Origin of the Different Activity and Selectivity toward Hydrogenation of

pubs.acs.org/Langmuir © 2010 American Chemical Society

Origin of the Different Activity and Selectivity toward Hydrogenation of Single Metal Au and Pt on TiO2 and Bimetallic Au-Pt/TiO2 Catalysts† Mercedes Boronat and Avelino Corma* Instituto de Tecnologı´a Quı´mica (UPV-CSIC), Universidad Polit
ecnica de Valencia, Consejo Superior de Investigaciones Cientı´ficas, Avenida de los Naranjos s/n, 46022 Valencia, Spain Received May 3, 2010. Revised Manuscript Received June 14, 2010 To improve the activity of Au/TiO2 catalysts toward hydrogenation without decreasing chemoselectivity, a number of isolated and supported Au, Pt, and Au-Pt nanoparticles have been investigated by means of density functional theory (DFT) calculations. H2 dissociation on Pt and Au-Pt nanoparticles is considerably faster than that on Au, because H2 adsorption on Pt atoms is dissociative and no activation barriers are involved. The high chemoselectivity of Au/TiO2 catalysts does not exist in Pt/TiO2 catalysts no matter how small the Pt particles are, but can be preserved in Au-Pt/TiO2 catalysts if the Au/Pt ratio is high enough as to keep Pt atoms isolated and not at the active metal-support interface.

1. Introduction Gold nanoparticles supported on metal oxides have shown interesting catalytic properties in the selective hydrogenation of aldehydes,1-5 olefins,6-9 and nitroaromatics.10-12 The chemoselective hydrogenation of substituted nitroaromatics to the corresponding aromatic amines is a very active research topic, since functionalized anilines are valuable intermediates in the industrial production of agrochemicals, pharmaceuticals, pigments, and dyes.13,14 We recently reported that gold nanoparticles supported on TiO2 or Fe2O3 are active and very selective catalysts for the hydrogenation of the nitro group in molecules containing other reducible groups such as -CO, CdC, or -CN.10 The chemoselective hydrogenation of nitrostyrene on Au/TiO2 was studied by combining in situ IR spectroscopy, quantum chemical modeling, and kinetic experiments, and it was found that there is a cooperation between gold and the oxide support so that H2 is dissociated on the gold nanoparticles and nitrostyrene adsorbs very selectively through the nitro group at the metal-support † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: [email protected].

(1) Claus, P. Appl. Catal. A 2005, 291, 222. (2) Bailie, J. E.; Hutchings, G. J. Chem. Commun. 1999, 2151. (3) Mohr, C.; Hofmeister, H.; Radnik, J.; Claus, P. J. Am. Chem. Soc. 2003, 125, 1905. (4) Milone, C.; Tropeano, M. L.; Guline, G.; Neri, G.; Ingoglia, R.; Galvano, S. Chem. Commun. 2002, 868. (5) Zanella, R.; Louis, C.; Giorgio, S.; Touroude, R. J. Catal. 2004, 223, 328. (6) Jia, J.; Haraki, K.; Kondo, J. N.; Domen, K.; Tamaru, K. J. Phys. Chem. B 2000, 104, 11153. (7) Yang, X. F.; Wang, A. Q.; Wang, Y. L.; Zhang, T.; Li, J. J. Phys. Chem. C 2010, 114, 3131. (8) Hugon, A.; Delannoy, L.; Louis, C. Gold Bull. 2008, 41, 127. (9) Hugon, A.; Delannoy, L.; Krafft, J. M.; Louis, C. J. Phys. Chem. C 2010, 114, 10823. (10) Corma, A.; Serna, P. Science 2006, 313, 332. (11) Cardenas-Lizana, F.; G
omez-Quero, S.; Perret, N.; Keane, M. Gold Bull. 2009, 42, 124. (12) Shimizu, K.; Miyamoto, Y.; Kawasaki, T.; Tnji, T.; Tai, Y.; Satsuma, A. J. Phys. Chem. C 2009, 113, 17803. (13) Blaser, H. U.; Steiner, H.; Studer, M. ChemCatChem 2009, 1, 210. (14) Blaser, H. U.; Siegrist, U.; Steiner, H. In Fine Chemicals through Heterogeneous Catalysis: Aromatic Nitro Compounds; Sheldon, R. A., van Bekkum, H., Eds.; Wiley: New York, 2001; p 389. (15) Boronat, M.; Concepci
on, P.; Corma, A.; Gonz
alez, S.; Illas, F.; Serna, P. J. Am. Chem. Soc. 2007, 129, 16230.

