Catalytic properties and surface modification of hexaaluminate microcrystals for combustion catalyst

Spinel oxide (Mn₃0₄) surface layers were produced on hexaaluminate microcrystals by means of the air-oxidation process. The surface layer completely covered the basal plane of hexagonal facets as evident from HREM observation and CO₂ chemisorption measurement. The catalytic activity of as prepared Mn₃0₄/hexaaluminate composites for methane combustion was evaluated as functions of the Mn₃0₄loading and the metal composition of the spinel surface layer. It was revealed that the air oxidationderived composites showed the excellent specific activities superior to those of the corresponding Mn₃0₄/hexaaluminates prepared from the conventional evaporation-to-dryness process. Partial substitution of Fe for Mn of the surface layer was effective in enhancing combustion activity in a whole temperature range.     

Modification of the surface reactivity of Cu-MFI during chemisorption and transformation of the reagents in the selective reduction of NO with propane and O2

The steady-and unsteady-state catalytic behaviour of Cu-MFI in the conversion of propane and NO in the presence of O₂ is reported, showing how the chemisorption and transformation of reactants may influence the surface reactivity. Various effects were observed: (i) a change in the surface reactivity and kinetics in going from low to high concentrations of NO or propane, (ii) the transformation of NO to N₂ and N₂O promoted at low temperature (250°C) by oxygen in the absence of hydrocarbon, (iii) the influence of NO over the surface reactivity of the catalyst in the conversion of propane and (iv) the influence of surface precoverage with oxidized nitrogen oxides (N_xO_y) or carboxylate species on the catalyst transient reactivity in the reduction of NO to N₂. In particular, Cu-MFI is initially more active when oxidized nitrogen oxides are present, suggesting that the active intermediate in the reduction of NO with propane is a complex formed by the reaction of nitrate with activated hydrocarbon. It is shown, however, that strongly bound oxidized nitrogen oxides may have also additional effects on the surface reactivity: (i) can promote the conversion of NO to N₂ and N₂O in transient conditions and (ii) can give a partial inhibition of the surface reactivity blocking copper ions due to their strong chemisorption. Furthermore, it is shown that NO reacts faster with oxygen than hydrocarbon forming N_xO_y species which are then the oxidizing agent for the hydrocarbon. It is thus suggested that the surface reactivity of Cu-MFI in the reduction of NO with propane/oxygen depends on the surface population of nitrogen oxide adspecies which influence not only the surface reactivity, but also the pathway of hydrocarbon oxidation.     

Modifications to the surface chemistry of low-rank coal-based carbon catalysts to improve flue gas nitric oxide removal

The effectiveness of carbons as low-temperature selective catalytic reduction (SCR) catalysts will depend upon their physical and chemical properties. Surface functional groups containing oxygen are closely related to the catalytic activity of carbons. These groups are expected to change the interaction between the carbon surface and the reactants through a variation in adsorption and reaction characteristics. This paper presents a more detailed study of the effects of either gas-phase sulfuric acid or oxygen oxidation treatments on the catalytic NO reduction by low-rank coal-based carbon catalysts. Raw and treated carbons were characterized by N₂ and CO₂ surface areas, TPD and ash content. NO removal capacity of carbons was determined by passing a flow containing NO, H₂O, O₂, NH₃ and N₂ through a fixed bed of carbon at 150°C and 4 s of residence time, the effluent concentration being monitored continuously during the reaction. The effects of varying the type and conditions of the treatment on the physicochemical features of carbons were studied. The gas-phase sulfuric acid treatment (corresponding to a first step SO₂ removal) markedly enhanced carbon activities for NO removal. On the contrary, oxygen oxidation enhanced NO removal capacity of chars to a lower extent. Therefore, the carbons studied could be used in a combined SO₂/NO removal process, because the use and regeneration of the carbon in the first step is beneficial for the performance in the second one.     

