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Langmuir 1998, 14, 3556-3564
Interaction of Carbon Dioxide with the Surface of Zirconia Polymorphs B. Bachiller-Baeza,† I. Rodriguez-Ramos,*,† and A. Guerrero-Ruiz‡ Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain, and Departamento Quı´mica Inorga´ nica, Facultad de Ciencias, UNED, 28040 Madrid, Spain Received August 1, 1997. In Final Form: April 14, 1998 A series of zirconia powder samples with different crystallographic phases and surface areas, either prepared at the laboratory or obtained commercially, have been studied in relation to the interaction of CO2 with their surfaces. Three complementary techniques such as infrared spectroscopy, adsorption microcalorimetry, and temperature-programmed desorption have been applied to the study of the CO2 adsorption. It has been found that the crystallographic structure of ZrO2 determines the number (or density) of CO2 adsorption sites on its surface, and consequently the type and stability of adsorbed species. Upon carbon dioxide adsorption on zirconia with monoclinic structure, hydrogen carbonates and monodentate and bidentate carbonates are formed, while bidentate and polydentate carbonates are generated on tetragonal zirconia. Although the bidentate carbonates are observed on monoclinic and tetragonal zirconia, they appear at different frequencies and they have different thermal stability, confirming that the surfaces of the two zirconia phases have different geometries. The results from the three techniques applied confirm that the monoclinic structure of zirconia brings about stronger surface adsorption sites concerning CO2 than the tetragonal structure. Moreover, for a given crystallographic structure the surface area and texture of the sample also affect the strength of the surface adsorption sites.
1. Introduction During the last 20 years, research on zirconium dioxide has been developed because of the excellent mechanical properties of this material.1 Properties such as low thermal conductivity, high resistance to corrosion, and high melting point (about 3000 K) have opened up new use fields of zirconia in refractories or fine ceramics. For these applications and with the aim of stabilizing the materials and of avoiding phase transformations during utilization, some dopants are generally added during material preparations.2,3 Among these dopants yttria is the most applied. On the other hand, zirconia has been claimed to be the only metal oxide whose surface exhibits four chemical properties: acidic, basic, oxidizing, and reducing.4 Two types of applications of the zirconia properties have attracted the interest of catalytic researchers: as a catalyst support due to its high thermal stability (i.e., in exhaust gas purification) and also because of its surface acid and base properties.5 Furthermore, ZrO2 has been used as a catalyst itself in many processes: hydrogenation of olefins or CO,6,7 isomerization of olefins8 or dehydration of alcohols,9 etc. These catalytic applications try to take * To whom correspondence may be addressed. E-mail:
[email protected]. † Instituto de Cata ´ lisis y Petroleoquı´mica, CSIC. ‡ Departamento Quı´mica Inorga ´ nica, UNED. (1) Garvie, R. C.; Hannink, R. H.; Pascoe, R. T. Nature 1975, 258, 703. (2) Lange, F. F. J. Am. Ceram. Soc.1986, 69, 240. (3) Alvarez, M. R.; Torralvo, M. J. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 113, 165. (4) Tanabe, K. Mater. Chem. Phys. 1985, 13, 347. (5) Yamaguchi, T. Catal. Today 1994, 20, 199. (6) He, M. Y.; Ekerdt, J. C. J. Catal. 1984, 90, 17. (7) Yamaguchi, T.; Hightower, J. W. J. Am. Chem. Soc. 1977, 99, 4201. (8) Nakano, Y.; Iizuka, T.; Hattori, H.; Tanabe, K. J. Catal. 1978, 57, 1. (9) Davis, B. H.; Lanesan, P. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18, 191.
advantage of the characteristic properties of ZrO2; that is, surface acid and base properties are found though their strength is rather weak. One example of the acid-base bifunctional character of zirconium dioxide is its capacity for CO2 and NH3 chemisoption. These acidic and basic sites on the surface of ZrO2 work independently11 and/or cooperatively12 in catalytic processes. Carbon dioxide chemisorption on zirconia has been greatly studied,8,10 particularly by application of infrared spectroscopy.13-17 In view of the importance of the knowledge of the surface properties of ZrO2, and particularly the basic sites, a systematic investigation of the interaction with CO2 has been carried out and it is presented in this article. Our aim is to detect whether the various polymorphs of ZrO2 induce different surface basic properties. For this study three different characterization methods have been used: (1) infrared spectroscopy (FTIR) for the analysis of the -OH groups after evacuation and for the adsorption of CO2; (2) microcalorimetry for the determination of the strength of the CO2 adsorption and the distribution of the surface sites; (3) temperature-programmed desorption of C18O2 to gain information about the type of surface sites and their reactivity. Other complementary characterization techniques such as X-ray diffraction, to know the crystal phases of the ZrO2 samples or N2 adsorption at 77 K to obtain the surface area, have been also applied. 2. Experimental Section Of the four studied zirconia samples, two of them are commercially available, ZrO2-h (from The Harshaw Chemical (10) Xu, B. Q.; Yamaguchi, T.; Tanabe, K. Chem. Lett. 1988, 1663. (11) Zhang, G.; Hattori, H.; Tanabe, K. Appl. Catal. 1988, 36, 189. (12) Ho¨lderich, W. F. In Proc. 10th Int. Cong. Catal. 1992, 127. (13) Tret’yakov, N. E.; Pozdnyakov, D. V.; Oranskaya, O. M.; Filimonov, V. N. Russ. J. Phys. Chem. 1970, 44, 596. (14) He, M. Y.; Ekerdt, J. G. J. Catal. 1984, 87, 381. (15) Kondo, J.; Abe, H.; Sakata, Y.; Maruya, K.; Domen, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1 1988, 84, 511. (16) Hertl, W. Langmuir 1989, 5, 96. (17) Morterra, C.; Orio, L. Mater. Chem. Phys. 1990, 24, 247.
