A Two-Step Reaction Mechanism of Oxygen Release from Pd/CeO2: Mathematical Modelling Based on Step Gas Concentration Experiments

A mathematical model has been developed to study the transient release of oxygen from a 1wt% Pd/CeO₂ catalyst in the 450–550 °C range based on alternate step concentration switches between CO and O₂. A two-step reaction mechanism that involves the reaction of gaseous CO with the oxygen species of PdO and of the back-spillover of oxygen from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the CO and CO₂ transient response curves. The proposed mathematical model allows the estimation of the transient rates of the CO oxidation reaction and of the back-spillover of oxygen process. It also allows the calculation of the intrinsic rate constant k ₁ (s^–1) of the Eley–Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO₂. An activation energy of 10.1 kJ/mol was estimated for this elementary reaction step. In addition, an apparent rate constant k ₂ ^app (s^–1) was estimated for the process of back-spillover of oxygen.     

Reactivity of Hydroxyl Species from Coadsorption of Oxygen and Water on Ni(110) Single-Crystal Surfaces

The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water–oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H₂O(ads) + O(ads) = 2OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.     

Quantum chemical study of the reactivity of boron-doped graphite layers towards water formation

Density functional calculations were used to study some fundamental features of boron-doped graphite layers (C_xB_y) and the boron influence on the mechanisms leading to the formation of water molecules on the C_xB_y graphite like layers. The Langmuir-Hinshelwood reactions leading to water formation on the graphite-like layers containing 12 at% boron take place with activation energies 3-5 times lower than on pure graphite ones. For the Eley-Rideal mechanism, the activation energies are always very low whether or not the graphite contains boron. Similar results were observed for 25 at% B doping. As a consequence, the oxygen and hydrogen can be more easily eliminated from the doped surfaces in the form of water molecules than from the corresponding pure ones. The C_xB_y layers with high boron content or having accumulations of boron, lose their planar structure. Two such parallel layers strongly interact through the boron atoms with the formation of a B-B bond and the displacement of the boron atoms into the inter-layer space. As a whole the system deviates from the graphite-like structure.     

Raman, FTIR and Theoretical Evidence for Dynamic Structural Rearrangements of Vanadia/Titania DeNOx Catalysts

Insight into the selective catalytic reduction (SCR) of NO by NH₃ over vanadia/titania catalysts is obtained from a combination of Raman and FTIR spectroscopic investigations and density functional theory (DFT) calculations. Studies of the V–OH and V=O functional groups under different conditions coupled with calculations of the stability and mobility of H atoms provide evidence that dynamic structural rearrangements may occur during the SCR reaction. Hydrogen atoms are bonded more strongly to oxygen atoms that are coordinated to a single vanadium atom (V=O species), compared to bonding at oxygen atoms that are coordinated to multiple vanadium atoms (e.g., V–O–V species); and, activation energy barriers for hydrogen transfer from a V=O species to another V=O species and to a V–O–V species are estimated from DFT calculations to be 60 and 130 kJ/mol, respectively. This dynamic nature of hydrogen transfer between oxygen atoms having different coordination environments also appears to explain some of the spectroscopic changes observed for vanadia/titania catalysts having different vanadia loadings.     

The reaction of hydroxyl radicals with carbon at 298 K

The reaction of free OH radicals with graphite was studied in a flow system by mass spectrometry, the OH being produced by the reaction H + NO₂ → OH + NO. The OH radicals react rapidly at 298 K to produce approximately equal amounts of CO and CO₂. The collision efficiency (γ) for gasification of the carbon is>5 × 10^?3. OH radicals are much more reactive than free oxygen atoms towards graphite at 298 K. Carbon is an efficient heterogeneous catalyst for the reaction H + OH → H₂O, and when free hydrogen atoms are present, this reaction is several times faster than the gasification of the carbon by OH. Carbon is also an efficient catalyst for the recombination of H atoms: 2H → H₂.     

水分子对初始碳烟颗粒形成过程影响的分子动力学模拟

采用LAMMPS软件，基于ReaxFF，以十氢化萘、萘、2-甲基蒽、1-乙基芘为催化油浆的模型化合物研究了600～2500 K温度下催化油浆形成初始碳烟颗粒的过程，考察了2500 K时水分子对初始碳烟颗粒形成过程的影响。研究表明温度在600 K时模型化合物分子主要是物理聚集成核。温度在900～1700 K时模型化合物分子处于聚集和分离的动态过程，无法从单体向碳烟颗粒转变。温度高于2100 K时主要是化学成核，模型化合物分子碳氢键先断裂，然后碳碳键断裂产生大量短碳链，碳链经成键和环化形成初始碳烟颗粒。温度在2500 K时水分子抑制模型化合物分子化学成核，随着体系氢碳比的增加，抑制初始碳烟颗粒形成的作用增强。水分子产生氢自由基和氢氧自由基，这些基团会直接导致模型化合物分子的侧链断裂和碳碳键断裂形成大量短碳链。碳链继续与氢自由基和氢氧自由基作用形成一氧化碳、二氧化碳、氢气、甲烷等而被消耗，水分子的作用为促进短碳链形成和抑制短碳链向形成初始碳烟颗粒的方向进行。     

Graphite: An active or an inactive anode?

