An investigation of the comparative reactivities of ethane and ethylene in the presence of oxygen over Li/MgO and Ca/Sm2O3 catalysts in relation to the oxidative coupling of methane.

In order to examine the importance of the further oxidation of the desired C₂ products in the oxidative coupling of methane, ethylene and ethane have been added to the feed (containing methane and oxygen) to a Li/MgO or Ca/Sm₂O₃ catalyst. The results of these measurements show that neither of these C₂ molecules is stable under these conditions with either of the catalysts. Additionally, the rates of the oxidation of ethane and of ethylene alone have been measured using a gradientless reactor for both catalysts as well as for a quartz bed. It was found that the Ca/Sm₂O₃ material had higher activities for the oxidation of C₂H₆ and C₂H₄ (and also of CH₄) than had the Li/MgO material. These higher activities result in a lower optimal reaction temperature for the oxidative coupling of methane and are (at least partially) responsible for the lower selectivity to C₂ products observed with the Ca/Sm₂O₃ catalyst compared to that with the Li/MgO catalyst.     

The selective oxidation of methane to ethane and ethylene over doped and un-doped rare earth oxides

A comparison has been made of the behaviour in the oxidative coupling of methane of the oxides of Sm, Dy, Gd, La and Tb with that of a Li/MgO material. All but the Tb₄O₇ (which gave total oxidation) were found to give higher yields than the Li/MgO material at temperatures up to approaching 750°C but the Li/MgO system gave better results at higher temperatures. The cubic structure of Sm₂O₃ was found to be responsible for its good performance while the monoclinic structure was relatively inactive and unselective. The addition of Na or Ca to cubic Sm₂O₃ gives a higher optimum C₂ yield than that of unpromoted Sm₂O₃. Sm₂O₃ and Ca/Sm₂O₃ catalysts are more stable than Li/MgO, Li/Sm₂O₃ or Na/Sm₂O₃. The addition of Li or Na to Sm₂O₃ causes the structure to change from cubic to monoclinic; the deactivation of the Na/Sm₂O₃ catalysts is caused by a loss of Na coupled with the formation of the monoclinic form of Sm₂O₃.     

Partial oxidation of methane over samarium and lanthanum oxides: a study of the reaction mechanism

A comparative study of methane oxidation using oxygen and nitrous oxide as oxidants over Sm₂O₃, 5% Li/Sm₂O₃, La₂O₃ and 5% Li/La₂O₃ is reported. In addition a study of the primary selectivities for the different systems is also reported in detail. Both ethane and ethene are confirmed as primary products under the conditions investigated. The results are discussed in terms of a mechanism for methane oxidation in which O⁻ is the selective oxidant for ethane formation and a dioxygen species is responsible for total oxidation of ethane and ethene.     

Activation of methane using solid oxide membranes

Dense membranes of mixed-conducting perovskite-type oxides La_0.6Sr_0.4,CO_0.8Fe_0.2O₃ and La_0.8Ba_0.2Co_0.8Fe_0.2O₃ were used for methane coupling by application of pressure-driven O₂ permeation. High operating temperatures, typically above 800°C, were needed to obtain reasonable oxygen fluxes. Conversions were small (1–3%). Both compositions showed comparable C₂ selectivities at low methane partial pressures. At higher pressures the selectivity to C₂ hydrocarbons for La_0.6Sr_0.4CO_0.8Fe_0.2O₃ increased to 67%, whereas La_0.8Ba_0.2CO_0.8Fe_0.2O₃ showed small C₂ selectivities. Strong surface segregation of Sr and Ba was shown by SEM for both compositions.     

