Oxygen Exchange Reaction Research Articles

The reactivity of molecular and atomic oxygen in oxygen-exchange reaction between NO and O2 coadsorbed on a Pt(111) surface

Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature-programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen-exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen-exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long-range order. On the contrary, the oxygen-exchange reaction is greatly suppressed when NO is coadsorbed on the surface with well-ordered p(2×2) oxygen atoms.

Oxygen electrodes of zirconia electrolytes: fundamentals and application to analysis of oxygen containing gases

The kinetics of the oxygen exchange reaction and the reduction of NO at La0.8Sr0.2CoO3−, La0.8Sr0.2MnO3− and Ag-electrodes on stabilized zirconia (8mol% Y203=YSZ) has been studied by means of electrochemical methods (impedance, I-U characteristics). For La0.8Sr0.2CoO3 electrodes the oxygen exchange was found to proceed via the bulk of the electrode with a rate limiting oxygen exchange at the electrode surface. Electrodes based on La0.8Sr0.2MnO3 change their electrode characteristics with the applied potential. At low cathodic polarization the electrode reaction is limited to the three-phase boundary electrode/YSZ/gas. At high cathodic potentials oxygen vacancies are created and consequently additional oxygen is exchanged via the electrode bulk. Furthermore, a significant NO reduction was observed which indicate a reaction with the oxygen vacancies at the electrode surface. For Ag a rate limiting transport of oxygen atoms through the bulk of the electrode was found. As a consequence the oxygen concentration at the electrode surface remains nearly constant. In this context, the observed inactivity for the NO reduction of Ag-electrodes may be explained.

Reactivity investigations of some chosen peroxo-vanadium(V), manganese(III) and chromium(VI) compounds

An interpretative account of the results of reactions in aqueous medium of a highly peroxygenated vanadium(V) complex, K [V(O23]·3H2O, with different organic and inorganic substrates is presented. The reactions were monitored by solution EPR spectroscopy and isolation of products at different stages of the reactions. Redox reactions between diperoxide, K[VO(O2)2(H2O)] and VOSO4 were conducted. The results of the investigation suggest that secondary oxygen exchange-reaction occurs which not only depends on but also utilises the intermediates in the primary reaction during diperoxovanadate-dependent oxidation of VOSO4.

Oxygen exchange reactions catalyzed by vacuolar H +-translocating pyrophosphatase Evidence for reversible formation of enzyme-bound pyrophosphate

Vacuolar membrane-derived vesicles isolated from Vigna radiata catalyze oxygen exchange between medium phosphate and water. On the basis of the inhibitor sensitivity and cation requirements of the exchange activity, it is almost exclusively attributable to the vacuolar H +-pyrophosphatase (V-PPase). The invariance of the partition coefficient and the results of kinetic modeling indicate that exchange proceeds via a single reaction pathway and results from the reversal of enzyme-bound pyrophosphate synthesis. Comparison of the exchange reactions catalyzed by V-PPase and soluble PPases suggests that the two classes of enzyme mediate P iHOH exchange by the same mechanism and that the intrinsic reversibility of the V-PPase is no greater than that of soluble PPases.

Oxygen-Exchange Reactions Accompanying Oxidation of Vanadyl Sulfate by Diperoxovanadate

The absorption spectrum in the visible range and the, ESR spectrum of vanadyl sulfate were lost on addition of diperoxovanadate. The V-51-NMR spectra revealed that diperoxovanadate was reduced to vanadate and its oligomers. With excess vanadyl, tetrameric vanadate was found to be the major product, During this reaction oxygen was released into the medium. The oxygen-release reaction was inhibited by a variety of organic ligands-imidazole, benzoate, formate, mannitol, ethanol, Tris, DMPO, malate, and asparagine. An oxygen-consuming reaction emerged at high concentrations of some of these compounds, e.g. benzoate and ethanol. Using DMPO as the spin-trap, an oxygen-radical species with a 1:2:2:1 type of ESR spectrum was detected in the reaction mixtures resulting from vanadyl oxidation by diperoxovanadate which was unaffected by addition of catalase or ethanol. The results showed that secondary oxygen-exchange reactions occur which depend on and utilize the intermediates in the primary reaction during diperoxovanadate-dependent oxidation of vanadyl sulfate.

