12410
Langmuir 2008, 24, 12410-12419
Adsorption and Protonation of CO2 on Partially Hydroxylated γ-Al2O3 Surfaces: A Density Functional Theory Study Yunxiang Pan,†,‡ Chang-jun Liu,*,‡ and Qingfeng Ge*,† Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901, and Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, China ReceiVed July 17, 2008. ReVised Manuscript ReceiVed August 25, 2008 Adsorption and protonation of CO2 on the (110) and (100) surfaces of γ-Al2O3 have been studied using density functional theory slab calculations. On the dry (110) and (100) surfaces, the O-Al bridge sites were found to be energetically favorable for CO2 adsorption. The adsorbed CO2 was bound in a bidentate configuration across the O-Al bridge sites, forming a carbonate species. The strongest binding with an adsorption energy of 0.80 eV occurs at the O3c-Al5c bridge site of the (100) surface. Dissociation of water across the O-Al bridge sites resulted in partially hydroxylated surfaces, and the dissociation is energetically favorable on both surfaces. Water dissociation on the (110) surface has a barrier of 0.42 eV, but the same process on the (100) surface has no barrier with respect to the isolated water molecule. On the partially hydroxylated γ-Al2O3 surfaces, a bicarbonate species was formed by protonating the carbonate species with the protons from neighboring hydroxyl groups. The energy difference between the bicarbonate species and the coadsorbed bidentate carbonate species and hydroxyls is only 0.04 eV on the (110) surface, but the difference reaches 0.97 eV on the (100) surface. The activation barrier for forming the bicarbonate species on the (100) surface, 0.42 eV, is also lower than that on the (110) surface (0.53 eV).
1. Introduction Global climate change caused by greenhouse gases such as CO2 has been a major concern over the past few years.1 Conversion of CO2 into useful products such as methanol and syngas not only will be economically beneficial but also will help to ease the CO2 emission into the atmosphere.1-13 Transition metal particles supported on oxides such as γ-Al2O3 have been shown to be effective for catalytic conversion of CO2.11-16 A good dispersion of the metal particles can be achieved on γ-Al2O3 due to the high porosity and specific surface area of the oxide.14,15 Furthermore, the presence of both acidic and basic sites on the γ-Al2O3 surfaces will undoubtly affect the reactions involving * To whom correspondence should be addressed. E-mail: ughg_cjl@ yahoo.com (C.-j.L.);
[email protected] (Q.G.). † Southern Illinois University. ‡ Tianjin University.
(1) Kno¨fel, C.; Hornebecq, V.; Llewellyn, P. L. Langmuir 2008, 24, 7963. (2) Song, C. Catal. Today 2006, 115, 2. (3) Zhu, X.; Zhang, Y.; Cheng, D.; Liu, C. Appl. Catal., B 2008, 81, 132. (4) Campbell, J. M.; Reiff, S.; Block, J. H. Langmuir 1994, 10, 3615. (5) Pan, Y.-X.; Liu, C.-J.; Shi, P. J. Power Sources 2008, 176, 46. (6) Ross, J. R. H. Catal. Today 2005, 100, 151. (7) Ruckenstein, E.; Wang, H. Y. Appl. Catal., A 2000, 204, 257. (8) Wang, S. B.; Lu, G. Q. Energy Fuels 1996, 10, 896. (9) Ruckenstein, E.; Hu, Y. H. J. Catal. 1996, 162, 230. (10) Wang, S.; Lu, G. Q. Energy Fuels 1998, 12, 248. (11) Cheng, D.; Zhu, X.; Ben, Y.; He, F.; Cui, L.; Liu, C. Catal. Today 2006, 115, 205. (12) Qi, G.-X.; Zheng, X.-M.; Fei, J.-H.; Hou, Z.-Y. Catal. Lett. 2001, 72, 191. (13) Keskitalo, T. J.; Veringa Niemela¨, M. K.; Krause, A. O. I. Langmuir 2007, 23, 7612. (14) Valero, M. C.; Digne, M.; Sautet, P.; Raybaud, P. Oil Gas Sci. Technol. 2006, 61, 535. (15) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. J. Catal. 2004, 226, 54. (16) Valero, M. C.; Raybaud, P.; Sautet, P. J. Catal. 2007, 247, 339. (17) Tai, J.; Ge, Q.; Davis, R. J.; Neurock, M. J. Phys. Chem. B 2004, 108, 16798. (18) Collins, S. E.; Baltana´s, M. A.; Bonivardi, A. L. J. Phys. Chem. B 2006, 110, 5498. (19) Lavalley, J. C.; Daturi, M.; Montouillout, V.; Clet, G.; Area´n, C. O.; Delgado, M. R.; Sahibed-Dinde, A. Phy. Chem. Chem. Phys. 2003, 5, 1301. (20) Vimont, A.; Lavalley, J. C.; Sahibed-Dinde, A.; Area´n, C.O.; Delgado, M. R.; Daturi, M. J. Phys. Chem. B 2005, 109, 9656.
