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Combinatorial Computational Chemistry Approach to the High-Throughput Screening of Metal Sulfide Catalysts for CO Hydrogenation Process Momoji Kubo,† Tsuguo Kubota,† Changho Jung,† Kotaro Seki,† Seiichi Takami,† Naoto Koizumi,‡ Kohji Omata,‡ Muneyoshi Yamada,‡ and Akira Miyamoto*,†,§ Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan, Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan, and New Industry Creation Hatchery Center, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan Received October 15, 2002
We have already proposed that a “Combinatorial Computational Chemistry” approach is very effective for performing the theoretical high-throughput screening of new catalysts, and its validity was strongly confirmed in various catalyst systems. In the present study, we applied our combinatorial computational chemistry approach to the design of new metal sulfide catalysts for the CO hydrogenation process and proposed new guidance for designing the highly selective catalysts for methanol synthesis. We investigated H2 and CO adsorption on a large number of metal and metal sulfide catalysts by first-principles calculations, and succeeded in clarifying the relationship between the metal species in the metal and metal sulfide catalysts and the products of the CO hydrogenation processes. Our results indicated that Co, Mo, Ru, Rh, Ir, and Pd sulfide catalysts selectively produce methanol, while Re and Os sulfide catalysts selectively produce hydrocarbons. The above results are in good agreement with the experimental results of Koizumi and co-workers. Moreover, we proposed that the Pd sulfide catalyst has the highest selectivity for methanol from the CO hydrogenation process. This result strongly supports the experimental results by Koizumi and co-workers. Moreover, we propose that the metal sulfide catalysts, which realize the bridge-site adsorption of the CO molecule on both the metal and sulfur atoms, have high selectivity for methanol. This proposed guidance for designing the highly selective metal sulfide catalysts for methanol may be useful for the experiments.
Introduction Recently, computational chemistry made great impacts on catalyst design and development. Especially significant is the recent advance of the first-principles calculation method, and it can calculate the formation energy of molecules and solids with a high accuracy of only 10 kJ/mol errors. However, computational chemistry is mainly employed to clarify the atomistic mechanism of the well-known catalytic reactions and to obtain electronic information on the catalysts of which property and reactivity are well-known experimentally. For example, recently there have been a lot of precise calculations for the CuZSM-5 to clarify the chemical reaction mechanism of deNOx processes. However, there are very few theoretical calculations to search for and design new catalysts over the CuZSM-5 for the deNOx reactions. We suggested that such traditional computational chemistry cannot contribute to the design and development of new catalysts at all. Hence, we recently * Corresponding author. † Department of Materials Chemistry, Graduate School of Engineering. ‡ Department of Applied Chemistry, Graduate School of Engineering. § New Industry Creation Hatchery Center.
introduced a concept of combinatorial chemistry used in drug development to computational chemistry and proposed a new concept “Combinatorial Computational Chemistry”.1-3 In this concept, the computational chemistry is employed to perform a high-throughput screening of the catalysts, including active metals, supports, and additives. The activity, selectivity, and functionality of many catalysts are calculated systematically, and the best catalyst is efficiently proposed. Moreover, our approach can propose new guidance for the design of highly active and highly selective catalysts by the investigation of many catalysts. We have already applied our combinatorial computational chemistry approach to the design of the zeolite catalysts for the deNOx reaction and proposed that IrZSM-5 has the highest tolerance to water poisoning.1 Moreover, after our proposal, the validity of IrZSM-5 was confirmed experimentally. Furthermore, we have also applied our (1) Yajima, K.; Ueda, Y.; Tsuruya, H.; Kanougi, T.; Oumi, Y.; Ammal, S. S. C.; Takami, S.; Kubo M.; Miyamoto, A. Appl. Catal. A 2000, 194/ 195, 183. (2) Yajima, K.; Sakahara, S.; Ueda, Y.; Belosludov, R.; Takami, S.; Kubo, M.; Miyamoto, A. Proc. SPIE 2000, 3941, 62. (3) Belosludov, R.; Ammal, S. S. C.; Inaba, Y.; Oumi, Y.; Takami, S.; Kubo, M.; Miyamoto, A.; Kawasaki, M.; Yoshimoto, M.; Koinuma, H. Proc. SPIE 2000, 3941, 2.
