Soot surface area evolution during air oxidation as evaluated by small angle X-ray scattering and CO2 adsorption

The total surface area of two diesel engine produced soots, a high volatile content NIST standard (termed NIST) and a low volatile content soot (termed NEU), were determined with CO₂ adsorption and small angle X-ray scattering (SAXS), as a function of the extent of oxidation. During initial volatilization of condensables of the NIST and NEU soots in a thermogravimetric analyzer, in helium at 1073 K, their CO₂ surface areas increased sharply from 49 m²/g to 273 m²/g and from 96 m²/g to 367 m²/g, respectively. During oxidation, the CO₂ surface area increased by an additional 100-150 m²/g, until 50% conversion was reached. Thereafter, the CO₂ surface area was relatively constant with conversion for the NIST soot, but decreased to 150 m²/g for the NEU soot. Three porosity regimes were assumed for the calculation of SAXS areas; they were based on (a) constant density (shrinking core), (b) constant diameter, and (c) an observed (with a TEM) diameter variation. The best agreement between the CO₂ and SAXS surfaces area occurred for the constant density assumption, in contrast to the actual measured diameter variation. By applying fractal surface analysis to the SAXS data, this discrepancy is ascribed to the opening up of internal volume to reaction volatilization of condensables and oxidation.     

A comparison of O2 and CO2 oxidation of glassy carbon surfaces

We have studied the reactive adsorption of O₂ on the edge surface of graphite. At 300°C the efficiency of oxygen uptake showed a strong coverage-dependent reactive adsorption coefficient. In general, the efficiencies were low (< 10⁻⁹) over the majority of the coverage range. In contrast, the uptake of oxygen from O₂ and H₂O on sputter-damaged graphite was far more rapid. Sputter-damaged carbon surfaces exhibit greatly enhanced reactivity and are poor models of edge carbon activity. Thermal stability studies on the resultant oxidized edge graphite surfaces provide information about the energetics of product formation in gasification reactions. CO was the dominant product. A fraction of the oxygen on the surface is very tightly bound with energies greater than 85 kcal/mole. The energy decreases to 70 kcal/mole over a wide coverage range. At the highest attainable coverages representing a small fractional population, the energy decreases further down to 58 kcal/mole. Our results show that increasing the amount of oxygen surface coverage decreases the energy barrier for gaseous CO formation but increases the barrier for O₂ dissociation.     

Description of active sites on molybdenum oxide as detected by isotope exchange between CO2 and MoO3

The oxygen exchange reaction between C¹⁸O₂ and MO¹⁶O₃ has been studied on well characterized MoO₃ crystallites. Temperature programmed desorption of adsorbed C¹⁸O₂ has shown that the extent of surface oxygen exchange is higher on partially reduced samples exposing the (010) face. This has been related with the easier reducibility of the (010) face. Moreover, experiments in a recycle reactor at moderate reaction temperature (623K) suggest that surface oxygen exchange is more rapid on the basal (010) face. On the other hand, when the experiment is run at higher reaction temperature (773K), using a higher ¹⁸O(C¹⁸O₂)/¹⁶O(MoO₃) ratio and for a longer period, a continuous oxygen exchange reaction occurs, specially with samples presenting lateral faces. In this latter conditions, two contributions to the exchange reaction can be distinguished. The first corresponds to a fast surface oxygen exchange. The second contribution which is more prominent over samples with (100) faces can be assigned to migration of oxygen atoms from lateral towards basal faces and/or to a slow migration of oxygen atoms from the bulk structure of MoO₃ towards the lateral faces. From the results of this study it is speculated on the importancë of a preferential oxygen migration from the bulk MoO₃ towards the lateral faces, which is related with the selective partial oxidation of olefins observed on these faces.     

CO2 adsorption on carbonaceous surfaces: a combined experimental and theoretical study

We present an experimental and theoretical study to provide further insight into the mechanism of CO₂ chemisorption on carbonaceous surfaces. The differential heat of CO₂ adsorption at low and high coverages was determined in the temperature range 553-593 K. We found that the heat profile has two distinct energetic zones that suggest two different adsorption processes. In the low-coverage region, the heat of adsorption decreases rapidly from 75 to 24 kcal/mol, suggesting a broad spectrum of binding sites. In the high-coverage region, the heat becomes nearly independent of the loading, from 9 to 5 kcal/mol. A systematic molecular modeling study of CO₂ chemisorption on carbonaceous surfaces was performed. Several of the carbon-oxygen complexes that have been proposed in the literature were identified and characterized. The calculated adsorption energies are within the experimental uncertainty of the heat of adsorption at low coverage. Pre-adsorbed oxygen groups decrease the exothermicity of CO₂ adsorption. In the high-coverage region, our theoretical results suggest that CO₂ molecules are likely to adsorb on surface oxygen complexes and on graphene planes.     

