J. Phys. Chem. B 1997, 101, 2917-2922
2917
Infrared Study of Ozone Adsorption on CaO K. M. Bulanin,† J. C. Lavalley,*,‡ and A. A. Tsyganenko† Institute of Physics, St. Petersburg UniVersity, St. Petersburg, 198904, Russia, and URA CNRS 414, ISMRA, UniVersite´ de Caen, 14050, Caen, Cedex, France ReceiVed: NoVember 21, 1996; In Final Form: February 5, 1997X
Ozone adsorption on CaO pretreated at different temperatures was studied by FTIR spectroscopy at 77-300 K. Besides physisorption effects, a weak complex was observed formed by ozone molecules with the OH surface groups, which have pronounced basic properties, as shown by a downward shift of the νOH band by about 21 cm-1 and by slight changes in ozone vibrational frequencies. Chemisorption of O3 occurred on CaO activated at 973 K as evidenced by the appearance of a band at 812 cm-1 accompanied by two weaker peaks at 1859 and 625 cm-1 attributed to the ν3, ν1 + ν3, and ν2 vibrational modes, respectively, of the surface ozonide O3- ions. Experiments with 18O substitution supported this assignment. Effect of CO preadsorption showed ozone chemisorption to occur on the same coordinate-unsaturated O2- surface sites that accounted for the formation of the “carbonite” CO22- ion from CO adsorbed on highly activated CaO.
Introduction Despite the great importance of ozone in the earth’s atmosphere, only few IR spectroscopic studies have been reported on O3 adsorbed on different surfaces. Since the first observation of the IR spectra of ozone adsorbed on MgO,1,2 detailed infrared studies were performed for SiO23 and TiO2 (anatase).4 Preliminary data were also obtained for several other oxides with different surface acidity.5 Ozone interaction with silica3 demonstrated the basic character of this molecule, since it is capable of forming a weak H-bond with the silanol groups. The vibrational frequencies of the hydrogen-bonded ozone were found to be almost the same as in the liquid or dissolved states, or for physically adsorbed ozone on surfaces of other oxides.3,4 It was concluded, from the analysis of the band half-widths for different isotopic modifications of ozone, that the O3 molecule interacts with the OH group proton via an O3 terminal oxygen atom. The effects caused by O3 interaction with the Lewis acid sites on metal oxides confirm the basic character of ozone.4,5 At liquid nitrogen temperature, ozone molecules form coordinated complexes bound with weaker sites via one of the terminal oxygen atoms. Large frequency shifts reveal strong distortion of these coordinatively bound molecules. Ozone dissociates on stronger Lewis sites, which results in release of oxygen atoms that can participate in further O3 decomposition or in the oxidation of other surface species. Basic oxides, like CaO, do not display Lewis acidity, nor do the surface OH groups have the ability to form even weak hydrogen bonds with basic molecules.6-8 However, CaO readily reacts at 300 K with acidic molecules, such as CO2, causing formation of bulk calcium carbonate, whereas at low temperatures, the reaction is restricted to the surface layer only.9 Interaction of CO2 with basic surface hydroxyl groups is known to lead to formation of bicarbonate HCO3- ions, while interaction with the surface oxygen ions yields different kinds of carbonate ions. Similarly, adsorption of SO2 leads to bisulfite and sulfite formation.10,11 Stronger basic sites appear on the CaO surface after its activation at 973 K, as shown after adsorption of CO by the appearance of “carbonite” CO22- ions.12,13 †
St. Petersburg University. Universite´ de Caen. X Abstract published in AdVance ACS Abstracts, March 15, 1997. ‡
S1089-5647(96)03879-5 CCC: $14.00
It is expected that ozone adsorption on basic sites will reveal acid properties of O3 and yield species different from those found on silica or titania. In the present work, we studied adsorption of 16O3 and 18O3 and of a mixture of partially substituted isotopic ozone modifications on calcium oxide, hydroxylated, activated at different temperatures or enriched with the 18O isotope. To elucidate the nature of the active sites, ozone adsorption was also carried out on surfaces exposed to CO. Experimental Section Ozone was prepared from commercial gaseous 16O2, 18O2 (CEA-ORIS, 99% isotopic purity), or their mixtures using an electric discharge and was purified and handled as previously reported.3-5 The stainless steel cell used for the IR studies of adsorbed species at liquid nitrogen temperature (77 K) was described elsewhere.13 The cell was fitted with KBr outer windows, while