Monoatomic Oxygen Research Articles

Mechanism and micro-kinetics of direct N2O decomposition over BaFeAl11O19 hexaaluminate and comparison with Fe-MFI zeolites

Mechanistic and kinetic aspects of direct N2O decomposition over BaFeAl11O19 hexaaluminate were investigated in the Temporal Analysis of Products (TAP) reactor and compared with those previously determined for Fe-MFI zeolites. The catalysts were chosen due to their de-N2O operation in significantly different temperature regimes. Several micro-kinetic models were evaluated for describing the transient responses of N2O, N2, and O2 obtained in N2O pulse experiments at 823–973K. Thorough discrimination between these models enabled us to conclude that the preferred models of N2O decomposition over BaFeAl11O19 and Fe-MFI zeolites differ in the reaction pathways leading to O2 and N2. Gas-phase N2 and O2 are simultaneously formed over BaFeAl11O19 upon interaction of gas-phase N2O with a bi-atomic surface oxygen (*–O2) species. Contrarily, the formation of O2 over Fe-MFI occurs via a sequence of three elementary heterogeneous steps and limits the overall rate of N2O decomposition. Despite the easy O2 formation, BaFeAl11O19 is less active for N2O decomposition below 973K than the Fe-MFI zeolites due to the low coverage by *–O2. According to our quantitative micro-kinetic analysis, this species is formed when gas-phase N2O reacts with a mono-atomic oxygen (*–O) species. This reaction pathway is strongly influenced by the degree of isolation of iron species. The higher the degree of iron isolation in the catalyst, the lower the de-N2O activity.

Mechanisms underlying dioxygen reduction in laccases. Structural and modelling studies focusing on proton transfer

BackgroundLaccases are enzymes that couple the oxidation of substrates with the reduction of dioxygen to water. They are the simplest members of the multi-copper oxidases and contain at least two types of copper centres; a mononuclear T1 and a trinuclear that includes two T3 and one T2 copper ions. Substrate oxidation takes place at the mononuclear centre whereas reduction of oxygen to water occurs at the trinuclear centre.ResultsIn this study, the CotA laccase from Bacillus subtilis was used as a model to understand the mechanisms taking place at the molecular level, with a focus in the trinuclear centre. The structures of the holo-protein and of the oxidised form of the apo-protein, which has previously been reconstituted in vitro with Cu(I), have been determined. The former has a dioxygen moiety between the T3 coppers, while the latter has a monoatomic oxygen, here interpreted as a hydroxyl ion. The UV/visible spectra of these two forms have been analysed in the crystals and compared with the data obtained in solution. Theoretical calculations on these and other structures of CotA were used to identify groups that may be responsible for channelling the protons that are needed for reduction of dioxygen to water.ConclusionsThese results present evidence that Glu 498 is the only proton-active group in the vicinity of the trinuclear centre. This strongly suggests that this residue may be responsible for channelling the protons needed for the reduction. These results are compared with other data available for these enzymes, highlighting similarities and differences within laccases and multicopper oxidases.

A unique tridecanuclear Co(II) cluster with derivatised salicylaldoxime ligands: Structure, electrospray ionization mass spectrometry analysis and preliminary magnetic studies

A unique [Co 13(mosao) 8(Hmosao) 6]·(ClO 4) 4·3DMF·MeOH·3H 2O ( 1, H 2mosao = 3-methoxysalicylaldoxime) supercluster has been synthesized by a method of solvothermal reaction. The [Co 13( μ 2-O) 8( μ 3-O) 6( μ 2-O–N) 2( μ 3-O–N) 8] 4+ core is highly conjugated butterfly-like disc, which can be considered as fourteen distorted defective cubes sharing the neighboring faces one another. The electrospray ionization mass spectrometry (ESI-MS) analysis of 1 strongly suggests that Co 13 cluster also exists in solution and maintains a conformation similar to that in the crystal structure, indicating that the significant stability of 1 in solution. Magnetic measurements of 1 reveal antiferromagnetic behaviors resulting from the cooperative magnetic coupling through multiplicate monoatomic oxygen and –N–O– exchange bridges and the odd number of spins result in relatively effective noncompensated moments at very low temperature.

