Abstract
This thesis investigates the degradation behavior of Pt/C catalysts under simulated
automotive conditions. By using the “tool box” synthesis method the Pt loading has been changed from low to high Pt loadings, therefore permitting to study the role of Pt on the degradation of high surface area (HSA) Pt/C catalyst. Diverse degradation mechanisms have been found to be responsible for the electrochemical surface area loss (ECSA). The different degradation mechanisms have been found to be dependent from the diverse potential windows applied during the stress test. Furthermore the synthesis approach allows changing between different carbon substrates, therefore enabling systematic studies on different carbon substrates. Concerning the role of the substrate on the degradation, a non conventional core shell TiO2@C substrate has been synthesized and compared with conventional carbon blacks (CBs).
Interestingly increasing the Pt to carbon ratio was found to enhance the ECSA loss during start/stop cycling up to a certain ratio. However, a further increase in the Pt to carbon ratio results in a decrease of the ECSA loss. Moreover a significant difference in the ESCA loss was found between two standard CBs, i.e. Vulcan XC72R and Ketjenblack EC-300J. The ECSA loss measured for Vulcan XC72R was significantly higher after start/stop than for Ketjenblack EC-300J. To this concern the Pt loaded Vulcan XC72R and Ketjenblack EC- 300J has been studied by Raman spectroscopy. The Raman spectra of the catalysts collected after start/stop cycling revealed significant differences between the two CBs used as Pt substrates. Vulcan XC72R resulted to be more prone than Ketjenblack EC-300J to get electrochemically oxidized and form C=O groups at its surface. This behavior explains the higher ECSA loss found for Vulcan XC72R. Furthermore the Raman spectra of the bare CBs exposed to start/stop cycling do not differ from the Pt loaded, thus not indicating a catalytic role of Pt towards the carbon corrosion.
The last part of this thesis is dedicated to the development and testing of a core shell TiO2@C support for Pt nanoparticles (NPs). TiO2@C was synthesized by heat treatment in C2H2 and subsequently loaded with Pt NPs. Pt/TiO2@C was tested and compared with Pt/C and Pt/TiO2 prepared using the same colloidal stock solution. Similar ECSA values were reached on Pt/TiO2@C and Pt/C while Pt/TiO2 fails to reach high ECSA values. Pt/TiO2@C showed significant improvements after start/stop cycling while after load cycles the same ECSA loss was detected.
Interestingly increasing the Pt to carbon ratio was found to enhance the ECSA loss during start/stop cycling up to a certain ratio. However, a further increase in the Pt to carbon ratio results in a decrease of the ECSA loss. Moreover a significant difference in the ESCA loss was found between two standard CBs, i.e. Vulcan XC72R and Ketjenblack EC-300J. The ECSA loss measured for Vulcan XC72R was significantly higher after start/stop than for Ketjenblack EC-300J. To this concern the Pt loaded Vulcan XC72R and Ketjenblack EC- 300J has been studied by Raman spectroscopy. The Raman spectra of the catalysts collected after start/stop cycling revealed significant differences between the two CBs used as Pt substrates. Vulcan XC72R resulted to be more prone than Ketjenblack EC-300J to get electrochemically oxidized and form C=O groups at its surface. This behavior explains the higher ECSA loss found for Vulcan XC72R. Furthermore the Raman spectra of the bare CBs exposed to start/stop cycling do not differ from the Pt loaded, thus not indicating a catalytic role of Pt towards the carbon corrosion.
The last part of this thesis is dedicated to the development and testing of a core shell TiO2@C support for Pt nanoparticles (NPs). TiO2@C was synthesized by heat treatment in C2H2 and subsequently loaded with Pt NPs. Pt/TiO2@C was tested and compared with Pt/C and Pt/TiO2 prepared using the same colloidal stock solution. Similar ECSA values were reached on Pt/TiO2@C and Pt/C while Pt/TiO2 fails to reach high ECSA values. Pt/TiO2@C showed significant improvements after start/stop cycling while after load cycles the same ECSA loss was detected.
| Originalsprog | Engelsk |
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| Forlag | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Antal sider | 117 |
| Status | Udgivet - 2014 |