10.1021/ef020245s CCC: $25.00 © 2003 American Chemical Society Published on Web 06/05/2003
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combinatorial computational chemistry approach to the methanol synthesis catalysts based on Cu/ZnO and to the Fischer-Tropsch catalysts based on the supported Fe metals and have succeeded in proposing new catalysts and additives.4-8 Hence, we have strongly confirmed the applicability and effectiveness of our combinatorial computational chemistry approach to catalyst design. On the other hand, the synthesis of the high-quality transportation fuels is expected in terms of the highly efficient utilization of the energy and low environmental impact. The development of highly active and highly selective catalysts with high resistance to sulfur for methanol synthesis and Fischer-Tropsch synthesis is strongly demanded in order to advance the industry of high-quality transportation fuels. Recently, Koizumi and co-workers reported that a lot of metal sulfide catalysts have high activity for the CO hydrogenation process9 and Rh sulfide, especially, has the highest activity and predominantly produces methanol.10 Moreover, they found that the Rh sulfide catalyst is not deactivated by H2S, while the regular Cu/ZnO/Al2O3 methanol synthesis catalyst is easily deactivated by H2S.10 Moreover, they also reported that the products of the CO hydrogenation process are strongly dependent on the metal species of the metal sulfide catalysts.9 For example, Rh and Pd sulfides selectively produce methanol in CO hydrogenation, while Re and Os sulfides predominantly produce hydrocarbons. In the present study, we applied our combinatorial computational chemistry based on the first-principles approach to the metal sulfide catalysts and investigated the relationship between the metal species in the metal sulfide catalysts and the CO hydrogenation process and their products. Moreover, the best metal sulfide catalyst with high selectivity for the methanol synthesis was designed theoretically. Methods First-Principles Calculation. Combinatorial computational chemistry based on the density functional theory (DFT) calculation11 is performed by using the ADF program12-14 developed in Vrije University, The Netherlands. In this program, linear combinations of Slater-type atomic orbitals are used in the Kohn-Sham formulation.15 The basic postulate in Kohn-Sham DFT (4) Belosludov, R.; Kubota T.; Sakahara, S.; Yajima, K.; Takami, S.; Kubo, M.; Miyamoto, A. Proc. SPIE 2001, 4281, 87. (5) Sakahara, S.; Yajima, K.; Belosludov, R.; Takami, S.; Kubo, M.; Miyamoto, A. Proc. SPIE 2001, 4281, 97. (6) Belosludov, R. V.; Takami, S.; Kubo, M.; Miyamoto, A.; Kawazoe, Y. Mater. Trans. 2001, 40, 2180. (7) Sakahara, S.; Yajima, K.; Belosludov, R.; Takami, S.; Kubo, M.; Miyamoto, A. Appl. Surf. Sci. 2002, 189, 253. (8) Belosludov, R. V.; Sakahara, S.; Yajima, K.; Takami, S.; Kubo, M.; and Miyamoto, A. Appl. Surf. Sci. 2002, 189, 245. (9) Koizumi, K.; Furukawa, T.; Miyazawa, A.; Ozaki, T.; Takahashi, Y.; and Yamada, M. International Symposium on Synthesis of Ecological High Quality Transportation Fuels 2000, 53. (10) Yamada, M.; Koizumi, N.; Miyazawa, A.; Furukawa, T. Catal. Lett. 2002, 78, 195. (11) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, 865. (12) Baerends, E. J.; Ellis, D. E.; Ros, D. P. Chem. Phys. 1973, 2, 41. (13) Baerends, E. J.; Ros, D. P. Chem. Phys. 1973, 2, 51. (14) Boerringt, P. M.; te Velde, G.; Baerends, E. J. Int. J. Quantum Chem. 1988, 33, 87. (15) Kohn, W.; Sham, L. Phys. Rev. A 1965, 140, 1133.
