Removal Of Lattice Oxygen Research Articles

Thermal Decomposition of a Chemical Warfare Agent Simulant (DMMP) on TiO2: Adsorbate Reactions with Lattice Oxygen as Studied by Infrared Spectroscopy

The thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on high surface area TiO2 nanoparticles (Degussa P25) has been studied by transmission infrared spectroscopy. The dominant reaction channel in the low-temperature regime from 295 to 400 K is the nucleophilic attack of adsorbed DMMP by neighboring oxygen to produce Ti−OCH3 and a variety of P−Ox surface bound groups. Arrhenius studies reveal an activation energy of ∼48 kJ mol−1 for the conversion of surface bound phosphoryl (P═O) groups into P−Ox species. Above 400 K, thermally activated lattice oxygen begins to play a dominant role in driving the oxidation of surface Ti−OCH3 groups. The electrons left behind following the extraction of lattice oxygen are observed by a broad rise in the infrared absorbance as the electrons are excited from shallow traps into the conduction band. Tracking lattice oxygen removal as a function of temperature reveals an activation energy of ∼33 kJ mol−1, over the temperature range ∼400−600 K, for the high-temperature oxidation of strongly bound surface adsorbates. These measurements can be employed to provide a more complete understanding of the key role that lattice oxygen plays in the degradation of adsorbates on the surface of active nanoparticulate semiconductor oxides.

Adsorption and Decomposition of Methanol on Gallium Oxide Polymorphs

The adsorption of methanol was studied on three gallia polymorphs (α, β, and γ), pretreated under oxygen or hydrogen at 723 K. Their Brunauer−Emmett−Teller surface areas were in the range 12−105 m2 g−1. Methanol (or methanol-d3) chemisorbs on the gallium oxides both molecularly, as CH3OHS (or CD3OHS,) and dissociatively, as methoxy (CH3O or CD3O) species, at 373 K. The quantification of the total amount of chemisorbed methanol at this temperature allowed us to determine the number of available surface active sites per unit area (NS), which is in the range 1−2 μmol m−2 for the oxygen pretreated oxides at 723 K. The density of active sites was moderately smaller (∼25%) after pretreating the oxides under hydrogen at 723 K. The temperature-programmed surface reaction of adsorbed methanol and methoxy was followed by mass spectrometry and infrared spectroscopy under He flow, up to 723 K. It was found that, upon heating above 473 K, methoxy oxidized to methylenbisoxi (H2COO) and, then, to formate (HCOO) species, and traces of dimethyl ether were also detected. Surface formate species further decompose to give CO(g) and CO2(g) at temperatures higher than 573 K, with the concurrent generation of OH and H species over the surface, which react toward H2(g). It is suggested that the CO2 production implies the removal of lattice oxygen, generating a surface oxygen vacancy, which can be restored by water molecules from the gas phase. Thus, gallia can be envisaged as a promising support for the steam reforming of methanol, as long as a (noble) metal officiates/acts as a rapid H2 releaser from the surface.

Vanadium Oxides on Aluminum Oxide Supports. 3. Metastable κ-Al2O3(001) Compared to α-Al2O3(0001)

Low-coverage vanadia species (monomers, dimers, trimers, and one-dimensional vanadia rows) as well as vanadium oxide films of varying thickness supported on the metastable κ-Al2O3(001) surface are investigated by density functional theory in combination with statistical thermodynamics. At low-vanadium chemical potentials and typical reducing conditions, species with V−O(3)−Al interface bonds are stable. These aggregates are partially reduced with vanadium in the VIII oxidation state. This correlates with defect formation energy values for the initial removal of lattice oxygen in the range of 1.3−2.7 eV. As the length of the polymeric species increases, the reduction energy decreases. We demonstrate that the support structure does affect the structure of the model catalyst and the lattice oxygen bond strength. On the α-Al2O3(0001) surface, the only stable low-coverage VOx species are dimers with V−O(2)−Al interface bonds and a defect formation energy of 2.8 eV. Reduction remains more facile for vanadia fil...

