Formation of Ce1-xPdxO2-delta solid solution in combustion-synthesized Pd/CeO2 catalyst: XRD, XPS, and EXAFS investigation

Pd/CeO2 (1 at. %) prepared by the solution-combustion method shows a higher catalytic activity for CO oxidation and NO reduction than Pd metal, PdO, and Pd dispersed over CeO2 by the conventional method. To understand the higher catalytic properties, the structure of 1 at. % Pd/CeO2 catalyst material has been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The diffraction lines corresponding to Pd or PdO are not observed in the high-resolution XRD pattern of 1 at. % Pd/CeO2. The structure of 1 at. % Pd/CeO2 could be refined for the composition of Ce0.99Pd0.01O1.90 in the fluorite structure with 5% oxide ion vacancy. Pd(3d) peaks in the XPS in I at. % Pd/CeO2 are shifted by 3 eV indicating that Pd is in a highly ionic +2 state. EXAFS studies show the average coordination number of 3 around Pd2+ ion in the first shell of 1 at. % Pd/CeO2 at a distance of 2.02 Angstrom, instead of 4 as in PdO. The second shell at 2.72 Angstrom is due to Pd-Pd correlation which is larger than 2.69 Angstrom in PdO. The third shell at 3.31 Angstrom having 7 coordination is absent either in Pd metal or PdO, which can be attributed to -Pd2+-Ce4+- correlation. Thus, 1 at. % Pd/CeO2 forms the Ce1-xPdxO2-delta type of solid solution having -Pd2+-O-2-Ce4+- kinds of linkages.

Oxidation and decomposition of NH3 over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts

The catalytic oxidation and decomposition of NH3 have been carried out over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts using temperature programmed reaction (TPR) technique in a packed bed tubular reactor. Metals are ionically dispersed over CeO2 and fine metal particles are found on Al2O3. NH3 oxidation occurs over 1% Pt/Al2O3, 1% Pd/Al2O3 and 1% Ag/Al2O3 at 175, 270 and 350 C respectively producing N-2, NO, N2O and H2O, whereas 1% Pt/CeO2, 1% Pd/CeO2 and 1% Ag/CeO2 give N-2 along with NO, N2O and H2O at 200, 225 and 250degreesC respectively. N-2 predominates over other nitrogen-containing products during the reaction on all catalysts. At less O-2 concentration, N-2 and H2O are the only products obtained during NH3 Oxidation. NH3 decomposition over all the catalysts occurs above 450degreesC.

Sandwich propellant combustion: Modeling and experimental comparison

This paper reports reacting fluid dynamics calculations for an ammonium percholrate binder sandwich and extracts experimentally observed features including surface profiles and maximum regression rates as a function of pressure and binder thickness. These studies have been carried out by solving the two-dimensional unsteady Navier-Stokes equations with energy and species conservation equations and a kinetic model of three reaction steps (ammonium perchlorate decomposition flame, primary diffusion flame, and final diffusion flame) in the gas phase. The unsteady two-dimensional conduction equation is solved in the condensed phase. The regressing surface is unsteady and two dimensional. Computations have been carried out for a binder thickness range of 25-125 mum and a pressure range of 1.4 to 6.9 MPa. Good comparisons at several levels of detail are used to demonstrate the need for condensed-phase two-dimensional unsteady conduction and three-step gas-phase reactions. The choice of kinetic and thermodynamic parameters is crucial to good comparison with experiments. The choice of activation energy parameters for ammonium percholrate combustion has been made with stability of combustion in addition to experimentally determined values reported in literature. The choice of gas-phase parameters for the diffusion flames are made considering that (a) primary diffusion flame affects the low-pressure behavior and (b) final diffusion flame affects high-pressure behavior. The predictions include the low-pressure deflagration limit of the sandwich apart from others noted above. Finally, this study demonstrates the possibility of making meaningful comparisons with experimental observations on sandwich propellant combustion.

Structural investigation of combustion synthesized Cu/CeO2 catalysts by EXAFS and other physical techniques: Formation of a Ce1-xCuxO2-delta solid solution