Cu II acetate complexes involving N,N,O donor Schiff base ligands: Mono-atomic oxygen bridged dimers and alternating chains of the dimers and Cu 2(OAc) 4

Two tridentate N,N,O donor Schiff bases, HL 1 (4-(2-ethylamino-ethylimino)-pentan-2-one) and HL 2 (3-(2-amino-propylimino)-1-phenyl-butan-1-one) on reaction with Cu II acetate in presence of triethyl amine yielded two basal-apical, mono-atomic acetate oxygen-bridging dimeric copper(II) complexes, [Cu 2L 1 2(OAc) 2] ( 1), [Cu 2L 2 2(OAc) 2] ( 2). Whereas two other similar tridentate ligands HL 3 (4-(2-amino-propylimino)-pentane-2-one) and HL 4 (3-(2-amino-ethylimino)-1-phenyl-butan-1-one) under the same conditions produced a mixture of the corresponding dimers and a one-dimensional alternating chain of the dimer and copper acetate moiety, [Cu 4L 3 2(OAc) 6] n ( 3) and [Cu 4L 4 2(OAc) 6] n ( 4), formed by a very rare μ 3 bridging mode of the acetate ion. All four complexes ( 1– 4) have been characterized by X-ray crystallography. The isotropic Hamiltonian, H = − JS 1 S 2 has been used to interpret the magnetic data. Magnetic measurements of 1 and 2 in the temperature range 2–300 K reveal a very weak antiferromagnetic coupling for both complexes ( J = −0.56 and −1.19 cm −1 for 1 and 2, respectively).

Mechanistic origin of the different activity of Rh-ZSM-5 and Fe-ZSM-5 in N 2O decomposition

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N 2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low (<550 K) and high (>650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N 2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N 2O, N 2, and O 2. Classical reaction schemes failed to describe the experimental data. The preferred models of N 2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O 2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N 2O participate in the decomposition of gas-phase N 2O. Gas-phase O 2 is formed directly on N 2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O 2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N 2O adsorption and, most importantly, the faster desorption of O 2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N 2O activity.

Micro-kinetic analysis of direct N 2O decomposition over steam-activated Fe-silicalite from transient experiments in the TAP reactor

Mechanistic and kinetic aspects of the direct decomposition of N 2O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N 2O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848 K. The transient responses of N 2O, N 2, and O 2 obtained upon N 2O decomposition were fitted to different micro-kinetic models. Through model discrimination it was concluded that both free iron sites and iron sites with adsorbed mono-atomic oxygen (* O) species are active for N 2O decomposition. Oxygen formation occurs via decomposition of bi-atomic (* O 2) oxygen species adsorbed over the iron site. This bi-atomic oxygen species originates from another bi-atomic oxygen species (O * O), which is initially formed via interaction of N 2O with iron site possessing mono-atomic oxygen species (* O). Based on our modeling, the recombination of two mono-atomic oxygen (* O) species or direct O 2 formation via reaction of N 2O with * O can be excluded as potential reaction pathways yielding gas-phase O 2. The simulation results predict that the overall rate of N 2O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700 K.

Mechanism and Kinetics of Direct N2O Decomposition over Fe−MFI Zeolites with Different Iron Speciation from Temporal Analysis of Products

The mechanism of direct N(2)O decomposition over Fe-ZSM-5 and Fe-silicate was studied in the temporal analysis of products (TAP) reactor in the temperature range of 773-848 K at a peak N(2)O pressure of ca. 10 Pa. Several kinetic models based on elementary reaction steps were evaluated to describe the transient responses of the reactant and products. Classical models considering oxygen formation via recombination of two adsorbed monoatomic oxygen species (*-O + *-O --> O(2) + 2*) or via reaction of N(2)O with adsorbed monoatomic oxygen species (N(2)O + *-O --> O(2) + N(2) + *) failed to describe the experimental data. The best description was obtained considering the reaction scheme proposed by Heyden et al. (J. Phys. Chem. B 2005, 109, 1857) on the basis of DFT calculations. N(2)O decomposes over free iron sites (*) as well as over iron sites with adsorbed monoatomic oxygen species (*-O). The latter reaction originates adsorbed biatomic oxygen species followed by its transformation to another biatomic oxygen species, which ultimately desorbs as gas-phase O(2). In line with previous works, our results confirm that the direct N(2)O decomposition is controlled by pathways leading to O(2). Our kinetic model excellently described transient data over Fe-silicalite and Fe-ZSM-5 zeolites possessing markedly different iron species. This finding strongly suggests that the reaction mechanism is not influenced by the iron constitution. The TAP-derived model was extrapolated to a wide range of N(2)O partial pressures (0.01-15 kPa) and temperatures (473-873 K) to evaluate its predictive potential of steady-state performance. Our model correctly predicts the relative activities of two Fe-FMI catalysts, but it overestimates the absolute catalytic activity for N(2)O decomposition.

