The effect of sulfur dioxide on the activity of hierarchical Pd-based catalysts in methane combustion

- Self-assembly synthesis of hierarchical Pd@CexZr1−xO2 methane combustion catalysts was obtained.
- Atomic Layer Deposition (ALD) was used to prepare Pd@CexZr1−xO2-based model catalysts suitable for morphological and chemical state studies.
- Below 450 °C, SO2 irreversibly poisons PdO, causing a partial reduction to Pd and the formation of sulfates in close proximity to the active phase.
- At higher temperatures, PdO is more resistant to sulfur poisoning, thanks to the spillover of sulfates from the active phase and/or the direct reaction of SO2 with the promoter.
- Pd@ZrO2 catalysts showed the best sulfur resistance.

SO2 poisoning of methane oxidation over alumina-supported, Pd@CexZr1−xO2 nanoparticle catalysts was systematically studied by means of advanced PhotoElectron Spectroscopy (PES) methods. The Pd@CexZr1−xO2 units were synthesized and deposited on two modified-alumina supports, i.e. high surface area modified alumina and a model alumina prepared by Atomic Layer Deposition (ALD) of alumina on Indium Tin Oxide (ITO)/quartz slides. The model support was designed to be suitable for PES analysis and was stable to high temperature treatments (850 °C). Characterization of the high-surface-area (HSA) catalysts by X-Ray Diffraction (XRD), N2 physisorption, CO chemisorption and Transmission Electron Microscopy (TEM) indicated formation of CeO2-ZrO2 (CZ) mixed-oxide crystallites that stabilize the Pd active phase against sintering. Correlation of methane-oxidation rates with PES results demonstrated two distinct mechanisms for deactivation by SO2. Below 450 °C, the presence of SO2 in the feed led to partial reduction of the active PdO phase and to the formation of sulfates on the Pd. Above 500 °C, poisoning by SO2 was less severe due to spillover of the sulfates onto the oxide promoter. Pd@ZrO2 catalysts showed the best resistance to SO2 poisoning, outperforming analogous Pd@CZ mixed-oxide catalysts, because there was less sulfate formation and the sulfates that did form could be removed during regeneration.

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