Structure and Stability of Formates and Carbonates on Monoclinic

Aug 13, 2008 - Olga A. Syzgantseva , Monica Calatayud , and Christian Minot. The Journal of Physical Chemistry C 2012 116 (11), 6636-6644. Abstract | ...
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J. Phys. Chem. C 2008, 112, 16096–16102

Structure and Stability of Formates and Carbonates on Monoclinic Zirconia: A Combined Study by Density Functional Theory and Infrared Spectroscopy Satu T. Korhonen,†,‡ Monica Calatayud,*,‡ and A. Outi I. Krause† Laboratory of Industrial Chemistry, Helsinki UniVersity of Technology, P.O. Box 6100, FI-02015 TKK, Finland, and Laboratoire de Chimie The´orique, CNRS, UPMC UniV Paris 06, UMR 7616, Paris F-75005, France ReceiVed: April 17, 2008

The structure and stability of formate and (bi)carbonate species were studied on monoclinic zirconia by theoretical and experimental methods. For the ab initio calculations, the most stable (1j11) surface containing one dissociatively adsorbed water molecule was used. Infrared spectroscopy measurements were carried out on a commercial catalyst. The calculated vibrational frequencies were in good agreement with species observed by in situ infrared spectroscopy. The most stable structures for both formates and bicarbonates were bidentate formed by an insertion of CO or CO2 to the terminal hydroxyl group. The most stable carbonates were formed on Lewis acid-base pairs by the adsorption of CO2 to the undercoordinated surface sites. 1. Introduction Zirconia has received considerable attention in the recent literature for applications ranging from catalysis to microelectronic gate dielectrics and ceramic engineering. A notable benefit of zirconia is its high thermal stability.1 Zirconia catalyzes the hydrogenation and isomerization of alkenes2 and the dehydrogenation of alkanes,3 and it is an active component in the synthesis of methanol from CO/H2 or CO2/H24-6 and in the water-gas shift reaction.7-9 The versatile reactivity of zirconia originates, among other things, from the amphoteric character of its hydroxyl groups.2 Zirconia exhibits three well-characterized phases: monoclinic, tetragonal, and cubic phase. Of these, the monoclinic phase is stable up to ∼1100 °C, the tetragonal between ∼1100 and 2400 °C, and the cubic up to the melting point of ∼2670 °C.10 The most stable surfaces for monoclinic, tetragonal, and cubic zirconia are (1j11),11 (101),12 and (111),11 respectively. The surface of zirconia contains hydroxyl groups of different coordination (terminal, 2-fold coordinated, and 3-fold coordinated), undercoordinated Zr4+ and O2- sites, and defects such as oxygen vacancies.2,13,14 The relative concentration of these species depends on the external conditions (temperature, pressure, gas-phase composition) and the phase of zirconia. Formates and carbonates are important intermediates in the synthesis of methanol from CO/H2 or CO2/H2.4-6 On Cu/ZrO2 catalysts, CO or CO2 reacts with the hydroxyl groups of zirconia to form formates or carbonates, while hydrogen is dissociated on Cu and spills over to the support.4,5 A series of hydrogenation reactions of adsorbed formate or carbonate species leads to the formation of methanol.4,5 (Bi)carbonates (bicarbonates and monodentate, bidentate, or polydentate carbonates) readily form on the surface of zirconia at room temperature, whereas formates form in an activated process requiring higher temperatures.15 The role of formates in the water-gas shift reaction (CO + H2O f CO2 + H2) is still under debate. Two reaction mechanisms have been proposed.7-9,16 In the “adsorptive * To whom correspondence should be addressed. E-mail: calatayu@ lct.jussieu.fr. Tel: +33 1-44-27-26-82. Fax: +33 1-44-27-41-17. † Helsinki University of Technology. ‡ UPMC University of Paris 06.

