N. Nag

Arizona State University, Phoenix, Arizona, United States

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Publications (8)20.4 Total impact

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    ABSTRACT: A nanoscale investigation was performed on the initial stages of the formation of nanoparticles during the preparation of NiCu/TiO2 bimetallic catalysts by incipient wetness impregnation. The evolution of the structure and chemistry of individual nanoparticles was followed in the reaction cell of an environmental transmission electron microscope. During reduction in hydrogen at 300 °C, nuclei quickly formed and grew mainly via Ostwald ripening or short-range particle–particle coalescence. The presence of Cu greatly enhanced the reducibility of the Ni species and about 85% of the particles were metallic. Most of the particles were uniform in composition but approximately 15% of the particles showed Ni enrichment on the surface. The surface enrichment of Ni was attributed to differential diffusion processes and demonstrated that, for the reduction temperature used for this experiment, the structure of the bimetallic particles was controlled by kinetics rather than by thermodynamics.
    Journal of Catalysis 02/2009; 262(1):73-82. · 6.07 Impact Factor
  • Microscopy and Microanalysis 08/2006; 12. · 1.76 Impact Factor
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    ABSTRACT: We have conducted a detailed investigation of the initial stages of the formation of metal nanoparticles during the incipient wetness synthesis process for both Co on γ-Al2O3 and Ru-promoted Co on γ-Al2O3 catalysts. The synthesis was performed in the reaction cell of an environmental transmission electron microscope so that the processes can be followed using in situ atomic resolution imaging, Z-contrast imaging and nanospectroscopy. The metal precursors were found to have a highly non-uniform distribution on the alumina support, which leads to a non-uniform distribution of metal particles in the final catalysts. In situ atomic resolution images and electron energy-loss nano-analyses showed that during the reduction process, the metal precursor material transformed into an intermediate phase of cubic CoO. With the addition of Ru, however, in addition to the presence of large CoO particles, small particles of either CoRu or pure Ru were also formed. Our experimental results directly show that the addition of Ru promotes the formation of individual CoRu bimetallic nanoparticles and enhanced the reducibility of small Co particles in the Co(Ru)/γ-Al2O3 systems.
    Applied Catalysis A General 07/2006; 307(2):212-221. · 3.67 Impact Factor
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    ABSTRACT: Nucleation and growth mechanisms of Ni nanoparticles synthesized via an incipient wetness technique on a high-surface area titania support (i.e., a mixture of anatase and rutile) are studied using environmental transmission electron microscope (ETEM). Most Ni nanoparticles are found to nucleate from the Ni precursor coated on the surface of the titania support. Even though both anatase and rutile supports are the nucleation sites for Ni nanoparticles, it was observed that the particles have different morphologies on the supports, i.e., a non-wetting morphology on the anatase support versus a wetting morphology on the rutile {1 0 1}. This is because the interfacial energy of Ni/rutile is lower than that of Ni/anatase. Titania clusters are found to nucleate on the surface of the Ni particles during in situ ETEM reduction, indicating that the presence of partial titania overlayers is directly related to the synthesis of the Ni/TiO2 catalysts. The growth mode of the Ni nanoparticles on the titania support is three-dimensional, while that of the rutile cluster on the surface of the Ni is two-dimensional layer-by-layer.
    Surface Science 02/2006; 600(3):693-702. · 1.87 Impact Factor
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005.
    Microscopy and Microanalysis 08/2005; 11(S02):1538-1539. · 1.76 Impact Factor
  • Microscopy and Microanalysis 08/2004; 10. · 1.76 Impact Factor
  • Microscopy and Microanalysis 08/2004; 10. · 1.76 Impact Factor
  • Microscopy and Microanalysis 07/2004; 10:456 - 457. · 1.76 Impact Factor