Combined Experimental and Theoretical Investigation on the

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Combined Experimental and Theoretical Investigation on the Selectivities of Ag, Au, and Pt Catalysts for Hydrogenation of Crotonaldehyde Xiaofeng Yang,†,‡,§ Aiqin Wang,† Xiaodong Wang,† Tao Zhang,*,† Keli Han,| and Jun Li*,‡ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China; Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China; Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China; and State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China ReceiVed: June 17, 2009; ReVised Manuscript ReceiVed: October 25, 2009

A parallel study on silver, gold, and platinum catalysts with inert silica support is conducted both experimentally and theoretically for the liquid-phase hydrogenation of crotonaldehyde (Me-CHdCH-CHdO). We find that the silver catalyst exhibits a uniquely high selectivity toward CdO hydrogenation and the selectivity remains constant even at the conversion close to 100%. The gold catalyst, however, shows only a moderate selectivity whereas the platinum catalyst has a rather poor selectivity. Such variation in selectivity is interpreted in terms of the varied adsorption geometries of the crotonaldehyde on different metals. According to our density functional calculations of the chemisorptions of crotonaldehyde on selected M19 (M ) Ag, Au, and Pt) model clusters and M(111) surface, the most favored adsorption mode for silver is the CdO oxygen atom being σ-bonded on low-coordinated silver atoms, which results in activation of the CdO bond. In contrast, so-called η4 and di-σCdC modes are preferred on Pt surface, while a πCdC adsorption mode is favored on low-coordinated gold atoms, which leads to the preference of CdC hydrogenation. Moreover, the calculations indicate that the selectivity to CdO hydrogenation is more favored on smaller silver nanoparticles. This implication has been further corroborated experimentally by investigation of silver catalysts with different particle sizes. 1. Introduction The selective hydrogenation of R,β-unsaturated aldehydes to their corresponding unsaturated alcohols, such as crotonaldehyde to crotyl alcohol, has attracted much interest.1-3 The presence of two conjugated double bonds in R,β-unsaturated aldehydes allows two competitive hydrogenation routes: the hydrogenation of the CdC bond to the saturated aldehydes and the hydrogenation of the CdO bond to the unsaturated alcohols (Scheme 1). Unfortunately, the desired selective hydrogenation of the CdO bond is thermodynamically unfavored, which necessitates the manipulation of the kinetic effects, particularly by choosing a suitable catalyst that favors CdO hydrogenation while keeping the CdC bond intact, in order to obtain the desired unsaturated alcohols. Pt, as one of the most commonly used catalysts for hydrogenation, has been intensively studied for the selective hydrogenation of R,β-unsaturated aldehydes. Experimentally, supported Pt catalyst was found to exhibit a poor selectivity to unsaturated alcohols.1-5 With electronic modification of Pt, for example, using an electron-donating support or adding a more electropositive metal, the selectivity to CdO hydrogenation over * To whom correspondence should be addressed. E-mail taozhang@ dicp.ac.cn; [email protected]. Tel: +86-411-84379015. Fax: +86-41184691570 . † State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Department of Chemistry, Tsinghua University. § Graduate University of Chinese Academy of Sciences. | State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.

SCHEME 1: Reaction Pathway for the Hydrogenation of r,β-Unsaturated Aldehyde

Pt catalyst can be greatly improved.5-8 Moreover, the particle size of Pt imposes an important effect on the selectivity to unsaturated alcohols. The larger the Pt particle size was used, the higher selectivity to CdO hydrogenation can be obtained.1-4,9 To theoretically understand the selectivity-structure relationship over the Pt catalysts, surface science experiments and density functional theory (DFT) calculations have been performed over the platinum crystal surface.10-27 Generally, the key factor governing the selectivity of the hydrogenation of R,β-unsaturated aldehydes is considered to be the chemisorption modes of the substrate,1-3,12,14,19 while Loffreda et al. proposed that the desorption of a specific product may also contribute to the selectivity over Pt catalyst based on the hydrogenation route of the most stable chemisorption mode of acrolein on Pt(111) surface.17,21

10.1021/jp905687g CCC: $40.75  2009 American Chemical Society Published on Web 11/12/2009

