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Adsorption and Thermally Induced Reactions of Methanol on Bimetallic X/Ru(0001) Layers (X = Cu, Pt)
This thesis summarizes the results of thermally induced methanol (CH_3OH) reactions on bimetallic Ru(0001)-based catalyst surfaces under ultrahigh vacuum conditions. Specifically, the following clean and oxygen covered surfaces were used: Ru(0001), (sub-) monolayer Cu/Ru(0001), multilayer Cu/Ru(0001...
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| Format: | Dissertation |
| Sprache: | Englisch |
| Veröffentlicht: |
Philipps-Universität Marburg
2012
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| Zusammenfassung: | This thesis summarizes the results of thermally induced methanol (CH_3OH) reactions on bimetallic Ru(0001)-based catalyst surfaces under ultrahigh vacuum conditions. Specifically, the following clean and oxygen covered surfaces were used: Ru(0001), (sub-) monolayer Cu/Ru(0001), multilayer Cu/Ru(0001), Pt_n/Ru(0001) layers, and Pt_xRu_{1-x}/Ru(0001) surface alloys.
The adsorption and reactions of methanol are of great technological relevance for the direct methanol fuel cell (DMFC). Thereby, it is desirable to influence chemical reactivity and selectivity of catalysts to convert methanol to CO_2 instead of CO which acts as a poison affecting a continued and stable operation. The stability of reaction intermediates and the height of the activation barriers of the various reaction steps critically depends on specific properties of the substrate material. Straightforward methods to design novel catalysts in a controlled way are the deposition of ultrathin metal films on a host material, the building of alloys or the addition of coadsorbates.
In the experiments performed in this work methanol was added at 20 or 80 K to the catalyst surfaces and slowly annealed with 1 K/s to increasingly higher temperatures. Thereby, the surface species were identified using Fourier transform infrared spectroscopy, i.e. the observed vibrational modes were analyzed in detail. For an unambiguous assignment of the observed peaks isotopic labeling was applied using different isotopes of methanol, oxygen, carbon monoxide, and hydrogen. The desorbing species, on the other hand, were analyzed by temperature programmed desorption using a quadrupole mass spectrometer. The desorption temperature provides information about the binding strength of an adsorbate and about the dissociation temperature of stable surface species which decays into gaseous products. Moreover, isotopic labeling (18-O and 16-O) allows the discrimination of reactions involving surface oxygen, e.g the formation of desorbing water. Similarly, the CD_3OH isotope allows to distinguish whether hydrogen from the CD_3 or the OH group contributes to a certain reaction. For the quantitative analysis of the chemical composition of the surface and the adsorbates, X-ray photoelectron spectroscopy was applied.
The experiments focus on the identification of fundamental reaction steps and stable intermediate species and, in the second step, on the variation of surface parameters, such as the sort and thickness of a deposited metal, the addition of coadsorbates, changing the adsorbate order and density or modifying the composition of the surface alloy.
The reactions on the investigated surfaces can be subdivided into two major pathways; (i): a total dehydrogenation pathway leading to CO, and (ii): an oxidation pathway which produces gaseous CO_2.
On the clean Ru(0001), Cu/Ru(0001) and Pt/Ru(0001) surfaces either the dehydrogenation pathways are observed or no reaction occurs at all. The CO_2 producing path, on the other hand, can be opened by the adsorption of oxygen. In parallel, the CO formation becomes reduced. In this context, the influence of oxygen on the yielded reaction products was investigated. Generally, it is found that only disordered and dilute oxygen promote methanol reactions; dense and ordered O-overlayers passivate the surface effectively.
A significant drawback of adding oxygen is the reaction of the oxygen atoms with hydrogen from methanol dehydrogenation to gaseous water. As hydrogen is the energy provider in a DMFC the desorbing water represents an unwanted drain of H atoms from the surface. Interestingly, the surfaces which produce the highest amount of CO_2 are also most efficient with respect to the formation of water.
As on oxygen covered Pt_xRu_{1-x}/Ru(0001) surface alloys the drain of H atoms is limited and they nonetheless exhibit CO_2 as a final product they represent a compromise regarding the ideal catalyst material for a DMFC. In particular, alloys with a Pt contents of 50 - 80% are found to be most suitable. |
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| DOI: | 10.17192/z2012.0047 |