Langmuir 2008, 24, 7963-7969
7963
Microcalorimetric Investigation of High-Surface-Area Mesoporous Titania Samples for CO2 Adsorption Christina Kno¨fel, Virginie Hornebecq, and Philip L. Llewellyn* Laboratoire Chimie ProVence, UniVersite´s d’Aix-Marseille I, II et IIIsCNRS, UMR 6264, Centre de Saint Je´roˆme, 13397 Marseille, France ReceiVed March 6, 2008. ReVised Manuscript ReceiVed May 1, 2008 Mesoporous titania powders were synthesized using the triblock copolymer F127 (PEO106PPO70PEO106) as a surfactant template. Two different procedures (ammonia and/or low-temperature treatment at 393 K) were successfully applied to stabilize the mesoporous structure, resulting in significantly increased surface areas and pore volumes with respect to those of the untreated titania powders. Three of these samples were chosen for further investigation by adsorption microcalorimetry. These samples are characterized by high surface areas (varying between 340 and 141 m2 g-1) and a varying degree of crystallization (anatase phase). The samples were compared to nanosized anatase particles treated to 873 K. The adsorption microcalorimetry was carried out using nitrogen and carbon dioxide at 77 and 303 K, respectively, to gain complementary information about the surfaces. Nitrogen at 77 K showed, for the three samples, adsorption enthalpies at low coverage of similar values, ∼-19 to -22 kJ mol-1, indicating that the probe gas interacts with similar energetic surface sites. Two distinct energetic regions are observed, the first of which increases with increasing pretreatment temperature, which can be related to increased sample crystallinity. The adsorption of carbon dioxide at 303 K showed high adsorption enthalpies (up to ∼65-80 kJ mol-1), highlighting strong interactions of the carbon dioxide with the titania surface at low pressures. Finally, the CO2 adsorption properties of the titania samples (adsorbed amount and enthalpies of adsorption) are compared with those of other nanosized adsorbents. This comparison shows the potentiality of mesoporous titania powders for the adsorption of CO2.
Introduction Carbon dioxide is of great strategic interest today due to its implication in the causes of global warming. It is a major combustion product in many industries ranging from energy production in power stations (fossil fuel and natural gas) to glass and cement fabrication. This industrial production of CO2 has led to a rise in atmospheric concentration from 280 ppm (ppm) at the start of the industrial revolution to around 379 ppm today.1 On the other hand, it is well established that the CO2 capture process represents 75% of the cost incurred in the global CO2 processes (i.e., capture, transport, and sequestration), and a strong need for new and cheaper technology is evident. The main solution to recover industrially produced CO2 involves the use of amine baths, which can have drawbacks in terms of additional processing with a dehydration unit, significant thermal regeneration of the solvent, and corrosion control.2 Another solution is to chemically capture CO2 using reactive oxides such as CaO.3 In this context, the use of adsorption-based processes is also envisaged involving microporous solids such as zeolites4 and activated carbons.5 In all of these cases though, regeneration of the materials, above 600 K for carbons and above 673 K for zeolites, represents a significant barrier in terms of process cost. There is thus a demand to develop energy-efficient strategies for * To whom correspondence should
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(1) Atmospheric Carbon Dioxide Record from Mauna Loa. http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm (accessed December 2007). (2) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Ind. Eng. Chem. Res. 1999, 38, 3917. (3) (a) Pereira, P. R.; Pires, J.; Carvalho, M. B. Langmuir 1998, 14, 4584. (b) Yong, Z.; Mata, V. G.; Rodrigues, A. E. J. Chem. Eng. Data 2000, 45, 1093. (4) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. J. Chem. Eng. Data 2004, 49, 1095. (5) Ranjani, V.; Siriwardane, M.-S. S.; Edward, M.; Fisher, P.; Poston, J. A. Energy Fuels 2001, 15, 279.
