R. Sasikala

Temperature-programmed reduction and CO oxidation studies over Ce–Sn mixed oxides

The reduction behaviour of Ce–Sn mixed oxides has been studied by a temperature-programmed hydrogen reduction technique and compared with that of pure SnO2 and CeO2. The mixed oxides were found to reduce at lower temperature as compared to that of individual oxides. Carbon monoxide oxidation studies showed that mixed oxides have better activity for CO oxidation reaction than the constituent oxides, which is in conformity with their surface reduction behaviour. The improved oxidation activity is attributed to a synergetic effect existing in these mixed oxides.

TRANSITION-METAL SACCHARIDE CHEMISTRY : SYNTHESIS, SPECTROSCOPY, ELECTROCHEMISTRY AND MAGNETIC SUSCEPTIBILITY STUDIES OF IRON(III) COMPLEXES OF MONO- AND DISACCHARIDES

Abstract Low molecular weight, soluble and characterizable saccharide complexes of iron(III) were synthesised from MeOH using stoichiometric quantities of saccharides and sodium metal. Monosaccharides such as d -glucose, l -sorbose, d -fructose, d -mannose and d -galactose, and disaccharides such as lactose, maltose and sucrose were used for complexation. The final iron(III)-saccharide complexes were isolated, purified and characterised by various analytical, spectroscopic methods including Mossbauer, magnetic susceptibility and electrochemical methods. While half of these complexes were found to be mononuclear, the rest exhibited characteristics close to hydroxo-bridged dinuclear species. The inherent stability and hydrolytically robust nature of these complexes over a wide range of pH was demonstrated. The reductive release of iron from these complexes was shown to be better than that of the iron-dextran complex reported in the literature. Based on their demonstrated properties these may be considered as potentially important in the oral nutritional supplementation of iron.

CO oxidation over Pd/SnO2 catalyst

Abstract Oxidation of CO over the surface of SnO2 and Pd/SnO2 catalysts has been investigated with a view to understand the role of Pd metal particles which are known to improve the catalytic activity. The reaction occurs through the incorporation of lattice oxygen and an oxidized surface of the catalyst facilitates the CO oxidation. For Pd/SnO2 significant CO oxidation occurs even at room temperature as monitored by CO pulse injection technique. Disproportionation of CO into CO2 and carbon has been observed in the region 350 K ⩽T⩽ 425 K. From 119Sn Mossbauer investigations the existence of Sn2+ species in the support matrix has been observed and its extent is affected by Pd incorporation. The effect of oxygen and/or hydrogen pretreatment of Pd/SnO2 catalyst at different temperatures on its catalytic activity has been discussed.

Improved photocatalytic activity of indium doped cadmium sulfide dispersed on zirconia

A novel composite photocatalyst of indium doped cadmium sulfide dispersed on zirconium oxide has been synthesized, which shows enhanced photocatalytic activity for hydrogen generation from water. In this system, cadmium sulfide exists as a separate dispersed phase on the zirconia support. Optical absorption spectra indicate a blue shift of absorption edge for CdS and In doped CdS dispersed on ZrO2 compared to pure CdS and indium doped CdS. Among the supported CdS, In doped CdS exhibits better optical absorption property. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdS dispersed on ZrO2 and indium doping in CdS enhances the activity further. Fluorescence lifetime studies indicate that, in the supported CdS, the charge carriers have higher lifetime than that in the unsupported CdS. Photocurrent response experiments show a relatively higher current output for the In doped CdS dispersed on ZrO2 support. The enhanced photocatalytic activity of this composite sample is attributed to a combination of factors like enhanced lifetime of the photogenerated charge carriers, increased photoresponse and improved surface area. The present study leads to a new observation that the photocurrent response and photocatalytic activity of CdS and indium doped CdS are enhanced when they are dispersed on a support like ZrO2. These composites with Pd as co-catalyst exhibit a large increase in the photocatalytic activity due to the increased availability of electrons on the metal surface by the interfacial transfer of electrons from CdS to Pd, when irradiated.