mechanism”, which operates through the formation of formates, water regenerates the hydroxyl groups and decomposes the formates, whereas in the “redox mechanism” water reoxidizes the reduced surface. On Pt/CeO2 catalyst the reaction mechanism was observed to depend on the reaction conditions:16 at 160 °C the formates were spectators, whereas at 220 °C they were possible main reaction intermediates. Both reaction mechanisms have also been suggested for Pt/ZrO2.7-9 In isotopic transient kinetic experiments7 formates on zirconia exchanged 12C for 13C slower than did the product CO at 200 °C. This result 2 indicated that under these reaction conditions the formates were spectators. In other experiments,9 however, alkali doping of the Pt/ZrO2 catalyst influenced both the formate C-H bond strength and the water-gas shift activity suggesting that here the formates were reaction intermediates. Only a few studies have described the structure and stability of formates and (bi)carbonates at molecular level. Most of the computational work related to formates has been performed on metal surfaces, for example, Pd(111),17 Cu(100), (110), and (111),18,19 and Zn/Cu(111).20 To our knowledge, for metal oxides only ZnO21 and TiO222-24 have been studied. In none of these studies were formates considered to form in a reaction between gaseous CO and the hydroxyl groups. Similarly, most of the computational work related to (bi)carbonates has been performed on metal surfaces: e.g., Ag(110)25 and Pt(111).26,27 For metal oxides, TiO2 has been studied,28 but again hydroxyl groups were not considered. In the present study, we report the structure and stability of formates and (bi)carbonates on hydrated monoclinic zirconia. For the formation of formates and bicarbonates an insertion of CO or CO2 into the hydroxyl groups was considered, whereas carbonates were formed by adsorbing CO2 on the undercoordinated surface sites. The computational work was performed on the most stable (1j11) surface11 containing one dissociatively adsorbed water molecule, as described previously.29 In parallel, we report results obtained by infrared spectroscopic measurements on a commercial zirconia catalyst. The experimental and theoretical results are compared, and the nature of the carbonaceous surface species is discussed.

10.1021/jp803353v CCC: $40.75  2008 American Chemical Society Published on Web 08/13/2008

Formates and Carbonates on Monoclinic Zirconia 2. Experimental Section 2.1. Computational Details. The Perdew-BurkeErnzerhof functional was used for all the calculations as implemented in the VASP code.30-32 The electron configurations [Kr]4d25s2, [He]2s22p4, and [He]2s22p2 were used for the zirconium, the oxygen, and the carbon atoms, respectively. The core electrons were kept frozen and replaced by PAW-generated pseudopotentials,33,34 whereas the valence electrons were described with a plane wave basis set with the cutoff of 400 eV. The distance between k-points in the reciprocal space was 0.05 Å-1. The methodology was tested for the bulk of monoclinic zirconia leading to an excellent agreement with previous experimental and theoretical results as described in ref 29. The most stable (1j11) surface of monoclinic zirconia11 was chosen for the calculations. A slab containing four ZrO2 layers was selected for the (1j11) surface, and this was repeated periodically in three dimensions. The (1 × 1) unit cell dimensions were 7.446 × 6.793 Å2 and the unit cell contained four ZrO2 units per layer (total of 48 atoms). A vacuum of 10 Å prevented the interaction between successive slabs. The surface contained one dissociatively adsorbed water molecule per unit cell. This dissociatively adsorbed water formed one terminal and one 2-fold coordinated hydroxyl. The adsorption of water is described in our previous publication.29 The low coverage of water was chosen for the following reasons: (i) thermodynamic analysis29 indicated that low coverage surfaces are stable at higher temperatures and (ii) low coverage enabled us to study the adsorption of CO/CO2 on the undercoordinated surface atoms using the same model surface. All carbonaceous species were studied on one side of the slab, and the full model was allowed to relax with the conjugate gradient algorithm until the difference in energy was smaller than 0.001 eV. The adsorption energies Eads were calculated as a difference between the total energy of the relaxed formate- or (bi)carbonate-containing slab and the energy of the reference units (the relaxed hydrated surface and gas-phase CO or CO2 molecules); negative values indicate exothermicity. The vibrational frequencies of the formate and (bi)carbonate species were calculated by finite differences within the harmonic approximation. The Hessian matrix was then constructed and diagonalized. Note that the intensities cannot be calculated with the code used. 2.2. In situ DRIFTS. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study the formation of formates and (bi)carbonates on monoclinic zirconia (Mel Chemicals, EC0100, BET surface area 47 m2/g3). The equipment consisted of a Nicolet Nexus FTIR spectrometer equipped with a Spectra-Tech high temperature and high pressure reaction chamber (ZnSe windows). A Pfeiffer Vacuums Omnistar mass spectrometer (MS) was used to monitor the gaseous products online. The sample was studied undiluted in powder form. The spectrum measured with an aluminum mirror (4 cm-1, 200 scans) was used as the background, and the gas flow was kept constant at 50 cm3/min. The sample was calcined with 10% O2/N2 (synthetic air 99.99% and N2 99.999% were from AGA) at 580 °C for 2 h, after which it was cooled to room temperature and flushed with nitrogen (purified with Oxisorb, Messer Griesheim GmbH) for 1 h. The formation and thermal stability of the formates and (bi)carbonates were studied by using dilute CO feed (5% CO/N2, CO from Messer Griesheim, 99.997%) for the formates and dilute CO2 feed (5% CO2/N2, CO2 from AGA, 99.99%) for the (bi)carbonates. Formate Studies by in situ DRIFTS. For the formate studies the calcined sample was slowly heated under dilute CO feed

