Balagopal N. Nair

Sol-gel synthesis of molecular sieving silica membranes

Abstract Polymeric silica sol was synthesized by the acid catalyzed hydrolysis and condensation of tetra-ethyl-ortho-silicate. Calcined unsupported membranes made from this sol showed microporous nature. Supported membranes on alumina were prepared by dipping and calcining. Helium showed activated diffusion with an apparent activation energy of 17 kJ mol−1. H2 permeation was comparable to that of helium under identical conditions. N2, Ar, O2, C3H6, C3H8, n-C4H10 and i-C4H10 permeation values were extremely small and therefore difficult to fit appropriate diffusion models. At 303 K hydrocarbon permeation was about 2 times higher than that of N2, Ar or O2. He N 2 permselectivity around 1000 and helium permeation in the order of 10−7–10−8 mol m−2 s−1 Pa−1 were measured in the temperature range of 303–460 K. Comparison of Eact, selectivity and He and N2 permeation of different samples evidenced the dependence of nitrogen flux on processing defects. Obviously permeation rate of nitrogen molecule was insignificant through majority pores of the membrane.

A Pore-Filling Electrolyte Membrane-Electrode Integrated System for a Direct Methanol Fuel Cell Application

To develop a high performance direct methanol fuel cell, a novel electrolyte membrane is needed. This electrolyte membrane should be durable up to 130°C to improve the catalytic reaction, and the methanol crossover should be reduced. Our approach was to design a pore-filling-type polyelectrolyte membrane, where the polyelectrolyte is filled into the pores of a porous substrate. This makes an integrated system with a membrane and a catalyst layer. The porous substrate was completely inert to aqueous methanol solution and was durable at high temperature. The substrate matrix could suppress membrane swelling to reduce methanol crossover, and showed mechanical strength at high temperatures. A radical polymerization technique was employed to fabricate the pore-filling membrane. A porous silica sol-gel thin base membrane on a carbon electrode was used as a membrane-electrode integrated system. The substrate pores were filled with a poly(acrylic acid-co-vinyl sulfonic acid) network. The membranes showed high proton conductivity, thermal stability, and low methanol permeation.

Enhanced CO2 absorption kinetics in lithium silicate platelets synthesized by a sol–gel approach

Platelet-shaped lithium orthosilicate particles synthesized by a sol–gel approach employing the precursors lithium nitrate and colloidal silica displayed enhanced absorption kinetics for CO2 compared to the powders prepared by a solid-state reaction process involving Li2CO3 and silica. The sol–gel samples showed a CO2 absorption capacity of 350 mg g−1 at an absorption rate of 22.5 mg g−1 min−1, a value 70% higher than the rate of 13.2 mg g−1 min−1 measured with the solid-state samples under similar conditions. The higher sorption kinetics of CO2 by the sol–gel derived lithium orthosilicate could be attributed to the unique platelet morphology of the particles, which have a very small thickness. A porous carbon mesh coated with the sol–gel based particles exhibited CO2 absorption capacity of 150 mg g−1 at an absorption rate of 37.5 mg g−1 min−1. This supported absorbent also showed stable absorption and desorption performance for the 8 cycles examined in this study. The excellent absorption characteristics of the sol–gel prepared powders, more specifically the coated strips, provide a successful pathway for the commercialisation of these materials.

Reaction control of tetraethyl orthosilicate (TEOS)/O3 and tetramethyl orthosilicate (TMOS)/O3 counter diffusion chemical vapour deposition for preparation of molecular-sieve membranes

Counter diffusion chemical vapour deposition (CVD) is an excellent way to prepare inorganic thin films or membranes on the surface of porous or non-porous supports. Modification of the pore size of porous supports by this method to prepare molecular-sieve membranes is another interesting opportunity. However, control of the deposition profile is very difficult because of the complex reaction kinetics. In this study, counter diffusion CVD of tetraethyl orthosilicate (TEOS)/O3 or tetramethyl orthosilicate (TMOS)/O3 systems was investigated. To control the deposition layer profile in a thin porous ceramic substrate, numerical simulations of the reaction sequences were performed. The reactant concentration and temperature were varied to control the deposition profile, and the simulation results were compared with experimental results. It is shown that high quality membranes can be processed under the optimised reaction conditions. In addition, the effect of step-coverage of the CVD reaction on final membrane performance was also investigated by comparison of the TEOS/O3 and TMOS/O3 reactions.

A facile one pot synthetic approach for C3N4–ZnS composite interfaces as heterojunctions for sunlight-induced multifunctional photocatalytic applications

Herein, we report a facile one pot synthetic protocol for the creation of C3N4–ZnS composite interfaces by the co-pyrolysis of a precursor mix containing zinc nitrate, melamine, and thiourea at 550 °C in air. The organic–inorganic semiconductor heterojunctions thus formed displayed increased absorbance in the longer wavelength region and facilitated broad absorption of visible light compared to pure ZnS, C3N4 and conventionally synthesized hybrid samples. The decreased emission intensity, increased photocurrent generation and decreased fluorescence lifetime revealed reduced exciton recombinations in the co-pyrolysed sample containing C3N4–ZnS heterostructures. The samples displayed sunlight driven photocatalytic reduction of nitrophenol as well as hydrogen generation (4 mmol g−1 h−1) by water splitting.

evolution of pore structure in microporous silica membranes: sol-gel procedures and strategies

Silica membranes exhibiting excellent molecular sieving capability, which would find applications in fuel‐cell electric vehicles with on‐board hydrogen generation, for example, are the aim of the sol‐gel strategies outlined here. It is shown that optimization of the sol‐gel synthesis parameters is important in order to achieve membranes with minimum defects and hence high selectivity. The preparation of the supported membranes is described and the gas permeation behavior of membranes made from different sol compositions reported.

