Sivanandi Rajadurai

Pathways for Carboxylic Acid Decomposition on Transition Metal Oxides

Abstract The concept of structure sensitivity is well established for reactions catalyzed by metals as it has been generally demonstrated by the use of supported metal catalysts exhibiting different particle size [l-71. The con-cept of structure sensitivity in catalysis by metal oxides is considerably less well developed than in catalysis by metals, in spite of the growing number of examples of such reactions. Characterization of oxide catalyst is generally more problematical than that of metal; it is difficult, for example, to titrate the active surface areas of supported oxides by chemisorption techniques. Carboxylic acid decomposition could be used as a probe to establish struc-tural dependence and selectivity on metal oxides. For example, in the case of formic acid decomposition, bimolecular decomposition of two adsorbed formates occurs on a surface with Ti4+ cation of fourfold oxygen coordi-znation while unimolecular decomposition occurs in the case of formates adsorbed on Ti4+ fivefold coordinated c...

Surface characterization and catalytic properties of La1−xAxMO3 perovskite type oxides. Part I. Studies on La0.95Ba0.05MO3 (M = Mn, Fe or Co) oxides.

Abstract Perovskite type La 0.95 Ba 0.05 MO 3 ( M = Mn , Fe or Co ) oxides were prepared by a liquid precursor method and characterized by X-ray diffraction, oxygen desorption, and X-ray photoelectron spectroscopic studies. XPS analysis revealed the characteristic changes of the O1s peak (∼530 eV) resulting from the substitution of Ba in LaMO 3 . The methane deep oxidation reaction was carried out in a plug flow reactor with 2% CH 4 in air in the temperature range of 200 to 700°C. Among the three compounds, the Mn perovskite showed the lowest temperature for the methane conversion with a maximum at 700°C. Temperature programmed desorption (TPD) experiments also indicated the maximum oxygen desorption for Mn perovskite in this temperature range. The results for oxygen chemisorption and surface area of these samples were found to be consistent with the observed variation in the catalytic oxidation of methane.

Evidence for non-radiative triplet-doublet energy transfer in the naphthalene-benzophenone system in tetrahydrofuran

Abstract Triplet-doublet energy transfer from the naphthalene triplet to the benzophenone ketyl radical has been studied in tetrahydrofuran at room temperature by measuring transient absorption and fluorescence spectra and decay profiles. The rate constant for the energy-transfer process has been determined to be 7 × 10 9 M −1 s −1 in tetrahydrofuran based on an analysis using a simple kinetic model.

Catalytic decomposition of nitrous oxide over perovskite type solid oxide solutions and supported noble metal catalysts

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been investigated over various solid oxide solutions (SOS), La0.8Sr0.2MO3−δ (M=Cr, Fe, Mn, Co or Y), La1.8Sr0.2CuO4−δ and supported Pd, Pt catalysts. The reaction was carried out in a gradientless recycle reactor at 1 atm pressure with a feed gas containing about 0.5% N2O (in helium). Among the various solid solutions, La0.8Sr0.2CoO3−δ showed a maximum N2O conversion of 90% at 600‡C. The order of activity observed for N2O decomposition was La0.8Sr0.2CoO3−δ>La0.8Sr0.2FeO3−δ>La1.8Sr0.2CuO4−δ> La0.8Sr0.2MnO3−δLa0.8Sr0.2CrO3−δ≈La0.8Sr0.2YO3−δ. The activity of La0.8Sr0.2CoO3−δ was compared with supported Pd, Pt and also with unsubstituted LaCoO3 catalysts under similar reaction conditions. Among all the catalysts tested in this study, Pd/Al2O3 showed the lowest light-off temperature for N2O decomposition. The activity of La0.8Sr0.2CoO3−δ was found to be comparable to Pd/Al2O3 catalyst at temperatures above 500‡ C. The influence of added oxygen (about 4%) in the feed was examined over La0.8Sr0.2CoO3−δ and Pd/Al2O3 catalysts and only in the case of cobalt catalyst was the conversion of N2O decreased by ∼13%. By choosing varied sintering conditions, La0.8Sr0.2CoO3−δ of different BET surface areas were prepared and the light-off temperature was found to decrease with increase in surface area. The results obtained over solid solutions are discussed on the basis of the cation mixed valency and oxygen properties of the catalyst.

Solid oxide solutions as catalysts —A comparison with supported Pt

Solid oxide solutions (SOS) of La, Sr and Cr of the formula La1-xSrxCrO3-β (where δ is the deviation from stoichiometry and 0 <x < 0.5) exhibit catalytic activity for oxidation (CO-oxidation) which rivals that of a highly dispersed 0.5% Pt/alumina fume abatement catalysts. Hydrogenation (of propylene) activity is also manifest with the SOS formulation. Exposure of the SOS catalyst to H2S does not reduce catalytic activity.

