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Adsorption of Carbon Dioxide on Mesoporous Zirconia: Microcalorimetric Measurements, Adsorption Isotherm Modeling, and Density Functional Theory Calculations Virginie Hornebecq,*,† Christina Kn€ofel,† Pascal Boulet,‡ Bogdan Kuchta,‡ and Philip L. Llewellyn† Laboratoire Chimie de Provence, †Equipe MATDIV, ‡Equipe Chimie Theorique, UMR 6264, Universite de Provence - CNRS, Centre de Saint Jer^ome, F-13397, Marseille, France ABSTRACT: Mesoporous zirconia was prepared using the solgel process and the EISA method. It presents a specific surface area of 90 m2 3 g1 and an interparticular porosity associated with a pore diameter of around 45 nm, and it crystallizes in the tetragonal symmetry. The determination of the CO2 adsorption properties (both the isotherm and adsorption enthalpies) coupled with isotherm modeling using a multi-Langmuir model and density functional theory (DFT) calculations on different representative clusters evidenced that the surface of zirconia is heterogeneous from an energetic point of view. It emerges from both the experimental and the theoretical results that (i) the adsorption sites associated with the lowest enthalpies of adsorption (between 24 and 34 kJ 3 mol1) represent nearly 6570% of the total number of the adsorption sites present on the zirconia surface. They correspond to the interactions (physisorption) between carbon dioxide and oxygen atoms or hydroxyl groups of the surface. (ii) The adsorption sites associated with higher enthalpies of adsorption (around 65 kJ 3 mol1) correspond to the interactions between carbon dioxide and Zr atoms; they represent around 5% of the total amount of adsorption sites. (iii) The adsorption sites associated with high enthalpies of adsorption (below 70 kJ 3 mol1) represent only a small fraction of the adsorption sites (around 10%) and correspond probably to the interaction of CO2 with structural surface defects or charged sites.
’ INTRODUCTION In the field of adsorption, studies are still devoted to the research of materials that will specifically interact with a given molecule. This is the case, for example, of research concerning nanoporous materials that will store hydrogen13 or recover carbon dioxide.4,5 The careful evaluation of various adsorption properties of these nanoporous materials, that are usually dependent in the synthesis procedure, represents a first part of the study. In order to obtain a fuller picture of the adsorption phenomena, it is necessary, in a second part, to develop a theoretical approach that can be useful at different levels. Indeed, quantum mechanics, using density functional theory (DFT), can be used to estimate the interaction energies of a given molecule with various cluster configurations representing the adsorbent surface. These interaction energies can be, then, directly compared with values obtained experimentally using microcalorimetry. Furthermore, the adsorption isotherm itself can be modeled using various macroscopic models, leading to a further estimation of the strength and number of different adsorption sites. Finally, the information obtained from molecular modeling can be used at the synthesis level to further target materials with a given surface chemistry. Several research groups investigated the adsorption properties of carbon dioxide on zirconia in order to characterize the surface from a chemical point of view for catalytic applications using mainly infrared spectroscopy and microcalorimetry.69 These studies were performed on the different zirconia polymorphs r 2011 American Chemical Society
(cubic, tetragonal, monoclinic, and even amorphous) and on samples calcined at different temperatures crystallizing in a specific structure. All these studies were done at very low carbon dioxide pressure (p < 0.1 bar). In the present work, mesoporous zirconia has been considered as a potential candidate for carbon dioxide recovery. In a first part of this work, the solgel synthesis of mesoporous zirconia using the EISA method is briefly described. After the standard characterization of the prepared materials using adsorption/desorption measurements at 77 K, transmission electron microscopy (TEM), X-ray diffraction (XRD), and the adsorption of carbon dioxide were carried out at 303 K and up to 1 bar. Using equipment developed in-house (coupling a manometer and a microcalorimeter), both the isotherm and the differential enthalpies of adsorption were obtained simultaneously. The experimental isotherm was modeled using a multi-Langmuir approach, and the experimental enthalpies were compared to the ones derived from DFT calculations.
