Labile Oxygen Research Articles

Synthesis, characterization, redox and photocatalytic properties of Ce1−xPdxVO4 (0 ≤ x ≤ 0.1)

Zircon-type CeVO4 and Ce1−xPdxVO4 (0.02≤x≤0.1) were synthesized by a single step solution combustion method. The materials were characterized by powder X-ray diffraction, magnetic measurement and temperature-programmed reduction (TPR) with H2. The redox properties of CeVO4 and Ce0.98Pd0.02VO4 have been investigated by temperature-programmed reduction. The gas-phase and liquid-phase catalytic properties of CeVO4 and Ce0.98Pd0.02VO4 were investigated. Ce0.98Pd0.02VO4 shows better catalytic activity than CeVO4 towards CO oxidation by O2, NO reduction by CO and the photocatalytic degradation of various dyes. This higher catalytic activity of Ce0.98Pd0.02VO4 can be attributed to the lattice labile oxygen and high surface area compared to that of CeVO4.

Oxygen nonstoichiometry and high-temperature transport in SrFe 1 − xW xO 3 − δ

The complex oxides in the series SrFe 1 − x W x O 3 − δ were synthesized and attested to have cubic structure. The oxygen content was measured by coulometric titration technique, and electrical conductivity was measured by four-probe d.c. method in the oxygen partial pressure range 10 − 18 –0.5 atm and at temperatures 650–950 °C. Thermal expansion tests were performed in air within temperature interval 20–1000 °C. The partial molar thermodynamic functions of labile oxygen ions were calculated using coulometric titration data. Conductivity analysis was utilized to separate partial contributions from oxygen ions, n-and p-type electron charge carriers. The concentration and mobility of p-type charge carriers were calculated by combining data from oxygen content and conductivity measurements. It is shown that partial replacement of iron for tungsten results in a strong disordering of the oxygen sub-lattice that renders the perovskite to brownmillerite phase transition suppressed. The observed ion conductivity level in the sample x = 0.1 at T < 850 °C occurs somewhat higher than in the undoped ferrite while electron contribution remains nearly the same. The increase in the concentration of the dopant results in filling of oxygen vacancies and in respective decrease of the oxygen ion conductivity level. The thermal expansion coefficient is observed to monotonously decrease with the increase in tungsten content.

Axinopsida serricata shell encrustation: A potential indicator of organic enrichment conditions in sediments in the southern Strait of Georgia, British Columbia, Canada

We evaluate the potential of the geographic and within-sediment distribution patterns of rust-coloured shell encrustations on an abundant subtidal bivalve, Axinopsida serricata (Carpenter, 1864) as geochemical indicators of organic enrichment from marine municipal outfalls. The progressive development of shell encrustation over the life of the animals is suggested by heavier encrustations on large shells compared to smaller shells regardless of the geochemical conditions of the habitat. Heavy encrustations decline in an exponential manner at sediment acid volatile sulphide (AVS) levels >7 μmol/g. Analyses show that the reddish shell encrustations are from an amorphous iron oxide or hydroxy-oxide likely micro-biologically mediated; the oxides appear to be embedded within the inner matrix of an organic layer, with a chemically distinct outer layer. A schematic model is proposed which shows how enrichment of labile organic carbon around outfalls affects the availability of dissolved iron in sediments and leads to less extensive encrustations on bivalve shells. Predominantly sandy sediments with low organic loading have much less potential for iron oxide deposition than silty sediments due to greater oxygen penetration into the surface sediment. Shell encrustation appears to be relatively persistent and indicative of long-term conditions, regardless of spatial and temporal fluctuations in sediment geochemistry. With more research on development over the life-cycle of the animal, the pattern of A. serricata shell encrustation has the potential to rapidly provide a map of cumulative labile organic loading and oxygen penetration of sediments around municipal outfalls on the west coast of North America where this species is common. However, caution must be used in interpreting results, since background sediment characteristics (substrate type, bottom currents and sediment transport) can affect encrustation patterns.

