One-Step Synthesis of Size-Tunable Rhodium Nanoparticles on

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J. Phys. Chem. C 2010, 114, 11364–11369

One-Step Synthesis of Size-Tunable Rhodium Nanoparticles on Carbon Nanotubes: A Study of Particle Size Effect on Hydrogenation of Xylene Horng-Bin Pan and Chien M. Wai* Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844 ReceiVed: February 12, 2010; ReVised Manuscript ReceiVed: May 26, 2010

Size-tunable rhodium (Rh) nanoparticles can be deposited uniformly on surfaces of carboxylate functionalized multiwalled carbon nanotubes (MWNTs) by a simple one-step sonochemical method in an ethanol solution containing RhCl3 and a series of boron-containing reagents of different reducing power. This technique provides a simple and rapid way of making rhodium nanoparticles of different size, which allows us to perform careful studies of particle size effect on catalysis. The hydrogenation of neat o-, m-, and p-xylene catalyzed by the MWNT-supported metallic Rh nanoparticle catalysts reveals a negative particle size effect (antipathetic structure sensitivity). The stereoselectivity of this reaction measured by the ratio of cis-/trans-dimethylcyclohexane increases with decreasing size of Rh nanoparticles. Introduction The development of metallic nanoparticle catalysts of controllable size and shape has been one of the most active research topics in nanoscience recently.1 It is well established now that the catalytic properties of metallic nanoparticles are significantly influenced by their size.1c,2 Van Hardeveld and Hartog reported that the ratio of (111) and (100) planes can vary depending on the size of metal particles.3 The biggest changes in facets, edges, and corners of metal nanoparticle surface occur between 1 and 5 nm. The exposed metal surface structure is a key factor that determines coordination to metal atoms on the corners or edges that influences the catalytic activity and selectivity of the nanoparticles.1b-d,f,2i,4 Therefore, control of monodisperse metal nanoparticles with size m-xylene > o-xylene. Smith and Pennekamp reported previously that the rate of hydrogenation of xylenes catalyzed by platinum at 30 °C revealed a symmetry effect on the reaction rate, the para position being the most reactive and the ortho position the least reactive.18 It is known that cisdimethylcyclohexane is the major product in heterogeneous hydrogenation of xylenes at relatively lower temperatures.19 According to the results given in Table 1, the cis/trans ratio of the product increases in the order p-xylene < m-xylene
meta (0.22) > para (0.16).20 The formation of trans-dimethylcyclohexane, a thermodynamically favored product, is usually explained by a rollover mechanism, i.e., dimethylcyclohexene, an intermediate of xylene hydrogenation, dissociates from the catalyst surface followed by a rollover and then reassociates for further hydrogenation to trans-dimethylcyclohexane. The formation of cis-dimethylcyclohexane, a thermodynamically less favorable product, is usually described by the addition of six hydrogen atoms (hydride form) from the catalyst surface to the benzene ring of the same face (without rollover of intermediate). Therefore, the o-xylene which has the least steric hindrance (the highest adsorption strength) to catalyst surface leads to the highest cis/trans ratio of the product (dimethylcyclohexane) in heterogeneous hydrogenation reaction at lower temperature. The theoretical turnover frequency (TOF) value, which is a very useful parameter to determine whether a given heterogeneously catalytic reaction is structural sensitive or insensitive to Rh nanocatalysts of varying size between 1 and 10 nm, is calculated from the dispersion value (the exposed catalyst surface) evaluated from the mean size of Rh nanoparticles found from the TEM measurement. The cis/trans ratio of product and theoretical TOF values given in Table 1 represent the order of relative catalytic activity of the different particle size of Rh/MWNTs tested for the hydrogenation of o-, m-, and p-xylene at room temperature. On the basis of the catalytic behavior, heterogeneously catalytic reactions are classified as either a structure-sensitive reaction if its TOF value depends on the particle size of the catalyst or a structure-insensitive reaction if its TOF value is independent of the particle size of the catalyst.2c,21 The structuresensitive reaction can be divided into three categories depending on how the TOF varies as a function of the particle size of the catalyst.2c The TOF value can increase (a negative particle size effect or antipathetic structure sensitivity), decrease (a positive

