Synthesis and applications of porous non-silica metal oxide submicrospheres

© 2016 Royal Society of Chemistry. Nowadays the development of submicroscale products of specific size and morphology that feature a high surface area to volume ratio, well-developed and accessible porosity for adsorbates and reactants, and are non-toxic, biocompatible, thermally stable and suitable as synergetic supports for precious metal catalysts is of great importance for many advanced applications. Complex porous non-silica metal oxide submicrospheres constitute an important class of materials that fulfill all these qualities and in addition, they are relatively easy to synthesize. This review presents a comprehensive appraisal of the methods used for the synthesis of a wide range of porous non-silica metal oxide particles of spherical morphology such as porous solid spheres, core-shell and yolk-shell particles as well as single-shell and multi-shell particles. In particular, hydrothermal and low temperature solution precipitation methods, which both include various structure developing strategies such as hard templating, soft templating, hydrolysis, or those taking advantage of Ostwald ripening and the Kirkendall effect, are reviewed. In addition, a critical assessment of the effects of different experimental parameters such as reaction time, reaction temperature, calcination, pH and the type of reactants and solvents on the structure of the final products is presented. Finally, the practical usefulness of complex porous non-silica metal oxide submicrospheres in sensing, catalysis, biomedical, environmental and energy-related applications is presented

A review of greywater characteristics and treatment processes

This paper presents a comprehensive literature review of different characteristics of greywater (GW) and current treatment methods. GW is domestic wastewater excluding toilet waste and can be classified as either low-load GW (excluding kitchen and laundry GW) or high-load GW (including kitchen and/or laundry). This review provides information on the quantity of GW produced, its constituents (macro and micro), existing guidelines for wastewater reuse, current treatment methods (from storage to disinfection) as well as related costs and environmental impacts. Moreover some successful examples from various countries around the world are examined. The current preferred treatments for GW use physical and biological/natural systems. Recently, chemical systems like coagulation, adsorption and advanced oxidation processes (AOPs) have been considered and have been successful for low to moderate strength GW. The presence of xenobiotic organic compounds (XOC), which are hazardous micropollutants in GW, is emphasised. Since conventional treatments are not efficient at removing XOC, it is recommended that future studies look at chemical treatment, especially AOPs that have been found to be successful at mineralising recalcitrant organic compounds in wastewater

Some aspects of photocatalytic reactor modeling using computational fluid dynamics

Design and analysis of photoreactors is significantly more challenging than conventional reactors due to participation of radiation in chemical reactions. This problem is further compounded in case of photocatalytic reactors because of presence of photocatalytic particles, which not only produce complex light scattering effects but, in case of slurry systems, also act as an additional phase, the hydrodynamics of which is essential to characterize for evaluating the phase distribution of photocatalyst particles without which it is not possible to calculate the light intensity distribution. This then necessitates the use of a computational fluid dynamics (CFD)-based simulation approach which can simultaneously take into account the hydrodynamics of multiple phases, light intensity distribution and reaction kinetics. This paper presents a sequential review of all steps for CFD simulations of photocatalytic reactors. The hydrodynamic modelling has been considered first with an emphasis on the Eulerian–Eulerian model because of its ability to handle large-scale photocatalytic reactor systems with only relatively moderate computational resources. This has been followed by a review of lamp emission models, which in CFD models are used as boundary conditions for solving the radiation transport equation (RTE). Before discussing the kinetics of photocatalytic reactors, are view of numerical models for solving the RTE has also been presented for both slurry and immobilized reactor systems. Finally, the paper discusses important factors for setting up the boundary conditions for CFD modeling of photocatalytic reactors

Light intensity distribution in multi-lamp photocatalytic reactors

A computational fluid dynamics approach has been used to investigate the effect of lamp separation (Xlamp) on the radiation intensity distribution in a multiple-lamp photocatalytic reactor. The optical parameters (absorption and scattering coefficients) of Aeroxide® P25 titanium dioxide (TiO2) were determined by performing experiments using a single lamp system. Since the optical properties are wavelength dependent, the range of wavelength from the UV lamp was divided into 4 bands, and optical properties in each of the bands were determined by matching the experimental observations with simulated values. Simulations were then carried on multiple-lamp (2 and 4 lamps) photoreactors as a function of lamp separation and catalyst loadings. In case of 2-lamp system, the maximum local volumetric rate of energy absorption () occurred at Xlamp=40mm, and it was independent of the catalyst loading. With 4 lamps however, optimum Xlamp was dependent on the catalyst loading. At low loads (up to Wcat=0.06gL-1), the optimum Xlamp was 80mm but as the catalyst concentration increased, the value of the optimum lamp separation decreased considerably, with 30mm for Wcat=0.07gL-1 and decreasing further as the concentration further increased. Because of the high absorption coefficient of the catalyst, the wall emissivity had a negligible effect on the for both configurations, even when the lamps were close to the wall. Finally, in both cases, the optimum lamp separation was independent of the lamp emissive power

Ca2+-doped ultrathin cobalt hydroxyl oxides derived from coordination polymers as efficient electrocatalysts for the oxidation of water

