Platinum Recovered from Automotive Heavy-Duty Diesel Engine Exhaust Systems in Hydrometallurgical Operation

The current study is focused on platinum recovery from the secondary sources of end-of-life heavy-duty diesel oxidation catalysts (DOCs) and heavy-duty catalyzed diesel particulate filters (c-DPFs) in order to reduce the supply–demand gap within the European Union. The extraction of platinum was based on a hydrometallurgical single-step low acidity leaching system (HCl-H2O2-NaCl) and a calcination step that takes place before the leaching process. The parameters of calcination and leaching process were thoroughly investigated in order to optimize recovery efficiency. The optimized results proved that a calcination step (at 800 °C for 2 h) improves the recovery efficiency by a factor of 40%. In addition, optimal Pt recovery yield was achieved after 3 h of leaching at 70 °C, with a solid-to-liquid (S/L) ratio of 70 g/100 mL (70%) and 3 M HCl-1% vol H2O2-4.5 M NaCl as leaching conditions. The optimum Pt recovery yield was 95% and 75% for DOC and c-DPF, respectively. Since the secondary feedstock investigated is highly concentrated in platinum, the pregnant solution can be used as a precursor for developing new Pt-based catalytic systems.

Crystalline ZnO Photocatalysts Prepared at Ambient Temperature: Influence of Morphology on p-Nitrophenol Degradation in Water

Since the Industrial Revolution, technological advances have generated enormous emissions of various pollutants affecting all ecosystems. The detection and degradation of pollutants has therefore become a critical issue. More than 59 different remediation technologies have already been developed, such as biological remediation, and physicochemical and electrochemical methods. Among these techniques, advanced oxidation processes (AOPs) have been popularized in the treatment of wastewater. The use of ZnO as a photocatalyst for water remediation has been developing fast in recent years. In this work, the goals are to produce ZnO photocatalysts with different morphologies, by using a green sol-gel process, and to study both the influence of the synthesis parameters on the resulting morphology, and the influence of these different morphologies on the photocatalytic activity, for the degradation of an organic pollutant in water. Multiple morphologies were produced (nanotubes, nanorods, nanospheres), with the same crystalline phase (wurtzite). The most important parameter controlling the shape and size was found to be pH. The photoactivity study on a model of pollutant degradation shows that the resulting activity is mainly governed by the specific surface area of the material. A comparison with a commercial TiO2 photocatalyst (Evonik P25) showed that the best ZnO produced with this green process can reach similar photoactivity without a calcination step.

Eco-Friendly Colloidal Aqueous Sol-Gel Process for TiO2 Synthesis: The Peptization Method to Obtain Crystalline and Photoactive Materials at Low Temperature

This work reviews an eco-friendly process for producing TiO2 via colloidal aqueous sol–gel synthesis, resulting in crystalline materials without a calcination step. Three types of colloidal aqueous TiO2 are reviewed: the as-synthesized type obtained directly after synthesis, without any specific treatment; the calcined, obtained after a subsequent calcination step; and the hydrothermal, obtained after a specific autoclave treatment. This eco-friendly process is based on the hydrolysis of a Ti precursor in excess of water, followed by the peptization of the precipitated TiO2. Compared to classical TiO2 synthesis, this method results in crystalline TiO2 nanoparticles without any thermal treatment and uses only small amounts of organic chemicals. Depending on the synthesis parameters, the three crystalline phases of TiO2 (anatase, brookite, and rutile) can be obtained. The morphology of the nanoparticles can also be tailored by the synthesis parameters. The most important parameter is the peptizing agent. Indeed, depending on its acidic or basic character and also on its amount, it can modulate the crystallinity and morphology of TiO2. Colloidal aqueous TiO2 photocatalysts are mainly being used in various photocatalytic reactions for organic pollutant degradation. The as-synthesized materials seem to have equivalent photocatalytic efficiency to the photocatalysts post-treated with thermal treatments and the commercial Evonik Aeroxide P25, which is produced by a high-temperature process. Indeed, as-prepared, the TiO2 photocatalysts present a high specific surface area and crystalline phases. Emerging applications are also referenced, such as elaborating catalysts for fuel cells, nanocomposite drug delivery systems, or the inkjet printing of microstructures. Only a few works have explored these new properties, giving a lot of potential avenues for studying this eco-friendly TiO2 synthesis method for innovative implementations.

