Noble metal ionic sites for catalytic hydrogen combustion: spectroscopic insights

A catalytic hydrogen combustion reaction was carried out over noble metal catalysts substituted in ZrO2 and TiO2 in ionic form. The catalysts were synthesized by the solution combustion technique. The compounds showed high activity and CO tolerance for the reaction. The activity of Pd and Pt ion substituted TiO2 was comparable and was higher than Pd and Pt ion substituted ZrO2. The mechanisms of the reaction over the two supports were proposed by making use of the X-ray photoelectron spectroscopy and FT infrared spectroscopic observations. The reaction over ZrO2 supported catalysts was proposed to take place by the utilization of the surface hydroxyl groups while the reaction over TiO2 supported catalysts was hypothesized to be a hybrid mechanism utilizing surface hydroxyl groups and the lattice oxygen.

Combustion Synthesis and Properties of Fine Particle Spinel Manganites

Fine particle spinel manganites have been prepared by thermal decomposition of the precursors N2H5M1/3Mn2/3(N2H3COO)3 · H2O (M = Co and Ni) and M1/3 Mn2/3(N2H3COO)2 · 2H2O (M = Mg and Zn), as well as by the combustion of redox mixtures containing M(II) nitrate (M = Mg, Co, Ni, Cu, and Zn), Mn(II) nitrate, and maleic hydrazide (MH) in the required molar ratio. Both the precursor and redox mixtures undergo self-propagating, gas-producing, exothermic reactions once ignited at 250-375°C to yield corresponding manganites in less than 5 min. Formation of single phase products was confirmed by X-ray powder diffraction patterns. The manganites are of submicrometer size and have surface area in the range 20-76 m2/g.

Effect of Li+-ion on enhancement of photoluminescence in Gd2O3:Eu3+ nanophosphors prepared by combustion technique

Gd2O3:Eu3+ (4 mol%) nanophosphor co-doped with Li+ ions have been synthesized by low-temperature solution combustion technique in a short time. Powder X-ray diffractometer (PXRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), UV-VIS and photoluminescence (PL) techniques have been employed to characterize the synthesized nanoparticles. It is found that the lattice of Gd2O3:Eu3+ phosphor transforms from monoclinic to cubic as the Li+-ions are doped. Upon 254 nm excitation, the phosphor showed characteristic luminescence D-5(0) -> F-7(J) (J= 0-4) of the Eu3+ ions. The electronic transition located at 626 nm (D-5(0) -> F-7(2)) of Eu3+ ions was stronger than the magnetic dipole transition located at 595 nm (D-5(0) -> F-7(1)). Furthermore, the effects of the Li+ co-doping as well as calcinations temperature on the PL properties have been studied. The results show that incorporation of Li+ ions in Gd2O3:Eu3+ lattice could induce a remarkable improvement of their PL intensity. The emission intensity was observed to be enhanced four times than that of with out Li+-doped Gd2O3:Eu3+. (C) 2010 Elsevier B.V. All rights reserved,

Synthesis and properties of willemite Zn2SiO4, and M2+: Zn2SiO4 (M = Co and Ni)

Fine particles of willemite, alpha -Zn2SiO4, were prepared by both solution combustion and sol-gel methods. Both processes yield single-phase, large-surface area (26- and 78-m(2)/g), sinteractive willemite powders. Thermal evolution of crystalline phases was studied using X-ray powder diffraction patterns. The combustion method favors low-temperature formation of willemite compared to the sol-gel method. The powders, when uniaxially pressed and sintered at 1300 degreesC, achieved 78-80% theoretical density. The microstructures of the sintered body show the presence of equiaxed 0.5- to 4-mum grains. Blue pigments of willemite doped with Co2+ and Ni2+ were also prepared by the combustion process.

Characterization and catalytic properties of combustion synthesized Au/CeO2 catalyst

Ceria-supported Au catalyst has been synthesized by the solution combustion method for the first time and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Au is dispersed as Au as well as Au3+ states on CeO2 surface of 20-30 nm crystallites. On heating the as-prepared 1% Au/CeO2 in air, the concentration of Au3- ions on CeO2 increases at the expense of Au. Catalytic activities for CO and hydrocarbon oxidation and NO reduction over the as-prepared and the heat-treated 1% Au/CeO2 have been carried out using a temperature-programmed reaction technique in a packed bed tubular reactor. The results are compared with nano-sized Au metal particles dispersed on alpha-Al2O3 substrate prepared by the same method. All the reactions over heat-treated Au/CeO2 occur at lower temperature in comparison with the as-prepared Au/CeO2 and Au/Al2O3. The rate of NO + CO reaction over as-prepared and heat-treated 1% Au/CeO2 are 28.3 and 54.0 mumol g(-1) s(-1) at 250 and 300 degreesC respeceively. Activation energy (E,) values are 106 and 90 kJ mol(-1) for CO + O-2 reaction respectively over as-prepared and heat-treated 1% Au/CeO2 respectively.

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.