S0743-7463(97)00856-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/28/1998
Interaction of CO2 with Zirconia Co.) and ZrO2-m (from Merck) and the other two, ZrO2-s and ZrO2-b, were synthesized by calcining in air at 773 K silicastabilized zirconium hydroxide (dried composition containing 3.5 wt % SiO2) and unstabilized zirconium hydroxide (both from MEL Chemical), respectively. The study of the ZrO2-s sample by X-ray photoelectron spectroscopy (XPS) fails to detect the presence of SiO2 at the surface. This fact indicates that the SiO2 is largely concentrated in the bulk solid. Specific surface areas of the samples were calculated by applying the BET method to the nitrogen adsorption isotherms, measured at 77 K on a Micromeritic ASAP 2000 instrument and taking a value of 0.162 nm2 for the cross section of the adsorbed nitrogen molecule. The crystalline phases present in these samples were determined from the X-ray diffraction patterns. The diffractograms were recorded on a Siefert 3000 P apparatus using Ni-filtered Cu KR radiation (λ ) 0.15406 nm) and a graphite monochromator. For each sample, Bragg’s angles between 5° and 75° were scanned at a rate of 2 deg/min. Infrared experiments were carried out using a portable glass infrared cell fitted with KBr windows and using an external furnace for heat treatments. Very thin self-supporting wafers (ca. 10 mg‚cm-2) were placed in the infrared cell and assembled with greaseless stopcocks to a vacuum system. The samples were first outgassed under high vacuum at 773 K for 2 h and then cooled to ambient temperature and exposed to CO2 (20 Torr, 1 Torr ) 133 Pa). A background spectrum prior to adsorption was made and subtracted from subsequent recorded spectra. A Nicolet SZDS spectrophotometer was used at resolution of 4 cm-1 and accumulating 100 scans. A calorimetric study of CO2 adsorption on these samples was further performed using a heat-flow microcalorimeter of the TianCalvet type (C80D from Setaram), linked to a Pyrex volumetric ramp equipped with greaseless stopcocks (SVT) that permitted the introduction of small pulses of CO2 gas. Samples were pretreated as above-described, and the adsorption temperature was maintained at 330 K. In some experiments the samples were treated under oxygen at 773 K and outgassed at 373 K for 0.5 h before cooling to the adsorption temperature. Calibration of the microcalorimeter was accomplished using the Joule effect, employing a standard cell with a Pt resistance and a EJ2 device, both supplied by Setaram (Lyon, France). The reference and laboratory cells were made of glass with SVT stopcocks. Successive doses of CO2 were sent onto the sample, until a final pressure of 7 Torr was reached. The equilibrium pressure relevant to each adsorbed amount was measured by means of a temperature-controlled pressure transducer heated at 373 K (MKS Baratron model 628). Both the calorimetric and volumetric data were stored and analyzed by microcomputer processing. The differential heats of adsorption (Qdiff) were obtained as the ratio between the exothermic integrated values of each pulse (∆Qint) and the adsorbed amount (na). For temperature-programmed desorption into vacuum experiments (TPD), the ZrO2 samples (0.05-0.250 g) were placed in an adsorption vessel and pretreated under vacuum at 773 K for 1 h. Then the sample was cooled to room temperature and a known amount of C18O2 (supplied by Isotec Inc., 95% isotopic purity) was introduced into the vessel and contacted with ZrO2 for 30 min. The residual pressure was negligible. Practically all of the C18O2 introduced into the vessel was adsorbed on the ZrO2. The TPD was run up to 773 K at a heating rate of 10 K‚min-1. The desorbed gases were analyzed by a quadrupole mass spectrometer, Balzers QMG 421C. The ion current of various products and the temperature of the sample were simultaneously collected in a personal computer.18 Also the pretreatment, consisting of heating to 773 K in an oxygen atmosphere and evacuation at 373 K for 0.5 h, was applied before some TPD experiments.