the inner windows were either BaF2 or ZnSe. The accessible spectral region was limited in the case of the ZnSe windows by the strong CaO absorption below 600 cm-1. The pressure inside the sample cell was monitored during experiments by a Barocel capacitance manometer (Datametrics 600). After introducing ozone into the cell at 77 K, a continuous pressure increase, detected for certain oxides, testifies for O3 decomposition catalyzed by the sample. When the cell is heated, the pressure always increases in accordance with the growing saturated vapor pressure of O3 condensed in the cell. Cooling the cell back to 77 K generally restores the initial pressure. If the pressure is higher than its initial value before heating, this can be considered as evidence for ozone decomposition. Activated CaO samples were prepared by treatment in vacuum at 973 K of Ca(OH)2 powder pressed into 10-40 mg/ cm2 pellets. To obtain a hydrated surface, pellets were brought into contact with saturated water vapor for several minutes at room temperature (r.t.) and then evacuated at 300 K for 1 h. Partial dehydration was achieved by evacuating the sample at 723 K. CaO enriched with 18O isotope was prepared by exposing the sample to H218O vapor at r.t. followed by pumping at 973 K. This procedure was repeated several times to obtain higher enrichment. Treatment by 18O-water vapor resulted in the appearance, apart from a strong OH-stretch absorption of Ca(OH)2, of two weak bands at 2675 and 2659 cm-1 assigned to 16OD- and 18OD- species formed due to presence of traces © 1997 American Chemical Society
2918 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Bulanin et al.
Figure 1. IR spectra of 16O3 adsorbed at 77 K on CaO activated at 723 K (1) and 973K (2-4). Curves 3 and 4 were recorded after heating at 100 K without (3) or after removal of weakly sorbed ozone by pumping at about 100 K for 5 min.4
of deuterium in the 18O-water sample. The intensity ratio of these bands, measured prior to final thermoevacuation of the sample, characterized the 18O content of the superficial layer of the oxide, which reached about 80%. IR spectra of ozone adsorbed on CaO from solution in liquid O2 were obtained by condensing oxygen gas in the sample cell and recording the spectrum of the CaO pellet immersed in oxygen. A portion of ozone mixed with oxygen was then introduced into the cell as described in ref 14. Changes in the spectra of the sample as well as that of the dissolved ozone were then determined. Spectra were recorded with a Nicolet FT-IR 710 spectrometer with 4 cm-1 spectral resolution. A germanium filter was installed in the beam before the cell to eliminate sample heating by the IR radiation. For better thermal contact of the sample with the cooled environment, about 0.5-1 Torr of helium was introduced in the sample compartment before recording spectra at the liquid nitrogen temperature. Results Effect of Surface Dehydration. IR spectra of the surface species produced by ozone adsorption on CaO strongly depend on the conditions of sample pretreatment (Figure 1). Adsorption of 16O3 at 77 K on the completely hydrated sample results in the appearance of a strong band at 1035 cm-1 accompanied by much less intense peaks at 1106 and 2106 cm-1 that correspond to the ν3, ν1, and ν1 + ν3 vibrational modes of the adsorbed molecules, respectively. These bands disappear after prolonged pumping at 77 K or after raising the sample temperature to about 120 K. The strong absorption in the OH-stretching region, evidently due to calcium hydroxide, prevents observation of the changes caused by ozone adsorption in this frequency range. No other indications for adsorbed ozone were detected on hydrated samples. Activation of the CaO sample at 723 K causes a dramatic decrease of the absorption in the OH-stretching region, leaving strong bands due to surface OH groups with several maxima between 3723 and 3710 cm-1. Ozone addition at 77 K does not affect these OH bands, while the spectrum of adsorbed O3 displays the same set of bands at almost unchanged wavenumbers and a weak absorption at about 810 cm-1 (Figure 1). In the spectrum of ozone adsorbed on CaO pretreated at 973 K, the bands appear at slightly different positions (1034.5, 1109, and 2113 cm-1) and a strong broad band emerges near 812 cm-1. A yellow coloration of the initially white sample becomes noticeable. The intensity of the 812 cm-1 band gradually grows with time, two much weaker bands show up at 625 and 1859 cm-1 (Figure 1), and the pellet color becomes deeper, turning to nearly brown.