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is that we can apply a one-electron formulation to the system of N interacting electrons by introducing a suitable local potential Vxc(r), in addition to any external potentials Vext(r) and the Coulomb potential of the electron cloud Vc(r), and solving the following equation.16
(- 21∇
2
)
+ Vext(r) + Vc(r) + Vxc(r) φi(r) ) iφi(r)
The potential Vxc is the functional derivative with respect to the density F of Exc[F], the exchange and correlation energy functional. The one-electron molecular orbitals φi with corresponding orbital energies i define the exact electronic charge density. In the present study, the structure optimization is carried out by local density approximation (LDA) with the Vosko-Wilk-Nusair (VWN) functional.17 The energy of the optimized structure is calculated by generalized gradient approximation (GGA) with Becke 88 and Perdew-Wang 91 functionals.18-19 The BFGS algorithm is employed for the structure optimization.20 The triple-ζ basis sets extended by polarization functions are used. The Mulliken method is employed to perform the atomic population analysis.21 The computer graphics pictures of catalyst models and adsorption structures of molecules are produced by the Cerius2 program developed in MSI Inc. Development of New Software for Combinatorial Computational Chemistry. In the present study, we developed new software to perform combinatorial computational chemistry. The new software realizes automatic modeling of a large number of catalysts by the continuous replacement of elements in the catalyst model. Moreover, it automatically performs the firstprinciples calculations of the above large number of catalyst models by using the ADF program, as well as realizes the automatic analysis of the optimized structure, bond distance, electronic states, electron transfer, bond population, and so on. All the modeling, calculation, and analysis for a large number of catalysts can be performed as only one job. This new software for combinatorial computational chemistry is very useful for performing a theoretical automatic screening of a large number of catalysts and for proposing a new highperformance catalyst among a large number of candidates. Models of Metal and Metal Sulfide Catalysts. To model the metal and metal sulfide catalysts, we employed diatomic molecules, M-M and M-S (M ) metal, S ) sulfur), respectively. Although it is better to employ the large catalyst model including more than 10-100 atoms, we employed the simplest models to perform high-throughput screening of catalysts. Since the calculation speed is most important for the high-throughput screening, the simplest model is adequate for the (16) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931. (17) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (18) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (19) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolliais, C. Phys. Rev. B 1992, 46, 6671. (20) Broyden, C. G. J. Inst. Math. Appl. 1970, 6, 76. (21) Mulliken, S. H. J. Chem. Phys. 1955, 23, 1833.
Combinatorial Computational Chemistry in Catalyst Screening Table 1. Binding Energy (Ebind), Charge on Metal, and Distance of Various Diatomic Metal and Metal Sulfide Catalysts
Energy & Fuels, Vol. 17, No. 4, 2003 859 Table 2. Adsorption Energy (Eads), Charge, and H-H Distance of the H2 Molecule Adsorbed on Various Metal and Metal Sulfide Catalysts
model
Ebind (kJ/mol)
metal charge
distance (nm)
model
Eads (kJ/mol)
H2 charge
H-H distance (nm)
Co-Co Co-S Mo-Mo Mo-S Ru-Ru Ru-S Rh-Rh Rh-S Ir-Ir Ir-S Pd-Pd Pd-S Re-Re Re-S Os-Os Os-S
-808.80 -795.00 -1209.26 -932.70 -818.81 -750.44 -600.11 -649.94 -795.92 -721.15 -131.75 -409.78 -1115.20 -819.23 -964.20 -785.76
0.000 0.386 0.000 0.508 0.000 0.494 0.000 0.429 0.000 0.660 0.000 0.338 0.000 0.658 0.000 0.694
