Defects in Al-Doped ZnO in Reactions with CO2 and H2: An In Situ/Operando cw ESR-Study

In this study, Al-doped ZnO (AZO) is investigated by combining in situ and operando continuous wave (cw) electron spin resonance (ESR) measurements with other methods, including microwave cavity perturbation technique (MCPT), magnetization, TGA, ²⁷Al-NMR, transmission electron microscopy, and XRD measurements. As Al-doped ZnO (AZO) is the support of Cu/ZnO:Al (CZA), an industrially used catalyst for methanol synthesis, studying the evolution of defects and changes in electronic structure under conditions used for reductive activation of the CZA catalyst or typical conditions for the r-WGS reaction is of interest to gain further insight into the complex interplay between the different components in the CZA catalyst. Next to paramagnetic defect sites, a ferromagnetic phase was observed forming upon activation in reducing atmospheres, which is modified, but not destroyed in (re)oxidizing conditions. The results prove the oxidizing effect of CO₂ on the paramagnetic defects as well as the ferromagnetic phase of an activated AZO. Comparison with prior results on CZA provides insight into the effect of Cu on AZO and enhances the understanding of the synergistic effects in the catalyst.