Figure 2. IR spectra of OH (a) and OD (b) groups on CaO activated at 973 K before (1) and after (2) ozone adsorption at 77 K.
Figure 2 shows the changes in the spectrum of the surface hydroxyl groups after admission of ozone on CaO pretreated at 973 K. The band due to residual OH groups situated at 3711 cm-1 (Figure 2a) is displaced to lower wavenumbers by approximately 21 cm-1. A similar effect takes place when ozone interacts with OD groups, the band shift being 15 cm-1 in that case (Figure 2b). Prolonged evacuation of the cooled cell or raising the sample temperature to about 150 K in a closed volume removes the bands of molecular ozone and restores the initial spectrum of the hydroxyl groups. However, the perturbation of hydroxyl groups persists until almost complete desorption of molecular ozone. At small coverage, ozone fundamentals, 1125, 1039, and 716 cm-1 , appear at somewhat different frequencies than those for physisorbed molecules. Thus, it seems that some more strongly bound O3 species are responsible for the residual perturbation of the surface hydroxyls. The intensity of the 812 cm-1 band, which is almost stable at 77 K, significantly increases upon heating the sample to 100 K without evacuation. Subsequent removal of O3 by pumping results in a splitting of this band into two poorly resolved peaks at 814 and 792 cm-1 (Figure 1). These are more thermostable compared with those due to molecular ozone and remain in the spectrum up to 110 K when they disappear simultaneously with the sample coloration. Raising the temperature results also in the appearance of bands at 1671, 1635, 1490, and 1381 cm-1. As shown below, these bands characterize surface carbonate ions formed from CO2 impurities in ozone. Disappearance of the 814-792 cm-1 features is accompanied by the evolution of a gas, presumably oxygen, because it cannot be frozen out in a liquid nitrogen-cooled trap. Provided other forms of adsorbed ozone have already been removed, the quantity of the liberated gas can be determined from the pressure increase in the known volume of the cell. If we assume that our sample preparation procedure, similar to one employed in ref 13, led to the same specific surface area of 40 m2 g-1, it becomes possible to estimate the number of the generated surface sites, which is ∼0.2 nm-2.