Langmuir, Vol. 14, No. 13, 1998 3557
Figure 1. X-ray diffraction patterns of the zirconia samples (A) ZrO2-h, (B) ZrO2-m, (C) ZrO2-s and (D) ZrO2-b. Table 1. Some Characteristics of the Studied Zirconia Samples sample
SBET (m2/g)
crystalline phase
ZrO2-h ZrO2-m ZrO2-s ZrO2-b
25 6 121 67
monoclinic monoclinic tetragonal and/or cubic monoclinic and tetragonal
3.1. Solid Characterizations. X-ray diffraction patterns revealed differences in the crystal structures of the four ZrO2 samples. The diffractograms of these samples
are shown in Figure 1. Commercial samples, ZrO2-h and ZrO2-m, exhibit a pure monoclinic structure.19 Synthetic zirconias, ZrO2-s and ZrO2-b, are poorly crystallized, as revealed by the broader peaks of the X-ray patterns. ZrO2-s sample can be considered as a tetragonal phase,20 whereas the recorded pattern of ZrO2-b shows the presence of both monoclinic and tetragonal phases. In the ZrO2-s pattern can be also observed a slight splitting of the reflections (200) and (220) of the tetragonal phase. These splits indicate a possible contribution of cubic phase.3 At this point, and considering the similarity of the diffractograms of cubic and tetragonal ZrO2 phase, we cannot conclude whether the ZrO2-s sample is constituted by pure tetragonal phase, pure cubic phase, or a mixture of both. It is important to note that the relative positions of Zr and O ions in the tetragonal structure of ZrO2 are very close to those in cubic structure20 while those in monoclinic ZrO2 are completely different. Table 1 summarizes these observations about the ZrO2 crystal phases. Also in this table are included the surface areas of the four ZrO2 samples, which vary from 6 m2‚g-1 for ZrO2-m up to 121 m2‚g-1 for ZrO2-s. As previously observed in the X-ray diffraction patterns, the synthetic samples are less crystalline than the commercial ones. This finding is corroborated by the higher surface area of the former. 3.2. Infrared Spectroscopy. Physicochemical characteristics of metal oxide surfaces are dependent on the pretreatment they are subjected to. Knowing this, all the samples have been similarly pretreated, that is, outgassed under high vacuum at 773 K, before any IR, TPD, or calorimetric measurement. Vacuum thermal activation of metal oxides may cause gradual surface dehydration and, at higher temperatures, the creation of surface oxygen
(18) Guerrero-Ruiz, A.; Rodrı´guez-Ramos, I. In Heterogeneous Hydrocarbon Oxidation; ACS Symposium Series 638; American Chemical Society: Washington, DC, 1996; p 347.
(19) Howard, C. J.; Hill, R. J.; Reichert, B. E. Acta Crystallogr. 1988, B44, 116. (20) Mudd, B. C.; Hannink, R. H. J. J. Am. Ceram. Soc. 1986, 69, 547.
3. Results and Discussion
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Figure 2. IR spectra of hydroxyl bands for zirconia samples outgassed at 773 K: (A) ZrO2-h and (B) ZrO2-s.
vacancies. To investigate the second aspect, we have applied an alternative pretreatment consisting in heating under oxygen at 773 K and evacuation at 373 K before the calorimetric and TPD experiments. To monitor the OH groups, we have analyzed the IR spectra of two samples with different crystalline structure in the region of O-H stretching. Many papers have reported on the hydroxyl band frequencies, either for ZrO2 monoclinic phase13,14,16,21 or for the ZrO2 tetragonal phase.16,22 It has been shown that highly hydrated zirconia is present on the surface: coordinated molecular water, which is eliminated upon evacuation at 450 K, and various types of surface hydroxyls that remain up to 870 K.17 Generally three types of O-H groups are observed with absorption bands at 3760-3780, 3730-3750, and 3650-3690 cm-1.13,16,21-25 These bands have been assigned to terminal, bibridged, and tribridged hydroxyl groups, respectively. The gradual thermal dehydration of the ZrO2 surface creates coodinatively unsaturated (cus) Zr4+ ions and coordinatively unsaturated anion-cation pairs.17 Figure 2 shows the IR spectra in the range of the hydroxyl groups (3500-3900 cm-1) for the ZrO2-h and ZrO2-s samples. The spectrum of ZrO2-h is typical of partially dehydrated monoclinic zirconia, with two bands of isolated OH groups at ca. 3770 and at ca. 3670 cm-1. These bands stem from terminal and tribridged hydroxyl groups.16,25 However, the spectrum of ZrO2-s sample with tetragonal structure reveals the virtual absence of surface terminal OH species, the bi(ca. 3735 cm-1) and tribridged (ca. 3670 cm-1) hydroxyls being dominant. The band at ca. 3735 cm-1 is typical of the tetragonal zirconia phase.16,22 Therefore, we found in agreement with earlier studies21-25 that the type of surface hydroxyls remaining upon evacuation at 773 K depends strongly on the structural modification. The nature of the OH groups present at the surface is relevant to the type of species formed upon CO2 uptake. (21) Tsyganenko, A. A.; Filimonov, V. N. Spectrosc. Lett. 1972, 5, 477. (22) Agron, P. A.; Fuller, E. L.; Holmes, H. F. J. Colloid Interface Sci. 1975, 52, 553. (23) Yamaguchi, T.; Nakano, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1978, 21, 2482. (24) Morterra, C.; Cerrato, G.; Ferroni, L.; Montanaro, L. Mater. Chem. Phys. 1994, 37, 243. (25) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579.
Figure 3. IR spectra of CO2 adsorbed on zirconia samples (A) ZrO2-h and (B) ZrO2-s. In curves are noted the outgassing temperatures.
