- Email: [email protected]
γ-Al2O3 surface in liquid paraffin solution
Applied Surface Science 290 (2014) 398–404
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Methanol synthesis by CO and CO2 hydrogenation on Cu/␥-Al2 O3 surface in liquid paraffin solution Zhi-Jun Zuo a , Le Wang a , Pei-De Han b , Wei Huang a,∗ a Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China b College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history: Received 2 May 2013 Received in revised form 5 November 2013 Accepted 18 November 2013 Available online 26 November 2013 Keywords: Density functional theory Methanol Cu/␥-Al2 O3 surface CO2 CO
a b s t r a c t Methanol synthesis by CO and CO2 hydrogenation over Cu-based ␥-Al2 O3 catalysts has been extensively studied. However, the reaction mechanism of this synthesis on Cu/␥-Al2 O3 in liquid paraffin solution is still unclear at the microscopic level. This work investigated the synthesis of methanol by CO and CO2 hydrogenation and water–gas-shift reaction on Cu/␥-Al2 O3 in liquid paraffin solution using density functional theory calculations. In CO hydrogenation, methanol was synthesized through the intermediates CHO, CH2 O, and CH3 O; the rate-limiting step was CHO hydrogenation. In CO2 hydrogenation, methanol was synthesized through the intermediates CHOO, CH2 OO, CH2 O, and CH3 O; the rate-limiting step was CHOO hydrogenation. A comparison of the activation energies of the rate-limiting steps in CO and CO2 hydrogenation (1.37 and 1.10 eV, respectively) at typical catalytic conditions (e.g., 573 K) revealed that the reaction rate of direct CO2 hydrogenation was faster than that of direct CO hydrogenation. This finding indicated that methanol was mainly produced by CO2 hydrogenation. The calculated results were consistent with the experimental ones. © 2013 Elsevier B.V. All rights reserved.
1. Introduction ␥-Al2 O3 plays an important role in industries because of its fine particle size, large surface area, surface catalytic activity, and other catalytic properties. Given these properties, ␥-Al2 O3 is widely used as a catalyst carrier in methane reforming [1], methanol and DME synthesis by CO and CO2 hydrogenation [2], as well as CO oxidation [3] and hydrodesulfurization processes [4]. Methanol synthesis by CO and CO2 hydrogenation has been extensively studied experimentally because methanol is a key material in the synthesis of other organic materials, such as dimethyl ether, formaldehyde, dimethyl sulfate, and acetic acid [5–8]. Currently, Cu/Zn/␥-Al2 O3 is one of the best catalysts for methanol synthesis by CO and CO2 hydrogenation in fixed or slurry bed reactions [9–11]. Given the important role of methanol, methanol synthesis from CO and CO2 hydrogenation has been studied by many researchers [12–15]. Raybaud et al. [16] and Ionescu et al. [17] studied the surface properties of ␥-Al2 O3 in detail. They suggest that the nondissociative and dissociative adsorptions of H2 O occur over the Lewis acid sites of the ␥-Al2 O3 surface, which indicates that the ␥-Al2 O3 surface is inevitably hydrated or hydroxylated under realistic reaction conditions. The influence of surface hydroxyls over
∗ Corresponding author. Tel.: +86 351 6018073; fax: +86 351 6018073. E-mail address: [email protected] (W. Huang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.092
␥-Al2 O3 has been theoretically studied [18–21]. For example, Pan et al. [18] and Zhang et al. [19] examined the effect of surface hydroxyls on selective CO2 hydrogenation over Ni/␥-Al2 O3 and Cu/␥-Al2 O3 , However, these papers are only focused on the influence of hydroxylation over ␥-Al2 O3 surface in the first step of methanol synthesis by CO and CO2 hydrogenation. Today, the synthesis of methanol and dimethyl ether (DME) from syngas in slurry bed has attracted more attention due to its advantages, such as, no diffusion limitations, low pressure drop over the reactor and caloric transfer [9,11]. Therefore, the reaction process of methanol synthesis by CO and CO2 hydrogenation in a slurry bed over Cu-based hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin solution was studied in this paper. Liquid paraffin solution is always used as an inert medium in a slurry bed. Meanwhile, the water–gas-shift (WGS) reaction (CO + H2 O → CO2 + H2 ) or reverse WGS (RWGS) reaction inevitably occur during methanol synthesis by CO and CO2 hydrogenation [2,9–11]. Thus, in the current paper, the processes of CO hydrogenation, CO2 hydrogenation, and WGS reaction on Cu/hydroxylated ␥-Al2 O3 (1 1 0) surface were studied in liquid paraffin. 2. Computational methods All calculations were performed using the DMol3 program of the Materials Studio software package [22]. Electronic structures were obtained by solving the Kohn–Sham equation under
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
self-consistently unrestricted spin conditions. The all-electron relativistic density functional theory (DFT) [23,24] was adopted for core electrons using PW91 [25] generalized gradient approximation of the exchange-correlation energy. The double numerical atomic orbital basis set plus polarization function [23] was also used. The hydroxylated ␥-Al2 O3 (1 1 0) model was similar to those used in previous studies [18–21], which was modeled by a six-layer slab containing twelve Al2 O3 molecular units. These units were sep˚ arated by a 15 A-thick vacuum. Brillouin-zone integrations were performed on 3 × 3 × 1 grid Monkhorst–Pack [26] special k-points. To simulate methanol synthesis over ␥-Al2 O3 catalysts in a slurry bed, the conductor-like screening model (COSMO) implemented in DMol3 was used [27,28]. COSMO is a continuum solvent model in which the solute molecule forms a cavity within the dielectric continuum of the permittivity ε of the solvent [29,30]. The dielectric constant of liquid paraffin was set to 2.06. Transition states (TS) were searched using the complete linear synchronous transit (LST)/quadratic synchronous transit (QST) method [31]. LST maximization was performed, followed by energy minimization in the directions conjugating to the reaction pathway. The approximated TS values were used for QST maximization. From this point, another conjugate gradient minimization was performed. The cycle was repeated until a stationary point was located [32]. The adsorption energy (Eads ) was defined as [33]:
399
Fig. 1. The side (a) and top views (b) for the stable model of Cu6 cluster adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
Compared with the net charge of metal Cu (−0.026) [38] and Cu+ (0.247) [39] by the same calculation methods, the results showed that the character of the top two Cu atoms is nearly metal Cu.
Eads = E(adsorbate/slab) − [E(adsorbate) + E(slab) ],
3.1. Methanol synthesis by CO hydrogenation
where E(adsorbate/slab) , E(adsorbate) , and E(slab) are the total energies of the slab with the adsorbate on its surface, of the free adsorbate, and of the slab surface, respectively. A negative Eads signifies an exothermic adsorption, and a positive one indicates an endothermic adsorption. The reaction energy H, e.g., A + B = C + D, was calculated as [33] follows:
As for CO adsorption on the Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, the adsorption energy is −1.18 eV. A comparison of the adsorption energies of CO adsorption on Cu2 O (1 1 1), Cu/ZnO, and Cu (1 1 1) surfaces reveals that the adsorption ability follow the order Cu/ZnO (−1.87 eV) > Cu2 O (1 1 1) (−1.50) > Cu/␥-Al2 O3 > Cu (1 1 1) (−0.98 eV) [37–39]. This finding indicates that the positive Cu ion benefits CO adsorption. However, no close correlation exists between the adsorption ability and Cu net charge. In the case of H2 adsorption, when H2 vertical adsorption occurs on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, the H H bond is broken after geometry optimization, indicating that H2 undergoes dissociative adsorption. This result has also been observed in a previous study on H2 adsorption on Cu2 O (1 1 1) and Cu (1 1 1) surfaces in liquid paraffin [39–41]. The first CO hydrogenation may occur through two pathways. One is hydrogenation to form COH, and the other is the formation of CHO (Fig. 3). A comparison of the activation energies and stability of CHO and COH formation by CO hydrogenation reveals that
H = [E(C/slab) + E(D/slab) ] − [E(A/slab) + E(B/slab) ], where E(C/slab) and E(D/slab) are the total energies of the slab with products on its surface, whereas E(A/slab) , and E(B/slab) are the total energies of the slab with reactants on its surface. A negative H signifies an exothermic reaction and a positive one indicates an endothermic reaction. The activation energy (Ea ) is calculated on the basis of the following formulas: Ea = ETS/slab − EA+B/slab where ETS/slab and EA+B/slab are the total energy of transition state on slab and the total energy of the slab with co-adsorbed reactants 3. Results and discussion As for Cu6 cluster adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin, some possible adsorption sites are studied. The steadiest adsorption model is used to study the CO and CO2 hydrogenation, as well as the WGS reaction. The side and top views of the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin are shown in Fig. 1, and the steadiest adsorption site is similar to Pd and Ni adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in a vacuum [18,20]. The net charge of the top layer of Cu is 0.023. When Cu2 cluster adsorbs onto hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin, the net charge of Cu is 0.082, which is the steadiest adsorption model as shown in Fig. 2. The result is similar to Cu deposition on the ZnO surface, in which the metallic character of Cu increases with increased amount of Cu deposited [34–36]. However, the charge transfer ability of Cu deposition on ␥-Al2 O3 is smaller than that on ZnO, in which the net charge of Cu ¯ [37]. is 0.155 when two Cu clusters are deposited on ZnO (1010)
Fig. 2. The side (left) and top (right) views for the stable model of Cu2 cluster adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
400
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
Fig. 3. Possible reaction pathways for the methanol synthesis from CO hydrogenation on Cu/␥-Al2 O3 in liquid paraffin solution.
