Kinetic Pathway of Palladium Nanoparticle Sulfidation
Process at High Temperatures
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Abstract
A significant
issue related to Palladium (Pd) based catalysts is
that sulfur-containing species, such as alkanethiols, can form a PdS<sub><i>x</i></sub> underlayer on nanoparticle surface and subsequently
poison the catalysts. Understanding the exact reaction pathway, the
degree of sulfidation, the chemical stoichiometry, and the temperature
dependence of this process is critically important. Combining energy-filtered
transmission electron microscopy (EFTEM), X-ray diffraction (XRD),
and X-ray absorption spectroscopy experiments at the S <i>K-</i>, Pd <i>K</i>-, and <i>L</i><sub>2,3</sub>-edges,
we show the kinetic pathway of Pd nanoparticle sulfidation process
with the addition of excess amount of octadecanethiol at different
temperatures, up to 250 °C. We demonstrate that the initial polycrystalline
Pd-oleylamine nanoparticles gradually become amorphous PdS<sub><i>x</i></sub> nanoparticles, with the sulfur atomic concentration
eventually saturating at Pd/S = 66:34 at 200 °C. This final chemical
stoichiometry of the sulfurized nanoparticles closely matches that
of the crystalline P<sub>16</sub>S<sub>7</sub> phase (30.4% S), albeit
being structurally amorphous. Sulfur diffusion into the nanoparticle
depends strongly on the temperature. At 90 °C, sulfidation remains
limited at the surface of nanoparticles even with extended heating
time; whereas at higher temperatures beyond 125 °C, sulfidation
occurs rapidly in the interior of the particles, far beyond what can
be described as a core–shell model. This indicates sulfur diffusion
from the surface to the interior of the particle is subject to a diffusion
barrier and likely first go through the grain boundaries of the nanoparticle