Pulse Response Operando Spectroscopy for Resolving Transient Dynamics in the CrO_x/Al₂O₃ Propane Dehydrogenation Catalyst

This work introduces an operando spectroscopy reactor based on the Temporal Analysis of Products (TAP) pulse response methodology. This “spectroTAP” reactor enables studies of complex catalyst processes by decoupling gas transport from kinetics under isothermal conditions, greatly simplifying the interpretation of both gas-phase and catalyst surface measurements. The spectroTAP is applied to a CrO_x/Al₂O₃ catalyst used in the dynamic CATOFIN process for propane dehydrogenation (PDH). Four distinct regimes during PDH are resolved by tracking the Cr oxidation state with UV-vis diffuse reflectance spectroscopy in tandem with gas-phase products and reactants. We find that oxygen not associated with the Cr oxidation state must be removed before propane can reduce Cr⁶⁺ present in the CrO_x lattice to Cr³⁺. After reduction, there is a transient period of rapid coking concomitant with the main PDH activity. This coking process diminishes much faster than the PDH activity, indicating that carbon deposited during this period does not block PDH active sites. Subsequent oxidative regeneration of the coked catalyst in spectroTAP resolves catalyst oxidation proceeding the carbon combustion. Regenerating the catalyst with large O₂ pulses into an inert flow reveals Cr⁶⁺ oxides, not gaseous O₂, as the species that directly react with surface carbon. These learnings gained from spectroTAP experiments provide a unique vantage point for the future design and operation of next-generation catalysts.