Mobility Of Oxygen Species Research Articles

Structure and Catalytic Properties of Ceria-based Nickel Catalysts for CO2 Reforming of Methane

Carbon dioxide reforming (CDR) of methane to synthesis gas over supported nickel catalysts has been reviewed. The present review mainly focuses on the advantage of ceria based nickel catalysts for the CDR of methane. Nickel catalysts supported on ceria–zirconia showed the highest activity for CDR than nickel supported on other oxides such as zirconia, ceria and alumina. The addition of zirconia to ceria enhances the catalytic activity as well as the catalyst stability. The catalytic performance also depends on the crystal structure of Ni–Ce–ZrO2. For example, nickel catalysts co-precipitated with Ce0.8Zr0.2O2 having cubic phase gave synthesis gas with CH4 conversion more than 97% at 800 °C and the activity was maintained for 100 h during the reaction. On the contrary, Ni–Ce–ZrO2 having tetragonal phase (Ce0.8Zr0.2O2) or mixed oxide phase (Ce0.5Zr0.5O2) deactivated during the reaction due to carbon formation. The enhanced catalytic performance of co-precipitated catalyst is attributed to a combination effect of nano-crystalline nature of cubic Ce0.8Zr0.2O2 support and the finely dispersed nano size NiO x crystallites, resulting in the intimate contact between Ni and Ce0.8Zr0.2O2 particles. The Ni/Ce–ZrO2/θ–Al2O3 also exhibited high catalytic activity during CDR with a synthesis gas conversion more than 97% at 800 °C without significant deactivation for more than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial pre-coating of Ce–ZrO2 resulting in the existence of stable NiO x species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. TPR results further confirmed that NiO x formation was more favorable than NiO or NiAl2O4 formation and further results suggested the existence of strong metal-support interaction (SMSI) between Ni and the support. Some of the important factors to optimize the CDR of methane such as reaction temperature, space velocity, feed CO2/CH4 ratio and H2O and/or O2 addition were also examined.

Interstitial oxygen molecules in amorphous SiO2. III. Measurements of dissolution kinetics, diffusion coefficient, and solubility by infrared photoluminescence

Concentration changes of interstitial oxygen molecules (O2) in amorphous SiO2(a-SiO2) thermally annealed in oxygen atmosphere were examined by the O2 photoluminescence at 1272 nm excited with 765-nm light of titanium sapphire laser. This highly sensitive technique allows the time- and temperature-dependent concentration changes of interstitial O2 due to their incorporation from an oxygen atmosphere to be directly measured. The data provide the dissolution rate, the diffusion coefficient, and the solubility of interstitial O2 in a-SiO2 and are able to exclude interferences from other forms of mobile oxygen species in a-SiO2. These observations confirm that O2 molecules are incorporated into a-SiO2 without separating into monoatomic species, diffuse in a-SiO2 without extensive interaction with the a-SiO2 network, and play a primary role in the thermal oxidation of silicon.

Correlation Between the Characteristics and Catalytic Performance of Ni–V–O Catalysts in Oxidative Dehydrogenation of Propane

Ni–V–O series catalysts for the oxidative dehydrogenation (ODH) of propane were prepared and characterized by BET, XRD, H2-TPR, O2-TPD-MS and electrical conductivity. At 425°C a C3H6 selectivity of 49.9% was observed on Ni0.9V0.1O Y at a C3H8 conversion of 19.4%, and the obtained selectivity is almost two times higher than that over NiO at the roughly same conversion of C3H8. The mobile oxygen species created by the interaction of NiO and V2O5 has been found in the composite catalysts by O2-TPD-MS and electrical conductivity studies, which seems to be responsible for the enhanced selectivity of the propane oxidative dehydrogenation.

Methane-reforming reactions over Ni/Ce-ZrO 2/ θ-Al 2O 3 catalysts

NiO/Ce-ZrO 2/ θ-Al 2O 3 catalysts were prepared and investigated after the reduction treatment in methane-reforming reactions such as steam reforming and oxy-steam reforming. Among the catalysts with various Ni loading values, 12% Ni loading exhibited not only the highest catalytic activity and selectivity but also remarkable stability. The TPR results reveal that strong interaction between Ni and support results in forming stable NiO x species. It is speculated that the populations of NiO x compared with NiO and/or NiAl 2O 4 play very important roles in the catalytic activity and stability. Metallic Ni sites reduced from NiO x species strongly interacting with the support are active sites for methane-reforming reactions and have high coke resistance. The high activity and stability of Ni/Ce-ZrO 2/ θ-Al 2O 3 is due to a Ce-ZrO 2 layer precoated on θ-Al 2O 3, which plays a role in forming stable NiO x species rather than free NiO or inactive NiAl 2O 4 and in giving mobile oxygen species.

