Continuous and batch hydrothermal synthesis of metal oxide nanoparticles and metal oxide-activated carbon nanocomposites
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Hydrothermal synthesis is a widely used technique for the preparation of fine particles. It can be carried out in batch or flow systems, although most studies have used batch reactors below 200 C. More recently, however, continuous hydrothermal synthesis has been employed in near- and supercritical water to obtain metal oxide particles. This technique offers tremendous promise for control of particle characteristics due to the rapidly changing properties of water with temperature and pressure in the critical region. However, the role of temperature in this process is not completely understood. Moreover, agglomeration of particles remains a problem in both batch and continuous hydrothermal techniques. This work is concerned with the use of continuous hydrothermal synthesis at near-critical and supercritical conditions to obtain iron oxide and lithium iron phosphate nanoparticles. Factors that affect size, size-distribution, and morphology of nanoparticles were investigated and the results have been used to resolve differences in the literature concerning the effect of temperature on particle size. It was shown that agglomeration can be minimized by using a protective polymer coating and this appears to be an effective method to control particle size. The continuous hydrothermal technique was also extended to materials other than metal oxides by synthesizing lithium iron phosphate. Differences in the particle sizes obtained using the batch and continuous methods were shown to be due to the different mechanisms of particle formation in the two techniques. Better particle characteristics (size, size distribution and morphology) were obtained using the continuous hydrothermal technique than using the batch hydrothermal method. Iron oxide nanoparticles were also deposited on the surface and in the pores of activated carbon pellets in a batch reactor in order to minimize agglomeration of particles. The resulting iron oxide activated carbon nanocomposites exhibited significant catalytic performance in the oxidation of propanal. Therefore, the use of supercritical water to deposit metal oxide particles on hydrophobic surfaces offers promise for carbon-supported catalyst preparation without the use of toxic or noxious solvents.