Ozone Adsorption on CaO
Figure 3. Effect of isotopic substitution on the spectrum of ozone adsorbed at 77 K on CaO pretreated at 973 K: (1) 16O3 on Ca16O; (2) 18 O3 on Ca16O; (3) 18O3 on CaO enriched with 18O
Because of the fast ozone decomposition on CaO activated at 973 K, enhancement of the 812 cm-1 band may become inhibited before complete saturation of all active sites. To reduce the rate of O3 decomposition, which significantly increases with temperature, we performed an experiment with adsorption of ozone on CaO from a liquid oxygen solution. This made it possible to study adsorption at temperatures below 77 K, when the pressure of gaseous O3 is negligible, whereas its solubility in liquid O2 remains high enough. Moreover, immersion of the sample in a liquid reduces light scattering, greatly improving the signal-to-noise ratio and the quality of the spectra obtained. In accord with an earlier observation,15 the band due to residual surface OH groups shifts after immersion in liquid O2 toward higher wavenumbers by about 5 cm-1, demonstrating no tendency for hydrogen bonding with O2. Strong bands of dissolved O3 present in the spectrum of solution obscure most of the absorptions due to adsorbed ozone. However, the 812 cm-1 band can be recorded. Its intensity gradually grows, and it becomes approximately the same or somewhat higher than the one observed after adsorption from the gas phase. Therefore, immersion in liquid oxygen inhibits ozone decomposition but does not impede (it favors) the chemisorption characterized by the 812 cm-1 band. Effect of Isotopic Substitution. Adsorption of 18O3 on the CaO sample pretreated at 973 K also results in perturbation of the hydroxyl band, which restores its initial position and shape after ozone removal. Similar reversible changes are found when 16O is adsorbed on the CaO sample enriched with 18O. In the 3 latter case, the center of the asymmetric OH band, initially shifted to 3698 cm-1 because of the presence of the predominant 18OH surface species, undergoes an additional adsorptioninduced shift to about 3683 cm-1. The observed reversibility shows that there is no isotopic exchange between ozone and oxygen atoms of the surface hydroxyl groups. The absorption bands due to adsorbed 18O3 molecules are located at 976 (ν3), 1044 (ν1), 665 (ν2), and 1992 (ν1 + ν3) cm-1. The band corresponding to that at 812 cm-1 appears at 765 cm-1. Its position remains unchanged in the spectrum of 18O adsorbed on 18O-enriched CaO. The frequencies of 3 adsorbed molecular ozone are similarly found to be insensitive to surface isotopic substitution (Figure 3). In the spectrum of an adsorbed isotopic mixture with a ratio 16O:18O close to 1:1, the bands of all six O3 isotopomers are observed at 1034, 1026, 1018, 1005, 991, and 980 cm-1. The absorption corresponding to the 812 cm-1 band emerges as a
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2919 broad feature with a maximum at 790 cm-1. Its profile cannot be fitted by a superposition of the two bands at 812 and 765 cm-1, suggesting the existence of at least one additional band. Effect of CO2 or CO Preadsorption. To obtain additional insight into the nature of the surface sites active in ozone chemisorption and decomposition, particularly those accounting for the band at 812 cm-1, we obtained spectra of ozone adsorbed on surfaces pre-exposed to CO2 or CO. Worth noting here is that, despite all the precautions, the prepared ozone samples invariably contain traces of CO2, as indicated by the appearance of carbonate bands upon raising the temperature after ozone adsorption. It was of interest to find out to what extent the CO2 impurity could affect the surface reactions on CaO samples studied in the present work. Two kinds of CO2 treatment were triedsone at 300 K and another one at low temperature. Admission of about 20 Torr of CO2 at r.t. for several minutes into the cell with the pellet pretreated in vacuum at high temperature followed by evacuation and cooling to 77 K yields a strong absorption in the 1550-1370 cm-1 range along with less intense bands at 1069, 860, and 730 cm-1, typical of bulk carbonate species. A band due to molecular CO2 is also detected at 2355 cm-1. Subsequent ozone admission results in the spectrum of physisorbed species only. In particular, no band is observed in the vicinity of 812 cm-1. In another experiment, a small quantity of CO2, sufficient to form roughly a monolayer on the surface (about 2.8 µmol), was first admitted into the cooled cell. The temperature was then raised until a slight pressure increase indicates the presence of carbon dioxide in the gas phase and the absorption bands due to adsorbed CO2 species emerge. After that, the cell was cooled again by liquid nitrogen and then ozone was introduced. The spectrum of adsorbed CO2 so obtained is quite different from that observed after CO2 adsorption at 300 K and presents a band at 2342 cm-1 with a shoulder at 2371 cm-1, assigned to adsorbed molecular species, and strong bands due to surface carbonates at 1627, 1382, 1311 (sh), and 860-840 cm-1. The marked dissimilarity of the above spectrum in comparison with that of bulk carbonate indicates that at low temperature, even when the adsorption proceeds from the gas phase, the reaction is limited to the surface of the adsorbent. A similar observation was made before for CO2 adsorbed from liquid oxygen on CaO.9 The same bands due to surface CO32- ions have usually been observed after heating the sample with adsorbed O3. These bands can be significantly suppressed by a more thorough ozone purification. Admission of ozone onto the sample exposed to CO2 at low temperature results in the appearance of the 812 cm-1 band, as well as of the bands due to absorbed molecular species, exactly identical with those observed in the case of ozone adsorption on pure CaO surfaces. We can thus conclude that at 77 K, CO2 impurities in ozone cannot reach the sample and do not influence the studied spectra of adsorbed O3. Moreover, if even the carbonate bands appear on heating the sample, this does not strongly affect the observed ozone adsorption. Unlike CO2, carbon monoxide is found to dramatically affect the spectra of adsorbed ozone. After a small amount of CO (approximately corresponding to a mololayer) was admitted into the cell at r.t., the bands due to surface “carbonite” arose at 1485, 890, and 743 cm-1, in agreement with the results of ref 13. Then the cell was cooled by liquid nitrogen. Introduction of ozone yielded only the bands due to physisorbed O3 molecules (ν3 ) 1037 cm-1) and carbonate species. The appearance of the latter already at 77 K is evidently a result of
2920 J. Phys. Chem. B, Vol. 101, No. 15, 1997 the “carbonite” species oxidation by ozone. No band appeared at 812 cm-1. Discussion Molecular Adsorption and OH-Group Perturbation. The wavenumbers of the bands due to ozone molecular adsorption on CaO are almost similar to those reported for O3 dissolved in liquid O214 or physisorbed on SiO2 and TiO2 surfaces.3-5 The intensity ratio of the ν1 and ν3 bands is quite small, as it is for the free or weakly perturbed ozone molecules. This form of adsorbed species, henceforth referred to as physisorbed O3, is the first to disappear on evacuation. No other features due to molecular ozone have been detected after pumping CaO at 77 K except the bands at 812, 625, and 1859 cm-1 discussed in detail below. The observed displacement of the OH (OD)-stretching band under the influence of O3 shows that at least some of the adsorbed molecules interact with the surface hydroxyl groups. Ozone interaction with acidic hydroxyls on SiO2 or TiO2 surfaces was interpreted earlier as a weak H-bond.3,4 In contrast, OH-groups on the CaO surface possess a very low protondonating ability and do not reveal any tendency for forming hydrogen bonds with organic bases, like pyridine or nitriles.6,7 Immersion in liquid oxygen or nitrogen results in a slight upward shift in the OH-stretching frequency, opposite to a more or less significant decrease typical for acidic surface OH groups.15 Neither the integrated intensity nor the half-width of the OH band changes appreciably under the influence of adsorbed ozone, showing that the perturbation of OH-groups by ozone adsorbed on CaO is not to be classified as hydrogen bonding. Noteworthy is that although the band of CaO surface hydroxyls is insensitive to adsorption of bases, it completely disappears after adsorption of SO2 with the simultaneous appearance of a broad absorption feature shifted to lower wavenumbers by about 160 cm-1.11 The origin of this feature, which remains in the spectrum after desorption of molecular SO2, was explained by formation of surface bisulfite species. The electronic structure of ozone molecule is close to that of SO2, making it likely that O3 can form a coordinative complex with the oxygen atom of the hydroxyl group:
