Plasma catalytic reaction of natural gas to C2 product over Pd-NiO/Al2O3 and Pt-Sn/Al2O3 catalysts

The decomposition of natural gas over Pd-NiO/Al₂O₃ and Pt-Sn/Al₂O₃ is carried out in a microwave catalytic reaction at room temperature. The decomposition of methane is caused by collision by excitation of unstable electronic state. Measuring the flow rate and plasma power can provide kinetic data and indicate the mechanism. The conversion of C₂ products increases from 47 to 63.7% in the microwave plasma catalytic reaction with electric field. Comparing the activities of catalysts, Pd-NiO/Al₂O₃ bimetallic catalyst is more active than Pt-Sn/Al₂O₃ catalyst because of modification of the surface of catalysts by carbon formation. The kinetic modeling of plasma of methane conversion seems related to the power of the electric discharge. It was also revealed that proper coking or polymeric carbon formation improves the catalytic activity; therefore, the conversion of methane may increase over Pd-Ni/Al₂O₃ catalyst in the plasma system.     

Oxidative conversion of methane and methanol on M/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/cordierite structured metal oxide catalysts (m = Ni,Cu, Zn)

A study was carried out on the properties of Ni/Al₂O₃ and Cu-ZnO/Al₂O₃ composites supported on ceramic honeycomb monoliths made from synthetic cordierite in the carbon dioxide conversion of methane and the partial oxidation of methanol. The structured nickel-alumina catalysts are significantly more efficient than the conventional granulated catalysts. The improved working stability of these catalysts was achieved by adjusting the acid-base properties of the surface by introducing sodium and potassium oxides, which leads to inhibition of surface carbonization. The hydrogen yield was close to 90% in the partial oxidation of methanol with a stoichiometric reagent ratio in the presence of the Cu-ZnO/Al₂O₃/cordierite catalyst. A synergistic effect was found, reducing the selectivity of CO formation in the presence of the Cu-ZnO catalyst relative to samples derived from the individual components Cu and ZnO. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 5, pp. 299–306, September–October, 2007.     

Carbon Dioxide Reforming of Methane over Ni Catalysts Supported on Al2O3 Modified with La2O3, MgO, and CaO

The preparation of synthesis gas from carbon dioxide reforming of methane (CDR) has attracted increasing attention. The present review mainly focuses on CDR to produce synthesis gas over Ni/MO_x/Al₂O₃ (X = La, Mg, Ca) catalysts. From the examination of various supported nickel catalysts, the promotional effects of La₂O₃, MgO, and CaO have been found. The addition of promoters to Al₂O₃-supported nickel catalysts enhances the catalytic activity as well as stability. The catalytic performance is strongly dependent on the loading amount of promoters. For example, the highest CH₄ and CO₂ conversion were obtained when the ratios of metal M to Al were in the range of 0.04–0.06. In the case of Ni/La₂O₃/Al₂O₃ catalyst, the highest CH₄ conversion (96%) and CO₂ conversion (97%) was achieved with the catalyst (La/Al = 0.05 (atom/atom)). For Ni/CaO/Al₂O₃ catalyst, the catalyst with Ca/Al = 0.04 (atom/atom) exhibited the highest CH₄ conversion (91%) and CO₂ conversion (92%) among the catalysts with various CaO content. Also, Ni/MgO/Al₂O₃ catalyst with Mg/Al = 0.06 (atom/atom) showed the highest CH₄ conversion (89%) and CO₂ conversion (90%) among the catalysts with various Mg/Al ratios. Thus it is most likely that the optimal ratios of M to Al for the highest activities of the catalysts are related to the highly dispersed metal species. In addition, the improved catalytic performance of Al₂O₃-supported nickel catalysts promoted with metal oxides is due to the strong interaction between Ni and metal oxide, the stabilization of metal oxide on Al₂O₃ and the basic property of metal oxide to prevent carbon formation.     

超声改性的CuO/Al2O3-MgO催化剂结构 及其超低浓度甲烷催化燃烧性能

采用普通浸渍和超声改性的方法分别制备了CuO/Al₂O₃-MgO催化剂,用于超低浓度甲烷的催化燃烧,并利用SEM、XRD、XPS、H₂-TPR等技术对催化剂进行表征,研究了超声改性作用对催化剂的结构和性能的影响.结果表明,与普通浸渍法制备的催化剂相比,在超声改性的CuO/Al₂O₃-MgO催化剂上,甲烷的转化率得到提高,燃烧特征温度降低.随着超声时间的延长和超声功率的增加,催化剂的催化活性均呈现先增大后减小的趋势;催化剂制备的最佳超声工况为功率150 W、时间20 min.超声改性可使催化剂的比表面积和孔容积增大,表面催化活性较高的Cu⁺浓度增加,活性组分CuO由晶相向非晶相转变、分散度增大,晶粒粒径变小、分布更均匀;这使得甲烷催化燃烧的表观活化能下降、催化剂活性得到增强.     