’ EXPERIMENTAL SECTION Chemicals. Zirconyl(IV) nitrate hydrate (97%) was purchased from Sigma-Aldrich. Pluronic F127 (PEO106PPO70PEO106; with PEO = poly(ethylene oxide), PPO = poly(propylene oxide)) was Received: February 1, 2011 Revised: April 18, 2011 Published: May 04, 2011 10097
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The Journal of Physical Chemistry C used as surfactant in the synthesis and was procured from Sigma. Aqueous ammonia (g20%) was purchased from Prolabo. All chemicals were used as received without further purification. Synthesis. Mesoporous zirconia was synthetized using conditions similar to the ones described in a previous paper for the synthesis of mesoporous titania.10 It is based on the EISA method followed by the stabilization of the inorganic zirconia network and the elimination of the surfactant template.11 Briefly, in the first synthesis step, zirconyl(IV) nitrate hydrate was dissolved in ethanol at room temperature. The surfactant template F127 was then added to the transparent solution and the mixture was stirred until gelation occurred. The transparent gel was aged for several days at room temperature. An ammonia treatment followed by a thermal treatment at 120 °C for 24 h was applied to stabilize the zirconia network. Finally, the structuring agent F127 was removed by calcination at 400 °C for 10 h in air atmosphere. This final material is named Z400. Characterization of the Samples. Nitrogen adsorption/ desorption experiments were carried out with an ASAP 2010 Micromeritics apparatus, at 77 K. Prior to adsorption, samples (∼4080 mg) were outgassed at 393 K overnight under vacuum of 103 mbar. TEM/high resolution TEM (HRTEM) micrographs were taken with a Jeol 2000FX microscope. The samples were ground and afterward suspended in ethanol. The suspension was added to a carbon grid and dried in air under a drying lamp. XRD (X-ray powder diffraction) patterns were measured on a Siemens D5005 XRD diffractometer using Cu KR radiation in the 2055° 2θ range, with a 0.02° step associated with a step time of 10 s. Carbon Dioxide Adsorption Measurements. Microcalorimetric measurements were carried out using a Tian-Calvet type isothermal microcalorimeter coupled with a manometric device built in-house.12 These combinations allowed us to obtain both the isotherms and the pseudo-differential enthalpies of adsorption as a function of the coverage in each system. Prior to each adsorption experiment, the sample was outgassed at 473 K overnight ensuring that the residual pressure fell below 5 103 mbar. The adsorption of carbon dioxide was done with point by point introduction of the pure carbon dioxide up to a pressure of ∼1 bar and at 303 K.
’ THEORETICAL APPROACH Density functional theory (DFT)13,14 has been used for the theoretical investigation of the CO2 adsorption onto ZrO2. The TPSS meta-GGA (Generalized Gradient Approach) functional has been used.15 This “ab initio” functional, which is based on the PBE16 and KPZB17 functionals, has showed to provide decent results for chemical properties, including intermolecular energies and distances in van der Waals complexes (see, e.g., ref 18). Since van der Waals (vdW) interactions may play a role in the adsorption of CO2 onto ZrO2, Grimme’s semiempirical corrections for vdW dispersion forces, the so-called DFT-D technique,19 have been included in the calculations. A triple-ζ basis set with polarization functions has been used to optimize the geometry of the structures. This TZVP basis set, developed by Ahlrichs et al.,20 has been supplemented by a set of polarization functions and a double set of diffuse functions (TZV(2df,2pd)þþ) in order to minimize the basis set superposition error while evaluating the adsorption energies. In order to speed up the calculations, the RI-J (resolution of the identity2126) has been applied to calculate the coulomb and exchange interactions. The Orca package has been utilized for all the calculations.27
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Figure 1. Nitrogen adsorption/desorption results for Z400 sample.
The adsorption of CO2 onto ZrO2 has been modeled using the cluster approach. Several clusters were chosen from the cubic structure of zirconia in order to represent various possible sites of adsorption. Although the cubic structure is stable at high temperature only, the fact that we used clusters instead of extended (surface) models should not change the results. Four types of clusters have been chosen with the formulas Zr2O14H20, Zr3O16H20, Zr3O19H26, and Zr5O24H28. These clusters are representatives of neutral and defect-free surface sites of adsorption as they exhibit either oxygen atoms, hydroxyl groups, or unsaturated zirconium atoms. Four clusters, of increasing size, are sufficient to account for these kinds of surface adsorption sites. As usual, for calculations using this type of surface model, the dangling bonds have been saturated with hydrogen atoms. As far as the authors know, no ab initio calculations dealing with the adsorption of CO2 onto ZrO2 have been reported so far in literature.
