Nano- and macro-structured cerium oxide – Carbon nanotubes composites for the catalytic ozonation of organic pollutants in water

Temperature and nitrogen-induced modification of activated carbons for efficient catalytic ozonation of salicylic acid as a model emerging pollutant

The occurrence of emerging pollutants on effluents of wastewater treatment plants makes unfeasible their reutilization and consequently to comply with the sixth goal of 2030 Agenda for sustainable development. Thus, it is extremely important to find ways to remove these pollutants without compromising the quality of reclaimed water. Ozonation has been successfully explored for this purpose, but it still presents limitations towards some oxidant-resistant pollutants. To surpass this, the conversion of ozone (O3) into more reactive species is required, which can be accomplished by using catalysts. Carbon catalysts, such as activated carbons (ACs), represent a more environmentally attractive option than traditional metal-based catalysts, with the advantage of being easily modified to tune their textural and surface properties to the reaction chemistry. In this study, two different sources of ACs were tested in the catalytic ozonation of a frequently detected emerging pollutant: salicylic acid (SalAc). These ACs were submitted to thermal treatment under H2 and functionalization with N precursors, such as melamine and poly(ethyleneimine), to induce changes in the surface properties, especially in the nitrogen content. Although no correlation was found between the N-content and catalytic activity, the thermal treatment under H2 increased the mesopores surface area (Smeso), which reflected in greater catalytic activity. As that, the best-performing AC was the one with the highest Smeso, which revealed also to be resistant to O3 and able to convert O3 into more reactive species, evidenced by the capacity of oxalic acid, a well-known ozone-resistant by-product. The same AC was then submitted to three consecutive reutilization cycles and a more significant activity loss was observed in terms of SalAc degradation rate (⁓ 40%) then total organic carbon removal (⁓ 25%), from the first to the third cycle. This decline in efficiency was ascribed to the presence of by-products adhered to the catalyst surface, which impede its ability to react effectively with O3.

Insight into Enhanced Microwave Heating for Ammonia Synthesis: Effects of CNT on the Cs-Ru/CeO2 Catalyst

Ammonia is emerging as a potential decarbonized H₂ energy carrier when produced from renewable energy. The on-site production of liquid ammonia from stranded renewable energy can solve the current energy transportation challenges. The employment of microwave technology can produce the desired ammonia product at milder conditions with the supply of intermittent renewable energy sources. Our previous studies have indicated that the Cs-Ru/CeO₂ catalyst is a promising catalyst for microwave-driven ammonia synthesis. In this study, the Cs-Ru/CeO₂ catalyst mechanically mixed with carbon nanotubes (CNT) and chemically synthesized using coprecipitation and a hydrothermal method is investigated systematically at low temperatures and atmospheric pressure for microwave-assisted ammonia synthesis. Additionally, the combination of two Ru-based catalysts (Cs-Ru/CeO₂ and Cs-Ru/CNT) is studied as well. Mechanical mixing of Cs-Ru/CeO₂ with CNT exhibited superior activity as compared to the chemically synthesized Cs-Ru/CeO₂-CNT catalyst. Besides the enhancement in dielectric property, the probable synergistic effect leads to increased interfacial polarization at the interface of the mechanically mixed catalyst, improving the overall heating and ammonia production rate. Moreover, the combined Ru-based catalyst also exhibited higher activity as compared to their individual activity toward ammonia synthesis. Numerous characterization techniques were performed, including thermal imaging camera and dielectric measurements, to better understand microwave interaction with the composite catalysts.

Mechanism of catalytic ozonation in expanded graphite aqueous suspension for the degradation of organic acids

In this study, expanded graphite (EG) was prepared by the oxidation and intercalation of the natural flake graphite using perchloric acid and potassium permanganate at different expansion temperatures (300, 400, 500, and 600°C), and were characterized by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). EG prepared at 500°C was found to be highly effective for the mineralization of oxalic acid aqueous solution during ozonation at pH 3, which was ascribed to the formation of hydroxyl radicals from the surface reaction of surface hydroxyl groups on EG with ozone. The performance of expanded graphite in this catalytic system was basically unchanged after three repeated use. The presence of Cl^-, SO42-, HPO42-/H2PO4- and NO3- could inhibit the degradation of oxalic acid in catalytic ozonation with EG. Degradations of oxamic acid and pyruvic acid in catalytic ozonation with EG were pH-dependent, which were lower than that of oxalic acid. The degradations of oxalic acid and oxamic acid were identified as mineralization process by the determination of TOC, while pyruvic acid may transform into organic products such as acetic acid by O₃/EG. Manganese ion (Mn²⁺) could promote the degradation of oxalic acid by O₃/EG at pH 3 because permanganate was produced by O₃/EG in oxalic acid solution and then reacted with oxalic acid readily at acidic pH. Catalytic ozonation by EG exhibited great application potential for the destruction of refractory organic compounds.

Reinforced Nafion Membrane with Ultrathin MWCNTs/Ceria Layers for Durable Proton-Exchange Membrane Fuel Cells

For further commercializing proton-exchange membrane fuel cells, it is crucial to attain long-term durability while achieving high performance. In this study, a strategy for modifying commercial Nafion membranes by introducing ultrathin multiwalled carbon nanotubes (MWCNTs)/CeO₂ layers on both sides of the membrane was developed to construct a mechanically and chemically reinforced membrane electrode assembly. The dispersion properties of the MWCNTs were greatly improved through chemical modification with acid treatment, and the mixed solution of MWCNTs/CeO₂ was uniformly prepared through a high-energy ball-milling process. By employing a spray-coating technique, the ultrathin MWCNTs/CeO₂ layers were introduced onto the membrane surfaces without any agglomeration problem because the solvent rapidly evaporated during the layer-by-layer stacking process. These ultrathin and highly dispersed MWCNTs/CeO₂ layers effectively reinforced the mechanical properties and chemical durability of the membrane while minimizing the performance drop despite their non-ion-conducting properties. The characteristics of the MWCNTs/CeO₂ layers and the reinforced Nafion membrane were investigated using various in situ and ex situ measurement techniques; in addition, electrochemical measurements for fuel cells were conducted.