Ultrafine Nanoparticle Rh/CeO₂–ZrO₂ Catalysts Synthesized via Spatial Confinement: Higher Three-Way Catalytic Activity Compared to Rh Single-Atom Catalyst

The synthesis of size-controlled ultrafine metal-based catalysts is vitally important for chemical conversion technologies. This study presents a spatial confinement strategy for the synthesis of Rh/CeO₂-ZrO₂ (0.5 wt % Rh) three-way catalysts with ultrafine Rh nanoparticles (1-3 nm). This strategy utilizes the self-confinement effect of Rh ions through the strong electrostatic adsorption between Rh ions and the surface of CeO₂-ZrO₂, as well as the spatial hindrance provided by the mesopores of the support during Rh particle growth. The nanoparticle catalyst (NPC) with a size of ∼2.19 nm exhibits high catalytic performance, surpassing the Rh single-atom catalyst (SAC) and the other NPCs with different Rh sizes in the three-way catalytic reaction under a gas mixture of carbon monoxide (CO), hydrocarbons (HCs), and nitric oxide (NO). Rh SAC displays higher CO oxidation activity and comparable C₃H₆ oxidation activity compared with Rh NPC in reaction atmospheres without NO gas molecules. However, the presence of NO molecules hinders the adsorption and reaction of CO and HCs on the Rh single-atom sites. The impact of NO on Rh NPC is weaker due to the multiatomic active center structure of the Rh nanoparticles, resulting in enhanced low-temperature catalytic activity in three-way reaction atmospheres. Additionally, NPC demonstrates better stability than SAC under hydrothermal aging condition.