Carburized Catalysts Research Articles

Effects of promoters on carburized fused iron catalysts in Fischer-Tropsch synthesis

The effects of K, Ru or La promoters on the structure, surface area, crystal phase, and catalytic behavior during FT synthesis of carburized and uncarburized fused Fe catalysts were studied by XRD, XPS, TPD, N2-physisorption and catalytic reaction evaluation techniques. Addition of K improved selectivity of C5+ products for both the carburized and uncarburized catalysts. Addition of Ru suppressed catalytic activity of the carburized catalyst, but had little influence on the uncarburized one. Addition of La led to the encapsulation of the iron carbide, which consequently severely inhibited the carburization and decreased the activity. While Ru and La promote the formation of light components due to their ability to promote hydrogen adsorption. The performance of the reaction in the experiment indicated that the U-K catalyst had the best product distribution, in which the methane selectivity was 4.04%, and the C5+ selectivity was 75.84%.

A highly active and stable bimetallic Ni-Mo2C catalyst for a partial oxidation of jet fuel

Mo2C is an attractive catalyst for fuel reforming reactions because it possesses both a high activity and a high coking resistance. However, Mo2C catalysts cannot guarantee sufficient long-term stability during fuel reforming reactions that operate at a high fuel flow rate due to their phase instability. The present investigation is focused on improving the phase stability and the performance of Mo2C by addition of Ni for the partial oxidation (POX) of jet fuel. Mo2C and Ni-Mo2C were first prepared from CH4/H2 carburization of synthesized MoO3 and NiMoO4, respectively. To investigate the catalytic activity of carburized catalysts for the POX of jet fuel, tests were conducted at 750 °C and 1 atm, with a weight-hourly-space-velocity (WHSV) of 42 h−1 and O2/C ratio of 0.6. The Mo2C sample without Ni shows a performance similar to that of a blank run (in the absence of catalyst) with 70% conversion, 12% H2 yield, and 53% CO yield. The poor performance of Mo2C is due to its partial phase transformation into the MoO2 phase at the high WHSV. For Ni-Mo2C, the catalyst exhibits excellent stability over the 24 h test period with carbon conversion of 90% and H2 and CO yields of 56 and 63%, respectively. There were no indications of bulk oxidation or surface coking. Temperature-programmed reaction and isotopic exchange experiments showed that Ni-Mo2C follows the “catalytic oxidation and re-carburization cycle.” In this cycle, molecular oxygen is activated over the Mo2C surface, and the activated oxygen species react with lattice carbons from Mo2C to produce both CO and carbon vacancies. Hydrocarbons are decomposed into H2 and surface carbons over the metallic Ni sites. To sustain the catalytic cycle, the Mo2C1-x phase (i.e., non-stoichiometric Mo2C with carbon vacancies) is re-carburized by the surface carbons deposited on the metallic Ni sites. Finally, our results also showed that NiMoO4 can be rapidly carburized in-situ to form a high performance Ni-Mo2C catalyst under a flowing mixture of n-dodecane and air.

Hindered methane decomposition on a coke-resistant Ni-In/SiO2 dry reforming catalyst

Decomposition of methane has been investigated by mass spectrometry assisted pulse chemisorption experiments at 600 °C on freshly reduced and on carburized Ni/SiO2 and Ni-In/SiO2 dry reforming catalysts. The results showed complete methane decomposition with stoichiometric hydrogen production on freshly reduced Ni/SiO2, while strong hydrogen chemisorption and partial methane decomposition was observed on Ni-In/SiO2. Hydrogen production decreased on both carburized catalysts and dissociative methane chemisorption without hydrogen formation was observed on the bimetallic catalyst. This difference in methane activation might be one of the reasons for the absence of coke on the bimetallic catalyst during dry reforming of methane.

Fe–Zn catalysts for the production of high-calorie synthetic natural gas

In this study, Fe–Zn catalysts were applied in a methanation reaction, where the C2–C4 hydrocarbon selectivity was controlled in order to increase the heating value of the synthetic gas product. Fe–Zn catalysts were prepared by the co-precipitation method and were activated via two different pretreatments: carburization (using synthesis gas) and reduction (using diluted hydrogen). The active species within the carburized Fe–Zn catalysts primarily consisted of iron carbides, while magnetite was the dominant species in the reduced Fe–Zn catalysts. The carburized Fe–Zn catalysts exhibited a higher CO conversion between 95.9% and 98.2% compared to the reduced Fe–Zn catalysts with same composition (81.7–89.9%) due to stronger interactions with CO and a higher BET area. The carbon chain growth on both catalysts was nearly identical (0.18–0.23), but reduced Fe–Zn catalysts exhibited higher paraffin-to-olefin ratio up to 4.30, while the carburized catalysts achieved relatively low ratio between 1.95 and 2.76, because magnetite was more efficient in olefin hydrogenation than the iron carbides. The carburized and reduced Fe–Zn catalysts produced high-calorie synthetic natural gas with a heating value of over 56MJ/Nm3.

Effect of Carbon Sources on the Catalytic Performance of Ni/β-Mo2C.