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interface.15 The complete mechanism of nitrostyrene hydrogenation was investigated, and it was proposed that H2 dissociation is the rate determining step of the reaction.16,17 Last year, an activation energy of 36.4 kJ mol-1 was reported for the H2-D2 exchange reaction on Au/TiO2 catalysts with different particle size, and it was proposed that the active sites are located at the perimeter interfaces around the gold nanoparticles.18 Despite the high chemoselectivity obtained with the Au/TiO2 catalyst, its activity toward hydrogenation is too low for practical applications,19,20 and attempts are being done to design a more efficient catalyst. Promising results with a Au/Al2O3 catalyst with a gold particle size of ∼2.5 nm diameter have been recently reported,12 and the higher hydrogenation activity as compared to Au/TiO2 has been attributed to a cooperation between the acid-base pair site on Al2O3 and the Au atoms to dissociate H2 into a Hþ/H- pair at the metal-support interface. A different way of improving the hydrogenation activity of gold based catalysts consists of alloying gold with more active transition metals, such as Pd21-24 or Pt.25 The catalytic behavior toward hydrogenation of Pt nanoparticules has been widely studied,26-28 and it is quite different from that of gold. Thus, while the activity of Pt nanoparticles toward ethylene hydrogenation is independent of particle size and shape, and comparable to Pt single crystals,29 the

(16) Corma, A.; Concepci
on, P.; Serna, P. Angew. Chem., Int. Ed. 2007, 46, 7266. (17) Serna, P.; Concepci
on, P.; Corma, A. J. Catal. 2009, 265, 19. (18) Fujitani, T.; Nakamura, I.; Akita, T.; Okumura, M.; Haruta, M. Angew. Chem., Int. Ed. 2009, 48, 9515. (19) Blaser, H. U. Science 2006, 313, 312. (20) Grirrane, A.; Corma, A.; Garcı´ a, H. Science 2008, 322, 1661. (21) Miura, H.; Terasaka, M.; Oki, K.; Matsuda, T. Stud. Surf. Sci. Catal. 1993, 75, 2379. (22) Sarkany, A.; Hargittai, P.; Horvath, A. Top. Catal. 2007, 46, 121. (23) Luo, K.; Wei, T.; Yi, C. W.; Axnanda, S.; Goodman, D. W. J. Phys. Chem. B 2005, 109, 23517. (24) Gao, F.; Wang, Y. L.; Goodman, D. W. J. Am. Chem. Soc. 2009, 131, 5734. (25) Bus, E.; van Bokhoven, J. A. Phys. Chem. Chem. Phys. 2007, 9, 2894. (26) Tao, F.; Dag, S.; Wang, L. W.; Liu, Z.; Butcher, D. R.; Bluhm, H.; Salmeron, M.; Somorjai, G. A. Science 2010, 327, 850. (27) Witham, C. A.; Huang, W. Y.; Tsung, C. K.; Kuhn, J. N.; Somorjai, G. A.; Toste, F. D. Nat. Chem. 2010, 2, 36. (28) Somorjai, G. A.; Park, J. Y. Top. Catal. 2008, 49, 126. (29) Tsung, C. K.; Kuhn, J. N.; Huang, W. Y.; Aliaga, C.; Hung, L. I.; Somorjai, G. A.; Yang, P. D. J. Am. Chem. Soc. 2009, 131, 5816.

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DOI: 10.1021/la101752a

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activity and selectivity toward crotyl alcohol in the hydrogenation of crotonaldehyde increase with increasing Pt particle size.30 In this paper, we analyze from a theoretical point of view the nature of the active sites involved in the rate determining step of the reaction, that is, H2 dissociation, and the possible ways in which Au/TiO2 catalysts can be modified in order to increase their hydrogenation activity without modifying their high chemoselectivity. The study of H2 dissociation on different catalyst models, including gold nanoparticles of different size, isolated and supported on TiO2, allowed us to conclude that neutral atoms located at corner or edge low coordinated positions can already dissociate H2,31,32 in agreement with different experimental information.33,34 The influence of the oxide support and of the reduction pretreatments with CO on the morphology of the supported gold nanoparticles and their hydrogenation activity was investigated by combining IR spectroscopy and density functional theory (DFT) calculations,35 and is now discussed and compared with new results obtained for nanoparticles of platinum and of gold doped with small amounts of platinum. On the other hand, it has been clearly established that the selectivity of Au/TiO2 toward the desired functionalized aniline is governed by the preferential adsorption of the nitroaromatic compound at the metal-support interface, with the nitro group interacting with the oxide surface.12,15 Altough the catalytic behavior of Pt is different from that of Au, we thought that in order to obtain highly active but still chemoselective Pt based catalysts it would be desirable to maximize the number of Pt atoms at the metal-support interface, by decreasing as much as possible the particle size. The adsorption of nitrostyrene at the interface between Pt and Au-Pt nanoparticles supported on TiO2 has now been theoretically studied, and it has allowed us to explain the low chemoselectivity of Pt/TiO2 and the higher activity of Au-Pt/TiO2 catalysts for the hydrogenation of substituted nitroaromatics.10,36

