On the structure and surface properties of NiO/MgO-La2O3 catalyst:: Influence of the support composition and preparation method

This work addresses the effect of catalyst preparation method and the carrier compositions (MgO-La2O3) over the NiO-support interaction, which affect the reducibility, textural properties and the different oxygen species chemisorbed at different temperatures over MgO-La2O3 supported NiO catalysts. The materials were prepared by wet sequential impregnation and wet co-impregnation with different Mg molar fractions [Mg/(La+Mg)]. The samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), infrared (IR) spectroscopy, scanning electron microscopy (SEM), changes of surface potential and BET surface area measurements. The total oxidation of methane was use as model reaction. It has been found that the catalyst formulations (i.e. NiO/MgO-La2O3) and the preparation methods not only affect the interaction among the catalyst components, but also the texture and material morphology as a result of different degrees of particle aggregation. The wet sequential impregnation-prepared catalysts showed a stronger MgO-La2O3 interaction than wet co-impregnation-prepared samples. A marked tendency of NiO to react with MgO rather than La2O3 following a mechanism of lattice substitution is observed. Mg-free catalyst showed LaNiO3 and NiO as major crystalline Ni-containing phases. The ternary Ni-Mg-La-O system, on the other hand, facilitates the formation of poorly reducible Ni phase, whereas the La-free catalyst (i.e. NiO/MgO) displayed the lowest content of Ni-reducible phase, owing to the formation of Ni1-xMgxO solid solution. Measurements of surface potential changes together with catalytic studies suggest that La-containing catalysts present oxygen vacancies, which markedly affect the chemical nature of the surface oxygen species and hence their catalytic behaviour.