TRANSIENT PHENOMENA IN THE PULSE RADIOLYSIS OF RETINYL POLYENES—7. RADICAL ANIONS OF VITAMIN A AND ITS DERIVATIVES

Abstract— Upon e‐‐pulse irradiation in nonprotic solvents, all‐trans retinol (ROH) and retinylmethyl ether (ROMe) form transient species (τ= 0.5–7μs, λmax=575–590 nm) identifiable as radical anions. Similar species are also formed upon laser pulse photoexcitation of these retinyl derivatives in the presence of N,N‐dimethylaniline in acetonitrile. In contrast, electron transfer or attachment to all‐trans retinyl acetate (ROAc) and palmitate (ROPa) results in ‘instantaneous’ loss of carboxylate anions from electron adducts giving the retinylmethyl radical (R‐, λmax= 395 nm, τk > 100 μ,s); the radical anions in these cases are too short‐lived to be detected by nanosecond pulse radiolysis. The lifetimes of radical anions of ROH and ROMe are very sensitive to water and alcohols (e.g. kq= 107M‐1 s‐1 with methanol as quencher for ROH‐ in tetrahydrofuran). Based on these findings, the spectral dissimilarity of the one‐electron reduction products from ROH and ROAc in alcohols and aqueous micelles becomes explainable in terms of fast formation of protonated radical anions (RH(OH), τ1/2, > 100 μs, λmax=370–375 nm) in the case of ROH and of retinylmethyl radical via loss of AcO‐ from radical anion in the case of ROAc. In tetrahydrofuran, the complexation of ROH‐ with cations such as Na+ and Bu4N+ affects the relative importance of its major decay modes, namely, protonation and dehydroxylation, the latter process being significantly enhanced by the presence of Na+.

Surface characterization and catalytic properties of La1 − xAxMO3 perovskite oxides. Part II. Studies on La1 − xBaxMnO3 (0 ⩽ x ⩽ 0.2) oxides

Abstract Barium substituted lanthanum manganite perovskite-type oxides were prepared by a liquid precursor method and characterized by X-ray diffraction, oxygen desorption, and X-ray photoelectron spectroscopic studies. XPS analysis revealed the significant changes in the doublet characteristics of the Ols peak (∼ 530 eV) resulting from the substitution of Ba in LaMnO 3 . A well defined Ba3d spectra was seen only with more than 10 mol% substitution of Ba in La site. Methane oxidation reaction was carried out in a flow reactor with a feed containing 0.28% CH 4 , 15% oxygen and balance helium. The conversion of methane was followed in the temperature range of 200 to 700 °C. The light-off temperature for 50% methane conversion was found to be the lowest (490 °C) for La 0.9 Ba 0.1 MnO 3 composition. Temperature Programmed Desorption (TPD) experiments of oxygen indicate two maxima centered around 320–550 °C and 650–760 °C respectively. In the low temperature oxygen desorption region, the peak maximum was found to shift with Ba substitution which also corresponded to the activity of methane. The results for oxygen chemisorption and surface composition of these samples are related to the methane oxidation properties.

Micellar effects on photoprocesses in retinyl polyenes

Abstract The effects of micellar solubilization on excited-state properties of several retinyl polyenes have been examined primarily by nanosecond laser flash photolysis. The relative intensity of band system III (254–256 nm) in the ground state absorption spectrum of 11-cis retinal decreases significantly on going from methanol to micellar solutions, suggesting that the 12-s-trans form of 11-cis retinal is relatively favored in the organized media. In addition to microsecond transient phenomena due to triplets, the laser flash photolysis of all-trans and 11-cis retinal and all-trans retinyl Schiff base incorporated into micelles leads to ‘permanent’ absorption changes attributable to photoisomerization (in the case of retinals) and protonation and/or complexation with water (in the case of Schiff base). All-trans retinol and retinyl acetate in micellar solutions undergo ionic photodissociation leading to long-lived retinyl carbocation (λmax = 585–600 nm), the process being monophotonic in the case of retinyl acetate and predominantly bipho-tonic in the case of retinol. The trends in the location of ground-state absorption maxima (IBu+*←IAg) and triplet yield of retinals, and photodissociation yield of retinyl acetate suggest that the polarity of the environment probed by the polyene systems increases in the order: Triton X-100

Synthesis, structural characterization and catalytic study of ZnCrFeO4 spinel

Abstract Mixed oxide catalyst of Zn, Cr and Fe in a 1:1:1 atomic concentration prepared by the slurry method and calcined at 873 K is found to be very active for decarboxylation of acetic acid. This system is multiphasic. One of the phases is ZnCrFeO4. The multiphasic oxide gives a single ZnCrFeO4 phase on heat treatment at 1523 K for 24 hours in the presence of oxygen. The variation of saturation magnetic susceptibility with temperature clearly indicates the crystallization of the spinel. The unit cell parameters of the face centered cubic lattice of the normal spinel ZnCrFeO4, calculated using the characteristic X-ray diffraction pattern, are given. The catalytic activity of this spinel for decarboxylation of acetic acid is determined and the results are discussed.

Catalytic activity of the 1:1:1 Zn, Cr and Fe mixed oxide: Mechanistic study of the ketonization of acetic acid

Abstract The catalytic activity of a ternary oxide mixture of Zn, Cr and Fe in 1:1:1 atomic concentration has been studied using ketonization of acetic acid. The Arrhenius plot for the reaction shows a break around 673 K indicating the dependence of the reaction mechanism on temperature. Electrical conductance measurement also shows a change in the electronic nature of the catalyst around 673 K. The utilization of dehydration and dehydrogenation active sites of the catalyst for the ketonization process is established by the mutual effect of isopropyl alcohol and acetic acid on each others reaction on the catalyst. Competitive reaction study shows that ketonization follows Langmuir-Hinshelwood mechanism where as esterification follows Rideal-Eley mechanism. Product inhibition studies and infrared spectral studies show that the reaction is between two adsorbed acetate ions above 673 K and between one adsorbed acetate ion and an adsorbed acyl carbonium ion below 673 K.