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Réactivité d'agrégats de palladium supportés sur MgO(100) : adsorption du NO, réactions CO+NO et CO+O2

Abstract : Nanometer-sized metal clusters show singular chemical properties comparatively to bulk materials, and are used as models for studying heterogeneous catalysis. The reactivity of palladium clusters, epitaxially grown on MgO(100) and characterised by transmission electron microscopy, has been investigated. In addition to NO adsorption, CO+NO and CO+O2 reactions experiments have been performed under ultrahigh vacuum, using pulsed molecular beam methods. Nitric oxide molecules can diffuse on the MgO support before being chemisorbed on the Pd clusters. We have measured the probability of NO physisorption on MgO, and analysed this diffusion-capture phenomenon in a quantitative way. Once adsorbed on palladium, NO either dissociates, producing nitrogen, or desorbs. The activation energy for NO desorption is 32 kcal/mol. An influence of the cluster temperature and morphology on the NO adsorption and dissociation capacity has been evidenced. The NO reduction by CO produces mostly CO2 and N2. At equal pressures of reactants, the catalytic activity reaches a maximum at about 260°C. The rate-limiting steps of the reaction are, in the low temperature regime, the dissociation of NO and, in the high temperature regime, the adsorption of CO. Considering the diffusion of the reactants on the support, we have been able to accurately determine the effect of the cluster size on the catalytic activity, revealing a correlation with NO adsorption experiments. Finally, the kinetics of the CO oxidation reaction has been analysed in the transient regime, with the help of simple simulations. Below about 200°C, the reaction rate is limited by the inhibition of oxygen adsorption by CO strongly bound to the cluster edges. Above 200°C, at high oxygen coverage, a precursor state of CO chemisorption induces a lowering of the CO2 production rate.
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https://tel.archives-ouvertes.fr/tel-00445979
Contributor : Laurent Piccolo <>
Submitted on : Monday, January 11, 2010 - 4:26:32 PM
Last modification on : Saturday, November 7, 2020 - 3:14:07 PM
Long-term archiving on: : Friday, June 18, 2010 - 12:41:56 AM

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  • HAL Id : tel-00445979, version 1

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Laurent Piccolo. Réactivité d'agrégats de palladium supportés sur MgO(100) : adsorption du NO, réactions CO+NO et CO+O2. Matériaux. Université de la Méditerranée - Aix-Marseille II, 1999. Français. ⟨tel-00445979⟩

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