Tracking Sulfur Poisoning of Pd/Al2O3 Catalysts for Methane Oxidation on Different Complexity Scales

Understanding the deactivation induced by sulfur
poisoning is crucial for designing more efficient palladium-based
monolithic catalysts for methane oxidation. This study employs
advanced characterization techniques, including X-ray absorption
spectroscopy (XAS), X-ray fluorescence (XRF), spatially resolved
activity measurements (SpaciPro), and synchrotron X-ray tomog-
raphy, to investigate the effects of sulfur poisoning and regeneration
from the atomic to the reactor scale. This includes structural
changes, i.e., oxidation state and chemical speciation in axial
direction of a catalyst bed/coated monolithic channel as well as in
the coated catalyst layer. Integral activity and spatially resolved
kinetic measurements revealed that sulfur significantly reduces the
catalytic activity for methane oxidation, in particular, from the beginning of the channel/catalyst bed. Gradients in sulfur
concentration were observed along the axial direction of the coated channel of the monolithic honeycomb catalysts by XRF,
supported by XAS. At the meso scale, X-ray holotomography provided three-dimensional (3D) maps (∼400 μm diameter)
uncovering some heterogeneous sulfur distribution within the catalyst layer due to different porosity/material structure. ... mehrThe sulfur
gradients correlate with the catalytic activity as a function of the axial position of the reactor as uncovered by SpaciPro.
Complementary XAS analysis at the S K-, Pd K-, and L3-edges uncovered details on the chemical structure changes (i.e., changes in
Pd and S oxidation states) during poisoning and regeneration leading to loss of catalytic activity. They show that it is challenging to
fully regenerate the Pd catalyst and its activity by a mild reductive treatment, particularly after prolonged sulfur exposure, as sulfur
predominantly remains in its oxidation state +6. The insights obtained allow for quantifying sulfur-induced deactivation in technical
catalysts as a function of composition and the location in the reactor. The insights may be used to develop and assess more stable catalysts in the future.