We have previously shown that the terminal oxygen atoms of O3 act as electron donors in hydrogen bonds with the surface OH groups3. Now we suggest that the acidic properties of O3 are concentrated at the central oxygen atom, which acts as an electron acceptor when interacting with the oxygen atom of a basic hydroxyl group. The moderate lowering of the OH frequency observed here points to much weaker acidic properties of ozone as compared with SO2. Indeed, the complex is weak and reversible. Moreover, the absence of any isotopic exchange between the hydroxyl oxygen and 18O-substituted ozone evidently implies that the proton is never transferred to an oxygen atom of the adsorbed O3 molecule. Hydroxyl perturbation by adsorbed ozone was observed for CaO samples pretreated at high temperature (973 K), but not for the samples activated at 723 K, presenting a strong band due to the surface hydroxyls. To our opinion, this suggests that the ability to bind ozone molecules depends on the OH-groups. According to ref 16, the band at 3711 cm-1 is assigned to type I OH groups on the (100) CaO plane. After dehydroxylation
Bulanin et al. at 973 K, most of the OH groups become removed from regular crystal faces, whereas those remaining should be localized on defects such as edges, steps, etc. The type I groups, i.e., bound to a single Ca2+ ion, would differ in the number of oxygens bound to this ion, and as a result, basic properties of the OH groups should be different. This is consistent with the structure of the OH-band observed in this study, and we propose that the above surface ozone complex could be formed only with the most basic OH groups persisting on CaO dehydroxylated at high temperature. The spectrum of ozone species responsible for the OH perturbation has not been distinctly separated. One may associate with these species the shoulders detected at about 1125, 1039, and 716 cm-1 on the high-frequency side of the physisorbed O3 fundamentals after pumping off most of the physisorbed ozone. Assuming the above assignment is correct, this points to a dissimilarity between ozone complexes formed with the weak Lewis acid sites (where the ν1 and ν3 vibrational modes move in the opposite direction4,5) and the complex formed with basic hydroxyl groups, where both fundamentals exhibit a slight frequency increase. Ozone Chemisorption on Basic Sites. Ozone is usually regarded as a source of atomic oxygen resulting from its dissociation into an oxygen molecule, which is liberated, and a residual oxygen atom that could stay attached to the surface. This was proposed4 on titania, the coordinatively unsaturated Ti4+ cations acting as strong Lewis sites and being centers of ozone dissociation. In the case of CaO, the band due to the chemisorbed species at 812 cm-1 cannot be attributed to a metal-oxygen mode for several reasons. (i) To form a bond with a metal cation, the oxygen atom should be reduced to O2-, an unlikely process for the oxidized CaO surface. (ii) The 812 cm-1 frequency is too high for a metal-oxygen stretching vibration of alkaline-earth elements not capable of forming double MdO bonds. (iii) The simultaneous appearance of bands at 1859 and 625 cm-1 and the results of the isotopically mixed ozone adsorption that points to the presence of more than two constituents due either to 16O- or to 18O-substituted products in the envelope of the corresponding band unequivocally testify that the structure in question contains at least two oxygen atoms. For basic oxides such as CaO, the sites involved in the adsorption of simple molecules, e.g., CO12,13 or NH3,17 are coordinatively unsaturated oxygen ions. Recent results of quantum chemical calculations18 show that these ions are the only sites of atomic oxygen adsorption on the CaO surface. Therefore, we may assume ozone dissociation to occur on surface oxygen ions and to result in formation of surface peroxide ions according to the scheme
O3 + O2- f O2 + O + O2- f O22- + O2v
(1)
It seems then natural to assign the 812 cm -1 band to surface peroxide O22- ions, as was suggested earlier.19 In that case, one should expect a mixed 16O18O2- peroxide species to be formed following adsorption of 18O3 on unchanged CaO, as well as the appearance of a completely substituted 18O18O2- peroxide species after adsorption of 18O3 on a surface enriched by 18O. This is not in agreement with our experimental data because completely substituted species are found after 18O3 adsorption on a nonsubstituted surface, indicating that the species formed do not contain oxygen atoms originating from the oxide surface. The above assignment has thus to be discarded. To explain our observations, we have to assume that the first step of the reaction leading to the peroxide formation is not a dissociation but an associative adsorption in which O3 forms an unstable O42- complex with the active surface oxygen:
