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J. Phys. Chem. C 2008, 112, 3840-3848
Adsorption and Dissociation of CO as Well as CHx Coupling and Hydrogenation on the Clean and Oxygen Pre-covered Co(0001) Surfaces Chun-Fang Huo,† Yong-Wang Li,† Jianguo Wang,† and Haijun Jiao*,†,‡ State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ReceiVed: NoVember 2, 2007; In Final Form: December 12, 2007
Spin-polarized density functional theory calculations were performed to investigate the oxidation of the Co(0001) surface and the effects of surface O on CO adsorption and dissociation as well as CHx coupling and hydrogenation. Under the realistic Fischer-Tropsch synthesis conditions (493 K, PH2O/PH2 ) 1-1.5), Co(0001) can be oxidized by H2O forming a 1/4 monolayer O-covered surface, while subsurface oxidation or high oxidized surface (1/2 monolayer) is thermodynamically not possible. Compared to clean Co(0001), the oxygen pre-covered surface lowers the ability for CO adsorption and activation, elevates the CO dissociation barrier, and favors CO2 formation, as well as elevates the CH/CH coupling barrier and favors the CH hydrogenation thermodynamically. Consequently, the catalytic activity is reduced, and the monomeric building block for chain growth changes from CH to CH2. Interestingly, the formation of CO2 will refresh the surface. Therefore, we suggest that either controlling H2O content to lower the possibility of oxidation or raising CO content to refresh the surface will maintain the surface activity.
1. Introduction As an efficient technology for converting syngas (CO + H2) to liquid fuels, Fischer-Tropsch synthesis (FTS) has been receivingwidespreadattentionbothacademicallyandindustrially.1-10 Although all group-VIII metals have noticeable activity in hydrogenation of CO to hydrocarbons, only iron and cobalt catalysts have found industrial applications.3,11 Because of the high pass conversion, low water-gas shift (WGS) activity, and paraffinic nature of the synthetic crude oil, cobalt catalysts are attractive in FTS industry, especially for converting natural gas with high H2/CO ratio (∼2) to diesel fuels.12-14 However, cobalt catalysts are relatively expensive as compared to iron and should have a long lifetime to balance the cost and performance. Therefore, understanding the deactivation mechanism at the molecular level is very crucial for developing efficient cobaltbased FTS catalysts with high activity and stability. Oxidation of metallic cobalt to cobalt oxide by water during FTS has long been postulated as a major deactivation mechanism.15 Several thermodynamiccalculations13,16 andconsiderableexperiments12,17-31 have been devoted to clarify this point, but to date there is no consistent picture. Thermodynamic analysis by van Berge et al.13 revealed that oxidation of bulk cobalt metal to CoO or Co3O4 is not spontaneous during FTS. However, a recent study16 showed that oxidation of nanosized cobalt crystallites ( CH2 > CHCH) is not changed. On the clean Co(0001) surface, the CH/CH coupling and CH hydrogenation reactions are competitive kinetically with very close energy barriers (0.52 vs 0.55 eV), while the CH/CH coupling reaction is more favored thermodynamically (-0.96 vs 0.17 eV). In addition, the endothermic character (0.17 eV) of CH hydrogenation suggests that the population of CH is much higher than that of CH2 on the clean Co(0001) surface (CH/CH2 ) 54). Therefore, CH is an important monomeric building block for chain growth, and the longer chain hydrocarbons are the main products for the cobalt catalyzed FTS. This is consistent with the early experimental observation by Steinbach et al.68 They found that CH2 dehydrogenation on Co surfaces would occur at a very low temperature of 180 K. Compared to those on the clean Co(0001) surface, the energy barrier of the CH/CH coupling reaction on Ohcp-p(2 × 2)-Co(0001) is raised by 0.19 eV, while the change of the reaction energy is very small (-1.00 vs -0.96 eV). Moreover, the presence of surface O has no influence on the energy barrier of the CH hydrogenation reaction (0.55 vs 0.55 eV) but strongly influences the reaction energy (-0.21 vs 0.17 eV). Thus, the CH hydrogenation reaction becomes more favored kinetically than the CH/CH coupling reaction (0.71 vs 0.55 eV), and CH2/ So becomes more stable than CH + H/So by 0.21 eV. At this stage, it is interesting to pay attention to the CH2 subsequent reactions on Ohcp-p(2 × 2)-Co(0001). As shown in Figure 8, the CH2/CH2 coupling reaction has no barrier and is highly exothermic by -1.19 eV. Alternatively, the other potential reactions, the CH/CH2 coupling and CH2 hydrogenation, are predicted to be exothermic by -0.85 and -0.55 eV, with the energy barriers of 0.52 and 0.47 eV, respectively. These indicate that the CH/CH2 coupling and CH2 hydrogenation are not competitive with the CH2/CH2 coupling, and CH2 becomes an important monomeric building block for chain growth on the Ohcp-p(2 × 2)-Co(0001) surface. In summary, it is suggested that the FT reaction and WGS reaction proceed on different active sites; metallic cobalt is the FT active phase, and the cobalt oxidized species is the WGS active phase. For well-reduced cobalt catalysts, the WGS activity is very low. Hence, the partial pressure of H2O increases gradually as the reactions proceed. When the PH2O/PH2 ratio is larger than 1 (at 493 K), the Co(0001) surface can be oxidized by H2O forming a 1/4 ML O-covered surface. Alternatively, CO can refresh the surface. Therefore, the formation of surface oxygen from H2O would not cause permanent deactivation but will affect the selectivity of the reactions. 4. Conclusions Employing the spin-polarized DFT method, we first studied the adsorption and diffusion of O atom on the Co(0001) surface, and then we discussed the effects of pre-covered O on CO adsorption, activation, and dissociation as well as on the CHx coupling and hydrogenation reactions.