the CHO formation is energetically compatible with COH formation. This finding is due to the lower activation energy (lower by 0.57 eV) and higher stability (higher by 0.32 eV) of CHO formation. Thus, CO hydrogenation is likely to proceed via the CHO pathway. Similarly, two possible products are considered for the further hydrogenation of CHO, i.e., CH2 O and HCOH (Fig. 3). The CH2 O pathway is energetically favored because it is exothermic with a reaction energy of −0.13 eV and an activation energy of 1.37 eV. By contrast, CHOH is less stable (0.31 eV) and has a higher activation energy (1.81 eV). Therefore, CH2 O is likely to be synthesized from CHO hydrogenation. CH2 O prefers to be hydrogenated to form CH3 O rather than being hydrogenated to form CH2 OH. A higher energy of 1.48 eV is obtained for CH2 OH synthesis than for CH3 O formation (0.46 eV). Therefore, CH3 O can be formed by CH2 O hydrogenation. Eventually, CH3 OH is produced from CH3 O hydrogenation by a energy of 0.63 eV. Thus, methanol is synthesized from CO by direct hydrogenation over Cu/␥-Al2 O3 through the intermediates CHO, CH2 O, and CH3 O, with the rate-limiting step being CHO hydrogenation. The TS are shown in Fig. 4. For the intermediates CHO, CH2 O, and CH3 O, the adsorption energies are −1.34, −0.66, and −2.97 eV, respectively. In the case of CHO, Liu et al. [12] showed that the activation energy of CHO hydrogenation to H2 CO is 15 times that of CHO dehydrogenation to form CO and H on a Cu29 cluster. Hence, the CHO intermediate
is unstable and quickly dissociates to CO and H before further hydrogenation. In this study, the activation energy of CH2 O formation from CHO hydrogenation is about 2.3 times that of CHO dehydrogenation, indicating that CHO dehydrogenation is easier than CHO hydrogenation synthesized over Cu/␥-Al2 O3 (1 1 0) surface. However, considering the concentrations of CO and H on Cu/␥-Al2 O3 (1 1 0) surface, the abundance of CO and H can inhibit CHO dehydrogenation to a certain extent. Therefore, CH2 O also can be synthesized from CHO hydrogenation. In fact, the mean-field microkinetic model under typical methanol synthesis conditions over Cu/ZnO/Al2 O3 (Cr2 O3 ) or Cu catalysts shows that methanol can be synthesized from both CO and CO2 hydrogenation pathways, and that two-thirds of methanol is produced by CO2 hydrogenation [21,42]. This result well agrees with our observation. CHO species have been recognized in many reactions associated with CO hydrogenation. A previous experiment shows that CHO formed from CO and H2 can be detected on Cu/ZnO [43] and Cu/ZnO/Al2 O3 surfaces [44–46], but not when pure Cu is used. For CH2 O, which is the least stable intermediate, the adsorption energy ranges from −0.01 eV to −0.33 eV on the Cu (h k l) surface [15,47,48]. CH2 O is theoretically a by-product during methanol synthesis on Cu (h k l) surface, but its presence is not easily observed in experiments [49,50]. In this study, the results show that the adsorption ability is improved by the catalyst carrier. Thus, CH2 O is preferentially hydrogenated to
Fig. 4. The TS of intermediates and involved in the methanol synthesis from CO hydrogenation reaction on the Cu/␥-Al2 O3 surface in liquid paraffin. (a), (b), (c) and (d): the TS of CO, CHO, CH2 O and CH3 O hydrogenation. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
401
Fig. 5. Possible reaction pathways for the methanol synthesis from CO2 hydrogenation on Cu/␥-Al2 O3 in liquid paraffin solution.
CHO rather than be desorbed. This finding is similar to the results of CH2 O hydrogenation on Cu/ZnO surface and Cu–ZrO2 interface [37,51]. 3.2. Methanol synthesis by CO2 hydrogenation For CO2 adsorption on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, the CO2 molecule moves away from Cu/␥-Al2 O3 (1 1 0) surface after geometry optimization, and the adsorption energy is −0.13 eV. These findings indicate that CO2 molecules undergo physisorption, consistent with previous DFT calculations showing that CO2 weakly adsorbs on Cu (h k l) surfaces [12,15]. Therefore, CO2 hydrogenation follows the Eley–Rideal mechanism, in which CO2 molecules directly react with an adsorbed H [12,15]. The first CO2 hydrogenation may occur through two pathways. One is hydrogenation to form COOH, and the other is CHOO formation (Fig. 5). The activation energies of CHOO and COOH syntheses are 0.56 and 1.7 eV, respectively. The reaction energies of CHOO and COOH syntheses are −1.17 and −0.28 eV, respectively. The lower activation energy and higher stability of CHOO synthesis show that CHOO is likely synthesized by CO2 hydrogenation. For further CHOO hydrogenation, Fig. 5 shows that the CH2 OO pathway is energetically favored because it is slightly endothermic with a reaction energy of 0.2 eV and activation energy of 1.10 eV. By contrast, the reaction energy of CHOOH formation is highly endothermic with a reaction energy of 0.94 eV and activation energy of 2.27 eV.
These results show that CHOO hydrogenation is likely to proceed via the CH2 OO formation pathway. Fig. 5 also shows two possible pathways for further H2 COOH hydrogenation: CH2 OOH formation through direct CH2 OO hydrogenation, as well as CH2 O and OH formation through H-guided dissociation. The adsorption ability of CH2 OOH on Cu/␥-Al2 O3 (1 1 0) surface is slightly stable than that of CH2 O and OH co-adsorption. The activation energy of CH2 OOH synthesis from CH2 OO is also much less than those of CH2 O and OH formation. Thus, the reaction prefers CH2 OOH formation through the direct CH2 OO hydrogenation rather than H-guided dissociation. Subsequently, the C O bond of CH2 OOH can easily break, leading o the formation of H2 CO and OH. Eventually, the H2 CO intermediate follows the same pathway shown in Fig. 3 to form CH3 OH. Therefore, methanol is synthesized from CO2 by direct hydrogenation over Cu/␥-Al2 O3 through the intermediates CHOO, CH2 OO, CH2 O, and CH3 O. The TS are shown in Fig. 6. For the process of methanol synthesis, the main intermediates are CHOO, CH2 OO, CH2 O, and CH3 O. The adsorption energies of CHOO and CH2 OO are −1.50 and −1.97 eV, respectively. The absorption ability of these main intermediates of methanol synthesis by CO2 /CO hydrogenation follows the order CH2 OO > CH3 O > CHOO > CHO > CH2 O. This finding may explain why main intermediates, except CH2 O, are always detected when using experimental techniques such as diffuse-reflectance infrared spectroscopy, thermal desorption spectroscopy, and temperatureprogrammed desorption [52].
Fig. 6. The TS of intermediates and involved in the methanol synthesis from CO2 hydrogenation reaction on the Cu/␥-Al2 O3 surface in liquid paraffin. (a), (b), (c) and (d): the TS of CO2 , CHOO, CH2 OO and CH2 O hydrogenation. The TSs of CH2 O and CH3 O hydrogenation, see Fig. 4(c) and (d). Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
402
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
Fig. 7. Possible reaction pathways for the WGS on Cu/␥-Al2 O3 in liquid paraffin solution.
