دانلود رایگان مقاله تاثیر دی اکسید گوگرد بر فعالیت مرتبه ای کاتالیزور مبتنی بر پالادیم

Abstract

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.

5. Conclusions

The self-assembly methodology described previously [32] was modified in order to synthesize nanostructured Pd@CexZr1−xO2 (Pd@MOx) units in the whole compositional range (0 < x < 1). The synthesis of dispersed Pd@MOx allowed the preparation of a series of high-surface-area Si-Al2O3 supported catalysts and model catalysts having similar nanostructure and surface chemistry. Comparison of results on the two types of catalysts allowed the SO2 poisoning ofmethane oxidationonPd-basedcatalysts to be systematically studied to elucidate the role of the MOx promoter and the aging conditions. At lower temperatures (<450 ◦C), the PdO active phase is irreversibly poisoned by SO2 due to interaction with sulfates which are not able to spillover to the support/promoter. At higher temperatures (>500 ◦C), poisoning is slowed by formation of sulfate species on the oxide promoter. Due to partial decomposition of sulfates at 500 ◦C, Pd@ZrO2-based catalysts showed the best sulfur-poisoning resistance, attaining complete regeneration even after prolonged aging, and thus they are the best candidates for real application. Pd@Ce0.6Zr0.4O2 catalysts showed intermediate sulfur tolerance compared to Pd@CeO2 and Pd@ZrO2, in agreement with previously reported results [23]. The high chemical sensitivity of PES techniques provided direct evidence for previously suggested formation of sulfate species on individual metal cations in CexZr1−xO2 mixed oxides [23]. Finally, the modelcatalyst approaches developed here should allow the study of metal-support interactions in other catalytically relevant systems by simply varying the ALD-deposited thin film composition.