Revealing the Active State of a Cu/ZnO:Al Catalyst During Reverse Water-Gas Shift Reaction in an Operando Microwave Absorption Study.

The industrially important Cu/ZnO:Al (CZA) catalyst is known as a dynamic system adapting to reaction conditions, which renders the application of in situ and operando methods key to establish structure function correlations. Herein, a CZA catalyst close to the industrially used compostion was studied using noninvasive and bulk-sensitive in situ/operando microwave cavity perturbation technique and electron paramagnetic resonance spectroscopy during activation and reverse water gas shift reaction. The transient changes of catalytic activity track with the transients of the dielectric properties providing evidence for the importance of bulk properties for catalytic activity. Furthermore, convincing support for the redox reaction mechanism is obtained, and it is shown that H2 and CO2 uptake is not competing kinetically with each other. In addition, the reservoir of H2 and CO2 transiently present in the catalyst during catalysis is determined by the chemical potential of the respective reactant, which is directly coupled to the catalytic activity of the system. The findings fit the model of a Schottky barrier at the Cu/ZnO:Al interface, altered by the gas phase composition which in turn alters the catalytic properties of the system.