This study presents clear and compelling experimental evidence for the significant beneficial effects of nitrogen-doping on the activity of Pt/C catalyst systems for the methanol oxidation reaction. This evidence is obtained through the deployment of geometrically well-defined model catalytic systems consisting of tunable assemblies of Pt catalyst nanoparticles deposited onto undoped, Ar-doped, and N-doped highly oriented pyrolytic graphite (HOPG) substrates. Both Ar-and N-doping were achieved via ion beam implantation, and Pt was electrodeposited from solutions of H 2 PtCl 6 in aqueous HClO 4 . Morphology from scanning electron microscopy (SEM) and aqueous electrochemical analysis of catalytic activity was utilized to examine the effect of N-doping compared to the undoped and Ar-doped control samples. The results strongly support the theory that doping nitrogen into a graphite support significantly affects both the morphology and the behavior of the overlying Pt nanoparticles. In particular, nitrogen-doping was observed to cause a significant decrease in the average Pt nanoparticle size, an increase in the Pt nanoparticle dispersion, and a significant increase in catalytic activity and durability for methanol oxidation. The model catalytic systems demonstrated here represent a versatile platform to study catalyst-support interactions in electrocatalytically relevant nanoparticle systems. Experimental HOPG substrate preparation, doping, and surface characterizationHighly oriented pyrolytic graphite (HOPG, 10 mm  10 mm  1 mm, grade 2, SPI Inc.) was used as a model graphitic carbon

N-doped graphite has been reported to provide enhanced catalytic properties as a support material for Pt catalysts in fuel cell applications. With use of a combined experimental and modeling approach, this work identifies the potential fundamental mechanisms for this enhancement effect. To ensure a well-defined experimental system, this work employs highly oriented pyrolitic graphite (HOPG) as a model analogue of the graphite support commonly used in fuel cell applications. Undoped, Ar-doped, and N-doped HOPG substrates have been investigated via electrochemical capacitance and X-ray photoelectron spectroscopy (XPS) measurements. The results indicate that doping, especially N-doping, induces significant modification to the electronic structure of the HOPG surface. A simplified model of the doping effects and band structures for the doped graphite surfaces are proposed to explain these results. When Pt nanoparticles are grown on top of these dopant-modified HOPG surfaces, the resulting Pt/surface-defect interactions significantly impact the Pt nanoparticle nucleation, growth, and catalytic activity.

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