One of the major challenges in chemistry today is to find more sustainable ways to activate the particularly inert N-N triple bond. In the N2 reduction subgroup at SUNCAT, our theoretical and experimental efforts to this end can be grouped into three main thrusts: electrochemical N2 reduction, thermochemical NH3 synthesis, and a hybrid approach that blends both electrochemical and thermochemical elements together in one process.
The activity and selectivity for the electrochemical reduction of N2 to NH3 (NRR) is limited by linear scaling relationships between the energetics of metal-N bond breakage and metal-N bond formation steps on transition metal catalysts. In particular, these scaling relationships result in the requirement of substantially larger overpotentials for NRR than for the competing hydrogen evolution reaction (HER), leading to extremely poor selectivity to NH3. Our theoretical efforts in this area are centered around the development of a quantitative microkinetic model that describes this selectivity challenge, based on density functional theory calculations of adsorption and transition state energies on close-packed transition metal surfaces in the presence of an explicit solvent layer. Results so far indicate that one promising way to increase NRR selectivity is to engineer the electrolyte rather than the catalyst. By reducing access to protons and/or electrons at the electrode surface, the HER rate can be suppressed dramatically while the NRR rate remains relatively unchanged. Experimentally, we are exploring various non-aqueous systems where the concentration of protons at the interface is drastically smaller than in traditional aqueous systems. Similarly, we are also trying to suppress HER by controlling the rate of electron transfer through a tunneling barrier grown using atomic layer deposition.
In thermochemical NH3 synthesis, we are seeking to circumvent two robust scaling relationships that limit the activity of state-of-the-art transition metal nanoparticle catalysts: (1) unfavorable EN-N vs. EN scaling, which results in poor activity under ambient conditions, leading to the requirement of high temperature and pressure for the industrial Haber-Bosch process, and (2) unfavorable EO vs. EN scaling, which causes industrial catalysts to be poisoned by oxygen, especially at lower temperatures. Theoretically, we have discovered that EN-N vs. EN scaling can be improved by the design of active sites that force the binding of N atoms onto relatively under-coordinated metal sites, destabilizing the final state relative to the transition-state, and that the inconvenient EO vs. EN scaling can be addressed by drastically reducing the O2/H2O content in the inlet gas mixture. Our experimental efforts in this area involve the identification and synthesis of active sites that explicitly force the under-coordinated dissociative adsorption of N atoms (interesting systems include dimers, trimers, and stripes of reactive metal atoms in unreactive lattices), as well as the use of an oxygen guard (such as Al or Na) that can help to limit oxygen poisoning.
Finally, our subgroup is exploring a cyclic hybrid approach to NH3 synthesis that exploits advantages of both the electrochemical and thermochemical processes described above. In this process, we avoid the high pressure traditionally needed to activate N2 by applying a strong reducing potential to generate a reactive metal surface that can easily break the N-N bond. The metal then forms a bulk nitride which releases NH3 spontaneously upon the addition of protons, completing the cycle. This approach explicitly circumvents the selectivity challenge that normally plagues the electrochemical process by physically (or temporally) separating the activation of N2 (formation of a nitride) from the addition of protons. Theoretically, we have identified lithium and the Group II metals as the most promising candidates for the cyclic process because they readily form a nitride upon exposure to N2 and release NH3 spontaneously when a proton source is added. Transition metal nitrides are not suitable for this process because they do not support facile diffusion of nitrogen or metal atoms through the lattice, making nitride formation difficult. We have realized this strategy experimentally at atmospheric pressure using a lithium-based molten salt electrolyte and are currently extending the process to the Group II metals.