2. Theoretical Basis The adsorption and dissociation of molecular hydrogen and the adsorption of nitrostyrene on isolated and supported metal nanoparticles were studied using periodic slab models, with supercells large enough to avoid interaction between the periodically repeated particles or between the adsorbates. Two isolated metal nanoparticles of different size were considered: a M38 cluster having a typical cuboctahedral shape and ∼1 nm diameter, and a initially hemispherical M13 cluster obtained by removing four atomic layers parallel to (100) from the M38 model. The isolated M38 clusters were placed in a 20  20  20 A˚3 cubic box, and, except when otherwise stated, the position of all atoms was always fully relaxed. The smaller M13 clusters were studied both isolated and supported on a model of the most reactive (001) facet of the anatase polymorph of TiO2. The TiO2 surface was represented by a (4  4) supercell slab containing nine atomic layers in the unit cell, that is, 144 atoms, and a vacuum region larger than 20 A˚ between vertically repeated slabs. Convergence with respect to slab thickness and degree of relaxation has been (30) Grass, M. E; Rioux, R. M.; Somorjai, G. A. Catal. Lett. 2009, 128, 1. (31) Corma, A.; Boronat, M.; Gonz
alez, S.; Illas, F. Chem. Commun. 2007, 3371. (32) Boronat, M.; Illas, F.; Corma, A. J. Phys. Chem. A 2009, 113, 3750. (33) Bus, E.; Miller, J. T.; van Bokhoven, J. A. J. Phys. Chem. B 2005, 109, 14581. (34) Mohr, C.; Hofmeister, H.; Radnik, J.; Claus, P. J. Am. Chem. Soc. 2003, 125, 1905. (35) Boronat, M.; Concepci
on, P.; Corma, A. J. Phys. Chem. C 2009, 113, 16772. (36) Corma, A.; Serna, P.; Concepci
on, P.; Calvino, J. J. J. Am. Chem. Soc. 2008, 130, 8748. (37) Calatayud, M.; Minot, C. Surf. Sci. 2004, 552, 169.

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Boronat and Corma Table 1. Calculated Adsorption, Activation, and Reaction Energies (in kcal mol-1) for H2 Dissociation on Different Gold and Platinum Catalyst Models Eads (H2)

Eact (H2 f 2H)

Au(111) -0.5 Au(100) -0.4 a -12.6 6.7 AuMAR -5.8 7.1 Au38 -7.2 6.0 Au25 -1.9 7.9 Au13 -27.2 Pt38 -35.6 Pt13 -15.4 Au37Pt -32.6 Au12Pt -3.9 2.2 Au13/TiO2 -21.5 Pt13/TiO2 -20.5 Au12Pt/TiO2 a Monatomic row model described in ref 31.

ΔE (H2 f 2H) 9.9 4.8 -7.5 2.3 (-21.5) -6.2 -11.4 -16.1 -12.1 -22.0

previously demonstrated for this surface model.37 The M13 clusters were placed on the TiO2 surface and the atomic positions of all 13 metal atoms and of the Ti and O atoms of the two uppermost layers of the support were fully relaxed. For a better comparison, the calculations of the isolated M13 nanoparticles were done using the same supercell employed for the M13/TiO2 systems. As will be mentioned later and was widely discussed in ref 32, deformation of the Au13 clusters is not energetically demanding and therefore their shape in vacuum and supported on TiO2 is quite different. For each model (isolated M38, isolated M13, and supported M13/TiO2), besides the pure gold and pure platinum systems, a third composition was considered. It was obtained by substituting one Au atom in Au38 and Au13 by one Pt atom, leading to Au37Pt and Au12Pt compositions, respectively. All calculations are based on DFT and were carried out using the Perdew-Wang (PW91)38,39 exchange-correlation functional within the generalized gradient approximation (GGA) as implemented in the VASP code.40,41 The Kohn-Sham orbitals used to obtain the electron density were expanded in a plane wave basis set with a kinetic energy cutoff of 415 eV, and the effect of the core electrons was taken into account by means of the projected augmented wave (PAW) method.42 Given the large size of the two unit cells employed, calculations were carried out at the Γ k-point of the Brillouin zone. The atomic positions were optimized by means of a conjugate-gradient algorithm until atomic forces were smaller than 0.01 eV/A˚. Transition states were located using the DIMER algorithm,43,44 and stationary points were characterized by pertinent frequency analysis calculations. Charge distributions were estimated using the theory of atoms in molecules (AIM) of Bader using the algorithm developed by Henkelman.45,46

3. Results and Discussion 3.1. Activity: Hydrogen Dissociation. 3.1.1. Isolated Nanoparticles. The adsorption and dissociation of molecular H2 on gold surfaces and isolated gold nanoparticles of different (38) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. (39) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (40) Kresse, G.; Furthm€uller, J. Phys. Rev. B 1996, 54, 11169. (41) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. (42) Bl€ochl, P. E. Phys. Rev. B 1994, 50, 17953. (43) Heyden, A.; Bell, A. T.; Keil, F. J. J. Chem. Phys. 2005, 123, 224101. (44) Henkelman, G.; J
onsson, H. J. Chem. Phys. 1999, 111, 7010. (45) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. J. Comput. Chem. 2007, 28, 899. (46) Henkelman, G.; Arnaldsson, A.; J
onsson, H. Comput. Mater. Sci. 2006, 36, 254.