J. Phys. Chem. C, Vol. 112, No. 41, 2008 16097 from room temperature to 100 °C. During the heating, spectra were recorded at 30, 50, 75, and 100 °C (4 cm-1, 100 scans). At 100 °C, the sample was flushed with nitrogen for 5 min, after which a spectrum was recorded under nitrogen flow (4 cm-1, 100 scans). The experiment was continued by redirecting the CO flow to the sample and heating to 200 °C. The spectrum was recorded under CO at 200 °C, after which the sample was again flushed with nitrogen. The experiment was continued by heating under CO and flushing with nitrogen at 300, 400, 500, and 580 °C. The spectra were recorded under CO every 25 °C and under nitrogen at 100, 200, 300, 400, 500, and 580 °C. After the experiment the sample was flushed with nitrogen at 580 °C for 30 min and reoxidized (2-10% O2/N2) at the same temperature for 30 min. (Bi)Carbonate Studies by in situ DRIFTS. For the (bi)carbonate studies the calcined dilute CO2 feed was directed to the calcined sample at room temperature for 5 min. Thereafter, the sample was flushed with nitrogen for 30 min to remove any physisorbed CO2. The spectra were recorded during the CO2 feed, during the first 5 min of the nitrogen flush once a minute (4 cm-1, 30 scans), and for the rest of the nitrogen flush once every 5 min (4 cm-1, 100 scans). Thereafter, the sample was slowly heated under nitrogen to study the thermal stability of the carbonate species. The heating was performed in sequences of heating under nitrogen and refeeding dilute CO2 to the sample. First the sample was slowly heated under nitrogen to 100 °C, and the spectra were recorded at 50, 75, and 100 °C (4 cm-1, 100 scans). Thereafter, the dilute CO2 feed was redirected to the sample for 5 min, and the spectra were recorded every minute (4 cm-1, 30 scans). After 5 min of CO2 feed, the sample was flushed with nitrogen for 15 min, and the spectra were recorded once a min (4 cm-1, 30 scans) for the first 5 min and thereafter once every 5 min (4 cm-1, 100 scans). After the flush, the heating was continued, and the spectra were recorded at 125, 150, 175, and 200 °C under nitrogen. The experiment was continued with CO2 directed to the sample at 200, 300, 400, and 500 °C, followed by 15 min nitrogen flush and heating under nitrogen. After the experiment the sample was flushed with nitrogen at 580 °C for 30 min and reoxidized (2-10% O2/N2) at the same temperature for 30 min. 3. Results and Discussion 3.1. The Hydrated Surface. Figure 1 presents the hydrated (1j11) surface of monoclinic zirconia. The surface and the adsorption of water are described in our previous publication.29 The surface contains four nonequivalent Zr atoms. The Zr atoms 1, 3, and 4 are undercoordinated (6-fold coordinated), while the Zr(2) is 7-fold coordinated as in the bulk. The undercoordinated O atoms are marked in Figure 1 with letters A, B, C, and D. Of these, the O(A) is 2-fold coordinated, while the O(B), O(C), and O(D) are 3-fold coordinated. The unmarked O atoms resemble those of the bulk zirconia. The most stable adsorption mode for one molecule of water is dissociative with hydroxyl groups at Zr(1) and O(A). Figure 1 also presents the distances (Å) between the undercoordinated Zr atoms. The distances were 3.145, 3.498, and 3.503 Å. On cubic zirconia (yttria stabilized), by comparison the distance between nearest Zr atoms is 3.64 Å.35 3.2. Adsorption of CO and Formate Formation. The adsorption of molecular CO was studied on the undercoordinated sites. However, no CO was found to adsorb molecularly on the zirconia surface. For the formation of formates, the insertion of CO into hydroxyl groups was considered. A reasonable number of formate structures (14) of formates was calculated with both hydroxyl groups, the terminal and

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Korhonen et al.

