Single-Molecule Investigation of Nanocatalysis

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0851257ChenThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Intellectual MeritNanoparticles catalyze many chemical transformations in organic synthesis, pollutant removal, and energy production; intense efforts are thus made to characterize the structure and understand the catalytic properties of nanoparticle catalysts. While structures of individual nanoparticles can be studied down to atomic resolution with advanced transmission electron microscopy and scanning probe microscopies, their catalytic properties have been mainly measured at the ensemble level, obtaining averaged properties. The catalytic properties of nanoparticles are intrinsically heterogeneous, however, due to their structural dispersions, heterogeneous distribution of surface sites, and surface restructuring dynamics. To address this heterogeneity challenge and to remove ensemble averaging in measurements, the PI has pioneered in studying nanoparticle catalysis at the single-particle and single-turnover resolution, using single-molecule fluorescence microscopy of fluorogenic reactions. Focusing on the redox catalysis by colloidal Au-nanoparticles, the PI's preliminary studies show it is possible to follow in real time in solution the catalytic turnovers of single Au-nanoparticles at single-turnover resolution. We found that for the catalytic product formation reaction, all Au-nanoparticles follow a Langmuir-Hinshelwood mechanism but with heterogeneous reactivity; for the product dissociation reaction, three nanoparticle subpopulations are present that show heterogeneous reactivity with distinct kinetics between two parallel product dissociation pathways. Moreover, individual Au-nanoparticles show large temporal activity fluctuations, attributable to catalysis-induced and spontaneous surface restructuring dynamics that occur with distinct timescales at the surface catalytic and product docking sites.Building on the achievements, the PI proposes to use the single-particle single-turnover approach to:1) probe the nature of the heterogeneous reactivity and surface restructuring dynamics of Au nanoparticles,2) determine the size dependence of single Au-nanoparticle catalysis, and 3) determine the shape dependence of single Au-nanoparticle catalysis. The results to be obtained will reveal the structural origins of their heterogeneous reactivity, unravel the nature of their surface restructuring dynamics, and gain insight into their structure-activity correlations at the single-particle level, much of which are beyond the reach of conventional ensemble measurements.Broader impactThe single-molecule nanocatalysis research described in this proposal opens new directions in both heterogeneous/nano-catalysis and single-molecule research. The single-particle methodology is also generalizable to study other nanoscale catalysts if a suitable fluorogenic probe reaction is designed. The knowledge to be gained will help understand the fundamental principles governing catalytic activities of nanoparticles, which will assist the efforts in improving current nanoscale catalysts and designing new ones for heterogeneous catalysis. Metal nanoparticles are widely used as catalysts for chemical transformations and in particular for energy conversion in fuel and solar cells. The fundamental knowledge from these studies will help understand the activity of nanoparticles used in the chemical industry and in fuel and solar cells, which will help in identifying possible means to improve them, thus benefiting the society as a whole.The PI's education efforts integrate his single-molecule research activities to promote interdisciplinary teaching and training in both undergraduate and graduate classes. The PI actively broadens the participation of women and minority students (both undergraduate and graduate) in research and offer training to undergraduate students, including those from four-year colleges. The scientific expertise and technical instrumentation of the PI's lab add strength to the research infrastructure of local community. The PI's outreach activities will advance the public understanding and support of frontier single-molecule research and promote science education in K-12 students.