3.3. WGS reaction Given that RWGS is the side reaction of methanol synthesis by CO and CO2 hydrogenation [2,12], extensive studies have been carried out to investigate the WGS reaction on both bulk surfaces such as Cu, Au, and Pd using DFT [15,53–57]. Two main reaction mechanisms have been proposed: one is a redox mechanism and the other is an associative mechanism. Thus, the WGS mechanism follows both of these mechanisms [15,54–56]. In the process of H2 O adsorption on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, H2 O moves away from the surface after geometry optimization, indicating that H2 O molecules undergo physisorption. The activation energy of H dissociation from H2 O is 1.26 eV, which results in a reaction energy of −0.22 eV. After the initial H2 O dissociation step, three phenomena may proceed during the WGS process: further OH dissociation into O and H (redox mechanism with direct OH dissociation), reaction with another OH* (redox mechanism with another OH), and reaction with CO to produce COOH (associative mechanism through the carboxyl intermediate). As shown in Fig. 7, a low energy of 0.65 eV is obtained for COOH synthesis by CO reaction with OH, whereas energies of 1.44 and 1.87 eV are calculated for the formation of H2 O + O and OH by further dissociation, respectively. Therefore, COOH formation seems likely to
proceed. The preference for the associative mechanism coincides with the results obtained for the WGS reaction on Cu (1 1 1) and Cu (3 2 1)-stepped surfaces, as well as Cu29 cluster [15,55,56]. For COOH in the associative mechanism, two possibilities exist for carboxyl dehydrogenation: one is the direct COO H bond breaking on Cu surface, and the other is COO H bond breaking assisted by an OH group. In the case of direct COOH dehydrogenation, the activation energy is 0.94 eV, which is higher than that of COO H bond breaking assisted by an OH group (0.71 eV). This result shows that CO2 formation by COO H bond breaking assisted by an OH group is likely to proceed. As shown in Fig. 7, the WGS on Cu/␥-Al2 O3 (1 1 0) adopts an associative mechanism via carboxyl, and then dissociates into CO2 and H. Therefore, H2 O dissociation is the rate-limiting step for the WGS on Cu/␥-Al2 O3 (1 1 0) surface. The TS are displayed in Fig. 8. The reaction mechanism and rate-limiting step for the WGS on Cu/␥-Al2 O3 (1 1 0) surface agree with the results obtained for the WGS on Cu (1 1 1) and Cu (3 2 1)-stepped surface, as well as Cu29 cluster [15,55,56]. In order to further understand the effect of liquid paraffin, the activation energies of the rate-limiting step for CO hydrogenation, CO2 hydrogenation and WGS reaction in vacuum are calculated. The activation energies of CH2 O formation, CH2 OO formation and H2 O dissociation are 1.50, 1.22 and 1.27 eV, respectively.
Fig. 8. The TS of intermediates and involved in the WGS on the Cu/␥-Al2 O3 surface in liquid paraffin. (a), (b) and (c): the TS of water dissociation, OH reaction with the CO and COO H bond breaking with an OH group assisted. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
Comparing with the corresponding activation energy in liquid paraffin, it can be seen that liquid paraffin influences the activation energies of CH2 OO formation (Ea = 0.12 eV) and CH2 O formation (Ea = 0.13 eV), but does not obviously effect the H2 O dissociation (Ea = 0.01 eV). 4. Conclusion Theoretical studies are carried out to investigate methanol synthesis by CO and CO2 hydrogenation and WGS on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin. The charge may be locally transferred from Cu to the ␥-Al2 O3 carrier, and the transfer of electron charge further decreases during Cu deposition. For CO hydrogenation on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, methanol is synthesized through the intermediates CHO, CH2 O, and CH3 O, and the rate-limiting step is CHO hydrogenation. For CO2 hydrogenation, methanol is synthesized through the intermediates CHOO, CH2 OO, CH2 O, and CH3 O, and the rate-limiting step is CHOO hydrogenation. Regarding WGS, it proceeds via the associative mechanism through the COOH intermediate by carboxyl dehydrogenation assisted by adsorbed OH, and the rate-limiting step is H2 O dissociation. The activation energies of the rate-limiting steps in CO and CO2 hydrogenation, as well as WGS follow the order Ea (CHO hydrogenation, 1.37 eV) > Ea (WGS, 1.26 eV) > Ea (CO2 hydrogenation, 1.10 eV). Liquid paraffin influences the activation energies of CH2 OO formation and CHO formation, but does not obviously effect the H2 O dissociation. Acknowledgements The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (20676087 and 21306125), China Postdoctoral Science Foundation Funded Project (2012M510784), Natural Science Foundation of Shanxi(Grant No. 2012011046-1 and 012021005-1), Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi(2013107), and Special/Youth Foundation of Taiyuan University of Technology (No. 2012L042). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2013.11.092. References [1] A.R. González, Y.J.O. Asencios, E.M. Assaf, J.M. Assaf, Dry reforming of methane on Ni Mg Al nano-spheroid oxide catalysts prepared by the sol–gel method from hydrotalcite-like precursors, Appl. Surf. Sci. 280 (2013) 876–887. [2] S. Park, B. Choi, H. Kim, J.H. Kim, Hydrogen production from dimethyl ether over Cu/␥-Al2 O3 catalyst with zeolites and its effects in the lean NOx trap performance, Int. J. Hydrogen Energy 37 (2012) 4762–4773. [3] C. Wang, G. Yi, H. Lin, Y. Yuan, Na+ -intercalated carbon nanotubes-supported platinum nanoparticles as new highly effective catalysts for preferential CO oxidation in H2 -rich stream, Int. J. Hydrogen Energy 37 (2012) 14124–14132. [4] Y. Jia, G. Li, G. Ning, Efficient oxidative desulfurization (ODS) of model fuel with H2 O2 catalyzed by MoO3 /␥-Al2 O3 under mild and solvent free conditions, Fuel Process. Technol. 92 (2011) 106–111. [5] A.C.P. Filho, R.M. Filho, Hybrid training approach for artificial neural networks using genetic algorithms for rate of reaction estimation: application to industrial methanol oxidation to formaldehyde on silver catalyst, Chem. Eng. J. 157 (2010) 501–508. [6] F. Zha, H. Tian, J. Yan, Y. Chang, Multi-walled carbon nanotubes as catalyst promoter for dimethyl ether synthesis from CO2 hydrogenation, Appl. Surf. Sci. 285 (2013) 945–951. [7] M. Farsi, M.H. Khademi, A. Jahanmiri, M.R. Rahimpour, Optimal conditions for hydrogen production from coupling of dimethyl ether and benzene synthesis, Int. J. Hydrogen Energy 36 (2011) 299–310.
