(558ah) Dynamic Catalysis and Surface Resonance for Turnover Frequency Enhancement

Ardagh, M. A. - Presenter, University of Minnesota
Abdelrahman, O., University of Massachusetts Amherst
Zhang, Q., University of Minnesota
Dauenhauer, P., University of Minnesota
Heterogeneous catalysis is an essential part of chemical processes to manufacture consumer goods, power transportation, and ensure a high standard of living. The goals of basic catalysis research can be distilled down to two grand challenges: (i) increasing the reaction rate towards desired products and (ii) overcoming limitations due to thermodynamic equilibrium. For many reaction and catalyst classes, the reaction rate has been shown to exhibit a “volcano plot” behavior as a function of various catalyst descriptors. This volcano peak rate, or Sabatier maximum, has traditionally been the speed limit for catalysis. The existence of a maximum is due to the inherent trade-offs between the reaction rate of the surface reaction and product desorption for a catalyst with static binding properties. Here, we demonstrate computationally that the Sabatier maximum can be exceeded (by up to 10,000x) by intentionally varying catalyst binding characteristics with time [1].

A model system was studied of A → B where the mechanism consists of: (i) adsorption of A, (ii) surface reaction of A* → B*, and (iii) desorption of B, with the corresponding reverse steps. The binding energies of A and B were varied using square, sinusoidal, triangle, or saw-tooth waveform with a specified oscillation frequency (fosc , [=] Hz) and amplitude (ΔU, [=] eV). In a CSTR model, the dynamic steady state rate was shown to be a strong function of frequency and amplitude. For moderate amplitudes from 0.5-1.5 eV, the CSTR has a resonance frequency range between 103 -107 Hz where the steady state rate levels off at a maximum value far above the volcano peak (100x-10,000x). Higher oscillation amplitudes > 1.5 eV lead to higher steady state rates and shift the resonance frequency range to higher frequencies. Finally, with practical implementation in mind, various oscillation waveform were implemented with the same frequency and amplitude to judge their effectiveness in enhancing the reaction rate. Square waveform led to the highest rate enhancement, with sinusoidal waveform also exhibiting high performance.

[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