Influence of the Metallic Precursor and of the Catalytic Reaction on the Activity and Evolution of Pt(Cl)/δ-Al2O3 Catalysts in the Total Oxidation of Methane

In the light-off temperature range of methane total oxidation (250–600°C), two types of Pt/δ-Al2O3 catalysts prepared from H2PtCl6 or Pt(NH3)4(OH)2 show different catalytic behaviors. Initially very active, fresh chlorine-free catalysts deactivate in isothermal conditions of reaction, while fresh chlorine-containing catalysts, initially poorly active, undergo an activation with time on stream. Consequences of dechlorination of chlorine-containing catalysts and of impregnation by various amounts of NH4Cl on both types of catalysts are examined. Chlorine is responsible for the inhibition observed on ex-H2PtCl6 catalysts, which could be attributed, more specifically, to the chloride ions remaining at the platinum–support interface after reduction of platinum. After aging of the catalysts under reaction, sintering of the platinum particles is observed on all samples, regardless of their preparation mode. This phenomenon is linked to the presence of oxygen in excess in the gas phase and can account for the deactivation of chlorine-free catalysts. The water produced during the reaction removes the inhibiting chlorine on all chlorine-containing catalysts. The activity of the largest sintered particles is shown to vary with the nature of the fresh catalyst, the aged ex-H2PtCl6 catalysts giving the best results: the molecular nature of the fresh catalyst would also affect the activity of the stable aged catalyst. Both the initial presence of chloride ions at the metal–support interface and the composition of the reacting mixture are key factors to this positive reconstruction.