COMBUSTION OF METHANE OVER A PD-AL2O3 SIO2 CATALYST, CATALYST ACTIVITY AND STABILITY/

Palladium supported on Si-stabilised alumina has been demonstrated to be an active and durable catalyst for the combustion of methane. Si is more effective in stabilising alumina than La or Ba when the elements are added through an impregnation technique. Multiple stabilisation w ith combinations of La, Ba and Si does not increase further the stabil ity against sintering. The stability increases logarithmically with th e amount of Si added (0.5-8 atomic%). The rate of sintering is not aff ected by an increase of the water vapour content of the atmosphere fro m 1 to 20 vol.%. Doping palladium with rhodium or platinum increases t he activity of the catalyst for methane combustion. The high-temperatu re stability of pure Pd is however superior to the stabilities of the Rh- and Pt-doped catalysts. Addition of La or Ce to the Pd-catalyst in creases its stability against thermal deactivation but leads to an ove rall decrease in activity. The activity of the as-prepared catalysts a re affected by the Pd-content below a value corresponding to 5% of the monolayer capacity. Thermally deactivated catalysts show a stronger a ctivity dependence of the Pd-content than as-prepared catalysts. The c ombustion reaction is first order with respect to methane and zero-ord er with respect to oxygen (>2 vol.% of oxygen). Carbon dioxide has no inhibitory effects on the combustion. The activity of the Pd-catalyst is decreased by a factor of 5 through deactivation at 1473 K for 768 h . The decrease in activity is linearly correlated to the decrease in s pecific surface area.