Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 559

& CHAPTER 16
Support and Particle Size Effects
in Electrocatalysis
BRIAN E. HAYDEN and JENS-PETER SUCHSLAND
School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
16.1 INTRODUCTION
The aim of catalysis is to lower the activation energy and/or increase the selectivity
[Somorjai and Rioux, 2005] for a particular chemical reaction. For heterogeneous
catalysis, at the gas/solid interface, activity is usually defined in terms of the turnover
frequency (TOF) per surface area of catalyst (or per active site: specific activity) at
fixed temperature. In the case of electrocatalysis, the electrochemical current per
catalyst area (or active site) at a fixed potential provides a measure of activity (or
specific activity). In order to produce high utilization of catalyst material (particularly
for precious metals), high dispersion of the active materials on a support will also yield
optimal mass activity. It is well established in heterogeneous catalysis, however, that
this does not take into account the apparent structural sensitivity of many catalytic
reactions [Henry et al., 1997]. A decrease in specific activity with increased dispersion,
for example, will counteract the positive effect of dispersion on mass activity. An
important example of a structurally sensitive electrocatalytic reaction is the apparent
deactivation of the oxygen reduction reaction (ORR) in acid media on platinum
with decreasing particle size [Guerin et al., 2004]. This results in an optimal mass
activity of carbon-supported Pt of about 3 nm. In addition to particle size effects,
the nature of the supporting substrate can strongly influence the overall catalytic
activity of a supported metal. This can result from stabilization of particle sizes and
morphologies, or from modification of the electronic properties of the supported
metal. A well-documented example of this effect in heterogeneous catalysis is the
induced activity of gold supported by a titania support in low temperature oxidation
reactions. Support effects are less well documented in the case of electrocatalysis.
This chapter summarizes a series of experiments carried out on model supported