’ EXPERIMENTAL RESULTS AND DISCUSSION Porous and Crystallographic Structures. Thermogravimetric analyses (TGA) performed on the calcined material (not shown) evidence a weight loss of 3% from room temperature to 1073 K. A weight loss of about 0.8% was detected from 473 to 1073 K, indicating that almost all organic species were removed during the thermal treatment performed at 673 K. The investigation of the porous structure was done using both nitrogen sorption measurements at 77 K and TEM. The shape of the isotherm (Figure 1) is of type IV associated with a hysteresis loop indicating the presence of mesoporosity.28 Additional investigations using t-plot analysis have confirmed that no microporosity was present. The specific surface area of the zirconia Z400 was found to be equal to 90 m2 3 g1, the mean pore diameter to 45 nm and the total pore volume to 0.08 cm3 3 g1. The TEM pictures of the sample are presented in Figure 2. The porosity of the sample can be attributed to interparticular pores, and no ordered porous structure was detected. The HRTEM picture (Figure 2b) shows that the sample is well crystallized and the size of the crystallized domains can be estimated to 510 nm. The X-ray diffraction pattern of the Z400 sample is presented in Figure 3; it presents well-defined peaks at 2θ values equal to 10098
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Figure 2. TEM (a) and HRTEM (b) images of Z400 sample.
Figure 3. X-ray diffraction pattern for the Z400 sample.
Figure 4. Adsorption isotherms of carbon dioxide of Z400 sample in mmol 3 g1 and in μmol 3 m2.
30, 35, 50, and 60°. These diffraction peaks can correspond either to the cubic or to the tetragonal structures of zirconia. Nevertheless, it is well-known that the tetragonal phase of zirconia is stabilized for nanosized powders with a dimension less than 30 nm.29 Thus, Z400 sample crystallizes in the tetragonal symmetry. CO2 Adsorption Properties at 303 K. CO2 adsorption properties were studied on Z400 using a microcalorimeter coupled with a manometric device at 303 K. The adsorption isotherm as the amount adsorbed per gram as a function of pressure is presented in Figure 4. In order to evaluate the adsorption capacity of the Z400 sample with other “high” surface area zirconia materials described in the literature, the CO2 adsorbed amount per BrunauerEmmettTeller (BET) surface area as a function of pressure is also presented (Figure 4). Isotherms are of Langmuir type; the high slope at very low pressure indicates a high affinity of CO2 with the zirconia surface. In order to compare our results with the ones found in the literature, one has to take into account several factors of great importance: the synthesis procedure, the outgassing temperature, and the pressure
range. Bolis et al. found an adsorbed amount of CO2 at p = 0.04 bar of 1 and 1.5 μmol 3 m2 for a zirconia sample outgassed at 473 and 673 K, respectively.6 Bachiller-Baeza et al. reported an adsorbed amount of CO2 at p = 0.01 bar of 1.5 μmol.m2 for a zirconia sample outgased at 773 K.7 In our case, the adsorbed amount of CO2 adsorbed onto mesoporous zirconia surface outgassed at 473 K at p = 0.01 and p = 0.04 bar is ∼3 and 4 μmol 3 m2, respectively. This value is quite above the ones reported. This is not due to the difference in the outgassing temperatures, as it is known that the adsorbed amounts per unit surface area increase as this temperature increases.6 It is certainly due to the difference in the synthesis procedure (progressive surface dehydration). Compared to other porous sorbents such as mesoporous silicas, zeolites, or metalorganic frameworks (MOFs),30 the adsorption capacity of the zirconia sample at 0.9 bar (∼ 5 μmol 3 m2) is very high (even doubled). Concerning the adsorption properties from an energetic point of view, the enthalpies of adsorption, also given as function of the specific surface area, are reported in Figure 5. Very high enthalpies 10099
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The Journal of Physical Chemistry C have been measured (124 kJ mol1) at very low carbon dioxide coverage that indicates that the gas and the zirconia surface develop strong interactions. In fact, these high values are characteristic for chemisorption. With increasing carbon dioxide coverage, the enthalpies of adsorption decreases quite smoothly to ∼ 40/-35 kJ mol1 (in the range of physisorption) than remains stable at this value. This evolution indicates that different adsorption sites from an energetic point of view are involved in the carbon dioxide adsorption. This heterogeneous energetic distribution of sites is a consequence of the surface heterogeneity and can be considered as typical for amphoteric oxides.7,8 Both the values of the adsorption enthalpy at low coverage and the shape of the curve are in agreement with the ones described in the literature.68 Concerning the chemical nature of the adsorption sites, it was shown using infrared spectroscopy that different species (carbonate, bicarbonate, and linearly coordinated) are formed during CO2 uptake depending on the synthesis procedure (precursors and calcination temperature), on the crystallographic structure of zirconia, and on the outgassing temperature (surface dehydration).69 The formation of carbonate-like species (strongly and irreversibly
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bound) on a tetragonal zirconia surface was associated with high energetic process (Δadsh < 100 kJ 3 mol1). Enthalpies in the 100/40 kJ 3 mol1 range were associated with the formation of bicarbonate (hydrogencarbonate) and linearly coordinated CO2 species, with the bicarbonate adsorption being only partially reversible. All strongly adsorbed carbonate species usually involve coordinative unsaturated (CUS) Zr4þ or pairs of CUS Zr4þO2- sites. As demonstrated by the experimental results, several adsorption sites associated with different enthalpies of adsorption are present at the mesoporous zirconia surface. In order to identify and quantify these different adsorption sites, the experimental adsorption isotherm will be first fitted with a multi-Langmuir model; this will allow the achieving of the corresponding distributions of energies. Then, using DFT calculations, several adsorption sites (zirconia clusters) will be used to investigate the adsorption of carbon dioxide.
