Catalytic Properties of Pd-Supported Hexaaluminate Catalysts for High-Temperature Catalytic Combustion

The catalytic properties of palladium supported an hexaaluminates (Sr 0.8La 0.2XAl 11O 19, X = Al and Mn) was studied for use in high-temperature catalytic combustion. The activity of Pd/Sr 0.8La 0.2A 12O 19 increased initially with a rise in temperature, hut decreased at high temperatures (ca. 700°C). The drop in catalytic activity became pronounced when the supported Pd particles were sintered into large agglomerates after calcination above 1000°C. From in situ XRD, TPD, and TPRO, it was revealed that the activity drop accompanies dissociated of PdO into metallic Pd species. Since the dissociation of PdO is a thermodynamic function of oxygen partial pressure, the temperature at the conversion drop appeared was influenced by the oxygen concentration in the combustion reaction. The dissociation of PdO appears to result in removal of adsorbed oxygen species necessary for the catalytic reaction. On the other hand, such a significant drop in catalytic activity can be avoided by the use of Mn-substituted hexaaluminate (X = Mn) as a catalyst support. The catalytic activity of Mn-substituted hexaaluminate is effective in compensating for the drop in activity of Pd so that a stable combustion reaction can be attained in a whole temperature range.