Research in the Miller group revolves around transformations relevant to global energy concerns, including the storage of solar energy in chemical fuels, proton-coupled electron transfer reactions, and hydrocarbon transformations. Our approach starts with the design and synthesis of transition metal catalysts, then shifts to examining catalyst performance with a focus on understanding reaction mechanism in order to inform catalyst improvements. Our catalysts feature multifunctional ligands: beyond simply supporting the metal center, the ligands position additional functionality in the secondary coordination sphere of the metal and work in concert with the metal center to enhance key steps in catalytic cycles.
Collaborative efforts play a major role in the Miller group. The Alliance for Molecular PhotoElectrode Design (AMPED) Energy Frontier Research Center (EFRC) is headquartered at UNC and features an integrated approach towards understanding how materials interface with molecular chromophores and catalysts for solar fuels applications. A collaborative project targeting electrochemical dinitrogen fixation is supported by the NSF’s Innovations at the Nexus of Food, Energy, and Water Systems (INFEWS), with efforts in both chemistry and economics at UNC, Yale, and Rutgers.
Pincer-crown Ether Complexes for Cation-Controlled Catalysis
Chemists are constantly seeking new ways to control catalytic activity, selectivity, and longevity. We have developed a new family of pincer ligands that utilize an aza-crown ether as one of the donor arms to promote cation-controllable reactivity based on cation-macrocycle interactions occurring near the active site of a transition metal catalyst. The cation-macrocycle interaction can tune chemical reactivity in two distinct ways. First, the presence of a cationic additive can alter ligand binding to favor low coordinate, reactive intermediates. The strength of the host-guest interaction alters the equilibrium constant for substrate binding, such that cation-specific tunable reaction rates can be achieved. Second, the macrocycle is capable of positioning a Lewis acid in the secondary coordination sphere of the catalyst, offering a mechanistic pathway for catalyst acceleration through acid-promoted steps. The Miller lab is exploring small molecule activation chemistry that takes advantage of the unique pincer-crown ether ligand motif.
Hydride transfer is a critical step in many reactions, including energy-storing reactions to produce chemical fuels, such as hydrogen evolution and CO2 reduction. What if the hydride transfer step could be controlled using visible light? The ability to trigger a hydride transfer at will, or generate a more reactive species using visible light, could lead to dramatically improved catalysts. We are working to realize this goal by understanding the thermodynamic and kinetic factors associated with excited state hydride transfer to a range of substrates. A family of Cp*Ir-based catalysts has shown excited light-promoted hydride transfer reactivity, due to enhanced excited state “hydricity” or hydride donor ability. This has led to the discovery of a molecular photoelectrocatalyst for hydrogen evolution, which operates with no electrochemical overpotential in aqueous media up to pH 10. In contrast, the catalyst shows no activity under these conditions in the dark. Current work is focused on gaining a deeper mechanistic understanding of photohydrides in parallel with the development of new catalytic reductions.
The electrochemical oxidation of water to dioxygen requires substantial energy input, but releases the electrons and protons required to drive fuel-forming reactions for solar fuels applications. The thermodynamic and kinetic challenges associated with an unfavorable 4H+/4e− are not insurmountable, but pose an exciting electrocatalytic challenge. In our group, we have focused on developing catalysts supported by an oxidatively robust tripodal ligand framework. Our interests lie in defining new modes of catalysis and new mechanistic pathways for water oxidation that can lead to faster rates, lower overpotentials, and longer catalyst lifetimes. This work is carried out as part of the UNC EFRC: Center for Solar Fuels, a collaborative team working towards a modular device that stores solar energy in the form of chemical fuels.
The Miller group can be found in beautiful Kenan Laboratories in the Carolina Physical Sciences Complex at UNC Chapel Hill. These completely renovated labs feature three double-wide gloveboxes and vacuum lines for air-free synthesis, a solvent purification system, and various electrochemical and analytical instrumentation.