S.K. Kulshreshtha

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.

SnO2:Eu3+ nanoparticles dispersed in TiO2 matrix: Improved energy transfer between semiconductor host and Eu3+ ions for the low temperature synthesized samples

SnO2:Eu3+ nanoparticles uniformly dispersed in TiO2 matrix were prepared at 185°C in ethylene glycol. Unlike in SnO2:Eu3+, significant improvement in the exciton mediated energy transfer between SnO2 and Eu3+ ions was observed when SnO2:Eu3+ nanoparticles are dispersed in TiO2 matrix, and this is attributed to effective shielding of surface Eu3+ ions present in SnO2:Eu3+ nanoparticles from the vibrations of stabilizing ligand by TiO2 matrix. Annealing the samples at high temperatures leads to formation of Sn1−xTixO2, without significantly affecting the energy transfer process between Eu3+ ions and semiconductor host.

Structural and electronic properties of Sin,Sin+, and AlSin−1 (n=2–13) clusters: Theoretical investigation based on ab initio molecular orbital theory

The geometric and electronic structures of Sin, Sin+, and AlSin−1 clusters (2⩽n⩽13) have been investigated using the ab initio molecular orbital theory under the density functional theory formalism. The hybrid exchange-correlation energy function (B3LYP) and a standard split-valence basis set with polarization functions [6-31G(d)] were employed for this purpose. Relative stabilities of these clusters have been analyzed based on their binding energies, second difference in energy (Δ 2E) and fragmentation behavior. The equilibrium geometry of the neutral and charged Sin clusters show similar structural growth. However, significant differences have been observed in the electronic structure leading to their different stability pattern. While for neutral clusters, the Si10 is magic, the extra stability of the Si11+ cluster over the Si10+ and Si12+ bears evidence for the magic behavior of the Si11+ cluster, which is in excellent agreement with the recent experimental observations. Similarly for AlSin−1 clusters...

The structural and magnetization studies of Co-doped ZnO co-doped with Cu: Synthesized by co-precipitation method

Co- and Cu-doped ZnO of nominal composition Zn0.95Co0.05O and Zn0.94Co0.05Cu0.01O were synthesized by a co-precipitation method in organic media. Rietveld refinement of X-ray diffraction data of the samples annealed at 725 K showed that they are single phase without any secondary phases. DC magnetization measurements of samples with Cu co-doping (Zn0.94Co0.05Cu0.01O) as a function of field at room temperature showed a ferromagnetic signature while the samples without Cu co-doping (Zn0.95Co0.05O) are paramagnetic in nature. Both the samples heated at 1075 K are found to be paramagnetic at room temperature. Resistivity measurements above room temperature clearly showed a semiconducting behavior with a decreased resistivity value for Zn0.94Co0.05Cu0.01O compared to Zn0.95Co0.05O and ZnO, confirming additional carriers in Zn0.94Co0.05Cu0.01O due to Cu co-doping. Our results are in agreement with a recent computational study that doping additional carriers are necessary for realizing room temperature ferromagnetism in Co-doped ZnO.

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.

Structural and magnetic properties of (In1−xFex)2O3 (0.0⩽x⩽0.25) system: Prepared by gel combustion method

(In1-xFex)2O3 polycrystalline samples with x = (0.0, 0.05, 0.10, 0.15, 0.20 and 0.25) have been synthesized by a gel combustion method. Reitveld refinement analysis of X raydiffraction data indicated the formation of single phase cubic bixbyite structure without any parasitic phases. This observation is further confirmed by high resolution transmission electron microscopy (HRTEM) imaging, and indexing of the selected-area electron diffraction (SAED) patterns, X-ray Absorption Spectroscopy (XAS) and Raman Spectroscopy. DC Magnetization studies as a function of temperature and field indicatethat they are ferromagnetic with Curie temperature (TC) well above room temperature.

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.

Structural and electronic properties of Sin, Sin−, and PSin−1 clusters (2⩽n⩽13): Theoretical investigation based on ab initio molecular orbital theory

The geometric and electronic structures of Si(n), Si(n)-, and PSi(n-1) clusters (2 < or = n < or = 13) have been investigated using the ab initio molecular orbital theory formalism. The hybrid exchange-correlation energy functional (B3LYP) and a standard split-valence basis set with polarization functions (6-31+G(d)) were employed to optimize geometrical configurations. The total energies of the lowest energy isomers thus obtained were recalculated at the MP2/aug-cc-pVTZ level of theory. Unlike positively charged clusters, which showed similar structural behavior as that of neutral clusters [Nigam et al., J. Chem. Phys. 121, 7756 (2004)], significant geometrical changes were observed between Si(n) and Si(n)- clusters for n = 6, 8, 11, and 13. However, the geometries of P substituted silicon clusters show similar growth as that of negatively charged Si(n) clusters with small local distortions. The relative stability as a function of cluster size has been verified based on their binding energies, second difference in energy (Delta2 E), and fragmentation behavior. In general, the average binding energy of Si(n)- clusters is found to be higher than that of Si(n) clusters. For isoelectronic PSi(n-1) clusters, it is found that although for small clusters (n < 4) substitution of P atom improves the binding energy of Si(n) clusters, for larger clusters (n > or = 4) the effect is opposite. The fragmentation behavior of these clusters reveals that while small clusters prefer to evaporate monomer, the larger ones dissociate into two stable clusters of smaller size. The adiabatic electron affinities of Si(n) clusters and vertical detachment energies of Si(n)- clusters were calculated and compared with available experimental results. Finally, a good agreement between experimental and our theoretical results suggests good prediction of the lowest energy isomeric structures for all clusters calculated in the present study.

Multiphoton ionisation of acetone at 355 nm: a time-of-flight mass spectrometry study

Abstract Multiphoton ionisation/fragmentation of acetone has been studied at λ=355 nm using time-of-flight mass spectrometry. A broad peak at m/e=43 is observed which shows a three-photon laser power dependence. The results suggest that three-photon ionisation at 355 nm produces vibrationally excited acetone ions which undergo fast dissociation. The broadening of the m/e=43 peak is found to depend on the laser power in the low laser intensity region. Under high laser intensity conditions, a bimodal distribution of m/e=43 peak was observed which has been assigned to two different isomeric product channels. The results provide clear evidence for partial isomerisation of the keto form of the acetone radical cation to the enol form. On ionisation at 355 nm, CH3COC2H5 fragments into two channels corresponding to (CH3CO+) and (C2H5CO+) in the ratio of approximately 3:1.

Radiolytic preparation and catalytic properties of nanophase nickel metal particles

Nanosized nickel particles produced in aqueous sol form were prepared by γ radiolysis of nickel sulfate solution, using gelatin as a stabilizer. The average size of these particles was estimated from the width of the powder X-ray diffraction pattern to be approximately 5 nm. The magnetization measurements were carried out from room temperature to 5 K. The ferromagnetic character of these particles was studied at 5 K. The room temperature catalytic activity of this nickel sol was demonstrated by microcalorimetry measurements of the heat evolved during chemisorption of hydrogen, and the reaction of hydrogen with oxygen and ethylene at room temperature was investigated.