A crucial role of surface oxygen mobility on nanocrystalline Y2O3 support for oxidative steam reforming of ethanol to hydrogen over Ni/Y2O3 catalysts

Y₂O₃ nanocrystals hydrothermally synthesized at different pH are found to have different morphologies and crystal sizes. These Y₂O₃ nanocrystals have been investigated as supports of Ni-based catalysts in oxidative reforming reaction of ethanol. Result of H₂-chemisorption shows that the particle size and dispersion of nickel on Y₂O₃ nanocrystals are not affected significantly by the crystal size of Y₂O₃. XPS and H₂-TPR results show that no association between nickel and the support-Y₂O₃ has taken place and the reducibility of nickel is not affected by the crystal size of Y₂O₃. However, it is found that the surface oxygen mobility of Y₂O₃ nanocrystal increases with decreasing crystal size. The surface oxygen mobility is believed to play an important role in promoting the H₂ production rate and selectivity of the Ni/Y₂O₃ catalyst over the oxidative steam reforming of ethanol.     

Diplatinum(III) [Cl2(μ2-amidinate)4] compound derived from air oxidation of mononuclear cis-[Pt(NH3)2(amidine-κ)2] complex

The dinuclear platinum(III) complex [Pt₂Cl₂{μ₂-N(H)C(Et)N(H)}₄] (2) has been prepared by heating cis-[Pt(NH₃)₂{NHC(NH₂)Et}₂](Cl)₂ (cis-1) under aeration conditions in an EtOH/H₂O mixture at 70 °C for 2 d and it was characterized by elemental analyses (C, H, N), ESI⁺-MS, IR, ¹H and ¹³C NMR spectroscopies and also by X-ray diffraction. Complex 2 represents the second Pt^III dimer stabilized by the amidinate ligand ever known and it has a lantern-type structure with four amidinate ligands bridging two Pt^III centers with Pt–Pt distance of 2.4809(2) Å.     

Toward selective electrochemical “E-tongue”: Potentiometric DO sensor based on sub-micron ZnO–RuO2 sensing electrode

Planar dissolved oxygen (DO) sensors based on thick-film ZnO–RuO₂ sensing electrodes (SEs) with different mol% of ZnO were prepared on the alumina substrates using a screen-printing method and their structural and electrochemical properties were closely studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electrochemical impedance spectroscopy (EIS) and energy-dispersive spectroscopy (EDS) techniques. Structural and electrochemical properties of ZnO–RuO₂–SEs have been investigated. Interference testing ascertained that the DO sensor based on sub-micron ZnO–RuO₂-SE is insensitive to the presence of various dissolved ions including Cl⁻, Li⁺, SO₄²⁻, NO³⁻, Ca²⁺, PO₄³⁻, Mg²⁺, Na⁺ and K⁺ within a concentration range of 10⁻⁷ to 10⁻¹ mol/L for DO measurement from 0.5 to 8.0 ppm in the test solution at a temperature range of 11–30 °C. These dissolved salts had practically no effect on the sensor's output potential difference response, whereas Br⁻ ions had some effects at concentration more than 10⁻³ mol/L. The relationship between DO and the sensor's potential difference was found to be relatively linear with the maximum sensitivity of −50.6 mV per decade was achieved at 20 mol% ZnO at 7.35 pH. The response and recovery time to pH changes for the planar device based on 20 mol% ZnO–RuO₂-SE was found to be 10 and 25 s, respectively, at a temperature of 25 °C, whereas the response time to DO changes at the same temperature was about 60 s. It has also been demonstrated that the appropriate amounts of mixed ZnO and RuO₂ nanostructures not only improves the structure of developed SE but also enhances the device's sensing performance.     