Because carbon dioxide is acidic, it adsorbs specially on basic sites of the metal oxide surfaces. In fact, the different surface species formed upon CO2 adsorption can yield information on the existence of surface basic sites (basic hydroxyl groups and cus O2- centers) or of acid-base pair sites (cus Zr4+-O2- centers).17,26-28 After CO2 uptake at room temperature on the ZrO2-h and ZrO2-s samples, and subsequent evacuation at the same temperature, the IR bands of adsorbed species were observed as shown in parts A and B of Figure 3, respectively. This figure also displays the spectra obtained after evacuation at increasing temperatures (373-573 K). A first inspection of the figure reveals that the distribution of bands on the two ZrO2 samples is quite different. In the case of ZrO2-h (monoclinic structure), five main bands appear at 1625, 1575, 1430, 1325, and 1220 cm-1. The bands at ca. 1625 and 1430 cm-1 (asymmetric and symmetric CO stretchings) and that at ca. 1220 cm-1 (δOH) stem from hydrogen carbonates species.27 Beside these, a band at ca. 3620 cm-1 (νOH) was also observed, but it is not shown for the sake of brevity. The hydrogen carbonate species are formed by adsorption of CO2 on basic hydroxyl groups. It is generally considered that, among the different types of (26) Lavalley, J. C. Trends Phys. Chem. 1991, 2, 305. (27) Lahousse, C.; Aboulayt, K.; Mauge´, F.; Bachelier, J.; Lavalley, J. C. J. Mol. Catal. 1993, 84, 283. (28) Lavalley, J. C. Catal. Today 1996, 27, 377.
Interaction of CO2 with Zirconia
OH groups persisting after activation of the metal oxides, OH groups coordinated to only one coordinatively unsaturated cation are mainly involved.28 The following mechanism has been proposed:29
The formation of hydrogen carbonate species on ZrO2-h is coherent with the presence of terminal hydroxyls on its surface (Figure 2). Moreover, the thermal stability of the hydrogen carbonate species (Figure 3A) is very limited, being practically eliminated by evacuation at 373 K. The broad bands that appear at ca. 1575 and ca. 1325 cm-1 have a spectral separation of 250 cm-1. On the basis of the literature,16,17,30 they can be assigned to bidentate carbonate species. Note that both bands may be partly resolved into two components at ca. 1575 and ca. 1560 cm-1 and ca. 1335 and ca. 1325 cm-1, respectively. This fact becomes particularly evident when the sample is evacuated at increasing temperatures, because the relative intensity of the bidentate carbonate species characterized by the bands at ca. 1575 and ca. 1325 cm-1 decreases. Therefore, there are two slightly different bidentate carbonates, most likely corresponding to two slightly different geometrical configurations and/or different crystal planes. According to the literature,17 the surface bidentate carbonate complexes involve acid-base pair sites (cus Zr4+-O2- centers), a tentative structure for these species being
Langmuir, Vol. 14, No. 13, 1998 3559
upon evacuation at 573 K, while hydrogen carbonates are eliminated at ca. 373 K. On the other hand, the spectral features of the ZrO2-s sample (tetragonal (cubic) structure) upon CO2 adsorption at room temperature and evacuation at the same temperature are shown in Figure 3B. It can be observed that the formation of hydrogenocarbonate complexes, which is best monitored by the δOH mode at ca. 1225 cm-1, is negligible. This result confirms that the formation of hydrogen carbonates occurs mainly at the expense of terminal hydroxyl groups, whereas the contribution of biand tribridged is negligible, since the ZrO2-s sample after pretreatment at 773 K in a vacuum (Figure 2) lacks terminal OH groups. It can also be seen in Figure 3B that two families of carbonate species form, which are characterized by (i) two broad bands at ca. 1550 and ca. 1355 cm-1 and (ii) a pair of strong bands at ca. 1450 and ca. 1425 cm-1. The bands of the former species can be resolved in two components at ca. 1550 and ca. 1570 cm-1 and at ca. 1325 and ca. 1350 cm-1. The spectral separation of the antisymmetric and symmetric CO stretching is 200 and 245 cm-1 for these carbonate species, which are typical of bidentate carbonate complexes on cus acid-base pair centers.30 Note that the bands of bidentate carbonates on the ZrO2-s sample appear at different frequencies and present a different spectral ∆νCO splitting compared with the bidentate carbonates of the ZrO2-h sample, confirming that the surface of the two zirconia phases (monoclinic and tetragonal) possesses somewhat different geometries. As for the strong bands at ca. 1450 and ca. 1425 cm-1, they could be assigned to monodentate carbonate species on the basis of their small spectral separation (25 cm-1). But, considerations about thermal stability, i.e., monodentate carbonates should be normally less stable on a surface than the corresponding bidentate structure,31 make this attribution quite unlikely in the present case, as their stability parallels that of the bidentate carbonates. Species with strong thermal resistance and a rather low separation between the two C-O stretching modes correspond to planar polydentate bridging carbonate complexes whose structure can be represented as follows: 31
Finally, we can distinguish on ZrO2-h (Figure 3A) small bands at ca. 1490 and ca. 1395 cm-1, which are overlapped with the band at 1430 cm-1 stemming from hydrogen carbonates. However, these bands became clearly visible when the latter species are eliminated at 373-473 K. The spectral separation (95 cm-1) of these bands suggests the formation of monodentate carbonates over cus O2- centers of high basicity, with a structure
This species has previously been observed on monoclinic zirconia.16 As for the thermal stability of the different carbonate species, we can observe (Figure 3A) that bidentate and monodentate species still remain at the ZrO2-h surface (29) Tsyganenko, A. A.; Trusov, E. A. Russ. J. Phys. Chem. 1985, 59, 2602.