Organotin adducts of indomethacin: synthesis, crystal structures and spectral characterization of the first organotin complexes of indomethacin

The complexes [Me 2(Indo)SnOSn(Indo)Me 2] 2, ( 1) [Bu 2(Indo)SnOSn(Indo)Bu 2] 2, ( 2), where Hindo is indomethacin, have been prepared and structurally characterized by means of 119Sn Mössbauer, vibrational, and NMR ( 1H and 13C) spectroscopy. The crystal structures of the complexes [Me 2(Indo)SnOSn(Indo)Me 2] 2 · 2C 4H 8O 2, 3 and 2 have been determined by X-ray crystallography. Each structure is centro-symmetric and features a central rhombus Sn 2O 2 unit with two additional tin atoms linked at the O atoms. Pairs of tin atoms are bridged by bidentate carboxylate ligands and by a monoatomic bridging oxygen. C–H→π, π→π, stacking interactions, inter and intramolecular hydrogen bonds stabilize the structures 2 and 3. Complexes 2 and 3 are self-assembled via π→π, C–H-π and stacking interactions.

Kinetics of the hydroxylation of benzene with N 2O on modified ZSM-5 zeolites

A detailed kinetic study of the hydroxylation of benzene to phenol using nitrous oxide (N 2O) is performed on different, well characterised modifications of ZSM-5 type zeolites. After process studies in a fixed bed reactor for the first time a recycle reactor (Berty-type) is successfully used in direct measurements of the molar rates of reactant consumption and product formation. From those data, an extended reaction network is developed which contained mechanistic steps derived from additional studies. Besides the formation of a chemisorbed, monoatomic oxygen species from N 2O and phenol formation, it can be shown that a consecutive reaction of phenol without the participation of oxygen and a non-selective oxygenation of strongly adsorbed hydrocarbons by oxygen play an important role. The former step is attributed to the accumulation of phenol inside the ZSM-5 crystal due to strong adsorption and slow diffusion of phenol. The latter step results in an accumulation of oxygen within the adsorbed hydrocarbon species until a critical oxygen content is reached and total oxidation starts. An isothermal two-site kinetic model is deduced containing the sorption of the reactants and products. The values of the model parameters allow a comparison of the efficiency of the catalysts on the rates of each step. Simulations of the selectivity–conversion behaviour impressively demonstrate the quality of the kinetic model as predictive tool for process studies.

Nucleation and Growth of Gallium Oxide Tubes, Nanopaintbrushes and Nanowires from Molten Gallium

ABSTRACTWe have synthesized highly crystalline β-gallium oxide tubes, nanowires, and unique one-dimensional structures in the form of nanopaintbrushes using molten gallium and microwave plasma containing a mixture of monoatomic oxygen and hydrogen. Multiple nucleation and growth of gallium oxide nanostructures occurred directly out of molten gallium upon exposure to an appropriate composition of hydrogen and oxygen in the gas phase. Gallium oxide nanowires were 20 to 100 nm thick and tens to hundreds of microns long. In addition to these morphologies, we also report for the first time, non-template based synthesis of novel 2-D networks of crystalline gallium oxide nanowires and nanotubes. Demonstration of this technique with gallium oxide certainly presents a new route for synthesis of nanostructures of other important metal oxides such as indium oxide, tin oxide, and zinc oxide.

Oxygen chemisorption at Cu(110) at 120 K : dimers, clusters and mono-atomic oxygen states

Three distinct states of oxygen have been observed at a Cu(110) surface at 120 K by scanning tunnelling microscopy (STM): isolated oxygen adatoms; pairs or dimers, separated by about 6 A and clusters of five or six atoms arranged anisotropically. There is also evidence for oxygen atoms undergoing ballistic motion as might be expected from “hot” oxygen atoms. Such states of oxygen have been central to the mechanistic models proposed earlier, and based on surface spectroscopic studies, for the oxidation of ammonia at copper surfaces under ammonia-rich conditions.