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3847 It is found that the O atom binds strongly with Co(0001) and prefers the hcp site at 1/4 ML. At 1/2 ML, there is no apparent preference for the adsorption modes on hcp sites (Ohh) as well as on hcp and fcc sites (Ohf). Subsurface adsorption is neither kinetically nor thermodynamically favored. Under the realistic FTS reaction conditions at 493 K and PH2O/ PH2 ) 1-1.5, the Co(0001) surface can be oxidized by H2O forming a 1/4 ML O-covered surface, while subsurface oxidation or high oxidized surface (1/2 ML O coverage) is thermodynamically not possible. On the clean Co(0001) surface, although the CH/CH coupling and CH hydrogenation reactions are competitive kinetically, the CH/CH coupling reaction is more favored thermodynamically. Considering its much higher population (CH/CH2 ) 54), CH is an important monomeric building block for chain growth. Compared to the clean Co(0001) surface, the 1/4 ML O precovered Co(0001) surface lowers the ability for CO adsorption (-1.42 vs -1.76 eV) and activation (1.179 vs 1.189 Å), raises the CO dissociation barrier (4.11 vs 2.79 eV), and favors the formation of CO2, as well as raises the energy barrier of the CH/CH coupling (0.71 vs 0.52 eV) and favors the CH hydrogenation thermodynamically (-0.21 vs 0.17 eV). Therefore, the activity of FTS is decreased, and the selectivity of CO2 is increased. Furthermore, the enhancement of the CH hydrogenation leads to CH2 becoming an important monomeric building block for chain growth on Ohcp-p(2 × 2)-Co(0001). It is interesting that the formation of CO2 on the oxygen pre-covered surface is advantageous because it will liberate and refresh the surface. Therefore, either reducing H2O content to lower the possibility of the surface oxidation or raising CO content to refresh the surface will be the method for maintaining the catalytic activity of the Co(0001) surface. Acknowledgment. This work was supported by the Chinese Academy of Science (No. 20069908), the National Natural Science Foundation of China (No. 20590361), and the National Outstanding Young Scientists Foundation of China (No. 20625620). References and Notes (1) Schulz, H. Appl. Catal., A 1999, 186, 3. (2) Geerlings, J. J. C.; Wilson, J. H.; Kramer, G. J.; Kuipers, H. P. C. E.; Hoek, A.; Huisman, H. M. Appl. Catal., A 1999, 186, 27. (3) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. ReV. 2007, 107, 1692. (4) Steynberg, A. P.; Dry, M. E. Stud. Surf. Sci. Catal. 2004, 152, 441. (5) Iglesia, E.; Reyes, S. C.; Madon, R. J. J. Catal. 1991, 129, 238. (6) Bianchi, C. L.; Ragaini, V. J. Catal. 1997, 168, 70. (7) Zheng, C.; Apeloig, Y.; Hoffmann, R. J. Am. Chem. Soc. 1988, 110, 749. (8) Turner, M. L.; Byers, P. K.; Long, H. C.; Maitlis, P. M. J. Am. Chem. Soc. 1993, 115, 4417. (9) Liu, Z. P.; Hu, P. J. Am. Chem. Soc. 2002, 124, 11568. (10) Khodakov, A. Y.; Lynch, J.; Bazin, D.; Rebours, B.; Zanier, N.; Moisson, B.; Chaumette, P. J. Catal. 1997, 168, 16. (11) Vannice, M. A. J. Catal. 1975, 37, 449. (12) Iglesia, E. Appl. Catal., A 1997, 161, 59. (13) van Berge, P. J.; van de Loosdrecht, J.; Barradas, S.; van der Kraan, A. M. Catal. Today 2000, 58, 321. (14) Eisberg, B.; Fiato, R. A. Stud. Surf. Sci. Catal. 1998, 199, 961. (15) Bartholomew, C. H. Appl. Catal., A 2001, 212, 17. (16) van Steen, E.; Claeys, M.; Dry, M. E.; van de Loosdrecht, J.; Viljoen, E. L.; Visagie, J. L. J. Phys. Chem. B 2005, 109, 3575. (17) Iglesia, E. Stud. Surf. Sci. Catal. 1997, 107, 153. (18) Kiss, G.; Kliewer, C. E.; De Martin, G. J.; Culross, C. C.; Baumgartner, J. E. J. Catal. 2003, 217, 127. (19) Krishnamoorthy, S.; Tu, M.; Ojeda, M. P.; Pinna, D.; Iglesia, E. J. Catal. 2002, 211, 422. (20) Saib, A. M.; Borgna, A.; van de Loosdrecht, J.; van Berge, P. J.; Niemantsverdriet, J. W. J. Phys. Chem. B 2006, 110, 8657.
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