Co3O4–C3N4 p–n nano-heterojunctions for the simultaneous degradation of a mixture of pollutants under solar irradiation

Environmental remediation employing sunlight-active semiconductor nano-heterostructures provides effective solutions for handling emerging contaminants through a greener approach. Herein, we report the creation of ultrafine dispersions of Co3O4 nanoparticles in a g-C3N4 matrix by a simple one-pot synthetic strategy involving the co-pyrolysis of constituent raw materials. Calcination of a homogeneous mixture of melamine and cobalt nitrate at 550 °C/2 h leads to the formation of Co3O4–C3N4 p–n nano-heterojunctions that displayed extended absorption in the visible wavelength region owing to the synergistic role of Co3O4 particles. Moreover, the surface area values of the composites reached 90 m2 g−1, a tenfold increase from the value of 8 m2 g−1 obtained for the pristine C3N4. The band bending, induced by the p–n nano-heterojunctions, leads to the formation of intimate interfaces having enhanced photophysical properties. The mass normalized photoluminescence spectra of the heterojunctions indicated reduced exciton recombinations that are validated further by the enhanced sunlight-induced photocatalytic degradation of a mixture of methylene blue and tetracycline organic pollutants.

C3N4 anchored ZIF 8 composites: photo-regenerable, high capacity sorbents as adsorptive photocatalysts for the effective removal of tetracycline from water

Materials combining the abilities of adsorption and photocatalysis provide a facile solution for pollutant disposal as secondary remediation processes are avoided. Herein, we report a simple strategy for the development of C3N4 anchored ZIF-8 microcrystals as sheathed architectures for the highly efficient adsorption and sunlight induced photocatalytic degradation of tetracycline from solution. An adsorption capacity as high as 420 mg g−1 of adsorbent was realized for a composition containing 60:40 wt% of C3N4 and ZIF. Subsequently, the adsorbed tetracycline was degraded to over 96% in 1 h of sunlight exposure. The effects of pH and adsorbate concentration are studied and valid adsorption and degradation kinetic models are arrived at. The bifunctional composite thus developed offers a photo-regenerable adsorbent for the effective removal of an emerging hazardous contaminant.

Hydrophobic and Metallophobic Surfaces: Highly Stable Non-wetting Inorganic Surfaces Based on Lanthanum Phosphate Nanorods.

Metal oxides, in general, are known to exhibit significant wettability towards water molecules because of the high feasibility of synergetic hydrogen-bonding interactions possible at the solid-water interface. Here we show that the nano sized phosphates of rare earth materials (Rare Earth Phosphates, REPs), LaPO4 in particular, exhibit without any chemical modification, unique combination of intrinsic properties including remarkable hydrophobicity that could be retained even after exposure to extreme temperatures and harsh hydrothermal conditions. Transparent nanocoatings of LaPO4 as well as mixture of other REPs on glass surfaces are shown to display notable hydrophobicity with water contact angle (WCA) value of 120° while sintered and polished monoliths manifested WCA greater than 105°. Significantly, these materials in the form of coatings and monoliths also exhibit complete non-wettability and inertness towards molten metals like Ag, Zn, and Al well above their melting points. These properties, coupled with their excellent chemical and thermal stability, ease of processing, machinability and their versatile photo-physical and emission properties, render LaPO4 and other REP ceramics utility in diverse applications.

Morphologically and compositionally tuned lithium silicate nanorods as high-performance carbon dioxide sorbents

The effective capturing of carbon dioxide using regenerable high capacity sorbents is a prerequisite for industrial applications aiming at CO2 capture and sequestration. The removal of CO2 directly from chemical reaction environments at high temperature is a less energy intensive method of its separation with the added benefit of improved efficiency in equilibrium limited reactions. However, the separation of CO2 at the typical reaction temperatures of 573–1073 K is a challenging task due to the non-availability of absorbents with kinetics comparable to reaction rates. Moreover their poor durability due to sintering and particle growth on prolonged use at high temperature is also an impediment to their practical application. Herein, we demonstrate the development of an efficient CO2 absorbent material, made of Li4SiO4 nanorods, with ultrafast sorption kinetics as well as remarkable durability. These nanorods enabled easier surface reaction with CO2 due to shorter diffusion pathways for lithium from the bulk to the surface of the rods permitting extremely fast absorption of CO2. Furthermore, the compositional tuning of the materials helped to realize absorbents with extraordinary CO2 absorption rates of 0.72 wt% s−1 at 100% CO2/923 K. The exceptional performance of these absorbents at lower temperatures (573–823 K) as well as lower CO2 pressures (0.15 atm) demonstrates their potential in practical CO2 separation applications.