403
[8] T. Ohnishi, T. Yamakawa, S. Shinoda, New preparation of Ru-Sn/Y zeolite catalyst for the formation of acetic acid (methyl acetate) from methanol alone, Appl. Catal. A 231 (2002) 27–33. [9] X. Zhang, Z. Li, Q. Guo, H. Fan, H. Zheng, K. Xie, Influence of the calcination on the activity and stability of the Cu/ZnO/Al2 O3 catalyst in liquid phase methanol synthesis, Fuel 89 (2010) 1348–1352. [10] J.H. Flores, D.P.B. Peixoto, L.G. Appel, R.R. de Avillez, M.I. Pais da Silva, The influence of different methanol synthesis catalysts on direct synthesis of DME from syngas, Catal. Today 172 (2011) 218–225. [11] Z. Li, Z. Zuo, W. Huang, K. Xie, Research on Si–Al based catalysts prepared by complete liquid-phase method for DME synthesis in a slurry reactor, Appl. Surf. Sci. 257 (2011) 2180–2183. [12] Y.X. Yang, J. Evans, J.Z. Rodriguez, M.G. White, P. Liu, Fundamental studies of methanol synthesis from CO2 hydrogenation on Cu(1 1 1), Cu clusters, and Cu/ZnO(0 0 0 1), Phys. Chem. Chem. Phys. 12 (2010) 9909–9917. [13] Y. Morikawa, K. Iwata, K. Terakura, Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfaces, Appl. Surf. Sci. 169–170 (2001) 11–15. [14] J. Yoshihara, C.T. Campbell, Methanol synthesis and reverse water–gas shift kinetics over Cu(1 1 0) model catalysts: structural sensitivity, J. Catal. 161 (1996) 776–782. [15] L.C. Grabow, M. Mavrikakis, Mechanism of methanol synthesis on Cu through CO2 and CO hydrogenation, ACS Catal. 1 (2011) 365–384. [16] M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat, Use of DFT to achieve a rational understanding of acid–basic properties of ␥-alumina surfaces, J. Catal. 226 (2004) 54–68. [17] A. Ionescu, A. Allouche, J.P. Aycard, M. Rajzmann, Structure and stability of aluminum hydroxides: a theoretical study, J. Phys. Chem. B 106 (2002) 5155–5162. [18] Y.X. Pan, C.J. Liu, Q.F. Ge, Effect of surface hydroxyls on selective CO2 hydrogenation over Ni4 /␥-Al2 O3 : a density functional theory study, J. Catal. 272 (2010) 227–234. [19] R.G. Zhang, B.J. Wang, H.Y. Liu, L.X. Ling, Effect of surface hydroxyls on CO2 hydrogenation over Cu/␥-Al2 O3 catalyst: a theoretical study, J. Phys. Chem. B 115 (2011) 19811–19818. [20] M.C. Valero, P. Raybaud, P. Sautet, Interplay between molecular adsorption and metal–support interaction for small supported metal clusters: CO and C2 H4 adsorption on Pd4 /␥-Al2 O3 , J. Catal. 247 (2007) 339–355. [21] Y.X. Pan, C.J. Liu, T.S. Wiltowski, Q.F. Ge, CO2 adsorption and activation over ␥-Al2 O3 -supported transition metal dimers: a density functional study, Catal. Today 147 (2009) 68–76. [22] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (2000) 7756–7764. [23] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys. Rev. B 136 (1964) 864–871. [24] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects, Phys. Rev. A 140 (1965) 1133–1138. [25] J.P. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy, Phys. Rev. B 45 (1992) 13244–13249. [26] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188–5192. [27] M. Tomonari, O. Sugino, DFT calculation of vibrational frequency of hydrogen atoms on Pt electrodes: analysis of the electric field dependence of the Pt–H stretching frequency, Chem. Phys. Lett. 437 (2007) 170–175. [28] A. Singh, B. Ganguly, DFT study of urea interaction with potassium chloride surfaces, Mol. Simul. 34 (2008) 973–979. [29] Z.J. Zuo, W. Huang, P.D. Han, Z.H. Gao, Z. Li, Theoretical studies on the reaction mechanisms of AlOOH- and ␥-Al2 O3 -catalysed methanol dehydration in the gas and liquid phases, Appl. Catal. A 40 (2011) 130–136. [30] T. Todorova, B. Delley, Wetting of paracetamol surfaces studied by DMol3 COSMO calculations, Mol. Simul. 34 (2008) 1013–1017. [31] T.A. Halgren, W.N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chem. Phys. Lett. 49 (1977) 225–232. [32] S.G. Wang, D.B. Cao, Y.W. Li, J.G. Wang, H.J. Jiao, Chemisorption of CO2 on the nickel surfaces, J. Phys. Chem. C 109 (2005) 18956–18963. [33] B. Xing, X.Y. Pang, G.C. Wang, C H bond activation of methane on clean and oxygen pre-covered metals: a systematic theoretical study, J. Catal. 282 (2011) 74–82. [34] S.V. Didziulis, K.D. Butcher, S.L. Cohen, E.I. Solomon, Chemistry of copper overlayers on zinc oxide single-crystal surfaces: model active sites for copper/zinc oxide methanol synthesis catalysts, J. Am. Chem. Soc. 111 (1989) 7110–7123. [35] K. Ozawa, T. Sato, Y. Oba, K. Edamoto, Electronic structure of Cu on ZnO(1 0 1 0): angle-resolved photoemission spectroscopy study, J. Phys. Chem. C 4256 (2007) 4256–4263. [36] J. Hu, W.P. Guo, X.R. Shi, B.R. Li, J.G. Wang, Copper deposition and growth over ZnO nonpolar (1 0 1 0) and (1 1 2 0) surfaces: a density functional theory study, J. Phys. Chem. C 113 (2009) 7227–7235. [37] Z.J. Zuo, P.D. Han, Z. Li, J.S. Hu, W. Huang, Can methanol be synthesized from CO by direct hydrogenation over Cu/ZnO catalysts? Appl. Surf. Sci. 261 (2012) 640–646. [38] Z.J. Zuo, L.L. Sun, W. Huang, P.D. Han, Z.H. Li, Surface properties of copper in different solvent mother solutions: a density functional theory study, Appl. Catal. A 375 (2010) 181–187. [39] Z.J. Zuo, W. Huang, P.D. Han, Z.H. Li, Solvent effects for CO and H2 adsorption on Cu2 O (1 1 1) surface: a density functional theory study, Appl. Surf. Sci. 256 (2010) 2357–2362.
404
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
[40] Z.J. Zuo, W. Huang, P.D. Han, Z.H. Li, CO and H2 molecules adsorption on Cu(1 1 1) surface and solvent effects, Acta Phys. Chim. Sin. 25 (2009) 2507–2512. [41] Z.J. Zuo, W. Huang, P.D. Han, A DFT study the solvent effects of H2 adsorption on Cu (h k l) surface, Appl. Surf. Sci. 258 (2012) 3364–3367. [42] E.M. Calverley, K.J. Smith, Kinetic model for alcohol synthesis over a promoted copper/zinc oxide/chromium oxide (Cr2 O3 ) catalyst, Ind. Eng. Chem. Res. 31 (1992) 792–803. [43] D. Liu, Q. Zhu, J. Li, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalysis, vol. 2, Chemical Institute of Canada, Ottawa, 1988, p. 577. [44] J.F. Edwards, G.L. Schrader, In situ Fourier transform infrared study of methanol synthesis on mixed metal oxide catalysts, J. Catal. 94 (1985) 175–186. [45] J. Saussey, J.C. Lavalley, An in situ FT-IR study of adsorbed species on a CuZnAl2 O4 methanol catalyst under 1 MPa pressure and at 525 K: effect of the H2 /CO/CO2 feed stream composition, J. Mol. Catal. 50 (1989) 343–353. [46] S. Fujita, M. Usui, E. Ohara, N. Takezawa, Methanol synthesis from carbon dioxide at atmospheric pressure over Cu/ZnO catalyst. Role of methoxide species formed on ZnO support, Catal. Lett. 13 (1992) 349–358. [47] J. Greeley, M. Mavrikakis, Methanol decomposition on Cu(1 1 1): a DFT study, J. Catal. 208 (2002) 291–300. [48] J.R.B. Gomes, J.A.N.F. Gomes, F. Illas, First-principles study of the adsorption of formaldehyde on the clean and atomic oxygen covered Cu(1 1 1) surface, J. Mol. Catal. A 170 (2001) 187–193. [49] P.B. Rasmussen, M. Kazuta, I. Chorkendorff, Synthesis of methanol from a mixture of H2 and CO2 on Cu(1 0 0), Surf. Sci. 318 (1994) 267–280.
[50] P.B. Rasmussen, P.M. Holmblad, T. Askgaard, C.V. Ovesen, P. Stoltze, J.K. Nørskov, I. Chorkendorff, Methanol synthesis on Cu(1 0 0) from binary gas mixtures of CO2 and H, Catal. Lett. 26 (1994) 373–381. [51] Q.L. Tang, Q.J. Hong, Z.P. Liu, CO2 fixation into methanol at Cu/ZrO2 interface from first principles kinetic Monte Carlo, J. Catal. 263 (2009) 114–122. [52] J.B. Hansen, P.E.H. Nielsen, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 13, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008, pp. 2920–2949. [53] A. Mondal, H. Seenivasan, A.K. Tiwari, Water dissociation on Cu (1 1 1): effects of molecular orientation, rotation, and vibration on reactivity, J. Chem. Phys. 137 (2012) 094708. [54] P. Liu, J.A. Rodriguez, Water–gas-shift reaction on metal nanoparticles and surfaces, J. Chem. Phys. 126 (2007) 164705. [55] A.A. Gokhale, J.A. Dumesic, M. Mavrikakis, On the mechanism of lowtemperature water gas shift reaction on copper, J. Am. Chem. Soc. 130 (2008) 1402–1414. [56] J.L.C. Fajín, M.N.D.S. Cordeiro, F. Illas, J.R.B. Gomes, Influence of step sites in the molecular mechanism of the water gas shift reaction catalyzed by copper, J. Catal. 268 (2009) 131–141. [57] J.L.C. Fajín, A. Bruix, M. Natália, D.S. Cordeiro, J.R.B. Gomes, F. Illas, Density functional theory model study of size and structure effects on water dissociation by platinum nanoparticles, J. Chem. Phys. 137 (2012) 034701.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Methanol synthesis by CO and CO2 hydrogenation on Cu/␥-Al2 O3 surface in liquid paraffin solution Zhi-Jun Zuo a , Le Wang a , Pei-De Han b , Wei Huang a,∗ a Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China b College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history: Received 2 May 2013 Received in revised form 5 November 2013 Accepted 18 November 2013 Available online 26 November 2013 Keywords: Density functional theory Methanol Cu/␥-Al2 O3 surface CO2 CO
a b s t r a c t Methanol synthesis by CO and CO2 hydrogenation over Cu-based ␥-Al2 O3 catalysts has been extensively studied. However, the reaction mechanism of this synthesis on Cu/␥-Al2 O3 in liquid paraffin solution is still unclear at the microscopic level. This work investigated the synthesis of methanol by CO and CO2 hydrogenation and water–gas-shift reaction on Cu/␥-Al2 O3 in liquid paraffin solution using density functional theory calculations. In CO hydrogenation, methanol was synthesized through the intermediates CHO, CH2 O, and CH3 O; the rate-limiting step was CHO hydrogenation. In CO2 hydrogenation, methanol was synthesized through the intermediates CHOO, CH2 OO, CH2 O, and CH3 O; the rate-limiting step was CHOO hydrogenation. A comparison of the activation energies of the rate-limiting steps in CO and CO2 hydrogenation (1.37 and 1.10 eV, respectively) at typical catalytic conditions (e.g., 573 K) revealed that the reaction rate of direct CO2 hydrogenation was faster than that of direct CO hydrogenation. This finding indicated that methanol was mainly produced by CO2 hydrogenation. The calculated results were consistent with the experimental ones. © 2013 Elsevier B.V. All rights reserved.
