Date of Award

2016

Document Type

Open Access Dissertation

Department

Chemical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

John R. Regalbuto

Abstract

Ultrasmall platinum and palladium nanoparticles supported on carbon are used in a wide variety of industrial catalytic processes including hydrogenation-dehydrogenation reactions, isomerization of hydrocarbons, ammonia and formic acid decomposition, the oxidation of carbon monoxide, alcohols and ammonia, and fuel cells. This dissertation covers three aspects of the chemistry of these ultrasmall, carbon supported nanoparticles.

In the first vein of work, the oft-observed discrepancy in Pd nanoparticle size estimation between chemisorption and other methods such as STEM and XRD is explored. It is demonstrated that lower-than-expected chemisorption uptake can stem from not only residual chloride, but also from the decoration of the Pd surface by the carbon support itself. The degree of decoration decreases with graphitization of the carbon supports due to stronger C-C interaction, whereas increased density of oxygen functional groups on the surface increases decoration, due to enhanced Pd-C interactions. A combined synthesis and chemisorption protocol featuring chloride free precursors and a mild oxidative pretreatment prior to chemisorption is established to eliminate the size discrepancy.

In the second vein, the ambient oxidation of ultra-small platinum nanoparticles was explored with a combination of powder XRD performed with a high sensitivity solid state detector, and aberration corrected electron microscopy with fast Fourier transform analysis. For the first time, the identity of the oxide phase is identified as Pt3O4, and the size window of oxidation is accurately outlined: below 1.5 nm, nanoparticles exist only as oxides; from 1.5 to 2.5 nm, metallic and oxide phases occur, while above 2.5 nm, particles are completely metallic. Carbon supports of high microporosity give rise to large particle sizes at high metal loading, which stabilizes the particles against oxidation.

In the last avenue of research, the application of Strong Electrostatic Adsorption for the synthesis of Pt nanoparticles was tested for specialty carbons: multi-walled nanotubes, nanofibers, graphene nanoplatelets, etc. These materials displayed volcano-shaped uptake curves typical of electrostatic adsorption for both Pt anions at low pH and Pt cations at high pH. However, the regimes of uptake often did not correspond to the measured point of zero charge (PZC). It was seen that the PZC of many of the carbons could be changed with washing, and so was likely affected by residual impurities of the manufacturing process. This renders the measured PZC of these specialty carbons unreliable for predicting anion and cation uptake. On the other hand, the anion and cation uptake curves provide an “effective” PZC and do indicate the optimal pH for the synthesis of ultrasmall nanoparticles.

Rights

© 2016, Ritubarna Banerjee

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