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
Highly oriented pyrolytic graphite (HOPG) implanted with N, Ar and B is studied as a support for platinum nanoparticle catalysts for fuel cells. Experimentally, we find that Pt supported by N-HOPG is more disperse, more catalytically active and suffers less particle ripening than native HOPG, while Pt supported on Ar-irradiated HOPG is slightly more active but ripens more than Pt on native HOPG. Defective HOPG supports are modeled by density functional theory (DFT) calculations that confirm and explain the above experimental results. First, defect energetics are studied to demonstrate that nitrogen doping at high doses likely causes agglomerated nitrogenous defect clusters, and irradiation with Ar ions creates vacancies that agglomerate in vacancy clusters. Second, Pt catalyst particle nucleation and agglomeration is studied. For Pt clusters supported on HOPG with nitrogen defects, calculations show a greater driving force for nucleation and greater particle tethering. For Pt clusters supported on HOPG with vacancy aggregations, this study shows a strong driving force for nucleation and a much enhanced tendency for particle ripening. Third, the electronic structure of Pt clusters on different supports is calculated. Finally, reaction energetics are calculated for two likely reaction pathways over Pt clusters supported on different HOPG substrates. Pt-N-HOPG shows modified electronic structure of the Pt catalyst and increased activity towards oxygen. Pt-Ar-HOPG shows slightly enhanced catalytic activity towards oxygen. In all respects, the findings agree with experiment. The calculations attribute the catalytic activity changes primarily to changes in the workfunction and secondarily to the d-band structure of supported Pt particles.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.