Enhanced photocatalytic hydrogen generation over ZrO2–TiO2–CdS hybrid structure

Hybrid photocatalysts with suitable band structures are expected to show enhanced photocatalytic activity as compared to their constituent single phase compounds due to their improved physico-chemical properties. Here, we report an enhanced photocatalytic activity of a new composite photocatalyst comprising of ZrO2, TiO2, and CdS. This hybrid catalyst exhibits increased photocatalytic activity for hydrogen generation from water as compared to their constituent compounds. The photocatalytic activity decreases in the order: ZrO2-TiO2-CdS>TiO2-CdS>ZrO2-CdS>CdS>ZrO2-TiO2≈TiO2>ZrO2. An apparent quantum efficiency of 11.5% is obtained for ZrO2-TiO2-CdS with Pd as co-catalyst. Absorption edge of the composite is slightly blue shifted compared with that of pure CdS. Photoluminescence lifetime studies indicate an increased lifetime for the charge carriers in the composite sample as compared to that of pure CdS. Transmission electron microscopy images reveal that the particle size of the composite is much less than that of single phase CdS. The enhanced photocatalytic activity of the composite is attributed to the decreased particle size of CdS and increased lifetime of the charge carriers resulting from the efficient interfacial transfer of photogenerated electrons at the CdS/TiO2 and CdS/ZrO2 interface.

Mineralization of malachite green dye over visible light responsive bismuth doped TiO2–ZrO2 ferromagnetic nanocomposites

We report enhanced visible light photocatalytic activity for ferromagnetic Bi doped (1% by atomic weight) TiO2–ZrO2 nanocomposites for the degradation of malachite green in aqueous solution. X-ray diffraction, Raman and High resolution Transmission electron microscopy (HRTEM) analysis indicate the presence of monoclinic ZrO2 and anatase TiO2 in the composite sample. The particle size of the nanocomposite is ∼20 nm. A significant absorption of visible light is observed for the doped TiO2 composite as compared to P25 and undoped TiO2–ZrO2. The photocatalytic activity of different concentrations of the TiO2–ZrO2 nanocomposite for the degradation of malachite green follows the order: 98% TiO2–2% ZrO2 < 80% TiO2–20% ZrO2 < 90% TiO2–10% ZrO2. The Bi doped TiO2–ZrO2 (90% TiO2–10% ZrO2) composite shows a still higher activity as compared to the undoped composite and commercial P25. TOC analysis exhibited 88% mineralization of malachite green over Bi doped TiO2–ZrO2 (90% TiO2–10% ZrO2). The enhanced photocatalytic activity of the doped TiO2–ZrO2 composite is attributed to improved visible light absorption and efficient separation of photogenerated charge carriers. The composite samples are found to be photo-stable and the photocatalytic activity remains almost the same for four cycles of degradation experiments. Testing with different quenchers suggest that hydroxyl radicals, holes and superoxide radicals play a considerable role in the photodegradation of malachite green. It is noteworthy that the Bi doped TiO2–ZrO2 nanocomposite exhibits defect induced room temperature ferromagnetism.

Studies on hydrogen storage material FeTi: Effect of Sn substitution

Abstract Effect of Sn substitution on hydriding characteristics of FeTi has been studied for FeTi 1−x Sn x system (x = 0.02 and 0.05). Sn substitution in place of Ti results in an increase of plateau pressures, ΔH and ΔS values, and decrease in saturation hydrogen contents. For FeTi .95 Sn .05 the ease of activation was found to be improved and a saturation hydride composition FeTi .95 Sn .05 H 1.4 could be achieved in two activation cycles. The samples have been characterized by X-ray diffraction, 57 Fe and 119 Sn Mossbauer spectroscopy and differential scanning calorimetry. Sn substituted samples showed the existence of an additional Fe 2 Ti phase which appears to be responsible for the easy activation. The sealing characteristics of FeTi .95 Sn .05 H 1.4 were found to be significantly improved as compared with FeTiH ∼2 . Unlike 57 Fe Mossbauer spectra which clearly revealed the formation of β and γ hydride phases through change in isomeric shift and quadrupole splitting values, 119 Sn spectra remained almost unaltered except the decrease in recoil free fraction on hydride formation.

