Several compositions of Pt-WO3 catalysts were synthesized and characterized for the electro-oxidation of methanol and CO. The surface morphologies of the catalysts were found to be dependent on the composition. X-ray energy dispersive spectroscopy and X-ray photoelectron spectroscopy results suggest a surface enrichment of WO3 in the codeposited Pt-WO3 catalysts. Cyclic voltammetry and chronoamperometry in methanol show an improvement in catalytic activity for the Pt-WO3 catalysts. A significant improvement in the poison tolerance toward CO and other organic intermediates was observed in the mixed metal-metal oxide catalyst. The catalytic performance of the different compositions was directly compared by normalization of the current to active sites. CO-stripping voltammetry suggests the involvement of WO3 in the catalytic process as opposed to a mere physical effect as suggested by previous work. A possible mechanism for this improvement is proposed based on the electrochemical data.
We describe a method for the characterization of electro-oxidation catalysts that involves the fabrication and reactivity mapping of samples possessing a catalyst gradient. The objective of this work is to demonstrate a method for catalyst preparation and screening that directly measures the activity of spatially localized catalyst samples toward electro-oxidation reactions relevant to the fuel cell anode in an effort to discover and characterize new catalyst formulations. In this report, a well-defined gradient in the surface coverage of platinum is created on an electronically conductive but catalytically inactive indium-tin-oxide (ITO) substrate by the application of a nonuniform electric field during platinum electro-deposition. A linear variation in applied potential is imposed on an ITO substrate to induce a nonuniform platinum deposition rate, which results in the formation of a coverage gradient. The reactivity of this catalyst gradient is measured directly as a function of spatial position using a scanning electrochemical microscope in the feedback mode. Surface imaging using a noncatalytic redox couple (Ru(NH3)6 3+/2+ ) depicts a uniform and highly reactive electrode surface over both ITO and platinum domains. In contrast, imaging with a catalytic probe (H + /H2), which senses variations in the substrate activity toward the hydrogen oxidation reaction, clearly illustrates a variation in surface reactivity that is a function of the local substrate composition. The presence of a nonuniform platinum coverage generates a variation in the hydrogen oxidation rate constant. The local reaction rate, as deduced by scanning electrochemical measurements, is proportional to the local platinum surface coverage as determined with electron microscopy. This work demonstrates a unique method for the preparation of catalyst gradient samples coupled with a characterization method that can measure catalytic activity for electro-oxidation reactions on a local scale.
We have manipulated raw and functionalized gold nanoparticles (with a mean diameter of 25 nm) on silicon substrates with dynamic atomic force microscopy (AFM). Under ambient conditions, the particles stick to silicon until a critical amplitude is reached by the oscillations of the probing tip. Beyond that threshold, the particles start to follow different directions, depending on their geometry and adhesion to the substrate. Higher and lower mobility were observed when the gold particles were coated with methyl- and hydroxyl-terminated thiol groups, respectively, which suggests that the adhesion of the particles to the substrate is strongly reduced by the presence of hydrophobic interfaces. Under ultrahigh vacuum conditions, where the water layer is absent, the particles did not move, even when operating the atomic force microscope in contact mode. We have also investigated the influence of the temperature (up to 150 degrees C) and of the geometrical arrangement of the particles on the manipulation process. Whereas thermal activation has an important effect in enhancing the mobility of the particles, we did not find differences when manipulating ordered versus random distributions of particles.
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