Research

 

Novel Materials as Catalysts for a Better World

The unifying theme of the research projects in our laboratory is CATALYSIS, more specifically heterogeneous catalysis (HC).

Catalysts are needed to meet many challenges we face such as creating alternative fuels, reducing toxic chemicals, remediating and preventing pollution, and producing safe pharmaceuticals, however the diversity and complexity of these catalysts and the mechanism by which they operate necessitate a thorough control of how they should be designed, synthesized, and evaluated

The most important challenge in catalysis is to understand how to design catalyst structures to control catalytic activity and selectivity. Success will be achieved only when the materials chemist working hands in hands with computational chemists and physicist can harness the methods of preparing molecular building blocks and assembling them into complex systems with predictable functions.

Four projects make up the backbone of our research initiatives. Each project is interdisciplinary in its nature and involves collaborators at other institutions and other countries. Each project involves a postdoc, and graduate and/or undergraduate students.

 

(1) Molecular Engineering of Hybrid Materials

(2) Materials for Energy Conversion and Storage

(3) Solid Acid Catalysts for Biofuel Production

(4) Materials for Selective Gas Adsorption

 

Engineering of Hybrid Materials for Catalysis

The main objective of this project is the systematic study of the synthesis and fundamental properties of crystalline hybrid inorganic/organic materials using the molecular building block approach. Incorporating metal clusters as building blocks of well structured materials leads to materials with the specific functions traditionally associated with metals, such as redox, magnetic, catalytic, and optical activity. The long-term goal is to reach an understanding of the relationships between the structure, bonding, and properties of these materials through iterative process between explorative synthesis and computational chemistry. The building blocks the group uses are nanosize octahedral metal clusters that are functionalized and assembled to form nanoscale spheres, sheets, rods, tubes, and three-dimensional frameworks glued together by hydrogen bonding, electrostatic interaction, or coordinative bonds.  An example of chemical reactions that illustrates this approach is given bellow:

    [Me4N]4[Nb6Cl12(CN)6] + 2[Mn(L)]+ +   MeOH solvent → → →          [Me4N]2{(Mn(L))2(Nb6Cl12(CN)6)} + 2(Me4N)+ 


The reaction leads to self assembly of a 2D material that has shown remarkable crystal-to-crystal ion exchange properties.

 

 

 

Materials for Energy Conversion and Storage

These projects are conducted in collaboration with Dr. Natalaie Holtzwarth and Dr. Richard Williams from the Physics Department, and Dr. Dave Carroll from the Center for Nanotechnology and Molecular Materials. The work is sponsored by the Wake Forest University Center for Energy, Environment, and Sustainability. In this work, we are primarily interested in two types of energy related materials that can have an impact on alternative energy field.

(A) Materials for Photocatalysis  

In this process, solar photon energy is catalytically converted into chemical energy by cleaving H2O. Solar energy is stored in molecular H2 and O2 which are the product of water splitting. To realize this energy conversion, it is important to develop effective photocatalysts. These catalysts must have band gaps that allow the material to harvest solar energies located in visible region (45%) of solar spectrum. An effective photocatalyst should possess properties such as sufficiently narrow bandgap, resistant to photocorrosion, high quantum efficiency, suitable thermodynamic potential, and simplicity. A simple schematic band diagram, which involves co-catalysts, is shown in Fig. 1 to depict the photocatalytic process.

 

Fig. 1. Schematic bandgap diagram that depicts water splitting photocatalysis mechanism

 

In our laboratory, we are currently engaged in the synthesis and development of metaloxynitride based materials as effective photocatalysts. In this work we investigate the preparation of metaloxynitride photocatalysts by different synthesis routes, and study the effect of introducing different types of co-catalysts on their activities.

 

(B) Solid Electrolytes for Li-ion Batteries

In a Li-ion battery, anode (typically graphite) and cathode (Li containing materials e.g LiCoO2, LiFePO4) is separated by an electrolyte, which can be a liquid, gel, or a solid-state material. The electrolyte conducts ions but acts as an insulator to electrons. In a charged cell, Li ions are concentrated at the anode. However, during the discharge process, lithium ions travel to the cathode across the electrolyte producing energy (Fig. 2). Lithium phosphorous oxynitride (LiPON) solid electrolytes have received much attention as solid state battery electrolyte owing to their high electrical conductivity and ability to tailor the electrical conductivity by controlling the N content. The structure of Li2PO2N has been predicted by carrying out first-principle simulations (Holtzwarth). In our laboratory, we are focused on the synthesis of the predicted Li2PO2N structure, primarily by solid state reaction, and understand its physicochemical properties.

Fig. 2. Simplified schematic diagram of a Lithium ion electrochemical cell. Adapted from www.snupeel.com

 

Fig. 3.  Calculated ball and stick model of Li2PO2N structure. Ref. Du, Y.; Holzwarth, N. A. W. Phys. Rev. B, 2010, 81, 184106