2019 AIChE Annual Meeting

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

Authors

M. Alexander Ardagh - Presenter, University of Minnesota
Shizhong Liu, University of Delaware
Gerhard R. Wittreich, University of Delaware
Omar Abdelrahman, University of Massachusetts Amherst
Qi Zhang, University of Minnesota
Dionisios G. Vlachos, University of Delaware
Paul Dauenhauer, University of Minnesota
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 (fosc , [=] 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