’ NUMERICAL ANALYSIS AND DISCUSSION Multi-Langmuir Model. Adsorption at 300 K makes only one layer of CO2 molecules with density lower than the complete layer coverage which corresponds to nm = 8.2 μmol 3 m-2. As has been mentioned before, the adsorption is of Langmuir type but with a distribution of adsorption site energies due to the heterogeneity of the surface as seen from measured enthalpies of adsorption (see Figure 5). Consequently, our analysis is based on a linear combination of Langmuir adsorptions:
n=nm ¼ Σxi Li ðPÞ
ð1Þ
where xi represents fractional contribution of adsorption on sites characterized by energy Ei < 0 (i = 1,...N), Li(P) are the Langmuir functions:31 exp βðμ Ei Þ Li ðPÞ ¼ 1 þ exp βðμ Ei Þ
Figure 5. Enthalpies of adsorption of carbon dioxide on Z400 sample.
where for the chemical potential μ we use the ideal gas formula: exp(βμ) = PβΛ3 (P is pressure of the CO2 gas, β = 1/kBT, Λ = h/(2πm/β)1/2, m is the CO2 molecular mass, and h is the Planck constant). We assume that the experimental adsorption isotherm may be approximated by a finite number of Langmuir functions. Initially, we assume that the number N (i = 1,...,N) of Li(P) contributions, values of surface fraction parameters xi, and
Figure 6. Comparison of experimental (solid squares) and multi-Langmuir models: (a) direct isotherm (eq 1), with adsorption energy distributions presented in the Figure 7; (b) linear representation (3-Langmuir model representation is not shown because it is visually identical with 7-Langmuir model one). 10100
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Figure 7. Distributions of energies fitted in 3-energy and 7-energy representation of the isotherm (eq 1). The heights of the columns are defined by the fractional parameters xi in eq 1. The width of the columns has no physical meaning; the dashed line is given only as an eye guide.
Figure 9. (a) Zr3O16H20 cluster; (b) CO2 adsorbed on Zr3O16H20.
Figure 8. (a) Zr2O14H20 cluster; (b) CO2 adsorbed on Zr2O14H20.
distribution of energies Ei are unknown and they are parameters of the fitting procedure. The fitting has been carried out numerically by iterative minimization of the sum of squares of errors between the experimental (measured for 13 pressures) and calculated (eq 1) values. A single Langmuir contribution cannot reproduce the experimental isotherm (see Figure 6). The minimum number of Langmuir contributions which is able to reproduce the experimental adsorption isotherm is N = 3 with the average absolute error equal 0.0062 (its maximal value is 0.0967 at very low pressure). This result is in agreement with the ones found by Keskitalo et al.32 Their kinetic model of the adsorption/desorption of CO2 onto ZrO2 catalyst includes three types of adsorption sites.
However, to obtain a better discrete representation of the adsorption energy distribution, we have used N = 7 energy values. Here, the average absolute error is 0.00121 and its maximal value is 0.0196 (also at very low pressure). Figure 6 shows the results. The comparison of isotherms is presented in the Figure 6a. Figure 6b compares the same isotherms but using the linear Langmuir representation (P/θ, where θ = n/nm). In the studied case, it is not a straight line because of the energetic heterogeneity of the adsorbing surface. This representation gives an additional verification of our fit that shows an excellent agreement between experimental and the model data. Figure 7 presents the energy distribution represented by fraction parameters xi. Both 3- and 7-energy distributions are shown; they display similar characteristics, that is, a strong contribution of energies in the range of 3060 kJ/mol and a weak contribution from energies above ∼70 kJ/mol. It is interesting to notice that even the simplest single Langmuir representation is consistent with this observation (the presented fit in the Figure 6a has been obtained for adsorption energy of 42 kJ/mol). Obviously, the quality of the fit has increased between 3- and 7-energy representations, especially for the 7-energy distribution of the energy of adsorption. This is consistent with the measured enthalpies of adsorption (formally, the model energies Ei should be increased by RT = 2.5 kJ/mol to be compared directly with the enthalpies from the Figure 5).