the three carbonate oxygens are bonded to metal ions. Therefore, the formation of polydentate carbonate species implies a large amount of closely spaced cus cationic centers. This picture is consistent with the ZrO2-s sample, which has low crystallinity (Figure 1) and then high surface heterogenity. When the temperature is raised (Figure 3B), all the bands resulting from CO2 adsorbed species are completely removed at 573 K. Then the carbonate species on the ZrO2-s sample (tetragonal structure) have lower stability than those on the ZrO2-h sample (monoclinic structure). The interaction with CO2 and the nature and variety of adspecies formed with CO2 at the surface of zirconia pretreated at 773 K in a vacuum depend on the crystalline (30) Morterra, C.; Cerrato, G.; Ferroni, L. J. Chem. Soc., Faraday Trans. 1995, 91, 125. (31) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89.
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Figure 4. Isotherms of carbon dioxide adsorption on ZrO2 samples at 330 K: (4) ZrO2-h, (3) ZrO2-m, (O) ZrO2-s; (0) ZrO2b.
bulk structure. The study by infrared spectroscopy of the structure of these species (hydrogen carbonates and monodentate, bidentate, or polydentate carbonates) as well as their thermal stability is essential for the interpretation of the results obtained by adsorption microcalorimetry and temperature-programmed desorption, which are discussed below. 3.3. Microcalorimetry of CO2 Adsorption. Volumetric isotherms corresponding to carbon dioxide adsorption on ZrO2 samples are given in Figure 4. The shape of the adsorption isotherms is in general characterized by a rapid increase in the amount adsorbed at negligible pressures (P < 1 Torr), well-defined knees, and a small increase that became almost constant at equilibrium pressures higher than 2 Torr. The amount of CO2 adsorbed for each sample is given per square meter of surface area (SBET (Table 1)). In this way, we have an average value of the density of adsorption sites (basic hydroxyl groups, cus O2- centers and cus Zr4+-O2- pair centers) present at the surface of the various ZrO2 samples. By a simple extrapolation of the linear part of the isotherm to zero pressure we can determine the density of sites (d) involved in the CO2 adsorption. The values of d are 1.2 µmol/m2 for ZrO2-s, 1.8 µmol‚m-2 for ZrO2-m, 2.5 µmol‚m-2 for ZrO2h, and 2.8 µmol‚m-2 for ZrO2-b. Considering apart the ZrO2-m sample whose SBET determination by N2 adsorption at 77K can have a somewhat higher error due to its rather low surface area (6 m2‚g-1), some conclusions can be drawn comparing the d values. First, d is not directly related to the surface area of ZrO2 samples, the minimum d being observed for the ZrO2-s sample that has the highest SBET (Table 1). Comparison of ZrO2-h and ZrO2-s samples seems to indicate that d is higher at the surface of a monoclinic structure. More complex is the situation of the ZrO2-b sample, which has a mixture of monoclinic and tetragonal structures and for which the d value is maximum. In short, the density of CO2 adsorption sites at the surface of ZrO2, which accounts for the number of basic sites, seems to depend on the crystallographic structure of the oxide and also on other aspects, such as the texture and preparation method of the ZrO2 sample. Further investigations are required to go deeply into this point. From Figure 4 it is seen that the shape of the CO2 adsorption isotherms is somewhat different for the distinct
Figure 5. Differential heat of carbon dioxide adsorption on ZrO2 samples: (∆) ZrO2-h, (∇) ZrO2-m, (O) ZrO2-s; (0) ZrO2-b.
zirconia samples. Clearly the isotherm knee of the ZrO2-s sample (tetragonal structure) is more pronounced than those of the isotherms obtained for zirconia samples (ZrO2-h and ZrO2-m) with monoclinic structure. The interpretation of these variations in the isotherm shape for a chemisorption phenomenon (irreversible) is not an easy task. However, the shape of the isotherm could be related to the amount and/or the energetic distribution of surface adsorption sites on the various ZrO2 samples. Anyway, the study of distribution of surface adsorption sites would be accomplished from the microcalorimetric measurements (see below). Figure 5 depicts the differential heats of CO2 adsorption (Qdiff) as a function of the adsorbed amount (µmol‚m-2) for the ZrO2 samples. Comparison of Qdiff curves for two ZrO2 samples, with the same crystallographic structure but exhibiting different surface areas (Table 1), that is, ZrO2-h and ZrO2-m, reveals that the energy distribution of surface CO2 adsorption sites on ZrO2 is largely controlled by the surface area of the metal oxide, that is, the higher the value of the surface area, the stronger the energy of the adsorption sites. As the surface area is (usually) directly related to the abundance of defective sites (edges, kinks, corners), it appears that more energetic surface CO2 species are formed on these defective sites. On the other hand, comparison of Qdiff profiles for ZrO2-h and ZrO2-s samples, with 25 and 121 m2‚g-1, respectively, shows that the former sample displays stronger adsorption modes than the second. Thus, it can be inferred that the crystallographic structure is the main factor determining the nature and density (d) of the surface sites involved in the adsorption of CO2. The ZrO2-b sample, with a mixture of monoclinic and tetragonal structures and higher surface area than ZrO2-h, is among all the zirconia samples studied in this work that possesses the strongest surface adsorption sites for CO2 (the strongest basic centers) (Figure 5).
