Synthesis of metal oxide nanoparticles using supercritical fluids from a green engineering perspective

Abstract

Nano-sized materials have special and unique properties which make them useful in various fields such as optical, electrical, magnetic, and catalytic science and engineering. Industrial use of nanomaterials will require many standards such as particle size uniformity, cost-effectiveness, environmental friendliness, and reproducibility. One of the most widely known methods of nanomaterials synthesis is chemical synthetic route which involves techniques such as inverse micelles and non-hydrolytic sol-gel. These chemical synthetic methods use toxic organic solvents, involve long reaction time, require complicated and inefficient separation and purification steps, and most of all, is only available in batch mode. This dissertation reports on the synthesis of metal and metal oxide nanoparticles using supercritical fluids. The aim is to develop more greener methods using supercritical fluids and natural bioresources to specifically synthesize certain metal or metal oxide nanoparticles. The advantages of the greener methods developed in this thesis are the low-cost of the reagents, short reaction time, simple reaction and downstream processing, and applicability of continuous production using plug flow reactors. In part one (Chapter 2), hydrothermal synthesis of cerium oxide nanocrystals was carried out with in-situ surface modification using edible vegetable oils such as soybean oil and palm oil. X-ray diffraction (XRD) patterns demonstrated that pure CeO2 was formed. Transmission electron microscopy (TEM) showed single crystalline nature with stable dispersion. Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA) results confirmed the chemical adsorption of fatty acids onto the CeO2 surfaces. This new technique employs vegetable oils as surface modifiers which are of low-cost and is abundant in nature. The second part (Chapter 3) involves a simultaneous synthesis of biodiesel and metal (oxide) nanoparticles using supercritical methanol. The precursor for metal (oxide) nanoparticles was in nitrate form. Thus formed metal (oxide) nanoparticles catalyzed the transesterification of vegetable oil into fatty acid methyl esters (FAME, biodiesel). In addition, the catalysts also reduced the reaction temperature and time. The highest FAME yield 96.88 wt% was obtained at 250 ˚C, 350 bar, 10 min with 3 oil wt% zinc nitrate loading. FT-IR spectra revealed that the metal (oxide) nanoparticles surfaces were modified by FAME molecules. The nanoparticles sizes were smaller than those synthesized in neat methanol. This newly developed simultaneous synthesis process provides an economic advantage as it reduces operational temperature and time, and also produces metal compounds as by-products. The third part (Chapter 4) utilizes glycerol as a reducing agent for synthesis of metals and metal oxide nanoparticles with supercritical water at 400 ˚C and 300 bar. Conventionally, formic acid or hydrazine were used as reducing agents for hydrothermal synthesis of inorganic materials. Formic acid and hydrazine are toxic, dangerous, chemically unstable and expensive. This chapter examines the applicability of glycerol as an environmentally benign and cheap reducing agent for hydrothermal syntheses. XRD patterns showed that silver, copper, and nickel nitrates were reduced to zero-valent metal nanoparticles. On the other hand, cobalt, iron, and manganese nitrates were partially reduced into low-valent metal oxides, which indicates glycerols role as an anti-oxidant. The prepared metals were generally smaller in size than their corresponding metal oxides. Glycerol is odorless, non-toxic, non-flammable, easy to handle, environmentally friendly, and readily available compared to other reducing agents. This new glycerol hydrothermal reduction method is yet, the greenest reductive materials processing in terms of chemical reagents. The three parts of this dissertation are devoted in producing metals, metal oxides, and biodiesel using the cleanest, safest, and natural precursors such as water, methanol, vegetable oil, and glycerol. The main goal of the study is to replace the conventional chemical syntheses that use toxic and dangerous chemical reagents.