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

CO oxidation by CeO(2)-Al(2)O(3)-CeAlO(3) hybrid oxides

A modified solution combustion technique was successfully used to synthesize sub-10 nm crystallites of hybrid CeO(2)-Al(2)O(3)-CeAlO(3). The fuel in the solution combustion was tuned to obtain mixed oxides and solid solutions of the compound. The compounds were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. XRD and TEM analysis showed the substitution of Al(3+) ions in the CeO(2) matrix when a combination of glycine, urea, hexamine and oxalyl dihydrazide was used as fuel for the synthesis. The compounds showed high activity for CO oxidation and the activity of the compounds was dependent upon the composition of the oxide.

Kinetics of carbon monoxide oxidation with Sn(0.95)M(0.05)O(2-delta) (M = Cu, Fe, Mn, Co) catalysts

Base metal substituted Sn(0.95)M(0.05)O(2-delta) (M = Cu, Fe, Mn, Co) catalysts were synthesized by the solution combustion method and characterized by XRD, XPS, TEM and BET surface area analysis. The catalytic activities of these materials were investigated by performing CO oxidation. The rates and the apparent activation energies of the reaction for CO oxidation were determined for each catalyst. All the substituted catalysts showed high rates and lower activation energies for the oxidation of CO as compared to unsubstituted SnO(2). The rate was found to be much higher over copper substituted SnO(2) as compared to other studied catalysts. 100% CO conversion was obtained below 225 degrees C over this catalyst. A bifunctional reaction mechanism was developed that accounts for CO adsorption on base metal and support ions and O(2) dissociation on the oxide ion vacancy. The kinetic parameters were determined by fitting the model to the experimental data. The high rates of the CO oxidation reactions at low temperatures were rationalized by the high dissociative chemisorption of adsorbed O(2) over these catalysts.

Enhanced photoluminescence of Gd2O3:Eu3+ nanophosphors with alkali (M = Li+, Na+, K+) metal ion co-doping

Gd1.95Eu0.4M0.01O3 (M = Li+ Na+ K+) nanophosphors have been synthesized by a low temperature solution combustion (LSC) method. Powder X-ray diffraction pattern (PXRD), scanning electron microscopy (SEM), UV-vis and photoluminescence (PL) measurements were carried out to characterize their structural and luminescent properties. The excitation and emission spectra indicated that the phosphor could be well excited by UV light (243 nm) and emit red light about 612 nm. The effect of alkali co-dopant on PL properties has been examined. The results showed that incorporation of Li+, Na+ and K+ in to Gd2O3:Eu3+ phosphor would lead to a remarkable increase of photoluminescence. The PL intensity of Gd2O3:Eu3+ phosphor was improved evidently by co-doping with Li+ ions whose radius is less than that of Gd3+ and hardly with Na+, K+ whose radius is larger than that of Gd3+. The effect of co-dopants on enhanced luminescence was mainly regarded as the result of a suitable local distortion of crystal field surrounding the Eu3+ activator. These results will play an important role in seeking some more effective co-dopants. (C) 2011 Published by Elsevier B.V.

An EDC-based turbulent premixed combustion model

In the present study, a new turbulent premixed combustion model is proposed by integrating the Coherent Flame Model with the modified eddy dissipation concept, and relating the fine structure mass fraction to the flame surface density. First, experimental results of turbulent flame speed available from literature are compared with the predicted results at different turbulence intensities to validate the flame surface density model. It is observed that the model is able to predict the turbulent burning speeds accurately. Then, a comprehensive validation is carried out utilizing data on a turbulent lifted methane flame issuing into a vitiated co-flow. Detailed comparison of temperature and species concentrations between experiment and simulation is performed at different heights of the flame. Overall, the model is found to predict both the spatial variation and peak values of the scalars at various heights satisfactorily.

Synthesis, characterization, thermo- and photoluminescence properties of Bi3+ co-doped Gd2O3:Eu3+ nanophosphors

Gd2O3:Eu3+ (4 mol%) co-doped with Bi3+ (Bi = 0, 1, 3, 5, 7, 9 and 11 mol%) ions were synthesized by a low-temperature solution combustion method. The powders were calcined at 800A degrees C and were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Fourier transform infrared and UV-Vis spectroscopy. The PXRD profiles confirm that the calcined products were in monoclinic with little cubic phases. The particle sizes were estimated using Scherrer's method and Williamson-Hall plots and are found to be in the ranges 40-60 nm and 30-80 nm, respectively. The results are in good agreement with TEM results. The photoluminescence spectra of the synthesized phosphors excited with 230 nm show emission peaks at similar to 590, 612 and 625 nm, which are due to the transitions D-5(0)-> F-7(0), D-5(0)-> F-7(2) and D-5(0)-> F-7(3) of Eu3+, respectively. It is observed that a significant quenching of Eu3+ emission was observed under 230 nm excitation when Bi3+ was co-doped. On the other hand, upon 350 nm excitation, the luminescent intensity of Eu3+ ions was enhanced by incorporation of Bi3+ (5 mol%) ions. The introduction of Bi3+ ions broadened the excitation band of Eu3+ of which a new strong band occurred ranging from 320 to 380 nm. This has been attributed to the 6s(2)-> 6s6p transition of Bi3+ ions, implying a very efficient energy transfer from Bi3+ ions to Eu3+ ions. The gamma radiation response of Gd2O3:Eu3+ exhibited a dosimetrically useful glow peak at 380A degrees C. Using thermoluminescence glow peaks, the trap parameters have been evaluated and discussed. The observed emission characteristics and energy transfer indicate that Gd2O3:Eu3+, Bi3+ phosphors have promising applications in solid-state lighting.