Fabrication of ZnO and TiO2 Nanotubes via Flexible Electro-Spun Nanofibers for Photocatalytic Applications

Web-like architectures of ZnO and TiO2 nanotubes were fabricated based on a three-step process of templating polymer nanofibers produced by electrospinning (step 1). The electrospun polymer nanofibers were covered by radio-frequency magnetron sputtering with thin layers of semiconducting materials (step 2), with FESEM observations proving uniform deposits over their entire surface. ZnO or TiO2 nanotubes were obtained by subsequent calcination (step 3). XRD measurements proved that the nanotubes were of a single crystalline phase (wurtzite for ZnO and anatase for TiO2) and that no other crystalline phases appeared. No other elements were present in the composition of the nanotubes, confirmed by EDX measurements. Reflectance spectra and Tauc plots of Kubelka–Munk functions revealed that the band gaps of the nanotubes were lower than those of the bulk materials (3.05 eV for ZnO and 3.16 eV for TiO2). Photocatalytic performances for the degradation of Rhodamine B showed a large degradation efficiency, even for small quantities of nanotubes (0.5 mg/10 mL dye solution): ~55% for ZnO, and ~95% for TiO2.

Yerba Mate (Ilex paraguarensis) as Bio-Adsorbent for the Removal of Methylene Blue, Remazol Brilliant Blue and Chromium Hexavalent: Thermodynamic and Kinetic Studies

Yerba mate (Ilex paraguarensis, YM) was used as biomaterial for the removal of anionic and cationic compounds from wastewater. Chromium hexavalent Cr(VI), Remazol brilliant blue (RBB) and methylene blue (MB) were selected as pollutants. A calcination step was performed after the washing and drying steps to evaluate its effectiveness at increasing the adsorption capacity of the solid. Both YM and calcinated YM (CYM) were characterized by means of scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR) and Brunauer–Emmett–Teller (BET) analysis. Adsorption batch tests revealed that YM was ineffective for the removal of Cr(VI) and RBB, while good results were obtained for MB (up to 80%) without pH dependency of the adsorption process, and CYM was able to remove Cr(VI) (up to 77%) and RBB (up to 65%) but not MB. The adsorption isotherm of MB on YM at 298 K was obtained experimentally and it is well represented by the Langmuir isotherm. YM’s adsorption capacity for MB was estimated to be 59.6 mg/g. Kinetic batch tests were conducted and the experimental results were fitted with a mathematical model. The low influence of temperature compared to the influence of the YM concentration on the adsorption rate was explained.

Efficient N, Fe Co-Doped TiO2 Active under Cost-Effective Visible LED Light: From Powders to Films

An eco-friendly photocatalytic coating, active under a cost-effective near-visible LED system, was synthesized without any calcination step for the removal of organic pollutants. Three types of doping (Fe, N and Fe + N), with different dopant/Ti molar ratios, were investigated and compared with undoped TiO2 and the commercial P25 photocatalyst. Nano-crystalline anatase-brookite particles were successfully produced with the aqueous sol-gel process, also at a larger scale. All samples displayed a higher visible absorption and specific surface area than P25. Photoactivity of the catalyst powders was evaluated through the degradation of p-nitrophenol in water under visible light (>400 nm). As intended, all samples were more performant than P25. The N-doping, the Fe-doping and their combination promoted the activity under visible light. Films, coated on three different substrates, were then compared. Finally, the photoactivity of a film, produced from the optimal N-Fe co-doped colloid, was evaluated on the degradation of (i) p-nitrophenol under UV-A light (365 nm) and (ii) rhodamine B under LED visible light (395 nm), and compared to undoped TiO2 film. The higher enhancement is obtained under the longer wavelength (395 nm). The possibility of producing photocatalytic films without any calcination step and active under low-energy LED light constitutes a step forward for an industrial development.