Oxygen Involvement in the Partial Oxidation of Methane on Supported and Unsupported V2O5

O exchange of O2 with V2O5/SiO2, V2O5/Al2O3, V2O5, SiO2, and Al2O3 was studied using steady-state isotopic transient kinetic analysis. It was found that bulk V2O5, V2O5/SiO2, V2O5/Al2O3, and Al2O3 exhibited some oxygen exchange capability, whereas SiO2 exhibited negligible oxygen exchange capability under the conditions used. Oxygen exchange reactions were also studied over supported vanadia catalysts under steady-state oxidation reaction conditions. O exchange of O2 with V2O5/SiO2 increased significantly in the presence of methane. This is attributed to a redox reaction making the surface more active for O exchange in the presence of the methane. The involvement of catalyst oxygen in the formation of the products HCHO, CO, and CO2 was demonstrated. However, an estimation of the contribution of the lattice oxygen in the formation of the products is complicated by the presence of secondary O exchange. The total amount of 16O exchanged with the feed O2 and the products indicates that the oxygen associated with silica or the vanadia-silica interface is also involved in the exchange process. In general, O exchange behavior of various product species with V2O5/Al2O3 was found to be similar to that with V2O5/SiO2. However, unlike V2O5/SiO2, the O exchange of O2 with V2O5/Al2O3 did not increase significantly in the presence of methane. This was attributed to the differences in the interactions of vanadia with silica and alumina. It is speculated that the O associated with highly dispersed tetrahedral surface vanadia is involved in a primary oxidation reaction, whereas O associated with the bulk-like vanadia and with the support is involved either in secondary oxidation reactions of HCHO or CO or in secondary O exchange of various O containing product species. Caution must be taken in making any conclusions about the source of reactive oxygen during partial oxidation on oxide catalysts based on isotopic oxygen studies due to the ease of secondary oxygen exchange.

Kinetic Studies of the Catalytic Oxygen Exchange of Chromate Ions with Water by Periodate Ions

Abstract The oxygen exchange reaction between chromate ions and water is catalyzed by the addition of a small amount of periodate ions. The catalytic reaction has been studied by using 18O tracer work (0 °C, pH = 8—12, [Cr(VI)] = 0.075 mol dm−3, [I(VII)] = 10−5 mol dm−3) and an 17O NMR line broadening technique(30 °C, pH = 4—6, [Cr(VI)] = 1 mol dm−3, [I(VII)] = 0.043 mol dm−3). The catalytic process has been interpreted in terms of the reversible condensation of chromate ions with periodate ions to form chromatoperiodate ions. The rate law is given as: Rc = k1[HCrO4−][H4IO6−] + k2[HCrO4−][H3IO62−] + k3[CrO42−][H4IO6−] + k4[CrO42−][H3IO62−], where Rc is defined as the increase in the rate of the oxygen exchange of chromate upon the addition of periodate ions. The catalytic rates were analyzed according to the rate law to obtain the values of the rate constants (k1−k4); k1, k2, and k3 are on the order of magnitude of 103—104 mol−1 dm3 s−1, and k4 = 5 mol−1 dm3 s−1 at 0 °C. For the reaction between H3IO62− and CrO42−, the energy and entropy of activation were obtained to be 26 kJ mol−1 and −143 J K−1 mol−1, respectively.

A study of oxygen interaction with high-temperature superconducting materials by the isotopic exchange method

The isotopic exchange method was used to determine the exchange rate of oxygen of YBa 2Cu 3O 7−δ, Bi 2Sr 2CaCu 2O 8+δ, Bi 1.6Pb 0.4Sr 1.6Ca 2.4Cu 3.4O 10.2+δ and Pb 2Sr 2Y 0.5Ca 0.5Cu 3O 8+δ with molecular oxygen. The measurements were made at temperatures of 600–1000 K and oxygen pressures of 3–15 Torr. The activation energy and the order of the gas-pressure dependence of the oxygen exchange reaction rate were determined.