LaMO3 (M = Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation

Two new series of perovskite-type oxides LaMO3 (M=Mg, Ti, Fe) with different ratio Mg/Fe (MF) and Ti/Fe (TF) in the B cation site were prepared by annealing the precursor, obtained by the mechanochemical activation (MCA) of constituent metal oxides, at 1000°C in air. In addition, two closely related perovskites LaFeO3 (LF) and LaTi0.5Mg0.5O3 (TM (50:50)) were synthesized in the similar way. Using MCA method, perovskites were obtained in rather short time and at room temperature. The samples were characterized by X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), temperature programmed desorption of oxygen (TPD), Mössbauer spectroscopy, BET surface area measurements and tested in methane deep oxidation. According to XRPD analysis all synthesized samples are almost single perovskite phase, with trace amounts of La2O3 phase. Data of Mössbauer spectroscopy identify Fe3+ in octahedral coordination. The activity of perovskite in methane deep oxidation increases in the order TM (50:50)<MF series<TF series. Higher activity of TF samples in respect to MF with similar Fe content can be related to the structural characteristic, mainly to the presence of predominantly most labile oxygen species evidenced by TPD at lowest temperature of oxygen evaluation. In used experimental conditions, the Fe substituted perovskite are thermal stable up to the temperature of 850°C. The stability of Fe active sites is probably the most important parameter responsible for thermal stability of perovskite, but the atomic surface composition also should be taken into account.

Effect of the redistribution of labile oxygen on the pseudogap state in untwinned single crystal YBa2Cu3O7−x

The influence of room-temperature annealing on the excess conductivity of untwinned single crystal YBa2Cu3O7−x with a small deviation of from oxygen stoichiometry is investigated. It is found that the range of existence of the pseudogap regime undergoes a relative expansion of the range of existence of the pseudogap regime during a four-day period of annealing. The absolute magnitude of the pseudogap decreases by approximately 10%.

Effect of high pressure on the fluctuation conductivity and the charge transfer of YBa2Cu3O7−δ single crystals

The influence of hydrostatic pressure of up to 11kbar on the fluctuation conductivity in the ab plane of the YBa2Cu3O7−δ (YBCO) single crystals with various oxygen concentrations is studied. In contrast to samples of stoichiometric composition, the application of high pressure on samples with oxygen deficiency, leads to the decrease of the coherence length perpendicular to the ab plane ξc(0). The pressure-induced redistribution of the labile oxygen causes a shift of the temperature interval in which three-dimensional superconducting fluctuations exist and a diffusion redistribution of the labile components from the phase of lower critical temperature to the high-temperature superconducting phase.

Redistribution of carriers in oxygen-underdoped Y1Ba2Cu3O7−δ at high hydrostatic pressure

The influence of high hydrostatic pressure up to 7kbar on the conductivity and charge transport in the ab plane is investigated in Y1Ba2Cu3O7−δ single crystals with an oxygen deficiency δ≈0.55 and a critical temperature Tc≈45K. It is shown that the pressure-induced redistribution of the labile oxygen leads to a change of the value of the activation energy for charge transport and to enhancement of phase separation in the sample. It is found that the relaxation behavior of the electrical resistance of a sample during a hold at room temperature after application and removal of pressure exhibits substantial asymmetry.

Study on SCR De NO x mechanism through in situ electrical conductivity measurements on V 2O 5-WO 3/TiO 2 catalysts

V 2O 5-WO 3/TiO 2 catalysts were prepared by impregnation method and in situ electrical conductivity measurements were carried out to investigate the reaction mechanism for ammonia SCR (selective catalytic reduction) of NO x . The electrical conductivity change with ammonia supply and the increase of electrical conductivity were mainly caused by reduction of the labile surface oxygen. The electrical conductivity change of catalysts shows close relationship with the conversion rate of NO x . Variation of conversion rate in atmosphere without gaseous oxygen also supports that the labile lattice oxygen is indispensable in the initial stage of the de NO x reaction. These results suggest that the liable lattice oxygen acts decisive role in the de NO x mechanism. They also support that De NO x reaction occurs through the Eley-Rideal type mechanism. The amount of labile oxygen can be estimated from the measurement of electrical conductivity change for catalysts with ammonia supply.