Structural Study of PbO–B2O3 Glasses by X-ray Diffraction and 11B MAS NMR Techniques

The structure of PbO–B₂O₃ glasses has been investigated in the composition range of 25–80 mol% PbO by using X-ray diffraction and ¹¹B NMR techniques. The well-separated peaks due to Pb–O and Pb–Pb pairs were first observed in the radial distribution function, and peak deconvolution was performed by using a pair function method. The average coordination number of lead atoms was determined to be about 6 in the low-PbO-content region and about 3 in the high-PbO-content region. With an increase of PbO content, the peaks at 0.24 and 0.40 nm shifted to shorter distances, and especially the latter peak drastically increased in intensity in the composition range >50 mol% PbO. It was suggested that PbO is a main glass former in the high-PbO-content region. Based on the results obtained, we propose structural models of lead borate glasses.     

Hydrophilicity transition of the clean rutile TiO2 (1 1 0) surface

We present contact angle measurements of water on single-crystal rutile TiO₂ (1 1 0) surfaces, exposed to ambient air, or protected in dry air. Our measurements indicate that the surfaces exposed to ambient air are hydrophobic, with a contact angle of θ = 61(5)°. However, the well-protected dry surface also exhibits some hydrophobic tendency, with θ = 32(5)°. It is known that UV irradiation transforms both surfaces superhydrophilic, with θ = 0° [R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431-432]. We also present preliminary X-ray crystal truncation rod measurements on the hydrophobic TiO₂ (1 1 0) surface, and of the effect of UV illumination on the surface.     

Mesoscale pore structure of a high-performance concrete by coupling focused ion beam/scanning electron microscopy and small angle X-ray scattering

This contribution couples (a) Small angle X-ray scattering (SAXS) experiments of a high-performance concrete (HPC) at the millimetric scale, and (b) Focused ion beam/scanning electron microscopy (FIB/SEM) of the cement paste of the HPC, with 10-20 nm voxel size. The aim is to improve the understanding of the 3D pore network of the HPC at the mesoscale (tens of nm), which is relevant for fluid transport. The mature HPC is an industrial concrete, based on pure Portland CEMI cement, and planned for use as structural elements for deep underground nuclear waste storage. Small angle X-ray scattering patterns are computed from the 3D pore images given by FIB/SEM (volumes of 61-118 μm³). They are positively correlated with SAXS measurements (volumes of 5 mm³). Aside from correlations with FIB/SEM data, experimental SAXS allows to investigate a wider range of effects on the pore structure. These are mainly the HPC drying state, the presence of aggregates (by analyzing data on cement paste alone), and the use of Poly Methyl MethAcrylate resin impregnation.     

Immobilization of manganese tetraphenylporphyrin on Au/SiO2 as new catalyst for cyclohexane oxidation with air

Manganese tetraphenylporphyrin was successfully immobilized on Au/SiO₂, by using mercaptopyridine with sulfhydryl and pyridyl groups as bridging agent. The synthesized catalyst with novel structure was characterized by FTIR spectroscopic technique, XPS measurement, TG–DTA analysis and so on. During aerobic oxidation of cyclohexane in the presence of this material, the conversion of cyclohexane and total selectivity to cyclohexanone and cyclohexanol were up to 5.39% and 88.74%, respectively.     

Further on the mechanism of the synergy between MoO3 and α-Sb2O4 in the selective oxidation of isobutene to methacrolein: Reconstuction of MoO3 via spillover oxygen

This paper concerns the synergetic effects between a (010) oriented MoO₃ and α-Sb₂O₄ in the oxidation of isobutene to methacrolein at 420°C. The catalysts were prepared by mechanical mixture of the pure oxides prepared separately. Important increases were observed both for the conversion of isobutene and for the selectivity to methacrolein, suggesting that new selective sites have been created. The characterization of the samples before and after the reaction by SEM and XRD shows an important reconstruction of the (010) face of MoO₃ into steps exposing (100) faces, when the reaction is carried out in the presence of α-Sb₂O₄. It is known from literature that the (100) face of MoO₃ performs selectively partial oxidation, while the (010) face is not selective. On the other hand, α-Sb₂O₄ has shown to produce spillover oxygen, which flows onto the MoO₃ surface. It is, therefore, concluded that the increase of selectivity is intimately connected to the reconstruction under the action of spillover oxygen. Spillover oxygen would favour the “selective coordination” of Mo atoms or groups of atoms, typical of (100), at the expense of the “non-selective one”, typical of (010). This result corresponds to the theoretical prediction of the Remote Control theory.     