The kinetics of electrochemical reactions on high temperature fuel cell electrodes

The rates of electrochemical reactions relevant for use in high-temperature solid oxide fuel cells (SOFC) has been investigated as a function of electrode potential, temperature and composition of the gas mixture. From Arrhenius plots, apparent activation energies, E a, and apparent pre-exponential factors, A, were calculated for the oxygen-reduction and oxygen-evolution reactions at La 0.84Sr 0.16MnO 3 cathodes. At low overpotentials (|η| ⩽ 0.2 V), both apparent activation energies and apparent pre-exponential factors are much higher in the temperature range T = 800−1000 °C ( E a ≈ 160−210 kJ/mol, log A ≈ 6−9) compared with those in the range T = 500−800 °C ( E a ≈ 80−110 kJ/mol, log A ≈ 2−4). For oxygen reduction, reaction orders of z e = 1 at p O 2 > 0.2 bar and z e = 0.5 at p O 2 < 0.2 bar were obtained. These values may be related to either oxygen adsorbed as molecules or atoms as the reacting species. From impedance spectroscopy, it follows that the rate of the oxygen-exchange reaction is determined not only by charge transfer, but also by another process, possibly the adsorption or surface diffusion of intermediates. For the nickel zirconia cermet anode fabricated by wet powder spraying (WPS), an increase in sintering temperature to 1400 °C results in an increase in current density. A current density of 0.27 A cm −2 at an overvoltage of 0.1 V may be achieved. From Arrhenius plots, an energy of activation of 130 ± 10 kJ mol −1 was determined.

Oxygen-exchange reaction between O 2 and NO coadsorbed on a Pt(111) surface: reactivity of molecularly adsorbed oxygen

The oxygen-exchange reaction between N 16O and 18O 2 coadsorbed on Pt(111) has been studied by temperature-programmed desorption (TPD). Reaction products of N 18O and 18O 16O are desorbed from Pt(111) initially saturated with 18O 2 at 94 K followed by exposure of N 16O. Three distinct desorption peaks are observed in N 18O TPD spectra at 145, 310, and 340 K, and two peaks in 18O 16O at 155 K and between 600 and 1000 K. In contrast, the exchange reaction is greatly suppressed when oxygen molecules are replaced with oxygen adatoms at three-fold hollow sites of Pt(111). These results strongly suggest that adsorbed oxygen molecules are responsible for the exchange reaction. NO 2 or NO 3 is postulated as a reaction intermediate. However, since desorption signals corresponding to these species are not detected, the oxygen-exchanged products are not due to the cracking processes of the higher order nitrogen oxides in the mass spectrometer. Thus, the reaction proceeds via the intermediate that is dissociated during the elevation of surface temperature.

Activity patterns in some oxidation and reduction reactions on oxides

The literature concerning the activity of the first long period oxides in the oxidation and reduction (oxygen removal) reactions is briefly reviewed and analysed. Essential in the discussion is that in the oxygen 16-oxygen 18 exchange the prevailing mechanism involves dissociation of O 2 by its dissolution in the oxide. Several reactions show the same activity pattern as the just mentioned oxygen exchange reactions and, it is speculated, therefore that a similar mechanism prevails. Another mechanism is the one upon which only a single oxygen atom is transfered between the solid and the gas phase, in one single elementary step. In the same group with such reaction are the reactions in which there is no transfer of oxygen between the phases whatsoever.

