One-Pot Synthesis of Pt@TiO2 Electrocatalysts for Methanol Oxidation

Abstract

Conducting metal@metal oxide structures are promising materials to overcome the limitations of state-of-the-art fuel cell catalysts based on carbon as support material, which suffer from carbon corrosion and metal nanoparticle sintering. One interesting metal oxide is titanium oxide, which shows high stability under fuel cell operating conditions [1-3]. Also due to electronic effects the catalytic properties of supported platinum nanoparticles can be tuned by metal oxide supports, leading to increased catalytic activity or better poisoning tolerance, which are the main research goals for methanol oxidation catalysts [4,5]. In this work, the one-pot synthesis of Pt@TiO2 nanostructures is presented. The synthesis is based on hydrophobic nanoreactor templating, using inverse block copolymer micelles as nanoreactor templates, allowing the concomitant structuration of the metal and metal oxide components respectively [6-8]. For the synthesis of platinum nanoparticles supported on titanium oxide, both metal and metal oxide precursor are loaded into the micelles. The micellar template is removed after solvent evaporation by calcination under air, leading, after a subsequent reduction step, to nanostructured composites made of finely distributed 2-4 nm platinum nanoparticles supported on titanium oxide particles. The overall size and shape of the Pt@TiO2 nanocomposite particles is thereby imprinted by the size and shape of the micellar template. Variation of the calcination temperature revealed that the number and size of the platinum nanoparticles can be controlled by the calcination temperature as well as the crystallinity of the oxide support. Using this approach thin film electrodes were made by dipcoating monolayers of Pt@TiO2 nanocomposites on ITO coated glass substrates as model electrocatalysts to study the catalytic properties of the Pt@TiO2 nanostructures regarding methanol oxidation. Cyclic voltammetry revealed a classical platinum CV, proving the electrochemical accessibility of the platinum nanoparticles. After the addition of methanol the Pt@TiO2 catalysts showed high specific activities, reaching values of up to 2 mA/cm²Pt (anodic peak current density) for the catalysts calcined at 500 °C. Compared to a commercial Pt/C catalyst (20wt.% Pt on Vulcan XC72), the specific activity increased by a factor of 2. The second oxidation peak in the cathodic scan direction, which is usually attributed to the oxidation of intermediates from the methanol oxidation, was used to characterize the poisoning tolerance of the catalysts by calculating the ratio of the peak currents from forward and backward scan (If /Ib ). In case of the Pt@TiO2 catalysts the I f/I b values are around 1.5, which is comparable to other Pt@metal oxide catalysts reported in literature [4,5]. In case of the commercial Pt/C catalyst the I f/I b value is 0.7, demonstrating the higher poisoning tolerance of the Pt@TiO2 catalyst. These results demonstrate that Pt@TiO2 is a promising electrocatalyst for methanol oxidation. The presented one-pot approach allows an easy synthesis of supported noble metal catalysts with full control over activity determining parameters like noble metal loading, nanoparticle size and composition.