Improvement of CO2 adsorption on ZIF-8 crystals modified by enhancing basicity of surface

The imidazolate framework ZIF-8 samples were modified separately by using ammonia impregnation and thermal treatment in atmosphere of N₂ or H₂ in order to improve its adsorption property toward CO₂, and the modified samples A-ZIF-8, N-ZIF-8 and H-ZIF-8 were correspondingly available. The modified ZIF-8 samples were characterized, and the surface chemical properties of the ZIF-8 samples were determined separately by FTIR, CO₂-TPD, NH₃-TPD and H₂O-TPD. The isotherms of CO₂ on the modified ZIF-8 samples were measured. Results showed that after surface modification, the total amounts of basicity of the modified samples significantly increased, and followed the order: A-ZIF-8>H-ZIF-8>N-ZIF-8>O-ZIF-8. The uptakes of CO₂ increased proportionally with the basic groups on the surfaces of the ZIF-8 samples due to CO₂ being an acidic molecule. As a consequence of that, the CO₂ adsorption capacity of the samples followed the order: A-ZIF-8>H-ZIF-8>N-ZIF-8>O-ZIF-8. The amount adsorbed of CO₂ on the modified ZIF-8 sample by ammonia impregnation is the highest, having an increase.     

Potential rare-earth modified CeO2 catalysts for soot oxidation part II: Characterisation and catalytic activity with NO + O2

Ceria (CeO₂) and rare-earth modified ceria (CeReO_x with Re = La³⁺, Pr^3+/4+, Sm³⁺, Y³⁺) supports and Pt impregnated supports are studied for the soot oxidation under a loose contact with the catalyst with the feed gas, containing NO + O₂. The catalysts are characterised by XRD, H₂-TPR, DRIFT and Raman spectroscopy. Among the single component oxides, CeO₂ is significantly more active compared with the other lanthanide oxides used in this study. Doping CeO₂ with Pr^3+/4+ and La³⁺ improved, however, the soot oxidation activity of the resulting solid solutions. This improvement is correlated with the surface area in the case of CeLaO_x and to the surface area and redox properties of CePrO_x catalyst. The NO conversion to NO₂ over these catalysts is responsible for the soot oxidation activity. If the activity per unit surface area is compared CePrO_x is the most active one. This indicates that though La³⁺ can stabilise the surface area of the catalyst in fact it decreases the soot oxidation activity of Ce⁴⁺. The lattice oxygen participates in NO conversion to NO₂ and the rate of this lattice oxygen transfer is much faster on CePrO_x. In general, the improvement of the soot oxidation is observed over the Pt impregnated CeO₂ and CeReO_x catalysts, and can be correlated to the presence of Pt°. The surface reduction of the supports in the presence of Pt occurred below 100 °C. The surface redox properties of the support in the Pt catalysts do not have a significant role in the NO to NO₂ conversion. In spite of the lower surface area, the Pt/CeYO_x and Pt/CeO₂ catalysts are found to be more active due to larger Pt crystal sizes. The presence of Pt also improved the CO conversion to CO₂ over these catalysts. The activation energy for the soot oxidation with NO + O₂ is found to be around 50 kJ/mol.     

Preparation of MnOx/TiO2 ultrafine nanocomposite with large surface area and its enhanced toluene oxidation at low temperature

The TiO₂ support materials were synthesized by a chemical vapor condensation (CVC) method and the subsequent MnO_x/TiO₂ catalysts were prepared by an impregnation method. Catalytic oxidation of toluene on the MnO_x/TiO₂ catalysts was examined with ozone. These catalysts had a smaller particle size (9.1 nm) and a higher surface area (299.5 m² g⁻¹) compared to MnO_x/P25-TiO₂ catalysts. The catalysts show high catalytic activity with the ozone oxidation of toluene even at low temperature. As a result, the synthesized support material by the CVC method gave more active catalyst.     