Formation of Nonplanar CuI(CO)3Tricarbonyls on CuI–ZSM-5:An FTIR Study at 80 K

The selective catalytic reduction of N2O with CH4 in the presence of excess O2 (CH4-SCRN2O) and CH4 + O2 reaction were studied on Ni-MOR, Co-MOR and Fe-MOR, prepared from H-MOR or Na-MOR, by ion-exchange or CVD.FTIR showed that transition metal ions (tmi) were well dispersed, mainly isolated with at least two coordinative vacancies. For CH4-SCRN2O, irrespective of Brønsted acid site and Na+ amount, catalysts were active in the order Fe-MOR > Ni-MOR > Co-MOR. Using a methane-poor mixture, on all samples CH4-SCRN2O consisted of two nearly independent reactions: CH4 + N2O and CH4 + O2. Using a methane-rich mixture, CH4-SCRN2O consisted of a CH4 + N2O + O2 reaction, whose stoichiometric ratios depended on the tmi. Whereas on Ni-MOR methane combustion occurred as side reaction, on Co-MOR and Fe-MOR no side reactions occurred. Accordingly, for CH4 + O2 Co-MOR and Fe-MOR were poorly active, whereas Ni-MOR were highly active.We conclude that in CH4-SCRN2O a monoatomic oxygen form activates methane on Co-MOR and Fe-MOR catalysts, whereas both a monoatomic oxygen form and a molecular oxygen form activate methane on Ni-MOR. Being active for the CH4-SCRN2O and CH4-SCRNOx and forming CO only in traces, Ni-MOR are promising catalysts for the simultaneous SCR of N2O and NOx using CH4 as reducing agent.

Characterization of solid oxide fuel cells based on solid electrolytes or mixed ionic electronic conductors

The relation between cell voltage ( V cell), applied chemical potential difference (Δμ(O 2)) and cell current ( I t ) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current ( I i) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce 0.8Sm 0.2O 1.9 and Ce 0.8Gd 0.2O 1.9 electrolytes with La 0.8Sr 0.16CoO 3 or Pt as the cathode and Ni/Ce 0.9Ca 0.1O 1.9− x or Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.

Selective Oxidation of Methane to Methanol over FePO4 Using N2O and H2 at Low Temperatures

Abstract Methane could selectively be converted to methanol with N2O over iron phosphate (FePO4) catalyst. Addition of hydrogen enhanced the conversion of methane and the yield of methanol. The formation of CH3OH initiated at ≥523 K and the one-pass yield for the sum of CH3OH and HCHO reached 5.3% at 723 K in the presence of hydrogen. A monoatomic oxygen species formed on FePO4 surface was suggeted to be responsible for this active and selective oxidation of methane.

Metal complexes of the anti-inflammatory drug sodium [2-[(2,6-dichlorophenyl)amino]phenyl]acetate (diclofenac sodium). Molecular and crystal structure of cadmium diclofenac

The reactions of ZnCl 2, CdCl 2 and Hg(NO 3) 2 · H 2O with deprotonated diclofenac (L) were studied in aqueous solutions. Complexes of the formulae [Zn(L) 2(H 2O)], [Cd(L) 2(H 2O)], [HgL 2] and (Cd 2(H 2O)(C 2H 5OH) 2(L) 4] n were isolated and characterized as solid products by elemental analyses and spectral (IR, 1H NMR) and thermal studies. The crystal structure of the complex [Cd 2(H 2O)(C 2H 5OH) 2L 4] n was also solved. The central feature of the macromolecule is a dinuclear moiety. Distorted octahedral coordination at Cd(1) is completed by oxygen atoms from one molecule of water, two molecules of C 2H 5OH, two oxygen atoms from one chelating ligand with a monoatomic bridging oxygen that bridges the atoms Cd(2)Cd(I) = 4.490(1) Å and one oxygen from a bidentate bridging carboxylato group that bridges the atoms Cd(1)Cd(2) = 4.300(1) Å. The geometry at Cd(2) is that of a distorted square pyramid with oxygen atoms from one chelating and one monodentate carboxylato group and the monoatomic bridging oxygen atom occupying equatorial positions. The apical site of the square pyramid is occupied by the oxygen atom of the bidentate briding carboxylato group. The crystal structure of the diclofenac acid is also reported.

Surface Oxygen Exchange Kinetics in Oxide-Ion Conducting Solids

AbstractThe surface oxygen exchange on oxides with high oxygen ion mobility is modelled with a two step reaction with adsorbed mono-atomic oxygen species as intermediate. Interpretation of the Po2 dependence of the exchange rates, following from this model, indicates that these adsorbed oxygen atoms are singly charged.For the stabilized δ-Bi2O3 solid electrolyte a good agreement has been found between the isotope exchange model and the electrochemical study of the oxygen exchange using gold electrodes. For the mixed La-Sr cobaltite perovskite a change in the surface exchange reaction is observed going from room temperature to 600°C. Indications are that above 450°C the bulk exchange is rate limiting with a (Po2)−1 dependence while below this temperature it is assumed that the dissociative adsorption is rate limiting with a (Po2)1/2 dependence.