Activation of Lattice Oxygen of TiO2 by Pd2+ Ion: Correlation of Low-Temperature CO and Hydrocarbon Oxidation with Structure of Ti1-xPdxO2-x (x=0.01-0.03)

Lattice oxygen of TiO2 is activated by the substitution of Pd ion in its lattice. Ti1-xPdxO2-x (x = 0.01-0.03) have been synthesized by solution combustion method crystallizing in anatase TiO2 structure. Pd is in +2 oxidation state and Ti is in +4 oxidation state in the catalyst. Pd is more ionic in TiO2 lattice compared to Pd in PdO. Oxygen storage capacity defined by ``amount of oxygen that is used reversibly to oxidize CO'' is as high as 5100 mu mol/g of Ti0.97Pd0.03O1.97. Oxygen is extracted by CO to CO2 in absence of feed oxygen even at room temperature which is more than 20 times compared to pure TiO2. Rate of CO oxidation is 2.75 mu mol g(-1) s(-1) at 60 degrees C over Ti0.97Pd0.03O1.97 and C2H2 gets oxidized to CO2 and H2O at room temperature. Catalyst is not poisoned on long time operation of the reactor. Such high catalytic activity is due to activated lattice oxygen created by the substitution of Pd ion as seen from first-principles density functional theory (DFT) calculations with 96 atom supercells of Ti32O64, Ti31Pd1O63, Ti30Pd2O62, and Ti29Pd3O61. The compounds crystallize in anatase TiO2 structure with Pd2+ ion in nearly square planar geometry and TiO6 octahedra are distorted by the creation of weakly bound oxygens. Structural analysis of Ti31Pd1O63 which is close to 3% Pd ion substituted TiO2 shows that oxygens associated with both Ti and Pd ions in the lattice show bond valence sum of 1.87, a low value characteristic of weak oxygen in the lattice compared to oxygens with valence 2 and above in the same lattice. Exact positions of activated oxygens have been identified in the lattice from DFT calculations.

Nanostructured Pd modified Ni/CeO2 catalyst for water gas shift and catalytic hydrogen combustion reaction

Nanostructured Pd-modified Ni/CeO2 catalyst was synthesized in a single step by solution combustion method and characterized by XRD, TEM, XPS, TPR and BET surface analyzer techniques. The catalytic performance of this compound was investigated by performing the water gas shift (WGS) and catalytic hydrogen combustion (CHC) reaction. The present compound is highly active and selective (100%) toward H-2 production for the WGS reaction. A lack of CO methanation activity is an important finding of present study and this is attributed to the ionic substitution of Pd and Ni species in CeO2. The creation of oxide vacancies due to ionic substitution of aliovalent ions induces dissociation of H2O that is responsible for the improved catalytic activity for WGS reaction. The combined H-2-TPR and XPS results show a synergism exists among Pd, Ni and ceria support. The redox reaction mechanism was used to correlate experimental data for the WGS reaction and a mechanism involving the interaction of adsorbed H-2 and O-2 through the hydroxyl species was proposed for CHC reaction. The parity plot shows a good correspondence between the experimental and predicted reaction rates. (c) 2012 Elsevier B.V. All rights reserved.

Particle size, morphology and color tunable ZnO:Eu3+ nanophosphors via plant latex mediated green combustion synthesis

Efficient ZnO:Eu3+ (1-11 mol%) nanophosphors were prepared for the first time by green synthesis route using Euphorbia tirucalli plant latex. The final products were well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), etc. The average particle size of ZnO:Eu3+ (7 mol%) was found to be in the range 27-47 nm. With increase of plant latex, the particle size was reduced and porous structure was converted to spherical shaped particles. Photoluminescence (PL) spectra indicated that the peaks situated at similar to 590, 615, 648 and 702 nm were attributed to the D-5(0) -> F-7(j(j=1,2,3,4)) transitions of Eu3+ ions. The highest PL intensity was recorded for 7 mol% with Eu3+ ions and 26 ml plant latex concentration. The PL intensity increases with increase of plant latex concentration up to 30 ml and there after it decreases. The phosphor prepared by this method show spherical shaped particles, excellent chromaticity co-ordinates in the white light region which was highly useful for WLED's. Further, present method was reliable, environmentally friendly and alternative to economical routes. (c) 2013 Elsevier B.V. All rights reserved.