1. Introduction ␥-Al2 O3 plays an important role in industries because of its fine particle size, large surface area, surface catalytic activity, and other catalytic properties. Given these properties, ␥-Al2 O3 is widely used as a catalyst carrier in methane reforming [1], methanol and DME synthesis by CO and CO2 hydrogenation [2], as well as CO oxidation [3] and hydrodesulfurization processes [4]. Methanol synthesis by CO and CO2 hydrogenation has been extensively studied experimentally because methanol is a key material in the synthesis of other organic materials, such as dimethyl ether, formaldehyde, dimethyl sulfate, and acetic acid [5–8]. Currently, Cu/Zn/␥-Al2 O3 is one of the best catalysts for methanol synthesis by CO and CO2 hydrogenation in fixed or slurry bed reactions [9–11]. Given the important role of methanol, methanol synthesis from CO and CO2 hydrogenation has been studied by many researchers [12–15]. Raybaud et al. [16] and Ionescu et al. [17] studied the surface properties of ␥-Al2 O3 in detail. They suggest that the nondissociative and dissociative adsorptions of H2 O occur over the Lewis acid sites of the ␥-Al2 O3 surface, which indicates that the ␥-Al2 O3 surface is inevitably hydrated or hydroxylated under realistic reaction conditions. The influence of surface hydroxyls over
∗ Corresponding author. Tel.: +86 351 6018073; fax: +86 351 6018073. E-mail address: [email protected] (W. Huang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.092
␥-Al2 O3 has been theoretically studied [18–21]. For example, Pan et al. [18] and Zhang et al. [19] examined the effect of surface hydroxyls on selective CO2 hydrogenation over Ni/␥-Al2 O3 and Cu/␥-Al2 O3 , However, these papers are only focused on the influence of hydroxylation over ␥-Al2 O3 surface in the first step of methanol synthesis by CO and CO2 hydrogenation. Today, the synthesis of methanol and dimethyl ether (DME) from syngas in slurry bed has attracted more attention due to its advantages, such as, no diffusion limitations, low pressure drop over the reactor and caloric transfer [9,11]. Therefore, the reaction process of methanol synthesis by CO and CO2 hydrogenation in a slurry bed over Cu-based hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin solution was studied in this paper. Liquid paraffin solution is always used as an inert medium in a slurry bed. Meanwhile, the water–gas-shift (WGS) reaction (CO + H2 O → CO2 + H2 ) or reverse WGS (RWGS) reaction inevitably occur during methanol synthesis by CO and CO2 hydrogenation [2,9–11]. Thus, in the current paper, the processes of CO hydrogenation, CO2 hydrogenation, and WGS reaction on Cu/hydroxylated ␥-Al2 O3 (1 1 0) surface were studied in liquid paraffin. 2. Computational methods All calculations were performed using the DMol3 program of the Materials Studio software package [22]. Electronic structures were obtained by solving the Kohn–Sham equation under
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
self-consistently unrestricted spin conditions. The all-electron relativistic density functional theory (DFT) [23,24] was adopted for core electrons using PW91 [25] generalized gradient approximation of the exchange-correlation energy. The double numerical atomic orbital basis set plus polarization function [23] was also used. The hydroxylated ␥-Al2 O3 (1 1 0) model was similar to those used in previous studies [18–21], which was modeled by a six-layer slab containing twelve Al2 O3 molecular units. These units were sep˚ arated by a 15 A-thick vacuum. Brillouin-zone integrations were performed on 3 × 3 × 1 grid Monkhorst–Pack [26] special k-points. To simulate methanol synthesis over ␥-Al2 O3 catalysts in a slurry bed, the conductor-like screening model (COSMO) implemented in DMol3 was used [27,28]. COSMO is a continuum solvent model in which the solute molecule forms a cavity within the dielectric continuum of the permittivity ε of the solvent [29,30]. The dielectric constant of liquid paraffin was set to 2.06. Transition states (TS) were searched using the complete linear synchronous transit (LST)/quadratic synchronous transit (QST) method [31]. LST maximization was performed, followed by energy minimization in the directions conjugating to the reaction pathway. The approximated TS values were used for QST maximization. From this point, another conjugate gradient minimization was performed. The cycle was repeated until a stationary point was located [32]. The adsorption energy (Eads ) was defined as [33]:
399
Fig. 1. The side (a) and top views (b) for the stable model of Cu6 cluster adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
Compared with the net charge of metal Cu (−0.026) [38] and Cu+ (0.247) [39] by the same calculation methods, the results showed that the character of the top two Cu atoms is nearly metal Cu.
Eads = E(adsorbate/slab) − [E(adsorbate) + E(slab) ],
3.1. Methanol synthesis by CO hydrogenation
where E(adsorbate/slab) , E(adsorbate) , and E(slab) are the total energies of the slab with the adsorbate on its surface, of the free adsorbate, and of the slab surface, respectively. A negative Eads signifies an exothermic adsorption, and a positive one indicates an endothermic adsorption. The reaction energy H, e.g., A + B = C + D, was calculated as [33] follows:
As for CO adsorption on the Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, the adsorption energy is −1.18 eV. A comparison of the adsorption energies of CO adsorption on Cu2 O (1 1 1), Cu/ZnO, and Cu (1 1 1) surfaces reveals that the adsorption ability follow the order Cu/ZnO (−1.87 eV) > Cu2 O (1 1 1) (−1.50) > Cu/␥-Al2 O3 > Cu (1 1 1) (−0.98 eV) [37–39]. This finding indicates that the positive Cu ion benefits CO adsorption. However, no close correlation exists between the adsorption ability and Cu net charge. In the case of H2 adsorption, when H2 vertical adsorption occurs on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, the H H bond is broken after geometry optimization, indicating that H2 undergoes dissociative adsorption. This result has also been observed in a previous study on H2 adsorption on Cu2 O (1 1 1) and Cu (1 1 1) surfaces in liquid paraffin [39–41]. The first CO hydrogenation may occur through two pathways. One is hydrogenation to form COH, and the other is the formation of CHO (Fig. 3). A comparison of the activation energies and stability of CHO and COH formation by CO hydrogenation reveals that
H = [E(C/slab) + E(D/slab) ] − [E(A/slab) + E(B/slab) ], where E(C/slab) and E(D/slab) are the total energies of the slab with products on its surface, whereas E(A/slab) , and E(B/slab) are the total energies of the slab with reactants on its surface. A negative H signifies an exothermic reaction and a positive one indicates an endothermic reaction. The activation energy (Ea ) is calculated on the basis of the following formulas: Ea = ETS/slab − EA+B/slab where ETS/slab and EA+B/slab are the total energy of transition state on slab and the total energy of the slab with co-adsorbed reactants 3. Results and discussion As for Cu6 cluster adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin, some possible adsorption sites are studied. The steadiest adsorption model is used to study the CO and CO2 hydrogenation, as well as the WGS reaction. The side and top views of the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin are shown in Fig. 1, and the steadiest adsorption site is similar to Pd and Ni adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in a vacuum [18,20]. The net charge of the top layer of Cu is 0.023. When Cu2 cluster adsorbs onto hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin, the net charge of Cu is 0.082, which is the steadiest adsorption model as shown in Fig. 2. The result is similar to Cu deposition on the ZnO surface, in which the metallic character of Cu increases with increased amount of Cu deposited [34–36]. However, the charge transfer ability of Cu deposition on ␥-Al2 O3 is smaller than that on ZnO, in which the net charge of Cu ¯ [37]. is 0.155 when two Cu clusters are deposited on ZnO (1010)
Fig. 2. The side (left) and top (right) views for the stable model of Cu2 cluster adsorption on the hydroxylated ␥-Al2 O3 (1 1 0) surface in liquid paraffin. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
400
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
Fig. 3. Possible reaction pathways for the methanol synthesis from CO hydrogenation on Cu/␥-Al2 O3 in liquid paraffin solution.