Positive polarization of a graphite anode in aqueous solution functionalizes the surface and releases soluble organic carbon to the solution concurrent with the electrolysis of water. Mineralization of the anode occurs at more positive potentials, and can be explained as a repetitive sequence involving functionalization, oxidation to carboxyl, and Kolbe decarboxylation, without recourse to hydroxyl radicals. Other lines of evidence against the intermediacy of hydroxyl radicals include the resistance of p-benzoquinone towards oxidation at graphite – i.e., graphite does not function as an inactive anode towards the oxidation of added substrates. A direct electron transfer mechanism operates for substrates that are oxidizable in the range of water stability, such as acetaminophen and sulfide ion. In the potential range of oxygen evolution we propose that graphite behaves as a modified active anode, at which the oxygen atom to be transferred to an oxidizable substrate first becomes bonded to the previously functionalized surface.     

Mechanistic Aspects of Hydrogen Production by Partial Oxidation of Methanol Over Cu/ZnO Catalysts

In this work, mechanistic aspects of the partial oxidation of methanol (POM) to hydrogen and carbon dioxide over Cu/ZnO catalysts have been investigated. The data obtained with different catalyst compositions and different Cu^o metal surface areas showed that the reaction depends on the presence of both the phases ZnO and Cu^o. On the other hand, for catalysts with Cu concentrations in the range 40-60 wt%, the copper metal surface area seems to be the main factor determining the reaction rate. Kinetic isotope effects using CH₃OH and CH₃OD showed that both C–H and O–H bonds are at least partially involved in the rate-limiting step. TPD experiments with pure Cu^o, pure ZnO and the catalyst Cu/ZnO showed that methanol can be activated by both ZnO and copper. On the ZnO surface methanol can form intermediates which in the presence of copper might react and desorb more easily probably via a reverse spillover process. The isotopic product distribution of H₂, HD, D₂, H₂O, HDO and D₂O in the temperature-programmed reaction of CH₃OD revealed a slight enrichment of the products with H, suggesting that during methanol activation on the ZnO some of the D atoms might be retained by the support. The effect of oxygen partial pressure suggests that oxygen atoms on the copper surface strongly promote methanol activation and H₂ and CO₂ formation. It is proposed that oxygen atoms participate in methanol activation by the abstraction of the hydroxyl H atom to form methoxide and OH_surf. This OH_surf species rapidly loses H to the surface regenerating the O_surf.     

On the so-called “semi-ionic” C–F bond character in fluorine–GIC

The short-range structures of fluorine–graphite intercalation compounds with stage-1 structures, C_xF (x = 2.47, 2.84, 3.61), were analyzed by means of neutron diffraction. It has been shown that the so-called “semi-ionic” C–F bond character in C_xF is essentially covalent with the bond length of 0.140 nm, and the original planar graphene sheets are buckled at the sp³-hybridized carbon atoms bound to fluorine atoms. Conjugated C–C double bonds with the bond length of 0.142 nm are preserved between the carbon atoms unbound to fluorine atoms in C_xF, while other C–C bonds are single bonds with the bond lengths of 0.153 nm. The C–F bond order in C_xF is slightly lower than those in poly(carbon monofluoride) ((CF)_n) and poly(dicarbon monofluoride) ((C₂F)_n), which is explained by the hyperconjugation involving the C–C bonds on the carbon sheets and C–F bond.     

Role of Hydrogen Peroxide in Selective OH Group Functionalization of Polypropylene Surfaces using Underwater Capillary Discharge

Plasma chemical methods are well suited for introducing functional groups to the surfaces of chemically inert polymers such as polyolefins. However, a broad variety of functional groups are often formed. Unfortunately, for further chemical processing such as grafting of molecules for advanced applications a highly dense monotype functionalized polyolefin surface is needed. Therefore, the main task was to develop a selective surface functionalization process, which formed preferably only a single type of functional groups at the surface in high concentration. Amongst the novel plasma methods, the underwater plasma process (UWP) is one of most attractive options to solve the problem of monotype functionalization. Such plasma is an efficient source of ions, electrons, UV-radiation, high-frequency shock waves, radicals such as hydroxyl radical, and reactive neutral molecules such as hydrogen peroxide. In contrast to established gas phase glow discharge processes, the water phase limits the particle and radiation energies and thus the energy input into the polymer. By virtue of the liquid water environment, which moderates highly energetic plasma species, extensive oxidation, degradation, cross-linking and radical formation on the polymer are more limited as compared to gas plasma exposure. The variety of plasma produced species in the water phase is also much smaller because of the limited reaction possibilities of the plasma with water. The possibility to admix a broad variety of chemical additives makes underwater plasma even more attractive. Hydrogen peroxide and the catalyst (Fe-ZSM5) should influence and increase the equilibrium concentration of OH radicals in the underwater plasma process. It was found that these radicals played a very important role in OH functionalization of polyolefin surfaces. Hydrogen peroxide was identified to be the most prominent precursor for OH group formation in the UWP. The catalyst would affect the steady state of OH radical formation and its reaction with the substrate surface and thus accelerates the functionalization process.     

