Effect of the phase transition of iron (III) oxide on the oxidative dehydrogenation of propane in the presence and absence of tetrachloromethane

The oxidative dehydrogenation of propane on α-Fe₂O₃ (hematite) was examined in the presence and absence of tetrachloromethane (TCM) at 723 K. The conversion of propane and the selectivity to propylene were evidently improved upon addition of TCM into the feedstream under oxygen limited conditions. It has been described that the suppression of the phase transition of hematite to maghemite (γ-Fe₂O₃) and a formation of chlorinated species on the catalyst together with gaseous chlorinated species contributes to the activities in the presence of TCM.     

Mechanistic aspects of the oxidative dehydrogenation of propane over an alumina-supported VCrMnWOx mixed oxide catalyst

Mechanistic and kinetic aspects of the catalytic oxidative dehydrogenation of propane (ODP) were studied within a wide range of temperatures (673–773 K), partial pressures of oxygen (0–20 kPa), propane (0–40 kPa) and propene (0–4 kPa) under both steady-state ambient-pressure and transient, vacuum conditions in the temporal analysis of products (TAP) reactor. A Mn_0.18V_0.3Cr_0.23W_0.26O_x–Al₂O₃ catalyst was identified as a selective catalyst for ODP by high-throughput experiments. For comprehensive catalyst characterization, XRD, BET, and in situ UV–visible techniques were applied. The results from transient experiments in combination with UV–visible tests reveal that selective and non-selective propane oxidation occurs on the same active surface sites, i.e., lattice oxygen. CO_x formation takes place almost exclusively via consecutive propene oxidation, which involves both lattice and adsorbed oxygen species, with the latter being active in CO formation. However, the adsorbed species play a minor role. CO₂ formation was found to increase in the presence of propene in the reaction feed. Optimized operating conditions for selective propane oxidation were derived and discussed based on the experimental observations with respect to the influence of temperature and partial pressures of O₂, C₃H₆ and C₃H₈ on the reaction. In co-feed mode with a propane to oxygen ratio of 2, optimal catalytic performance is achieved at low partial pressures of oxygen and high temperature. Propene selectivity can be also improved by carrying out the ODP reaction in a periodic mode; that is an alternate feed of propane and air.     

BiOCl催化剂的丙烷氧化脱氢催化性能

以_L-赖氨酸为模板剂,采用沉淀法制备了BiOCl催化剂,对催化剂进行了X射线衍射、N₂吸附-脱附和H₂-TPR等表征,并测试了BiOCl催化剂对丙烷氧化脱氢制丙烯反应的催化性能。结果表明,制备的BiOCl催化剂为四面体结构,500 ℃焙烧3 h后,催化剂比表面积为11.2 m²·g^-1,未完全还原氧物种的含量较多。随着反应温度升高,丙烷转化率和丙烯选择性增加,丙烷转化率为20%时,丙烯选择性达64.5%。     

Niobium oxide as support material for the oxidative dehydrogenation of propane

The ability of using niobium oxide (Nb₂O₅) as a support for preparing surface metal oxide species and testing the catalyst for the ODH of propane was done in the present study. Chromium oxide was used as the representative surface metal oxide species. To test the objective several loadings of Cr₂O₃/Nb₂O₅ were prepared by the incipient wetness impregnation technique. Surface area analysis, Raman, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy and TPR studies were used to characterize the sample. It was observed that surface chromium oxide species are formed similar to those on other oxide supports and similar monolayer coverages were achieved. However, the reduction characteristic (T_max temperature) was different due to the change in the Cr–O–support bond. The ODH of propane over the Cr₂O₃/Nb₂O₅ catalysts revealed that the activity increased up to monolayer coverage and then decreased due to the presence of Cr₂O₃ crystals. Similar observations were seen for the V₂O₅/Nb₂O₅ and MoO₃/Nb₂O₅ catalysts. The turnover frequency (TOF) was independent of coverage for the surface chromium, vanadium and molybdenum oxide species on Nb₂O₅. The constant TOF suggests the structure insensitivity of this type of reaction. The propene selectivities were high and relatively constant for the Cr₂O₃/Nb₂O₅ catalysts revealing the higher yields that can be obtained on this series of catalysts compared to the Cr₂O₃/Al₂O₃ and Cr₂O₃/TiO₂ catalysts. Additional studies involving tungsten and molybdenum oxide additives on 1% Cr₂O₃/Nb₂O₅ reveal the effect of exposed support surface on the propene selectivities.     