Figure 2. The most stable formate structures in order of stability for the (1j11) surface of monoclinic zirconia. Adsorption energies are presented in eV. (I) Bidentate formate bonded to two Zr atoms, (II) bidentate formate bonded to one Zr atom, (III) bidentate formate bonded to Zr and lattice O, and (IV) monodentate formate. See Table 1 for bond distances.

TABLE 1: Adsorption Energies (eV and kJ/mol) and Bond Distances (Å) for the Different Formate Structures I-IV (see Figure 2)a Eads (eV) Eads (kJ/mol) Zr-O (Å) C-H (Å) O-C-O (Å) O-C-O (°) Figure 1. The relaxed (1j11) surface containing one dissociatively adsorbed water molecule per unit cell (top and side views). The Zr atoms are presented in yellow and the O atoms in red, and the atoms of the adsorbed water molecule are dark blue. The lattice O atoms taking part in the dissociative adsorption are colored turquoise. The hydrogen bonds are shown with a dashed line. The unit cell is presented by the gray lines.

2-fold coordinated, considered. Both hydroxyl groups, the terminal and the 2-fold coordinated, were considered for the formate formation. The most stable formate structures for the (1j11) surface are presented in Figure 2, and the adsorption energies and bond distances are presented in Table 1. The vibrational frequencies were calculated for the different structures and are compared with those observed by infrared spectroscopy in Table 2. Structure and Stability of Formate Species. Structure I in Figure 2 was the most stable (Eads ) -1.59 eV, 1 eV ∼ 96.5 kJ/mol). In this structure, the formate bridges two Zr atoms (Zr(1) and Zr(4) with distance of 3.498 Å between them). The two Zr-O bonds and the two C-O bonds were equivalent. The C-O distance was elongated from the calculated bond distance of 1.144 Å of a free CO molecule. Other bidentate formates bound to two different Zr atoms (Zr(1)-Zr(3)) were less stable: Eads ) -1.57 and -1.35 eV for stretching over distances of 3.145 and 3.503 Å, respectively. In the second most stable structure (II in Figure 2) the formate was again bidentate but now coordinated to the same Zr atom (Zr(1)) (Eads ) -1.32 eV). The structure was somewhat more

I

II

III

IV

-1.59 -151 2.255-2.251 1.110 1.269-1.269 127

-1.33 -127 2.316-2.444 1.106 1.273-1.275 121

-1.11 -107 2.282 1.103 1.261-1.285 120

-0.64 -62 2.102 1.116 1.214-1.342 125

a The total energies for the hydrated (1j11) surface and the CO molecule are -467.38 and -14.78 eV, respectively.

TABLE 2: Comparison of Calculated and Experimentally Observed Infrared Vibrational Frequencies (cm-1) for Formate Species; the Numbers Refer to the Species in Figure 2 νas(COO) + νs(COO) ν(CH) νs(COO) + δ(CH) a νas(COO) δ(CH) νs(COO) out of plane ∆ν ) νas - νs

a

exp

I

II

III

IV

2964 2881 2747 1564 1384 1365 n.o. b 199

2915 2974 2754 1555 1394 1360 1010 195

2845 2997 2676 1544 1375 1301 995 243

2765 3138 2504 1585 1324 1180 958 406

2843 2892 2473 1717 1347 1126 983 591

a Combination modes are not observed in the calculations. The values were obtained by summing the corresponding frequencies. b n.o. ) not observed.

strained than structure I (the O-C-O angle was 121°, as compared with 127° in structure I). The greatest effect was on the two Zr-O bond distances: both were longer than the Zr-O bond distances of structure I. The C-O bond distances were only slightly affected. This structure was less stable than the bidentate formate structures described above. It would therefore seem that the formation of bidentate formates bound to two different Zr atoms is possible and preferred on monoclinic zirconia. This is in contrast to what was proposed35 for the cubic

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Figure 4. Proposed reaction scheme for the formation of formates by insertion of CO into a hydroxyl group.