Molecular structure and reactivity of the Group V metal oxides

The physical, electronic and reactivity properties of bulk and supported Group V metal oxides (V, Nb, Ta and Db) were compared at the molecular level. Dubnium is a very short-lived element, 60 s, whose properties have not been extensively studied, but can be predicted from knowledge of the other members of the Group V metal oxides. Bulk V₂O₅ possesses platelet morphology with the active surface sites only located at the edges: primarily surface redox sites and some surface acidic sites. Bulk Nb₂O₅ and Ta₂O₅, as well as to be expected for bulk Db₂O₅, possess isotropic morphologies and the active surface sites relatively homogeneously dispersed over their surfaces: only surface acidic sites. However, the bifunctional bulk V₂O₅ was found to exhibit a much higher specific acidic catalytic activity than the acidic bulk Nb₂O₅ and Ta₂O₅, the latter being almost identical in their specific acidic catalytic activity. The bulk properties of the Group V metal oxides were essentially transferred to the analogous supported Group V metal oxides, where the active Group V metal oxides were present as a two-dimensional monolayer on various oxide supports (e.g., Al₂O₃, TiO₂, ZrO₂ as well as Nb₂O₅ and Ta₂O₅). For supported vanadia catalysts, the active surface sites were essentially redox sites, with the exception of supported V₂O₅/Al₂O₃ that also contained strong acidic sites. For supported niobia and tantala catalysts, as well as to be expected for supported dubnia catalysts, the active surface sites were exclusively acidic sites. However, the TOF_redox for the supported vanadia catalysts and the TOF_acidic for the supported niobia and tantala catalysts varied over several orders of magnitude as a function of the specific oxide support with the electronegativity of the oxide support cation. However, the TOF_redox varied inversely to that of the TOF_acidic variation because of the opposite requirements of these active surface sites. Surface redox sites are enhanced by reduction and surface acidic sites are enhanced by stabilization (lack of reduction). The current fundamental understanding of the Group V metal oxides allows for the molecular engineering of their metal oxide applied catalytic materials.     

Enhanced cycling stability of LiMn2O4 by surface modification with melting impregnation method

The surface of as-prepared LiMn₂O₄ was modified with Al₂O₃ by a melting impregnation method. X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) studies indicated that Al₂O₃ nano-particles are distributed around the spinel. X ray absorption fine structure analysis (XAFS) further demonstrated that Al atoms were also doped to the surface of LiMn₂O₄ particles. The nano-Al₂O₃ particle coating improves the capacity retention of spinel LiMn₂O₄ efficiently at both room temperature and 55 °C. The mechanism of improvement for surface modified LiMn₂O₄ can be attributed to the inhibition of a surface Jahn-Teller distortion and the decrease of manganese dissolution, leading to good electric contact among particles.     

滑石、CaCO3、BaSO4填充PP复合材料力学性能及界面相互作用对比

以3种白色矿物粉体［颗粒状CaCO₃、BaSO₄,片层状滑石（talc）］为填料,聚丙烯（PP）为基体树脂,通过熔融共混法制备PP复合材料,研究3种矿物粉体在不同含量时对PP复合材料的力学性能、流动性能与断裂形貌的影响规律,并采用Turcasanyi半经验公式计算了矿物填料与PP复合材料力学性能界面相互作用。结果表明,3种矿物粉体的加入均降低了PP的拉伸强度,PP/talc复合材料的拉伸强度明显高于PP/CaCO₃ 与PP/BaSO₄,且talc的加入明显增强了PP的拉伸模量与弯曲模量;CaCO₃ 与BaSO₄的加入使复合材料弯曲强度降低,talc的加入使复合材料弯曲强度提高;CaCO₃对PP断裂伸长率与悬臂梁缺口冲击强度的提升最为明显;talc的加入使复合材料流动性能得到提高,而颗粒状的CaCO₃与BaSO₄的加入对加工性能影响较小;CaCO₃ 与BaSO₄在PP中均存在一定团聚现象,且与PP相容性较差,存在明显界面缺陷;talc与PP间界面较模糊,二者之间有较强的黏结作用。     

Enhanced photocatalytic degradation of maleic acid by Fe(III) adsorption onto the TiO2 surface

The role of Fe(III) on the TiO₂-assisted photocatalytic degradation of maleic acid has been investigated. The study on the kinetics of the removal of organic matter, as well as the identification of all the stable mineralization intermediates, demonstrates that the presence of Fe(III), adsorbed onto the TiO₂ surface, renders a more efficient process through a cleaner mineralization pathway compared to bare TiO₂. Furthermore, the behaviour of the system does not depend on the iron source. On the other hand, the study of the Fe(III) influence on the interaction of maleic acid with TiO₂ in the solid phase, by means of ATR–FT-IR technique, has been performed. From all the reported results, it is concluded that the Fe(III) effects are mainly due to surface phenomena. The enhanced photocatalytic activity is interpreted under different perspectives, including adsorbed Fe–maleic complexes, enhanced adsorption of oxygen and lower geminate recombination yields.     

Surface organometallic chemistry on metals in water: Chemical modification of platinum catalyst surface by reaction with hydrosoluble organotin complexes: application to the selective dehydrogenation of isobutane to isobutene

Surface organo-metallic chemistry on metals can be a new route to generate supported bimetallic catalysts. According to previous works on Pt–Sn catalysts, the reaction of tetra n-butyl-tin on the reduced platinum surface leads to well-defined bimetallic catalysts which are very active and selective in the dehydrogenation of isobutane into isobutene. The presence of tin not only isolates the surface platinum atoms from each other (EXAFS) and thus prevents a fast deactivation by decreasing the processes of C–C bond cleavage but also favors the regeneration processes under air. So far the catalyst preparations were carried out either in the gas phase or in organic solution (e.g. heptane). However, in order to meet the industrial criteria of process simplicity, there is a need to carry out such catalyst preparation in water.

