Browsing by Author "Li, Hao"

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    Design of a Pd–Au Nitrite Reduction Catalyst by Identifying and Optimizing Active Ensembles
    (American Chemical Society, 2019) Li, Hao; Guo, Sujin; Shin, Kihyun; Wong, Michael S.; Henkelman, Graeme
    Nitrate (NO3–) is a ubiquitous contaminant in groundwater that causes serious public health issues around the world. Though various strategies are able to reduce NO3– to nitrite (NO2–), a rational catalyst design strategy for NO2– removal has not been found, in part because of the complicated reaction network of nitrate chemistry. In this study, we show, through catalytic modeling with density functional theory (DFT) calculations, that the performance of mono- and bimetallic surfaces for nitrite reduction can be rapidly screened using N, N2, and NH3 binding energies as reactivity descriptors. With a number of active surface atomic ensembles identified for nitrite reduction, we have designed a series of “metal-on-metal” bimetallics with optimized surface reactivity and a maximum number of active sites. Choosing Pd-on-Au nanoparticles (NPs) as candidate catalysts, both theory and experiment find that a thin monolayer of Pd-on-Au NPs (size: ∼4 nm) leads to high nitrite reduction performance, outperforming pure Pd NPs and the other Pd surface compositions considered. Experiments show that this thin layer of Pd-on-Au has a relatively high selectivity for N2 formation, compared to pure Pd NPs. More importantly, our study shows that a simple model, based upon DFT-calculated thermodynamic energies, can facilitate catalysts design relevant to environmental issues.
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    Spectroscopic imaging of surfaces—Sum frequency generation microscopy (SFGM) combined with compressive sensing (CS) technique
    (American Institute of Physics, 2020) Li, Hao; Kelly, Kevin F.; Baldelli, Steven
    Surface chemistry is notoriously difficult to study, in part, due to the decreased number of molecules that contribute to the properties compared to the bulk phase but often has significant effects on the chemical activity of the material. This is especially true in topics such as corrosion, catalysis, wetting, and many others in nature and industry. Sum frequency generation (SFG) spectroscopy was developed for interface studies due to its high molecular selectivity and surface sensitivity, which is quite useful to study the effects of structural inhomogeneity in microscopy. Compressive sensing (CS) combined with SFG spectroscopy minimizes the imaging time while still producing quality images. Selected systems are presented here to demonstrate the capability of CS-SFG microscopy. CS-SFG microscopy successfully distinguished the static monolayer molecular mixtures, the orientations and adsorption of adsorbed molecules by the dip-coating technique, and the localized CO behaviors on polycrystalline Pt electrodes. Further discussion includes dynamic imaging as a future direction in CS-SFG microscopy. As materials and surfaces become more complex, imaging with chemical contrast becomes indispensable to understanding their performance and CS-SFG microscopy seems highly beneficial in this respect.