In this study, nickel supported on various metal oxides were prepared by simple impregnation and the performance for CO₂ methanation was tested. The oxide supports were all prepared by thermal decomposition of metal salts to provide comparable oxide properties such as surface area. Among the investigated oxides, nickel supported on CeO₂ and Y₂O₃ showed the highest CO₂ conversion of 90% at 320 °C with highest CH₄ selectivity of 99%. The order of catalyst activity (X_CO2@320°C) was reported: Ni/CeO₂ ~ Ni/Y₂O₃ > > Ni/La₂O₃ > Ni/ZrO₂ > Ni/Al₂O₃ > Ni/MgO > Ni/CaO > > Ni/MnO. The physicochemical properties of the catalysts were analyzed by TEM, BET, XRD, ICP, H₂-TPR, CO₂-TPD, H₂ chemisorption, TGA, Raman, and XPS. From the characterization results, the catalyst activity was independent to specific surface area of catalyst and crystallite size of Ni. The amount of oxygen vacancies and weak-to-medium basic sites exhibited major roles for enhancing catalyst activity. The CeO₂ and Y₂O₃ as reducible oxide supports not only provided abundant oxygen vacancies / basic sites, but also promoted Ni dispersion with appropriate interaction between metal and support, resulting in higher reducibility at low temperature. The reduction of catalyst at high temperature can significantly improve the performance of Ni supported on non-reducible MgO. However, the Ni/CeO₂ and Ni/Y₂O₃ reduced at high temperature suffered from coalescence of CeO₂ and Y₂O₃, though Ni crystallite sizes are well preserved from sintering.

Natural kaolin-based Ni catalysts have been developed for low-temperature CO₂ methanation. The catalysts were prepared via a one-step co-impregnation of Ni and Ce onto a natural kaolin-derived metakaolin using a microwave-assisted hydrothermal method as an acid-/base-free synthesis method. The influences of microwave irradiation and Ce promotion on the catalytic enhancement including the CO₂ conversion, CH₄ selectivity, and CH₄ yield were experimentally investigated by a catalytic test of as-prepared catalysts in a fixed-bed tubular reactor. The relationship between the catalyst properties and its methanation activities was revealed by various characterization techniques including X-ray fluorescence, X-ray diffraction, Brunauer–Emmett–Teller, scanning electron microscopy, selected area electron diffraction, transmission electron microscopy, elemental mapping, H₂ temperature-programmed reduction, and X-ray absorption near-edge structure analyses. Among the two enhancement methods, microwave and Ce promotion, the microwave-assisted synthesis could produce a catalyst containing highly dispersed Ni particles with a smaller Ni crystallite size and higher catalyst reducibility, resulting in a higher CO₂ conversion from 1.6 to 7.5% and a better CH₄ selectivity from 76.3 to 79.9% at 300 °C. Meanwhile, the enhancement by Ce addition exhibited a great improvement on the catalyst activities. It was experimentally found that the CO₂ conversion increased approximately 7-fold from 7.5 to 52.9%, while the CH₄ selectivity significantly improved from 79.9 to 98.0% at 300 °C. Though the microwave-assisted synthesis could further improve the catalyst activities of Ce-promoted catalysts, the Ce addition exhibited a more prominent impact than the microwave enhancement. Cerium oxide (CeO₂) improved the catalyst activities through mechanisms of higher CO₂ adsorption capacity with its basic sites and the unique structure of CeO₂ with a reversible valence change of Ce⁴⁺ and Ce³⁺ and high oxygen vacancies. However, it was found that the catalyst prepared by microwave-assisted synthesis and Ce promotion proved to be the optimum catalyst in this study. Therefore, the present work demonstrated the potential to synthesize a nickel-based catalyst with improved catalytic activities by adding a small amount of Ce as a catalytic promoter and employing microwave irradiation for improving the Ni dispersion.

This work aims to enhance the value of palm empty fruit bunches (EFBs), an abundant residue from the palm oil industry, as a precursor for the synthesis of luminescent carbon dots (CDs). The mechanism of fIuorimetric sensing using carbon dots for either enhancing or quenching photoluminescence properties when binding with analytes is useful for the detection of ultra-low amounts of analytes. This study revealed that EFB-derived CDs via hydrothermal synthesis exceptionally exhibited luminescence properties. In addition, surface modification for specific binding to a target molecule substantially augmented their PL characteristics. Among the different nitrogen and sulfur (N and S) doping agents used, including urea (U), sulfate (S), p-phenylenediamine (P), and sodium thiosulfate (TS), the results showed that PTS-CDs from the co-doping of p-phenylenediamine and sodium thiosulfate exhibited the highest PL properties. From this study on the fluorimetric sensing of several metal ions, PTS-CDs could effectively detect Fe³⁺ with the highest selectivity by fluorescence quenching to 79.1% at a limit of detection (LOD) of 0.1 µmol L⁻¹. The PL quenching of PTS-CDs was linearly correlated with the wide range of Fe³⁺ concentration, ranging from 5 to 400 µmol L⁻¹ (R² = 0.9933).