In this work, Pt–Sn/Al₂O₃ and Pt–Sn/SiO₂ catalysts was prepared by reacting tris n-butyl-tin hydroxide on the platinum surface, in water solution under atmospheric pressure of hydrogen. The kinetics of the reaction was followed by measuring the amount of butane evolved as a function of time. The solids obtained were characterized by CO, O₂ or H₂ chemisorption and electron microscopy (CTEM and EDAX). Clearly, the (n-Bu)₃Sn(OH) reacts selectively on the platinum surface and not with the support, with evolution of butane, leading to a bimetallic catalyst where the platinum atoms are isolated from each other by the tin atoms. Very high selectivities (>95%) and activities were obtained for the reaction of isobutane dehydrogenation into isobutene and it was concluded that surface organo-metallic chemistry on metal in water can be an alternative route to prepare well-defined supported bimetallic Pt–Sn catalysts.     

Mechanism for Environmental Control of Drilling in MgO and CaF2 Monocrystals

The effects of certain liquid environments on the penetration of a carbide spade drill and on the mobility of near-surface edge dislocations in freshly cleaved MgO and CaF₂ monocrystals were determined at room temperature. The environments were water, toluene, DMF, DMSO, DMF-DMSO solutions, n -alkanes ranging from heptane to hexadecane, and aqueous solutions of AlCl₃. Drilling efficiency and near-surface dislocation mobility are directly related, and "softening" environments facilitate crack initiation in MgO. The n -alkanes are extremely surface-active with respect to MgO and CaF₂. It is proposed that adsorbed surface-active species control the drilling behavior of MgO and CaF₂ (and perhaps of other notch-sensitive nonmetallic solids (minerals) also) as a consequence of their influence on the flow and flow-dependent fracture properties of the near-surface regions of these solids. Possible effects of adsorption-induced reductions in surface free energy are of minor importance.     

Selective adsorption of carbon dioxide, methane and ethane by porous clays heterostructures

The separation of binary mixture representatives of natural and landfill gases by selective adsorption using adsorbent porous clay heterostructures has been investigated. Using Wyoming clay as a starting material, the solids are prepared by the gallery-templated approach from the polymerization of two silica sources, tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhOS), at several molar ratios. In this way, the surface chemistry and the porosity characteristics of the samples are modified. The adsorbents presented specific surface areas up to 634 m² g⁻¹, micropore-size distributions with maxima near to 0.6 nm, thermal stability up to 400 °C and also hydrophobic characteristics. The selectivity for the separation of various binary mixtures such as CO₂/CH₄, CO₂/C₂H₆ and C₂H₆/CH₄, estimated by a methodology based on the determination of the Gibbs free energy, is discussed.     

Physicochemical surface and catalytic properties of CuO–ZnO/Al2O3 system

A series of CuO–ZnO/Al₂O₃ solids were prepared by wet impregnation using Al(OH)₃ solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O₂ at temperatures ranged between 125°C and 200°C.

The results showed that CuO interacts with Al₂O₃ to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl₂O₄ at 600°C while stimulates its production at 800°C. The treatment of CuO/Al₂O₃ solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol⁻¹ in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature.     

纳米Fe3O4磁性颗粒表面改性及其在医学和环保领域的应用

纳米Fe₃O₄磁性材料在生物医学、环保、催化及电子信息等领域有巨大的应用潜力,但单独的纳米Fe₃O₄颗粒存在一些弊端,难以直接使用,在生物医药领域尤其如此。对Fe₃O₄磁性纳米粒子进行表面改性,可以改善其结构与性能,因此,备受科学界关注。对近年来Fe₃O₄磁性纳米颗粒的表面改性方法及其在生物医学、环境工程两大领域中的应用做了综述,并对今后发展趋势做了初步的展望。     

Surface nano-aggregation and photocatalytic activity of TiO2 on H-type activated carbons

The main objective of this work is to detect any associative or synergistic effects between TiO₂ and activated carbon in 4-chlorophenol photodegradation. Different activated carbons (AC) were prepared from Tabebuia pentaphyla wood by means of physical activation with CO₂ or by extensive carbonization under N₂ flow at temperatures from 450 °C up to 1000 °C during 1 h. Results indicate a clear correlation between photocatalytic activities of titania with texture and surface chemistry of AC. Kinetic results of 4-chlorophenol photodegradation indicate that for most of mixed TiO₂ and AC solids a synergistic effect between both solids is observed. Surface nano-aggregation of TiO₂ on AC was observed by scanning electronic microscopy and dispersion of TiO₂ nanoparticles was improved as a function of more basic surface pH of AC. In conclusion, an increase of electronic density in carbon support clearly introduces an enhancement in titania's photoactivity for 4CP photodegradation. This beneficial effect indicates that it is possible to obtain clean water in a much shorter period of time by employing some selected AC in conjunction with TiO₂.     

