High Temperature Stable Pt Nanoclusters Formation in Alumina-Encapsulated Pt Catalysts

Abstract

Metal nanoparticle sintering in high-temperature oxidizing atmospheres remains a primary deactivation pathway for platinum-group metal catalysts on alumina supports. Here, atomic layer deposition (ALD) was used to deposit alumina on supported Pt catalysts and develop alumina-encapsulated Pt/Al2O3 catalysts that were aged at 800 °C under different atmospheres and characterized by time-resolved high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), CO-probe diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), and propene combustion kinetics. Within the alumina-encapsulated system, a sub-2 nm Pt cluster population formed under all oxidizing atmospheres tested but not under inert atmosphere, and this population persisted for at least 12 h at 800 °C under propene complete combustion (PCC) conditions. Unencapsulated Pt/Al2O3 controls, by contrast, developed only a transient sub-2 nm population, detected within 1 min of aging in air at 800 °C and no longer present afterward, consistent with the rapid disappearance of small particles predicted by Ostwald ripening theory. CO-DRIFTS confirmed the presence of undercoordinated Pt surface sites in all oxidatively aged samples, with no detectable signal when the sample was aged under inert atmosphere. Propene combustion rates measured after aging for an alumina-encapsulated catalyst were approximately two and five times higher than the fresh encapsulated catalyst for catalysts aged under inert atmosphere and under oxygen and steam, respectively. DFT calculations of Pt diffusion through defective Al2O3 identified kinetically accessible diffusion pathways at 800 °C, with activation barriers as low as 0.76 eV. Furthermore, Pt surface binding was calculated to be favored in the presence of O2. These results identify oxygen-driven surface segregation, mediated by defect-assisted diffusion through the ALD overcoat, as a possible mechanism responsible for nanocluster formation in this system. The persistence of the sub-2 nm cluster population for at least 12 h at 800 °C under PCC conditions demonstrates the thermal stability of these sites within the encapsulated architecture.