Synthesis, Characterization and Energy Applications of Supported Mixed-Metal Oxides

Abstract

The researchers are continuously investigating sustainable energy sources because of the significant rise in the usage of fossil fuels and the associated environmental risks caused by greenhouse gases. Hydrogen and biodiesel are among the sustainable fuel routes to encounter rapidly growing energy demands. Noble metal-based catalysts have so far been developed as stateof- the-art materials for hydrogen production while strong acids/bases are used for biodiesel production from different feedstocks. However, the high cost of noble metals and the corrosion/soap formation by strong acids/bases limit the practical applications of these catalysts. This research work focuses on the synthesis of environmentally friendly, cost effective and stable mixed-metal oxides over different porous materials as catalysts for water splitting and biofuel production applications. For the water splitting application, bifunctional composites based on post transition-metal oxide (Bi2O3) and 3d-transition metal oxides (Ni, Co, Fe, Mn) were synthesized and characterized through multitude analytical techniques including powder X-ray diffraction analysis (PXRD), scanning electron microscopy coupled energy dispersive X-ray analysis (SEM/EDX) and X-ray photoelectron spectroscopy (XPS). Composites were supported on nickel foam (NiF) and carbon nanotube fiber (CNTF) as electrode materials for oxygen evolution reaction (OER) in basic medium (1M KOH) while OER/hydrogen evolution reaction (HER) in neutral (1M PBS) medium. The electrochemical OER results revealed that Bi2O3/NiO-CNTF based electrodes outperformed in 1M KOH (467 mV@150 mA/cm2) compared to other NiF and CNTF based fabricated electrodes. Similarly, for OER and HER in 1M PBS, Bi2O3/NiO-CNTF also outperformed (164 mV@20 mA/cm2 and 268 mV@50 mA/cm2, respectively) than the other fabricated electrodes. Keeping in view the performance of mixed metal oxides, three different transition metal-based polyoxometalates (POMs) were also synthesized and fabricated on the surface of CNTF. The electrochemical water oxidation activity in 1M KOH revealed that Ni based POM was more efficient (260 mV@10 mA/cm2) than other synthesized analogues. For biodiesel production, bifunctional catalysts based on tungsten oxide (WO3) over mesoporous material MCM-41 impregnated with alkaline earth metal (Mg, Ca, Sr, Ba) oxides were synthesized and characterized through PXRD, SEM-EDX and Brunauer-Emmett-Teller (BET) and Barrett-Joyner- Halenda (BJH) surface area, pore size and volume analysis. These composites were tested for biodiesel production using corn and waste cooking oil as feedstocks. The results revealed that xvi strontium-based composite was an efficient bifunctional (esterification/trans-esterification simultaneously) catalyst for biodiesel production with 96% and 93% yield from corn oil and waste cooking oil, respectively. Through 1H-NMR spectroscopy, the obtained biodiesel was also characterized.