(419e) Electrocatalytic Ethanol Oxidation On Platinum-Tin Oxide Atomic Clusters

Despite intensive research spanning four decades, identification of a sustainable electrocatalyst for proton exchange membrane fuel cells (PEMFCs) remains a pressing need.  PEMFC commercialization is precluded because of the costly Pt catalysts required to accelerate the sluggish oxygen reduction reaction (ORR).  The concomitant use of hydrogen fuel to prevent catalyst de-activation by strongly adsorbed species such as CO further impacts the sustainability of this Pt-based technology due to the need for high-energy purification (to less than 10 ppm CO) as well as an entirely new high-pressure storage and delivery infrastructure.  Rational catalyst design based on highly active nanoparticles (NPs) and the use of biomass-derived liquid fuels, such as ethanol, is a credible strategy to circumvent these problems. 

Single crystal studies have previously demonstrated that that the electrocatalytic ethanol oxidation reaction (EOR) to CO₂ is inhibited by strong chemisorption of the intermediate CO on all crystal facets of Pt.  However, similar studies have also demonstrated that EOR activity can be enhanced by reduced Sn atoms on the Pt surface, with Pt(110) exhibiting the highest anodic current at optimum Sn coverage.  CO migration from Pt and subsequent oxidation on Sn may be occurring since the Pt(110) surface is more active for C-C bond cleavage.  This hypothesis is consistent with the strong intermetallic bonding in Pt-Sn alloys that weakens CO adsorption on Pt sites and activates the more crowded Pt₃Sn(111). Application of these single-crystal results to interpret the behavior of dispersed systems is not presently possible due to the polydispersity of the NPs. 

We have recently demonstrated continuous control over Pt nanoparticle size across the cluster-to-crystal transition region (ca. 0.5–2.7 nm) using a colloidal synthesis approach that involves autoreduction of a Sn-Pt complex in hydrochloric acid solution.  This approach yields particle size distributions less than 10% and permits fine atomic structure contrast in this critical size range.  We here present results concerning the structure dependency of the ethanol oxidation reaction (EOR) at the atomic cluster to single crystal NP transition (ca. 0.5–3 nm) for Pt and Pt-SnO core-shell nanoparticle catalysts.  Chronoamperometry measurements at 0.45 V in 0.5 M ethanol/0.1 M HClO₄ at 60°C have demonstrated that the long-term EOR activity loss for the Pt-SnO core-shell atomic clusters synthesized is 0.1 mA/cm_Pt²-hr.  This loss is the lowest ever reported in the literature. These atomic clusters also exhibit the highest EOR activity relative to Sn-free clusters, single crystal NPs and commercial NPs.