Promotional Effect of Bismuth as Dopant in Bi-Doped Vanadyl Pyrophosphate Catalysts for Selective Oxidation of n-Butane to Maleic Anhydride

Bismuth-promoted (1% and 3%) vanadyl pyrophosphate catalysts were prepared by refluxing Bi(NO 3) 3· 5H 2O and VOPO 4· 2H 2O in isobutanol. The incorporation of Bi into the catalysts lattice increased the surface area and lowered the overall V oxidation state. Profiles of temperature programmed reduction (TPR) in H 2 show a significant shift of the maxima of major reduction peaks to lower temperatures for the Bi-promoted catalysts. A new peak was also observed at the low temperature region for the catalyst with 3% of Bi dopant. The addition of Bi also increased the total amount of oxygen removed from the catalysts. The reduction pattern and reactivity information provide fundamental insight into the catalytic properties of the catalysts. Bi-promoted catalysts were found to be highly active (71% and 81% conversion for 1% and 3% Bi promoted catalysts, respectively, at 703 K), as compared to the unpromoted material (47% conversion). The higher activity of the Bi-promoted catalysts is due to that these catalysts possess highly active and labile lattice oxygen. The better catalytic performance can also be attributed to the larger surface area.

Synthesis of Methylene- and Difluoromethylenephosphonate Analogues of Uridine-4-phosphate and 3-Deazauridine-4-phosphate

Cytidine triphosphate synthetase (CTPS) catalyzes the formation of cytidine triphosphate from glutamine, uridine-5'-triphosphate (UTP), and adenosine-5'-triphosphate. Inhibitors of CTPS are of interest because of their potential as therapeutic agents. One approach to potent enzyme inhibitors is to use analogues of high energy intermediates formed during the reaction. The CTPS reaction proceeds via the high energy intermediate UTP-4-phosphate (UTP-4-P). Four novel analogues of uridine-4-phosphate (U-4-P) and 3-deazauridine-4-phosphate (3-deazaU-4-P) were synthesized in which the labile phosphate ester oxygen was replaced with a methylene and difluoromethylene group. The methylene analogue of U-4-P, compound 1, was prepared by a reaction of the sodium salt of tert-butyl diethylphosphonoacetate with protected, 4-O-activated uridine followed by acetate deprotection and decarboxylation. It was found that this compound undergoes relatively facile dephosphonylation presumably via a metaphosphate intermediate. The difluoromethylene derivative, compound 2, was prepared by electrophilic fluorination of protected 1. This compound was stable and did not undergo dephosphonylation. Synthesis of the methylene analogue of 3-deazaU-4-P, compound 3, was achieved by ribosylation of protected 4-(phosphonomethyl)-2-hydroxypyridine. Electrophilic fluorination was also employed in the preparation of protected 4-(phosphonodifluoromethyl)-2-hydroxypyridine which was used as the key building block in the synthesis of difluoro derivative 4. These compounds represent the first examples of a nucleoside in which the base has been chemically modified with a methylene or difluormethylenephosphonate group.

MoVW mixed metal oxides catalysts for acrylic acid production: from industrial catalysts to model studies

(MoVW)5O14-type oxides were identified as the active and selective components in industrial acrylic acid catalysts. Tungsten is suggested to play an important role as a structural promoter in the formation and stabilization of this oxide. Vanadium is responsible for high catalytic activities but is detrimental for the stability of this oxide at the necessary high concentrations for optimum catalytic performance. The activity of mixed MoVW oxide catalysts for methanol, propene, and acrolein partial oxidation could be considerably improved, when the amount of the (MoVW)5O14-type oxide was increased by thermal annealing. A model is proposed on the basis of the correlation between Raman wavenumber and bond order and degree of reduction, which explains the observed different selectivities of MoO3−x and the (MoVW)5O14-type oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. According to this model, the (MoVW)5O14 mixed oxide catalyses partial oxidation because of its intermediate C–H activation and oxygen releasing oxygen functionalities. However, these (MoVW)5O14-type industrial oxidation catalysts are heterogeneous and highly complex systems. Their physicochemical characterization also revealed that their chemical bulk and surface compositions vary with thermal activation and oxygen potential. A core-shell model is suggested to describe the active catalyst state, the shell providing a high number of active centers, the core high electronic conductivity and ion mobility. The fact that the surface composition of such catalysts is considerably different from their bulk compositions, most probably implies that the “molecular structure” at their surface differs too considerably from their bulk crystal structure. Hence, the posed question about the active catalyst structure and its relation to its catalytic performance cannot unambiguously be explained by the crystallographic structure, but still remains unsolved.