Highly active CoMo HDS catalyst for the production of clean diesel fuels

HY–Al₂O₃-supported CoMo catalysts with a chelating agent and phosphorus for the hydrodesulfurization (HDS) of diesel fractions were prepared. The activity measurements with the prepared catalysts were carried out with straight-run light gas oil feedstocks in a pilot plant under industrial hydrotreating conditions. As a result, Cosmo Oil Co., Ltd. developed a new CoMoP/HY–Al₂O₃ catalyst, C-606A, which had three times higher HDS activity than the conventional CoMoP/Al₂O₃ catalyst. Commercial operations to produce ultra-low sulfur diesel (ULSD) with C-606A have successfully demonstrated its high performance and high stability. This catalyst has an extremely high activity, which enables to achieve <10-ppm sulfur in products in diesel hydrotreater designed to produce 500-ppm sulfur diesel fuels. Mo K-edge EXAFS, TEM and FT-IR of adsorbed NO were performed to investigate the nature of the active sites on the developed catalysts. The results showed that the new catalyst has multiple layers of MoS₂ slabs and the edges of MoS₂ are mainly occupied by Co–Mo–S phases. XPS and FT-IR were used to investigate the sulfiding behavior of Co and Mo in the formation process of the active sites during sulfidation. The results showed that addition of carboxylic acid to the impregnation solution postponed the sulfidation of Co at low temperatures, thereby increasing formation of the Co–Mo–S phase.     

Titania supported copper catalysts for methanol synthesis from carbon dioxide

The effect of co-catalyst (ZnO or ZrO₂) has been tested for hydrogenation of CO₂ on CuO/TiO₂ and CuO/Al₂O₃. CuO−ZnO/TiO₂ catalyst showed the highest activity for methanol synthesis. Kinetic parameters were also determined.     

Effect of the Electric Conductivity of a Catalyst on Methane Activation in a Dielectric Barrier Discharge Reactor

The influence of catalyst electric conductivity on methane activation in a planar-type dielectric barrier discharge reactor is investigated by empirically comparing the degree of methane conversion of bare Al₂O₃ with that of Pt/Al₂O₃; from this, it is determined that the latter catalyst converts less methane owing to the presence of Pt. Calculations and comparisons of electric fields with and without Pt show that the presence of a Pt catalyst results in a lower electric field than does bare Al₂O₃. An analysis of product gases based on the correlation between the fragmentation of radicals and the electric field also indicates that the electric field is decreased by using Pt. From these results, it can be concluded that the synergies between the plasma and the conductive catalysts need to be reassessed for different electric field conditions, and that further studies of non-conductive catalysts that can enhance methane activation and synergistic effects are needed.     

Structure and Catalytic Properties of Ceria-based Nickel Catalysts for CO<Subscript>2</Subscript> Reforming of Methane

Carbon dioxide reforming (CDR) of methane to synthesis gas over supported nickel catalysts has been reviewed. The present review mainly focuses on the advantage of ceria based nickel catalysts for the CDR of methane. Nickel catalysts supported on ceria–zirconia showed the highest activity for CDR than nickel supported on other oxides such as zirconia, ceria and alumina. The addition of zirconia to ceria enhances the catalytic activity as well as the catalyst stability. The catalytic performance also depends on the crystal structure of Ni–Ce–ZrO₂. For example, nickel catalysts co-precipitated with Ce_0.8Zr_0.2O₂ having cubic phase gave synthesis gas with CH₄ conversion more than 97% at 800 °C and the activity was maintained for 100 h during the reaction. On the contrary, Ni–Ce–ZrO₂ having tetragonal phase (Ce_0.8Zr_0.2O₂) or mixed oxide phase (Ce_0.5Zr_0.5O₂) deactivated during the reaction due to carbon formation. The enhanced catalytic performance of co-precipitated catalyst is attributed to a combination effect of nano-crystalline nature of cubic Ce_0.8Zr_0.2O₂ support and the finely dispersed nano size NiO _x crystallites, resulting in the intimate contact between Ni and Ce_0.8Zr_0.2O₂ particles. The Ni/Ce–ZrO₂/θ–Al₂O₃ also exhibited high catalytic activity during CDR with a synthesis gas conversion more than 97% at 800 °C without significant deactivation for more than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial pre-coating of Ce–ZrO₂ resulting in the existence of stable NiO _x species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. TPR results further confirmed that NiO _x formation was more favorable than NiO or NiAl₂O₄ formation and further results suggested the existence of strong metal-support interaction (SMSI) between Ni and the support. Some of the important factors to optimize the CDR of methane such as reaction temperature, space velocity, feed CO₂/CH₄ ratio and H₂O and/or O₂ addition were also examined.     