0.1954 0.1962 0.1973 0.2114 0.2103 0.2075 0.2199 0.2050 0.2216 0.2074 0.2416 0.2088 0.1883 0.2154 0.2080 0.2102
Co-Co Co-S Mo-Mo Mo-S Ru-Ru Ru-S Rh-Rh Rh-S Ir-Ir Ir-S Pd-Pd Pd-S Re-Re Re-S Os-Os Os-S
-55.90 -47.03 -277.91 -97.46 -102.62 -104.47 -119.78 -126.80 -122.41 -164.95 -129.85 -117.64 -31.22 -31.37 -87.51 -184.77
-0.010 -0.008 -0.318 -0.075 -0.300 -0.014 -0.222 0.009 -0.520 -0.115 -0.316 0.008 -0.363 -0.112 -0.535 -0.124
0.0880 0.3293 0.2452 0.2097 0.2322 0.2521 0.2254 0.2366 0.2590 0.1914 0.1469 0.2191 0.2300 0.3268 0.2573 0.2234
present purpose. In the present study, we investigate the H2 and CO adsorption on the Co-Co, Mo-Mo, RuRu, Rh-Rh, Ir-Ir, Pd-Pd, Re-Re, and Os-Os as metal catalysts and Co-S, Mo-S, Ru-S, Rh-S, Ir-S, Pd-S, Re-S, and Os-S as metal sulfide catalysts. Moreover, to check the validity of the simplest clusters for the present purpose, we constructed a Rh6 cluster and compared the CO adsorption results on the Rh2 and Rh6 clusters. Results and Discussion H2 Adsorption on Metal and Metal Sulfide Catalysts. The dissociative adsorption of H2 molecules on catalyst surfaces is one of the key steps in the CO hydrogenation process. The investigation on the adsorption structure of the H2 molecule is important to design the highly active and highly selective catalysts for the CO hydrogenation. Hence, we applied our combinatorial computational chemistry approach to the H2 adsorption on various metal sulfide catalysts. The H2 adsorption on various metal catalysts was also calculated for comparison. Before the calculation of the H2 adsorption on the various catalysts, we optimized the structures of metal and metal sulfide catalysts, which are represented by the diatomic clusters. Table 1 shows the binding energy, charge on metal, and distance of various diatomic metal and metal sulfide catalysts. Here, it is interesting to see that the binding energies of the PdPd and Pd-S are extremely smaller than other metal and metal sulfide catalysts. After the optimization of the catalyst structures, we calculated the adsorption structure of the H2 molecule on the various catalysts. Since the elongation of the H-H distance is required for the CO hydrogenation, we paid attention to the H-H distance of the H2 molecule adsorbed on the various metal and metal sulfide catalysts. Table 2 shows the adsorption energy, charge, and H-H distance of the H2 molecule on the various catalysts. Here, the H-H distance of the H2 molecule in the vapor phase is 0.074 nm. We found that the H-H distance of the H2 molecule adsorbed on all the metal and metal sulfide catalysts is elongated, compared to the H-H distance in the vapor phase. It indicates that the H2 molecule on all the catalysts is activated by the adsorption. Only on the Co metal catalyst did the H2 molecule not dissociate, although on the other metal and
metal sulfide catalysts the dissociative adsorption of the H2 molecule is observed. Hence, we concluded that all the metal and metal sulfide catalysts, except Co metal catalyst, dissociate the H2 molecule and hence those catalysts are effective for the CO hydrogenation processes. Moreover, the H2 molecule on all the metal catalysts has negative charge. It indicates that the electron donation from the metal catalysts to the H2 molecule was carried out and it leads to the H2 dissociation, except on the Co metal catalyst. Compared to the H2 molecule on the metal catalysts, the H2 molecule adsorbed on the metal sulfide catalysts has low negative charge or plus charge. This is due to the positive charge of the H atom adsorbed on the sulfur atom of the catalysts, although the H atom adsorbed on the metal atom has negative charge. Hence, we suggested that the different mechanism of the H2 dissociation takes place on the metal and metal sulfide catalysts. CO Adsorption on Metal and Metal Sulfide Catalysts. Our combinatorial computational chemistry