PhD Defense of Amaury Decoster

Pr. PIRAULT ROY Laurence

Thesis reviewer

Université de Poitiers, Institut de Chimie des Milieux et Matériaux de Poitiers

Pr. DATURY Marco

Thesis reviewer

Université de Caen, Laboratoire de Catalyse et Spectrochimie

Pr. ESPECEL Catherine

Member

Université de Poitiers,  Institut de Chimie des Milieux et Matériaux de Poitiers

Pr.  GRANGER Pascal

Unité de Catalyse et Chimie du Solide (UMR 8181), Université de Lille

MC. DHAINAUT Fabien

Co-supervisor

Unité de Catalyse et Chimie du Solide (UMR 8181), Centrale Lille

This thesis concerns the development of automotive post-combustion catalysts for natural gas-fuelled engines. Unburned methane leaving the engine must be post-treated on a catalyst to prevent it from being released into the atmosphere, because of its global warming potential, which is around 20 times greater than that of CO₂.

The noble metals contained in catalytic converters are extensively used. The metal of choice for methane combustion is palladium. Its most active form in an oxidising environment is PdO. For three-way operation close to stoichiometric conditions and at high temperatures, PdO is unstable and tends to decompose into its inactive metallic form. An alternative approach is to disperse the palladium on an oxide support capable of producing active oxygen species in a medium depleted of gaseous oxygen, or even to replace the palladium completely with a metal oxide such as Co₃O₄ or Mn₃O₄ known for its good intrinsic activity comparable, under certain operating conditions, to that of noble metals.

However, this type of oxide has poor thermal stability. We therefore developed mixed oxides by introducing an iron heteroatom to improve their performance in methane combustion. The structural study of mixed oxides of composition Co_xFe_3-xO₄ and Mn_xFe_3-xO₄ by X-ray diffraction, Raman and Mössbauer spectroscopies revealed the presence of different phases enriched in iron, cobalt or manganese and identified different interstitial sites with octahedral or tetrahedral symmetry. Surface analysis by X-ray induced photoelectron spectroscopy, nitrogen physisorption and oxygen thermodesorption provided insights to explain their catalytic behaviour and, more specifically, to characterise the catalyst pre-treatment conditions conducive to enhancing their activity.

In the second part, the influence of the addition of palladium was studied, looking more specifically at the method of addition, sequential by impregnation, or during the spinel production process. The first method, which disperses the palladium on the surface, proved to be the most effective, producing more active catalysts that are more resistant to deactivation at high temperatures.

Finally, the kinetic study of the methane combustion reaction with O₂ enabled the functionalities of the catalyst to be identified, in particular the role of the metal-support interface in the development of catalytic activity. The reaction rate measurements were compared with two reaction mechanisms that differ in the nature of the reactive oxygen, which comes either from the gas phase or from the spinel lattice, and their contribution was examined under different operating conditions, particularly in an oxygen-depleted environment.