Research Interests

Alkanechalcognenol Protected Metal Nanoparticles
Polyoxometalate Surface Chemistry
Fuel Cell Electrochemical & heterogeneous catalysis
Methodology Development

Fuel Cell Electrochemical & Heterogeneous Catalysis

    The objective of this project is to use a novel interdisciplinary approach, which draws diverse strengths from in situ surface electrochemical nuclear magnetic resonance (EC-NMR), surface-enhanced infrared spectroscopy (SEIRAS), surface-enhanced Raman spectroscopy (SERS), and interfacial electrochemistry, to investigate the physical and chemical properties of electrochemically-engineered nanoscale bimetallic surfaces in order to establish relationships between surface electronic/structural/dynamic properties and catalytic reactivity in these real-world bimetallic catalysts with many industrial applications. This project will provide unique insights into electronic structure –– reactivity relationships for real-world bimetallic catalysts and make significant contributions toward establishing a bridge between low-surface-area model and high-surface-area industrial catalysts, thereby furthering our understanding of surface science in general and bimetallic electrocatalysis in particular.
    As pointed out above, our approach draws strengths from different fields. First, the materials-engineering and environment-controlling power of interfacial electrochemistry offers a truly elegant way to prepare nanoscale bimetallic surfaces via electroless or underpotential deposition (UPD), and electrolysis or electro-dissolution. The relatively mild electrochemical approach in preparing bimetallic surfaces can rival the widely-used vapor deposition technique used in UHV and wet impregnation methods used industrially, both of which involve procedures that are rigorous and often technically demanding. Second, EC-NMR technique possesses technical versatility and chemical specificity for in-situ investigations in general and the analytical strength of high-field solid-state NMR in investigating real-world supported nanoscale (gas phase) metal and alloy catalysts in particular. Third, IR spectroscopy and Raman spectroscopy are a well-established major research techniques of the surface/interface science community and can provide quick access to important information regarding the bond strength and geometry of molecular probes such as CO or NO. A joint in situ SEIRAS/SERS/NMR approach will provide new, mutually complementary information on the electronic/molecular structure of the bimetallic surfaces by combining bond strength/geometry (IR/Raman) and quantitative electronic/dynamic (NMR) results. Finally, this electrochemically-based approach offers an extra bonus ability to control the metal surface potential, opening a new route by which processes of practically important, namely alkali promotion/halogen poison and the newly discovered electrochemical pumping promotion in heterogeneous catalysis, may be investigated in detail. In addition, this interdisciplinary project will play an important role in graduate and undergraduate education by providing unique opportunities for training in materials-engineering, interfacial electrochemistry, condensed matter chemistry and physics, heterogeneous and electrocatalysis, solid-state NMR, IR/Raman spectroscopy, and quantum chemistry, all directed towards the improved engineering of novel bimetallic systems.
    Specifically, the following questions will be the main focuses of this project:
    Another project that we are interested in is the peculiar catalytic reactivity of oxide-supported Au nanoparticles, in particular CO oxidation. Gold metal is probably the most noble material that us inert to chemical reactions. However, nanoscale Au particles supported on TiO2 or some other oxides show unusual high catalytic reactivity for CO oxidation, even at low temperatures. The exact mechanism is still subject to animated debate. We are planning to use 13CO as a molecualr probe to carry out 13C NMR and CO IR investigation. We believe such investigation will provide  much  needed mechanistic information for a better understanding of the system.

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Alkanechalcogenol-Protected Metal Nanparticles and Ramifications in Molecular Electronics

    A metal QD is an isolated nanoparticle containing hundreds or thousands of metal atoms which forms a small enough geometric 3-dimension confinement of electrons leading to resolvable discrete electronic energy levels, as opposed to the quasi-continuum of band structures in its bulk counterpart. Such metal QDs are the fundamental building blocks for many nanostructured materials expected to show unprecedented physical and chemical properties which are inaccessible using existing materials. This project is directed toward investigation of local electronic/structural properties of ligand-protected metal quantum dots (QDs) as a function of QD size, number of excess electrons that the QD carries (i.e., electric potential), and inter-QD spacing in metal QD superlattices. This project will provide critical data for understanding the physics and chemistry of these systems, in particular the  bonding interactions between the metal surface and the ligand. Such an understanding is crucial for the ultimate rational design of novel nanostructured materials which will be the basis for many future technological applications, for instance, molecular electronics.
    Currently, we are focusing on octanethiol- and octaneselenol-protected metal nanoclusters and using 13C1 of octane carbon chain and 77Se as proximal NMR probe to invesigate the physical and chemical properties of underlying metal particles. Simultaneously, we are also using 195Pt and 109Ag NMR to investigate directly the metal nanoparticels themselves. The following are some specific targets:
    Alkaneselenol-Protected Metal QDs. Selenol offers numerous NMR advantages over thiol in that 77Se is a much better NMR nucleus than 33S and offers potentially a valuable NMR probe that is most proximal to the underlying metal surfaces. Since no commercial alkaneselenols is available, we started synthesizing dioctyl-diselenide via selenolbenzamide. Our current yield is 20%. Although this represents good progress from a yield of 8% from our first tries, it is still far lower than the 67% range reported in the literature. So, our first step is to continue to improve the yield of dioctyl-diselenide. Then we will synthesize octaneselenol-protected metal QDs by following the published method and explore the 77Se NMR by following the footsteps elaborated for the octanethiol-protected metal QDs. This direction will be of high payoff because 77Se has the real potential to become a major local probe, as 13CO for nanoscale heterogeneous catalysts, for investigating chemical/physical properties of ligand-protected metal QDs