Characterization and Activity of Cu‐MnOx/γ‐Al2O3 Catalyst for Hydrogenation of Carbon Dioxide

The effect of manganese on the dispersion, reduction behavior and active states of surface of supported copper oxide catalysts have been investigated by XRD, temperature‐programmed reduction and XPS. The activity of methanol synthesis from CO₂/H₂ was also investigated. The catalytic activity over CuO‐MnO_x/γ‐Al₂O₃ catalyst for CO₂ hydrogenation is higher than that of CuO/γ‐Al₂O₃. The adding of manganese is beneficial in enhancing the dispersion of the supported copper oxide and make the TPR peak of the CuO‐MnK_x/γ‐Al₂O₃ catalyst different from the individual supported copper and manganese oxide catalysts, which indicates that there exists strong interaction between the copper and manganese oxide. For the CuO/γ‐Al₂O₃ catalyst there are two reducible copper oxide species; α and β peaks are attributed to the reduction of highly dispersed copper oxide species and bulk CuO species, respectively. For the CuO‐MnO_x/γ‐Al₂O₃ catalyst, four reduction peaks are observed, α peak is attributed to the dispersed copper oxide species; β peak is ascribed to the bulk CuO; γ peak is attributed to the reduction of high dispersed CuO interacting with manganese; δ peak may be the reduction of the manganese oxide interacting with copper oxide. XPS results show that Cu⁺ mostly existed on the working surface of the Cu‐Mn/γ‐Al₂O₃ catalysts. The activity was promoted by Cu with positive charge which was formed by means of long path exchange function between Cu? O? Mn. These results indicate that there is synergistic interaction between the copper and manganese oxide, which is responsible for the high activity of CO₂ hydrogenation.     

Syngas conversion beyond chemical equilibrium by in situ bimolecular reaction

An in situ bimolecular reaction, in which syngas is fed with toluene as a secondary reactant (hereafter Tol in situ methylation), was studied over bifunctional catalysts comprised of methanol synthesis catalyst and H-ZSM-5 in a fixed-bed down-flow reactor at 460 psig. When physically mixed with H-ZSM-5 to form bifunctional catalysts, CrZ_HZ (Cr₂O₃/ZnO + HZSM-5) catalyst showed much higher activity than CZA_HZ (CuO/ZnO/Al₂O₃ + H-ZSM-5) in the Tol in situ methylation, while CrZ catalyst exhibited substantially lower activity than CZA in methanol synthesis. CO conversion to methanol in the Tol in situ methylation was estimated by Bz in situ methylation. The CO conversion to methanol was calculated to be in the range of 11–27 %, while that in methanol synthesis over CrZ was about 5 % at most due to chemical equilibrium limitation. By employing a silicalite-coated H-ZSM-5 (Sil/HZ) in bifunctional catalyst, xylene selectivity and para-xylene yield were much improved in the Tol in situ methylation.     

Effect of rare earth oxides (CeO2, La2O3) on properties of Pd/Al2O3 catalysts for reduction of nitrogen oxides by methane

We have established that the thermal stability of supported Pd/Al₂O₃ catalysts is increased after they are modified by rare earth oxides (La₂O₃, Ce₂O₃). We have observed the effect of thermal activation of an aluminopalladium catalyst modified by lanthanum oxide. This effect is apparent in the increase of the specific catalytic activity in the reaction of high-temperature reduction of nitrogen oxides by methane after heat treatment of the catalyst at 850 °C. We have used X-ray photoelectron spectroscopy (XPS) to show that the reason for the thermal activation effect is stabilization of palladium in the Pd¹⁺ state. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 44–48, January–February, 2006.     