The interaction of ultrathin nickel films with yttria-stabilized zirconia ( [formula omitted

Increasing amounts of nickel, from 0.2 up to 5 equivalent monolayers, were vapour-deposited on a clean YSZ (1 0 0) surface at room temperature and then annealed at 600 K for up to 19 h. The nickel deposit was gradually oxidized at a rate depending on the deposited amount. For submonolayer coverages, it was completely converted into NiO within a few hours, whereas for multilayer quantities only a small fraction was oxidized. Carbon monoxide TPD experiments showed an intensity attenuation of the CO desorption peak, down to extinction for complete oxidation of the nickel surface, accompanied by a decrease in the TPD peak maximum temperature. This unexpected nickel oxidation upon prolonged heating was attributed to an interfacial interaction of the deposited Ni with mobile oxygen species present on YSZ at elevated temperature, due to its ionic conductor character.

Structure and morphology of aluminium-oxide films formed by thermal oxidation of aluminium

The structure and morphology of thin aluminium-oxide films grown by the dry, thermal oxidation of a bare Al(4 3 1) substrate at a partial oxygen pressure of 1.33×10 −4 Pa in the temperature range of 373–773 K were studied using X-ray photoelectron spectroscopy and high resolution electron microscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures ( T⩽573 K), the mobility of the oxygen species is very low, and an amorphous oxide film of relatively uniform, limiting thickness develops. X-ray photoelectron spectroscopic analysis established the occurrence of a surface-oxide species at the very surface of these films. At higher temperatures ( T>573 K) an initially amorphous oxide film of less uniform thickness develops that gradually transforms into crystalline γ-Al 2O 3. At these temperatures an amorphous-to-γ-Al 2O 3 transition oxide phase occurs.

Growth kinetics and mechanisms of aluminum-oxide films formed by thermal oxidation of aluminum

The growth kinetics and mechanisms of thin aluminum-oxide films formed by the dry, thermal oxidation of a bare Al(431) substrate at a partial oxygen pressure of 1.33×10−4 Pa in the temperature range of 373–773 K were studied using x-ray photoelectron spectroscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (T⩽573 K), an amorphous oxide film develops that attains a limiting (uniform) thickness. At high temperatures (T>573 K), growth is not impeded at a limiting thickness. Kinetic analysis established the occurrences of two different oxide-film growth regimes: an initial regime of very fast oxide-film growth and a second, much slower oxidation stage that is observed only at T>573 K. These results could be discussed in terms of electric-field controlled, interstitial, outward transport of Al cations through a close packing of O anions in the amorphous films, and inward diffusion of O along grain boundaries in the crystalline films, respectively. For the electric-field controlled Al cation motion, a value of 2.6 eV was determined for the rate-limiting energy barrier, which is located at the metal/oxide interface. This corresponds with a Mott potential of −1.6 V.

A Highly Active and Stable Catalyst for Carbon Dioxide Reforming of Methane: Ni/Ce–ZrO2/θ-Al2O3

A novel catalyst, Ni/Ce-ZrO2/θ-Al2O3 has been designed and examined in carbon dioxide reforming of methane. It gives synthesis gas with CH4 conversion more than 97% at 800 °C and the activity was maintained during the reaction for longer than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial precoating effect of Ce-ZrO2 resulting in the existence of stable NiOx species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. From TPR results, it has been confirmed that NiOx formation is more favorable than NiO or NiAl2O4 formation, resulting in strong interaction between Ni and the support.

Propane Oxidative Dehydrogenation on Molybdenum Doped LaVO<sub>4</sub> Catalyst

the catalytic behavior of molybdenum doped lanthanum vanadate for propane oxidative dehydrogenation was investigated. it was found that the addition of molybdenum showed a positive promoting effect to propane oxidative dehydrogenation. 56% and 43% of c3h6 selectivities were obtained over lamo0.1v0.9o4.05 at 10% and 20% c3h8 conversion respectively, while the selectivities on lavo4 were only 36% and 22% at the isoconversion, which can be attributed to an increase of the mobile oxygen species beneficial to propene formation and to the change of the redox properties of the molybdenum doped catalysts.