particle size effect or sympathetic structure sensitivity), or go through a maximum with increasing particle size.2c In this study, we observed that the theoretical TOF values vary as a function of the particle size and reveal a negative particle size effect (antipathetic structure sensitivity) of Rh/MWNTs for the hydrogenation of o-, m-, and p-xylene, respectively. Van Hardeveld and Hartog reported the results of many calculations for the arrangements of the atoms in small metal particles.22 There are three types of surface atoms indicated by the symbols C4 (corner atoms), C7 (edge atoms), and C9 (face atoms). Rhodium nanoparticles have an fcc closed packed arrangement. According to this report,22 the ratio N(Cj)/Ns of face atoms (C9) increases with increasing the particle size but the ratio N(Cj)/ Ns of corner atoms (C4) and edge atoms (C7) decrease with increasing the particle size, where N(Cj) is the number of atoms of coordination Cj and Ns is the number of surface atoms. According to this prediction, the hydrogenation of xylenes in our system takes place on the face atoms C9 (atoms of high coordination) not on the edge atoms C7 and corner atoms C4 (atoms of low coordination). Recently Jackson et al. reported that a Rh/SiO2 catalyst exhibited a negative particle size effect for hydrogenation of p-toluidine in 2,2,4-trimethylpentane under mild conditions.2e Moreover, benzene hydrogenation over Rh/ Al2O323 and Pt/Al2O3,24 and toluene hydrogenation over platinum25 and iridium26 catalysts are also known to have a negative particle size effect. These reports have proven a negative particle size effect on aromatic ring hydrogenation, which is consistent with our results. In terms of stereoselectivity, the cis/trans ratio of the product dimethylcyclohexane increases with decreasing particle size of Rh/MWNTs for the hydrogenation of o-, m-, and p-xylene (Table 1). This phenomenon may be explained by a difference in the ratio of (111) and (100) planes, which is known to vary with particle size3 and can affect hydrogenation mechanism.1d,e,2c For example, Somorjai’s group has reported that the Rh (111) planes favor the complete hydrogenation of benzene to cyclohexane while the Rh (100) planes favor the semihydrogenation to form cyclohexene.1d,e As mentioned above, the dimethylcyclohexene intermediate has a possibility to undergo a rollover mechanism and then reassociates to the metal surface for further hydrogenation to trans-dimethylcyclohexane. Hydrogenation of xylene on the Rh (111) plane probably produces cis-dimethylcyclohexane only but on the Rh (100) plane it has a possibility to form trans-dimethylcyclohexane. According to the regular fcc cubooctahedral model of metal crystals,3,27 the calculated ratios of the (111)/(100) planes are 6.3, 4.8, and 4.4 for Rh nanoparticles of 2.3, 4.5, and 7.8 nm diameter, respectively. On the basis of this calculation, the smallest Rh nanoparticles (2.3 nm) should have the largest (111)/(100) ratio relative to the other two Rh/MWNTs catalysts. A higher ratio of the (111)/ (100) plane should favor formation of more cis-isomer in the hydrogenation of xylene according to our reasoning. Therefore the cis/trans ratio of the dimethylcyclohexane product increases in the order 7.8 nm < 4.5 nm < 2.3 nm in the hydrogenation of xylenes. Conclusion A simple one-step sonochemical method for preparation of sizetunable rhodium nanoparticles on carbon nanotube has been developed which requires only an ethanol solution containing carboxylate functionalized MWNTs, rhodium(III) ions and a series of boron-containing reducing agents with different reducing powers. The particle size of Rh can be controlled by the reducing power of the boron-containing reagents used in the sonochemical synthesis. Different sizes of rhodium nanoparticles with a narrow size

