DOI
https://doi.org/10.25772/MYWM-F487
Author ORCID Identifier
https://orcid.org/0000-0001-6527-879X
Defense Date
2020
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Nanoscience and Nanotechnology
First Advisor
Dr. Hani M. El-Kaderi
Abstract
Precise control of metal nanoparticles’ size, composition, and dispersity over high surface area supports are highly desirable to address current challenges in energy storage and conversion as well as catalytic processes involving precious metals. Therefore, developing viable synthetic routes that enable new catalytic systems derived from inexpensive transition metals or limited use of precious metals is vital for clean energy applications such as fuel cells and rechargeable batteries or affordable drugs in the pharmaceuticals arena. In addition to metal components of heterogeneous catalysts, the catalyst support is an integral part of catalyst design as it can impart both physical stability and catalytic enhancement through strong metal-support interactions. In particular, recent studies have shown that the incorporation of heteroatoms like nitrogen and phosphorus in high surface area carbon supports is an effective approach for tailoring the textural and electronic properties of carbon supports.
Here we introduce different supported metal nanoparticles on high surface area supports, with their characteristic tuned toward different applications. In the first project, we developed an iron phosphide doped porous carbon system (PFeC) and used it as a cathode catalyst for oxygen reduction reaction (ORR) in fuel cells. The conversion of chemical energy to electrical energy is a sustainable approach for energy production achieved by fuel cells. Currently, the noble metal platinum, in the form of 20 wt% Pd deposited on carbon support (Pt/C) is the commercially available catalyst for the ORR. Sluggish ORR mechanism and lack of long-term stability demand for a more sustainable, inexpensive, and kinetically efficient replacement catalyst. Here iron phosphide nanoparticles (NPs) incorporated in a phosphorus-doped porous carbon, with a high specific area (SABET = 967 m2 g−1) was synthesized using inexpensive reactants, triphenylphosphine and iron chloride by a facile carbonization/chemical activation method via zinc chloride. PFeC selectively reduces O2 via an efficient reaction pathway and exhibits superior long-term stability than Pt/C. The superior electrocatalytic performance is credited to the synergistic effects between the P and Fe which, form well-defined and well-distributed nanoparticles confined in highly porous carbon nanosheets.
In the second project, supported palladium-based ultra-small bimetallic NPs deposited on mesoporous fumed silica support (SABET = 350 m2 g−1) were synthesized and used as a catalyst for Suzuki -Miyaura cross-coupling (SCC) reactions. Bimetallic NPs consisting of active metal Pd and base metals (Cu, Ni, and Co) were deposited on the silica support through strong electrostatic (SEA) synthesis method yielding homogeneously alloyed nanoparticles with an average size of 1.3 nm. All bimetallic catalysts were found to be highly active toward SCC surpassing the activity of monometallic Pd/SiO2. In particular, the catalyst consisting of Cu and Pd (CuPd/SiO2), performed the SCC with a remarkable turn over frequency of 248000. The combination of Pd with base metals helps in retaining the Pd0 status by charge donation from base metals to Pd and thus facilitating the SCC, in specific lowering the activation energy of the aryl halide oxidative addition rate-limiting step.
In the third and last project, functionalized supports are widely utilized in energy conversion and energy storage applications. High surface area porous carbon materials have been introduced as a highly active cathode material for Lithium-sulfur batteries (LSB). The electrochemical performance of the LSB can be largely improved by the efficient reversible conversion of lithium polysulfides to Li2S during discharge and to elemental sulfur during charge. Nickel NPs deposited on high surface area nitrogen-doped carbon support (Ni/BIDC-900, SABET = 3560 m2 g−1) act as active centers for the adsorption of polysulfides during the discharge process and rapidly convert them to Li2S while catalyzing Li2S oxidation to sulfur in the reverse process. The addition of Ni NPs improves the reaction kinetics and activity retention of the LSB.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-16-2020