The development of highly efficient oxygen evolution reaction (OER) electrocatalysts is critical because the sluggish reaction kinetics of this reaction usually hinders the efficiency of water electrolysis for the generation of H2. Herein, we report that ultrathin cobalt-based coordination polymers (Co-CPs) plates are exceptional electrocatalysts for the OER as they show one of the most lowest Tafel slopes among the cobalt-based OER catalysts reported to date. It has been found that the Co-CPs can be transformed into Co(OH)2 hexagonal nanoplates and then further oxidized into CoOOH, which acts as a catalytic active site for the oxidation of water. Interestingly, when Ca2+, an inactive ion, was introduced into the precursors, thinner CoOOH nanosheets with more exposed active sites were formed, which displayed significantly higher OER activity than the Co-CPs. Furthermore, the content of Ca2+ in the Co-based coordination polymers (CoxCay-CPs) and the corresponding OER activity of these CPs were systematically investigated, and Co0.89Ca0.11-CPs exhibited highest OER activity. This study provides a fundamental understanding of the catalytic mechanism of cobalt-based coordination polymer electrocatalysts and the effect of inactive species, such as Ca2+, on the OER in an alkaline electrolyte, which may promote the design of highly active electrocatalysts

Mesoporous MnO2 hollow spheres for enhanced catalytic oxidation of formaldehyde

International audienceIn this work, hollow MnO2 spheres were synthesized via a sacrificial templating method using SiO2 nanospheres as hard templates. The MnO2 coating on SiO2 was achieved by a newly devised low-temperature, controlled precipitation by redox (CPR) method which is based on the controlled redox reaction between KMnO4 and Mn(NO3)2 at 30 °C. After calcination of the SiO2@MnO2 core shell spheres at 300 °C, hollow spheres were obtained following template removal by NaOH etching. The hollow spheres were characterized by XRD, Raman spectroscopy, TEM, HRTEM, N2 adsorption, XPS and H2-TPR. Two different MnO2 crystal phases (γ- and δ-MnO2) could be obtained by making simple changes in the precursor addition protocols during synthesis. All the samples had high BET surface areas between 104 m2 g−1 and 236 m2 g−1, hierarchical pore size distribution with high mesoporosity and the presence of oxygen vacancies for high mobility of oxygen species on the catalyst surface. The catalysts were used for the complete catalytic oxidation of formaldehyde in dry air (100 ppmv; GHSV: 30,000 h−1). Long-term stability tests showed that γ-MnO2 deactivated gradually with time potentially due to structural collapse of the hollow spheres. The interlayer spacing in the δ-MnO2 sample, however, was responsible for increasing the expected conversion due to the regeneration of oxygen/hydroxyl species from adsorbed water formed from the oxidation of formaldehyde on the catalyst surface

Synthesis of colloidal mesoporous silica spheres with large through-holes on the shell

Colloidal silica spheres with controllable large through-holes and mesopores on the shell were synthesized by using polystyrene (PS) spheres as a hard template and cationic surfactant hexadecyl trimethylammonium bromide (CTAB) as a soft template. Through modulating the synthetic conditions, including the volume ratio of ethanol (EtOH)/water, the amount of ammonia hydroxide, and the dosage of CTAB, SiO spheres can transform among hollow structure, through-hole structure, and no large pore structure. The investigation suggests that the hydrolysis rate of the silica source and the interaction strength between the PS sphere template and SiO may determine the large pore structure of the final product. The moderate hydrolysis rate of tetraethyl orthosilicate (TEOS) and strong interaction between the PS sphere template and SiO is conductive to the formation of large through-holes in SiO spheres. To further investigate the pore structure of through-holes of SiO spheres, the lysozyme (Lz) was selected as a model molecule for adsorption experiments. The Lz adsorption experiments show that SiO spheres with through-hole structure exhibit a much faster adsorption rate than SiO spheres with hollow structure and higher adsorption capacity than SiO with no large pore structure. Such a behavior could find interesting applications in the fields that require a fast-loading characteristic

Excellent tribological properties of epoxy-Ti3C2 with three-dimensional nanosheets composites

As a widely used engineering polymer, epoxy resin has been successfully employed in high-performance components and setups. However, the poor thermal and friction properties of traditional epoxy resin greatly limit its application in many extreme environments. In this work, a new kind of epoxy-Ti3C2 with three-dimensional nanosheets (3DNS) composite which was designed by freeze-drying method showed up excellent thermal and friction properties. As a result, the coefficient of thermal expansion (CTE) of epoxy-Ti3C2 3DNS 3.0 composites was 41.9 ppm/K at 40 degrees C, which was lower than that of the traditional epoxy resin (46.7 ppm/K), and the thermal conductivity (TC) was also improved from 0.176 to 0.262 W/(m center dot K). Meanwhile, epoxy-Ti3C2 3DNS 1.0 composites showed up the best friction property, with wear rate 76.3% lower than that of epoxy resin. This work is significant for the research of high-performance composite materials

Pillar-free TiO2/Ti3C2 composite with expanded interlayer spacing for high-capacity sodium ion batteries

The lamellar transition metal oxides, sulfides and carbides with expanded interlayer spacing have attracted wide attention due to their enlargeable interlayer diffusion channels and larger contact areas. However, the existence of pillars between the interlayer would occupy the inter layer voids, which would hinder the accommodation of more lithium ion, sodium ion and so on. Pillar-free TiO2/Ti3C2 composite with expanded interlayer spacing was prepared by sintering the pre-intercalated pristine Ti3C2 with TMAOH under N2 atmosphere. The obtained TiO2/Ti3C2 composite showed remarkable capacity (237.8 mAh g−1 at 100 mA g−1) and long-term stability (153 mAh g−1after 100 cycles at current density of 600 mA g−1) as anode material for sodium ion batteries. These remarkable electrochemical properties of pillar-free TiO2/Ti3C2 were ascribed to its effective expanded interlayer distance fixed by TiO2 nanoparticles attached on the edge plane of Ti3C2, pseudocapacitance contribution of TiO2 nanoparticles, and the synergistic effect between TiO2 and Ti3C2. This work offered a general strategy for fabricating pillar-free MXene-based composites with enlarged interlayer spacing