Particle Size Effect on Hydrogenation of Xylene distribution can be uniformly attached to the surfaces of the MWNTs, which allows us to perform careful studies of particle size effect in catalysis. In this study, the hydrogenation of neat o-, m-, and p-xylene catalyzed by CNT-supported metallic Rh nanoparticle catalysts reveals a negative particle size effect (antipathetic structure sensitivity), indicating the aromatic ring hydrogenation reaction takes place on the face atoms (C9), not on the edge atoms (C7) and corner atoms (C4). Moreover, the stereoselectivity (cis/ trans ratio) of the product increases with decreasing particle size of Rh for the hydrogenation of o-, m-, and p-xylene, respectively, suggesting increasing in the (111)/(100) plane ratio with reducing Rh particle size favors complete saturation of the aromatic ring and hence a higher cis-/trans-diemthylcyclohexane ratio. Acknowledgment. This work was supported by AFOSR (FA9550-06-1-0526). We thank Dr. Thomas J. Williams and Mr. J. Franklin Bailey for assistance with XRD, TEM, and SEM/ EDX analysis of our samples. Supporting Information Available: The XRD spectrum of carboxylic acid functionalized MWNTs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Tao, A. R.; Habas, S.; Yang, P. D. Small 2008, 4, 310. (b) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (c) Corain, B.; Schmid, G.; Toshima, N. Metal Nanoclusters in Catalysis and Materials Science: The Issue of Size Control; Elsevier: Boston, MA, 2008. (d) Somorjai, G. A.; Park, J. Y. Angew. Chem., Int. Ed. 2008, 47, 9212. (e) Somorjai, G. A.; Frei, H.; Park, J. Y. J. Am. Chem. Soc. 2009, 131, 16589. (f) Somorjai, G. A.; Park, J. Y. Chem. Soc. ReV. 2008, 37, 2155. (g) 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. (h) Xia, Y.; Xiong, Y. J.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (i) Bhattacharjee, S.; Dotzauer, D. M.; Bruening, M. L. J. Am. Chem. Soc. 2009, 131, 3601. (j) den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Froseth, V.; Holmen, A.; de Jong, K. P. J. Am. Chem. Soc. 2009, 131, 7197. (k) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (l) Zhang, Q. B.; Xie, J. P.; Yang, J. H.; Lee, J. Y. ACS Nano 2009, 3, 139. (m) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (n) Chen, J. Y.; Lim, B.; Lee, E. P.; Xia, Y. N. Nano Today 2009, 4, 81. (2) (a) Hoxha, F.; van Vegten, N.; Urakawa, A.; Krurneich, F.; Mallat, T.; Baiker, A. J. Catal. 2009, 261, 224. (b) Prieto, G.; Martinez, A.; Concepcion, P.; Moreno-Tost, R. J. Catal. 2009, 266, 129. (c) Che, M.; Bennett, C. O. AdV. Catal. 1989, 36, 55. (d) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X. D.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956. (e) Hindle, K. T.; Jackson, S. D.; Stirling, D.; Webb, G. J. Catal. 2006, 241, 417. (f) Watzky, M. A.; Finney, E. E.; Finke, R. G. J. Am. Chem. Soc. 2008, 130, 11959. (g) Zhou, W. J.; Lee, J. Y. J. Phys. Chem. C 2008, 112, 3789. (h) Arai, M.; Takada, Y.; Nishiyama, Y. J. Phys. Chem. B 1998, 102, 1968. (i) Van Santen, R. A. Acc. Chem. Res. 2009, 42, 57. (j) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Ghosh, S. K.; Pal, T. J. Phys. Chem. C 2007, 111, 4596. (3) Van Hardeveld, R.; Hartog, F. Surf. Sci. 1969, 15, 189. (4) (a) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (b) Bowker, M. ACS Nano 2007, 1, 253. (c) Zaera, F. Acc. Chem. Res. 2009, 42, 1152. (d) Meunier, F. C. ACS Nano 2008, 2, 2441. (e) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2004, 126, 7194. (f) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (g) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P. D.; Somorjai, G. A. Nano Lett. 2007, 7, 3097. (5) (a) Humphrey, S. M.; Grass, M. E.; Habas, S. E.; Niesz, K.; Somorjai, G. A.; Tilley, T. D. Nano Lett. 2007, 7, 785. (b) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 7824. (c) Zhang, Y. W.; Grass, M. E.; Habas, S. E.; Tao, F.; Zhang, T. F.; Yang, P. D.; Somorjai, G. A. J. Phys. Chem. C 2007, 111, 12243. (d) Stowell, C. A.; Korgel, B. A. Nano Lett. 2005, 5, 1203. (e) Grass, M. E.; Joo, S. H.; Zhang, Y. W.; Somorjai, G. A. J. Phys. Chem. C 2009, 113, 8616. (f) Grass, M. E.; Zhang, Y. W.; Butcher, D. R.; Park, J. Y.; Li, Y. M.; Bluhm, H.; Bratlie, K. M.; Zhang, T. F.; Somorjai, G. A. Angew. Chem., Int. Ed. 2008, 47, 8893. (g) Zhang, Y. W.; Grass, M. E.; Kuhn, J. N.; Tao, F.; Habas, S. E.; Huang, W. Y.; Yang, P. D.; Somorjai, G. A. J. Am. Chem. Soc. 2008, 130, 5868. (h) Fan, F. R.; Liu, D. Y.; Wu, Y. F.; Duan, S.; Xie, Z. X.; Jiang, Z. Y.; Tian,

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