The Effect of Embedding Conditions on the Thermal Conductivity of .BETA.-Si3N4

Dense β-Si3N4 ceramics were prepared from the α-Si3N4 raw powder by sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa, using a mixture of Y2O3 and MgSiN2 as the sintering additives. The effect of embedding conditions on the thermal conductivity of β-Si3N4 ceramics was investigated by changing the number of samples and the embedded fraction in BN packing powder with BN crucible. It was found that the embedment had no effect on the densification, but had an effect on the weight loss, crystalline secondary phase and microstructure. The thermal conductivity was found to increase linearly with the weight loss, but independent of the grain size. The complete embedment and the increased number of sintered samples tended to suppress the weight loss and thus lowered the thermal conductivity. The optimum condition led to a significant improvement in the thermal conductivity from 96 to 117 Wm-1 K-1. It is proposed that the enhanced thermal conductivity is attributed to the removal of lattice oxygen by the reaction Si3N4(s)+3 SiO2(l)⇔6 SiO(g)+2N2(g). In addition, the decreased amount of secondary phases also contributes to the thermal conductivity. This work suggests that the increased weight loss may also be an important strategy for enhancing the thermal conductivity of β-Si3N4 ceramics by controlling the sintering atmosphere.

Identification of Surface Sites on Monoclinic WO3 Powders by Infrared Spectroscopy

The dehydroxylation/dehydration and Lewis acidity of the surface of monoclinic tungsten oxide (m-WO3) powder as a function of evacuation temperature was investigated by infrared spectroscopy. At room temperature, the m-WO3 surface contains both isolated and hydrogen bonded hydroxyl groups along with equal amounts of strongly and weakly adsorbed layers of water. Most of the surface hydroxyl groups and the weakly adsorbed water layer are eliminated by evacuation at room temperature. The strongly adsorbed water is removed by evacuation above 200 °C. Adsorption of D2O shows that the surface hydroxyl groups and adsorbed water are accessible and easily exchanged. However, the removal of the strongly held water is not related to the number of Lewis or Brønsted acid sites on the surface. While there is little change in the amount of adsorbed water between a room temperature sample and a sample evacuated at 150 °C, pyridine adsorption shows that there is a corresponding 50% reduction in the number of Lewis acid sites. Furthermore, the strongly held water is eliminated by evacuation between 200 and 400 °C, whereas there is little change in the number of Brønsted or Lewis acid sites. The changes in Lewis/Brønsted acidity are not related to the dehydration but rather attributed to reduction of the oxide due to removal of lattice oxygen.

Tetragonal structure, anionic vacancies and catalytic activity of SO 42−-ZrO 2 catalysts for n-butane isomerization

An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO 4 2−-ZrO 2 with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO 4 2−-ZrO 2 catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H 2 was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work. A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO 4 2−-ZrO 2 is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.

Electron paramagnetic resonance study of interaction between MoO3 and propene

Interaction of MoO3 with propene at the temperature range 420–520 K results in the appearance of several EPR signals. Four of them have been assigned to different Mo(V) centres, one to localised conduction electrons. The analysis of the signals' parameters and the sequence of their appearance allows the conclusion that the allyl species formed in the first stage of interaction are adsorbed in a certain distance from the simultaneously created Mo(V). This is an important finding, modifying the existing hypotheses on the reaction mechanism. In the next stage of reduction the precursors of shear structures are formed due to the removal of lattice oxygen by the desorbing oxidation products.

