CeO2-W-Mn/SiO2 catalyst is found to be effective for the oxidative coupling of methane (OCM); ca. 30 % of C-2+ and 22 % of C2H4 yields have been obtained in single pass at 800 degrees C. During the 500 h continuous reaction, no significant decrease of activity and selectivity is observed.

Temperature-programmed desorption (TPD) of CH4, C2H6, C2H4, and CO and temperature-programmed pulse surface reactions (TPSR) of CH4, C2H6, C2H4, CO, and CO/H2 over a Co/MWNTs catalyst have been investigated. The TPD results indicated that CH4 and C2H6 mainly exist as physisorbed species on the Co/MWNTs catalyst surface, whilst C2H4 and CO exist as both physisorbed and chemisorbed species. The TPSR results indicated that CH4 and C2H6 do not undergo reaction between room temperature and 450oC. Pulsed C2H4 can be transformed into CH4 at 400 oC whilst pulsed CO can be transformed into CO2 at 100 or 150oC. In gaseous mixtures of CO and H2 containing excess CO, the products of pulsed reaction were CH3CHO and CH3OH. When the ratio of CO and H2 was 1:2, pulsed CO and H2 were transformed into CH3CHO, CH3OH and CH4. In H2 gas flow, pulsed CO was transformed into a mixture of CH3CHO and CH4 between 200 and 250oC and was transformed into CH4 only above 250oC.

The 5 wt% Na2WO4-2 wt% Mn/SiO2 catalysts have been prepared by the incipient wetness impregnation method, mixture slurry method and sol-gel method, and their catalytic performances for the oxidative coupling of methane (OCM) are evaluated in a continuous micro-reactor. 30% of CH4 conversion and 70% of C-2 selectivity have been obtained in methane-oxygen co-feed without any dilutes over these catalysts. X-ray diffraction (XRD) studies indicate that the main crystallite phases of catalysts are Mn2O3, Na2WO4 and alpha-cristobalite, and different precursors of silica are transformed ultimately into highly crystallite alpha-cristobalite. XPS results show that Na, W and Mn are mainly distributed on the surface of catalyst prepared by the incipient wetness impregnation method, but more uniform between the surface and bulk on the catalysts prepared by other two methods. Comparing with the catalyst prepared by the incipient wetness impregnation method, that of prepared by the mixture slurry method has an excellent stability and can alleviate the loss of active components during a 500 h run. Structure changes indicate that a-cristobalite is not indispensable for effective Na2WO4-Mn/SiO2 catalysts. (c) 2005 Elsevier B.V. All rights reserved.

A series of BaCO3/La2O3 catalysts have been prepared using urea combustion method, and tested their activity in oxidative conversion of methane to ethane and ethylene below 600 °C. For 5% BaCO3/La2O3 catalyst, under integral conditions, CH4 conversion of 31.8% with C2 selectivity of 45.9% have been obtained at 320 °C. With the increase of BaCO3 contents, CH4 conversion and C2 selectivity are improved. The temperature profiles of reactor have been described and found that the distribution of hot spots on catalyst bed is related to the low-temperature activity of catalyst.

A Mo2C and an alumina-supported Mo2C catalysts were prepared and their catalytic performance for the direct decomposition of NO to N2 and O2 was evaluated in a continuous-flow microreactor. The structural properties of the catalysts have been studied using X-ray diffraction as well as BET surface area measurement. The results show that the bulk molybdenum carbide and the Mo2C/Al2O3 catalysts exhibit an excellent catalytic activity for NO direct decomposition. There is a stronger interaction between the β-Mo2C phase and γ-Al2O3 support in the Mo2C/Al2O3 catalysts when Mo content is higher. It is suggested that the base sites of Mo2C and Mo2C/Al2O3 catalysts may have played an important role in catalyzing the decomposition of NO.

The progress in the study of early transition metal carbides in heterogenous catalysis, especially molybdenum and tungsten carbides is reviewed. The supported molybdenum or tungsten carbides or doped with a second metal shows excellent catalytic activity, especially for the hydrotreatment of petroleum distillates, such as hydrodenitrogenation (HDN), hydrodesulfurization (HDS) and isomerization, which makes them the promising substitutes for commercial catalysts. The preparation of carbide as well as its effects on the catalysis is also reviewed.