Figure 3. In situ DRIFT spectra for adsorption of CO on monoclinic zirconia. The spectra were recorded under nitrogen.

zirconia with a slightly longer distance between the neighboring Zr atoms. For the cubic zirconia, the formate species are bidentate but coordinated to one surface Zr atom (i.e., structure II).35 The preference to bridge between two undercoordinated surface atoms has previously been reported for ZnO21 and TiO222 based on theoretical studies. Structure III was obtained by inserting CO into the 2-fold coordinated hydroxyl group (Eads ) -1.11 eV). This caused a still larger strain in the structure: the O-C-O angle was only 120°. However, the Zr-O and C-O distances were similar to those observed for the most stable structure I. The energy difference (0.48 eV) of structures I and III would suggest that the terminal hydroxyls are more reactive than the 2-fold coordinated. The monodentate structure IV was the least stable (Eads ) -0.64 eV). For this structure, the shortest Zr-O bond distance of 2.102 Å was observed. In addition, in contrast to the other structures, the C-O bond distances were not equivalent. This is in good agreement with the presence of single (1.342 Å) and double (1.214 Å) bonds in this formate structure. In the other structures, both C-O bonds had some double bond character. Comparison of Calculated and Experimental Vibrational Frequencies. Figure 3 presents the in situ DRIFT spectra recorded during an experiment where commercial zirconia sample was heated under CO from room temperature to 580 °C with periodic flushing with nitrogen. The presented spectra were recorded under nitrogen. The nitrogen flushing had little effect on the spectra. The bands were identified according to previous reports36-38 and the band positions were in good agreement with literature. In the experiments, the formates began to form at 175 °C and were stable up to 500 °C under CO feed. When the feed was switched to pure nitrogen the formates disappeared from the spectrum at 500 °C. The formation of gaseous CO2 was observed by MS (not shown) as the formates decomposed. Table 2 compares the observed and calculated vibrational frequencies. Since the calculations are unable to produce the combination modes νas(COO) + νs(COO) and νs(COO) + δ(CH), these are calculated for structures I-IV by summing the respective calculated vibrational frequencies. The calculated frequencies for structure I were in excellent agreement with the experimental results. However, the calculated vibrations for structure II were also in reasonable agreement with the

experimentally observed vibrations. In addition, the difference in the adsorption energies for the two structures was only 0.26 eV (Table 1). Hence, the results do not rule out the presence of both structure I and structure II. At high formate coverage, structure II can be expected to dominate owing to the lack of available neighboring Zr atoms. Neither structure III nor structure IV is likely to be stable. Comparison of the calculated and experimental vibrational frequencies suggests that these species are in the minority or are rapidly transformed to other more stable formate structures. A reaction scheme for the formation of formates is proposed in Figure 4. First, CO is inserted in a hydroxyl group forming a monodentate species IV. This step involves an energy barrier and would explain the absence of formates at room temperature. This structure (IV) would then rotate with a small or negligible barrier to the most stable bridged structure (I or II). As observed by in situ DRIFTS, the bridged structures accumulate between 200 and 400 °C. At 500 °C, the barrier to the decomposition to CO2 is overcome. It is unlikely that such decomposition takes place from the bridged species owing to their high thermodynamic stability. A more plausible scenario would be interconversion to the monodentate formate IV and its decomposition to CO2 with the H atom being transferred to a surface proton acceptor such as a hydroxyl group. Both bidentate and monodentate formate species have been observed for Ga2O3 (gallia),39 and the interconversion of the bidentate species to monodentate formates has been suggested to be essential for methanol synthesis from CO2/H2. The monodentate species could also be the key intermediate on zirconia, but it was not detected by infrared spectroscopy because of its low stability during the formation step at 200-400 °C and its rapid decomposition at higher temperatures. 3.3. Adsorption of CO2 and (Bi)Carbonate Formation. The adsorption of molecular CO2 was studied on the undercoordinated sites. In contrast to CO adsorption, the CO2 adsorbed molecularly on the undercoordinated surface sites forming carbonates. All possible structures of carbonates were tested by calculating their total energies. Both hydroxyl groups, the terminal and the 2-fold coordinated, were considered for the bicarbonate formation. In total, 19 different structures were considered for the (bi)carbonates. Figure 5 presents the most stable (bi)carbonate structures for the (1j11) surface, and Table 3 lists the adsorption energies and bond distances. A comparison of the calculated vibrational frequencies with bands observed by infrared spectroscopy is presented in Table 4. Structure and Stability of (Bi)Carbonate Species. Figure 5 presents the most stable (bi)carbonates for the (1j11) surface in the order of stability and Table 3 their adsorption energies (eV and kJ/mol) and bond distances (Å). Structure I, a bidentate

