the chemisorption of CO2 on ZnO nanoparticles, we also studied the interaction of CO2 with ZnO at lower temperatures. Figure 6 presents UHV-FTIR spectra recorded after CO2 adsorption on ZnO nanoparticles at 100 K. Figure 6A again shows the presence of a clean, adsorbate-free ZnO powder sample prepared by heating to 850 K and subsequently cooling to 100 K. After exposing the clean ZnO sample to 1 × 10-5 mbar of CO2 at 100 K, four intense bands show up at 1617, 1343, 1000, and 842 cm-1. They can be unambiguously assigned to the tridentate carbonate species formed on the mixed-terminated ZnO(101j0)

Noei et al. surfaces. The frequencies are in a perfect agreement with the HREELS data obtained for a full (1 × 1) monolayer on ZnO(101j0) (see Table 1), indicating that the (101j0) facets of ZnO nanoparticles are fully covered by the tridentate carbonate species formed upon CO2 adsorption at 100 K. In addition, the spectrum in Figure 6B displays the bands at 1643 and 1295 cm-1, which are characteristic for the bidentate carbonate formed at oxygen vacancy sites within the polar O-ZnO(0001j). Finally, a weak band is visible at about 1543 cm-1 as a shoulder on the low frequency side of the main band at 1617 cm-1, which is also detected for the room-temperature sample and is assigned to a monodentate carbonate species formed on defects. The fact that the carbonate vibrations are seen for CO2 exposure at these low temperatures reveals that the corresponding activation energy must be fairly low. In addition to the carbonate-related bands, exposure of the ZnO nanoparticles to CO2 at 100 K leads to the occurrence of IR bands at 2352 and 638 cm-1. These frequencies are very close to that of the νas(OCO) and π(OCO) vibrations observed for free CO2 molecules and are, accordingly, assigned to linear CO2 species physisorbed on the surfaces of ZnO powder particles. The fact that these vibrational bands disappear upon heating to 180 K (Figure 6C) is fully consistent with an assignment to a physisorbed species. In a previous work, it has been proposed that the physisorbed CO2 is weakly coordinated to the Zn cation sites.21 Interestingly, after large exposure to CO2, the νas(OCO) band is found to split into a doublet with the two components located at 2366 and 2352 cm-1. We speculate that the splitting of the νas(OCO) band originates from a coupling of two linear CO2 species adsorbed on the same Zn cation site. However, we cannot definitely exclude that the doublet results also from physisorption of CO2 on different Zn cation sites. 3.3. Interaction of CO2 with Hydroxylated ZnO Nanoparticles. In an additional set of experiments, we have studied the interaction of CO2 with hydroxylated ZnO nanoparticles. Prior to CO2 adsorption, clean ZnO powder samples prepared as described above were first exposed to H2O at a pressure of 1 × 10-6 mbar at 300 K and then heated to 350 K to remove partially the weakly bound water molecules. A typical UHVFTIR spectrum recorded for such samples is presented in Figure 7A. Two sharp OH stretch vibrations are detected at 3672 and 3620 cm-1. On the basis of the HREELS data reported for different single crystal ZnO surfaces, these bands are assigned to hydroxyl species formed by water dissociation on the mixedterminated ZnO(101j0) and the polar O-ZnO(0001j) surfaces, respectively.33 This assignment is also consistent with relative intensity of the two OH bands, considering that among the facets exposed by the ZnO nanoparticles those with (101j0) orientation provide the largest area.34 The 3672 cm-1 vibration originates from OH species with coadsorbed H2O molecules on mixedterminated ZnO(101j0), as confirmed by the observation of the scissoring mode of H2O at 1615 cm-1.33 After the hydroxylated ZnO nanoparticles were exposed to CO2 at 300 K, the IR data (Figure 7B) reveal the formation of three different carbonate species, similar to the situation for the clean surface: (1) a tridentate carbonate on ZnO(101j0) (1581 and 1331 cm-1), (2) a bidentate carbonate on O-ZnO(0001j) (1620 and 1292 cm-1), and (3) monodentate carbonates on defect sites (1520 and 1323 cm-1). Compared with CO2 adsorption on the clean ZnO nanoparticles (Figure 1), however, the intensity of carbonate-related bands is substantially smaller, indicating an inhibiting effect induced by the presence of hydroxyl groups. This conclusion is further supported by the

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Figure 6. UHV-FTIR spectra obtained after exposing the clean ZnO nanoparticles (A) to 1 × 10-5 mbar CO2 at 100 K and then heating to (B) 100 K, (C) 180 K, (D) 200 K. (a) Physisorbed CO2 and (b) carbonate regions.