The investigation on MoO3/Al2O3 sample or its modifiers with nickel, copper or potassium was performed using temperature programmed surface reaction (TPSR) technique and measurements of BET surface area. The results indicate that addition of nickel promotes the methane reduction, further the carburization, of MoO3, and addition of nickel also promotes the activation of methane over the surface of oxycarbide or carbide due to the increase of active sites per unit area and intrinsic activity of catalytic centers. This is favorable to the conversion of methane. The addition of copper promotes the methane reduction, further the carburization, of MoO3 to some extent, while the introduction of copper also accelerates the sintering of catalyst to a degree. Thus copper doped carbide catalyst exhibits its exceptionally catalytic performance. However, potassium prevents the MoO3 from reduction with methane, which is unfavorable to the carburization. Potassium also restrains methane from being activated over the surface of oxycarbide or carbide. MoO3/Al2O3 doped with potassium is of lower specific area, which originates from its boosting sintering of catalyst. This causes the inferior methane conversion over potassium doped carbide catalyst.

Investigation on molybdenum carbide catalyst, its oxide precursor and its modifiers was performed using temperature programmed surface reaction (TPSR) technique, BET surface specific area and X-ray diffraction (XRD) measurement. The carbide catalyst and its modifiers were evaluated using microreactor apparatus. The evaluation result showed that the addition of nickel promoted the POM to syngas, the addition of copper exhibited weaker promotion at the initial stage, but it turned unfavorable to the POM to syngas hereafter. As for potassium doped catalyst, it was unfavorable to the POM. According to the characterizations, the effect of added metals on the carburization, especially under the condition of POM, was elucidated. (C) 2003 Elsevier B.V. All rights reserved.

The performance of supported and unsupported molybdenum carbide for the partial oxidation of methane to synthesis gas was studied. Bulk molybdenum carbide had a higher methane conversion during the initial stage but a lower selectivity to CO and H2/CO ratio in the products. The supported molybdenum carbide catalyst exhibited a much higher methane conversion, increased selectivity, and significantly improved catalytic stability. The bulk or the supported molybdenum carbide existed in the β-Mo2C phase, while it was transformed into molybdenum dioxide postcatalysis, which is an important cause of molybdenum carbide deactivation. The catalyst with a loading of 35.4 wt % had the best selectivity. The catalytic pyrolysis of methane could occur over the molybdenum carbide and explained the high methane conversion and high ratio of H2/CO during the initial stage. However, the low selectivity to CO could be due to the low capacity of molybdenum carbide to activate oxygen.

The supported Mo2C/Al2O3 catalyst was prepared and modified by a trace amount of Ni, Co, Cu or K, and evaluated under the conditions of n(CH4)/n(O-2) = 2.05, GHSV = 20.5 L/(g(.)h) and theta = 850 degreesC in a microreactor for partial oxidation of methane (POM) to syngas. The results showed that the Ni-MO2C/Al2O3 exhibited the highest activity and selectivity, and its catalytic performance was particularly stable. Co had the functions, though to less extent, similar to Ni. The addition of Cu promoted the CH, conversion at initial stage, but then the promotion disappeared rapidly. However, the addition of Cu was unfavorable to the selectivity for CO and H-2. The addition of K was unfavorable to POM to syngas. The catalytst samples were characterized by temperature-programmed surface reaction (TPSR) technique. The TPSR result indicated that the carbonization Of MoO3 was carried out via two steps, namely, reduction by H-2 at first and then carbonization by CH4. The addition of Ni or Co promoted the activation of CH4, the reduction of MoO3 by H-2 and the deposition of carbon, and then promoted the carbonization. Thus, there was possibility for the modified catalyst to hold more carbide or oxycarbide as active phase in the process of POM. Whilst for K-modified catalyst, the K prevented the catalyst from reduction by H-2, carburization by CH4 and activation of CH4. Thus, the K was unfavorable to POM. As for Cu-modified catalyst, Cu promoted the reduction of MoO3 and activation of CH4, thus promoted POM to syngas. Meanwhile, the Cu was favorable to the activation Of O-2 Molecule, being unfavorable to the reaction selectivity. For the Cu-modified catalyst, the disappearance of promotion to CH4 conversion after initial period might be due to the change of valence state of Cu ion.

Alumina-supported molybdenum carbide and its modifier with small amounts of nickel were evaluated and characterized. The evaluation results showed that a nickel-doped carbide catalyst exhibited a much higher conversion of methane and selectivity to CO and H2for the partial oxidation of methane (POM) to syngas. Also, the addition of a small amount of nickel resulted in significant improvement of catalysis stability. Characterization by temperature programmed surface reaction (TPSR) indicated that the addition of nickel promoted the carburization of molybdenum oxide. Especially under the conditions of the POM to syngas, this promotion resulted in more active phases being held. The nickel doped catalyst had small particles, larger specific area and greater ability to resist sintering. All these improved the catalytic stability. The other characterizations by XRD, XPS, SEM as well as BET surface area measurement also confirmed these promotions of nickel. In addition, the doped nickel made possible the increase of the intrinsic activity of catalytic centers over molybdenum carbide catalyst.