Energy efficient microwave synthesis of mesoporous Ce0.5M0.5O2 (Ti, Zr, Hf) nanoparticles for low temperature CO oxidation in an ionic liquid - a comparative study

Ce0.5M0.5O2 (M = Ti, Zr, Hf) nanoparticles have been successfully synthesized by microwave irradiation in the ionic liquid [C(4)mim][Tf2N] (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide). The morphology, crystallinity, and chemical composition of the obtained materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and N-2-adsorption measurements. XRD and Raman spectroscopy analyses confirmed the formation of solid solutions with cubic fluorite structure. The catalytic activities of the Ce0.5M0.5O2 (M = Ti, Zr, Hf) nanoparticles were investigated in the low-temperature oxidation of CO. Ce0.5Zr0.5O2 nanospheres exhibit the best performance (100% conversion at 350 degrees C), followed by Ce0.5Hf0.5O2 (55% conversion at 360 degrees C) and Ce0.5Ti0.5O2 (11% conversion at 350 degrees C). Heating the as-prepared Ce0.5Zr0.5O2 to 600 degrees C for extended time leads to a decrease in surface area and, as expected decreased catalytic activity. Depending on the ionic liquid the obtained Ce0.5Zr0.5O2 exhibits different morphologies, varying from nano-spheres in [C(4)mim][Tf2N] and [P-66614][Tf2N] (P66614 = trishexyltetradecyl-phosphonium) to sheet-like assemblies in [C(3)mimOH][Tf2N] (C(3)mimOH = 1-(3-hydroxypropyl)-3-methylimidazolium). The microwave synthesis superiority to other heating methods like sonochemical synthesis and conventional heating was proven by comparative experiments where the catalytic activity of Ce0.5Zr0.5O2 obtained by alternate methods such as conventional heating was found to be poorer than that of the microwave-synthesised material.