Study of special cases where the enhanced photocatalytic activities of Pt/TiO2 vanish under low light intensity

Surface platinized TiO₂ (Pt/TiO₂) has been widely used and investigated but their properties are yet to be understood fully. Although it is known that the Pt effects depend on many experimental parameters and the kind of substrates, this study newly finds that the Pt effects could be also influenced by the light intensity. As for the photocatalytic degradation of trichloroethylene (TCE), the Pt effect was positive at high light intensity but was negative at low light intensity. A similar behavior was also observed in the photocurrent collection in the Pt/TiO₂ suspension with polyoxometalate (POM: PW₁₂O₄₀³⁻) used as an electron shuttle. The photocurrent collection in the Pt/TiO₂ suspension was less efficient than in TiO₂ suspension when the light intensity was low. Such abnormal light intensity-dependent behaviors were not observed in the photocatalytic degradation of dichloroacetate on Pt/TiO₂ and the Fe³⁺-mediated photocurrent collection in the Pt/TiO₂ suspension. It is proposed that the photochemical interactions between the Pt surface and reactive intermediates (TCE radical anions and reduced POM anions) induce a null reaction favorably at low light intensity condition.     

Reaction and Fired-Property Studies of Cordierite Compositions

Ceramic bodies approaching the theoretical composition of cordierite were prepared from mixtures of talc and clay; talc, clay, and varying amounts of MgCO₃; talc, clay, and alumina; and prochlorite and clay. Additions of BaCO₃ and PbSiO, were made. Differential thermal analysis and X-ray diffraction methods were used in reaction studies of each composition. Bodies had thermal expansions from 0.344 to 0.109% (25° to 900°C.). BaCO₃ and PbSiO₃ affected thermal expansion. Dielectric properties were improved by additions of BaCO₃ and were markedly improved by additions of both BaCO₃ and PbSiO_l. Cordierite started to crystallize at about 1250°. Additions of PbSiO₃ lowered this temperature about 50°C. With sufficient BaCO₃, the Ba-C phase, as described by Wisely, crystallized in place of cordierite. The crystallization temperature for the Ba-C phase was about 1200°C. With insufficient BaCO₃, both cordierite and Ba-C formed. Additions of PbSiO₃ reduced the crystallization temperature of the Ba-C phase by 50°C. and increased the glassy phase. Several of the compositions should haw considerable practical value.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

Preparation and characterisation of vanadium catalysts supported over alumina-pillared clays

Supported vanadium-containing catalysts were prepared by impregnation of Al-pillared clays with NaVO₃ precursor aqueous solutions. A montmorillonite and a saponite, both natural as well as previously pillared with Al₁₃-Keggin polycations, were used as supports, being impregnated with solutions of the precursor by means of the incipient wetness technique. The layered structure of the supports is maintained after impregnation, and even after calcination of the impregnated solids at 500 °C, although with a significant worsening of the textural properties of the solids, in particular, a loss of specific surface area. Three crystalline vanadium-containing phases were identified in the impregnated solids, namely, NaV₃O₈, AlVO₄ and V₂O₅ (shcherbinaite). The reduction of these phases to V³⁺ species was observed as wide processes in the 450–750 °C range, all of them centred at ca. 600 °C.     

EXAFS study and photocatalytic properties of un-doped and iron-doped ZrO2-TiO2 (photo-) catalysts

The local zirconium and iron arrangements of the iron-doped ZrO₂-TiO₂ system, prepared by sol–gel impregnation method, were studied by EXAFS spectroscopy. Only a tetragonal ZrO₂ structure is located on TiO₂ surface. For the iron-doped ZrO₂-TiO₂ system, the presence of the Fe-O-Fe species as well as and Fe-O-Zr species located on the surface/pre-surface region are shown; it seems that iron is heterogeneously distributed, forming small iron oxide nanoclusters and Fe_x/ZrO₂ (tetragonal) spots at the catalyst surface. The photocatalytic activity of the un-doped and iron-doped binary system ZrO₂-TiO₂ was investigated in two kind of photoreactions: the salicylic acid photooxidation and the photocatalytic reduction of Cr(VI). Different photocatalytic behaviour has been found for the un-doped and iron-doped ZrO₂-TiO₂ systems which have been explained in terms of the EXAFS study.

This study represents an example of attempt to prepare a new potential photoactive mixed oxide system, containing two ions (Ti⁴⁺ and Zr⁴⁺) with good photocatalytic activity if it is compared with commercial TiO₂ (Degusssa P25) calcined at 600 °C.