Termodynamic considerations of strong metal-support interaction in a real PtTiO2 catalyst
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AbstractThe nature of lattice defects induced by strong metal-support interaction (SMSI) treatment in titania supporting platinum has been studied by in situ electrical conductivity measurements as a function of oxygen pressure. The variations between 425 and 500 °C of the electrical conductivity σ vs PO2 (σ = k · PO2−14) are indicative of the presence of singly ionized vacancies. Their heats of formation are equal to twice the eneration determined by microcalorimetry corresponds, in absolute value, to the heat of filling two anionic vacanciactivation energy of conduction (ΔH = +81 kcal mol-1). The heat of PtTiO2 reges by one molecule of O2. It is substantially lower than the heats of titanium suboxide reoxidation, which rules out the presence of such species on PtTiO2 real catalysts in the SMSI state. Titania has a stoichiometry of TiO1.996, i.e., still close to that of anatase. These anionic vacancies can be filled by the oxygen of water molecules arising from the methanation reaction (ΔH = −23 kcal mol−1), thus explaining the suppression of the SMSI state under working conditions. To each singly ionized anionic vacancy corresponds one free electron of conduction, which increases the Fermi level of titania and makes possible an electron transfer to the metal. The electron transfer can inhibit H2 or CO chemisorption, if these molecules chemisorb as dipoles with a donor character to the metal as evidenced by conductivity.

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