SEMI-EMPIRICAL (PM3) COMPUTATIONAL STUDIES OF THE ASSOCIATION OF ALKYL PYRIDINE MONOCARBOXYLATES WITH METHANOL AND WATER IMPLICATION FOR INTERFACIAL ACTIVITY AND THE EFFECT OF PHASE MODIFIERS

ABSTRACT

Alkyl pyridine monocarboxylates can form associates with alcohols. The alcohol hydroxyl group is mainly bonded with oxygen atoms of the ester group, while the pyridine nitrogen is only slightly blocked. The degree of hydrogen-bonding at the different acceptor sites depends significantly on the position of the carboxyl group in the pyridine ring. Alkyl pyridine monocarboxylates can form hydrates with water molecules. Again the hydrogen bonds are mainly formed with the oxygen atoms of the ester group. Such hydration is limited in extraction systems containing alkyl isonicotinate and alkyl nicotinate, and only weak monohydrates with one water molecule bonded with the pyridine nitrogen can be formed. The oxygens of the ester group of alkyl picolinate can be, however, easily hydrated. This phenomenon is reflected in interfacial activity and interfacial concentration of the considered extractants.     

浸渍活性炭脱除H2S的反应动力学

提出了浸渍活性炭脱除Ｈ２Ｓ的反应过程机理模型，认为脱硫过程主要是Ｈ２Ｓ在活性炭的吸附水膜内离解后被活化的Ｏ２分子以及活性氧原子氧化生成单质硫逐渐沉积在活性炭表面，同时应用最小二乘法回归实验数据得到了脱硫动力学方程ｄＷｓ／ｄｔ＝ｋ１ｋ２／ａｐｏ２／ｐＨ２Ｓ／１＋ｋ２ｐｏ２×（１－αＷｓ）＾２其中动力学参数ｋ１＝３．７５３×１０＾５ｅｘｐ「－３００６．５／Ｔｒ」ｋ２／０．２６３ｅｘｐ「１９４５．６／Ｔ     

Isotopic Study of N2O Decomposition on an Ion-Exchanged Fe-Zeolite Catalyst: Mechanism of O2 Formation

N₂O decomposition on an ion-exchanged Fe-MFI catalyst has been studied using an ¹⁸O-tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O₂-treated Fe-MFI catalyst at 693 K, and the O₂ molecules produced were monitored by means of mass spectrometry. The ¹⁸O fraction in the produced oxygen had almost half the value of that on the surface oxygen, and ¹⁸O¹⁸O was not detected. The result shows that O₂ formation proceeds via the Eley–Rideal mechanism (N₂ ¹⁶O + ¹⁸O(a) N₂ + ¹⁶O¹⁸O).     

Isotopic study of nitrous oxide decomposition on an oxidized Rh catalyst: mechanism of oxygen desorption

N₂O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O/oxidized Rh catalyst at 493 K and desorbed O₂ molecules were monitored. The ¹⁸O fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N₂O decomposition reaction at low temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen Isotope Effects in the Radiolysis of Water

The tritium fractionation between water and radicals formed in the radiolysis of dilute solutions of monomerous methyl methacrylate in H₂O-HTO and D₂O-DTO mixutres was studied. A parallel determination of the tritium content in molecular hydrogen was performed. Also, the isotopic composition of the initial molecular hydrogen was measured in the concentration range 10 to 90 Mol% D₂O of H₂O-HDO-D₂O mixtures. Hydrogen atoms, hydroxyl radicals and molecular hydrogen produced in the radiolysis of these solvents were found to be depleted in heavier hydrogen isotopes. The isotope effects on the composition of hydrogen atoms are discussed in terms of the rate isotope effects in proton transfer. The isotope effects on the composition of hydroxyl radicals are close to those on molecular hydrogen and their extent suggests ionic dissociation of water as a rate-determining step in the process of formation of both products.     