Ozone Adsorption on CaO
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2921
O3 + O2- f O42- f O22- + O2v
(2)
If the first step is fast and reversible, it should lead, in the presence of an excess of 18O3, to substitution of the surface oxygen sites by 18O. The slower second step may yield completely substituted peroxide species. On the other hand, the assignment of the 812 cm-1 band to surface peroxide ions is still not entirely satisfactory because calcium peroxide is a relatively stable compound that does not decompose on heating below 523 K.20 Surface species normally resist heating to higher temperatures as compared with the corresponding bulk compounds. This is consistent with the recently reported observation of peroxides on MgO modified by barium by means of Raman spectroscopy.21 The band of O22- detected at 843 cm-1 appears after heating in oxygen at 573 K and persists up to 773 K. Moreover, peroxides are usually not colored, so the observation of the sample coloration in the present study remains unexplained. A more probable though less evident process is the formation of the ozonide O3- ions on activated CaO. Ozonides of alkali metals are known to be formed in ozone reaction with metal oxides. These compounds are colored. The strongest ν3 bands in their IR spectra is situated in the 818-802 cm-1 range, whereas weaker ones are noted at about 1020-1008 (ν1) and 620 (ν2) cm-1.22,23 In the present study, the ν1 band of the O3species would be masked by the strong ν3 band of physisorbed ozone. The observed absorption at 625 cm-1 could be attributed to the ν2 mode, while the weak maximum at 1859 cm-1 that appears in unison with the 812 cm-1 band may be assigned to the ν1 + ν3 combination. From the latter value, we can estimate the frequency of the ν1 vibration assuming the anharmonicity does not exceed that for the O3 molecule. The result of such estimate is 1050-1080 cm-1, very close to the values reported for the ν1 vibration of alkali metal ozonides.22,23 The mechanism for the ozonide formation is not obvious. According to the ESR data,24 electron-donor sites exist on the activated CaO but the estimated concentration of the radical sites capable of donating a single electron is 7 × 10-3 nm-2, much lower than the value of 0.2 nm-2 determined in the present study. The latter almost coincides with the earlier deduced concentration of oxygen ions capable of forming “carbonite” ions on CaO at 300 K.13 It seems surprising, however, that every basic surface oxygen accounting for the “carbonite” formation acts as a donor of a single electron toward ozone. The slow rate of ozonide formation also points to a more complex mechanism of this reaction. To our opinion, a charge disproportionation reaction involving two O3 molecules could provide an explanation of the observed spectra. One molecule adsorbed on a basic surface oxygen ion would act as a donor of electron, transferring it to another ozone molecule:
O2- + 2O3 f O42- + O3 f O4- + O3- f O2- + O3- + O2v (3) This mechanism involves the O4- intermediate. Such ions have been detected by ESR on titania25 and, according to refs 26 and 27, should also absorb in the 1000-800 cm-1 region. One would expect, however, these species to be unstable, decomposing into gaseous dioxygen and the more common superoxide O2- ion. Absorption bands of the latter were observed near 1129 cm-1 in the spectrum of O2 adsorbed on CeO2.28 After removal of most of molecular ozone, a weak band at nearly the same position is observed in the present study (Figure 1), which could be assigned to superoxide species produced as a result of this reaction. Moreover, such a
mechanism accounts well for the observed pressure increase that always accompanies an increase of the 812 cm-1 band intensity. On the other hand, in the presence of molecular oxygen always formed as a result of ozone decomposition, a charge disproportionation between dioxygen and ozone could be realized as well, leading to the appearance of two ozonide ions:
O2- + O2 + O3 f O32- + O3 f 2O3-