approach was applied to the CO adsorption on various metal sulfide catalysts. We also calculated the CO adsorption on various metal catalysts for comparison, by using our combinatorial computational chemistry. The elongation of the C-O distance may lead to the formation of hydrocarbons, while the shrinkage of the C-O distance may lead to the formation of methanol. Hence, we paid attention to the C-O distance of the CO molecule adsorbed on the various metal and metal sulfide catalysts. Figure 1 shows the adsorption energy, charge, and C-O distance of the CO molecule adsorbed on various catalysts. Here, the C-O distance of the CO molecule in the vapor phase is 0.113 nm. We found that the C-O distance of the CO molecule adsorbed on all the metal catalysts and metal sulfide catalysts is elongated, compared to the C-O distance in the vapor phase. It indicates that CO molecule on all the catalysts is activated by the adsorption. Moreover, the C-O distance was found to be strongly depending on the metal species. The C-O distance is shortened on the Co, Mo, Ru, Rh, and Ir sulfide catalysts compared to their metal catalysts. It indicates that these metal sulfide catalysts selectively produce methanol, which is in agreement with the experimental results by Koizumi and co-workers.9 On the other hand, since the C-O distance is elongated on the Re and Os sulfide catalysts
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Figure 1. Adsorption energy, charge, and C-O distance of the CO molecule adsorbed on various metal and metal sulfide catalysts.
Figure 2. Adsorption structures of the CO molecule on the metal sulfide catalysts. (a) Except Pd sulfide catalysts, and (b) Pd sulfide catalysts.
compared to their metal catalysts, these metal sulfide catalysts are suggested to be very effective in producing hydrocarbons selectively, which is also in good agreement with the experimental results by Koizumi and coworkers.9 Moreover, we clarified that the Pd sulfide catalysts have the specific characteristics compared to the other metal sulfide catalysts. We suggested that the Pd sulfide has the highest selectivity of the methanol synthesis among all the metal sulfide catalysts, which is in agreement with the experimental result by Koizumi and co-workers.22 The CO molecule has the plus charge on the Pd sulfide catalysts, while the CO molecule has the minus charge on the other metal and metal sulfide catalysts, as shown in Figure 1. It was suggested that this difference is due to the different adsorption structure of the CO molecule on the Pd sulfide and the other metal sulfide catalysts as shown in Figure 2. CO molecule adsorbs on the on-top site of the metal species (22) Koizumi, N.; Miyazawa, A.; Furukawa, T.; Yamada, M. Chem. Lett. 2001, 1282.
in the case of all the metal sulfide catalysts except the Pd sulfide catalyst. Hence, electrons transfer from metal species to the CO molecule and then CO gains a minus charge on most metal sulfide catalysts. Similarly, the CO molecule surely adsorbs on the metal species in the case of all the metal catalysts, and then electrons transfer from the metal to the CO molecule and the CO molecule gains a minus charge. On the other hand, the CO molecule adsorbs on the bridge-site of the Pd sulfide catalyst and is in contact with both the Pd and sulfur atoms of the Pd sulfide catalyst. Hence, electrons transfer from the CO molecule to the sulfur atom and CO gains a plus charge. Koizumi and co-workers employed Fourier transform infrared spectroscopy technique to observe the adsorption structure of CO molecules on the Pd sulfide and Rh sulfide catalysts.23 They reported that all the CO molecules adsorbed on the Rh sulfide catalyst have single coordination to the catalyst, (23) Koizumi, N.; Murai, K.; Tamayama, S.; Kato, H.; Ozaki, T.; Yamada, M. Prepr. Pap.sAm. Chem. Soc., Fuel Chem. Div. 2002, 47, 519.