CO2.16 In fact, adsorption of CO2 has been used to characterize the basicity of solid-base catalysts.17-20 In order to better understand the role that γ-Al2O3 plays in CO2 conversion and elucidate the key factors that affect the selectivity and activity of the catalysts, we performed a detailed characterization of CO2 interaction with the γ-Al2O3 surfaces using the density functional theory (DFT) method. The adsorption of CO2 on γ-Al2O3 has been a subject of numerous experimental studies.21-28 Manchado et al.21 conducted a series of volumetric and microcalorimetric measurements and suggested that three energetically different species, including weakly adsorbed CO2, carbonate, and bicarbonate species, can be formed upon CO2 adsorption. The bicarbonate species was attributed to the reaction between CO2 and the surface hydroxyls. Combining Fourier transform infrared (FTIR) spectroscopy with isotope labeling, Baltrusaitis et al.22 also observed the formation of carbonate and bicarbonate species upon CO2 adsorption. Furthermore, they suggested that the bicarbonate species bound to the surface in a bridged structure. A nucleophilic attack of adsorbed CO2 by the surface hydroxyls followed by a rearrangement on the surface was proposed to be responsible for the formation of the bridged bicarbonate structure. Rosynek23 measured the adsorption isotherms and isosteric heats for CO2 in the temperature range of 100-300 °C. The author identified that the carbonate species was the dominant surface species at high temperatures and low CO2 coverages whereas bicarbonate was formed at low temperatures ( Cs2O2 > CsO2, in agreement with the traditional ranking of basicity. Casarin et al.29 studied CO2 adsorption on R-Al2O3 using density functional molecular cluster calculations. They found that CO2 interacts with the R-Al2O3(0001) surface to form a bidentate-chelating carbonate species. The process involves CO2 acting as both electron acceptor and donor: (i) the CO2 2πu (LUMO) orbitals accepted electrons from the occupied states of the surface basic sites; (ii) the CO2 1πg (HOMO) orbital donated its electrons to the empty states of the surface acidic sites. There have been many studies on CO2 interaction with different oxides.17,29-35 However, theoretical studies of using γ-Al2O3 as a substrate were not common due to its structural complexity.34 Both spinel and nonspinel structural models have been proposed for γ-Al2O3.14,15,34-46 The nonspinel model has been well characterized both experimentally and computationally.14,15,36-43 Moreover, the nonspinel model produces diffraction patterns that are close to the characteric diffaction patterns of γ-Al2O3.38-40 Digne et al.15 performed a detailed thermodynamical analysis for different surfaces of the nonspinel γ-Al2O3 as a function of temperature at various hydroxyl coverages. The analysis helped to establish that the (110) and (100) surfaces with varying degrees of hydroxylation are dominant. Herein, we chose the (110) and (100) surfaces and