(4)
The processes 3 and 4 are in better agreement with the results of isotopic substitution experiments, since both should yield completely substituted 18O3- ozonide species, than the direct peroxide formation according to reaction 1. The presence of two bands at 814 and 792 cm-1 after annealing at 100 K could be explained either by the difference between ozonide species formed according to reaction 4 or by formation of ozonide and peroxide species as in reaction 3. Conclusion Interaction of ozone with the activated CaO surface is found to differ significantly from interactions with silica3 and titania4 surfaces. Besides physisorption, which occurs irrespective of the pretreatment conditions employed, ozone interacts with most basic hydroxyl groups that resist pumping at 973 K and bound to coordinatively unsaturated Ca2+ ions. This interaction causes a downward shift of the bands due to OH and OD surface groups by about 21 and 15 cm-1, respectively, and slightly increases the O3 fundamental vibrational frequencies. It is suggested that the bond is formed between the hydroxyl oxygen atom and the positively charged central atom of the O3 molecule. On CaO activated at 973 K, O3 chemisorption occurs both from the gaseous phase or from a solution in liquid oxygen. It leads to a yellow-brown coloration of the sample and to the appearance of a band at 812 cm-1 and weaker ones at 1859 and 625 cm-1, which could be attributed to the ν3, ν1 + ν3, and ν2 vibrations of surface ozonide O3- ions. The isotopic shifts observed from experiments with 18O3 adsorption on CaO surfaces of different isotopic composition are consistent with this assignment. Preadsorption of carbon dioxide in small quantities at low temperature does not affect ozone chemisorption, which, however, becomes impeded by larger doses of CO2 added at 300 K. The effect of carbon monoxide preadsorption shows ozone chemisorption to occur on the same coordinately unsaturated O2- surface species that account for the “carbonite” CO22- ions formation from CO on thermally activated CaO. To explain the mechanism of ozonide formation on such sites, a charge disproportionation between two adsorbed molecules is suggested, with the formation of superoxide O2- ions and simultaneously of molecular oxygen. The present study shows that the acidity of ozone is quite weak. It does not chemisorb or dissociate at 77 K on CaO unless the oxide is first activated at high temperature (973 K), e.g., when the strongest surface basic sites are formed. Thus, either strong Lewis acid sites, as those found on activated TiO2 surfaces,4 or strong basic sites can provoke the O3 catalytic transformation into dioxygen. Acknowledgment. The authors gratefully acknowledge invaluable assistance by J. Lamotte in performing the experiments. Part of this work was supported by the RFFI (Russian Foundation for Fundamental Researches) under Grant 94-0308550-a. K.M.B. is grateful to the Ministe`re des Affaires Etrange`res Franc¸ ais for a grant.
2922 J. Phys. Chem. B, Vol. 101, No. 15, 1997 References and Notes (1) Alekseev, A. V.; Babaeva, M. A.; Bystrov, D. S.; Tsyganenko, A. A.; Yushkov, V. A. All-Union symposium on photochemical processes in the Earth atmosphere. Moscow, Tchernogolovka, 1986; p 27 (Russian). (2) Alekseev, A. V.; Babaeva, M. A.; Bystrov, D. S.; Tsyganenko, A. A.; Yushkov, V. A. In Photochemical processes in the Earth atmosphere; Nauka: Moscow, 1990; p 20 (Russian). (3) Bulanin, K. M.; Alekseev, A. V.; Bystrov, D. S.; Lavalley, J. C.; Tsyganenko, A. A. J. Phys. Chem. 1994, 98, 5100. (4) Bulanin, K. M.; Lavalley, J. C.; Tsyganenko, A. A. J. Phys. Chem. 1995, 99, 10294. (5) Bulanin, K. M.; Lavalley, J. C.; Tsyganenko, A. A. Colloids Surf. 1995, 101, 153. (6) Tret’yakov, N. E.; Filimomov, V. N. Kinet. Catal. 1970, 11, 990(Russian). (7) Tret’yakov, N. E.; Filimomov, V. N. Kinet. Catal. 1972, 13, 735. (8) Tret’yakov, N. E.; Filimomov, V. N. Kinet. Catal. 1973, 14, 803 (Russian). (9) Tsyganenko, A. A. Adsorbtsiya i adsorbenty, No. 7; Naukova Dumka: Kiev, 1979; p 53 (Russian). (10) Low, M. J. D, Goodsel, A. J.; Takezawa, N. EnViron. Sci. Technol. 1971, 5, 1191. (11) Babaeva, M. A.; Tsyganenko, A. A.; Filimomov, V. N. Kinet. Catal. 1984, 25, 787. (12) Babaeva, M. A.; Tsyganenko, A. A. React. Kinet. Catal. Lett. 1987, 34, 9. (13) Babaeva, M. A.; Bystrov, D. S.; Kovalgin, A. Yu.; Tsyganenko, A. A. J. Catal. 1990, 123, 396.
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