Partial Pressure Modulation as a Method to Access Non-Equilibrium States in Heterogeneous Catalysis

Under steady-state conditions, heterogeneous catalyst surfaces are often represented by thermodynamically favoured surface configurations defined by temperature and pressure. However, catalyst surfaces are inherently dynamic and can populate non-equilibrium states that differ from those expected based on the reaction environment. Herein, we show how partial pressure modulation can be utilised to sustain access to non-equilibrium states. Using CO oxidation over Pd/Al 2 O 3 as a model system, we demonstrate how oscillating the partial pressures of CO and O 2 drives the catalyst surface into a non-equilibrium state, accessing CO* coverages and reactivities unattainable under steady-state conditions. We find that accessing this state enhances catalytic rate up to three times above the steady-state maximum at a given temperature and induces activity under conditions where no steady-state activity can be recorded. Numerical simulations reproduce similar rate enhancements, demonstrating that accessing non-equilibrium states via dynamic operation can enhance activity without invoking catalyst reconstruction or alternative kinetic pathways. We conclude that the rate enhancement is directly related to disrupting the thermodynamically expected relationship between the gas and surface concentrations. More broadly, these findings suggest that dynamic reactor operation provides a powerful strategy for accessing non-equilibrium states and revealing otherwise inaccessible catalytic behaviour.