(506g) Transcending the Volcano Plot: Enhancing NH3 Synthesis with Dynamic Catalysis

Natural and artificial nitrogen fixation to NH₃ is essential to life, agriculture, and the manufacture of fertilizer. The development of the Haber-Bosch process for metal-catalyzed NH₃ synthesis was a key step forward to increasing agricultural yields and feeding a growing population. However, several economic and scientific challenges in improving NH₃ synthesis remain, and these derive from the trade-off between reaction rate and thermodynamic equilibrium. At low temperature, equilibrium favors NH₃ production, while few catalysts have an observable rate under these conditions. Raising the temperature lowers equilibrium NH₃ production; however, most catalysts need to operate at > 400 ^oC where NH₃ is highly disfavored.

Above 400 ^oC, NH₃ synthesis rates exhibit a volcano plot relationship with the binding energy of N* as a descriptor. The NH₃ synthesis speed limit exists at the volcano peak, which has been attributed to the trade-off between the rate of N₂ adsorption and N₂ dissociation for a catalyst with static binding properties. In this talk, we demonstrate that these kinetic and thermodynamic constraints can be surpassed by varying catalyst binding properties as a function of time [1]. Using CSTR and batch reactor models in Matlab, a model system with A → B and three elementary steps: (i) adsorption of A, (ii) reaction of A*→ B*, and (iii) desorption of B was simulated. Binding energies were varied using square, sinusoidal, triangle, or saw-tooth waveform with specified oscillation frequencies (f_osc , [=] Hz) and amplitudes (ΔU, [=] eV). Dynamic steady state rates were found to be highly dependent on the frequency and amplitude. Rate enhancement over the Sabatier maximum was observed for a system with Brønsted-Evans-Polanyi parameters comparable to NH₃ synthesis.

[1] M. A. Ardagh, O. A. Abdelrahman, P. J. Dauenhauer, “Principles of Dynamic Heterogeneous Catalysis: Surface Resonance and Turnover Frequency Response” ChemRxiv Preprint, 2019. doi.org/10.26434/chemrxiv.7790009.v1