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Figure 1. Calculated energy profile and optimized structures involved in the dissociative adsorption of molecular H2 on Au13, Au12Pt, and Pt13 isolated nanoparticles. Distances in A˚. Au atoms are yellow, Pt atoms blue, and H atoms small white balls.

size and shape have been theoretically investigated,31,47-49 and it has been shown that the active sites are low coordinated gold atoms, no matter whether they belong to defective extended surfaces or to large or small isolated nanoparticles. Energies of adsorption of molecular H2 on gold as well as activation and reaction energies involved in the H2 dissociation process catalyzed by gold reported in previous work31 are summarized in Table 1 together with new values obtained for Pt and Au-Pt nanoparticles. In all cases, H2 adsorption energies have been calculated as Eads ¼ EM- H2 - EM þ EH2




with EM-H2 being the total energy of the adsorption complex and EM þ EH2 being the sum of the total energies of the corresponding catalyst model and the H2 molecule placed in a 20  20  20 A˚3 cubic box. Activation energies Eact are obtained as the energy difference between the transition state and the initial adsorption complex, and reaction energies ΔE are obtained as the energy difference between the final state with two hydrogen atoms adsorbed on the catalyst and the initial adsorption complex. As previously discussed and in agreement with experiment,1 calculations indicate that molecular H2 neither adsorbs nor dissociates on perfect gold surfaces. On the other hand, calculated adsorption energies of H2 on isolated or supported gold nanoparticles of different size range between -2 and -7 kcal mol-1. The calculated energy profile and the optimized geometry of all structures involved in the H2 dissociation process on a small isolated Au13 nanoparticle are depicted in Figure 1. The first step is adsorption of a H2 molecule on a low coordinated corner gold atom of the nanoparticle, forming a stable complex in which the two H atoms are bonded to the same Au atom at a distance of (47) Andrews, L. Chem. Soc. Rev. 2004, 33, 123. (48) Varganov, S. A.; Olson, R. M.; Gordon, M. S.; Metiu, H. J. Chem. Phys. 2004, 120, 5169. (49) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. Top. Catal. 2007, 44, 27.

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∼1.9 A˚, while the H-H bond is slightly elongated from 0.75 A˚ in the gas phase to 0.80 A˚. In the transition state, the H-H bond length increases to 1.5 A˚ while the Au-H distances are ∼0.3 A˚ shorter than those in the adsorption complex. A local minimum 1.3 kcal mol-1 more stable than the transition state, in which the H2 molecule is already dissociated and the two H atoms are still bonded to the same corner Au atom and oriented in opposite directions, was localized on the potential energy surface. This intermediate evolves without activation energy to a more stable situation in which the two H atoms are forming two Au-H-Au bridges. It should be mentioned that during the process the particle shape changes and the coordination of some of the gold atoms is modified. As summarized in Table 1, the calculated activation energy is 7.9 kcal mol-1, only slightly higher than that on defective surfaces (AuMAR, monatomic row model described in ref 31) or larger nanoparticles, and the reaction is exothermic by 11.4 kcal mol-1. The interaction of H2 with Au12Pt and Pt13 nanoparticles is quite different, as depicted in Figure 1, right side. H2 adsorbs dissociatively on both systems, yielding very stable structures in which the two H atoms are bonded to the same Pt atom at ∼1.6 A˚, and with calculated H-H distances of ∼1.9 A˚. The optimized geometries of these intermediates are similar to that obtained on the Au13 nanoparticle, but the calculated adsorption energies are considerably larger, -32.6 kcal mol-1 on Au12Pt and -35.6 kcal mol-1 on Pt13. As on Au13, these structures lead to still more stable complexes with the H atoms occupying bridge positions without activation energy. The global process is highly exothermic, -12.1 and -16.1 kcal mol-1 on Au12Pt and Pt13, respectively, and does not imply any activation barrier. This is in agreement with experimental results from van Bokhoven and co-workers,25,33 indicating that the Au-H interaction is weaker than the Pt-H interaction and that the H2 dissociation on gold is activated. While the corner Au atom with which H2 interacts on Au13 is bonded to four other Au atoms, the coordination number of a DOI: 10.1021/la101752a

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Figure 2. Calculated energy profile and optimized structures involved in the dissociative adsorption of molecular H2 on Au38, Au37Pt, and Pt38 isolated nanoparticles. Distances in A˚.

corner Au atom in the Au38 nanoparticle is 6, and H2 binds to it in a different conformation, as depicted in Figure 2. At this point, it should be mentioned that the dissociation of H2 on the Au38 model always involves an important structural rearrangement of the gold cluster, so that the calculated energies are mainly related to this particle rearrangement and not to the H2 dissociation process itself. In order to obtain an approximate activation energy value that could be associated to the dissociation of H2, we performed a transition state search in which the positions of the Au atoms were kept fixed and only the H atoms were allowed to relax. With this approach, we obtained an upper limit for the activation energy of 7.1 kcal mol-1 and the structure depicted in Figure 2 labeled as “partially optimized”. In this transition state, the H-H bond length is 1.3 A˚, suggesting that molecular dissociation has not occurred yet, and the Au-H distances are about 1.7 A˚. The product obtained from this reaction with only partial geometry optimization is 2.3 kcal mol-1 less stable than adsorbed H2, and it corresponds to a structure in which the two H atoms are forming two almost symmetrical Au-H-Au bridges with Au-H bond lengths between 1.8 and 1.9 A˚. If this structure is reoptimized without any restriction, it evolves to the system depicted in Figure 2, bottom, labeled as “fully optimized”. It can be seen that the H atoms are placed in the same bridge positions, with Au-H distances being slightly shorter, between 1.7 and 1.8 A˚. However, the particle geometry has been modified, and the Au atom directly bonded to the two H atoms has broken some of the Au-Au bonds in which it was involved and its coordination number has decreased to 4. This structural rearrangement implies an important stabilization of the system, and the process becomes exothermic by 21.5 kcal mol-1. Attempts to find a “fully optimized” transition state for H2 dissociation on Au38 have not succeeded yet. In all cases, the H2 dissociation process is strongly coupled with important changes in the coordination of the gold atoms and in the particle shape, and therefore, the reaction coordinate and the energies involved are mainly associated to the structural rearrangement of the particle and not to the reaction of interest. Thus, the reported value of 7.1 kcal mol-1 obtained with the partial optimization described above is just an 16610 DOI: 10.1021/la101752a