An evolutionary approach in the combinatorial selection and optimization of catalytic materials

A methodical basis of the evolutionary method for selection and optimization of heterogeneous catalytic materials was developed. For validation, the oxidative dehydrogenation of propane was used as a model reaction. Various oxides (V₂O₅, MoO₃, MnO₂, Fe₂O₃, GaO, MgO, B₂O₃, La₂O₃) were chosen as primary components for the generation of catalytic materials. The first generation consisting of 56 catalytic materials was created by combination of the primary components in a stochastic manner. The materials of each preceding generation were selected based on the catalytic results obtained and subjected to an evolutionary procedure applying mutation and crossover operators to create further generations of catalytic materials of different qualitative and quantitative compositions. For illustration, four generations were created with a total number of tested catalytic materials of 224. As a result of the preliminary optimization procedure an increase in the propene yield was achieved with increasing number of generations; the results can be certainly improved by screening further generations of catalytic materials. Under standard conditions used for testing (T=500°C, C₃H₈/O₂=3, p(C₃H₈)=30 Pa), the highest C₃H₆ yield amounted to 9.0% (S=57.4%) in the 3rd generation on V_0.22Mg_0.47Mo_0.11Ga_0.20O_x.     

Preparation of CexZr1−xO2 (x = 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts

Nanoparticles of Ce_xZr_1−xO₂ (x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H₂O₂ as an oxidant, and characterized by N₂ adsorption, XRD and H₂-TPR. Ce_xZr_1−xO₂ prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce_xZr_1−xO₂ increased, but thermal stability of Ce_xZr_1−xO₂ decreased. The surface area of Ce_0.62Zr_0.38O₂ was 41.2 m²/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO₂, and the reduction properties of Ce_xZr_1−xO₂ were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce_0.75Zr_0.25O₂ was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C₃H₈ was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C₃H₈ was induced by the steam reform and dissociation adsorption reaction of C₃H₈. Pd-only catalyst using Ce_0.75Zr_0.25O₂ as active washcoat showed high light off activity, the reaction temperatures (T₅₀) of 50% conversion of CO, C₃H₈ and NO were 180, 200 and 205 °C, respectively. However, the conversions of C₃H₈ and NO showed oscillation with continuously increasing the reaction temperature. The presence of La₂O₃ in washcoat decreased the light off activity and suppressed the oscillation of C₃H₈ and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La₂O₃ in active washcoat can enhance the thermal stability of catalyst significantly.     

Catalytic oxidation of saturated hydrocarbons on multicomponent Au/Al2O3 catalysts: Effect of various promoters

The performance of unpromoted and MO_x-(M: alkali (earth), transition metal and cerium) promoted Au/Al₂O₃ catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH₄ (above 400 °C), compared with C₃H₈ (above 250 °C). The addition of various MO_x to Au/Al₂O₃ improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al₂O₃ are FeO_x and MnO_x. For C₃H₈ oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation.     

Partial oxidation of ethane to synthesis gas over Co-loaded catalysts

Attention has been increasingly paid to the partial oxidation of lower alkanes to synthesis gas, due to its intrinsic energy saving process. We studied the partial oxidation of ethane (POE) on Co loaded on various supports. The POE performance varied as follows: Y₂O₃, CeO₂, ZrO₂, La₂O₃  SiO₂, Al₂O₃, TiO₂ > MgO. Comparing Y₂O₃ and CeO₂, the carbon deposition during the POE was negligible on CeO₂ and therefore CeO₂ was the most preferable support. By changing space velocity and O₂ partial pressure, reaction mechanism of POE was studied and it was revealed that two-step mechanism was prevailing; combustion of ethane to H₂O and CO₂ and subsequent reforming of ethane with H₂O and CO₂ to synthesis gas. Co/CeO₂ catalyst exhibited high and stable catalytic activity for 10 h; high ethane conversion of 18% (maximum ethane conversion 20% at O₂/C₂H₆ = 0.2) with H₂ and CO selectivities of 93 and 84%, respectively.     