Oxygen-17 Magnetic Resonance Study of the Oxygen Exchange of Methyl- and Butylarsonic Acids and Dimethylarsinic Acid with Solvent Water

Abstract The rates of the oxygen-exchange reactions of methyl- and butylarsonic acids and dimethylarsinic acid with solvent water were measured in the pH range below 6, and at temperatures between 50 and 90 °C, by using the 17O NMR line-broadening technique. For methylarsonic acid, the rate RMAA is expressed by the following rate law: RMAA = kh1[H+][H2A] + k1[H2A] + kh2[H+][H2A]2 + k2[H2A]2 + k3[H2A][HA−], where H2A and HA− designate CH3AsO3H2 and CH3AsO3H−, respectively. For butylarsonic acid, the rate law is expressed as: RBAA = kh1[H+][n–C4H9AsO3H2]. For dimethylarsinic acid, its conjugated acid is involved in the rate law as: RCAA = kh1′[(CH3)2AsO2H2+] + khh1′[H+][(CH3)2AsO2H2+]. The exchange rates of methyl- and butylarsonic acids and dimethylarsinic acid (50—90 °C) were analyzed in terms of the respective rate law to obtain the values of the rate constants involved in the rate laws. The activation parameters of these rate constants were obtained; for both the first- and second-order rate constants, the activation energies are characteristic of their small values (from 45 to 20 kJ mol−1), while the activation entropies have large negative values, which are found in the range between −92 and −145 J K−1 mol−1.

Oxygen exchange between an O2 adsorbate and CaO surfaces.

Thermal desorption of $^{16}\mathrm{O}^{18}$O and $^{16}\mathrm{O}_{2}$ gases after $^{18}\mathrm{O}_{2}$ gas adsorption on CaO has been observed in the temperature range 120--1100 K, indicating that single and double oxygen exchange reactions occur on CaO surfaces even at low temperatures. To elucidate the mechanism of the two reactions, an ab initio molecular-orbital calculation is performed by use of a ${\mathrm{Ca}}_{7}$${\mathrm{O}}_{5\mathrm{\ensuremath{-}}}$${\mathrm{O}}_{2}$ cluster model. The reactions are found to take place via ${\mathrm{O}}_{3}$ and ${\mathrm{O}}_{4}$ intermediates, respectively. Their selectivity is ascribed to the absence or to the presence of molecular oxygen ion pairs at a kink site on a single-atom stepped (111) surface before $^{18}\mathrm{O}_{2}$ adsorption.

Characterization of the Light-Induced Oxygen Gas Exchange from the IC 2 Deletion Mutant of Synechocystis PCC 6803 Lacking the Photosystem II 33 kDa Extrinsic Protein

Abstract The absence of the extrinsic Mn-stabilizing 33 kDa protein in the IC 2 mutant of Synechocystis PCC 6803 disturbs the redox cycling of the water splitting system and retards the formation of its higher S-states (I. Vass, K. Cook, S. Deak, S. R. Mayes, and J. Barber, Biochim. Biophys. Acta 1102, 195-201 (1992)). We have performed analyses of the flashinduced oxygen exchange in the mutated cyanobacterium to clarify further the role of the 33 kDa protein. Under aerobic conditions, both the wild type and IC2 mutant show a relatively slow signal of oxygen rise on the first flash which is increased about twice by the addition of 10 μᴍ DCMU and significantly diminished by lowering the oxygen concentration in the medium. According to action spectra measurements, this mode of apparent oxygen release is mediated by PS I and can be attributed to a light induced inhibition of respiratory activity. In contrast to the wild type, having the usual oxygen evolution flash pattern with a periodicity of four, the IC2 mutant shows a binary oscillation pattern of flash-induced respiratory oxygen exchange at a flash frequency 10 Hz, being dampened with DCMU or by a lower flash frequency (&lt; 1 Hz). Oxygen evolution due to water splitting is clearly seen in the IC2 mutant when background far-red illumination is applied to saturate the signal due to respiratory inhibition, but a quadruple oscillatory component of flash-induced oxygen evolution appears only in the presence of artificial electron acceptors under partial aerobic conditions. The mutant possesses a higher PS I/PS II ratio compared to the wild type, as judged from both the flashinduced yields and quantum efficiencies of the steady-state rates of the oxygen exchange reactions. Estimates of antenna sizes indicate about a 20% decrease of optical cross-section at 675 nm of the PS II unit in IC2 mutants in comparison with the wild type. It is suggested that the absence of the 33 kDa protein leads to a modification of the PS II assembly and because of the slowing down of the S-state cycle, the rate of cyclic electron flow around PS II is enhanced. It seems that the absence of the 33 kDa protein in Synechocystis 6803 also disturbs energy transfer between adjacent PS II core complexes and may also alter their association with the phycobilisomes.