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Figure 5. The most stable (bi)carbonate structures in order of stability for the (1j11) surface of monoclinic zirconia. Adsorption energies are presented in eV. (I) Bidentate bicarbonate bonded to two Zr atoms, (II) polydentate carbonate, (III) bidentate carbonate, (IV) bidentate bicarbonate bonded to Zr and lattice O, (V) monodentate bicarbonate, and (VI) bidentate bicarbonate bonded to one Zr atom. Color coding is as in Figure 2. See Table 3 for bond distances.

bicarbonate bound to two surface Zr atoms (Zr(1) and (Zr(3)), was the most stable species. Neither the Zr-O nor the C-O bonds were equivalent for this structure. The adsorption energy was -0.44 eV, significantly lower than that for the bidentate formate species on this surface. This bidentate bicarbonate was formed on the Zr(1) and Zr(3) surface sites with the shortest distance between them (see Figure 1), whereas the most stable bidentate formate stretched between the Zr(1) and Zr(4) species. However, the O-C-O angles were similar: 128 and 127° for the bicarbonate and the formate, respectively. Structure II, a polydentate carbonate, was the second most stable species and was formed upon the adsorption of CO2 on three undercoordinated surface sites (Eads ) -0.32 eV). The CO2 was bound to the surface by two Zr-O bonds and one C-O bond (Zr(3), Zr(4), and O(D)). Structure III, a bidentate carbonate, bound to one undercoordinated Zr(4) and one undercoordinated O(D) (i.e., a Lewis acid-base pair), had an adsorption energy of -0.23 eV. Comparison of structures II and III suggests that the carbonate structure can be further stabilized by an additional Zr-O bond. The small energy difference (0.09 eV) would, however, suggest that the two structures are present simultaneously. Structure II is proposed to form when the linear CO2 approaches the surface horizontally, whereas structure III forms when the CO2 molecule approaches the surface at an angle with one of the oxygen atoms pointing toward the surface. Structure IV was formed by the insertion of the CO2 into the 2-fold coordinated hydroxyl. The adsorption energy (Eads )

Korhonen et al. -0.10 eV) for this bicarbonate structure was noticeably lower than that for the bicarbonate formed by the insertion of CO2 into the terminal hydroxyl (structure I). The energy difference is in agreement with the conclusion drawn on the basis of the formate structures that terminal hydroxyl groups are more reactive than 2-fold coordinated ones. However, adsorption energies and stabilities are also affected by the strain in the structure: the more strained the structure the lower the stability. The O-C-O angle in structure IV was only 120° as compared with 128° in structure I. Structures V and VI were obtained by inserting CO2 into the terminal hydroxyl. Neither structure was stable. The adsorption energies for the monodentate bicarbonate and the bidentate bicarbonate bound to the same Zr(1) atom were endothermic: 0.05 and 0.09 eV, respectively. In agreement with findings for the formate structures, the stability of the carbonaceous structures appears to be affected both by the number of coordinating surface atoms and by the internal strain of the structure. Comparison of Calculated and Experimental Vibrational Frequencies. Figure 6 presents the spectra recorded at room temperature under CO2 for the commercial zirconia. Immediately after the start of the CO2 feed, the formation of bicarbonates and carbonates was observed. Simultaneously, the hydroxyl bands, especially those assigned to terminal hydroxyls2,3,13,29 at 3772 cm-1, decreased in intensity. The nitrogen flush at room temperature had no effect on the presented bands. The experimentally observed bands and their assignments are listed in Table 4 together with the calculated vibrational frequencies for structures I-VI. The experimental bands were identified according to previous reports13,15 and the band positions were in good agreement with the literature. Heating the sample under nitrogen caused the bicarbonate species to disappear form the spectra at ∼175 °C, whereas the carbonate bands were observed up to ∼400 °C (not shown). The thermal stability of the (bi)carbonate species was in good agreement with literature findings.13,15 Bands assignable to the bicarbonate species13 were observed at 3611 (ν(OH)), 1619 (νas(CO)), 1436 (νs(CO)), and 1223 (δ(OH)) cm-1. These bands have previously been assigned to the monodentate bicarbonate species on zirconia.13 For gallia, the monodentate and the bidentate bicarbonate species have been reported40 to give bands at similar (νs(CO)) vibrational frequencies, that is, 1431 for monodentate and 1455 cm-1 for bidentate bicarbonate species. On the basis of the stability of the different bicarbonate structures and the calculated vibrational frequencies, we assign our experimentally observed bands to the bidentate bicarbonate species (structure I). The calculated vibrational frequencies for structure VI were also in good agreement with the experimentally observed bands and those calculated for structure I. However, structure VI was energetically the least favored one, and such a species could hardly exist on the surface for extended periods of time. More likely, the structural similarity of IV with the stable structure I caused the good agreement in vibrational frequencies. The bidentate bicarbonate species are probably formed via the monodentate bicarbonates, analogously with the formates species (see Figure 4), but the reaction is evidently too fast, even at room temperature, to allow us to observe this experimentally. However, both the experimental and theoretical results suggest that the formation of bicarbonate species requires terminal hydroxyl groups. This is in agreement with the literature13 and also with our above description of the formation of formates. CO is slightly basic, and commonly used as a probe molecule for the acidic sites of metal oxides,15 whereas CO2 is

