Experimental Assessments and Kinetic Models of Palladium Nanoparticle Sintering Behavior on Alumina Supports

Palladium
nanoparticles supported on alumina are widely used as
catalysts in emissions after-treatment. The evolution of Pd particle
size distributions during sintering is influenced by the initial Pd
nanoparticle size distribution, surface heterogeneities of Pd and
the oxide support, and the reduction or oxidizing gas environments.
Here, we synthesized Pd nanoparticles of similar average particle
sizes and different distributions using either colloidal synthesis
(normally distributed) or incipient wetness impregnation (log-normally
distributed) on different amorphous Al2O3 supports
(γ-Al2O3, La-doped γ-Al2O3). Under oxidative (O2, 1073 K) and reductive
(H2, 1073 K) aging conditions, Pd showed similar sintering
rates on γ-Al2O3 and La-doped Al2O3 (quantified by TEM, O2 chemisorption and
XRD). Starting from the same average Pd particle sizes, reductive
conditions resulted in a greater extent of Pd sintering for samples
prepared by incipient wetness impregnation, while oxidative conditions
resulted in a greater extent of sintering for samples prepared by
colloidal synthesis methods. This contrast between samples was rationalized
using Ostwald ripening kinetic models that show colloidally synthesized
particles are formed in different regions of the support and have
a lower chemical potential upon exposure to dihydrogen rather than
dioxygen. Consistent with these findings, samples prepared using incipient
wetness impregnation with the Pd precursors used for colloidal synthesis
approaches show reductive and oxidative sintering rates similar to
those of colloidally prepared materials. Our results show that the
average particle size is an incomplete predictor for particle sintering
behavior, and colloidal synthesis methods may alter nanoparticle deposition
and surface chemistry along with particle size distributions.