Influence of Temperature on Gas-Phase Toluene Decomposition in Plasma-Catalytic System

The decomposition of toluene on γ-alumina, MnO₂-alumina and Ag₂O-alumina catalysts in a plasma-catalytic reactor is tested. A comparison between catalytic, catalyst-after-plasma and catalyst-in-plasma systems is made in 150–400 °C temperature range. An Arrhenius plot is made in order to deduce the mechanism of plasma activation. It was found that there is no difference between the measured activation energy for catalytic and catalyst-after-plasma systems. On the other hand it was found that plasma could activate catalyst placed inside of the discharge. Plasma treatment decreases the activation energy for the silver-alumina catalyst but does not increase the number of active centers on the surface of Ag₂O-alumina. In case of MnO₂-alumina, the activation mechanism is different: plasma does not change the activation energy and but does increase its efficiency due to formation of additional active centers. The mechanism of catalyst activation in plasma, which includes the structural change of manganese ions, is suggested.     

The role of urea in Cu–Zn–Al catalysts for methanol steam reforming

Abstract  

Impregnated Cu–Zn over Al₂O₃ exhibits high activity with the use of a lower amount of active metal relative to conventional co-precipitation catalysts. The activity of the catalyst could be enhanced by addition of urea to the metal salt solution during impregnation. The H₂ yield from Cu–Zn catalysts with urea is 42%, while the H₂ yield from catalyst without urea is only 28% in a continuous system at 250 °C and 1.2 atm. The H₂ yield of the catalyst with urea in this study could compete with that of commercial catalysts. The role of urea in the Cu–Zn catalysts was investigated. X-ray diffraction (XRD) analysis of the catalysts shows that the crystal size of CuO could be reduced by the addition of urea. The XRD diffractogram of the catalyst prior to calcination also shows the formation of NH₄NO₃, which could aid in dissociation of metal clusters. Scanning electron microscopy (SEM) images of catalysts show the size of Cu–Zn compound clusters and also their dispersion over the Al₂O₃ surface on the impregnated catalysts. The addition of urea could also yield smaller Cu–Zn compound clusters and better dispersion compared with the impregnated catalyst without urea. Such impregnated Cu–Zn catalysts with urea could be alternative novel catalysts for methanol steam reforming.     

Synergistic effects of potassium promoter and carbon fibers on direct synthesis of isobutanol from syngas over Cu/ZnO/Al2O3 catalysts obtained from hydrotalcite-like compounds

The Cu/ZnO/Al₂O₃ catalysts (CuZnAl) can be utilized to directly synthesize higher alcohols from syngas under mild conditions. Carbon fibers (CFs) are widely used as a catalyst supporter, and potassium is usually used as a good electron assistant for charge transfer to the active phase of the catalyst. However, little is known about the combined effects of CFs and potassium on Cu/ZnO/Al₂O₃ catalysts. In this work, the CuZnAl catalysts supported on activated carbon fibers (ACFs) were prepared by a co-precipitation method, and then the catalysts were modified by potassium. The catalytic performances of K-modified CuZnAl and composites containing ACFs and CuZnAl were evaluated. Addition of ACFs and/or potassium increased CO conversion and selectivity for isobutanol compared with pure CuZnAl. All the samples were characterized by BET, XRD, SEM–EDS, CO–TPD, and Raman spectroscopy to further disclose the reason for better catalytic performance of the catalysts with ACFs and/or potassium. We found that addition of ACFs or potassium promotes moderate CO adsorption and formation of the active phase (CuO/ZnO solid solution) during alcohol synthesis, which facilitates synthesis of higher alcohols and CO conversion. As a result, ACFs and potassium exhibited synergistic effects on improvement of CO conversion and selectivity for isobutanol.     

Plasma treatment of Ni and Pt catalysts for partial oxidation of methane

Summary In this work DBD (dielectric barrier discharge) plasma treatments of 10%Ni/Al₂O₃and 1%Pt/Al₂O₃catalysts have been conducted to study the principles of plasma treatment of supported catalysts. It was found that 10%Ni/Al₂O₃and 1%Pt/Al₂O₃catalysts treated by plasma exhibit a higher catalytic activity and a better stability than the catalysts prepared without plasma treatment. Methane conversion over the plasma treated catalyst is 3-5% higher than on untreated catalysts. The metal species dispersion also increased after plasma treatment, which leads to improvement of the interaction between active species and supports, the catalytic activities and the resistance to carbon deposition.</o:p>     

Influence of Support Type and Metal Loading in Methane Decomposition over Iron Catalyst for Hydrogen Production