the separation/storage of carbon dioxide using processes with minimal environmental impact and affordable costs. In the academic literature, the use of metal organic framework (MOF) type materials for the recovery of CO2 is being suggested.6 Other materials such as amine-modified mesoporous silicas7 also show potential to adsorb significant quantities of CO2. In many of these studies, the authors highlight the capacity of these materials to adsorb large amounts of CO2. However, one has to bear in mind that the energetic aspects also have a significant role to determine how the CO2 will be adsorbed as well as for an evaluation of ease of regeneration. The adsorption energies can be obtained after the measurement of isotherms at several temperatures and the calculation of the isosteric enthalpies of adsorption. It is also possible to evaluate the adsorption enthalpies directly using microcalorimetry. In the present study, the synthesis of high-surface-area mesoporous titania has been carried out using a triblock copolymer as the structure-directing agent. Several synthesis strategies are proposed with an evaluation of the stabilization of the inorganic network by ammonia8 and/or thermal treatment.9 Nitrogen adsorption “BET” measurements of all samples were first investigated to select only those presenting a high surface area. The physicochemical properties of various selected materials were then measured using TGA, XRD, and TEM. Finally, the mesoporous solids were characterized using adsorption micro(6) (a) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998. (b) Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Millange, F.; Loiseau, T.; Fe´rey, G. J. Am. Chem. Soc. 2005, 127, 13519. (7) (a) Leal, O.; Bolivar, C.; Ovalles, C.; Garcia, J. J.; Espidel, Y. Inorg. Chim. Acta 1995, 240, 183. (b) Hiyoshi, N.; Yogo, K.; Yashima, T. Microporous Mesoporous Mater. 2005, 84, 357. (c) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2006, 45, 3248. (8) (a) Cassiers, K.; Linssen, T.; Meynen, V.; Van Der Voort, P.; Cool, P.; Vansant, E. F. Chem. Commun 2003, 1178. (b) Cassiers, K.; Linssen, T.; Mathieu, M.; Bai, Y. Q.; Zhu, H. Y.; Cool, P.; Vansant, E. F. J. Phys. Chem. B 2004, 108, 3713. (c) Beyers, E.; Cool, P.; Vansant, E. F. J. Phys. Chem. B 2005, 109, 10081. (9) Grepaldi, E. L.; A. A. Soler-Illia, G. J.; Grosso, D.; Sanchez, C. New. J. Chem 2003, 27, 9.
10.1021/la800710q CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
7964 Langmuir, Vol. 24, No. 15, 2008 Scheme 1. Synthesis Procedure
calorimetry with both nitrogen and carbon dioxide probe gases at 77 and 303 K, respectively. To compare the surface properties, the adsorption of nitrogen at 77 K for the mesoporous titania materials was compared with that of a well-characterized sample of anatase nanoparticles treated to 873 K, detailed elsewhere,10 and with that of mesoporous silica. Finally, the adsorption properties of CO2 with the mesoporous titania were compared with those of these two materials.
Experimental Section Chemicals. Titanium(IV) isopropoxide (97%) was purchased from Sigma-Aldrich. Pluronic F127 (PEO106PPO70PEO106, with PEO ) poly(ethylene oxide), PPO ) poly(propylene oxide)) was used as the surfactant in the synthesis and was procured from Sigma. 2-Butanol was purchased from Fluka. Nitric acid (g68%) and aqueous ammonia (g20%) were purchased from Prolabo. All chemicals were used as received without further purification. Synthesis of the Titania Materials. The synthesis procedure is presented in Scheme 1 and can be divided into two main parts. The first one is the preparation of the as-prepared gel based on the EISA method.11 Thus, 2-butanol, nitric acid, and F127 were mixed at room temperature using a magnetic stirrer until the surfactant was completely dissolved. Then titanium(IV) isopropoxide was added under vigorous stirring. The molar ratio of the final mixture (2butanol:nitric acid:F127:titanium(IV) isopropoxide) was 4:0.2:0.005: 1. Stirring was continued until the evaporation of the volatile species led to gelation. The as-prepared transparent mixture was aged for several days at room temperature until the weight stayed approximately constant and a transparent yellow gel was formed. After the preparation of the as-synthesized gel, the removal of the surfactant can be carried out using either a calcination or an extraction procedure. However, the removal of the surfactant by calcination is accompanied by crystallization processes which can lead to a collapse of the porous structure. Therefore, the application of additional procedures is of interest to increase the stability of the porous network. Thus, the second part of the synthesis procedure concerns the stabilization of the inorganic network using two methods. We have applied the ammonia treatment which was already used by the group of Vansant.8 The ammonia activation is known to promote the condensation of (10) Weibel, A.; Bouchet, R.; Boulc’h, F.; Knauth, P. Chem. Mater. 2005, 17, 2378. (11) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. AdV. Mater. 1999, 11(7), 579.