High oxygen pressure (Po2 &lt; 3000bar) synthesis and properties of YBaCuO phases

Abstract An overview is presented of our phase diagram, crystal growth, structural, surface and physical characterization studies of double chain compounds YBa2Cu4O8 (124) and Y2Ba4Cu7O15−x (247). The stability regions of 123, 247 and 124 phases (up to 3000 bar O2) are appreciably different for different compositions. The thermodynamics of the oxygen exchange reaction of the orthorhombic 123 with gaseous oxygen has been investigated in the pressure range 10−3–100 bar. Thermodynamics and mechanism of crystal growth have been studied by STM, AFM, SEM, X-ray and ED AX. New results of flux growth phase diagram studies are also presented. Melting of bulk 124 samples under Po2 = 1000 − 2700 bar results in ingots with Jc=4×104A/cm2 at 5K. X—ray single crystal structure analysis of 247 shows, that structural changes and incorporation of impurities occur only in the single chain “123” unit, whereas the double chain “124” unit remains stable.

Oxygen exchange between adsorbed CO2 and MgO surfaces

By using temperature-programmed desorption gas analysis, thermal desorption of C16O2 and C18O16O gases after C18O2 adsorption on Mgo powders was observed in the temperature range 350–100 K, suggesting double oxygen exchange reactions to occur on the surfaces.

Lateral lipid distribution is a major regulator of lipase activity. Implications for lipid-mediated signal transduction.

Pancreatic carboxylester lipase catalyzes the exchange of 18O between water and 13,16-cis,cis-doco-sadienoic acid (DA) in monolayers at the argon-buffer interface (Muderhwa, J.M., Schmid, P.C., and Brockman, H.L. (1992) Biochemistry 31, 141). In mixed monolayers of 18O, 18O-DA and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), both the extent and mechanism of 18O exchange show characteristics of a critical transition in the range of 0.5-0.6 mol fraction of DA (Muderhwa, J.M., and Brockman, H. L. (1992) Biochemistry 31, 149). To determine if the regulatory behavior exhibited on this type of surface is limited to members of the carboxylester lipase gene family (cholinesterases), comparable experiments were performed with a genetically and functionally unrelated lipase, pancreatic colipase-dependent lipase (PL). PL readily catalyzed the exchange of 18O between water and the carboxyl group of DA with enzyme at either monolayer or catalytic levels in the fatty acid-buffer interface. The oxygen exchange reaction obeyed a random, sequential mechanism, indicating that the dissociation of the enzyme.DA complex is much faster than the rate-limiting step in the overall exchange process. Kinetic analysis of oxygen exchange in pure DA monolayers showed a first-order dependence on interfacial PL and DA concentrations from which kcat/Km values were calculated. The oxygen exchange reaction proceeded with a rate constant of 16 x 10(-2) cm2 pmol-1 s-1, a value comparable to that for hydrolysis of the ester substrate, 1,3-dioleoylglycerol. With a monolayer of PL adsorbed to the interfacial phase, kcat/Km for oxygen exchange was about 600-fold lower than the value obtained with catalytic levels of adsorbed enzyme, indicating a possible restriction of substrate diffusion in the protein-covered fatty acid monolayer. With constant bulk PL concentration and mixed lipid monolayers containing DA and the non-substrate lipid, POPC, the extent of oxygen exchange increased abruptly as the abundance of DA in the interface was increased from 0.5 to 0.6 mol fraction. Concomitant with this critical transition was a change in the apparent mechanism of oxygen exchange from coupled to random, sequential. For both the extent of oxygen exchange and its mechanism shift, the critical transition was independent of the lipid packing density, i.e. surface pressure, of the interface. These results show that PL responds similarly to carboxylester lipase with respect to changes in interfacial lipid mole fraction in DA-POPC surfaces.(ABSTRACT TRUNCATED AT 400 WORDS)

