Size Control of Iron Oxide Nanoparticles Using Reverse Microemulsion Method: Morphology, Reduction, and Catalytic Activity in CO Hydrogenation

Iron oxide nanoparticles were prepared by microemulsion method and evaluated in Fischer-Tropsch synthesis. The precipitation process was performed in a single-phase microemulsion operating region. Different HLB values of surfactant were prepared by mixing of sodium dodecyl sulfate (SDS) and Triton X-100. Transmission electron microscopy (TEM), surface area, pore volume, average pore diameter, pore size distribution, and XRD patterns were used to analyze size distribution, shape, and structure of precipitated hematite nanoparticles. Furthermore, temperature programmed reduction (TPR) and catalytic activity in CO hydrogenation were implemented to assess the performance of the samples. It was found that methane and CO2 selectivity and also the syngas conversion increased as the HLB value of surfactant decreased. In addition, the selectivity to heavy hydrocarbons and chain growth probability ( ) decreased by decreasing the catalyst crystal size. 1. Introduction Nanoscale materials exhibited novel properties including quantum size effect on photochemistry, nonlinear optical properties of semiconductor and new catalytic properties of metallic nanoparticles [1]. Therefore, synthesis of nanoparticles is of a great interest field during the past few years. As compared to their bulk counterparts, nanoparticles often have superior or even new catalytic properties following from their nanometer size [2]. Hence, the size of the nanoparticles is pivotal in determining the catalytic properties and understanding how size may affect the catalytic properties remains a central goal in nanocatalysis research [3]. Many methods have been reported for preparation of nanoparticles [2, 3]. Preparation of metallic nanoparticles using microemulsion has been well established [4–7]. Due to the specific structure of the microemulsion system, it was expected as a suitable environment for producing small metal nanoparticles with narrow size distribution. A microemulsion is defined as a system of water, oil, and amphiphile (surfactant) [4–7]. In some respects, microemulsions can be considered as small-scale versions of emulsions, that is, droplet type dispersions either of oil-in-water (o/w) or of water-in-oil (w/o), with a size range in the order of 5？50？nm in drop radius. In the hydrophilic interior of these droplets, a certain amount of water-soluble material can be dissolved, for example, transition metal salts that then serve as precursor(s) for the final metal particles. As for simple aqueous systems, microemulsion formation is dependent on surfactant type and structure. A