approximate estimation of the activation energy necessary to dissociate H2 on Au38 nanoparticles. As in the case of the smaller Au13 cluster, doping with Pt leads to an enhancement of the interaction between H2 and the metal nanoparticle. H2 preferentially adsorbs on the Pt atom of the Au37Pt particle, with optimized Pt-H distances of 1.7 A˚. However, in this case, the H-H bond length is not as long as that on the Au12Pt model, and the adsorption energy is also smaller. On the Pt38 particle, however, the interaction is equivalent to that found on the smaller Pt13 cluster. The calculated adsorption energy is -27.2 kcal mol-1, and H2 is directly dissociated into two H atoms. From the results reported up to this point concerning H2 activation on isolated nanoparticles, it can be concluded that Pt is considerably more active than gold and that the best way to improve the activity of hydrogenation catalysts is to directly substitute gold with platinum. 3.1.2. Supported Nanoparticles. In real catalysts, the metal nanoparticles are supported on different materials, mainly inorganic oxides such as SiO2, MgO, Al2O3, TiO2, Fe2O3, or CeO2, and it is known that the nature of the support has an influence on the catalytic properties of the supported metal particles. Depending on the reaction considered, the role of the support may be limited to the stabilization of small particles with large amounts of low coordinated sites, or may help to stabilize or activate some of the reactants at the metal-support interface or on the support itself. In the case of hydrogenation on gold catalysts, it is possible that the activity of sites at the metal-support interface is different from that of sites at the top of the particle.18 In a recent paper,32 we investigated from a theoretical point of view the activity toward H2 dissociation of several Au nanoparticles of different shape supported on stoichiometric and reduced TiO2. We found that interaction with the support causes noticeable changes in the shape of the gold nanoparticles and a certain degree of charge transfer: gold particles become globally positively charged when supported on stoichiometric TiO2 and negatively charged when adsorbed on the reduced surface. The net charge is not equally distributed among all atoms but clearly localized on some of Langmuir 2010, 26(21), 16607–16614

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Figure 3. Optimized structures involved in the dissociative adsorption of molecular H2 on Au13/TiO2: (a) adsorbed H2, (b) transition state, and (c) reaction product; (d) dissociative adsorption of H2 on Pt13/TiO2; (e) H2 adsorbed on Au12Pt/TiO2 and (f ) dissociated H2 on Au12Pt/ TiO2. Distances in A˚. O atoms are red, Ti atoms dark blue, Au atoms yellow, Pt atoms blue, and H atoms small white balls.

them. Cationic gold atoms are always directly bonded to an O atom forming a Au-O-Ti bridge, while anionic gold atoms are usually highly coordinated inside or terrace-type atoms. None of them is able to coordinate H2, and therefore, they are inactive for hydrogenation. The main conclusion from this work is that only neutral corner or edge gold atoms in low coordination state and not directly bonded to the support through Au-O-Ti linkages are active for H2 dissociation. This hypothesis was further confirmed experimentally by combining IR spectroscopy of CO adsorption, H/D isotopic exchange, and DFT calculations.17,35 A number of Au/TiO2 catalysts with similar gold particle sizes were characterized by IR spectroscopy of CO adsorption, and different distributions of sites related to differences in particle morphology were found. With the help of DFT calculations, IR bands between 2070 and 2110 cm-1 were assigned to CO adsorbed on low coordinated neutral Au atoms, bands at ∼2120 cm-1 were assigned to CO interacting with positively charged Au atoms involved in Au-O-Ti bonds at the metal-support interface, and a band at 2135 cm-1 was related to CO weakly adsorbed on positively charged Au atoms in contact with O adatoms. Then, the rate of H/D exchange on the different catalysts studied by IR spectroscopy was plotted against the concentration of the different gold species identified. It was found that there is a linear correlation between the rate of H/D exchange and the amount of neutral Au sites characterized by IR bands between 2070 and 2110 cm-1, but not with the amount of positively charged Au atoms associated with the bands at 2120 and 2135 cm-1. It was thus demonstrated that Au atoms directly bonded to O are not able to dissociate H2 and are therefore inactive for hydrogenation reactions. Another important conclusion from that work35 was the discovery that CO adsorption on supported gold nanoparticles can irreversibly modify the particle morphology and, in some cases, can even break some of the Au-O-Ti bridges, generating new low coordinated Au sites able to adsorb and dissociate H2. This finding can be related to the strong influence that calcination and reduction catalyst pretreatments with CO have on the hydrogenation (50) Haruta, M. Catal. Today 1997, 36, 153.

