Sucrose synthesis of nanoparticulate alumina

γ -Al2O3 is frequently an unwanted intermediate phase in the conversion of nanoparticulate amorphous alumina into nanoparticulate α-Al2O3. γ -Al2O3 has a relatively disordered structure, but usually requires temperatures in the order of 1100 ◦C to transform completely to the stable α-Al2O3 phase [2]. Dispersion of cations in a polymer matrix has been widely used as a “chimie douce” method for the production of oxide materials at low temperatures. The “Pechini” process, where citric acid and ethylene glycol are co-polymerized to form a resin is a particularly common variant [3]. A highly acidic sucrose solution can self-polymerize or cross-link with polyvinyl alcohol (PVA) to form a heavier and more 3 dimensional resin [4]. The charred polymer also acts as a fuel during calcination, raising the local temperature briefly to much higher temperatures than the equilibrium temperature in the furnace. The extent of cation dispersion in a matrix has been seen to affect phase composition and crystallite size in other systems [3]. Consequently a study was undertaken to ascertain the effect of drastically increasing the dispersion of Al3+ cations in the matrix by increasing the sucrose content. This would be expected to increase the surface area of the material, but as the sucrose : Al3+ molar ratio is greater than 18 : 1, calcining at 600 ◦C should still produce α-Al2O3 according to the hypothesis of Das et al. [1]. The resin was produced by adding 250 ml of 10 M sucrose solution acidified to a pH of approximately 1 with nitric acid, to 3.75 g of Al(NO3)3 · 9H2O. This formulation yielded a 250 : 1 molar ratio. The intimate mixture was caramelized on a hot plate at approximately 250 ◦C to reduce the aqueous solution to a thick syrup. Afterwards, it was placed in an air-circulating oven at 200 ◦C for 18 h resulting in a dehydrated charred resin. The resulting carbon-rich precursor was calcined in a muffle furnace at 600 ◦C for 24 h to produce the aluminum oxide material. Samples of this material were further calcined in a zirconia crucible in air at temperatures of 1050, 1150 and 1250 ◦C to examine the ease of α-Al2O3 formation from this very high surface area γ -Al2O3. X-ray diffraction data were obtained using a Bruker D8 diffractometer equipped with a HISTAR 2dimensional area detector at a sample-detector distance of 15.3 cm. The tube angle was fixed at 10◦ using a 1 mm diameter snout collimator, and frames taken using Cu Kα radiation at detector angles of 15, 30 and 45◦ for 30 min durations. The samples were mounted on a quartz zero background holder. 1-D powder patterns were obtained by integrating the 2-D data across a fixed chi range, and merging the three resulting patterns into one. The instrument broadening function was determined by obtaining frames from the NIST 660a LaB6 standard, and crystallite size obtained by peak fitting with the Bruker TOPAS software [5]. Scanning electron microscope (SEM) micrographs were taken of the alumina using a Cambridge Stereoscan 250. Samples were prepared with carbon adhesives on aluminum stubs, and then sputter-coated with gold. The surface area values of the powders were obtained using BET analysis with nitrogen gas as the absorbate. A Quantachrome Quantasorb system was used for the sorption measurements. After calcination of the charred precursor at 600 ◦C, the material obtained was a white, fluffy powder with a very low density. On examination of this material by SEM, a foil-like microstructure (Fig. 1) was observed. This microstructure probably resulted from preferential growth around the resin pore surfaces. Attempts to resolve the individual particles with a high resolution Field emission gun (FEG) SEM tentatively suggested a particle size of 10–20 nm, however it was not possible to obtain well-resolved images due to charging of the sample. The surface area of the material was found to be approximately 290 m2/g, as opposed to the 194 m2/g reported by Das et al. [1]. Given the greater organic content in this precursor, this increase was not unexpected, but may be of interest given the use of γ -Al2O3 as a catalyst support. X-ray diffraction of the 600 ◦C material yielded a poor crystalline pattern (Fig. 2), which could be assigned as γ -Al2O3. Firing at 1050 ◦C for 30 min yielded a more ordered γ -Al2O3 pattern, while firing for 60 min gave rise to a number of spots corresponding to a few crystallites of α-Al2O3. The significance of spots in a frame is discussed below. Increasing the temperature to 1150 ◦C for 60 min led to the formation of a two phase mixture of γ -Al2O3 and nanocrystalline α-Al2O3 powder with a 17.5 nm crystallite size. Firing at 1250 ◦C for 60 min led to the formation of 25 nm α-Al2O3 with β-Al2O3 present as a minor phase. β-Al2O3 does not exist as a pure alumina polymorph [6], so the presence of β-Al2O3 could be due to trace sodium impurities, producing phases such as NaAl11O17.