Kevin M. Metz, PhD , Albion College
In our first year of funding, our research efforts perused two similar avenues simultaneously. These were the creation and characterization of shaped palladium nanoparticles grown in situ on carbon substrates, and the creation and characterization of gold-palladium bimetallic nanoparticles grown in situ on carbon substrates. Both avenues used molecular monolayers to direct the synthesis of the nanoparticles on the surfaces.
In our second year, our focus shifted slightly due to difficulties in instrumental access. We moved away from molecular monolayer modified carbon substrates, and began using porous polycarbonate filtration membranes. This transition allowed for qualitative analysis of the synthesis of our nanoparticles, i.e., the membrane changed color when nanoparticles were fabricated, allowing us to transition to the study of catalytic properties while we waited to access to microscopes. As a result of this transition, we have now fully created an assay for the screening and characterization of catalytic behavior.
In the first two years, seven undergraduate research students have actively participated in this research during the academic year. Four of these students performed research over the summer. Their efforts have resulted in presentations at Wayne State University’s (Detroit, MI) Research Symposium in Chemistry, two presentations at the 239th National Meeting of the American Chemical Society (San Francisco, CA), three posters at the 241st National Meeting of the American Chemical Society (Anaheim, CA), and numerous presentations and posters at Albion College’s annual Elkin Honors Day Symposium.
A brief summary of accomplishments during the past year is included below.
Fabrication of Palladium Nanoparticles on Polymeric Membrane Supports
Polycarbonate membranes have been used as templates for nanomaterial synthesis for over two decades now. They are believed to have terminal carboxylic moieties that can act as chelating sites for metal ions. Leveraging electroless deposition processes that were developed for plating metals on plastics we have been able to fabricate metal nanoparticles on these membranes.
In this process, the surface is sensitized with tin ions, then activated with palladium ions. A galvanic reaction occurs between the tin and palladium ions resulting in atomic palladium nucleation sites on the surface. Following activation, the membrane is exposed to a palladium plating bath and palladium nanoparticles are grown from the nucleation sites.
In the past year we have utilized two palladium plating bathes. First we used a stabilized hypophosphite bath. We have also utilized a green bath that uses coffee as the reductant. The hypophosphate bath has some irreproducibility, but creates highly catalytic particles (vide infra) that are < 50 nm in diameter. The coffee reduced bath is stable, simple to use, and highly reproducibility. It produces particles that are in the 30-60 nm diameter range.
Catalytic Characterization of Palladium Nanoparticles
Palladium is used in a number of cross-coupling reactions, including the Suzuki, Heck, Sonogashira, Negishi, and Stille reactions. It can also be used as a catalyst in dechlorination reactions. In the past year we have explored the catalytic activity of our particles in Suzuki coupling reactions, and in the hydrogenation of trichloroethene (TCE). We had limited success with the TCE reactions due to instrumentation difficulties. However, we have had great success with the Suzuki reaction.
Our exploration of the Suzuki reaction created a collaborative project with Prof. Clifford E. Harris, an organic chemist in the Chemistry Department at Albion College. Dr. Harris, an expert in boron chemistry, was happy to work with my students to teach them the Suzuki reaction. Our initial studies were promising enough that Dr. Harris changed his sabbatical research plans during the spring 2011 semester to explore the use of our supported palladium nanoparticles in a wide array of cross coupling reactions. His results are currently being completed and will soon be submitted for publication.
While Dr. Harris explored the use of the supported palladium nanoparticles in a traditional organic reaction set-up, my students sought to take full advantage of the porous support and explored a flow-through reactor geometry. They are currently optimizing reaction conditions such as temperature, flow rate, and pore diameter. It is expected that publications will sought when the results are finalized.