Nolan T. Flynn and Andrew A. Gewirth. Department of Chemistry. University of Illinois at Urbana-Champaign. 600 S. Mathew Ave., Urbana, IL 61801. Recently ...
Molybdate-Stabilized Platinum Nanoparticles: Synthesis, Characterization, and Electrocatalytic Behavior Nolan T. Flynn and Andrew A. Gewirth Department of Chemistry University of Illinois at Urbana-Champaign 600 S. Mathew Ave., Urbana, IL 61801
Figure 1. STEM image of molybdate/platinum nanoparticles on holey carbon support obtained with a magnification of 500,000× at 100 kV.
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Recently, for a number of reasons, there has been renewed interest in synthesis and characterization of nanoparticles.1 Although quantum confinement effects are among the most interesting, much of the practical motivation remains centered on the catalytic properties exhibited by nanoscale metal particles. One goal of catalytic research is the modulation of particle properties through altering the type of capping agent introduced during synthesis. Often, however, this stabilizing agent must be removed following synthesis through processes such as annealing in a reactive environment to yield a catalytically active surface.2,3 Although much work in this area has occurred, the bulk of research has focused on the utilization of organic molecules (e.g., polyvinyl alcohol, alkanethiols) or small, charged inorganic species (borohydride, citrate). Our work employs precursors to inorganic analogues of organic polymers—namely, oxoanions such as molybdate—to serve as stabilizers and catalytic modifiers. Following synthesis of the particles using simple, solution-based chemical reduction methods,4,5 we have utilized a number of techniques to characterize the physical and chemical properties of the resultant molybdate-stabilized platinum nanoparticles. The alteration in mean size and distribution were probed using scanning transmission electron microscopy (STEM); see image in Figure 1. Energy dispersive X-ray spectroscopy (EDX) coupled to the STEM allowed both point and cross particle chemical characterization. These techniques indicated a smaller particle with detectable quantities of molybdate resulted from introducing this stabilizing agent to the reaction vessel. The presence of molybdenum along with platinum in the sample was confirmed by the subsequent X-ray photoelectron spectroscopy (XPS) analysis. Additionally, XPS enabled assignment of the oxidation state of these two species to hexavalent molybdenum and zerovalent platinum, as would be expected. To probe the alteration of catalytic properties imparted by the presence of the molybdate adlayer, the molybdate-capped platinum nanoparticles were tested for activity toward methanol electro-oxidation and compared with uncapped platinum particles. This latter system typically suffers from the hindrance of strongly adsorbed, incompletely oxidized side products such as carbon monoxide.6 The molybdate adlayer was found to alter both the synthetic results and the methanol oxidation behavior, specifically with regards to the duration of resistance to deactivation processes as seen in the i-t curves in Figure 2. This modification demonstrates the ability to control particle behavior by stabilization with a new class of compounds—namely, oxoanions and polyoxoanions such as molybdate.
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Figure 2. Current-time curves for uncapped (solid) and molybdate-capped (dashed) platinum nanoparticles following a potential step from 0.0 V to 0.4 V. References 1)Bradley, J. S. “The Chemistry of Transition Metal Colloids” In Clusters and Colloids: from Theory to Applicationis; Schmid, G., Ed.; VCH: Weinheim, 1994, pp 459-544. 2)Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Brijoux, W.; Bönnemann, H. Langmuir 1997, 13, 2591-2595. 3)Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Bönnemann, H. J. Electrochem. Soc. 1998, 145, 925-931. 4)Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Chem. Mater. 1996, 8, 1161-1163. 5)Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924-1926. 6)Jarvi, T. D.; Stuve, E. M. “Fundamental Aspects of Vacuum and Electrocatalytic Reactions of Methanol and Formic Acid on Platinum Surfaces” In Electrocatalysis; Lipkowski, J. and Ross, P. N., Ed.; Wiley-VCH: New York, 1998, pp 75-152.