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Polyoxometalate Surface Chemistry

    Polyoxometalates (POMs) are discrete, nanoscale (0.6 -2.5 nm) molecular oxygen-metal clusters containing early transition metal cations M (=V, Nb, Ta, Mo, or W) in an oxygen-coordinated octahedra, MO6. By sharing edges and corners, these octahedra usually form a highly symmetrical structure of general formula XmMxOyn-, where X (=B, Si, Ge, P(V), As(V), and some other elements) are the so-called heteroatoms. POMs adsorbed on metal surfaces have many promising technological applications. These include, among others, new heterogeneous catalysts for industrial oxidation of hydrocarbons, new electrocatalysts for hydrogen production and oxygen reduction in fuel cell applications, new electron transfer mediators for chemical sensors, and new corrosion inhibitors for replacing the still widely-used yet toxic chromate inhibitors. However, POM surface chemistry is poorly understood. This project is directed toward the heart of POM surface chemistry. the chemsorption of POMs on electrode surfaces. Electrochemical NMR and infrared spectroscopies will be developed to investigate metal-POM bonding interaction as a function of local chemical environment and of electrode potential. The potential societal impacts of this project are numerous.
   The unique primary structures of POM result in many "value-adding" properties advantageous to processing that distinguish POMs from metallic oxides and conventional compounds. These include diverse molecular composition and aesthetically appealing geometric shape, redox reversibility and tunable redox potentials, strong acidity, high solubility, and good structural and thermal stability. Thus, POM chemistry is of fundamental importance for its current and future practical applications that spread over diverse fields such as catalysis, chemical/biological sensing, corrosion inhibition, geochemistry, and environmental chemistry as well as clinical chemistry. Among many current focuses of POM chemistry, POM-metal surface bonding is of particular interest because it lies at the heart of POM surface chemistry–a relatively young yet very technologically relevant subfield of POM chemistry that encompasses using POMs as building units for the rational design of novel surface inorganic materials for new and less expensive electro/heterogeneous catalyts, chemical sensors, and corrosion protection.
   Despite all these very promising technologically-related advances, the fundamentals of the POM surface chemistry are still far from well-understood11, in particular, POM-metal bonding, a key surface chemical property that governs the formation of strongly adsorbed adlayers on metal (electrode) surfaces. Few detailed mechanistic investigations have been published thus far. Answers to many questions are still very evasive. For instance, how does the POM bind to the metal surface? how does the POM-metal bonding depend on the type of underlying metal and that of POM? how are the chemical properties of POMs modified through POM-metal bonding while their overall primary structure is retained? and what are the key factors that make adsorbed POMs more active and that influence the stability of the POM-metal bonding? just to name a few. In addition, few modern molecular/atomic level characterization techniques have been devised so far to study general POM surface chemistry in general and POM-metal bonding in particular.
    Mindful of the current status of the POM surface chemistry and challenges and opportunities so presented, we are combining electrochemistry and nuclear magnetic resonance (NMR) approach to investigate specifically the POM-metal bonding by using POM-protected metal nanoparticle systems. Our approach embodies at least two overall novelties. One is to use electrochemistry to precisely control and monitor the reduction state of POM anions which will be used as reductant as well as protecting ligand for synthesizing POM-protected metal nanoparticles. The other is the application of NMR to the POM surface chemistry under active electrochemical control of POM-protected metal nanoparticle systems that are also an intriguing 3D analogous of 2D POM SAMs on single crystal surfaces. NMR is an attractive technique that possesses many virtues including local structural, electronic, and motional sensitivity, unmatchable chemical specificity and the ability to see the buried interfaces. Also, the intrinsic high surface/interface area of POM-metal nanoparticle systems largely offsets the technical burden imposed by a low mass detection limit of NMR. Thus, not only will our investigation of metal-POM interaction provide much needed information in a timely fashion regarding a fundamental yet poorly understood aspect11 of the surface chemistry of POMs, i.e., the chemisorption of POMs7,12 on metal surfaces, but also bring the powerful NMR spectroscopy into play, thus enhancing significantly the arsenal of modern molecular/atomic level characterization techniques in the field of POM surface chemistry.

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Methodolody Development

    Enhancing mass detection sensitivity is a constant challenge for applying solid-state NMR to surface, interface, and nanoscience. New venues, such as low-temperature preamplifier system, microcoil and toroidal detection, polarization transfer, as well as coupling NMR with other sensitive techniques, will be explored along the progress of above research projects.
    We are also developing new IR spectroelectrochemistry method to expand our capability to investigate dynamic aspect of direct methanol oxidation on Pt based electrocatalyst surfaces.

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