Different Sized Platinum Nanoparticles Supported on Carbon: an Xps Study on These Methanol Oxidation Catalysts

Abstract

Different sized platinum nanoparticles on carbon supports have been prepared using PtCl4 (catalyst Ia) and H2PtCl6 (catalyst lia) as starting materials and 1-hexanethiol as a surfactant and then heated to 200°C (catalyst Ib and catalysts IIb), 300°C (catalyst Ic and catalysts IIc), and 400°C (catalyst Id and catalysts IId) for 4 h under argon gas. All the catalysts showed a face-centered cubic (fee) crystal structure as determined by X-ray diffraction. X-ray diffractograms and transmission electron microscopy results reveal that the platinum nanoparticles are homogeneously dispersed on the carbon support, exhibit narrow particle size distribution, and show no appreciable aggregation. The average platinum particle size as determined from XRD data was found to be ∼2.00, ∼2.56, ∼4.23, ∼4.52, ∼2.13, ∼2.77, ∼4.29, and ∼4.62 nm for catalysts Ia-d and IIa-d, respectively. X-ray photoelectron spectra of all the catalysts indicated that most (>70%) of the platinum nanoparticles have an oxidation state of zero and a small amount (<30%) have a +4 oxidation state with binding energies of 71.2-72.2 (Pt 4f7/2) and 74.3-75.5 eV (Pt4f7/2), respectively. Increasing the temperature during the heat treatment process results in an enlargement of platinum particle size, a positive shift in the binding energies of Pt(0) and Pt(IV), and a decrease in catalytic activity in the methanol oxidation reaction. © 2007 American Chemical Society.