The objective of this research is to advance understanding of heterogeneous catalysis using experiments where catalytic metal nanoparticles are characterized one at a time in an electrochemical cell. Nanoparticles (NPs) are collections of matter that are between 1 and 1000 nanometers in size. They have received great attention because of their unique physical and chemical properties. Due to their high surface area and sizedependent reactivity, metal NPs have found applications in catalysis, sensors, and spectroscopy. We are primarily studying heterogeneous catalysis in the context of an electrochemical cell, where reactions are controlled in part by electricity. Heterogeneous catalysis, especially using metal catalysts, is critical for the development of fuel cells, batteries, and sensors. A common example of heterogeneous catalysis is found in catalytic converters in cars. Using transition metals such as rhodium, platinum, and palladium, catalytic converters catalyze the reaction of harmful carbon monoxide to form carbon dioxide and water. NPs are especially efficient in this application due to their high surface area.
The main experimental focus of this work is a technique called electrocatalytic amplification, where chemical reactions may be recorded at individual nanoparticles striking a larger area electrode. The strength of this measurement is that information is provided on a per-particle basis, rather than averaged over many particles as in a conventional measurement. In this project, platinum, rhodium, and iridium nanoparticles were first synthesized by the reduction of their chloride salts, under varying conditions. We have worked towards the synthesis of various transition metal nanoparticles in facile, aqueous, one-pot syntheses using citrate as a capping agent and sodium borohydride as the reducing agent. The synthesized particles were then characterized using transmission electron microscopy (TEM). Of these particle syntheses, iridium has proved the most challenging due to the comparatively low stability of citrate-capped iridium NPs in solution.
To date, our group has been able to synthesize rhodium, iridium, platinum, and palladium nanoparticles with diameters of less than 10 nm that were successfully used in electrocatalytic amplification detection experiments. In these experiments, the hydrazine oxidation reaction (a chemical reaction with relevance to fuel cells) was observed at individual nanoparticles of the different metals in an electrochemical cell. Individual NP/electrode collision events are recognized by spike or step shaped current transients in plots of the electrode current vs. time. The size of these transients provides information about the nanoparticle size and reactivity. In application, this technique could be used to assist in identifying which NPs in a sample are most effective at catalyzing a desired reaction.
The overall goal of this project is to extend the electrocatalytic amplification experiment to new nanoparticle types and new reactions, in particular systems relevant to the design of chemical sensors and energy devices. The bulk of literature on NP/electrode collisions has appeared in just the last 5 years, with the majority of reports limited to platinum nanoparticles. Extending this method to new materials will both advance fundamental understanding of catalysis and push the technique closer to practical applications in catalyst development.
*This scholar and faculty mentor have requested that only an abstract be published.