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activity of Au/TiO2 catalyts,50 and further confirms the role of low coordinated Au atoms in the dissociation of H2. Among the different supported Au13 particles investigated in our group, we have now selected the model with the largest number of active sites and have calculated the complete reaction path for H2 dissociation. It can be seen in Figure 3 that the particle consists of one bottom layer of gold atoms in direct contact with the support and therefore inactive, and one top layer separated from the support with low coordinated gold atoms on which H2 is adsorbed and activated. The calculated adsorption energy is -3.9 kcal mol-1, slightly higher than that obtained for the isolated Au13 cluster, and the optimized bond lengths also reflect a slightly higher degree of interaction. H2 dissociates through a transition state whose geometry is quite similar to that obtained on the isolated Au13 cluster. The two H atoms are bonded to the same gold atom at ∼1.6 A˚ and oriented in opposite directions, and the H-H distance has increased to 1.5 A˚. The final product, that is 22 kcal mol-1 more stable than adsorbed H2, consists of two H atoms located on bridge positions at the edge of the gold nanoparticle, with Au-H optimized distances around 1.8 A˚. The main difference in relation to the isolated particles is that the activation energy is lower, only 2.2 kcal mol-1, although in this case the support does not play any direct role in adsorbing or activating the H2 molecule. This is in agreement with recent catalytic results8 showing that a series of gold catalysts supported on different oxides (alumina, titania, zirconia, ceria) and containing the same gold loading (1 wt % Au) with the same gold particle size (∼2 nm in average) exhibited similar catalytic properties in the selective gas phase hydrogenation of 1,3-butadiene in excess of propene. If a direct implication of the support atoms in the dissociation reaction existed, some differences would be expected in the catalytic behavior of the above-mentioned systems, but not if the only role of the support is to stabilize small nanoparticles with a given morphology. On the other hand, it has also been unequivocally demonstrated that the rate of HD formation from H2 and D2 on Au/TiO2 catalysts directly correlates with the mean gold particle diameter, suggesting that the active sites for H2 dissociation are located at the periphery around the gold DOI: 10.1021/la101752a

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Table 2. Calculated Adsorption Energies (in kcal mol-1) of Nitrostyrene Interacting Mainly through the -NO2 or the CdC Groups with Different Gold and Platinum Catalyst Models

AuMAR Au38 Pt38 TiO2 Au13/TiO2 Pt13/TiO2 Au12Pt/TiO2

Eads (-NO2)

Eads (CdC)

-8.7 -20.0 -46.0 -42.4 -19.1 -24.2 -30.5

-12.2 -28.9 -62.2 -30.5

particle,18 that is, atoms at the metal-support interface or atoms in corner or edge positions in layers above the metal-support interface. Our calculations discard Au atoms directly involved in Au-O-Ti bridges as potential active sites for H2 dissociation, but not the rest of the low coordinated Au atoms of the Au13/TiO2 model, that, due to its small size, are all located at the periphery of the particle. We have also observed that the activation energy calculated using the Au13/TiO2 model is lower than that obtained with the isolated nanoparticles, suggesting a possible role of the oxide support that is not related to a direct interaction with the H2 molecule. In order to definitely clarify whether the morphology of gold nanoparticles with increasing diameter is such that low coordinated gold atoms only exist at the particle periphery or if, on the contrary, there is a different reactivity of low coordinated gold atoms placed on top of the particle, a theoretical study should be done using considerably larger models for the supported nanoparticles, but this is beyond the scope of this paper. When a gold atom in the Au13/TiO2 model is substituted with a Pt atom and the Au12Pt/TiO2 model is generated, the optimized shape of the supported particle is not modified and the net charge on the particle decreases from þ0.51 e in Au13/TiO2 to þ0.39 e. When the Pt13 nanoparticle is placed on the TiO2 support, the net charge on the particle increases to þ0.65 e, and, as in the case of Au13/TiO2, it is clearly localized on two Pt atoms that are directly bonded to the support through Pt-O-Ti bridges. Moreover, its geometry changes as depicted in Figure 3, and it cannot be further described as a bilayer structure. Nevertheless, since the objective of this study is not the morphology of supported Pt nanoparticles, we did not consider other isomers of Pt13/TiO2, and H2 was adsorbed on top of the obtained model. The adsorption was dissociative, and the structure obtained, which is 21.5 kcal mol-1 more stable than separated reactants, corresponds to two H atoms forming two Pt-H-Pt bridges. Adsorption of H2 on the Pt atom of the Au12Pt/TiO2 model is also highly exothermic, with a calculated energy of -20.5 kcal mol-1, but the geometry of the adsorption complex corresponds to a molecule of H2 strongly bonded and highly activated but still not dissociated. A second minimum was obtained, 17.9 kcal mol-1 more stable, in which H2 is already dissociated and the two H atoms are forming two Au-H-Pt bridges. It can then be concluded that the catalytic behavior toward hydrogenation of supported Au13, Au12Pt, and Pt13 nanoparticles is quite similar to that of the corresponding isolated clusters, and that the support does not play any active role in the H2 dissociation step. 3.2. Selectivity: Nitrostyrene Adsorption. 3.2.1. Isolated Nanoparticles. Nitrostyrene adsorption on gold catalysts was theoretically investigated in previous work,15 and the most relevant results reported there together with new values obtained now for Pt and Au-Pt catalysts are summarized in Table 2. It should be initially noted that the adsorption mode of nitrostyrene on Au(111) and Pt(111) surfaces is completely different. Nitrostyrene interacts weakly with Au(111); the adsorbed molecule lies parallel 16612 DOI: 10.1021/la101752a