Characterization of transient response curves in heterogeneous catalysis—II Estimation of the reaction mechanism in the oxidation of ethylene over a silver catalyst from the mode of the transient response curves

The reaction mechanism and the kinetic structure of the oxidation of ethylene over a silver catalyst at 91°C have roughly been elucidated from the mode of the transient response curves of ethylene oxide, carbon dioxide and water, due to the concentration jump of reactants. The typical overshoot mode with an instantaneous maximum for C 2H 4O clearly indicates two things. First that the surface reaction controlling in the reaction between adsorbed diatomic oxygen species and gaseous C 2H 4 (an Eley—Rideal type mechanism) with the rapid desorption of C 2H 4O formed and, second, that the slower regeneration of the adsorbed oxygen species. The typical S-shape mode with an overshoot for CO 2 strongly suggests the existence of an intermediate ( In) and, the presence of the ( In) has been actually confirmed by the transient response of its decomposition in an O 2-He stream. The ( In) can desorb as acetic acid only when the surface is reduced by H 2. Much transient data seems to suggest that the ( In) is formed from the reaction of adsorbed monoatomic oxygen species with C 2H 4 and, is decomposed by the reaction with diatomic oxygen species. The false start mode for the response curve of ( In) decomposition indicates the inhibitory effect of adsorbed ethylene on the reaction.

The crystal and molecular structure of Methyltriethylammonium μ-8,8′-oxa-3,3′-commo-bis(undecahydro-1,2-dicarba-3-cobalta-closo-dodecaborate) (1-), [N(C2H5)3CH3]+· [O(C2B9H10)2Co]−

Abstract. The structure of the title compound, C11H38B18CoNO was solved using X-ray diffraction techniques. The crystals are orthorhombic, space group Pna21 with a = 12.508(7), b = 15.935(2),c = 12.618(2) Å, Z = 4. The structure has been determined by the heavy-atom method and refined by the full-matrix least-squares method up to R = 0.066 for 1621 counter reflections. The ionic structure consists of the [N(C2H5)3CH3]+ cation and the [O(C2B9H10)2Co]− anion. The anion is built from two dicarbollide ligands sharing the cobalt atom as a common apex. Moreover, these two ligands are linked by a monoatomic oxygen bridge. The pentagonal planes bound to the cobalt atom are not parallel in the anion, but inclined to each other by an angle of 28.3°, which is caused by the oxygen bridge.

Calculations of Energy Levels of Oxygen and Silicon Vacancies at the Si-SiO2 Interface

In order to understand physical origins of interface states in St-SiO2 system, calculations of interface band structures on the unreconstructed and oxidized surface of Si are carried out simulating the Si-SiO2 system by silicon surface epitaxially covered with monoatomic oxygen layer. Then modifying the model to include oxygen and silicon vacancies, localized levels associated with the vacancies are calculated using perturbation theory. It is shown that so called surface-states on a clean surface whose density is of the order of 1014 cm-2 are completely removed away from the energy gap of Si under the complete oxidation and that the same number of localized levels exist in the energy gap of Si as that of dangling bonds of Si atom associated with the oxygen and silicon vacancies at the interface.

Diffusivity and solubility of oxygen in platinum and Pt-Ni alloys

The diffusivity and solubility of oxygen in metal specimens were determined from measurements of permeation through thin wall tubes containing oxygen and heated by electrical resistance. The permeating oxygen desorbed in vacuum as monoatomic oxygen and the flux was monitored mass-spectrometrically. A known helium leak rate and experimentally-determined sensitivities of the two gases were used for flow calibration. The diffusion coefficient of oxygen in pure platinum, calculated from the time lag to reach a steady state flux, is: {fx065-01} in the temperature range of 1435∮ to 1504°. The solubility of oxygen in pure platinum was obtained from the steady-state flux using the previously determined diffusivity. The solubility is proportional to p1/2O2 and at 1 atm of oxygen the solubility is: CsO = (0.2 ± 0.1) × 1012 exp {fx065-02}, wt pct. Small amounts of nickel, less than required for internal oxidation, had a negligible effect on the oxygen solubility and diffusivity in platinum alloys.