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Langmuir 2009, 25, 574-581
Electrooxidation of CO on Uniform Arrays of Au Nanoparticles: Effects of Particle Size and Interparticle Spacing Sachin Kumar and Shouzhong Zou* Department of Chemistry and Biochemistry, Miami UniVersity, Oxford, Ohio 45056 ReceiVed August 22, 2008. ReVised Manuscript ReceiVed October 16, 2008 Uniform arrays of Au nanoparticles with controlled size and interparticle distance were synthesized by using polystyrene-b-poly(2-vinylpyridine) as a template and an Ar plasma treatment. These uniform arrays of nanoparticles are ideal model systems for studying the effects of particle size and interparticle distance on their catalytic activity. Electrooxidation of carbon monoxide (CO) on these particle arrays in CO-saturated 0.1 M NaOH was examined. On particle arrays with a particle size of ca. 4 nm and an interparticle distance varying from 28 to 80 nm, rotating disk electrode (RDE) voltammetric results show that the half-wave potential for CO oxidation shifted to more positive potentials as the interparticle distance increased. This apparent kinetic difference can be explained by the CO diffusion pattern change with the interparticle distance. On particle arrays with a similar interparticle distance but varying size from 2.4 to 9.0 nm, the electrooxidation of CO shows a particle size-dependent activity, with the highest activity obtained on 4.2 nm Au particles, as revealed by the Tafel plot. The Tafel slope also depends on the particle size, with the smallest slope obtained on 4.2 nm particles. The particle size-dependent catalytic activity was tentatively explained in terms of the size-dependent adsorption properties. A brief comparison was made with the results from gas phase CO oxidation on Au nanoparticles.
Introduction The catalytic oxidation of carbon monoxide (CO) is of both fundamental and practical importance. In heterogeneous gas phase and homogeneous solution phase reactions, CO oxidation by water to form carbon dioxide and hydrogen, the so-called “water gas shift” reaction, is an important process for producing hydrogen from various organic fuels.1,2 In addition, CO oxidation with dioxygen is a process of relevance to CO removal in automobile exhaust.3 In direct alcohol fuel cells, CO is an intermediate of oxidation of the carbon containing fuels, such as methanol and ethanol.4,5 The strong adsorption of CO on Pt catalysts for both alcohol and hydrogen fuel cells poisons the catalysts and presents a challenge for fuel cell technology.4,5 Recent advancement in synthesizing nanoparticles with uniform size distribution and shape control6-11 has renewed interests in studying the relationship between the catalytic activity and particle structure and composition.9,12-22 One highly cited and extensively studied example is gas phase carbon monoxide (CO) oxidation on Au nanoparticles supported on different materials.20-22 In most of the gas phase reactions, Au was considered to be a poor heterogeneous catalyst.21 In the past decade or so it has been * To whom correspondence should be addressed. Tel: 513-529 8084. Fax: 513-529 5715. E-mail:
[email protected]. (1) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935–938. (2) Rodriguez, J. A.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Angew. Chem., Int. Ed. 2007, 46, 1329–1332. (3) Lin, P. Y.; Skoglundh, M.; Lowendahl, L.; Otterstedt, J. E.; Dahl, L.; Jansson, K.; Nygren, M. Appl. Catal. 1995, 6, 237–254. (4) Iwasita, T. In Handbook of Fuel Cells; Vielstich, W., Gasteiger, H. A., Lamm, A., Eds.; John Wiley & Sons: New York, 2003; Vol. 2, pp 603-624. (5) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9–45. (6) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924–1926. (7) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161. (8) Murphy, C. J.; San, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. (9) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663– 12676. (10) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648–8649. (11) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176–2179.
demonstrated that small (