Natural gas resources, stimulate the method of catalytic methane decomposition. Hydrogen is a superb energy carrier and integral component of the present energy systems, while carbon nanotubes exhibit remarkable chemical and physical properties. The reaction was run at 700 °C in a fixed bed reactor. Catalyst calcination and reduction were done at 500 °C. MgO, TiO₂ and Al₂O₃ supported catalysts were prepared using a co‐precipitation method. Catalysts of different iron loadings were characterized with BET, TGA, XRD, H₂‐TPR and TEM. The catalyst characterization revealed the formation of multi‐walled nanotubes. Alternatively, time on stream tests of supported catalyst at 700 °C revealed the relative profiles of methane conversions increased as the %Fe loading was increased. Higher %Fe loadings decreased surface area of the catalyst. Iron catalyst supported with Al₂O₃ exhibited somewhat higher catalytic activity compared with MgO and TiO₂ supported catalysts when above 35% Fe loading was used. CH₄ conversion of 69% was obtained utilizing 60% Fe/Al₂O₃ catalyst. Alternatively, Fe/MgO catalysts gave the highest initial conversions when iron loading below 30% was employed. Indeed, catalysts with 15% Fe/MgO gave 63% conversion and good stability for 1 h time on stream. Inappropriateness of Fe/TiO₂ catalysts in the catalytic methane decomposition was observed.     

Gold catalysts supported on ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for low-temperature CO oxidation

Gold catalysts with loadings ranging from 0.5 to 7.0 wt% on a ZnO/Al₂O₃ support were prepared by the deposition–precipitation method (Au/ZnO/Al₂O₃) with ammonium bicarbonate as the precipitation agent and were evaluated for performance in CO oxidation. These catalysts were characterized by inductively coupled plasma-atom emission spectrometry, temperature programmed reduction, and scanning transmission electron microscopy. The catalytic activity for CO oxidation was measured using a flow reactor under atmospheric pressure. Catalytic activity was found to be strongly dependent on the reduction property of oxygen adsorbed on the gold surface, which related to gold particle size. Higher catalytic activity was found when the gold particles had an average diameter of 3–5 nm; in this range, gold catalysts were more active than the Pt/ZnO/Al₂O₃ catalyst in CO oxidation. Au/ZnO/Al₂O₃ catalyst with small amount of ZnO is more active than Au/Al₂O₃ catalyst due to higher dispersion of gold particles.     

A Brief Catalyst Study on Direct Methane Conversion Using a Dielectric Barrier Discharge

A series of metal catalysts was used for methane conversion to higher hydrocarbons and hydrogen in a dielectric barrier discharge. The main goal of this study is to identify the metal catalyst components which can influence the reactions in room‐temperature plasma conditions. The catalysts supported by γ‐Al₂O₃ and zeolite (ZSM 5x) were prepared by the incipient wetness method with solutions containing the metal ions of the second component. Among the catalysts tested, only Pt and Fe catalysts showed a unique result of catalytic reaction in a reactor bed packed with glass beads.     

溶剂极性对微波辐射老化制备前驱体及CuO/ZnO/Al2O3催化剂结构的影响

在制备CuO/ZnO/Al2O3催化剂的老化过程中,采用微波辐射老化技术,着重研究了溶剂极性对前躯体物相组成,烧后CuO/ZnO/Al2O3催化剂结构及其在浆态床合成甲醇工艺中催化性能的影响。通过XRD、DTG、H2-TPR,FTIR、HR-TEM和XPS对前驱体及催化剂表征表明,沉淀母液在微波辐射条件下进行老化,溶剂的极性对前躯体物相组成及催化剂结构影响显著。随着溶剂极性的增大,Zn2+/Cu2+取代Cu2(CO3)(OH)2/Zn5(CO3)2(OH)6中Cu2+/Zn2+的取代反应增强,使得前躯体中(Cu,Zn)5(CO3)2(OH)6和(Cu,Zn)2(CO3)(OH)2物相的含量增多,结晶度提高,导致烧后CuO/ZnO/Al2O3催化剂中CuO-ZnO协同作用增强,且CuO晶粒减小,表面Cu含量增加,催化剂活性和稳定性提高。水溶剂的极性最大,制备的催化剂活性和稳定性最好,甲醇的时空收率(STY)和平均失活率分别为320 mg.g-1.h-1和0.11%.d-1。     

Role of rare-earth element oxides in catalysts for the oxidative conversion of methane based on Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

It has been established that oxides of the rare-earth elements with moderate redox potentials (La₂O₃, CeO₂) increased the activity and working stability of Ni-Al₂O₃/cordierite catalysts in the reactions of deep and partial oxidation of methane. In the presence of the (NiO + La₂O₃ + Al₂O₃)/cordierite catalyst the process of carbon dioxide conversion of methane can be intensified by introduction of oxygen into the reaction gas mixture which decreases the temperature to achieve high conversion to 75–100 °C and has practically no effect on selectivity with respect to H₂. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 6, pp. 359–364, November–December, 2008.