The impact of catalysis on the economy and technology of industrialized countries is impossible to overstate. It is surprising that our current knowledge of catalyst design that leads to a catalyst capable of desired product formation with minimal environmental impact is often quite rudimentary. Heterogeneous catalysis is extremely complex and the dynamics at the catalyst surface sites during catalytic turnover influences the kinetic outcome of the reaction. Studying nanoparticle catalysts a single particle at a time with single turnover resolution will improve our molecular level understanding of the origins of kinetic dispersion, and will provide insights on how to utilize this information to design catalysts with optimized activity and selectivity. Robert M. Rioux of The Pennsylvania State University will provide this level of study under a NSF Faculty Early Career Development (CAREER) Program Award. Rioux analogizes the situation to that in biological systems. Single molecule measurements have revolutionized how biologists think about structure and function since it was revealed structure is dynamic rather static with changes in structure occurring while functioning. In heterogeneous catalyst systems, experimental studies have also provided direct evidence that surface atoms responsible for catalytic turnover are dynamic, rather than static. However, measured reactivity is due to an ensemble of surface atoms, and structural dynamics have not been coupled with reactivity measurements. Understanding how dynamic structural changes (fluxionality) couple to function is critical to the design of next generation catalysts since this is direct insight into the catalytic entity responsible for catalytic turnover.
Rioux will investigate structure-function relationships utilizing single nanoparticle methods with single turnover resolution coupled with characterization of the catalytic solid-liquid interface with novel calorimetric methods and chemical titration. Experimental studies utilizing pro-fluorescent molecules which convert to fluorophores will be utilized to examine the relationship between dynamic structural changes ? whether these changes are associated with the nanoparticle itself or the primary solvation layer -- and single molecule turnover trajectories. Catalyst variables such as particle size and their subsequent modification with adsorbates of varying affinity and chemical character will be examined. The rate of reaction is influenced drastically by temperature and the influence of temperature-dependent fluxionality on the catalytic processes will be evaluated. Distribution of kinetic and thermodynamic parameters associated with proposed rate expressions will be assessed at the single molecule level and compared with ensemble equivalents. This work will additionally examine the spatial dependence of simultaneous turnover on activity and selectivity using a non-fluorescent reactant that produces two products with different emission characteristics. The relationship between reaction selectivity and location on the catalytic nanoparticle will be assessed with correlative microscopy.
Broader Impacts
The results of the proposed research will provide the catalysis/nanoparticle community with unambiguous structure-function relationships regarding the kinetics and dynamics of catalytic turnover. The fundamental insight into catalytic processes gained from single nanoparticle measurements should enable more efficient catalyst design by providing a dynamic, rather than static picture of the influence of structure on function.
Educational activities supported by this NSF CAREER proposal focus on the development of a first year seminar (FYS) to attract and retain particularly undergraduate female students in the chemical engineering major. The newly-developed FYS will integrate the PI?s current involvement in AIChE ChemE car project and provide a ?hands-on? mentored experience for freshmen/sophomore female students. The FYS will include a component of outreach to the commonwealth campuses of Penn. State, where female (and male) students do not have the opportunity to participate in a chemical engineering themed FYS. The PI along with his graduate students will travel to the commonwealth campuses for on-site, ?hands-on? demonstrations.
