Isolated MnV-N-MnIII species.

Energy Storage: Probing Ammonia Oxidation

Fossil fuels have specific energies ranging from 30-55 MJ/kg making them ideal for the transportation sector. However, the combustion of these and resulting unmitigated CO2 emissions are exacerbating current climate change. Transitioning to clean, renewable energy will require additional energy storage capability, due to the fluctuating nature of these sources. Hydrogen has a high specific energy (143 MJ/kg) and may be used as an energy storage medium when generated photochemically (water splitting), or by coupling to a renewable energy source (water electrolysis). H2 can be electrochemically or chemically (Haber-Bosch) stored in N2 to generate ammonia (NH3). Central to using NH3 as a fuel – either for release of energy rich H2, or for direct use in fuel cells – is understanding the multi-electron oxidation process.

 

Our group is currently investigating the mechanism of oxidation using homogeneous first-row metal catalysts as molecular probes. Possible pathways are shown to generate N2 from NH3. Known ligand platforms, such as salens and phthalocyanines, with first-row metals are used to probe NH3 oxidation, either through stepwise H+/e- elimination, or through H-atom abstraction routes to generate H2. In several of our systems, metal-nitrides are formed and can subsequently couple to generate N2. To understand these mechanisms, we use a suite of spectroscopic, magnetometric, and crystallographic techniques to probe and potentially isolate important or interesting intermediates, such as the mixed valent MnV-N-MnIII species shown. We also use gas chromatographic techniques to measure evolved N2, with the goal of working towards catalytic NH3 oxidation. Please see our preliminary published results for more on this project.

 

Redox-Flow Batteries for Energy Storage

Redox flow batteries (RFBs) are widely applicable energy storage devices which have modular design, fast response times, and are easily scalable to meet the demands from stationary renewable energy sources. RFBs contain two electrolyte tanks (catholyte and anolyte) containing redox-active molecules capable of accepting or delivering multiple electron equivalents through a membrane. The energy density of RFBs (Ê) is dependent on the concentration (Cactive) and number of electrons transferred (n) per redox active molecule, as well as the potential window of the solvent used (Vcell). While some RFBs have been commercialized, most suffer from low energy density limiting their applicability.

We are investigating new, non-aqueous soluble molecular frameworks capable of cycling through multiple oxidation states in a fully reversible fashion, and with wide potential windows. These molecules, produced from abundant starting materials, target many of the current deficiencies in RFB applications.

 

 

 

Redox-Active Main-Group Reactivity

Most p-block main group compounds are closed-shell, diamagnetic, and generally redox inactive, serving as ligands, co-catalysts, or solid-state supports. With the exceptions of open-shell radical species (ex. boryls, phosphinyls, etc.), single electron processes tend to be rare. However, some transiently generated main-group multiple bonds bearing O-centered radical behavior (ex. P=O·) have shown remarkable reactivity, for instance in the room temperature C–H activation of methane. Recent theoretical studies by Goddard have also shown similar C–H chemistry emanating from the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst for butane oxidation to maleic anhydride.

Due to the transient nature of many reported main-group radical species, we are currently investigating strategies to stabilize these intermediates through redox delocalization to pendent metal electron reservoirs. Controlled C–H bond reactivity at redox-active main group multiple bonds is targeted. Expanding to multi-metallic frameworks may provide access to multi-electron transformations of relevant small molecules (CO2, H2O, NH3) at a main group center. We have synthesized and are currently characterizing the reactivity of several of these species.

 

 

 

The Ménard Group is based in the Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510

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