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 tomography, 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. The 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.