Determining the State of a Pd/γ-Al₂O₃ Catalyst Using Pulsed Flow and Transient Spectroscopy

A precise understanding of heterogeneous catalyst structure and activity is required to design more efficient, greener, industrial chemical processes. While it is possible to resolve both the structure and activity of a catalyst, their innate complexity means it is nontrivial to unambiguously assign one to the other. Herein, we demonstrate how transient measurements, coupled with kinetic modeling, can be used to resolve kinetics with sufficient precision such that it is possible to resolve the identity of an "active site" over technical catalysts. We find that under CO rich conditions the transient activity and coverage dependencies measured over a Pd/γ-Al2O3 catalyst during pulsed flow and transient spectroscopy CO oxidation experiments can be quantitatively predicted using a kinetic model derived from ultrahigh vacuum surface science experiments over Pd(111) single crystals. Thus, we conclude that the state (i.e., structure and composition of the active site) of the Pd/γ-Al2O3 catalyst under reaction conditions is equivalent to that of a Pd(111) surface. Instead of determining a catalyst's structure and assigning it to activity, we instead propose a new approach: using precise kinetic measurements to infer the catalyst's state. This approach enables the identification of active sites that have been challenging to resolve using traditional spectroscopic or microscopic methods, offering a powerful and complementary tool for designing new catalysts.