The structure and chemical environment of Cu in Cu/CeO2 catalysts synthesized by the solution combustion method have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and extended X-ray fine structure (EXAFS) spectroscopy. High-resolution XRD studies of 3 and 5 atom % Cu/CeO2 do not show CuO lines in their respective patterns. The structure could be refined for the composition Ce1-xCuxO2-delta (x = 0.03 and 0.05; delta similar to 0.13 and 0.16) in the fluorite structure with 5-8% oxide ion vacancy. High-resolution TEM did not show CuO particles in 5 atom % Cu/CeO2. EPR as well as XPS studies confirm the presence of Cu2+ species in the CeO2 matrix. Redox potentials of Cu species in the CeO2 matrix are lower than those in CuO. EXAFS investigations of these catalysts show an average coordination number of 3 around the Cu2+ ion in the first shell at a distance of 1.96 Angstrom, indicating the O2- ion vacancy around the Cu2+ ion. The Cu-O bond length also decreases compared to that in CuO. The second and third shell around the Cu2+ ion in the catalysts are attributed to -Cu2+-O2--Cu2+ - at 2.92 Angstrom and -Cu2+-O2--Ce4+- at the distance of 3.15 Angstrom, respectively. The present results provide direct evidence for the formation of a Ce1-xCuxO2-delta type of solid solution phase having -square-Cu2+-O-Ce4+- kind of linkages.

Synthesis of LiCo1-xNixO2 from a low temperature solution combustion route and characterization

Nickel substituted lithium-cobalt oxides, LiCo1-xNixO2 (0 < x < 0.4), have been synthesized in a very short time by a solution combustion method at 350 degreesC using diformyl hydrazine as a fuel. Pure phases with hexagonal lattice structure have been obtained. These compounds facilitate reversible insertion/extraction of Li+ ions with good discharge capacity between 3.0 and 4.4 V versus Li/Li+. Results of the studies by powder X-ray diffraction, scanning electron microscopy, cyclic voltammetry, galvanostatic charge-discharge cycling and ac impedance measurements are presented. (C) 2002 Elsevier Science B.V. All rights reserved.

Combustion synthesis and properties of Ce1-xPrxO2-delta red ceramic pigments

Praseodymium-doped ceria red pigments, Ce1−xPrxO2−δ, x=0–0.5 have been prepared by the thermal decomposition of the redox compound Ce1−xPrx(N2H3COO)3·3H2O as well as by the combustion of aqueous solutions containing cerous nitrate, praseodymium nitrate and oxalyl dihydrazide (ODH)/ammonium acetate. Formation of the pigment has been confirmed by its characteristic red colour and reflectance spectra which shows the reflection edge not, vert, similar690 nm corresponding to charge transfer from the ligand orbitals to the localised 4f1 of Pr4+. The particulate properties of praseodymium-doped ceria pigments obtained from the combustion of redox compounds and redox mixtures are compared.

Solution combustion synthesis of gamma(L)-Bi(2)MoO(6) and photocatalytic activity under solar radiation

The facile method of solution combustion was used to synthesize gamma(L)-Bi(2)MoO(6). The material was crystallized in a purely crystalline orthorhombic phase with sizes varying from 300 to 500 nm. Because the band gap was 2.51 eV, the degradation of wide variety of cationic and anionic dyes was investigated under solar radiation. Despite the low surface area (< 1 m(2)/g) of the synthesized material, gamma(L)-Bi(2)MoO(6) showed high photocatalytic activity under solar radiation due to its electronic and morphological properties. (C) 2011 Elsevier Ltd. All rights reserved.

Combustion Synthesized Vanadia Rods for Environmental Applications

Rod-shaped V(2)O(5) was synthesized using the solution combustion technique, and the morphology of the compound was confirmed by TEM. Rods of an average diameter of 500 nm and length 3-6 times the diameter were obtained after the calcination of freshly prepared V(2)O(5) at 550 degrees C for 24 h. Pd metal nanoparticles of 20 nm size were deposited onto the rods using the wet impregnation technique. The as-synthesized, calcined and Pd impregnated V(2)O(5) were characterized by a wide variety of techniques including energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. These compounds were tested for CO oxidation, adsorption, and photocatalytic degradation of dyes. The 1% Pd/V(2)O(5) showed a high activity for CO oxidation, the as-synthesized compound showed activity for the adsorption of cationic dyes, whereas the calcined V(2)O(5) sample showed high rates of photocatalytic degradation of dyes. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2215-2228, 2011

The effect of nature of raw coal on the adhesion of bacteria to coal surface

The surface properties of coal and solution pH play a major role in determining the adhesion of microorganisms. In this study, three Indian coal samples with different compositions have been used and the adhesion of the bacterium Bacillus polymyxa to these coals has been investigated. It was found that due to the high ash content of coal, the zeta-potential was negative over most of the pH range which is close to the values exhibited by pure quartz as well as B. polymyxa. Similarly, the surface free energy components of coal (derived from contact angle measurements) showed that the electron-donor component increased with ash content. Adhesion experiments revealed that maximum adhesion of the bacterium B. polymyxa occurred on to the coal samples around the point-of-zero-charge of the coal and the bacterium i.e. about pH 2. Further, adhesion was found to be dependent on the ash content and the surface free energy of the coals. (C) 2002 Published by Elsevier Science Ltd.