Numerical simulation of mixing process of gas–liquid impinging jet in ballast water discharge pipe

When a gas–liquid turbulent jet with hydroxyl free radicals is jetted into a ballast water discharge pipe to kill the invasive microbes, there is a challenge for hydroxyl free radicals to contact a large number of microbes in ballast water. In this paper, the Eulerian–Eulerian two-equation model is employed to simulate the mixing process of gas–liquid being jetted into the ballast water crossflow. The results show that the upstream wall-surface vortices lead to counter-rotating scarf vortex pairs (CVSPs) at downstream in confined crossflow, which enhance the mixing process. However, it is hard for the gas in the downstream wall-surface jet to diffuse due to the fact that the CVSPs have no influence on the downstream wall-surface jet. Therefore, the higher the momentum ratio is, the lower the value of weighted coefficient of variation of gas volume fraction at outlet is, however, when the momentum ratio is constant, the weighted coefficient of variation first drops and then goes up with increasing diameter ratio.     

Effects of water–cement solutions on the composition of organic compounds leached from oxidized Callovo–Oxfordian argillaceous sediment

Both oxygen penetration, due to excavation, and the use of hydraulic cement as engineering barrier has led to the investigation of effects of high pH solutions on the oxidized host rock formation. A Callovo–Oxfordian sediment has been oxidized in a ventilated drying oven at 130 °C between 2 and 1024 h. The geochemical characterization of oxidized samples shows that oxidation induces transformations in extractable organic matter similar to those observed during thermal maturation. Rock-Eval analyses of extracted solid residues reveal a loss of hydrogen while oxygen is incorporated into the kerogen structure during oxidation experiments. The oxidized samples have been leached with deionized water and cement solution. Comparisons between basic and near-neutral leaching treatments showed that the amounts and nature of leached organic species are pH-dependent. The dissolved organic compound amounts are higher at basic pH than at pH close to neutrality. Gas chromatography–mass spectrometry (GC-MS) analyses show that aromatic carboxylic acids are preferentially leached by deionized water, while n-carboxylic acids predominate in cementitious solution. The data reveal that kerogen oxygen content may constrain the dissolved concentration of organic species. The DOC concentration and the C=O functionality of leachable organic molecules increase with increasing duration of oxidation.     

IR study of the interaction of hydroxyl groups of silica gel with Cr species of Phillips' type catalysts

Spectroscopic evidence for the interaction of hydroxyl groups and chromium ions was obtained using a catalyst prepared from chromyl chloride. A new OH peak, observed at 3705 cm^–1 after pumping away CO gas, is attributed to the direct interaction of OH with the low-valent chromium. This peak shifts to 3590 cm^–1 on contact with O₂ at room temperature and it is assigned to a hydroxyl interacting with the oxidized chromium. New assignments are also proposed for IR bands of CO presorbed on the catalyst. The peak due to CO at 2188 cm^–1 decreases as the OH intensity at 3705 cm^–1 increases, suggesting that the former peak arises from adsorption on Cr(II) species to which two oxygen atoms are attached.     

Hartree–Fock periodic study of the chemisorption of small molecules on TiO2 and MgO surfaces

We present ab initio periodic Hartree–Fock calculations (CRYSTAL program) of the adsorption of small molecules on TiO₂ and MgO. These may be molecular or dissociative, depending on the acidic and basic properties of the molecules in gas phase and of the nature of the surface oxide. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages of the molecule; they also depend on the surface oxide; the oxygen atom of MgO, in spite of its large charge, is poorly reactive and dissociation on MgO is not favorable.

The surface hydrosyl of MgO are more basic than the O of the lattice and water is not dissociated under adsorption. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs basic behavior, but in terms of hard and soft acidity. MgO surface is a “soft” acidic surface that reacts preferentially with the soft base, CO₂.

Another important factor is the adsorbate–adsorbate interaction: favorable cases are the sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface, so that their mutual orientation resembles the geometry of the CO₂ dimer.     

Etude de la formation d'un complexe de surface graphite-eau relation avec la reaction d'oxydation

Using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection and an apparatus operating under low pressure with a mass spectrometer for analysis, we have found the following: The chemisorption of water becomes appreciable above 200°C. The water chemisorbed on graphite previously degassed at 1000°C forms a surface complex which on raising the temperature decomposes with simultaneous evolution of CO and H₂, thus showing that hydrogen enters into the composition of the complex. The programmed thermal decomposition of the complex shows two peaks. The first peak corresponds to the sites which disappear with the degassing of the CO and H₂; these sites are formed by the ‘labile’ carbon atoms created by the prior degassing. The second peak corresponds to the sites which are constantly renewed after degassing the CO molecules, and which are therefore formed by graphite atoms normally linked to the graphite lattice. The reaction rate of the oxidation of graphite by water shows two transient rates before attaining a stationary value. Correlations between the CO thermo-desorption and these reaction rates are found which indicate that the formation of a surface complex is a necessary step in graphite-H₂O reaction.