Thermodynamics of the oxygen defects and phase stability of orthorhombic YBa2Cu3O7−x at oxygen pressures 10-3-100 bar

The thermodynamics of the oxygen exchange reaction of the orthorhombic YBa2Cu3O7−x phase with gaseous oxygen has been investigated in the pressure range 10-3-100 bar. Assuming that the oxygen exchange takes place at the O(4) and O(5) sites of the structure, thermodynamic functions of this reaction have been calculated as a function of the oxygen pressure. The thermodynamic stability range of the orthorhombic “123” is also discussed.

The Kinetic Study of the Oxygen-Exchange Reaction of Some Alkylarsonates and Dialkylarsinates with Water

Abstract The rates (RS) of the oxygen exchange reaction of alkylarsonates (methyl-, butyl-, hexyl-, and benzyl-) and dialkylarsinates (dimethyl- and dibutyl-) with solvent water were measured in the pH range greater than 6, and at temperatures between 30 and 0 °C. For arsonates, the rate (R) is expressed by the following rate law: R = k1[A2−] + k2[HA−] + k3[H2A] + k4[HA−]2 + k5[HA−][H2A] + k6[H2A]2, where A2−, HA−, and H2A designate R′AsO32−, R′AsO3H−, and R′AsO3H2 (R′ = alkyl), respectively. For arsinates, R = k1[R′2AsO2−] + k2[R′AsO2H] (pH &amp;gt; 9). The exchange rates of arsonates at 0 and 30 °C were analyzed in terms of the rate law to obtain the values of the rate constants (k1–k6). The activation parameters were obtained for the k1 paths of both arsonates and arsinates, which are the reaction between R′AsO32− or R′2AsO2− and water. It is noticeable that the values of the activation entropies of the arsonates were found to be negative, while those of the arsinate were found to be positive. It was suggested that the oxygen exchange of arsonate ions proceeds via an SN2 mechanism, while that of arsinates preceeds via SN1.

Methane formation in H 2,CO mixtures over carbon-supported potassium carbonate

The rates of CH 4 and CO 2 formation over K 2CO 3/carbon have been studied in H 2,CO mixtures as a function of temperature (600–1000 K), total pressure (1.5–10 bar), H 2/CO feed ratio (0.3–8), and the K 2CO 3 loading (0–20 wt%). In H 2,CO mixtures both the rate of CH 4 and CO 2 formation are enhanced by the presence of the potassium catalyst and show the same dependence on the alkali loading as that observed for gasification of carbon in H 2O and CO 2. Below 900 K the CH 4 formation has an apparent order of ≌1.2 in p H 2 and ≌0.3 in p co. Under most experimental conditions the rate of CO 2 formation ( E a(app) = 40–50 kJ mol −1) is higher than that of CH 4 formation ( E a(app) = 130–150 kJ mol −1), resulting in carbon deposition. Only at pressures above 5 bar can CO be selectively converted with H 2 into CH 4 and CO 2 above 900 K. Hydrogenation of the support and of the carbon deposited by the CO disproportionation is not catalysed by potassium. A reactive carbon intermediate is proposed, formed by catalysed dissociation of CO. This can either react with hydrogen to form methane or form the carbon deposit. The oxygen is removed by CO in a manner similar to that in potassium-catalysed oxygen exchange reactions. The observed deactivation is ascribed to a combined effect of blocking of the active sites by carbon deposition and migration of metallic potassium into the carbon matrix, leading to H 2 adsorption on the liberated carbon edge sites, as revealed by temperature-programmed desorption. Potassium is hardly lost at all during the methanation experiments.