The utility of Bacillus subtilis as a bioflocculant for fine coal

The application of Bacillus subtilis as a flocculant for fine coal has been reported here. Zeta-potential measurements showed that both the coal and bacteria had similar surface charge as a function of pH. Surface free energy calculations showed that the coal was hydrophobic while the bacterium was hydrophilic. The adhesion of the bacteria to coal and subsequent settling was studied in detail. Adhesion of bacteria to coal surface and subsequent settling of coal was found to be quick. Both adhesion and settling were found to be independent of pH, which makes the process very attractive for field applications. The presence of an electrolyte along with the bacterium was found to not only enhance adhesion of bacteria, but also produce a clear supernatant. Further, the settled fraction was more compact than with bacteria alone. Interaction energy calculations using the extended DLVO theory showed that the electrical forces along with the acid-base interaction energy play a dominant role in the lower pH range. Above pH 7, the acid-base interaction energy is the predominant attractive force and is sufficient enough to overcome the repulsive forces due to electrical charges to brine about adhesion and thus settling of fine coal. With increase in electrolyte concentration, the change in total interaction energy with pH is minimal which probably leads to better adhesion and hence settling. (C) 2003 Elsevier Science B.V. All rights reserved.

Effect of inorganic ions, H(2)O(2) and pH on the photocatalytic inactivation of Escherichia coli with silver impregnated combustion synthesized TiO(2) catalyst

The photocatalytic antibacterial activity of Ag impregnated combustion synthesized TiO(2) (0.25 g/L) was studied against Escherichia coil in presence of UV irradiation. The effect of various parameters, such as anions, canons, hydrogen peroxide and pH, on the photocatalytic inactivation was investigated. The addition of inorganic ions showed a negative effect on inactivation. Among anions, the presence of chloride ions was observed to have a maximum negative effect and reduced the inactivation considerably. Among cations, the bacterial inactivation reduced significantly in the presence of Ca(2+) ions. Hydrogen peroxide addition in combination with Ag/TiO(2) photocatalysis, however, improved the inactivation. Photocatalysis with high concentration of H(2)O(2) yielded complete bacterial inactivation within few minutes. The photocatalytic inactivation of E. coil was not affected by variation in pH. (C) 2011 Elsevier B.V. All rights reserved.

Explicit filtering LES for turbulent non-premixed combustion

Two models for large eddy simulation of turbulent reacting flow in homogeneous turbulence were studied. The sub-grid stress arising out of non-linearities of the Navier-Stokes equations were modeled using an explicit filtering approach. A filtered mass density function (FMDF) approach was used for closure of the sub-grid scalar fluctuations. A posteriori calculations, when compared with the results from the direct numerical simulation, indicate that the explicit filtering is adequate in representing the effect of sub-grid stress on the filtered velocity field in the absence of reaction. Discrepancies arise when reactions occur, but the FMDF approach suffices to account for sub-grid scale fluctuations of the reacting scalars, accurately.

Single Particle and Packed Bed Combustion in Modern Gasifier Stoves—Density Effects

This article is concerned with a study of an unusual effect due to density of biomass pellets in modern stoves based on close-coupled gasification-combustion process. The two processes, namely, flaming with volatiles and glowing of the char show different effects. The mass flux of the fuel bears a constant ratio with the air flow rate of gasification during the flaming process and is independent of particle density; char glowing process shows a distinct effect of density. The bed temperatures also have similar features: during flaming, they are identical, but distinct in the char burn (gasification) regime. For the cases, wood char and pellet char, the densities are 350, 990 kg/m(3), and the burn rates are 2.5 and 3.5 g/min with the bed temperatures being 1380 and 1502 K, respectively. A number of experiments on practical stoves showed wood char combustion rates of 2.5 +/- 0.5 g/min and pellet char burn rates of 3.5 +/- 0.5 g/min. In pursuit of the resolution of the differences, experimental data on single particle combustion for forced convection and ambient temperatures effects have been obtained. Single particle char combustion rate with air show a near-d(2) law and surface and core temperatures are identical for both wood and pellet char. A model based on diffusion controlled heat release-radiation-convection balance is set up. Explanation of the observed results needs to include the ash build-up over the char. This model is then used to explain observed behavior in the packed bed; the different packing densities of the biomass chars leading to different heat release rates per unit bed volume are deduced as the cause of the differences in burn rate and bed temperatures.