Hydrodechlorination catalysis of Pd-on-Au nanoparticles varies with particle size

Trichloroethene (TCE), a common carcinogen and groundwater contaminant in industrialized nations, can be catalytically degraded by Au nanoparticles partially coated with Pd (“Pd-on-Au NPs”). In this work, we synthesized Pd-on-Au NPs using 3, 7, and 10 nm Au NPs with Pd surface coverages between 0–150% and studied how particle size and composition influenced their TCE hydrodechlorination (HDC) activity. We observed volcano-shape dependence on both Au particle size and Pd surface coverage, with 7 nm Au NPs with Pd coverages of 60–70% having maximum activity. Using extended X-ray absorption fine-structure spectroscopy, we found a strong correlation between catalytic activity and the presence of 2-D Pd ensembles (as small as 2–3 atoms). Aberration-corrected scanning transmission electron microscopy further confirmed the presence of Pd ensembles. The Pd dispersion and oxidation state generally changed from isolated, metallic Pd atoms to metallic 2-D Pd ensembles of varying sizes, and to partially oxidized 3-D Pd ensembles, as Pd surface coverage increased. These changes occurred at different surface coverages for different Au particle sizes. These findings highlight the importance of controlling particle size and surface coverage in bimetallic catalysts.