Effect of the preparation conditions of carbon-supported Pt catalyst on PEMFC performance

Carbon-supported Pt catalysts were prepared using NaBH₄ as a reducing agent in either ethylene glycol or water for use as a cathode catalyst in PEMFCs (polymer electrolyte membrane fuel cells). Aqueous NaBH₄ solution was used to reduce Pt precursor and to produce the Pt-W catalyst, while Pt-E and Pt-E-base catalysts were synthesized using NaBH₄ in ethylene glycol for the reduction of Pt. Compared to Pt-W catalyst, Pt-E and Pt-E-base catalysts have higher Pt dispersion and larger EAS (electrochemically active surface area) due to the stabilizing effect of ethylene glycolic NaBH₄ solution on Pt particles. In addition, increasing pH of the preparation solution improved the Pt dispersion (Pt-E-base). In unit cell tests the performance of Pt catalysts decreased in the following order: Pt-E-base > Pt-E > Pt-commercial > Pt-W. Higher metal dispersion and larger EAS are believed to be responsible for the superior performance of Pt-E catalysts, particularly Pt-E-base, compared to other catalysts.     

Effect of iron oxide on the phase composition of zirconia refractories

Conclusions It has been established that in the compositions ZrO₂Y₂O₃Fe₂O₃ and ZrO₂Nd₂O₃Fe₂O₃ the processes of formation of rare earth ferrites and ZrO₂ cubic solid solutions occur simultaneously. At temperatures of up to 1300°C the ferrite-formation process is fastest; with a further increase in the temperature the synthesis of ZrO₂ cubic solid solutions is significantly intensified.In the mixture of CaO, ZrO₂, and Fe₂O₃, the calcium oxide reacts only with ZrO₂.The greatest stability in relation to the action of Fe₂O₃ is clearly found in the case of the zirconia-calcium oxide solid solutions.In the mixtures of cubic solid solutions of Nd₂Zr₂O₇-ZrO₂ or ZrO₂-Y₂O₃ and Fe₂O₃, an interaction is only possible in the case where the synthesis of ferrites is not accompanied by the restructuring of the crystal lattice of ZrO₂, i. e., by destabilization of the material.The presence of Fe₂O₃ prevents the decomposition of zirconia-calcium oxide and zirconia-neodymium oxide solid solutions under conditions of prolonged heating in air at 1300°C. The zirconia-yttria cubic solid solution is the most stable and does not decompose, regardless of the concentration of Fe₂O₃, after a dwell of 250 h at 1300°C.Translated from Ogneupory, No. 1, pp. 45–49, January, 1980.     

Influence of preparation method on the performance of vanadia-niobia catalysts for the oxidative dehydrogenation of propane

The influence of various preparation methods on the performance of V-Nb-0 catalysts has been investigated. It was found that the activity and selectivity of a vanadium site depend on the nature of the neighbouring atoms. Vanadium neighbours provide activity, while niobium neighbours provide selectivity. Careful preparation of these catalysts ensures a homogeneous distribution and good mixing of the vanadium and niobium. It was also found that the vanadium becomes mobile upon reduction and this results in better distribution of vanadium in used catalysts.     

The oxidative dehydrogenation of ethane over molybdenum-vanadium-niobium oxide catalysts: the role of catalyst composition

The oxidative dehydrogenation of ethane has been studied at atmospheric pressure using molybdenum-vanadium-niobium oxide catalysts in the temperature range of 350–450 °C. The presence of all three oxides together is necessary in order to have active and selective catalysts. The best results have been obtained using a mixture having a Mo V Nb ratio of 19 5 1. Our studies of the variation of oxide composition suggest that the active phase is based on molybdenum and vanadium. Niobium enhances the intrinsic activity of the molybdenum-vanadium combination and improves the selectivity by inhibiting the total oxidation of ethane to carbon dioxide. The apparent activation energies for the conversion of ethane to ethylene, carbon monoxide and carbon dioxide were 18, 27 and 17 kcal/mol, respectively. The addition of water vapor to the gas stream does not affect the product distribution on this catalyst.     