Figure 4. Optimized geometry of nitrostyrene adsorbed on Au38 (a,b) and Pt38 (c,d) nanoparticles, interacting mainly trough the nitro group (a,c) or the CdC bond (b,d). Distances in A˚. O atoms are red, N atoms green, C atoms orange, Au atoms yellow, Pt atoms blue, and H atoms small white balls.

to the Au surface at a distance of nearly 4 A˚, with optimized NO and CC bond lengths almost identical to those obained for the isolated molecule in the gas phase (1.24 and 1.40 A˚, respectively), and with a calculated value for the adsorption energy of only -2.5 kcal mol-1. On the contrary, nitrostyrene adsorbs on Pt(111) forming strong bonds between the surface Pt atoms and the C atoms of the benzene ring and of the CdC group. The molecule is placed parallel to the surface with optimized Pt-C distances between 2.1 and 2.4 A˚, the NO and CC bond lengths increase to 1.30 and 1.48 A˚, respectively, and the calculated adsorption energy is -21.6 kcal mol-1. This mode of interaction with Pt implies the activation of both groups and explains the lack of selectivity of Pt/C catalysts.10 Nitrostyrene adsorbs moderately on low coordinated gold atoms in defective surfaces and stronger on isolated nanoparticles, with the interaction involving either the nitro group or the CC double bond. As depicted in Figure 4, when nitrostyrene interacts with the Au38 model through the nitro group, one of the NO bond lengths increases to 1.3 A˚ and the O atom of this bond is at only 2.3 A˚ from a Au atom. When the molecule interacts through the CC group, each of the two C atoms is bonded to a different Au atom and the CC bond length indicates that the initially double bond is now a single bond. It is also important to note that the shape of the gold nanoparticle is distorted by these interactions. Nitrostyrene interaction with a Pt38 particle is stronger than that with Au38, and it involves the formation of a large number of Pt-C bonds. There are also two different ways of adsorption, one of them preferentially activating the nitro group and the other one activating the CC bond (see Figure 4c and d). However, in all cases, both on gold and on platinum, the most stable adsorption mode involves the preferential activation of the CC bond, and therefore, the theoretical results presented up to this moment are not able to explain the unique chemoselectivity of the Au/TiO2 catalyst for hydrogenation of nitrostyrene. It is necessary to include the TiO2 support in the calculations and to Langmuir 2010, 26(21), 16607–16614

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Figure 5. Optimized geometry of nitrostyrene adsorbed (a) on the TiO2 support, (b) close to Au13 or Au12Pt nanoparticles supported on TiO2, front view. (c) Side view for Au13/TiO2. (d) Side view for Au12Pt/TiO2. Distances in A˚.

study the interaction of nitrostyrene with realistic models of supported nanoparticles. 3.2.2. Supported Nanoparticles. When nitrostyrene is adsorbed on the TiO2 surface, the interaction through the CC bond is only possible if the molecule lies parallel to the surface. However, the repulsion between the aromatic ring and the O atoms of the oxide support is so strong that in all cases the geometry optimization converged to the structure depicted in Figure 5a, in which the molecule is perpendicular to the surface and interacts through the nitro group. More precisely, the O atoms of the nitro group interact with two Ti atoms of the surface, with optimized Ti-O distances of 2.1 and 2.4 A˚. The adsorption energy calculated within the DFT approach used, which does not take into account dispersion forces, is high, -42.4 kcal mol-1, and suggests that these strongly chemisorbed species might not be the reactive species, but just spectators. When nitrostyrene is adsorbed near the supported Au13 nanoparticle, it stays perpendicular to the oxide surface, as depicted in Figure 5b and c. One of the O atoms of the nitro group is directly bonded to a Au atom located at the nanoparticle edge in contact with the support, and the other one is interacting weakly with a Ti atom of the support. The calculated adsorption energy at this site is -19.1 kcal mol-1, considerably smaller than that on the extended TiO2 support and almost as large as that obtained for the interaction of nitrostyrene with the isolated Au38 particle. In this situation, the NO bond is activated while the CC bond is unaltered because it remains far from the nanoparticle and from the support. The same type of interactions exist when nitrostyrene is adsorbed at the interface between the Au12Pt nanoparticle and the oxide support, as depicted in Figure 5d. The interaction between the nitro group and the Au12Pt nanoparticle is somewhat stronger, as indicated by the larger adsorption energy, -30.5 kcal mol-1. The two O atoms of the NO bonds are now closer to the Au atoms in the nanoparticle and at a distance larger than 4 A˚ from the Ti of the oxide support, and the nitro group is slightly more activated than on Au13/TiO2. Langmuir 2010, 26(21), 16607–16614

Figure 6. Different modes of adsorption of nitrostyrene on a Pt13 nanoparticle supported on TiO2. Distances in A˚.