Kno¨fel et al. the inorganic titania network, but it can also be used to dope titania with nitrogen atoms.12 For nitrogen doping the samples are treated to at least 773 K under an ammonia stream, and the subsequent samples are reported to be yellow to blue in color depending on the nitrogen atom concentration.12 In the present case, the samples were calcined to a maximum temperature of 673 K and were white in color, thus excluding the formation of any significant amounts of nitrogen in the structure. The second method chosen for the stabilization of the inorganic network is a long thermal treatment at low temperatures (393 K for 24 h).9 Both methods are known to stabilize the porous structure of mesoporous titania by increasing the condensation of the inorganic network. Finally, for some samples, these two procedures were applied together. Both post synthesis treatments were carried out prior to surfactant elimination, which was done directly by calcination or by extraction with water followed by calcination to ensure a maximum elimination of the surfactant. All samples prepared in the present study are summarized in Scheme 1. Characterization of the Samples. Nitrogen adsorption/desorption experiments were carried out with an ASAP 2010 Micromeritics apparatus, at 77 K. Prior to adsorption, the samples (∼40-80 mg) were outgassed at 393 K overnight under a vacuum of 5 × 10-3 mbar. 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 diffraction) powder patterns were measured on a Siemens D5005 XRD diffractometer using Cu KR radiation in the 20-50° 2θ range, with a 0.02° step associated with a step time of 10 s. Microcalorimetry Measurements. The calorimetry experiments were carried out using two different set ups. Both use Tian-Calvettype calorimeters which are adapted to work at either 77 or 303 K. The calorimeter used at 77 K consists of two thermopiles of around 800 thermocouples each, mounted in electrical opposition.13 This system is immersed in a liquid nitrogen cryostat. Around 100 mg of sample is placed in a sample cell, which after outgassing is attached to a simple manometric device coupled to the calorimeter. A continuous procedure of nitrogen introduction is employed which is slow enough (approximately 2 cm3 h-1) to be close to equilibrium. Once the equilibrium conditions are verified, this procedure leads to a high resolution in both the isotherm and differential enthalpies of adsorption. The experiments carried out at 303 K again use a Tian-Calvettype calorimeter (with over 1000 thermocouples in each thermophile) which is placed in a thermostat which is regulated to within 0.01 K.13 A discontinuous gas dosing procedure is used up to a pressure of ∼1 bar. Prior to each adsorption experiment, the sample was outgassed at 393 K overnight, ensuring that the residual pressure fell below 5 × 10-3 mbar. The carbon dioxide and nitrogen were obtained from Air Liquide and are of 99.9999% minimum purity (N60).
Results and Discussion Selection of Titania Samples Presenting High Surface Areas. As was shown in the Experimental Section, different synthesis conditions were applied to study their influence on the porous network, the final aim being to obtain titania samples with high surface area. All samples listed in Scheme 1 have thus been characterized using nitrogen sorption manometry. The shapes of the isotherms for all the samples are relatively similar and can be considered as essentially of type IV, indicating the presence of mesoporosity.14 The adsorption branches were characterized by very large capillary condensation steps between p/p° ) 0.4 and p/p° ) 0.8 which suggest pore size distributions (12) Martinez-Ferrero, E.; Sakatani, Y.; Boissie`re, C.; Grosso, D.; Fuertes, A.; Fraxedas, J.; Sanchez, C. AdV. Funct. Mater. 2007, 17, 3348. (13) Llewellyn, P. L.; Maurin, G. C. R. Chim. 2005, 8, 283. (14) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. IUPAC 1985, 57(4), 603.
InVestigation of Titania for CO2 Adsorption
Langmuir, Vol. 24, No. 15, 2008 7965 Table 1. Nitrogen Physisorption Results for the Mesoporous Titania Samples sample name BET surface area, m2 g-1 total pore volume, cm3 g-1 R573 R673 AR573 AR673 AS573 S673 ASE573
Figure 1. Nitrogen adsorption/desorption results for (a) samples calcined at 573 K and (b) samples calcined at 673 K.
(determined using the BJHads method) in the range of 4-6 nm, respectively. The slope of the plateau in the isotherms at high relative pressures is characteristic of samples with small external surface areas (