Oxygen desorption and oxidation-reduction kinetics with methane and carbon monoxide over perovskite type metal oxide catalysts

Selected perovskite-based, metal oxide catalysts of composition La 1−x Sr xB yB' zO 3 (B: Mn,.Fe; B': Fe or Cu) containing, in some cases, an excess of one or more of the B-position elements (i.e. y+z >1) were investigated for their use in methane oxidation. A flow reactor/mass spectrometer arrangement was used to characterize the supported monolith catalyst samples. The catalysts possess significant activity for methane oxidation and also are able to exchange bulk oxygen in quantities corresponding to several hundred monolayers. Much greater quantities of oxygen could be removed under reducing reaction conditions than by desorption in an inert argon atmosphere. With one catalyst, LaFe 0.84Cu 0.16O 3, oxygen desorption occurred already at 200 ° C; otherwise typical temperatures for desorption onset were 500–600 °C. Lattice oxygen removal under reducing conditions and oxygen uptake by reduced catalysts were both found to exhibit a rather complex temperature dependence. Investigated catalysts with an excess of B-position metals had a greater capacity to exchange oxygen than the stoichiometric perovskites LaMnO 3 and LaFe 0.84Cu 0.16O 3. Reoxidation of the reduced catalysts occurred at lower temperatures than reduction. Oxidation of carbon monoxide consistently occurred at considerably lower temperatures than oxidation of methane. Complete oxidation of methane to carbon dioxide and water was observed under most conditions with little carbon monoxide or higher hydrocarbon formation. Only for strongly reduced catalysts was carbon monoxide production detected.

Propene adsorption on Cu 2O single-crystal surfaces

The interaction of propene with Cu 2O surfaces was investigated using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). Propene adsorption was studied on clean, deuterium-predosed, and oxygen-predosed Cu 2O(100) and (111) surfaces. Clear differences in the numbers and temperatures of the desorption states on the (111) and (100) surfaces demonstrate that propene adsorption is structure-sensitive on Cu 2O. Propene adsorption at low temperature is primarily molecular on both the (100) and (111) surfaces. However, evidence for a small amount of propene dissociation to allyl (C 3H 5) was found for the (111) surface, and is believed to occur at surface oxygen vacancies and defects. CO was the only desorption product besides propene, and was detected after propene adsorption at low temperature on the (111) surface only. A change in the (111) surface structure from a nearly stoichiometric (1 × 1) surface to an oxygen-deficient (√3 × √3) R30° surface was observed due to the removal of lattice oxygen to form CO. These changes in surface structure were accompanied by changes in the propene desorption spectra, and demonstrated that the highest temperature propene desorption feature is associated with lattice oxygen vacancies. For adsorption at 300 K, evidence for propene dissociation to allyl was found for both the (100) and (111) surface. However, the coverages of adsorbed propene were small, and the sticking coefficient at 300 K was estimated to be less than 10 −5.

Thermal decomposition of hydrogen molybdenum bronze, H0.25MoO3, in a nitrogen atmosphere: defects and phase transformations

The dehydrogenation and subsequent phase transformations of hydrogen molybdenum bronze, H0.25MoO3, have been investigated in a nitrogen atmosphere. Two dehydrogenation processes were identified, which depend on the density of defects. For low defect density, the removal of lattice oxygen accompanied by dehydrogenation results in disordered shear defects in the parent Mo–O framework, while for a high defect density the removal of lattice oxygen leads to an MoO2 phase as the Mo–O framework can not maintain the structure based on MoO3. The phase transformations after the dehydrogenations are, according to the literature, different from those at equilibrium of MoO3-MoO2 or MoO3–Mo(MoO2.875) mixtures. However, they were found to be similar to that of MoO3–MoO2 under non-equilibrium conditions. It was observed that in a stream of nitrogen (i. e. in an open system) Mo18O52 was formed from a mixture of MoO3 and MoO2 at temperatures > 1023 K, although it had been known to transform to Mo9O26 at these temperatures. The formation temperature range of γ-Mo4O11 was found to be ca. 20 K lower than the former literature value.

