(5e) Tuning Catalytic Activity with Forced Dynamic Operation

While world-scale chemical plants are restricted to steady-state operating conditions, the emergence of smaller, modular reactor concepts opens new opportunities to operate chemical reactors with externally imposed dynamics. The potential of such dynamic operation has been reported by Dauenhauer et al., showing that even for the simplest A → B reaction a periodic change of binding properties can amplify the reaction rate beyond the Sabatier limit.¹ Alternatively, one can impose dynamics by varying the reaction conditions, such as the partial pressure. The benefits of this approach were reported for methane oxidation, showing a 85°C light off temperature reduction for modulated feeds compared to time invariant feed.²

Both cited examples explored a range of frequencies and amplitudes to improve activity, but for more complicated systems governed by multiple time constants, a guided optimization approach is needed. To this end, we used a kinetic Monte Carlo (kMC) model for CO oxidation on Pd (111), which considers two different active sites, surface diffusion, and many-body lateral interactions between all surface species.³ The interaction terms modify binding energies and activation barriers depending on local coverages, resulting in highly complex transient kinetic behavior. We analyzed the transient catalytic responses of this kMC model to changes in partial pressure at constant temperature and observed a strong dependence of time constants on surface coverage (Figure 1). In lieu of an analytical solution, we have described a heuristic approach to systematically improve the time-averaged conversion by varying partial pressures. This study forms the basis for the development of a rigorous approach to find optimal dynamic operating conditions for arbitrarily complex reaction systems.

(1) Ardagh, M. A. et al., ACS Catal. 2019, 9 (8), 6929–6937.

(2) Kang, S. B. et al., Catal. Today 2021, 360, 284–293.

(3) Piccinin, S. et al., Top. Catal. 2017, 60, 141–151.