Enhancement of palladium in wet-lean methane combustion

  • Author / Creator
    Nassiri, Hanieh
  • The beneficial feature of the application of natural gas in the automotive industry is the lowest carbon content of any fossil fuel, which results in the lowest CO2 emissions. On the other hand, the concern of atmospheric pollution due to methane emissions from natural-gas vehicles (NGV) engines evokes the stringent regulations of vehicular pollution from NGVs, and specific considerations in designing the catalytic converters with expensive and scarce noble metal catalysts. Pd is conventionally used for the catalytic combustion of methane but its activity is inhibited by water, and a considerable work needs to be done to increase its stability. Adding a co-metal like Pt in order to produce a bimetallic alloy catalyst or adding a metal-oxide promoter is desirable to stabilize the catalytic conversion. However, the chemical states of Pd in bimetallic catalysts is expected to be completely different from that of the mono Pd under the reaction conditions. Therefore, an in situ X-ray absorption spectroscopy (XAS) study can also be a viable investigation to shed light on the effect of Pt on Pd/PdOx distribution in bimetallic Pd-Pt catalysts under reaction conditions at low temperatures (200-550ºC) with or without the presence of water as a catalytic poison. Furthermore, loss of activity can be avoided by choosing a SnO2 promoter as a non-noble metal oxide support material with a low metal-oxygen bond. This thesis develops an understanding of the effect of the addition of Pt in bimetallic Pd-Pt/Al2O3 catalysts and also the benefits of using SnO2 as a support for Pd-based catalysts encountering the effect of water in a low-temperature methane combustion reaction. The synthesis method of controlled-size nanoparticles via a colloidal solution technique and evaluating the effect of different synthesis parameters helps to control interactions between catalytic phases in bimetallic catalysts, and also assures the existence of intrinsic bimetallicity to get desired activity and stability in order to study the structure-property relationship and understand not only the role of Pt as a secondary catalytic phase, but also SnO2 as a metal-oxide promoter in methane combustion reactions. Besides, in situ XAS studies demonstrate the effect of Pt on the state of the active Pd surface under low-temperature, dry and wet lean methane combustion conditions. Pt addition promotes Pd reduction even in a reactive oxidizing environment with a significantly lower activity in a dry condition as compared with the oxidized Pd. However, the presence of water leads to the increased fraction of metallic Pd due to the lack of surface oxygen, resulting in Pt atoms available for methane dissociation, which does not occur in the dry methane-lean feed in which oxygen poisons Pt. Evaluation of the effect of the Pd:Pt ratio on the stability of the bimetallic Pd-Pt/Al2O3 catalysts during and after 40-hour in-situ hydrothermal ageing at 400-550ºC with 5% water in a low-temperature lean methane combustion revealed that the ratio of Pd/Pt had the governing effect, not the catalyst preparation method. The Pd:Pt 1:1 (atomic) ratio catalyst was found to provide the most optimal combination of activity and stability. Significant structural changes also occurred in-situ regardless of the preparation method. Finally, as a potential replacement of Pt in catalytic converters for NGVs, the beneficial role of SnO2 as a metal oxide support was also studied. SnO2 enhanced the activity of PdO sites by providing oxygen, and overrode the Pt-promoting role in bimetallic Pd-Pt/Al2O3 catalysts.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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