NO reduction by CH4 in the presence of O2 over La2O3 supported on Al2O3

Dispersing La₂O₃ on δ- or γ-Al₂O₃ significantly enhances the rate of NO reduction by CH₄ in 1% O₂, compared to unsupported La₂O₃. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La₂O₃ precursor used, the pretreatment, and the La₂O₃ loading. The most active family of catalysts consisted of La₂O₃ on γ-Al₂O₃ prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m²) occurred between the addition of one and two theoretical monolayers of La₂O₃ on the γ-Al₂O₃ surface. The best catalyst, 40% La₂O₃/γ-Al₂O₃, had a turnover frequency at 700°C of 0.05 s⁻¹, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La₂O₃/Al₂O₃ catalysts exhibited stable activity under high conversion conditions as well as high CH₄ selectivity (CH₄ + NO vs. CH₄ + O₂). The addition of Sr to a 20% La₂O₃/γ-Al₂O₃ sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO⁼₄ to the latter Sr-promoted La₂O₃/Al₂O₃ catalyst decreased activity although sulfate increased the activity of Sr-promoted La₂O₃. Dispersing La₂O₃ on SiO₂ produced catalysts with extremely low specific activities, and rates were even lower than with pure La₂O₃. This is presumably due to water sensitivity and silicate formation. The La₂O₃/Al₂O₃ catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications.     

稀土镧添加氧化锆的合成与物相分析研究

以硝酸锆、硝酸镧和柠檬酸为原料,原位合成了稀土镧（La）掺杂氧化锆（ZrO₂）和锆酸镧/氧化锆（La₂Zr₂O₇/ZrO₂）复合材料。采用X射线衍射（XRD）和拉曼光谱（Raman）作为分析和表征手段,研究了稀土La掺杂ZrO₂和La₂Zr₂O₇/ZrO₂复合材料的物相组成以及La₂Zr₂O₇/ZrO₂复合材料的高温热稳定性。研究结果表明：稀土La很难进入ZrO₂晶格形成固溶体,当稀土La掺杂量较少时,生成以m-ZrO₂为主相的m-ZrO₂和La₂Zr₂O₇的复合材料;当稀土La掺杂量较大时,生成以La₂Zr₂O₇为主相的t-ZrO₂和La₂Zr₂O₇的复合材料。在1 400 ℃煅烧6 h条件下,LZZ11（按La₂Zr₂O₇与ZrO₂物质的量比为1∶[KG-*2]1称取原料制备的样品）中的t-ZrO₂转变为m-ZrO₂相,此时La₂Zr₂O₇失去了对ZrO₂的稳定作用。     

OXIDATIVE COUPLING OF METHANE ON SUPERCONDUCTOR-TYPE CATALYTIC MATERIALS

Oxidative coupling of methane to higher hydrocarbons was investigated using alkali and rare earth cuprates such as YBa₂Cu₃O_7-x, La_1.8Ba_0.2CuO and La₂CuO₄. Oxygen and methane in helium were fed to the reactor in a cofeed mode. Approximately, 5 g of catalyst in powder form was placed in a quartz flow reactor in all experiments. Oxygen partial pressure was changed as a parameter (P₀ = 0.5 to 9.1 kPa) while keeping methane partial pressure and temperature constant at 18kPa and 1023 K respectively. Investigation of catalytic activity in terms of overall methane conversion and C₂ + (C₂H₄+C₂H₆) product selectivity indicated that higher conversions and lower selectivities were obtained as O₂ partial pressure was increased at a constant methane partial pressure of 8kPa. In comparing the performance of the two catalysts, La_1.8Ba_0.2CuO and La₂CuO₄; the selectivity results indicated a positive influence of incorporation of Ba into La₂CuO₄ structure. Similarly, selectivity values substantially increased teaching 86.3% at a reaction temperature of 1023 K and at P_CH4./P_O3, = 6 when La was replaced by Y and Ba.     