Interaction of CO2 with Zirconia
Figure 6. Differential heat of carbon dioxide adsorption on ZrO2-h sample (2) after treatment under vacuum at 773 K and (∆) after treatment in oxygen atmosphere at 773 K and outgassing at 373 K.
At very low coverages, the values of initial differential heats of CO2 adsorption follow the order ZrO2-b > ZrO2-h > ZrO2-s > ZrO2-m, varying between 139 and 100 kJ/mol. Figure 5 also shows that the energetic distribution of the adsorption sites concerning CO2 is very heterogeneous for all the samples. These results agree with those published in the literature for the CO2 interaction with ZrO2.32 The heterogeneous energetic distribution of sites is a consequence of the surface heterogeneity and can be considered as typical of amphoteric oxides. The shape of the differential heat curves also reflects the repulsive interactions between the adsorbate molecules. On the other hand, these results corroborate those from the infrared spectroscopic study, that is, the ZrO2-h sample presents at the surface stronger CO2 adsorption sites compared with ZrO2-s (Figure 3). According to infrared spectroscopy results the stronger adsorption mode on the ZrO2-h sample can be referred to bidentate carbonate species. Therefore, the adsorption sites with higher basicity are associated to cus Zr4+-O2- pairs. In conclusion, the microcalorimetric study shows that the bulk structure of zirconia seems to determine the number and the energetic distribution of the surface carbon dioxide adsorption sites. Another important point in the study of metal oxide surfaces is the effect of treatments on the generated reactive sites. In Figures 4 and 5 all samples were pretreated under high vacuum at 773 K before the adsorption. To gain information on the nature of the stronger adsorption sites, sample ZrO2-h was also submitted to treatment in an oxygen atmosphere at 773 K followed by vacuum at 373 K. The second experimental condition was avoiding the formation of anionic vacancies at the ZrO2 surface. Figure 6 displays the differential heats of CO2 adsorption on the ZrO2-h sample pretreated under the two above-described conditions. It can be seen that the differential heats of CO2 adsorption fully match in the field of medium and low values. However, the stronger sites for CO2 adsorption disappear when the evacuation at relatively high temperature (773 K) is precluded. Infrared study of CO2 has shown that for this (32) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371.
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Figure 7. Profiles corresponding to temperature-programmed desorption of CO2 from zirconia samples with a surface coverage of 0.25 µmol‚m-2.
sample, the more stable CO2 adsorption complexes are bidentate carbonate species. We can deduce that the formation of the stronger bidentate carbonates requires an acid-base pair center associated to an anionic vacancy, which will generate electron-rich zirconium cations. As basicity is related with electron-donor properties, the stronger the electron-donating power, the larger the basicity
3.4. Temperature-Programmed Desorption of C18O2. The temperature-programmed desorption (TPD) of adsorbed species is another useful method to gain information about the strength of the adsorbateadsorbent interactions.33 When these experiments are carried out using various surface coverages, the relative distribution of surface sites for a given adsorption process can be obtained. For this reason we have determined the TPD profiles for the different ZrO2 samples after contacting with amounts of CO2 leading to surface coverages of 0.25, 1.25, and 2.5 µmol‚m-2. Figure 7 shows the TPD profiles corresponding to a coverage of 0.25 µmol‚m-2 CO2. These experiments were carried out over the higher surface area samples, that is, ZrO2-s, ZrO2-h, and ZrO2-b. Similar curves were obtained for higher surface coverages but for the sake of brevity they are not given. In a first approximation it is possible to estimate desorption energies (Ed) from the temperature of the desorption peaks using a semiempirical correlation.34 If Tm is the temperature of the peak maximum, then Ed (kJ‚mol-1) ≈ 0.23 Tm (K). Using this equation we have determined the desorp(33) Falconer, J. L.; Shwarz, J. A. Catal. Rev.-Sci. Eng. 1983, 25, 141. (34) Knorr, Z. In Catalysis Science and Technology; SpringerVerlag: Berlin, 1982.
