Supported chromium catalysts find broad application due to metal's ability to assume a range of oxidation states and structures. However, the behavior of chromium species on reducible metal oxides under reactive conditions remains poorly understood, despite the potential for unique reactivity through metal-support interactions. Recent studies on CeO2-supported Crn+ single atoms for selective NH3 oxidation to N2O underscore this potential, attaining high performance through cocatalytic action with CeO2, but also suffering from deactivation through agglomeration of isolated sites into Cr2O3. To address this challenge, we investigate how distinct CeO2-supported chromium species evolve under varying reactive environments. We show that under oxidative conditions redispersion of Cr2O3 occurs, serving as a catalyst regeneration strategy and enabling the recovery of both structure and performance. Combining advanced microscopy, in situ Raman, UV-vis, electron paramagnetic resonance and X-ray absorption spectroscopies, we follow the transformation from crystalline Cr2O3 nanoparticles to isolated chromium species. The redispersion is proposed to proceed via particle amorphization and oxidation of Cr3+ to mobile Cr6+, which diffuse over the support and stabilize as chromate species or as Cr5+ upon reduction by oxygen vacancies. A slower redispersion rate is observed over less reducible ZrO2 and TiO2, with minimal changes on γ-Al2O3 and Nb2O5, highlighting support reducibility as the driver of the process and reflecting the potential for generating Cr6+ sites from nontoxic Cr2O3 by controlling support properties. These results demonstrate how redox-active supports facilitate reversible changes to metal nanostructure, offering a promising strategy for regenerating catalysts and tuning metal speciation through rational support design for various applications.
Surin et al. (Sun,) studied this question.