’ THEORETICAL CALCULATIONS USING DENSITY FUNCTIONAL THEORY AND DISCUSSION The clusters used in the calculations are presented in Figures 8a, 9a, 10a and 11a. Cluster Zr2O14H20 (Figure 8a) exhibits an oxygen atom bridging two zirconium ones and some 10101
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Figure 10. (a) Zr3O19H26 cluster; (b) CO2 adsorbed on Zr3O19H26.
OH groups onto which CO2 may interact with during adsorption. In Zr3O16H20 (Figure 9a), a central oxygen atom is capped between the three zirconium ones which are unsaturated. This cluster was used in order to study the adsorption of carbon dioxide on oxygen. By contrast, in the Zr3O19H26 cluster (Figure 10a), all the zirconium atoms are saturated with either a OH group or a water molecule (chemisorbed). Therefore CO2 may only interact with these species upon adsorption. Finally, in the Zr5O24H28 cluster, a naked zirconium atom is pointing outward the surface and may accommodate a CO2 molecule (Figure 11a). In Table 1 are given the enthalpies of adsorption of CO2 onto the clusters. The enthalpy of adsorption ranges between 65 and 25 kJ 3 mol1 depending on the nature of the atom with which CO2 interacts. The lowest energy (24.9 kJ 3 mol1) is obtained when carbon dioxide adsorbs on an oxygen of the surface, which is observed for Zr2O14H20. In this case, CO2 adsorbs parallel to the surface and it is aligned with the ZrOZr axis in order to maximize the interactions between the CO2 oxygen atoms, as can be seen in Figure 9b. The distance between the adsorbate and the surface is large (around 270 pm). The adsorption of CO2 onto an oxidized surface that exhibits hydroxyl groups is slightly stronger than that when no OH groups are involved. It is the case of Zr2O14H20 and Zr3O19H26 clusters where the adsorption enthalpy amounts to 32.6 and 33.9 kJ 3 mol1, respectively. Still, the distance between CO2 and the surface is quite large; it varies between 210 and 260 pm (see Figures 8b and 10b). Therefore, as
Figure 11. (a) Zr5O24H28 cluster; (b) CO2 adsorbed on Zr5O24H28.
Table 1. Calculated Adsorption Enthalpies of CO2 on the Zirconium Oxyde Clustersa structures
enthalpy of adsorption (kJ 3 mol1)
Zr2O14H20
32.6
Zr3O16H20
24.9
Zr3O19H26
33.9
Zr5O24H28
64.6
a
The energies are estimated at the TPSS/TZVP//TPSS/TZV(2df,2pd)þþ level of theory.
with the previous cluster, the interactions between the adsorbate and the surface are weak. This is typical of a physisorbed form of CO2. It is therefore expected that neither the sorbate nor the cluster are strongly electronically perturbed during the adsorption process On the Zr5O24H28 cluster, carbon dioxide adsorbs in apical conformation (see Figure 11b). The position where the carbon atom would interact with the zirconium one, hence forming a carbonate-like structure ZrCO2δ-, is unstable, and the optimization leads to the apical form. The adsorption enthalpy is the 10102
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The Journal of Physical Chemistry C strongest one among the studied structures, and it amounts to about 65 kJ 3 mol1. The ZrO(CO) distance is 239 pm, and CO2 is slightly tilted by 16° with respect to the normal of the plane formed by the four underlying zirconium atoms. When comparing the theoretical results with the experimental values of enthalpies of adsorption (Figure 5), we observe that the sites modeled by clusters Zr2O14H20, Zr3O16H20, and Zr3O19H26 correspond to those available at high CO2 loading where the curve exhibits a plateau. They are less energetic but probably more abundant on the surface. With the multi-Langmuir model (7-energy distribution), we found that these sites represent 6570% of adsorption sites. It is therefore likely that hydroxyl groups onto which CO2 physisorbs largely cover the surface. The adsorption on an unsaturated zirconium atom such as the one modeled by Zr5O24H28 may occur at lower coverage (