The formation of ketones in the presence of carbon monoxide over CuO/ZnO/Al 2O 3

The reaction of linear primary alcohols over CuO/ZnO/Al 2O 3 was studied under nitrogen and under carbon monoxide at 285 °C and 1 to 65 atm. The principal products for alcohols with n carbon atoms were, in addition to the corresponding aldehydes, esters with 2 n carbon atoms, ketones with 2 n carbon atoms, and ketones with 2 n−1 carbon atoms. Under nitrogen the esters predominated, but under 65 atm of carbon monoxide the principal condensation products were ketones with 2 n carbon atoms. Isotopic labeling studies showed that carbon from CO was not incorporated into the esters and ketones, that oxygen, on the other hand, was exchanged between CO and the reaction products, and that the ketones were formed by an aldol-type mechanism. The changes in selectivity observed under carbon monoxide are attributed to changes in the catalyst surface resulting from the removal of lattice oxygen by the CO and the formation of oxygen vacancies.

A study of the activity and selectivity of molybdenum crystallographic shear compounds in the oxidation of C 4 hydrocarbons

Oxides which are catalysts in the selective oxidation of olefins are thought to form crystallographic shear (CS) structures. These structures can form as a result of the removal of lattice oxygen which leads to a change from corner to edge sharing of some of the metal-oxygen octahedra. Oxygen removal also results in reduction of some of the metal atoms to form a mixed valence solid. This work was undertaken with the goal of illuminating the role of these reduced molybdenum-oxygen structures in selective oxidation. In particular, several molybdenum CS compounds were studied and compared with MoO{sub 3}.

Kinetic analysis of surface reduction in transition metal oxide single crystals

Abstract A mathematical treatment of the kinetics of metal oxide reduction is presented that is particularly applicable to single crystal substrates under conditions which promote slow, orderly removal of lattice oxygen at the crystal surface. The crystal is modeled as a semi-infinite plane with oxygen removal at the surface only and with Fickian diffusion from subsurface oxide into the depleted surface region. Removal of oxygen at the surface has been related to oxide surface concentration, as well as to oxygen vacancy concentration in an attempt to model the commonly observed induction period to reduction onset. Solutions to the models are given in closed analytic form for zeroth and first-order oxygen surface concentration dependencies and as an infinite, but converging, series in reaction time for the first and higher reaction orders. A numerical method is also used to extend the fit to longer times for which the infinite series method converges too slowly to be practically useful. As an example of the relevance of the kinetic analysis, the model is applied to data obtained on a NiO(100) substrate reduced under 1.0×10−7 to 1.3×10−6 Torr H2.

Kinetic analysis of surface reduction in transition metal oxide single crystals

A mathematical treatment of the kinetics of metal oxide reduction is presented that is particularly applicable to single crystal substrates under conditions which promote slow, orderly removal of lattice oxygen at the crystal surface. The crystal is modeled as a semi-infinite plane with oxygen removal at the surface only and with Fickian diffusion from subsurface oxide into the depleted surface region. Removal of oxygen at the surface has been related to oxide surface concentration, as well as to oxygen vacancy concentration in an attempt to model the commonly observed induction period to reduction onset. Solutions to the models are given in closed analytic form for zeroth and first-order oxygen surface concentration dependencies and as an infinite, but converging, series in reaction time for the first and higher reaction orders. A numerical method is also used to extend the fit to longer times for which the infinite series method converges too slowly to be practically useful. As an example of the relevance of the kinetic analysis, the model is applied to data obtained on a NiO(100) substrate reduced under 1.0×10 −7 to 1.3×10 −6 Torr H 2.

Lattice oxygen removal from nearly stoichiometric V2O5

The activation energy of the removal of lattice oxygen connected with slight thermal dissociation of V2O5 was measured by the TPD method. The small value (24.7 kcal/mol O2) of this energy is discussed as the result of bivariant equilibria V2O5−x−O2 within the range x<0.01.

Lattice oxygen removal from the V2O5−x oxide system

Lattice oxygen removal from the V2O5−x oxide system has been studied by the TPD method in relation to the catalytic activity of this system. The differences between the removal of oxygen from the oxidized and reduced V2O5 phase are discussed.

Lattice oxygen removal from V6O13 by the TPD method

Removal of the lattice oxygen from the V6O13 phase has been measured and the activation energy determined.