Al18B4O33 aluminium borate: A new efficient support for palladium in the high temperature catalytic combustion of methane

Well crystallised aluminium borate Al18B4O33 has been synthesised from alumina and boric acid with a BET area of 18 m2/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al18B4O33 was prepared by conventional impregnation of Pd(NO3)2 aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al18B4O33 in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al2O3. Pd/Al18B4O33 exhibited a much lower activity than Pd/Al2O3 when treated in hydrogen at 500 °C or aged in O2/H2O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al18B4O33 which became as active as Pd/Al2O3. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al2O3 and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al18B4O33. The existence of two distinct types of PdO species formed on Al18B4O33 and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O2.