Catalysts for Hydrogenation of Crotonaldehyde In contrast to the intensive study of platinum, both gold and silver nanoparticles are emerging as new promising catalysts for the selective hydrogenation of R,β-unsaturated aldehydes.28-49 It is noted that gold particles, when deposited on a reducible support, exhibit intriguing selectivities to unsaturated alcohols.28-39 For example, Bailie and Hutchings found that the selectivity to crotyl alcohol was greater than 50% over Au/ZrO2 and Au/ ZnO, whereas it was zero on Au/SiO2.28 The reducibility of the support can induce a modification of the electron density of gold, which in turn leads to changes in its adsorption properties. It is assumed that an electron transfer occurs from the reduced support to gold, creating more electron-enriched gold particles. Thereby, the binding energy of the olefinic double bond on gold surface decreases due to increased repulsive four-electron interaction, whereas the back-bonding with the π* CdO orbital is favored, leading to an enhanced reaction rate for the selective hydrogenation of CdO bond.29,30 Alternatively, the reducible supports may also affect the morphology and size of gold particles, which then influence the selectivity in the hydrogenation of unsaturated aldehydes.50 Ag nanoparticles, however, exhibit quite different dependence on the underneath support.42-49 Claus and colleagues found that Ag nanoparticles, either on a nonreducible (SiO2 and Al2O3) or a reducible (TiO2) support, had a rather high selectivity to the even with acrolein unsaturated alcohols,43-49 (H2CdCH-HCdO) as the substrate.44-49 These results appear to suggest that Ag may have an intrinsically different property from Au or Pt with regard to the preferential hydrogenation of the CdO bond. It is therefore important to elucidate the nature of chemisorptions and/or hydrogenation of R,β-unsaturated aldehydes on silver nanoparticles. However, a comparative study of the three typical metal catalysts, Ag, Au, and Pt, for the selective hydrogenation of crotonaldehyde, both experimentally and theoretically, is still lacking. We carry out here a combined experimental and theoretical study on the chemisorptions and hydrogenation of crotonaldehyde on Ag, Au, and Pt nanoparticles and bulk surfaces, with particular focus on the catalysts themselves without contribution from a reducible support to gain a better understanding of the intrinsic properties and particle size effect of Ag, Au, and Pt toward the selective hydrogenation of the CdO bonds. To this end, we prepared silica supported metal nanoparticles with comparable particle sizes and investigated their catalytic performances for the selective hydrogenation of crotonaldehyde. Our results have shown that the Ag/SiO2 catalyst presented a uniquely higher selectivity to the unsaturated alcohols than the Au/SiO2 and Pt/SiO2. To understand the origin of the differences in selectivities toward the CdO hydrogenation over Ag, Au, and Pt catalysts, DFT calculations were conducted for crotonaldehyde adsorption on Ag, Au, and Pt clusters. Our calculations indicate that the CdC bond is preferentially activated either via σ-coordinating on Pt or via π-bonding on low-coordinated Au atoms, while the activation of CdO group is only favored on low-coordinated silver atoms at the edges and corners. This featured adsorption pattern of crotonaldehyde on silver leads to an effective activation of the CdO group while keeping the CdC bond almost intact. The unusual selectivity of the silver catalyst can thus be attributed to the unique adsorption geometries of the conjugated CdO bond on the silver surface. Finally, on the basis of our DFT calculations, smaller silver particles might be favorable for the hydrogenation. We therefore prepared smaller silver nanopartilces on the amine-modified silica surface and indeed an enhanced selectivity toward CdO hydrogenation was achieved, exemplifying the importance of

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20919 interplay between experiment and theory in designing highperformance catalysts. 2. Experimental and Computational Details 2.1. Preparation of Ag/SiO2, Au/SiO2, and Pt/SiO2 Catalysts. As a silver catalyst tends to possess low activity toward the crotonaldehyde hydrogenation, a high loading of the silver catalyst was used. Both the Ag/SiO2 and Pt/SiO2 catalysts were prepared by incipient wetness impregnation of the silica support (surface area of 391 m2/g, Qingdao Ocean Chemical Plant) with an aqueous solution of AgNO3 or H2PtCl4. After being dryed overnight at 373 K, calcined in air at 673 K for 3 h, and reduced with hydrogen at 673 K for 1 h, the final catalysts with Ag loading of 12 wt % and Pt loading of 5 wt % were obtained. To prepare 4 wt % Au/SiO2 with particle sizes Ag, which is also in accordance with their reaction rates (Table 1) for the selective hydrogenation of crotonaldehyde. 5. Conclusions We have performed a comparative study of the hydrogenation of crotonaldehyde over Pt, Au, and Ag catalysts using both experimental and theoretical methods. It is shown that in the hydrogenation of R,β-unsaturated aldehydes the silver catalysts possess an intrinsically high selectivity toward formation of unsaturated alcohols. Our theoretical calculations have revealed that this high selectivity can be attributed to the preference of the CdO atop adsorption mode for the low-coordination sites of the catalysts. Namely, the oxygen atom of the CdO group is preferentially σ-coordinated on low-coordinated silver atoms located at the edges and corners of catalytic nanoparticles. In contrast, the gold catalysts present only a moderate selectivity toward unsaturated alcohols when no reducible support is involved, while the platinum catalysts are the least selective