Influence of high pressure on the fluctuation conductivity of YBa2Cu3O7−δ single crystals with an oxygen deficiency

The influence of high hydrostatic pressure of up to 11kbar on the fluctuation conductivity of YBa2Cu3O7−δ single crystals with an oxygen deficiency δ≈0.5 and a critical temperature tc≈50K is investigated. It is found that for these samples, unlike samples of stoichiometric composition, the application of high pressure leads to a decrease of the coherence length perpendicular to the ab plane, ξc(0). It is shown that the pressure-induced redistribution of the labile oxygen causes a shift of the temperature interval in which three-dimensional superconducting fluctuations exist.

The Role of Lattice Oxygen in the Selective Catalytic Reduction of NOx on V2O5/TiO2 Catalysts

In situ electrical conductivity measurements on <TEX>$V_2O_5WO_3/TiO_2$</TEX> catalysts were carried out at between 100 and <TEX>$300^{\circ}C$</TEX> under pure oxygen, NO and <TEX>$NH_3$</TEX> to investigate the reaction mechanism for ammonia SCR (selective catalytic reduction) de NOX. The electrical conductivity of catalysts changed irregularly with supply of NO. It was, however, found that the electrical conductivity change with ammonia supply was regular and the increase of electrical conductivity was mainly caused by reduction of the labile surface oxygen. The electrical conductivity change of catalysts showed close relationship with the conversion rate of NOx. Variation of conversion rate in atmosphere without gaseous oxygen also showed that labile lattice oxygen is indispensable in the initial stage of the de NOx reaction. These results suggest that liable lattice oxygen acts decisive role in the de NOx mechanism. They also support that de NOx reaction occurs through the Eley?Rideal type mechanism. The amount of labile oxygen can be estimated from the measurement of electrical conductivity change for catalysts with ammonia supply. This suggests that measurement of the change can be used as a measure of the de NOx performance.

Role of nitrogen dioxide in the oxidation of diesel soot on promoted mixed catalysts with fluorite and perovskite structures

The oxidation of soot on catalysts with the perovskite and fluorite structures (including platinum-promoted catalysts) in the presence and in the absence of NO2 was studied using in situ IR spectroscopy and temperature-programmed techniques (TPR, TPD, and TPO). It was found that, as a rule, the temperature of the onset of soot oxidation considerably decreased upon the addition of NO2 to a flow of O2/N2, whereas the amount of oxygen consumed in soot oxidation considerably increased. To explain these facts, we hypothesized that the initiation of soot combustion in the presence of NO2 was related to the activation of the NO2 molecule through the formation (at a low temperature) and decomposition (at a high temperature) of nitrate structures on the catalyst. Superequilibrium amounts of NO2 resulted from the decomposition of nitrate complexes immediately on the catalyst for soot combustion. Based on a comparison between catalyst activities and data obtained by TPR and the TPD of oxygen, a conclusion was drawn that the presence of labile oxygen in the catalyst is a necessary but insufficient condition for the efficient occurrence of a soot oxidation reaction in the presence of NO2. The introduction of platinum as a constituent of the catalyst increased the amount of labile oxygen and, as a consequence, increased the amount of highly reactive nitrate complexes. As a result, this caused a decrease in the temperature of the onset of soot combustion.

The thermodynamic properties of weakly bound oxygen and electron transport in Sr3Fe2O6+δ under oxidizing conditions

The content of oxygen, electrical conductivity, and thermal electromotive force were measured for ferrite Sr3Fe2O6+δ over the oxygen partial pressure range 10−4–0.5 atm at 650–950°C. The partial molar thermodynamic functions of weakly bound oxygen in the oxide structure were determined. Labile oxygen ions were characterized as an ensemble of weakly interacting particles. The predominant charge carriers under experimental conditions were electron holes. An analysis of conductivity was performed using the data on the oxygen content. The concentration, activation energy, and mobility of charge carriers were determined. The results can be satisfactorily interpreted using the polaron transfer model of conductivity taking into account the charge disproportionation reaction for iron ions, 2Fe4+ = Fe3+ + Fe5+.

