This paper presents a study regarding the preparation of MgCr2O4 from waste tannery solution, and chromium leaching behavior is also investigated with varying amounts of sulfate, chloride and calcium. The phase transformation, crystallinity index and crystallite diameter were characterized using XRD, FT-IR and thermal analysis. A well-crystallized MgCr2O4 was successfully prepared at 1400 °C. The sintering temperature had a major impact on the formation of MgCr2O4 compared with sintering time. The MgCr2O4 phase was observed initially at 400 °C and its crystallite diameter increased with increasing temperature. The concentration of total chromium leached and Cr(VI) decreased gradually with increasing temperature. The considerable amount of Cr(VI) was found in the leachate at 300–500 °C caused by Cr(VI) intermediary products. Sulfate and chlorine could impact the transformation efficiency of chromium adversely, and chlorine has a more significant effect than sulfate. The presence of calcium disturbed the formation of MgCr2O4 and new chromium species (CaCrO4) appeared, which resulted in a sharp increase in the concentration of leached Cr(VI). Incorporating Cr(III) into the MgCr2O4 spinel for reusable products reduced its mobility significantly. This was demonstrated to be a promising strategy for the disposal of chromium containing waste resource.

In this work, Ag+ and Ag0 were absorbed onto the surface of 3-mercaptopropyltriethoxysilane modified fly ash cenospheres (FACs) in two Ag activation processes. The activation methods, avoiding traditional surface sensitization by SnCl2, successfully initiated electroless copper particles deposition for the preparation of buoyant Cu-FAC and CuAg-FAC composites. The CuAg-FAC had a much more uniform morphology than the Cu-FAC. The catalytic performance of the Cu-FAC and CuAg-FAC was examined by the reduction of Orange IV azo dye with the presence of NaBH4. 98.4% of Orange IV was rapidly reduced within 25 min by the CuAg-FAC, whereas 76.4% of Orange IV was removed by the Cu-FAC. The results reveal that the degradation processes matched well with the pseudo-first-order kinetics model, and rate constants of 0.057 and 0.186 min−1 were obtained for the Cu-FAC and CuAg-FAC, respectively. Moreover, two other dyes of Orange II and Reactive Black 5 were also efficiently reduced by the CuAg-FAC which could be easily recycled and stably reused at least four times. These buoyant metal-coated FAC composites would be very useful in various catalytic reductions.

A novel fly ash cenospheres (FACs)-supported CeO2 composite (CeO2/FACs) was successfully synthesized by the modified pyrolysis process. The prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflection spectra (DRS) techniques. XRD results indicated that the CeO2 film coated on cenospheres was a face-centered cubic structure. SEM images confirmed that the CeO2 film was relatively compact. XPS results showed that Ce was present as both Ce4+ and Ce3+ oxidation states in CeO2 film coated on FACs substrate. The bandgap of the composite was narrower compared with the pure CeO2. The as-prepared material exhibited good photocatalytic activity for the decolorization of methylene blue (MB) under visible light irradiation, and the first-order reaction rate constant (k) of 0.0028 min−1 for CeO2/FACs composite was higher than 0.0015 min−1 of pure CeO2. The fact that they floated on water meant that CeO2/FACs composites were easily recovered from water by filtration after the reaction. The recycling test revealed that the composites were quite stable during the MB photocatalytic decolorization. The CeO2/ FACs catalyst was therefore promising for practical use in the degradation of pollutants or water cleanup.SEM micrographs of a pristine FAC (a); some FACs (b) at low magnification; CeO2/FAC at low (c) and high (d) magnification; cross-section of CeO2/FACs (e); some pieces of coatings of CeO2/FACs (f); EDX spectra of pristine FACs (g) and CeO2/FACs samples (h)

•The Pd/SnO2–TiO2/MWCNT catalyst is synthesized for DMFCs and DEFCs.•Sn(II) could significantly improve the dispersion of the Pd nanoparticles on TiO2.•The possible synergetic effect of Pd and SnO2 for alcohol oxidation is proposed.•Addition of SnO2 has strong promoting effect on catalyst activity and CO tolerance.Pd/tin oxide-titanium dioxide/multiwalled carbon nanotubes (Pd/SnO2–TiO2/MWCNT) have been synthesized according to a facile and controllable in situ chemical method and their performances in methanol and ethanol oxidation reactions evaluated. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses revealed the beneficial effects of the SnO2 component in the Pd/SnO2–TiO2/MWCNT, indicating that the metallic-state Pd nanoparticles were uniformly dispersed onto the SnO2–TiO2 precursor anchored onto the MWCNT. Electrochemical characterization techniques, including cyclic voltammetry (CV), chronoamperometry and electrochemical impedance spectroscopy (EIS) measurements were used to analyze the electrochemical performance of the Pd/SnO2–TiO2/MWCNT relative to Pd/MWCNT. The results revealed that the Pd/SnO2–TiO2/MWCNT provided excellent and superior levels of performance in terms of their electrocatalytic activity, electrochemical active surface (EAS), carbon monoxide (CO) tolerance and stability for both methanol and ethanol oxidation in alkaline solution.

A simple and controllable in situ chemical method for preparing Palladium/Tin oxide-titanium dioxide/multi-walled carbon nanotube (Pd/SnO2–TiO2–MWCNTs) catalysts for direct formic acid oxidation is proposed in this study. The electro-oxidation of formic acid can be promoted by the addition of SnO2. Transmission electron microscopy (TEM) showed that Pd particles were homogeneously distributed on the surface of the SnO2–TiO2 surface. Cyclic voltammetry indicated that a much greater enhancement in catalytic activity (3.6 times) was observed when compared with the improvement in ESA value (1.8 times). The results of the electrochemical impedance spectroscopy showed that the enhanced CO tolerance may be attributed to the synergetic effect between the Pd and Sn oxides.Graphical abstractHighlights► Pd based on the SnO2–TiO2 precursor anchored on MWCNTs synthesized for DFAFCs. ► Uniform dispersion of the Pd particles attributed to the addition of the Sn (II). ► Introduction of the Sn has promoting effect on catalyst activity and CO tolerance. ► The synthesized catalyst exhibits excellent performance for formic acid oxidation.

Fly ash cenospheres (FACs) coated with nitrogen-doped TiO2 (N-TiO2) composites were prepared by the sol–gel method for use as photocatalysts. The photocatalytic activity and kinetics of the composites produced at different calcination temperatures and with different nitrogen doping contents were studied under visible light irradiation by monitoring the environmental degradation of methylene blue (MB). The MB degradation ratio of N-TiO2/FAC (25%, 450 °C) was 10% higher than that of N-TiO2 and 40% higher than that of TiO2/FAC. These N-TiO2/FAC composites floated in water and could be recovered after the photocatalytic reaction by phase separation.