Rationalization of the Catalytic Behavior of Lanthanide Oxides and Praseodymium Molybdates in Total and Selective Oxidation of Isobutene

The purpose of this work is to examine critically the correlations between ionization potential or absolute hardness and the catalytic behavior of binary and ternary oxides containing lanthanides (Ln) in alkene oxidation. Five lanthanide oxides (La2O3, Sm2O3, Pr6O11, Tb4O7, and CeO2) and three praseodymium molybdates (Pr-2-Mo3O12, Pr6MoO12, and Pr2MoO6, were used as pure phases or in mixtures with MoO3 in the partial and total oxidation of isobutene. The catalytic performances of their mixtures with MoO3 are described in the context of the remote mechanism, according to which the lanthanide-based oxides are identified as donors of spillover oxygen. Their donor ability is evaluated through the synergetic effects on methacrolein selectivity and correlated with the physical properties mentioned above. In the case of ternary oxides such as praseodymium molybdates, the concepts of average ionization energy and average absolute hardness are proposed to take into account the fact that the electron density and therefore the reactivity of mobile surface oxygen species are influenced by the presence of both metals in the lattice. The results obtained with the pure Ln-based oxides are in line with the literature concepts of ionization energy and absolute hardness. As far as the mechanical mixtures are concerned, it is shown that the above correlations, when combined with the remote control concepts, provide a comprehensive view of the influence of ionization energy or absolute hardness for both single phase and biphasic catalysts exhibiting phase cooperation.

Hexane total oxidation on LaMO3 (M = Mn, Co, Fe) perovskite-type oxides

LaMO3 (M=Mn, Fe, Co) perovskites were prepared by calcining the citrate precursors at 800°C for 5h. PdO 3% /Al2O3, a largely used combustion catalyst, was also studied for comparison. The catalytic behavior was examined in the hexane total oxidation. The following activity scale was found: LaFeO3>LaCoO3>LaMnO3. >3% PdO/Al2O3, the three perovskites activating the CH bond cleavage of hexane better than PdO/Al2O3. At temperature around 250–300°C mobile surface oxygen species are involved in the oxidation process, whereas at T>300°C oxygen mobile species are no more present and so, particularly for LaCoO3 and PdO/Al2O3, the activation mechanism becomes more energetic. The activity values were compared with those obtained from the total oxidation of methane.

Characterization of Ag and Ba-modified manganese oxide catalysts: unraveling the factors leading to their enhanced CH4 oxidation activity

Catalysts based on manganese oxide and modified with Ag (AgMn0.10) or Ba (BaMn0.10) were prepared and applied to CH4 deep oxidation. The catalysts were characterized by means of N2-BET, XRD, XPS, CH4-TPR and O2-TPD techniques. Catalytic evaluation results show that both AgMn0.10 and BaMn0.10 exhibit higher CH4 oxidation activity than the unmodified MnOx. However, comparative studies on CO oxidation reveal that only AgMn0.10 displays markedly improved activity for this reaction; the activity of BaMn0.10 remains unchanged compared to unmodified MnOx. In AgMn0.10, a large amount of more mobile and reactive oxygen species is formed, which is thought to be the main reason for its enhanced CH4 and CO oxidation activity. Whereas, in BaMn0.10, it is found that the mobility of the lattice oxygen is not increased, but somewhat decreased, thus resulting in no enhanced CO oxidation activity on this catalyst. However, the basicity of the lattice O2− is thought to be increased ia Ba addition, which is favorable for the activation and rupture of the first C–H bond in CH4, the rate-determining step for this reaction. Indeed, this modification effect of Ba is believed to be the predominant factor responsible for the enhanced CH4 oxidation activity of BaMn0.10.