Catalytic reduction of SO2 over supported transition-metal oxide catalysts with C2H4 as a reducing agent

This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO₂ with C₂H₄ as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C₂H₄/SO₂, and pretreatment are all important factors affecting catalyst activity. Fe₂O₃/γ-Al₂O₃ was found to be the most active catalyst among six γ-Al₂O₃-supported metal oxide catalysts tested. With Fe₂O₃ as the active species, of the supports tested, CeO₂ is the most suitable one. Using this Fe₂O₃/CeO₂ catalyst, we found that the optimal Fe content is 10 wt.%, the optimal feed ratio of C₂H₄/SO₂ is 1:1, and the catalyst presulfidized by H₂+H₂S exhibits a higher performance than those pretreated with H₂ or He. Although the feed concentrations of C₂H₄:SO₂ being 3000:3000 ppm provide a higher conversion of SO₂, the sulfur yield decreases drastically at temperatures above 300 °C. With higher feed concentrations, maximum yield appears at higher temperatures. The C₂H₄ temperature-programmed desorption (C₂H₄-TPD) and SO₂-TPD desorption patterns illustrate that Fe₂O₃/CeO₂ can adsorb and desorb C₂H₄ and SO₂ more easily than can Fe₂O₃/γ-Al₂O₃. Moreover, the SO₂-TPD patterns further show that Fe₂O₃/γ-Al₂O₃ is more seriously inhibited by SO₂. These findings may properly explain why Fe₂O₃/CeO₂ has a higher activity for the reduction of SO₂.     

Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

A series of palladium-substituted La₂CuO₄, corresponding to the formula La₂Cu_{1 −x}Pd_xO₄ (x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m²/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C₃H₆ decreased markedly when increasing the palladium content, the activity of catalysts La₂Cu_0.9Pd_0.1O₄ and La₂Cu_0.8Pd_0.2O₄ being comparable to that of a Pt-Rh/CeO₂–Al₂O₃ catalyst for NO reduction, and higher for CO and C₃H₆ oxidation.

All the La₂Cu_{1 − x} Pd_xO₄ catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd⁰ ions atomically dispersed, surrounded by Cu⁺ and Cu²⁺ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La₂Cu_{1 −x}Pd_xO₄ structure.     

The NO selective reduction on the La1−xSrxMnO3 catalysts

La_1−xSr_xMnO₃ (x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La_0.7Sr_0.3MnO₃ calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C₃H₈ could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O₂ concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C₃H₈, leading to a decrease of the NO reduction from 100% to zero at 560 °C.     

The reduction phase in NOx storage catalysis: Effect of type of precious metal and reducing agent

The effect of different reducing agents (H₂, CO, C₃H₆ and C₃H₈) on the reduction of stored NO_x over PM/BaO/Al₂O₃ catalysts (PM = Pt, Pd or Rh) at 350, 250 and 150 °C was studied by the use of both NO₂-TPD and transient reactor experiments. With the aim of comparing the different reducing agents and precious metals, constant molar reduction capacity was used during the reduction period for samples with the same molar amount of precious metal. The results reveal that H₂ and CO have a relatively high NO_x reduction efficiency compared to C₃H₆ and especially C₃H₈ that does not show any NO_x reduction ability except at 350 °C over Pd/BaO/Al₂O₃. The type of precious metals affects the NO_x storage-reduction properties, where the Pd/BaO/Al₂O₃ catalyst shows both a high storage and a high reduction ability. The Rh/BaO/Al₂O₃ catalyst shows a high reduction ability but a relatively low NO_x storage capacity.     

Stable titanium metal-organic framework with strong binding affinity for ethane removal

Direct separation of high purity ethylene (C₂H₄) from an ethane (C₂H₆)/ethylene mixture is a critical and challenging task owing to the very similar molecular size and physical properties of the two components. While some studies have attempted this separation, there is a lack of excellent porous materials with strong binding affinity for C₂H₆-selective adsorption via an energy-efficient adsorptive separation process. Herein, we report a titanium metal-organic framework with strong binding affinity and excellent stability for the highly efficient removal of C₂H₆ from C₂H₆/C₂H₄ mixtures. Single component adsorption isotherms demonstrated a larger amount of adsorbed ethane (1.16 mmol·g^-1 under 1 kPa) and high C₂H₆/C₂H₄ selectivity (2.7) for equimolar C₂H₆/C₂H₄ mixtures, especially in the low-pressure range, which is further confirmed by the results of grand canonical Monte Carlo simulations for C₂H₆ adsorption in this framework. The experimental breakthrough curves showed that C₂H₄ with a high purity was collected directly from both 1:9 and 1:15 C₂H₆/C₂H₄ (volume ratio) mixtures at 298 K and 100 kPa. Moreover, the unchanged adsorption and separation performance after cycling experiments confirmed the promising applicability of this material in future.     