Selective Catalytic Oxidation of Benzyl Alcohol by MoO2 Nanoparticles

Selective oxidation of benzyl alcohol to benzaldehyde was carried out with MoO2 nanoparticles (MoO2 NPs). MoO2 NPs were synthesized by two different approaches and characterized by several techniques. The synthesis was done by a hydrothermal procedure using ethylenediamine and either Fe2O3 or hydroquinone. In the latter case, an additional calcination step under N2 was performed to eliminate passivating agents at the surface of the nanoparticles. The synthesized nanocatalysts showed similar catalytic properties, being efficient catalysts in the oxidation of benzyl alcohol. High substrate conversion and product selectivity were achieved under all tested conditions. Studies were conducted using two different oxidants: tert-butyl hydroperoxide and hydrogen peroxide, in our continuous effort to obtain more efficient catalysts for more sustainable catalytic processes. When H2O2 was used as the oxidant, 94% yield was achieved with 100% selectivity for benzaldehyde, which was a very promising result to undergo other studies with this system. Moreover, to elucidate some aspects of the reaction mechanism, a study was conducted, and it was possible to conclude that the reaction undergoes, to some extent, through a radical mechanism with both oxidants.

Conductive Electrospun Nanofiber Mats

Conductive nanofiber mats can be used in a broad variety of applications, such as electromagnetic shielding, sensors, multifunctional textile surfaces, organic photovoltaics, or biomedicine. While nanofibers or nanofiber from pure or blended polymers can in many cases unambiguously be prepared by electrospinning, creating conductive nanofibers is often more challenging. Integration of conductive nano-fillers often needs a calcination step to evaporate the non-conductive polymer matrix which is necessary for the electrospinning process, while conductive polymers have often relatively low molecular weights and are hard to dissolve in common solvents, both factors impeding spinning them solely and making a spinning agent necessary. On the other hand, conductive coatings may disturb the desired porous structure and possibly cause problems with biocompatibility or other necessary properties of the original nanofiber mats. Here we give an overview of the most recent developments in the growing field of conductive electrospun nanofiber mats, based on electrospinning blends of spinning agents with conductive polymers or nanoparticles, alternatively applying conductive coatings, and the possible applications of such conductive electrospun nanofiber mats.

Heterogeneity Challenges in Multiple-Element-Modified Lead-Free Piezoelectric Ceramics

We report on a heterogeneity study, down to the atomic scale, on a representative multiple-element-modified ceramic based on potassium sodium niobate (KNN): 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3–0.05CaZrO3 with 2 wt % MnO2. We show that different routes for incorporating the MnO2 (either before or after the calcination step) affect the phase composition and finally the functionality of the material. According to X-ray diffraction and scanning electron microscopy analyses, the ceramics consist of orthorhombic and tetragonal perovskite phases together with a small amount of Mn-rich secondary phase. The addition of MnO2 after the calcination results in better piezoelectric properties, corresponding to a ratio between the orthorhombic and tetragonal perovskite phases that is closer to unity. We also show, using microscopy techniques combined with analytical tools, that Zr-rich, Ta-rich and Mn-rich segregations are present on the nano and atomic levels. With this multi-scale analysis approach, we demonstrate that the functional properties are sensitive to minor modifications in the synthesis route, and consequently to different material properties on all scales. We believe that detecting and learning how to control these modifications will be a step forward in overcoming the irreproducibility problems with KNN-based materials.

Structure and Electrochemical Properties of Li4Ti5O12 Prepared via Low-Temperature Precipitation

This work aimed to prepare the spinel phase Li4Ti5O12 by a combination of the low-temperature precipitation technique and assisted calcination step. X-ray diffraction (XRD) revealed that the intermediated phase was Li2TiO3, and the spinel phase could be evidently formed at 700°C for 12 to 20 hours. The morphology of spinel powder, determined by SEM and TEM, exhibited a good distribution at the submicrometric scale that promoted a fast kinetic of Li migration and an excellent performance at the high-rate cycling test. The stable performances were achieved in the charge-discharge test at different current densities: 80 mA/g (165 mAh/g), 320 mA (160 mAh/g), and 1600 mA (145 mAh/g) upon 100 cycles. Moreover, we observe a capacity retention of 48% (corresponding 80 mA/g) at a high rate of 5000 mAh/g. The cyclic voltammetry measurement displayed a reversible system and revealed the lithium diffusion coefficient of 1.15 × 10−11 cm2/s.