the CHO formation is energetically compatible with COH formation. This finding is due to the lower activation energy (lower by 0.57 eV) and higher stability (higher by 0.32 eV) of CHO formation. Thus, CO hydrogenation is likely to proceed via the CHO pathway. Similarly, two possible products are considered for the further hydrogenation of CHO, i.e., CH2 O and HCOH (Fig. 3). The CH2 O pathway is energetically favored because it is exothermic with a reaction energy of −0.13 eV and an activation energy of 1.37 eV. By contrast, CHOH is less stable (0.31 eV) and has a higher activation energy (1.81 eV). Therefore, CH2 O is likely to be synthesized from CHO hydrogenation. CH2 O prefers to be hydrogenated to form CH3 O rather than being hydrogenated to form CH2 OH. A higher energy of 1.48 eV is obtained for CH2 OH synthesis than for CH3 O formation (0.46 eV). Therefore, CH3 O can be formed by CH2 O hydrogenation. Eventually, CH3 OH is produced from CH3 O hydrogenation by a energy of 0.63 eV. Thus, methanol is synthesized from CO by direct hydrogenation over Cu/␥-Al2 O3 through the intermediates CHO, CH2 O, and CH3 O, with the rate-limiting step being CHO hydrogenation. The TS are shown in Fig. 4. For the intermediates CHO, CH2 O, and CH3 O, the adsorption energies are −1.34, −0.66, and −2.97 eV, respectively. In the case of CHO, Liu et al. [12] showed that the activation energy of CHO hydrogenation to H2 CO is 15 times that of CHO dehydrogenation to form CO and H on a Cu29 cluster. Hence, the CHO intermediate
is unstable and quickly dissociates to CO and H before further hydrogenation. In this study, the activation energy of CH2 O formation from CHO hydrogenation is about 2.3 times that of CHO dehydrogenation, indicating that CHO dehydrogenation is easier than CHO hydrogenation synthesized over Cu/␥-Al2 O3 (1 1 0) surface. However, considering the concentrations of CO and H on Cu/␥-Al2 O3 (1 1 0) surface, the abundance of CO and H can inhibit CHO dehydrogenation to a certain extent. Therefore, CH2 O also can be synthesized from CHO hydrogenation. In fact, the mean-field microkinetic model under typical methanol synthesis conditions over Cu/ZnO/Al2 O3 (Cr2 O3 ) or Cu catalysts shows that methanol can be synthesized from both CO and CO2 hydrogenation pathways, and that two-thirds of methanol is produced by CO2 hydrogenation [21,42]. This result well agrees with our observation. CHO species have been recognized in many reactions associated with CO hydrogenation. A previous experiment shows that CHO formed from CO and H2 can be detected on Cu/ZnO [43] and Cu/ZnO/Al2 O3 surfaces [44–46], but not when pure Cu is used. For CH2 O, which is the least stable intermediate, the adsorption energy ranges from −0.01 eV to −0.33 eV on the Cu (h k l) surface [15,47,48]. CH2 O is theoretically a by-product during methanol synthesis on Cu (h k l) surface, but its presence is not easily observed in experiments [49,50]. In this study, the results show that the adsorption ability is improved by the catalyst carrier. Thus, CH2 O is preferentially hydrogenated to
Fig. 4. The TS of intermediates and involved in the methanol synthesis from CO hydrogenation reaction on the Cu/␥-Al2 O3 surface in liquid paraffin. (a), (b), (c) and (d): the TS of CO, CHO, CH2 O and CH3 O hydrogenation. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
401
Fig. 5. Possible reaction pathways for the methanol synthesis from CO2 hydrogenation on Cu/␥-Al2 O3 in liquid paraffin solution.
CHO rather than be desorbed. This finding is similar to the results of CH2 O hydrogenation on Cu/ZnO surface and Cu–ZrO2 interface [37,51]. 3.2. Methanol synthesis by CO2 hydrogenation For CO2 adsorption on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, the CO2 molecule moves away from Cu/␥-Al2 O3 (1 1 0) surface after geometry optimization, and the adsorption energy is −0.13 eV. These findings indicate that CO2 molecules undergo physisorption, consistent with previous DFT calculations showing that CO2 weakly adsorbs on Cu (h k l) surfaces [12,15]. Therefore, CO2 hydrogenation follows the Eley–Rideal mechanism, in which CO2 molecules directly react with an adsorbed H [12,15]. The first CO2 hydrogenation may occur through two pathways. One is hydrogenation to form COOH, and the other is CHOO formation (Fig. 5). The activation energies of CHOO and COOH syntheses are 0.56 and 1.7 eV, respectively. The reaction energies of CHOO and COOH syntheses are −1.17 and −0.28 eV, respectively. The lower activation energy and higher stability of CHOO synthesis show that CHOO is likely synthesized by CO2 hydrogenation. For further CHOO hydrogenation, Fig. 5 shows that the CH2 OO pathway is energetically favored because it is slightly endothermic with a reaction energy of 0.2 eV and activation energy of 1.10 eV. By contrast, the reaction energy of CHOOH formation is highly endothermic with a reaction energy of 0.94 eV and activation energy of 2.27 eV.
These results show that CHOO hydrogenation is likely to proceed via the CH2 OO formation pathway. Fig. 5 also shows two possible pathways for further H2 COOH hydrogenation: CH2 OOH formation through direct CH2 OO hydrogenation, as well as CH2 O and OH formation through H-guided dissociation. The adsorption ability of CH2 OOH on Cu/␥-Al2 O3 (1 1 0) surface is slightly stable than that of CH2 O and OH co-adsorption. The activation energy of CH2 OOH synthesis from CH2 OO is also much less than those of CH2 O and OH formation. Thus, the reaction prefers CH2 OOH formation through the direct CH2 OO hydrogenation rather than H-guided dissociation. Subsequently, the C O bond of CH2 OOH can easily break, leading o the formation of H2 CO and OH. Eventually, the H2 CO intermediate follows the same pathway shown in Fig. 3 to form CH3 OH. Therefore, methanol is synthesized from CO2 by direct hydrogenation over Cu/␥-Al2 O3 through the intermediates CHOO, CH2 OO, CH2 O, and CH3 O. The TS are shown in Fig. 6. For the process of methanol synthesis, the main intermediates are CHOO, CH2 OO, CH2 O, and CH3 O. The adsorption energies of CHOO and CH2 OO are −1.50 and −1.97 eV, respectively. The absorption ability of these main intermediates of methanol synthesis by CO2 /CO hydrogenation follows the order CH2 OO > CH3 O > CHOO > CHO > CH2 O. This finding may explain why main intermediates, except CH2 O, are always detected when using experimental techniques such as diffuse-reflectance infrared spectroscopy, thermal desorption spectroscopy, and temperatureprogrammed desorption [52].
Fig. 6. The TS of intermediates and involved in the methanol synthesis from CO2 hydrogenation reaction on the Cu/␥-Al2 O3 surface in liquid paraffin. (a), (b), (c) and (d): the TS of CO2 , CHOO, CH2 OO and CH2 O hydrogenation. The TSs of CH2 O and CH3 O hydrogenation, see Fig. 4(c) and (d). Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
402
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
Fig. 7. Possible reaction pathways for the WGS on Cu/␥-Al2 O3 in liquid paraffin solution.
