Poisoning of platinum thin film catalyst by hexamethyldisiloxane (HMDS) for thermoelectric hydrogen gas sensor

The evolution of the catalytic activity of platinum (Pt) thin film towards hydrogen oxidation has been studied as a function of the exposure to hexamethyldisiloxane (HMDS) at various concentrations and temperatures. HMDS, decomposes by the thermally activated reaction and the catalyst surface, is progressively covered by silica and the number of active sites decreases. The interaction of Pt thin film with HMDS produces the formation of a silica overlayer and the mechanism of the catalytic oxidation of hydrogen is deeply modified. The consequences on the surface temperature of catalytic films are analyzed as a function of the operating temperature of the catalyst. The XPS analysis was performed on the Pt surface after exposure to HMDS. The peaks of Si 2p and O 1s in XPS profiles were identified the SiO2 layer on the Pt surface. The thickness of SiO2 on the Pt after exposure to HMDS was estimated to 1.8 nm considering from the AES depth profiles. After heat treatment following exposure to HMDS, the catalytic activity was recovered to some extent, because the corresponding AES depth profile enables us to conclude that the thickness of SiO2 was decreased by heat treatment. For the practical use of the thermoelectric gas sensor, Pt/SiGe/alumina, its hydrogen selectivity was improved by the exposure to HMDS with the help of the SiO2 layer on the Pt surface.