Matías Gastón Rinaudo

In this work, Cu/TiO₂ catalysts were prepared by several high-energy ball milling strategies (dry and semi-wet milling) using different copper reagents and compared with a sample synthesized by a conventional impregnation method. Crystal structures were identified by means of X-ray Diffraction (XRD), including anatase, rutile, high-pressure TiO₂ (II) and W species due to mill vial erosion at some conditions, highlighting the effect of a copper precursor on the rate of titania polymorphic transformation. Specific Surface Area (S_BET) values were calculated from N₂ physisorption, showing a correlation between the energy supplied to the powder and the milling conditions. Moreover, Scanning Electron Microscopy (SEM) was able to display the morphologies while a semi-quantification of present elements could be performed by Electron Dispersive X-ray Spectroscopy (EDS). Catalysts obtained through this green and one-pot process could be suitable for a variety of reactions, including CO₂ hydrogenation and glycerol valorization.

In this study, commercial Multi-Walled Carbon Nanotubes (MWCNTs) were subjected to two treatments: oxidation in a liquid phase using concentrated nitric acid and oxidation in a gas phase using air flow. Cu-based catalysts were prepared by ultrasound-assisted impregnation of each support in order to obtain a metal composition of 1 wt%, followed by drying and calcination under an inert atmosphere. Commercial MWCNTs without treatment were used as references. The crystalline structure of the samples was characterized by X-ray diffraction (XRD), while their textural properties were analyzed by N₂ physisorption (BET). The distinctive physicochemical properties achieved in the MWCNT supports could be related to the nature of the functionalization treatment that was used, with consequent effects on the features of the Cu catalysts.

How important is the support during the rational design of a catalyst? Herein, doped ceria (Zr; Pr and Tb) was used as an active support to prepare Pt catalysts (0.5 wt%) for glycerol selective oxidation. A thorough characterization of achieved catalytic systems showed that the nature of doping elements led to different physicochemical properties. The presence of surface Pr³⁺ and Tb³⁺ not only increased oxygen vacancies but also electron mobility, modifying the oxidation state of platinum particles. The redox properties of the catalyst were also affected, achieving a close interaction between the support and metal particles even in the form of Pt-O-Pr(Tb) solid solutions. Furthermore, the combination of medium-sized metal particle dispersion, strong metal–support interaction and a synergy between the amount of oxygen vacancies and Pt⁰, observed in the Pt/CeTb catalyst, led to a high turnover frequency (TOF) and increased selectivity to glyceric acid. Thus, the present study reveals how a simple structural modification of active supports, such as cerium oxide, by means of doping elements is capable of improving the catalytic performance during glycerol selective oxidation, avoiding the cumbersome methods of synthesis and activation treatments.

Eggshell waste is a biodegradable residue composed of more than 90% CaCO₃, which makes it a great candidate to be converted into functional materials for diverse applications. Herein, domestic hen eggshell waste was dried and calcined in muffle under air at different temperatures (300 to 900 °C) and times (1 or 3 h) to achieve distinctive calcium species, compositions, and solid-phase transformations. The crystal structures achieved were characterized by X-ray diffraction (XRD), evidencing the transformation from CaCO₃ (calcite) to CaO (lime) at high temperatures and the formation of Ca(OH)₂ (slaked lime) due to the hydration of CaO facilitated by ambient water molecules. Considering this preliminary results, prepared solids could be useful as low-cost and metal-free (unsupported) catalysts for different heterogeneous reactions, such as the transesterification of vegetable oil and/or glycerol, where the presence of basic sites are needed.

Mechanochemical activation, by means of high-energy ball milling, was applied to CeO₂ as a strategy to enhance its physicochemical properties. Different milling times were screened to evaluate their effect on ceria. The fluorite-type structure of cerianite was maintained in all cases, no matter the amount of energy introduced by the milling process, as observed via X-ray diffraction (XRD). A decrease in crystallite sizes and a consequent increase in specific surface area (S_BET) were observed through XRD and N₂ sorption (BET method). Pore diameters and total pore volumes were also in line with the duration of CeO₂ milling. Moreover, redox properties and oxygen mobility studied through H₂ temperature-programmed reduction (H₂- TPR) showed an increase in reducibility with milling time, including signals of both bulk and surface ceria, due to the greater number of defects and/or oxygen vacancies achieved by mechanochemical activation. The obtained features could play an essential role in terms of metal–support interaction, reactant adsorption and/or oxygen supply during catalytic reactions. Thus, high-energy ball milling is a useful, simple and green method for the design of materials with catalytic applications.

Palladium nanoparticles were supported on unusual mixtures of anatase, TiO₂ (II) and rutile titania phases by wet impregnation, obtaining catalysts with metal contents of ca. 0.25 wt % labeled Pd/Ti5, Pd/Ti45, and Pd/Ti120. Crystalline structures were confirmed by X-ray diffraction. Pd particle sizes in the range of 4–20 nm were observed by scanning-transmission electron Microscopy. External surface areas (S_BET) in the range 10–17 m² g⁻¹ were higher enough to achieve a good distribution of palladium over titanium oxide outer surface, as evidenced by energy-dispersive X-ray spectroscopy elemental profiles. Pd⁰/Pd^δ+ atomic ratio measured by X-ray photoelectron spectroscopy showed a decrease from Pd/Ti5 to Pd/Ti120, in line with the decrease in anatase phase present in the catalysts. This behavior suggested that palladium tended to form more TiPd_xO structures in Pd/Ti5 whilst PdO_x structures were more likely to be present on supports with greater amounts of TiO₂ (II) and rutile, due to the distinct metal–support interactions. An increase in reducibility and oxygen mobility from Pd/Ti5 to Pd/Ti120 was observed by temperature programmed measurements and associated to the different high-energy ball milled supports. Catalysts with improved properties reported herein could exhibit an excellent performance in oxidation reactions, e.g., glycerol selective oxidation.