3.3. WGS reaction Given that RWGS is the side reaction of methanol synthesis by CO and CO2 hydrogenation [2,12], extensive studies have been carried out to investigate the WGS reaction on both bulk surfaces such as Cu, Au, and Pd using DFT [15,53–57]. Two main reaction mechanisms have been proposed: one is a redox mechanism and the other is an associative mechanism. Thus, the WGS mechanism follows both of these mechanisms [15,54–56]. In the process of H2 O adsorption on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, H2 O moves away from the surface after geometry optimization, indicating that H2 O molecules undergo physisorption. The activation energy of H dissociation from H2 O is 1.26 eV, which results in a reaction energy of −0.22 eV. After the initial H2 O dissociation step, three phenomena may proceed during the WGS process: further OH dissociation into O and H (redox mechanism with direct OH dissociation), reaction with another OH* (redox mechanism with another OH), and reaction with CO to produce COOH (associative mechanism through the carboxyl intermediate). As shown in Fig. 7, a low energy of 0.65 eV is obtained for COOH synthesis by CO reaction with OH, whereas energies of 1.44 and 1.87 eV are calculated for the formation of H2 O + O and OH by further dissociation, respectively. Therefore, COOH formation seems likely to
proceed. The preference for the associative mechanism coincides with the results obtained for the WGS reaction on Cu (1 1 1) and Cu (3 2 1)-stepped surfaces, as well as Cu29 cluster [15,55,56]. For COOH in the associative mechanism, two possibilities exist for carboxyl dehydrogenation: one is the direct COO H bond breaking on Cu surface, and the other is COO H bond breaking assisted by an OH group. In the case of direct COOH dehydrogenation, the activation energy is 0.94 eV, which is higher than that of COO H bond breaking assisted by an OH group (0.71 eV). This result shows that CO2 formation by COO H bond breaking assisted by an OH group is likely to proceed. As shown in Fig. 7, the WGS on Cu/␥-Al2 O3 (1 1 0) adopts an associative mechanism via carboxyl, and then dissociates into CO2 and H. Therefore, H2 O dissociation is the rate-limiting step for the WGS on Cu/␥-Al2 O3 (1 1 0) surface. The TS are displayed in Fig. 8. The reaction mechanism and rate-limiting step for the WGS on Cu/␥-Al2 O3 (1 1 0) surface agree with the results obtained for the WGS on Cu (1 1 1) and Cu (3 2 1)-stepped surface, as well as Cu29 cluster [15,55,56]. In order to further understand the effect of liquid paraffin, the activation energies of the rate-limiting step for CO hydrogenation, CO2 hydrogenation and WGS reaction in vacuum are calculated. The activation energies of CH2 O formation, CH2 OO formation and H2 O dissociation are 1.50, 1.22 and 1.27 eV, respectively.
Fig. 8. The TS of intermediates and involved in the WGS on the Cu/␥-Al2 O3 surface in liquid paraffin. (a), (b) and (c): the TS of water dissociation, OH reaction with the CO and COO H bond breaking with an OH group assisted. Pink, red, dark gray and white spheres represent Al, O, Cu and H atoms, respectively. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
Comparing with the corresponding activation energy in liquid paraffin, it can be seen that liquid paraffin influences the activation energies of CH2 OO formation (Ea = 0.12 eV) and CH2 O formation (Ea = 0.13 eV), but does not obviously effect the H2 O dissociation (Ea = 0.01 eV). 4. Conclusion Theoretical studies are carried out to investigate methanol synthesis by CO and CO2 hydrogenation and WGS on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin. The charge may be locally transferred from Cu to the ␥-Al2 O3 carrier, and the transfer of electron charge further decreases during Cu deposition. For CO hydrogenation on Cu/␥-Al2 O3 (1 1 0) surface in liquid paraffin, methanol is synthesized through the intermediates CHO, CH2 O, and CH3 O, and the rate-limiting step is CHO hydrogenation. For CO2 hydrogenation, methanol is synthesized through the intermediates CHOO, CH2 OO, CH2 O, and CH3 O, and the rate-limiting step is CHOO hydrogenation. Regarding WGS, it proceeds via the associative mechanism through the COOH intermediate by carboxyl dehydrogenation assisted by adsorbed OH, and the rate-limiting step is H2 O dissociation. The activation energies of the rate-limiting steps in CO and CO2 hydrogenation, as well as WGS follow the order Ea (CHO hydrogenation, 1.37 eV) > Ea (WGS, 1.26 eV) > Ea (CO2 hydrogenation, 1.10 eV). Liquid paraffin influences the activation energies of CH2 OO formation and CHO formation, but does not obviously effect the H2 O dissociation. Acknowledgements The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (20676087 and 21306125), China Postdoctoral Science Foundation Funded Project (2012M510784), Natural Science Foundation of Shanxi(Grant No. 2012011046-1 and 012021005-1), Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi(2013107), and Special/Youth Foundation of Taiyuan University of Technology (No. 2012L042). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2013.11.092. References [1] A.R. González, Y.J.O. Asencios, E.M. Assaf, J.M. Assaf, Dry reforming of methane on Ni Mg Al nano-spheroid oxide catalysts prepared by the sol–gel method from hydrotalcite-like precursors, Appl. Surf. Sci. 280 (2013) 876–887. [2] S. Park, B. Choi, H. Kim, J.H. Kim, Hydrogen production from dimethyl ether over Cu/␥-Al2 O3 catalyst with zeolites and its effects in the lean NOx trap performance, Int. J. Hydrogen Energy 37 (2012) 4762–4773. [3] C. Wang, G. Yi, H. Lin, Y. Yuan, Na+ -intercalated carbon nanotubes-supported platinum nanoparticles as new highly effective catalysts for preferential CO oxidation in H2 -rich stream, Int. J. Hydrogen Energy 37 (2012) 14124–14132. [4] Y. Jia, G. Li, G. Ning, Efficient oxidative desulfurization (ODS) of model fuel with H2 O2 catalyzed by MoO3 /␥-Al2 O3 under mild and solvent free conditions, Fuel Process. Technol. 92 (2011) 106–111. [5] A.C.P. Filho, R.M. Filho, Hybrid training approach for artificial neural networks using genetic algorithms for rate of reaction estimation: application to industrial methanol oxidation to formaldehyde on silver catalyst, Chem. Eng. J. 157 (2010) 501–508. [6] F. Zha, H. Tian, J. Yan, Y. Chang, Multi-walled carbon nanotubes as catalyst promoter for dimethyl ether synthesis from CO2 hydrogenation, Appl. Surf. Sci. 285 (2013) 945–951. [7] M. Farsi, M.H. Khademi, A. Jahanmiri, M.R. Rahimpour, Optimal conditions for hydrogen production from coupling of dimethyl ether and benzene synthesis, Int. J. Hydrogen Energy 36 (2011) 299–310.