Photocatalytic hydrogen generation from water using a hybrid of graphene nanoplatelets and self doped TiO2–Pd

Nanohybrids of self doped (Ti3+ doped or reduced TiO2–TiO2R) TiO2–graphene nanoplatelets (TiO2R–G) of different compositions are synthesized by a facile soft chemical method. A decrease of bandgap and improved visible light absorption is exhibited by TiO2R–G. Based on current–voltage (I–V) measurements, it is concluded that the hybrid material possesses improved electron transport properties compared to TiO2R and pure TiO2. A detailed characterization of the composites indicated that TiO2R exists as a dispersed phase on graphene nanoplatelets (graphene). Among different compositions of the composites, the catalyst containing 3 weight% of graphene (TiO2R–3G) shows enhanced photocatalytic activity for hydrogen generation from water compared to both TiO2 and TiO2R. When Pd is used as co-catalyst in this composite, a large increase in the activity is observed. The increased efficiency of the nanocomposite is attributed to factors like: (i) improved visible light absorption promoted by G and Ti3+ dopant (ii) increased lifetime of the charge carriers assisted by the enhanced electron transporting properties of G (iii) increased number of active sites for hydrogen evolution provided by the Pd co-catalyst. This work highlights the role of TiO2 based hybrid materials as efficient photocatalysts for solar energy utilization.

Pd–TiO2–SrIn2O4 heterojunction photocatalyst: enhanced photocatalytic activity for hydrogen generation and degradation of methylene blue

A novel heterojunction photocatalyst of TiO2–SrIn2O4 is found to be active for hydrogen generation from water as well as for the degradation of methylene blue under sunlight type radiation. Photocatalytic activity for hydrogen generation increases with increase in SrIn2O4 concentration and the optimum concentration is found to be 40% (by weight). The presence of a Pd co-catalyst enhances the photocatalytic activity and the catalyst is found to be stable after repeated cycles of photocatalysis experiments. A higher rate of degradation of methylene blue is observed when the composite is used as photocatalyst compared to pure TiO2 and SrIn2O4. Detailed characterization of the composite revealed that TiO2 exists as a dispersed phase on SrIn2O4 and the particle size of TiO2 and TiO2–SrIn2O4 is around 20 nm and 15 nm, respectively. Photocurrent experiments show a relatively higher current output for the composite compared to pure TiO2. The enhanced photocatalytic activity of the composite is attributed to a synergistic effect of the increased lifetime of the charge carriers in the composite and increased surface area of the sample. The dispersion of TiO2 nanoparticles on SrIn2O4 results in the formation of hetero-junctions between TiO2 and SrIn2O4 leading to efficient interfacial transfer of photogenerated electrons from TiO2 to SrIn2O4 and enhancing the lifetime of the charge carriers. The Pd co-catalyst enhances the activity of the composite further by increasing the availability of electrons in it and providing active sites for hydrogen evolution.

Carbon monoxide methanation over FeTi1+x intermetallics

Abstract FeTi1+x intermetallics (0 ≤ x ≤ 0.15), well-known hydrogen absorption materials, have been found to be active catalysts for CO methanation. The initial activity for H2 absorption, CO disproportionation, and CO hydrogenation increased significantly with increasing Ti content. However, the catalysts rapidly lost their activity because of carbon layer deposition at the surface. Mossbauer and X-ray diffraction studies indicate that no surface or bulk carbides are formed during CO H 2 reaction. Auger electron spectroscopy data have revealed that for all the titanium compositions, the surface is rich in iron, and conversion electron Mossbauer results showed that the surface becomes enriched with iron metal clusters during catalytic reaction. Thus, the catalytic activity is attributed to α-iron centers at the surface which are responsible for the formation of different carbonaceous precursor species. Excess Ti concentration results in the formation of secondary iron titanium suboxide phases which help in the generation of additional active sites during the activation process. The activity of surface carbon species and the reaction routes involved in CO methanation are discussed in detail.