The situation is completely different when nitrostyrene is adsorbed near the supported Pt13 nanoparticle. Starting from an initial geometry in which nitrostyrene was placed perpendicular to the oxide surface as in the Au13/TiO2 model, geometry optimization lead to the structure depicted in Figure 6a. The O atoms of the nitro group are displaced far from the Pt atoms and are interacting weakly with Ti atoms of the support. On the other hand, the aromatic ring of nitrostyrene tends to bind the Pt nanoparticle, whose structure is highly distorted by these interactions. The adsorption energy of the system depicted in Figure 6a is -24.2 kcal mol-1, and neither the nitro nor the CC bond are activated in this situation. A second structure with similar stability was obtained when nitrostyrene was placed on top of the Pt13 nanoparticle, as depicted in Figure 6b. In this situation, the aromatic ring is bonded to Pt, with several Pt-C distances between 2.1 and 2.3 A˚, while neither the nitro nor the CC bonds interact with the particle. Finally, we considered a third situation DOI: 10.1021/la101752a

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in which nitrostyrene was placed on top of the Pt13 nanoparticle but with the CC bond being allowed to interact with the Pt atoms, as depicted in Figure 6c. The adsorption energy of this conformation in which only the CC bond is activated is the highest, -30.5 kcal mol-1, and indicates that no matter how small the Pt nanoparticles are, they will never be able to hydrogenate the nitro group in nitrostyrene in a highly chemoselective way. From these results, it can be concluded that the high chemoselectivity of Au/TiO2 catalysts is due to the selective adsorption and proper activation of the nitro group at the interface between the Au nanoparticle and the titanium oxide support. This selectivity is completely lost in Pt/TiO2 catalysts due to the strength of the interaction between the aromatic ring of nitrostyrene and the Pt particle. However, the chemoselectivity of Au/ TiO2 catalysts can be preserved in Au-Pt/TiO2 catalysts provided that the Au/Pt ratio is high enough as to keep Pt atoms isolated and not at the active metal-support interface. Moreover, the results discussed before for H2 adsorption on supported nanoparticles showed that the presence of just one Pt atom in the Au13/TiO2 catalyst modifies the energy profile and H2 dissociation occurs without activation barrier. Altogether, it can be concluded that bimetallic Au-Pt/TiO2 catalysts show an activity toward hydrogenation comparable to that of Pt/TiO2, while preserving the high chemoselectivity of Au/TiO2. Indeed, recently a series of bimetallic catalysts prepared by impregnating increasing amounts of Pt onto the Au/TiO2 catalyst provided by the World Gold Council were tested in the hydrogenation of nitrostyrene, and an optimum composition with 1.5% Au and 0.01% Pt was found that enhanced the catalytic activity of Au/TiO2 by 1 order of magnitude while maintaining the high chemoselectivity of gold.17

4. Conclusions The mechanism of the chemoselective hydrogenation of nitrostyrene catalyzed by Au/TiO2 involves the dissociation of H2

16614 DOI: 10.1021/la101752a

on the gold nanoparticle and the preferential adsorption and activation of the nitro group at the metal-support interface. In order to improve the activity of Au/TiO2 catalysts without decreasing their high chemoselectivity, we have investigated by means of DFT calculations the nature of the active sites involved in the H2 dissociation and in the preferential activation of the nitro group on a series of catalyst models in which Au atoms are substituted with Pt. It has been found that while H2 dissociation on low coordinated Au atoms involves activation energies between 3 and 8 kcal mol-1 depending on the model, the presence of just one Pt atom in the Au nanoparticle is enough to modify the energy profile so that the dissociative chemisorption becomes highly exothermic and barrierless. On the other hand, selectivity is governed by the preferential adsorption and proper activation of the nitro group of nitrostyrene at the metal-support interface. It has been found that the modes of adsorption on nitrostyrene on Au13/TiO2 and Pt13/TiO2 are completely different. No matter how small the Pt particles are, the aromatic ring of nitrostyrene always interacts strongly with Pt atoms. The molecule is preferentially adsorbed on top of the particle and the two groups are simultaneously activated, explaining the lack of selectivity of Pt catalysts. However, when only small amounts of highly dispersed Pt are added to the catalyst, as simulated by the Au12Pt/TiO2 model, the activity toward hydrogenation increases considerably while the properties of the metal-support interface and the mode of adsorption of nitrostyrene remain similar to those found for Au13/TiO2, and therefore the chemoselectivity is maintained. Acknowledgment. We thank Consolider-Ingenio-2010 (project MULTICAT) and Red Espa~ nola de Supercomputaci
on (RES) and Centre de C alcul de la Universitat de Val encia for computational resources and technical assistance. We thank M. Calatayud for providing the TiO2 slab model.

Langmuir 2010, 26(21), 16607–16614