Oxygen reduction crystallite size effects

For fuel-cell technology development, it has been important to understand the characteristics and operation of highly dispersed platinum and platinum alloy electrocatalysts. A series of papers on platinum crystallite size determinations in acid environments for oxygen reduction and hydrogen oxidation was published together by Bert, Stonehart, Kinoshita and co-workers.5 The conclusion from these studies was that the specific activity for oxygen reduction on the platinum surface was independent of the size of the platinum crystallite and that there were no crystallite size effects. 

Markovic NM, Gasteiger HA, Ross PN (1997) Kinetics of oxygen reduction on Pt(hkl) electrodes implications for the crystallite size effect with supported Pt electrocatalysts. J Electrochem Soc 144 1591... 

In this chapter we review studies, primarily from our laboratory, of Pt and Pt-bimetallic nanoparticle electrocatalysts for the oxygen reduction reaction (ORR) and the electrochemical oxidation of H2 (HOR) and H2/CO mixtures in aqueous electrolytes at 274—333 K. We focus on the study of both the structure sensitivity of the reactions as gleaned from studies of the bulk (bi) metallic surfaces and the resultant crystallite size effect expected or observed when the catalyst is of nanoscale dimension. Physical characterization of the nanoparticles by high-resolution transmission electron microscopy (HRTEM) techniques is shown to be an essential tool for these studies. Comparison with well-characterized model surfaces have revealed only a few nanoparticle anomalies, although the number of bimetallics... 

With differences in activity between crystal faces of only a factor of five or less for oxygen reduction in perchloric acid, the particle models of Kinoshita indicate that there would be little or no crystallite size effect for the ORR in a nonadsorbing acid, such as trifluoromethane sulfonic acid or closely related derivatives [44]. In KOH, because the (111) face is the most active, an increase in activity with the smallest particles (e.g., 1 nm)—ideally (uniform size, perfect geometry) would be predicted by the Vt hkl) data—would be by about a factor of five. With real catalysts, where all particles are neither uniformly sized nor perfectly facetted, the effect might be much less, perhaps only a factor of two or so. Unfortunately, we do not know of any studies of crystallite size effects for supported Pt catalysts in either KOH or... 

G. Tamizhmani, J.P. Dodelet, D. Guay, Crystallite size effects of carbon-supported platinum on oxygen reduction in liquid acids. J. Electrochem. Soc. 1996, 143, 18-22. 

N. Markovic, H. Gasteiger, and P. N. Ross, Kinetics of Oxygen Reduction on Pt(hkl) Electrodes Implications for the Crystallite Size Effect with Supported Pt Electrocatalysts, /. Electrochem. Soc., 144, 1591 (1997). 

The effect of Pt crystallite size on the kinetics of the oxygen reduction reaction is a long-standing problem in electrocatalysis. An excellent review of experimental work with carbon-supported Pt was presented by Kinoshita [33] for work prior to about 1990. Kinoshita concluded that the change in fraction of surface atoms on the... 

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