403
[8] T. Ohnishi, T. Yamakawa, S. Shinoda, New preparation of Ru-Sn/Y zeolite catalyst for the formation of acetic acid (methyl acetate) from methanol alone, Appl. Catal. A 231 (2002) 27–33. [9] X. Zhang, Z. Li, Q. Guo, H. Fan, H. Zheng, K. Xie, Influence of the calcination on the activity and stability of the Cu/ZnO/Al2 O3 catalyst in liquid phase methanol synthesis, Fuel 89 (2010) 1348–1352. [10] J.H. Flores, D.P.B. Peixoto, L.G. Appel, R.R. de Avillez, M.I. Pais da Silva, The influence of different methanol synthesis catalysts on direct synthesis of DME from syngas, Catal. Today 172 (2011) 218–225. [11] Z. Li, Z. Zuo, W. Huang, K. Xie, Research on Si–Al based catalysts prepared by complete liquid-phase method for DME synthesis in a slurry reactor, Appl. Surf. Sci. 257 (2011) 2180–2183. [12] Y.X. Yang, J. Evans, J.Z. Rodriguez, M.G. White, P. Liu, Fundamental studies of methanol synthesis from CO2 hydrogenation on Cu(1 1 1), Cu clusters, and Cu/ZnO(0 0 0 1), Phys. Chem. Chem. Phys. 12 (2010) 9909–9917. [13] Y. Morikawa, K. Iwata, K. Terakura, Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfaces, Appl. Surf. Sci. 169–170 (2001) 11–15. [14] J. Yoshihara, C.T. Campbell, Methanol synthesis and reverse water–gas shift kinetics over Cu(1 1 0) model catalysts: structural sensitivity, J. Catal. 161 (1996) 776–782. [15] L.C. Grabow, M. Mavrikakis, Mechanism of methanol synthesis on Cu through CO2 and CO hydrogenation, ACS Catal. 1 (2011) 365–384. [16] M. Digne, P. Sautet, P. Raybaud, P. Euzen, H. Toulhoat, Use of DFT to achieve a rational understanding of acid–basic properties of ␥-alumina surfaces, J. Catal. 226 (2004) 54–68. [17] A. Ionescu, A. Allouche, J.P. Aycard, M. Rajzmann, Structure and stability of aluminum hydroxides: a theoretical study, J. Phys. Chem. B 106 (2002) 5155–5162. [18] Y.X. Pan, C.J. Liu, Q.F. Ge, Effect of surface hydroxyls on selective CO2 hydrogenation over Ni4 /␥-Al2 O3 : a density functional theory study, J. Catal. 272 (2010) 227–234. [19] R.G. Zhang, B.J. Wang, H.Y. Liu, L.X. Ling, Effect of surface hydroxyls on CO2 hydrogenation over Cu/␥-Al2 O3 catalyst: a theoretical study, J. Phys. Chem. B 115 (2011) 19811–19818. [20] M.C. Valero, P. Raybaud, P. Sautet, Interplay between molecular adsorption and metal–support interaction for small supported metal clusters: CO and C2 H4 adsorption on Pd4 /␥-Al2 O3 , J. Catal. 247 (2007) 339–355. [21] Y.X. Pan, C.J. Liu, T.S. Wiltowski, Q.F. Ge, CO2 adsorption and activation over ␥-Al2 O3 -supported transition metal dimers: a density functional study, Catal. Today 147 (2009) 68–76. [22] B. Delley, From molecules to solids with the DMol3 approach, J. Chem. Phys. 113 (2000) 7756–7764. [23] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys. Rev. B 136 (1964) 864–871. [24] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects, Phys. Rev. A 140 (1965) 1133–1138. [25] J.P. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas correlation energy, Phys. Rev. B 45 (1992) 13244–13249. [26] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188–5192. [27] M. Tomonari, O. Sugino, DFT calculation of vibrational frequency of hydrogen atoms on Pt electrodes: analysis of the electric field dependence of the Pt–H stretching frequency, Chem. Phys. Lett. 437 (2007) 170–175. [28] A. Singh, B. Ganguly, DFT study of urea interaction with potassium chloride surfaces, Mol. Simul. 34 (2008) 973–979. [29] Z.J. Zuo, W. Huang, P.D. Han, Z.H. Gao, Z. Li, Theoretical studies on the reaction mechanisms of AlOOH- and ␥-Al2 O3 -catalysed methanol dehydration in the gas and liquid phases, Appl. Catal. A 40 (2011) 130–136. [30] T. Todorova, B. Delley, Wetting of paracetamol surfaces studied by DMol3 COSMO calculations, Mol. Simul. 34 (2008) 1013–1017. [31] T.A. Halgren, W.N. Lipscomb, The synchronous-transit method for determining reaction pathways and locating molecular transition states, Chem. Phys. Lett. 49 (1977) 225–232. [32] S.G. Wang, D.B. Cao, Y.W. Li, J.G. Wang, H.J. Jiao, Chemisorption of CO2 on the nickel surfaces, J. Phys. Chem. C 109 (2005) 18956–18963. [33] B. Xing, X.Y. Pang, G.C. Wang, C H bond activation of methane on clean and oxygen pre-covered metals: a systematic theoretical study, J. Catal. 282 (2011) 74–82. [34] S.V. Didziulis, K.D. Butcher, S.L. Cohen, E.I. Solomon, Chemistry of copper overlayers on zinc oxide single-crystal surfaces: model active sites for copper/zinc oxide methanol synthesis catalysts, J. Am. Chem. Soc. 111 (1989) 7110–7123. [35] K. Ozawa, T. Sato, Y. Oba, K. Edamoto, Electronic structure of Cu on ZnO(1 0 1 0): angle-resolved photoemission spectroscopy study, J. Phys. Chem. C 4256 (2007) 4256–4263. [36] J. Hu, W.P. Guo, X.R. Shi, B.R. Li, J.G. Wang, Copper deposition and growth over ZnO nonpolar (1 0 1 0) and (1 1 2 0) surfaces: a density functional theory study, J. Phys. Chem. C 113 (2009) 7227–7235. [37] Z.J. Zuo, P.D. Han, Z. Li, J.S. Hu, W. Huang, Can methanol be synthesized from CO by direct hydrogenation over Cu/ZnO catalysts? Appl. Surf. Sci. 261 (2012) 640–646. [38] Z.J. Zuo, L.L. Sun, W. Huang, P.D. Han, Z.H. Li, Surface properties of copper in different solvent mother solutions: a density functional theory study, Appl. Catal. A 375 (2010) 181–187. [39] Z.J. Zuo, W. Huang, P.D. Han, Z.H. Li, Solvent effects for CO and H2 adsorption on Cu2 O (1 1 1) surface: a density functional theory study, Appl. Surf. Sci. 256 (2010) 2357–2362.
404
Z.-J. Zuo et al. / Applied Surface Science 290 (2014) 398–404
[40] Z.J. Zuo, W. Huang, P.D. Han, Z.H. Li, CO and H2 molecules adsorption on Cu(1 1 1) surface and solvent effects, Acta Phys. Chim. Sin. 25 (2009) 2507–2512. [41] Z.J. Zuo, W. Huang, P.D. Han, A DFT study the solvent effects of H2 adsorption on Cu (h k l) surface, Appl. Surf. Sci. 258 (2012) 3364–3367. [42] E.M. Calverley, K.J. Smith, Kinetic model for alcohol synthesis over a promoted copper/zinc oxide/chromium oxide (Cr2 O3 ) catalyst, Ind. Eng. Chem. Res. 31 (1992) 792–803. [43] D. Liu, Q. Zhu, J. Li, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalysis, vol. 2, Chemical Institute of Canada, Ottawa, 1988, p. 577. [44] J.F. Edwards, G.L. Schrader, In situ Fourier transform infrared study of methanol synthesis on mixed metal oxide catalysts, J. Catal. 94 (1985) 175–186. [45] J. Saussey, J.C. Lavalley, An in situ FT-IR study of adsorbed species on a CuZnAl2 O4 methanol catalyst under 1 MPa pressure and at 525 K: effect of the H2 /CO/CO2 feed stream composition, J. Mol. Catal. 50 (1989) 343–353. [46] S. Fujita, M. Usui, E. Ohara, N. Takezawa, Methanol synthesis from carbon dioxide at atmospheric pressure over Cu/ZnO catalyst. Role of methoxide species formed on ZnO support, Catal. Lett. 13 (1992) 349–358. [47] J. Greeley, M. Mavrikakis, Methanol decomposition on Cu(1 1 1): a DFT study, J. Catal. 208 (2002) 291–300. [48] J.R.B. Gomes, J.A.N.F. Gomes, F. Illas, First-principles study of the adsorption of formaldehyde on the clean and atomic oxygen covered Cu(1 1 1) surface, J. Mol. Catal. A 170 (2001) 187–193. [49] P.B. Rasmussen, M. Kazuta, I. Chorkendorff, Synthesis of methanol from a mixture of H2 and CO2 on Cu(1 0 0), Surf. Sci. 318 (1994) 267–280.
[50] P.B. Rasmussen, P.M. Holmblad, T. Askgaard, C.V. Ovesen, P. Stoltze, J.K. Nørskov, I. Chorkendorff, Methanol synthesis on Cu(1 0 0) from binary gas mixtures of CO2 and H, Catal. Lett. 26 (1994) 373–381. [51] Q.L. Tang, Q.J. Hong, Z.P. Liu, CO2 fixation into methanol at Cu/ZrO2 interface from first principles kinetic Monte Carlo, J. Catal. 263 (2009) 114–122. [52] J.B. Hansen, P.E.H. Nielsen, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 13, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008, pp. 2920–2949. [53] A. Mondal, H. Seenivasan, A.K. Tiwari, Water dissociation on Cu (1 1 1): effects of molecular orientation, rotation, and vibration on reactivity, J. Chem. Phys. 137 (2012) 094708. [54] P. Liu, J.A. Rodriguez, Water–gas-shift reaction on metal nanoparticles and surfaces, J. Chem. Phys. 126 (2007) 164705. [55] A.A. Gokhale, J.A. Dumesic, M. Mavrikakis, On the mechanism of lowtemperature water gas shift reaction on copper, J. Am. Chem. Soc. 130 (2008) 1402–1414. [56] J.L.C. Fajín, M.N.D.S. Cordeiro, F. Illas, J.R.B. Gomes, Influence of step sites in the molecular mechanism of the water gas shift reaction catalyzed by copper, J. Catal. 268 (2009) 131–141. [57] J.L.C. Fajín, A. Bruix, M. Natália, D.S. Cordeiro, J.R.B. Gomes, F. Illas, Density functional theory model study of size and structure effects on water dissociation by platinum nanoparticles, J. Chem. Phys. 137 (2012) 034701.