Tracking Chromium Evolution on Ceria from Particles to Single Atoms: A Catalyst Regeneration Strategy for Ammonia Oxidation

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.