Combinatorial Computational Chemistry in Catalyst Screening
Figure 3. Molecular orbitals of the CO molecule.
while the CO molecules with double coordination were observed on the Pd sulfide catalyst. This result strongly supports our calculation results, which suggest the double coordination of the CO molecule on both the Pd and sulfur atoms of the Pd sulfide catalysts. Figure 3 shows the molecular orbital of the CO molecule. The lowest unoccupied molecular orbital (LUMO) and the second LUMO of the CO molecule are antibonding orbitals, and the electron has to be transferred to the above two orbitals of the CO molecule from catalysts in order to dissociate the CO molecule. However, the CO molecule on the Pd sulfide catalyst has a plus charge and then a large number of electrons have to be transferred to the CO molecule from the catalysts after the adsorption in order to dissociate the CO molecule. Hence, the dissociation of the CO molecule is much more difficult on the Pd sulfide catalyst than on the other metal sulfide catalysts. This is the reason we suggested that the Pd sulfide catalyst has the highest selectivity toward methanol. Moreover, we proposed that the metal sulfide catalysts, which realize the bridge-site adsorption of the CO molecule on both the metal and sulfur atoms, have the high selectivity toward methanol. This proposed guidance to designing the highly selective metal sulfide catalysts for methanol may be useful for the experiments. In the previous papers,6-10 we have proved that our combinatorial computational chemistry approach is very effective to design new catalysts with high activity and high resistance to poisons. Moreover, we indicated that our combinatorial computational chemistry is also applicable to predict the catalytic selectivity and to design new catalysts with high selectivity. Hence, we concluded that our combinatorial computational chemistry approach is a powerful tool to design new catalysts. Validity of the Simplest Cluster Model. To check the validity of our simplest cluster models for the present purpose, we constructed a Rh6 cluster model and the adsorption of CO molecule on the Rh6 cluster was
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calculated. The obtained adsorption energy of CO molecule on the Rh6 cluster is -223.51 kJ/mol. This value is significantly similar to the adsorption energy of -253.55 kJ/mol on the Rh2 cluster. The C-O distance and the charge of the CO molecule on the Rh6 cluster are 0.1169 nm and -0.347, which are also extremely close to the 0.1176 nm and -0.324 on the Rh2 cluster, respectively. Hence, we strongly confirmed the validity of our simplest cluster models to perform the highthroughput screening of the catalysts. Effect of Sulfur Vacancy on CO Adsorption. Generally, a metal sulfide catalyst is not stable in the CO and H2 atmosphere, and then some vacancy of sulfur atom is possible to form in the metal sulfide catalysts. Hence, to clarify the effect of sulfur vacancy on the catalytic activity and selectivity, we modeled a Rh2S cluster and the CO adsorption on that catalyst was calculated. The calculation results show that the CO adsorption energy is -208.28 kJ/mol and the CO charge is -0.178 on the Rh2S cluster. These results are not significantly different from the CO adsorption on the RhS cluster. On the RhS cluster, the CO adsorption energy is -216.97 kJ/mol and the CO charge is -0.115. The coordination of the CO molecule to the sulfur atom is not observed on the Rh2S cluster, similar to that on the RhS cluster. Hence, we suggest that the sulfur vacancy does not greatly influence our conclusions in the present study. Conclusions In the present paper, we applied our combinatorial computational chemistry approach to the design of the metal sulfide catalysts for the CO hydrogenation process. We succeeded in clarifying the relationship between the metal species in the metal sulfide catalysts and the products of the CO hydrogenation process. This result was in good agreement with the experimental results by Koizumi and co-workers. Moreover, we proposed that the Pd sulfide catalyst has the highest selectivity toward methanol from the CO hydrogenation process. This result strongly supports the experimental results by Koizumi and co-workers. Furthermore, we proposed that the metal sulfide catalysts, which realize the bridge-site adsorption of the CO molecule on both the metal and sulfur atoms, have the high selectivity toward methanol. This proposed guidance to design the highly selective metal sulfide catalysts for methanol may be useful for the experiments. Finally, we concluded that our combinatorial computational chemistry approach is effective and useful to design new catalysts with high activity and selectivity. We are going to expand the application of our combinatorial computational chemistry to many catalyst systems. Acknowledgment. This work was supported by Research for the Future Program of Japan Society for the Promotion of Science under the Project “Synthesis of Ecological High Quality Transportation Fuels”, (JSPS-RFTF98P01001). EF020245S