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TABLE 3: Adsorption Energies (eV and kJ/mol) and Bond Distances (Å) for the Different (Bi)Carbonate Structures I-VI (see Figure 5) on the Hydrated (1j11) Surfacea Eads (eV) Eads (kJ/mol) Zr-O (Å) O-H (Å) CdO (Å) O-C-O (Å) O-C-O (°) a

I

II

III

-0.44 -43 2.142-2.436 0.973

-0.32 -31 2.326-2.440 O-C: 1.408

1.264-1.280 128

1.250-1.269 135

IV

-0.23 -23 2.152

-0.10 -10 2.204 0.975

1.216 1.299-1.451 108

1.268-1.287 120

V

VI

0.05 4 2.070 0.979 1.227 1.311-1.373 112

0.09 9 2.392-2.505 0.974 1.257-1.284 122

The total energies for the hydrated (1j11) surface and the CO2 molecule are -467.38 and -22.97 eV, respectively.

4. Conclusions The formation of formates and (bi)carbonates was studied on hydrated monoclinic zirconia. Comparison of the adsorption energies suggested that the favored structures for the formates and bicarbonates on zirconia are of bridged type. Additional support for these structures was provided by the good agreement between the calculated vibrational frequencies and the vibrational frequencies observed by in situ DRIFTS. The adsorption energies further suggested that the terminal hydroxyl groups are more reactive than the 2-fold coordinated species. Acknowledgment. We gratefully acknowledge the financial support provided by the Academy of Finland and the aid provided by the COST action D36 in organizing a researcher exchange between TKK and Paris6. Computational facilities by IDRIS, CINES, and CCRE are acknowledged. We thank Professor C. Minot for stimulating discussions Figure 6. In situ DRIFT spectra for adsorption of CO2 on monoclinic zirconia at room temperature. The spectra were recorded under CO2.

TABLE 4: Comparison of Calculated and Experimentally Observed Infrared Vibrational Frequencies (cm-1) for (Bi)Carbonate Species; the Numbers Refer to the Structures in Figure 5 experimental bidentate bicarbonate carbonate ν(OH) νas(CO) νs(CO) δ(OH) ∆ν ) νas - νs

3611 1619 1436 1223 183

1559 1322 237

calculated I

II

III

IV

3686 3750 1600 1667 1791 1579 1386 1239 1158 1428 1204 1194 213 428 633 151

V

VI

3708 1728 1307 1126 421

3764 1619 1412 1197 207

acidic and used as a probe molecule for basic surface sites.13 Our results on the structure and stability of the formates and bicarbonates, therefore, support the amphoteric character2 of the hydroxyl groups of zirconia. The carbonate species13 were observed experimentally at 1559 ((νas(CO)) and 1322 ((νs(CO)) cm-1. These bands are assigned to bidentate carbonates formed in the interaction of CO2 with a Lewis acid-base pair, whereas a polydentate carbonate would be observed from bands at 1450 ((νas(CO)) and 1425 ((νs(CO)) cm-1. 13 However, the calculated vibrational frequencies for structure II, a polydentate carbonate, seem to be in best agreement with the experimental ones. Nevertheless, the experimental bands were broad and the energy difference for structures II and III was small. Therefore, no distinction can be made between the two structures based on the here presented results. However, the formation of both structures requires the presence of a Lewis acid-base pair and is therefore in agreement with the literature.13

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