氧分级对药渣O2/CO2燃烧NOx生成规律与反应机理的影响

在我国目前能源结构中,化石能源尤其是煤炭资源占比很高,造成了极大的环境压力。抗生素发酵药渣为近年来产量迅速升高的固体废弃物,也是一种生物质燃料资源,但目前对药渣的能源化利用研究较少。以CH4等气体来模拟药渣可燃成分,利用Chemkin模拟软件中的PFR反应器构建了药渣在O₂/CO₂气氛下氧气分级燃烧及非分级燃烧模型,对2种情况下NO_x生成特性进行了模拟研究,探求了氧气分级及非分级燃烧时各种因素的影响,并利用生成速率分析法和敏感性分析法对结果进行了反应机理分析。研究结果表明,在氧气非分级条件下,NO_x转化率随燃烧温度升高先升高后降低,在1500℃左右达到峰值;NO_x转化率随过量氧气系数增加而升高,在过量氧气系数由0.9增至1.1时,增幅显著。在氧气分级条件下,主燃区燃烧温度对NO_x转化率的影响较为复杂;NO_x转化率随燃尽风率增加先降低后升高,随燃尽风位置推后降低。氧气分级条件下,还原气氛促进了NO_x中N向其他组分转化,能够明显降低NO_x生成。当燃烧温度低于1500℃,燃尽风率为0.35左右时,NO_x转化率最低。首次对药渣在O₂/CO₂气氛下的燃烧进行了反应动力学模拟研究,探求了各种因素的影响,为实现药渣能源化利用提供了指导。     

Impact of surface mobility in selective oxidation. Isotopic exchange of O2 with O2 on various oxides: MoO3, SnO2 and Sb2O4. Effect of a reducer gas

Isotopic oxygen exchange experiments were carried out on simple oxides (Sb₂O₄, MoO₃ and SnO₂) and mechanical mixtures of MoO₃ or SnO₂(acceptor phase) with Rh//Al₂O₃ or Sb₂O₄ (donor phase). With Rh//Al₂O₃ a synergy effect is found: the amount of oxygen exchanged on the mixture is higher than the sum of oxygen exchanged on the two separate phases. This is a prove of oxygen mobility between donor and acceptor phases. With Sb₂O₄ this effect is less marked suggesting that the presence of a reducer gas is required. In presence of hydrocarbon, no isotopic exchange is observed: only total oxidation occurs. The presence of Sb₂O₄ decreases the rate of total oxidation by blocking, by oxygen spillover, the most active sites for oxidation.     

Atomic force microscopy studies of surface and dimensional changes in LixCoO2 crystals during lithium de-intercalation

An in situ electrochemical atomic force microscopy (EC-AFM) cell was developed to study surface and dimensional changes of individual Li_xCoO₂ crystals during lithium de-intercalation. Discrete Li₂CO₃ particles having 50-250 nm in diameter and 5-15 nm in height were observed on the surface of stoichiometric LiCoO₂ crystals and they were shown to gradually dissolve into the LiPF₆-containing electrolyte. The dimensional change of individual Li_xCoO₂ crystals along the c_hex. axis was monitored in situ during lithium de-intercalation. Evidence of surface instability or structural instability was not found in Li_xCoO₂ single crystals upon de-intercalation to 4.2 V versus Li.     

Improving the quality of nanocrystalline MgAl2O4 spinel coating on graphite by a prior oxidation treatment on the graphite surface

This paper aims to report the effect of surface oxidation treatment on graphite flakes and its effect on the improvement of MgAl₂O₄ spinel coating developed by sol–gel citrate process. The graphite surface was oxidized by hydrogen peroxide and the coating was subsequently applied. The coating structure, water-wettability and oxidation resistance of coated samples were taken as criteria to evaluate the coating integrity. It was clarified that the oxidation treatment developed hydrophilic functional groups on graphite surface. This helped the formation of an even and smooth MgAl₂O₄ spinel coating texture on graphite flakes. The water-wettability and oxidation resistance were also found to be improved significantly compared to non-treated samples. The results were supported by thermogravimetric analysis, contact angle and zeta-potential measurements.     

Novel synthesis method and microstructure evolution of Ti3C2(OH)2/K2Ti8O17 nanocomposites as an effective surface enhanced Raman scattering substrate

In this paper, we have developed a mild and facile approach using KOH solution as etchants to corrode Ti₃SiC₂ at room temperature, fabricating novel Ti₃C₂(OH)₂/K₂Ti₈O₁₇ nanocomposites with open network structures and abundant surface functional groups. The as-produced products obtain the excellent surface enhanced Raman scattering (SERS) performance in the determination of crystal violet (CV), rhodamine 6G (R6G) and methylene blue (MB) dye molecules. The enhancement factors of the three can be calculated to be 3.98 × 10⁵, 1.78 × 10⁵ and 5.27 × 10⁵, and the Raman signals can be detected at the concentrations as low as 10^?6 M, 10^?8 M and 10^?6 M, respectively, suggesting the potential for reliable SERS substrates. In this regard, this work provides a new strategy for exploring and synthesizing MAX phase-derived nanomaterials as promising SERS substrates for high-sensitive molecular detection.