Interaction of CO and methane with platinum, Low-temperature oxidation and sensor response mechanisms

Licentiate thesis, 2008

The aim of this thesis project was to increase the understanding of the mechanisms for low-temperature oxidation of carbon monoxide and methane over supported platinum catalysts. In situ spectroscopic methods together with mass spectrometry were used to study the catalysts during both steady state and transient conditions. The results show that the interaction between oxygen and platinum is crucial, but this interaction differs depending on the reaction and/or application. In specifc, for CO oxidation, when the reaction is self-poisoned by carbon monoxide, alternative channels of supplying oxygen to the reaction exist, e.g. by additional compounds, or via the support material. For oxidation of methane at oxygen excess, however, the catalyst surface is most likely covered with oxygen, thus hindering methane from adsorption and dissociation. By transient operation of the feed gas composition, it is possible to increase the activity of methane oxidation at lean conditions. In situ XANES spectroscopy was used to monitor the surface O/Pt ratio during transient methane oxidation, and a new method was developed and applied for analysis of the in situ XANES spectra. The time-resolved XANES data show that there likely is an optimal catalyst state with a maximum activity for methane oxidation. Further, the sensing mechanism of platinum based metal insulator silicon carbide sensors towards carbon monoxide was investigated. Infrared spectroscopy studies performed using model sensors show that the sensor response is likely related to the CO coverage of the sensor surface.