Effect of operating conditions on the reduction of nitrous oxide by propane over a Fe-zeolite monolith

Fluidised bed combustion is an important source of nitrous oxide emissions. The influence of different operating parameters, such as catalyst volume, temperature, gas hourly space velocity, and hydrocarbon addition, on the activity, selectivity, and poisoning tolerance of a Fe-ZSM-5 monolith for the nitrous oxide selective catalytic reduction, has been investigated under realistic conditions, at bench scale.

Both in the absence or in the presence of poisons, such as H₂O, NO, and SO₂, the optimisation of operating conditions gives rise to a broadening of the temperature window for N₂O reduction, making it more compatible with real application conditions, with a simultaneous reduction in hydrocarbon fugitive emission, resulting in an environmental friendly process.

Excessively high reaction temperatures seem to be needed to obtain an acceptable level of N₂O decomposition. On the contrary, high N₂O reduction conversions are obtained, even in the presence of poisons and at relatively low temperatures, which is the preferred situation in the processes of pollutants removal from stationary combustion sources.

The optimum value of C₃H₈/N₂O ratio to be used for reducing N₂O over the catalyst system seems to be about the unity, since higher N₂O and C₃H₈ conversions and lower hydrocarbon unwanted emissions are attained, with a low consume of propane as selective reductant.     

Effect of the crystallization conditions on the phase behavior of syndiotactic polystyrene/poly(phenylene oxide) blends

Syndiotactic polystyrene (sPS) and poly(phenylene oxide) (PPO) blends, miscible in the melt state, were crystallized from the melt and the quenched state at different temperatures. The effect of the crystallization temperature on the phase behavior of the blends and the polymorphic changes in sPS was investigated by dynamic mechanical analysis (DMA), wide‐angle X‐ray diffraction (WAXD), and density measurements. In most blends, the crystallization of sPS induced segregation into two homogeneous amorphous phases of different compositions. The temperatures of the DMA relaxations of the neat homopolymers and crystallized blends were fit by the Gordon–Taylor relation to calculate the compositions of these phases. In melt‐crystallized blends, with slower crystallization, the major amorphous phase became sPS‐rich, whereas the minor phase became PPO‐rich. These major and minor amorphous phases could be tentatively assigned to interfibrillar and interlamellar regions, respectively. In cold‐crystallized blends, slower crystallization decreased the sPS concentration in both phases, and the scale of segregation was much smaller. WAXD studies and density measurements indicated a complex polymorphic behavior of sPS after it was blended with PPO. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1975–1983, 2003     

Structural characterisation of a VMgO catalyst used in the oxidative dehydrogenation of propane

A VMgO catalyst (containing 14 wt% vanadium) that is used in the oxidative dehydrogenation of propane (ODHP) reaction has been examined in detail by in situ EXAFS, in situ XRD and HREM. These characterisation techniques have revealed that, as prepared, the catalyst is in effect a three-component system comprising discrete magnesium orthovanadate (Mg₃V₂O₈) particles, magnesium oxide and a disordered vanadium-containing overlayer supported on the MgO. When the catalyst is exposed to typical ODHP reaction conditions at $500^\circ {\text{C}}$ the in situ EXAFS indicates a change in vanadium oxidation state from 5+ to 3+. Under the same conditions, in situ XRD suggests that Mg3V2O8 transforms to a cubic spinel type structure with a lattice parameter of 8.42 Å. These changes are reversible on exposure to air at $500^\circ {\text{C}}$ . HREM shows that the overlayer on MgO changes from a disordered state to a weakly ordered structure after exposure to normal reaction conditions whilst pure propane (strongly reducing conditions) induces pronounced structural ordering of the overlayer. Image simulations have led us to the conclusion that the ordered layer comprises a cubic spinel (MgV₂O₄) phase in parallel epitaxy with the MgO support. The surface regions of the bulk Mg₃V₂O₈ particles are also found to undergo structural modification under typical reaction conditions decomposing to a mixture of MgO crystallites and MgV₂O₄; strong reduction causes a complete conversion to MgV₂O₄.     

Kinetic modelling of the gas phase ethane and propane oxidative dehydrogenation

Gas phase thermal and oxidative dehydrogenation of ethane and propane are explored experimentally and mikrokinetically. Kinetic data for the elementary reactions postulated were adopted from the literature. The results show that oxygen plays a dominating role in the primary activation of the alkanes by producing an HO₂ and a hydrocarbon radical. The kinetic sequences developed predict satisfactorily conversion and products yields for ethane and propane oxidative dehydrogenation.     

