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Activity of carbon supported platinum nanoparticles catalysts toward methanol oxidation reaction : role of metal precursor and a new surfactant

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2008
Şen, Selda
In this thesis, carbon supported platinum nanoparticle catalysts were prepared using PtCl4 and H2PtCl6 as starting materials and 1-heptanethiol, tert-nonyl mercaptan, 1-hexadecanethiol, 1-octadecanethiol as surfactants. These new catalysts were employed for methanol oxidation reaction which are used for direct methanol fuel cells. Tert-nonyl mercaptane was used for the first time in this type of reaction and the other surfactants were used for comparison of the catalysts performance. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used in order to determine the nature of the catalysts. The average platinum crystallite particle sizes of all prepared catalysts were determined by both X-ray diffraction and transmission electron microscopy. It was found that platinum crystallizes in face-centered cubic structure and the surfactant play an important role on the size of platinum nanoparticles, branch surfactant, such as tert-nonyl mercaptane, causes an increase in the size of platinum nanoparticles, about 3 nm, compared to linear surfactant, such as 1-heptanethiol, about 2 nm. The oxidation states of platinum and their ratios were determined by XPS technique. These results indicated that platinum has two different oxidation states, zero and +4, and Pt(0) to Pt(IV) ratio is about 7.5 to 2.5. In addition to this, O 1s region of XPS was also examined and found that the surface of all of the catalysts covered by adsorbed hydroxide except the catalyst which was prepared by PtCl4 and tert-nonyl mercaptane (Catalyst IIa), where adsorption of water were observed and the catalyst which was prepared by H2PtCl6 and tert-nonyl mercaptane (Catalysts IIb), where adsorption of 65% of hydroxide and 35% of water were identified. Electrochemical studies indicated that Catalyst IIa has the maximum activity (342 A/gPt at 0.612 V) towards methanol oxidation reaction while Catalyst IIIb (H2PtCl6 and 1-hexanethiol were used to prepare this catalyst) has the minimum activity (91A/gPt at 0.580V). XRD, TEM and XPS results indicated that the optimum catalyst for methanol oxidation reaction contains about 3 nm of platinum nanoparticles, adsorbed hydroxide and water on the surface of catalyst, but sulphur. These results are in agreement with the proposed mechanism.