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Table 2. Temperature of the Desorption Peaks and Desorption Energies Calculated from the CO2 TPD Profiles at Different Surface Coverages sample
coverage
Tm (K)
Ed (kJ‚mol-1)
Qaver (kJ‚mol-1)a
ZrO2-h
0.25 1.25 2.5 0.25 1.25 2.5 0.25 1.25 2.5 0.25 1.25 2.5
575 485 423 530 410
132 112 97 122 94
127 115 105 120 100
635 523 470
146 120 108
400 400
92 92
137 126 113 101 92 72
ZrO2-s ZrO2-b ZrO2-m
a
Data from Figure 5.
tion energy values reported in Table 2. To compare these data with those from microcalorimetric measurements, we have chosen the average heats of adsorption and not the initial or integral heats that are less representative of the average value of the adsorption site strength. The average heats of CO2 adsorption are also given in Table 2 for the different surface coverages of the zirconia samples. In general, there is a good agreement between Ed values and Qaver determinations. Therefore, both techniques (TPD and adsorption calorimetry) concur in the order of strength of adsorption sites (ZrO2-b > ZrO2-h > ZrO2-s) and in the density of adsorption sites (µmol‚m-2). TPD results confirm that the monoclinic structure of zirconia brings about stronger surface adsorption sites concerning CO2 than the tetragonal structure. Moreover, for a given crystallographic structure the surface area and texture of the sample also affect the strength of the surface adsorption sites (ZrO2-b vs ZrO2-h). The profiles shown in Figure 7 for CO2 desorption from ZrO2 samples correspond to the total amount of CO2, that is, the addition of contributions of various isotopic labeled molecules (C16O2, C16O18O, and C18O2). However, as indicated in the Experimental Section we have adsorbed pure gas C18O2 on the samples. Complementary information on the nature of adsorbed species and their reactivity35,36 can be achieved by the analysis of the isotopic distribution of the three labeled carbon dioxides. Figure 8 shows for ZrO2-h sample the different labeled CO2 species produced during TPD as a function of the ZrO2 surface coverages. For the lowest coverage (0.25 µmol‚m-2) a practically total exchange between the oxygen atoms of the C18O2 adsorbed and the ZrO2 surface oxygens takes place arising C16O2 as the major TPD product. This extensive exchange corresponds with the strongest adsorbed species. For the highest coverage (2.5 µmol‚m-2) can be clearly distinguished the three isotopic CO2 (C18O2, C16O18O, and C16O2), with relative contributions that depend on the desorption temperature range. Close relationships were observed for the other ZrO2 samples, concerning the effect of the concentration of adsorbed C18O2 over the resulting isotopic distributions. Usually, there is a stepwise occupation of the various adsorption sites; that is, at lower coverages the more strongly bound sites are preferred. Moreover, different kinds of adsorption sites bring about different isotopic distributions. Therefore, species that are observed by infrared spectroscopy and whose desorption temperature is well-known can (35) Tsuji, H.; Shishido, T.; Okamura, A.; Gao, Y.; Hattori, H.; Kita, H. J. Chem. Soc., Faraday Trans. 1994, 90, 803. (36) Yanagisawa, Y.; Takaoka, K.; Yamabe, S. Stud. Surf. Sci. Catal. 1993, 90, 201.
Figure 8. Isotopic distribution of carbon dioxide species evolved during temperature-programmed desorption for ZrO2-h sample after different initial surface coverages: (A) 0.25 µmol‚m-2, (B) 1.25 µmol‚m-2; (C) 2.5 µmol‚m-2. Signals from mass spectrometer are as follows: ‚ - ‚, C18O2, - - -, C16O18O; ‚‚‚ C16O2; s, total CO2 .
become more clearly defined by the use of isotopic CO2 and the measurement of the isotopic distribution.35 To compare the oxygen exchange reaction between adsorbed CO2 and ZrO2 surfaces for the various zirconias, in Figure 9 are given the TPD profiles corresponding to the various isotopic-labeled CO2 evolved, for an initial surface coverage of 1.25 µmol‚m-2. It can be observed that under these conditions the maximum oxygen exchange occurs on ZrO2-b and the minimum for ZrO2-m. Anyway, the extent of the oxygen exchange reaction seems to depend mainly on the desorption temperature range considered, that is, the strength of the adsorption site. In a general view, Figure 9 shows the following order for the extent of the oxygen exchange reaction: ZrO2-b > ZrO2-h > ZrO2-s > ZrO2-m. This order is in full agreement with the stability of the surface CO2 species detected by FTIR and with the values of initial differential heats of CO2 chemisorption revealed from adsorption microcalorimetry. Correlation of the CO2 chemisorbed species identified by IR to a desorption region; that is, an isotopic distribution, is somewhat risky, but some point can be raised. For the sake of clarity the TPD profile is divided in three regions. The first region ranges from room temperature to 373 K, the second one from 373 to 573 K, and the third above 573 K:
Interaction of CO2 with Zirconia
Langmuir, Vol. 14, No. 13, 1998 3563
Figure 9. Comparison of isotopic distribution of carbon dioxide species evolved during TPD after an initial surface coverage of 1.25 µmol‚m-2 C18O2. Samples were (A) ZrO2-h, (B) ZrO2-m, (C) ZrO2-s, and (D) ZrO2-b. Signal from mass spectrometer as in Figure 8.
