A two-step seeded-growth method was refined to synthesize
Au@Pd
core@shell nanoparticles with thin Pd shells, which were then deposited
onto alumina to obtain a supported Au@Pd/Al2O3 catalyst active for prototypical CO oxidation. By the strict control
of temperature and Pd/Au molar ratio and the use of l-ascorbic
acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is
formed around the Au core, as evidenced by transmission electron microscopy
and energy-dispersive spectroscopy. The core@shell structure and the
Pd shell remain intact upon deposition onto alumina and after being
used for CO oxidation, as revealed by additional X-ray diffraction
and X-ray photoemission spectroscopy before and after the reaction.
The Pd shell surface was characterized with in situ infrared (IR)
spectroscopy using CO as a chemical probe during CO adsorption–desorption.
The IR bands for CO ad-species on the Pd shell suggest that the shell
exposes mostly low-index surfaces, likely Pd(111) as the majority
facet. Generally, the IR bands are blue-shifted as compared to conventional
Pd/alumina catalysts, which may be due to the different support materials
for Pd, Au versus Al2O3, and/or less strain
of the Pd shell. Frequencies obtained from density functional calculations
suggest the latter to be significant. Further, the catalytic CO oxidation
ignition-extinction processes were followed by in situ IR, which shows
the common CO poisoning and kinetic behavior associated with competitive
adsorption of CO and O2 that is typically observed for
noble metal catalysts.