Pt and Pt-Ir Nanoparticles Supported on Carbon Allotropes for Ammonia Oxidation
Cunci Perez, Lisandro F.
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This thesis summarizes the development, preparation and characterization of novel noble metal electrocatalysts using the improved dispersion, conductivity, and activity provided by three carbon supports, boron doped diamond nanoparticles (BDDnp), Vulcan and rGOx. After detailed experimental procedures presented in Chapter 2, Chapter 3 depicts the doping of diamond nanoparticles ranged between 4 and 13 nm with substitutional boron atoms which increased their conductivity and decreased their capacitance. The fabrication of BDDnp was shown to be successful and by using sol-gel synthesis, Pt nanoparticles presented a change in morphology after the doping procedure. Chapter 4 presents the use of supercritical CO2, an easily scalable method, in the synthesis of Vulcan supported Pt and Pt0.8-Ir0.2 nanoparticles using three different reactor settings. Starting with Vulcan, a reliable material for testing new deposition methods, and metal precursors in dry powder form, we demonstrated the deposition of Pt and Pt0.8-Ir0.2 alloy nanoparticles without the need of dispersing these materials in a solvent. Testing the synthesized materials, two reactor settings were found to be the best ones to produce the most active electrocatalysts for the ammonia oxidation. In Chapter 5, rGOx was used as a support for Pt, Ir and Pt0.5-Ir0.5 alloy nanoparticle depositions. rGOx micrometers sized layers were used with a sol-gel method to deposit finely dispersed metal and alloy nanoparticles in two different atmospheres, air and dry N2, producing nanoparticles ranged between 2 and 30 nm, and 1 to 4 nm, respectively. After the material characterizations, the activity towards the ammonia oxidation was tested in a biocompatible pH with rGOx/Pt resulting the best electrocatalyst. Chapter 6 presents a comparison in a biocompatible pH between the sol-gel method used in previous chapters and a shape controlled method known for its capacity to deposit cubic Pt nanoparticles with (100) facets exposed. Micrometers sized layers of GOx were used as support, reduced during the shape controlled Pt deposition by bubbling H2 gas in aqueous solution. While well faceted Pt nanoparticles were synthesized with a mixture of different shapes, the current density for the oxidation of ammonia improved in about 40% compared to sol-gel synthesized rGOx/Pt electrocatalyst. Finally, after the analysis of rGOx supported electrocatalysts, layers with sizes greater than many micrometers were found to fold themselves easily covering many metal nanoparticles inside preventing them to be used in the reactions. Therefore, GOx sized up to 250 nm were synthesized by a different method with better dispersibility, longer times dispersed in polar solutions even in acid mediums, and higher number of nucleation sites. Using the rotating disk slurry electrode (RoDSE) technique, Pt nanoparticles/clusters/atoms were deposited on electrochemically reduced graphene oxide (erGOx) and a detailed material characterization was performed. Then, its activity for oxidizing ammonia was compared with commercially available Vulcan/Pt and a polycrystalline Pt disk, at high pH with the goal of using it for fuel cells and water treatments. This material showed a 5-fold increase and a 20% increase in current density compared to Vulcan/Pt and Pt disk, respectively.