Mechanism for catalytic partial oxidation of methane to syngas over a Ni/Al 2O 3 catalyst

The mechanism of catalytic partial oxidation of methane to syngas (POM) over a Ni/α-A1 2O 3 catalyst was studied by using a pulse reactor and temperature-programmed surface reaction (TPSR) techniques. Over a reduced nickel catalyst (Ni 0/A1 2O 3), methane activation follows the dissociation mechanism; while on oxidic nickel catalysts (NiO/Al 2O 3), methane is first oxidized to carbon dioxide and water, and simultaneously, NiO is reduced to Ni 0. CH 4 dissociation occurs over Ni 0 active sites, generating hydrogen and surface C species. A transient process was observed during the CH 4/O 2 reaction. The nickel valence was transformed from NiO to Ni 0 at a certain critical temperature and simultaneously, the reaction was transformed rapidly from the complete oxidation of methane to the partial oxidation of methane. It has been found that the POM reaction takes place over a thin layer of the catalyst bed. This reaction zone is nearly isothermal, over which almost 100% of oxygen and more than 90% of methane are converted. The temperature drop in the downstream of the catalyst bed does not imply that the steam or carbon dioxide reforming reaction occurs in the lower part of the bed. Ni 0 species constitute the active sites for the partial oxidation of methane to syngas. Both methane and oxygen are activated on Ni 0 sites, generating surface Ni⋯C and Ni δ+ ⋯O δ− species. These two kinds of intermediates have been proposed to account for the mechanism of methane partial oxidation. The Ni δ+ ⋯O δ− species over Ni 0 catalyst surface is considered to be a kind of weakly bounded, mobile oxygen species. The reaction between Ni δ+ ⋯O δ− and Ni⋯C intermediates generate the primary product of CO. However, the presence of NiO over the catalyst surface significantly reduces the CO selectivity. Thus, the NiO species are not possible to be the intermediate for the POM reaction. The mechanism of partial oxidation of methane should follow the direct oxidation route.

Characterization of oxygen phases created during oxidation of Ru(0001)

Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction (LEED), and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru(0001). Emission of RuOx (x<4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (Tp<850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular RuxOy phase takes place within the temperature region of 900–1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relatively low amount of oxygen atoms adsorbed on the bare Ru surface can be achieved, neither oxides nor subsurface oxygen have been found. The presence of a RuO2 coating layer manifests itself by LEED patterns characteristic for a particular RuO2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules reactively scattered at these oxygen-rich surfaces proceeds as long as mobile oxygen atoms are present in the subsurface region. The reaction is entirely quenched when the subsurface oxygen is replaced by an uniform film of RuO2.

Elemental steps in the growth of thin, amorphous gallium oxide films on CoGa(001)

The interaction of oxygen (O 2) with CoGa(001) at 300 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. For low O 2 exposures (≤ 0.3 L), oxygen is dissociatively adsorbed on the surface. In STM images these oxygen adatoms appear as defect-like depressions which are mainly located at the boundaries of the two perpendicularly oriented c(4 × 2) domains of the reconstructed clean CoGa(001) surface. The oxygen adatoms are found to be immobile at 300 K. The observed distribution of the oxygen atoms on the surface can be explained by a short-lived and mobile atomic precursor oxygen species (model of “hot adatoms”) and assuming the domain boundary defects to be reactive centers for oxygen adsorption. For O 2 exposures > 0.3 L, nuclei of amorphous gallium oxide start to form. In STM images they appear as pairs or clusters of protruding features. For an exposure ≥ 25 L, the CoGa(001) surface is entirely covered with an amorphous gallium oxide film. In EELS spectra this film is characterized by intense and broad loss structures around 400 and 690 cm −1 originating from Fuchs-Kliewer modes within the oxide film.

New aspects of the cooperation between phases in vanadium phosphate catalysts

The possibility of a cooperation between two different phases in the phosphorous vanadium catalysts during the oxidation of butane to maleic anhydride is discussed. The results deal with mechanical mixtures of separately prepared catalysts. A conspicuous synergy is observed between a phosphorous poor and a phosphorous rich VPO catalyst, as well as between a VPO and Sb 2O 4 or BiPO 4. Both activities and selectivities are increased. The synergy corresponds to a decrease of sintering during the catalytic reaction, an increase of acidity, and a higher oxido-reduction rate. Our results lead to speculate that in phosphorous vanadium catalysts, at least two separate phases could cooperate during the catalytic reaction. This cooperation would take place thanks to mobile oxygen species. One of these phases (probably that corresponding to a low P V ratio) acts as activator (or donor) of oxygen and the other (probably that with a high P V ratio) plays the role of acceptor of oxygen.