(Sm1-xLa_x)_2(Zr1-yCe_y)_2O_7陶瓷材料介电性能与热导率

以Sm_2O_3, La_2O_3,ZrO_2和CeO_2等氧化物为原料,采用固相反应法制备双位掺杂(Sm_1-xLa_x)_2(Zr_1-yCe_y)_2O_7(x=0.5;y=0.1,0.2,0.3,0.4)陶瓷材料,研究了其晶体结构、显微组织、介电性能和热导率。结果表明,随着离子掺杂变化,该系列氧化物的结构发生了从焦绿石向萤石的转变。其显微组织比较致密,晶界清洁,但晶粒大小分布不均匀;且随掺杂离子元素含量的增加制备陶瓷材料的介电常数增大,热导率下降,热导率与介电常数成反比。     

Reversed ethane/ethylene adsorption in a metal-organic framework via introduction of oxygen

Separation of ethane from ethylene is a very important but challenging process in the petrochemical industry. Finding an alternative method would reduce the energy needed to make 170 million tons of ethylene manufactured worldwide each year. Adsorptive separation using C₂H₆-selective porous materials to directly produce high-purity C₂H₄ is more energy-efficient. We herein report the “reversed C₂H₆/C₂H₄ adsorption” in a metal-organic framework Cr-BTC via the introduction of oxygen on its open metal sites. The oxidized Cr-BTC(O₂) can bind C₂H₆ over C₂H₄ through the active Cr-superoxo sites, which was elucidated by the gas sorption isotherms and density functional theory calculations. This material thus exhibits a good performance for the separation of 50/50 C₂H₆/C₂H₄ mixtures to produce 99.99% pure C₂H₄ in a single separation operation.     

Oxidative coupling of methane over K/Ni/Ca oxide and K/Ni/Mg oxide catalysts

The oxidative coupling behaviour of a series of K/Ni/Ca oxide catalysts with low nickel-to-calcium ratios has been examined and the results are compared with those for a magnesium-based catalyst. The effect of gas composition and the stability of ethylene under reaction conditions have also been studied. The catalysts were calcined at 1200°C unless otherwise stated. Potassium was added after the calcination stage. It is found that a high calcination temperature of 1200°C is necessary to give a Ca-based catalyst with high activity and selectivity. The catalysts based on MgO were less selective. Substitution of K for Li in the MgO based catalyst gave a slight improvement in the selectivity. A series of experiments was carried out with the K_0.1Ni_0.012 Ca material with the aim of optimising the yield. It was found that the selectivity could be improved by increasing the concentration of CH₄ or by adding CO₂ to the feed. However the addition of CO₂ decreased the activity of the catalyst. The activity could be increased by increasing the H₂O concentration. An increase of the O₂ concentration in the feed from 10.85 to 13% with 31% of CH₄ and 21% H₂O increased the C₂ yield from 15.1% to 17.8%. In a series of experiments in which different concentrations of C₂H₄ were added to the feed, it was found that the main oxidation product of ethylene was CO₂. The formation of ethane was unaffected by the addition of ethylene. It is therefore proposed that two different sites are required for the oxidation of ethylene and the activation of methane to form ethane.     

Effect of hydrogen on reaction intermediates in the selective catalytic reduction of NOx by C3H6

Selective catalytic reduction of NO_x by C₃H₆ in the presence of H₂ over Ag/Al₂O₃ was investigated using in situ DRIFTS and GC–MS measurements. The addition of H₂ promoted the partial oxidation of C₃H₆ to enolic species, the formation of –NCO and the reactions of enolic species and –NCO with NO_x on Ag/Al₂O₃ surface at low temperatures. Based on the results, we proposed reaction mechanism to explain the promotional effect of H₂ on the SCR of NO_x by C₃H₆ over Ag/Al₂O₃ catalyst.