studied the adsorption and protonation of CO2 on these surfaces using the DFT slab calculations. Stable adsorption structures as well as transition states for both the hydroxylation of dry γ-Al2O3 surfaces and the protonation of adsorbed CO2 have been determined. We anticipate that results from the study of CO2 adsorption and protonation on the ideal surfaces will provide some insights into chemistry of the γ-Al2O3 surfaces. This paper is organized as follows. Computational methods and models are described in section 2. Results and discussion are given in section 3. Finally, the main conclusions are presented in section 4. (29) Casarin, M.; Falcomer, D.; Glisenti, A.; Vittadini, A. Inorg. Chem. 2003, 42, 436. (30) Mei, D.; Deskins, N. A.; Dupuis, M.; Ge, Q. J. Phys. Chem. C 2007, 111, 10514. (31) Pejov, L.; Skapin, T. Chem. Phys. Lett. 2004, 400, 453. (32) Maresca, O.; Allouche, A.; Aycard, J. P.; Rajzmann, M.; Clemendot, S.; Hutschka, F. J. Mol. Struct. 2000, 505, 81. (33) De Vito, D. A.; Gilardoni, F.; Kiwi-Minsker, L.; Morgantini, P.-Y.; Porchet, S.; Renken, A.; Weber, J. J. Mol. Struct. 1999, 469, 7. (34) Trueba, M.; Trasatti, S. P. Eur. J. Inorg. Chem. 2005, 17, 3393. (35) Cai, S.; Sohlberg, K. J. Mol. Catal., A 2003, 193, 157. (36) Paglia, G.; Buckley, C. E.; Rohl, A. L.; Hunter, B. A.; Hart, R. D.; Hanna, J. V.; Byrne, L. T. Phys. ReV. B 2003, 68, 144110. (37) Paglia, G.; Buckley, C. E.; Rohl, A. L.; Byrne, L. T. J. Australas. Ceram. Soc. 2002, 38, 92. (38) Menendez-Proupin, E.; Gutierrez, G. Phys. ReV. B 2005, 72, 035116. (39) Paglia, G.; Bozı˘n, E. S.; Billinge, S. J. L. Chem. Mater. 2006, 18, 3242. (40) Paglia, G.; Buckley, C. E.; Rohl, A. L. J. Phys. Chem. B 2006, 110, 20721. (41) Digne, M.; Raybaud, P.; Sautet, P.; Rebours, B.; Toulhoat, H. J. Phys. Chem. B 2006, 110, 20719. (42) Krokidis, X.; Raybaud, P.; Gobichon, A. E.; Rebours, B.; Euzen, P.; Toulhoat, H. J. Phys. Chem. B 2001, 105, 5121. (43) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. J. Catal. 2002, 211, 1. (44) Sohlberg, K.; Pennycook, S. J.; Pantelides, S. T. J. Am. Chem. Soc. 1999, 121, 10999. (45) Sohlberg, K.; Pennycook, S. J.; Pantelides, S. T. J. Am. Chem. Soc. 1999, 121, 7493. (46) Sohlberg, K.; Pennycook, S. J.; Pantelides, S. T. J. Am. Chem. Soc. 2001, 123, 26.
Langmuir, Vol. 24, No. 21, 2008 12411
2. Methodology and Models All the calculations were performed in the framework of DFT by using the Vienna ab initio simulation package (VASP).47,48 The projector augmented wave method49,50 was used to describe the interaction between ions and electrons. The nonlocal exchangecorrelation energy was evaluated using the Perdew-Burke-Ernzehof functional.51 A plane wave basis set with a cutoff energy of 400 eV and a 2 × 2 × 1 k-point grid determined by the Monkhorst-Pack method52 were found to give converged results. The atomic structures were relaxed using either the conjugate gradient algorithm or quasiNewton scheme as implemented in the VASP code until the forces on unconstrained atoms were