Oxygen effects on anisotropic photopolymerization in polymer matrices

The influence of oxygen on polymerization kinetics in photopolymer films is studied using a fluorescence-based technique developed in this laboratory. Fluorescent N-vinylcarbazole monomer is used to monitor photopolymerization kinetics in the presence and absence of oxygen. Effects of oxygen at pressures up to 3 atm (3 × 10 5 Pa) on photopolymerization in various inert polymer matrices are studied. A reduction in the photopolymerization rate without induction period is observed in the presence of oxygen. A detailed computer analysis of the kinetics is presented. The unusual polymer spatial distribution is calculated and attributed to the anisotropy of illumination and the diffusion and reaction of the mobile oxygen and monomer species.

Phase cooperation between tin and antimony oxides in selective oxidation of isobutene to methacrolein I. Mechanical mixtures of SnO 2 and α-Sb 2O 4

Results are reported concerning the cooperation between SnO 2 and α-Sb 2O 4 particles in the selective oxidation of isobutene to methacrolein. The catalysts were prepared by mechanically mixing the corresponding powders. A conspicuous catalytic synergy was observed when methacrolein production and selectivity to methacrolein formation were considered. The catalysts, both fresh and used, were characterized by XRD, 119Sn Mössbauer spectroscopy, XPS, analytical electron microscopy (AEM), and ESR in order to investigate the origin of the synergy observed. The joint use of these techniques yielded no indication that a new phase (or solid solution) formed or that mutual surface contamination during either the preparation of the mixture or the catalytic test took place. Within the sensitivity limits of the techniques used, the mechanical mixtures can be considered as composed of two pure separate phases in good contact. The origin of the observed synergy and the other experimental observations is explained in a satisfactory manner by the existence of a “remote control” mechanism, i.e., that α-Sb 2O 4 produces a mobile oxygen species, namely spillover oxygen, which, by flowing onto the surface of SnO 2, creates on the surface of the latter new selective sites and/or regenerates those which have become deactivated. Spillover oxygen produced by α-Sb 2O 4 seems to control the selective catalytic sites on SnO 2 by inhibiting their transformation to reduced, nonselective sites. Spillover oxygen also inhibits the formation of carbonaceous deposits.

Evidence of the migration of oxygen species from Sb 2O 4 to MoO 3 in MoO 3-Sb 2O 4 selective oxidation catalysts

Mechanical mixtures of Mo 16O 3 and 18O-labelled (≅25%) α-Sb 2O 4 and the pure oxides are contacted with propene in the absence of molecular oxygen. α-Sb 2O 4 alone is totally inactive. Acrolein, CO and CO 2 are formed on MoO 3 and the mixtures. The oxidation products are 18O labelled when α-Sb 2O 4 is present, proving that a migration of oxygen from α-Sb 2O 4 to MoO 3 takes place. The mobile (spillover) oxygen species brings about two effects: (i) direct reaction with propene and (ii) increased rate of incorporation of MoO 3 lattice oxygen. This second effect corresponds to a remote control mechanism, namely the creation and/or regeneration of reaction sites by spillover oxygen.

The role of mobile oxygen species in the selective oxidation of isobutene

A simple experiment can introduce this contribution: SB{sub 2}O{sub 4} particles on the one hand, and SnO{sub 2} particles on the other hand, are prepared independently, and then mixed at room temperature. A comparison of the methacrolein yields in isobutene oxidation over identical weights of mixtures of different compositions indicates a dramatic synergy: for adequate proportions (Sb{sub 2}O{sub 4}/(Sb{sub 2}O{sub 4} + Sno{sub 2}) = 0.3-0.6) the yield is multiplied by a factor of ten, compared to the added yields which would be reached with the corresponding weights of the simple oxides, if isolated. There is no indication of formation of any mixed phase during the catalytic experiments. All industrial oxide catalysts used for selective catalytic oxidation are composed of more than one phase. The above mentioned results suggest that there is a fundamental reason for that. But the majority of the fundamental studies have dealt with single phases and all the published mechanisms consider phenomena taking place in a single phase. The factors influencing activity and selectivity which are considered are those corresponding to the characteristics of this single phase. Our objective was to investigate the role of external phases.