纤维状BPO4/SiO2催化剂的制备及其丙烷氧化脱氢性能

通过静电纺丝法制备了一种高温焙烧后具有纤维状结构的磷酸硼/二氧化硅（BPO₄/SiO₂）催化剂,并考察了BPO₄负载量和焙烧温度对该催化剂的结构和催化丙烷氧化脱氢性能的影响。研究发现焙烧过程使纤维直径减小。随着BPO₄负载量的增加,丙烷氧化脱氢活性增高。当BPO₄负载量为7%（质量分数）,焙烧温度为600℃时,BPO₄/SiO₂催化剂具有最佳的催化性能;在反应温度为480℃下,丙烷转化率和丙烯产率分别达到17.0%和13.0%,且催化剂稳定性良好。当焙烧温度较低时（550℃）,催化剂中的有机物分子未被除尽,导致烯烃的选择性偏低;当焙烧温度较高时（700℃）,SiO₂结构收缩紧密,抑制了活性相的暴露。由于纤维结构可暴露更多活性位点,该催化剂较相同条件下粉末状BPO₄催化剂有着更高的催化活性。     

Effect of focused ion beam sample preparation on the phase composition of zirconia

The investigation of phase transformations in metastable ceramic systems such as zirconia often requires local phase analysis within the areas of interest. Electron backscatter diffraction is a suitable method in combination with focused ion beam sample preparation. The interaction between ion beam and sample has to be carefully considered. In case of metastable Y-PSZ and Mg-PSZ, phase transformations were observed after FIB preparation with 30?kV, 30?nA and 5° incidence angle. Damage was the dominating effect for angles of 72°. The expected local temperature increase due to the ion bombardment with 30?kV and 30?nA is 700?K for ZrO₂. Thus, the observed phase transformations can be explained on the basis of the temperature increase in the corresponding Y-PSZ phase diagram. In case of Mg-PSZ, the transition temperature is 1083?°C. The local temperature increase was obviously lower. The excitation energy for the observed phase transformation was smaller than expected from the phase diagrams of the thermodynamic equilibrium. Using 5?kV, 4.8?nA and 5° incidence angle, no phase transformations and no damage were observed. Thus, these conditions are well suited for the FIB preparation of metastable zirconia.     

The effect of Nb or Ta substitution into the M1 phase of the MoV(Nb,Ta)TeO selective oxidation catalyst

We use aberration corrected high-angle annular dark field (HAADF) imaging to systematically study, atomic column by atomic column, the effects of substituting Nb or Ta into the M1 phase of the MoV(Nb,Ta)TeO propane (amm)oxidation catalyst. The HAADF results indicate that the x,y coordinates of the metal sites within the M1 framework are unaffected by the substitution of either Nb or Ta for Mo. The HAADF analysis of the Ta-substituted catalyst demonstrated that the Ta preferentially substitutes into the pentagonal bipyramidal site, and by analogy, we anticipate that Nb substitutes similarly. Compositional analysis of the entire framework suggests that Ta/Nb behaves as a director of V among the octahedra that link the pentagonal rings, and the variable V occupancy may be correlated with variations in catalytic activities and selectivities. Finally, HAADF imaging provided evidence of coexistence of Ta-rich and Ta-poor domains. Similar phase segregation behavior may be present in Nb-substituted specimens, but would be very difficult to detect.     

Role of potassium on the structure and activity of alumina-supported vanadium oxide catalysts for propane oxidative dehydrogenation

The influence of potassium on the structure and properties of alumina-supported vanadium oxide catalysts has been studied by in situ Raman spectroscopy, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), a probe reaction of acid/base–redox sites (methanol chemisorption) and tested in oxidative dehydrogenation (ODH) of propane. Potassium coordinates to surface vanadium oxide species altering its structure but does not form bulk compounds, possibly because the total V+K coverage does not reach the monolayer coverage on alumina. The interaction of K with V weakens the terminal V=O bond. K-doped alumina (KAl)-supported vanadia catalysts show lower acidity, a decrease of reducibility and a decrease of propane conversion values. These trends do not correspond with the changes in the terminal V=O bond energy. Thus, it appears that the terminal V=O bond of surface vanadium oxide species is not the active site for propane ODH, oxidation of methanol to formaldehyde and for the reduction of surface vanadium oxide species by hydrogen. Potassium also changes the acid–base characteristic of the system and decreases the acidic character of surface vanadia. This shift in the acid–base character to a more basic system must also account for the better selectivity in propane ODH due to a variation in the interaction between the intermediates and the surface.     