Reduction of N 2O over carbon fibers promoted with transition metal oxides/hydroxides

Reductions of N 2O on oxides/hydroxides of nickel, cobalt or manganese supported on carbon fibers were studied. Supports and catalysts were characterized by low temperature argon sorption, temperature-programmed desorption (TPD) of surface oxides and temperature-programmed desorption of ammonia (TPAD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). Conversion of N 2O depended on the initial pretreatment of the support, the origin of the effect lying in a different distribution of active material. The differences in distribution influenced also the mobility of surface oxygen. The presence of more labile oxygen resulted in its easier transfer and faster formation of CO 2.

Nature of the Catalytically Labile Oxygen at the Active Site of Xanthine Oxidase

In this paper we report the results of molybdenum K-edge X-ray absorption studies performed on the oxidized active site of xanthine oxidase at pH 6 and 10. These results indicate that the active site possesses one terminal oxygen ligand (Mo=O), two thiolate ligands (Mo-S), one terminal sulfido ligand (Mo=S), and one Mo-OH moiety. EXAFS analysis demonstrates that the Mo-OH bond shortens from 1.97 A at pH 6 to 1.75 A at pH 10, which is consistent with the generation of a Mo-O- moiety. This study provides convincing structural evidence that the catalytic oxygen donor at the oxidized active site of xanthine oxidase is Mo-OH rather than the Mo-OH2 ligation previously suggested by X-ray crystallography. These results support a mechanism initiated by base-assisted nucleophilic attack of the substrate by Mo-OH.

Oxidising CO to CO2 using ceria nanoparticles

We calculate, using simulated amorphisation and recrystallisation (A&R), that ceria (CeO2) nanoparticles, about 8 nm in diameter, comprise a high concentration of labile surface oxygen species, which we suggest will help promote the oxidation of CO to CO2. In particular, the ceria nanoparticle contains a high proportion of reactive {100} surfaces, surface steps and corner sites. When reduced to CeO1.95, the associated Ce3+ species and oxygen vacancies decorate step, corner and {100} sites in addition to plateau positions on {111}. The energetics of CO oxidation to CO2, catalysed by a ceria nanoparticle, is calculated to be lower compared with CO oxidation associated with the lowest energy surface (i.e. CeO2(111)) of the corresponding 'bulk' material. Our calculated morphologies for the ceria nanoparticles are in accord with experiment.

Identification of Surface Species on Titania-Supported Manganese, Chromium, and Copper Oxide Low-Temperature SCR Catalysts

TiO2-supported transition metal oxides (Mn, Cr, and Cu) for the SCR of NO with NH3 have been synthesized by wet impregnation. The adsorption and coadsorption of NH3, NO, and O2, in conjunction with in situ FT-IR spectroscopy, was used to elucidate the reaction mechanism as the samples were heated from 323 to 673 K. While Cr was the only transition metal that generated significant amounts of Bronsted acidity, strong Lewis acid sites were present over all of the materials. The peak strength corresponding to the δs(NH3) coordinated to Lewis acid sites decreased in the following order: Ti > Mn > Cr ∼ Cu. Similarly, the peak strength corresponding to the δas(NH3) coordinated to Lewis acid sites decreased as follows: Mn > Cr ∼ Cu. Exposing the catalysts to oxygen before the introduction of NO did not impact the adsorption of NO as nitrates on the catalysts, suggesting that labile lattice oxygen plays an important role in the formation of nitrates. Three types of nitrates were observed after the adsorption of ...

The Difluoromethylene Group as a Replacement for the Labile Oxygen in Steroid Sulfates: a New Approach to Steroid Sulfatase Inhibitors.

Abstract Several estrone sulfate and estradiol sulfate analogues, in which the sulfate group was replaced with an α,α-difluoromethylenesulfonate group or an α,α-difluoromethylenetetrazole group, were examined as inhibitors of steroid sulfatase (STS). These compounds were 4.5–10.5 times more potent than their non-fluorinated analogues. Moreover, the presence of the fluorines changed the mode of inhibition from mixed to competitive. The inhibitor bearing the α,α-difluoromethylenetetrazole group exhibited an affinity for STS approaching that of the natural STS substrate, estrone sulfate. Possible reasons for the enhanced affinity of the fluorinated compounds compared to their non-fluorinated counterparts are discussed.