Figure 7. UHV-FTIR spectra obtained after exposing the clean ZnO nanoparticles to H2O (1 × 10-6 mbar) at room temperature (A). The sample is then heated up to 350 K, cooled again to 300 K and exposed to 1 × 10-5 mbar CO2 at 300 K (B). The sample is further heated to (C) 325, (D) 350, (E) 400, (F) 450, (G) 500, (H) 550, (I) 600, (J) 650, and (K) 800 K. (a) Hydroxyl and (b) carbonate regions.

observation of frequency shifts of carbonate-related bands, which is known to result from interactions with other carbonate-species at adjacent sites.25 In addition, exposure of the hydroxylated ZnO sample to CO2 at 300 K leads to the appearance of two additional weak bands at 1466 and 1386 cm-1 that are not seen for the hydroxyl-free surface. Their intensity depends on surface temperatures and reaches the maximum at ∼450 K. This pair of bands shows a small splitting of 80 cm-1 and is characteristic

for the formation of a polydentate carbonate species.41–44 It is found from the UHV-FTIRS data that this species is more stable than the carbonates formed on the ZnO(101j0) and O-ZnO(0001j) surfaces but is less stable than the monodentate carbonates formed on defect sites. (See Figure 7.) This minority carbonate species was not observed on the ZnO single crystal surfaces and must originate from the activation of CO2 on special defect sites not present on the clean, hydroxyl-free surface. In none of

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the present experiments did we find any indications for the interaction of activated CO2 species with surface hydroxyls under UHV conditions. 4. Conclusions The interaction of CO2 with clean and hydroxylated ZnO nanoparticles was investigated using UHV-FTIRS. After exposure of clean ZnO samples to CO2 at 300 K, a number of carbonate-related bands are detected. On the basis of the vibrational and thermal stability data available for single crystal ZnO surfaces, four kinds of carbonate species formed on the ZnO nanoparticles are clearly identified: (1) an unusual tridentate carbonate as a majority species formed via CO2 activation on the perfect mixed-terminated ZnO(101j0) surface exposing coordinatively unsaturated surface Zn and O atoms; (2) a bidentate carbonate species on the polar O-ZnO(0001j) surface resulting from activation of CO2 on oxygen vacancy sites; (3) a monodentate carbonate formed via interaction of CO2 with defect sites; and (4) a polydentate carbonate on the hydroxylated ZnO nanoparticles related to CO2 activation on defect sites. The high reactivity of the ZnO powders for CO2 adsorption is further confirmed by the low-temperature CO2 adsorption on the clean ZnO nanoparticles at 100 K leading to the formation of a full monolayer tridentate carbonate on ZnO(101j0). The IR spectra provide evidence of the formation of other minority carbonates at 100 K, and the activation of CO2 is found to be inhibited by the presence of hydroxyl groups. The UHV-FTIRS data for CO2 adsorption on the clean and hydroxylated ZnO nanoparticles clearly demonstrate that CO2 activation on the exposed ZnO surfaces involves coordinatively unsaturated surface oxygen species. Acknowledgment. This work was supported by the German Research Foundation (DFG) through the Sonderforschungsbereich SFB558 “Metal-Substrate-Interactions in Heterogeneous Catalysis” and the ANR-DFG French-German cooperation project “ACTCO2”. H.N. thanks the German Academic Exchange Service (DAAD) for a research grant. References and Notes (1) Freund, H.-J.; Roberts, M. W. Surf. Sci. Rep. 1996, 25, 225. (2) Catalytic ActiVation of Carbon Dioxide; Ayers, W. M., Ed.; ACS Symposium Series 363; American Chemical Society: Washington, D.C., 1988. (3) Behr, A. Carbon Dioxide ActiVation by Metal Complexes; Verlag Chemie: Weinheim, Germany, 1988. (4) Solymosi, F. J. Mol. Catal. 1991, 65, 337. (5) Carbon Dioxide Chemistry: EnVironmental Issues; Paul, J., Pradier, C.-M.; Eds.; Royal Society of Chemistry: London, 1994. (6) Schenk, S.; Notni, J.; Ko¨hn, U.; Wermann, K.; Anders, E. Dalton Trans. 2006, 4191. (7) Hansen, J. B. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; VCH: Weinheim, Germany, 1997.

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