VO_x/焦磷酸镧催化剂的丙烷氧化脱氢制丙烯催化性能

以嵌段共聚物P188为模板剂制备VO_x/焦磷酸镧催化剂。采用比表面分析仪、透射电镜、X射线光电子能谱和H_2-TPR对催化剂结构进行表征,并评价其丙烷氧化脱氢制丙烯的催化性能。结果表明,焦磷酸镧有一定的催化活性,但催化性能不高。当负载钒氧物种后,VO_x/焦磷酸镧催化剂催化活性有所增加,反应温度600℃时,丙烯产率达16.6%。原因主要是钒氧物种和可移动氧在丙烷氧化脱氢过程中起到重要作用。     

Optimization of V2O5–MgO/TiO2 catalyst for the oxidative dehydrogenation of propane effect of magnesia loading and preparation procedure

The effect of magnesia loading and preparation procedure of vanadia on titania catalysts on the physicochemical characteristics and the performance in propane oxidative dehydrogenation were investigated. A series of magnesia promoted vanadia catalyst (5 wt% V₂O₅) with varying amounts of MgO (1.9--10 wt%) were synthesized by synchronous and sequential deposition on titania support. The catalysts were characterized using several techniques (BET, XRD, H₂-TPR and NH₃)-TPD). Both MgO loading and preparation procedure affect the catalyst surface properties and the behavior in the oxidative dehydrogenation of propane. Magnesia addition results in drastic increase in propene selectivity, while the effect on activity is negative. The activity is inversely related with the magnesia loading. Deposition of V₂O₅ on previously prepared MgO/TiO₂ presents a beneficial effect in the activity of the sample. The role of acidity and reducibility is explored. There is no correlation between reducibility and activity of the catalysts, whereas the acidity seems to influence the catalytic performance. Catalyst containing 5 wt% V₂O₅ and 1.9 wt% MgO prepared by sequential deposition of V₂O₅ on already doped with MgO titania exhibits the most interesting results.     

High-performance poly(butylene oxide)/poly (ethylene oxide) block copolymer surfactants for the preparation of water-in-oil high internal phase emulsions

High-performance surfactants have been developed for the preparation of water-in-oil high internal phase emulsions (HIPE), particularly for the preparation of polymerized HIPE foams. High-efficiency surfactants with poly(butylene oxide)/poly(ethylene oxide) (BO/EO) block copolymer backbones have been developed that can stabilize an HIPE through polymerization at concentrations as low as 0.006 wt% based on total emulsion weight. Polymerizable versions have been developed that bind into the polymeric foam backbone. BO/EO block copolymer surfactants also allow preparation of polymerized HIPE foams without salt in the aqueous phase. HIPE with the BO/EO surfactants have been prepared at room temperature and polymerized at temperatures exceeding 90°C. By minimizing the required amount of surfactant, allowing the surfactant to react during HIPE polymerizations, eliminating the need for salt, and stabilizing over a broad range of temperatures, BO/EO block copolymer surfactants have demonstrated their place as high-performance HIPE surfactants.     

CO高温变换催化剂浸渍条件研究

采用γ-Al₂O₃作载体,Ni^2＋、Cu^2＋和Mn^2＋水溶液为浸渍液,进行制备无铬CO高温变换催化剂浸渍条件的研究。由实验数据回归出Ni^2＋、Cu^2＋和Mn^2＋活性组分在载体上的浸渍动力学方程,得到Ni^2＋、Cu^2＋和Mn^2＋组分在载体上浸渍的速率常数为：k_Ni2＋= 0.000 2,k_Cu2＋=0.000 2,k_Mn2＋=0.001 6。Freundlich等温吸附表达式为：lnC_Ni2＋=－0.000 2 t＋4.447 4,lnC_Cu2＋=－0.000 2 t＋4.590 9和lnC_Mn2＋=－0.001 6 t＋3.589 5。考察了催化剂中活性组分含量与共浸渍组分浓度的关系,并得出从浸前液浓度来预测和控制催化剂成品中Ni^2＋、Cu^2＋和Mn^2＋活性组分含量的经验式。