A study on enhancing thermal stability of alumina

Abstract

The use of temperature resistant support is a prerequisite for successful catalysts prepared to be used in reactions which need high temperature, like Dry and Mixed Reforming of Methane, catalytic oxidation, and combustion reactions. Alumina is a common and cheap catalyst support for achieving high metal dispersion, e.g., γ-Al2O3 has a surface area of 220 m2/g. However, as high temperature leads to phase change from high- to low-SA alumina phases, i.e. γδθα, preventing surface area loss requires to shift the phase change temperatures up to higher levels. In the current study, the thermal stability of commercial γ-Al2O3 samples was enhanced via doping with stabilizers La, Ba and Pt through application of various preparation and pre-treatment parameters such as; pre-doping temperature, vacuum pressure, drying temperature, pH of the precursor solution, and heat treatment applied upon doping. Total of 142 samples were prepared in 19 sets of samples, and all samples were characterized by their surface area, pore volume and pore size distribution. Selected samples chosen to represent each set were furtherly tested by XRD and Raman spectroscopy. Reference γ-Al2O3 calcined at the highest temperature applied, 1150 oC, for 2 hours had BET surface area of 8.81 m2/g. Under the same conditions, thermally resistant 1%La/Al2O3 and 1%Ba/Al2O3 samples prepared by incipient-to-wetness impregnation have BET surface areas of 61.74 m2/g and 38.26 m2/g, respectively. Results indicated that the effect of vacuum pressure and drying temperature applied during preparation were ambiguous. Changing only the pH of impregnation solution from pH1(acidic) to pH2(basic) led to decreased surface area values. While combination of pH regulation and post heat treatment significantly have a stabilizing effect leading 68.27 m2/g surface area for 1%La/Al2O3 upon calcination at 1150 °C for 2 hours, further elaboration of pre-doping temperature applied led to the highest BET surface area of 70 m2/g. XRD analysis confirmed the effect of pre-doping temperature on shifting the phase transfer temperature of alumina.