Electrocatalytic O₂ reduction at glassy carbon electrodes electrochemically
modified with dendrimer-encapsulated Pt nanoparticles

In 1998 we reported a strategy for preparing metallic dendrimer-encapsulated nanoparticles (DENs) using poly(amidoamine) (PAMAM) dendrimers as templates (J. Am. Chem. Soc. 1998, 120, 4877). DENs are normally prepared by a two-step process. First, metal ions are extracted into the interior of the dendrimers. Second, the metal-ion/dendrimer composite is chemically reduced using NaBH₄ to produce encapsulated metal nanoparticles.  In this approach the dendrimer is a more-or-less monodisperse template, and consequently the nanoparticles that result from this synthesis (the replicas) are themselves highly monodisperse.The dendrimer also stabilizes the encapsulated nanoparticles, but because it is porous the surface of the nanoparticles are partially exposed to reactants in the solution.  This means that DENs have interesting catalytic properties, and indeed we and others have shown that they are effective as both homogeneous catalysts (e.g., hydrogenations and carbon-carbon coupling reactions) and as supported heterogeneous catalysts (e.g., CO oxidation). 

Recently, research group member Heechang Ye expanded the scope of DEN-based catalysis by immobilizing Pt DENs on the surface of glassy carbon electrodes (GCEs) and showing that this results in electrocatalytic activity for the O₂ reduction reaction (ORR). This result is significant because there are few alternative methods for immobilizing well-defined nanoparticle catalysts on electrode surfaces.

Scheme 1 shows the general synthetic route for preparing Pt-DEN monolayers on GCEs and their application to the ORR. Our approach consists of two steps.  First, Pt DENs containing an average of 40 Pt atoms each are prepared using fourth-generation, hydroxyl-terminated PAMAM dendrimers (G4-OH).  Transmission electron microscopy indicates that these particles have a diameter of 1.4 ± 0.3 nm.  Second, these Pt DENs are immobilized onto a GCE using an electrooxidative coupling method. X-ray photoelectron spectroscopy confirms the presence of Pt DENs on the GCE following immobilization.

The resulting Pt DEN films are electrocatalytically active for the O₂ reduction. Figure 1 shows cyclic voltammograms (CVs) for O₂ reduction at three different types of electrodes: a naked GCE, a GCE modified with Pt-free G4-OH dendrimers, and a GCE modified with G4-OH(Pt₄₀) DENs. The electrode modified with Pt DENs yields an onset current at about 0.5 V and a well-defined peak at 0.22 V in an O₂-saturated 0.5 M H₂SO₄ electrolyte solution.  However, the other two electrodes, which lack Pt nanoparticles, result in an onset current of about -0.1 V and a well-defined peak at –0.39 V. The 610 mV positive shift in the O₂ reduction peak for Pt DENs indicates a significant electrocatalytic effect. Importantly, these DEN monolayers are robust: they retain their electrocatalytic properties even after 50 consecutive CVs over the O₂ reduction wave and sonication for 10 min in a 0.5 M H₂SO₄ electrolyte solution.

To verify that the Pt nanoparticles remain within the dendrimers during the electrochemical immobilization procedure, a selective Pt DEN poisoning experiment was performed. The top CV in Figure 2 was obtained after exposing at Pt DEN-modified electrode to to a solution of CH₂Cl₂ containing 3 mM C12SH.  We hypothesized that the dendrimer would collapse around the nanoparticle in a poor solvent like CH₂Cl₂, and that this would protect its surface from the thiol poison.  A comparison of the top CV in Figure 1 with the top CV in Figure 2 confirms this hypothesis.  When the same experiment was carried out using ethanol, which is a good solvent for the dendrimer, the surface of the encapsulated nanoparticle was poisoned (bottom CV in Figure 2).  In this case, the dendrimer does not collapse, and the thiol is able to penetrate to the catalyst surface and passivate it. If the Pt nanoparticles were not associated with the dendrimers, then they would become poisoned by the thiol regardless of the nature of the solvent.  We conclude, therefore, that the nanoparticles remain within the dendrimers even after immobilization on the GCE surface and subsequent electrocatalytic reduction of O₂.

The next step for this project is an exploration of the catalytic properties of other types of DENs, including other monometallics and particularly bimetallic alloys and core/shell nanoparticles. This study will include a quantitative examination of electrode kinetics as a function of catalyst size, composition, and structure.

The results described in this brief summary are more fully elucidated in a research publication submitted to the Journal of the American Chemical Society in October, 2004.

Scheme 1

  

Figure 1. CVs for the reduction of O₂ using (top to bottom) a GCE modified with G4-OH(Pt₄₀) DENs, a GCE modified with Pt-free G4-OH dendrimers, and a naked GCE. The data were obtained in an aqueous 0.5 M H₂SO₄ electrolyte solution saturated with O₂. The scan rate was 50 mV/s.

 

  Figure 2. CVs obtained using G4-OH(Pt₄₀)-modified GCEs. Prior to obtaining the CVs, the modified electrodes were exposed to C12SH dissolved in either CH₂Cl₂ (top) or ethanol (bottom) for 20 min. The CVs were obtained in an aqueous 0.5 M H₂SO₄ electrolyte solution saturated with O₂. The dashed line shows the position of O₂ reduction peak before the modified electrodes were exposed to C12SH solution. The scan rate was 50 mV/s.