Collaborative Research: Detailed Mechanistic Pathways of Surface Catalysis using SERS Spectroscopy: A Joint Theoretical and Experimental Synergistic Approach

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With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, Professors Stephen Cronin at the University of Southern California (USC) and Lasse Jensen at Pennsylvania State University (PSU) will investigate the physical mechanisms underlying a measurement technique called surface-enhanced Raman scattering (SERS). This work closes the loop between theory and experiment, providing a detailed quantum mechanical picture of the molecules adsorbed on metal surfaces during catalytic reactions. The project uses a general SERS-based approach to study reaction mechanisms associated with surface catalysis, and also toward the goal of imaging chemical reactivity of catalytic surfaces with sub-micron spatial resolution. The information provided by experimental SERS spectra is somewhat limited without the aid of supporting theoretical work to provide atomistic insight on the underlying processes. Therefore, it is crucial to carry out theoretical calculations in parallel with the experiments in order to obtain a detailed picture of the molecules and their reactions on surfaces, including the charge transfer between a molecule and the metal surface, to distinguish between physisorption and adsorption, to establish key catalytically active sites, and to identify surface-bound intermediate species and the relative importance of other key aspects of catalytic reactions. In this collaborative project, the experimental research group led by Professor Stephen Cronin (USC) will record the SERS spectra of electrode surfaces under electrochemical working conditions using a water immersion lens, and the theoretical research group led by Professor Lasse Jensen (PSU) will use time-dependent density functional theory (TD-DFT) and related methods to simulate adsorbed chemical species on metal surfaces. The direct correspondence between the theoretical calculations and experimental measurements in this effort promises to provide a robust test of the basic hypotheses underlying charge flow between the adsorbate and metal surface. The team is testing their hypothesis that dynamic charge flow between a molecule and the metal surface plays an important role in the SERS enhancement process and can be used to extract important chemical information from the mode-dependent SERS enhancement factors. The researchers are testing this hypothesis by correlating product formation with in situ SERS spectra that they record under electrochemical working conditions. They will also examine a hypothesis that the SERS enhancement is either correlated or anti-correlated with catalytic activity. This hypothesis stems from the idea that both the enhancement and the catalytic activity depend on the degree to which a given reactant or intermediate is bound to the metal surface, which is ultimately based on the amount of charge transfer that occurs at the interface. Additionally, the team plans to use the relative SERS enhancement of different vibrational modes to image reactivity and/or reactive sites based on the information they learn about the relationship between dynamic charge flow and SERS activity.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.