(1) Hydrogen carbonates were detected by FTIR on aZrO2-h sample, with a thermal stability limited to 373 K under vacuum (Figure 3A). These species only appear at higher surface coverages (Figure 8C), and C16O18O seems to be the major isotopic species evolved during TPD. It is obvious that one of the oxygen atoms of the hydrogen carbonate structure C-OH and /or CdO is able to be exchanged with the oxygen surface ions, at temperatures as low as 340 K. (2) By infrared spectroscopy were detected bidentate carbonates both on ZrO2-h and on ZrO2-s samples, the CO2 being adsorbed on a cus acid-base pair center of the surface. In the first case the bidentate carbonates still remain at 573 K under vacuum (Figure 3A) while in the second they are eliminated at temperatures above 473 K (Figure 3B). Observation of CO2 TPD profiles of these two samples for an intermediate surface coverage (parts A and C of Figure 9) reveals that bidentate structures give rise to C16O2 and C16O18O in a ratio close to 1:1 in the temperature range of 373-473 K. The isotopic distribution expected for the decomposition of a species bound to the surface through two bonds, one between the C in CO2 and the surface O and the other between an O in CO2 and Zr, is a 1:1 mixture of C16O18O and C18O2 if both C-O bonds are equivalent. On ZrO2-h the only adsorbed species in this temperature range are bidentate carbonates, and then C16O2 should not have been included in the desorbed CO2. We can conclude that the type of adsorbed species is not the only factor controlling the extension of oxygen isotopic exchange] and that processes other than simple adsorption-desorption are involved. Two possibilities for the explanation of the multiple exchange reaction can be considered. A repetitive adsorption-desorption of CO2 molecules or a CO2 migra-
tion over the surface without leaving the ZrO2 surface.35 The first interpretation is not consistent with that found in Figure 8, that is, an enhanced oxygen-exchange at lower surface coverage. In the experimental conditions for Figure 8A, the partial pressure of CO2 after admission of one molecule per 664 Å2 was negligible, which is evidenced by the absence of a desorption below 473 K. Therefore, it is excluded that the multiple oxygen exchange between CO2 and the surface oxygen occurs through repetitive adsorption-desorption of CO2 molecules. Thus, we can assume that the extensive exchange results from the adsorbed CO2 migrating over the surface without leaving it.18 On ZrO2-s, carbonate species have been detected with high stability assimilated to polydentate carbonates. The high extent of the oxygen exchange reaction in the region between 473 and 573 K supports the presence of polydentate species on the surface of the ZrO2-s sample. (3) At temperatures higher than 573 K the major isotopic form of CO2 was C16O2. The absence of partially exchange species indicates that the adsorbed CO2 species migrate over the surface as the temperature is raised in the TPD. Finally, to analyze how the pretreatment conditions affect the oxygen isotopic exchange reaction, and particularly to distinguish the nature of the highest stable carbonates on ZrO2-s, we have applied an alternative pretreatment to this sample. In Figure 10 are shown the TPD plots for each type of isotopic labeled CO2 after C18O2 adsorption on the ZrO2-s sample pretreated: (i) under vacuum at 773 K (general condition used in this work) and (ii) in oxygen at 773 K and outgassed at 373 K. We found that the high-temperature pretreatment under oxygen brings about a decrease in the number of the stronger adsorption sites. A similar effect was evidenced over the ZrO2-h sample by adsorption microcalorimery
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dioxide adsorption on ZrO2-s sample would imply a large amount of closely spaced cus cationic centers. This picture agrees with the increasing intensity of these carbonate species over the sample treated in a vacuum at 773 K on which the cus surface cations should increase.
Figure 10. Comparison of isotopic distribution of carbon dioxide species evolved from ZrO2-s during TPD (A) after treatment under vacuum at 773 K and (B) after treatment in oxygen atmosphere at 773 K and outgassing at 373 K. Surface coverage was 1.25 µmol‚m-2 C18O2.
(Figure 6) and by TPD (not shown for the sake of brevity). The formation of polydentate carbonates upon carbon
4. Conclusions This study shows that the zirconia polymorphs exhibit different adsorption properties concerning CO2 on their surfaces. From the application of complementary techniques such as infrared spectroscopy, adsorption microcalorimetry, and temperature-programmed desorption, it can be deduced that the crystallographic structure of ZrO2 determines the number (or density) of CO2 adsorption sites on its surface and, consequently, the type and stability of adsorbed species. The uptake of carbon dioxide on zirconia with monoclinic structure yields hydrogen carbonates and monodentate and bidentate carbonates, reflecting that its surface basicity is due to terminal hydroxyl groups, cus O2- centers, and cus Zr4+-O2- pairs. Upon carbon dioxide adsorption on tetragonal zirconia, bidentate and polydentate carbonates are generated, which reflects the presence at the surface of cus Zr4+-O2pairs and closely spaced cus Zr4+ centers. Although the bidentate carbonates are observed on monoclinic and tetragonal zirconia, they appear at different frequencies and they have different thermal stability, indicatings that the basicity of the cus Zr4+-O2- pair centers is different on both surfaces. This fact confirms that the surfaces of the two zirconia phases have different geometries. The results from the three techniques applied confirm that the monoclinic structure of zirconia brings about stronger surface adsorption sites concerning CO2 than the tetragonal structure. These adsorption sites with stronger basicity are associated to cus Zr4+-O2- pairs. Moreover, for a given crystallographic structure the surface area and texture of the sample also affect the strength of the surface adsorption sites, that is, the higher the value of the surface area, the stronger the energy of the adsorption sites. Acknowledgment. The authors thank the CICYT and DGICYT of Spain for finantial support under projects MAT96-08559-CO2-O2 and PB94-0077-CO2-O2, respectively. LA970856Q