toward unsaturated alcohols. The low selectivity of Pt and Au catalysts is interpreted in terms of the most favored adsorption geometries, which prefer the CdC bond activation to the CdO bond activation. The distinctive size effect presented by the silver catalyst, i.e., the selectivity to unsaturated alcohols increases with decreased particle size, paves the way for simultaneously enhancing the selectivity and activity of silver catalysts by largely decreasing the particle size. Acknowledgment. This work was supported by the National Natural Science Foundation of China (NSFC) for Distinguished Young Investigators (20325620) (T.Z.) and NSFC (20773124) (A.Q.W.) and by NKBRSF (2006CB932305, 2007CB815200) and NSFC (20525104, 20933003) (J.L.). The calculations were performed using a Linux cluster computer from Shanghai Supercomputing Center. Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gallezot, P.; Richard, D. Catal. ReV. Sci. Eng. 1998, 40, 81. (2) Claus, P. Top. Catal. 1998, 5, 51. (3) Ma¨ki-Arvela, P.; Ha´jik, J.; Salmi, T.; Murzin, D. Yu. Appl. Catal., A 2005, 292, 1. (4) Englisch, M.; Jentys, A.; Lercher, J. A. J. Catal. 1997, 166, 25. (5) Gebauer-Henke, E.; Grams, J.; Szubiakiewicz, E.; Farbotko, J.; Touroude, R.; Rynkowski, J. J. Catal. 2007, 250, 195. (6) Margitfalvi, J. L.; Tompos, A.; Kolosova, I.; Valyon, J. J. Catal. 1998, 174, 246. (7) Marinelli, T. B. L. W.; Nabuurs, S.; Ponec, V. J. Catal. 1995, 151, 431. (8) Rechard, D.; Ockelford, J.; Girior-Fendler, A.; Gallezot, P. Catal. Lett. 1989, 3, 53. (9) Grass, M. E.; Rioux, R. M.; Somorjai, G. A. Catal. Lett. 2009, 128, 1. (10) Kliewer, C. J.; Bieri, M.; Somorjai, G. A. J. Am. Chem. Soc. 2009, 131, 9958. (11) de Jesu´s, J. C.; Zaera, F. Surf. Sci. 1999, 430, 99. (12) Birchem, T.; Pradier, C. M.; Berthier, Y.; Cordier, G. J. Catal. 1994, 146, 503. (13) Murillo, L. E.; Goda, A. M.; Chen, J. G. J. Am. Chem. Soc. 2007, 129, 22. (14) Delbecq, F.; Sautet, P. J. Catal. 2002, 211, 398. (15) Delbecq, F.; Sautet, P. J. Catal. 2003, 220, 115. (16) Haubrich, J.; Loffreda, D.; Delbecq, F.; Jugnet, Y.; Sautet, P.; Krupski, A.; Becker, C.; Wandelt, K. Chem. Phys. Lett. 2006, 433, 188. (17) Loffreda, D.; Delbecq, F.; Vigne´, F.; Sautet, P. J. Am. Chem. Soc. 2006, 128, 1316. (18) Haubrich, J.; Loffreda, D.; Delbecq, F.; Sautet, P.; Jugnet, Y.; Krupski, A.; Becker, C.; Wandelt, K. J. Phys. Chem. C 2008, 112, 3701. (19) Hirschl, R.; Delbecq, F.; Sautet, P.; Hafner, J. J. Catal. 2003, 217, 354. (20) Loffreda, D.; Delbecq, F.; Sautet, P. Chem. Phys. Lett. 2005, 405, 434. (21) Loffreda, D.; Delbecq, F.; Vigne, F.; Sautet, P. Angew. Chem., Int. Ed. 2005, 44, 5279. (22) Loffreda, D.; Jugnet, Y.; Delbecq, F.; Bertolini, J. C.; Sautet, P. J. Phys. Chem. B 2004, 108, 9085. (23) Delbecq, F.; Sautet, P. J. Catal. 1995, 152, 217.

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