Synthesis of Nanoporous Metal Oxide Particles by a New Inorganic Matrix Spray Pyrolysis Method

In this paper we describe a new approach for the formation of nanoporous particles in which a thermally stable, water-soluble, and therefore easily leached second component is employed during spray pyrolysis to produce a composite particle. We demonstrate the new synthetic approach for synthesizing nanoporous aluminum oxide (Al 2 O 3 ) by spray pyrolysis of aluminum nitrate (Al(NO 3 ) 3 ·9H 2 O) and sodium chloride (NaCl) solutions. The characteristics of the nanoporous material such as surface area, pore volume, and particle morphology are investigated as a function of relative humidity, temperature, and salt fractions. Specific surface area of the product particles increased significantly from 10 m 2 /g for the produced composite particles to over 370 m 2 /g for the remaining Al 2 O 3 particles following aqueous leaching to remove the NaCl filler. Nanoporous particles could be produced at relatively low temperature (<550 °C) and high relative humidity (or low evaporation rate of aerosolized droplets) when followed by the aqueous leaching process of final product powder. However, we observed only solid particles at reactor temperatures above 700 °C and residence times of about 1 s. We attribute this behavior to evaporation of salt from the composite particles as evidenced through particles size distribution measurements, which showed the formation of a nuclei mode at these temperatures. At these higher temperatures we suspect a collapse of the porous network in the absence of the supporting salt network and results in particles with low surface area. Intermediate levels of salt fractions result in the formation of nanoporous particles, while high salt fractions result in a loosely connected network and the formation of individual nanoparticles in the salt matrix.