In this paper, Ni/β-Mo2C(S) and Ni/β-Mo2C(G) were prepared from solution-derived precursor with two different carbon sources (starch and glucose) and tested as anodic noble-metal-free catalysts in air-cathode microbial fuel cells (MFCs). The carburized catalyst samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer-Emmett-Teller (BET). The activity of the electrocatalyst towards the oxidation of several common microbial fermentation products (formate, lactate, and ethanol) was studied for MFC based on Klebsiella pneumoniae conditions. The composite MFC anodes were fabricated, and their catalytic behavior was investigated. With different carbon sources, the crystalline structure does not change and the crystallinity and surface area increase. The electrocatalytic experiments show that the Ni/β-Mo2C(G) gives the better bio- and electrocatalytic performance than Ni/β-Mo2C(S) due to its higher crystallinity and BET surface area.

Fischer−Tropsch Synthesis: Influence of Mn on the Carburization Rates and Activities of Fe-Based Catalysts by TPR-EXAFS/XANES and Catalyst Testing

Fe-based catalysts containing different amounts of Mn were tested for Fischer−Tropsch synthesis using a stirred tank reactor at 270 °C, 1.21 MPa, and H2:CO = 0.7. Catalyst activation by carburization with 10% CO/He was followed by Temperature Programmed Reduction/X-ray Absorption Spectroscopy (TPR-EXAFS/XANES) from room temperature to 300 °C. γ-Fe2O3 was converted into iron carbides, whereas MnOx was reduced to oxygen deficient MnO. Mn hindered Fe carburization, such that the carburized catalyst displayed higher Fe3O4 content than the catalyst without Mn. EXAFS fitting indicates that the carburized catalyst contained a mixture of Hägg carbide, Fe3O4, and Mn oxides. Increasing Mn content led to higher CH4 and light product selectivities, and lower light olefin selectivities. Higher and stable conversions were obtained with a catalyst containing an almost equimolar Fe/Mn ratio relative to the catalyst without Mn. Selectivity trends are attributed to the higher WGS rates observed on the FeMn catalysts, consistent with the structural differences observed.

Adsorption and reaction of CO and hydrogen on iron-based Fischer–Tropsch synthesis catalysts

H 2, CO chemisorptions and the carbon hydrogenation on promoted iron Fischer–Tropsch synthesis (FTS) catalysts (Fe/SiO 2 and FeK/SiO 2) were investigated using temperature programmed surface reaction with X-ray photoelectron spectroscopy and laser Raman spectroscopy. It is found that potassium, used as promoter, does not lead to a distinct variation in the carbon species but changes the surface H/C ratio of carburized catalysts. Besides bulk iron carbide, several carbon species with different cluster sizes exist in the carburized catalysts, which have different reactivities towards hydrogen. The surface atomic carbon, the oligomerized carbon species and the bulk iron carbide are more reactive to hydrogen, whereas the large-size amorphous carbons are relatively inert to hydrogen. There is a good correlation between the chemisorption of H 2 or CO and the corresponding feed gas conversion activity in FTS reaction. Meanwhile, the methane selectivity is correlated with the hydrogenation capability of catalysts, indicating that the surface H concentration has an important effect on the selectivity of hydrocarbons.

Cobalt molybdenum carbides as anode electrocatalyst for proton exchange membrane fuel cell

Cobalt molybdenum (Co-Mo) carbides were prepared by the carburization of Co-Mo oxides at temperatures of 723–973 K in a stream of CH 4/H 2 gas. The carburized catalysts were evaluated using a single-stack fuel cell and three-electrode cell. The results showed high activities for the anodic electrooxidation of hydrogen over the Co-Mo catalysts carburized at 873 and 923 K. The 873 K carburized Co-Mo catalyst had the highest activity and achieved 10.9% of the performance of a commercial Pt/C catalyst in a single-stack fuel cell. The XRD, TPC, TPR and XPS results showed that the Co-Mo oxycarbide in the bulk and on the surface are the active species for the hydrogen oxidation reaction.

Nano-structured nickel–molybdenum carbide catalyst for low-temperature water-gas shift reaction

The water-gas shift (WGS) reaction over nickel–molybdenum (Ni–Mo) carbide catalysts was investigated at 453 K. The oxide precursors and the carburized catalysts were characterized based on the adsorption of N 2 and CO, XRD, high-resolution tunnel electron microscopy (HRTEM), temperature-programmed reaction with 20% CH 4/H 2 or with He, and X-ray photoelectron spectroscopy. The 873 K-carburized Ni 0.25Mo 0.75 catalyst (Ni 0.25Mo 0.75C-873) exhibited the highest rate of CO conversion among the catalysts carburized at 823, 923, and 973 K with various Ni contents. The Ni 0.25Mo 0.75C-873 catalyst contained β-Mo 2C and amorphous Ni–Mo oxycarbide (NiMoC 3.6O 1.4) which turned to Ni 6Mo 6C after being heated under flowing He. The TEM image showed that Ni 0.25Mo 0.75C-873 consisted of Ni particles dispersed on plate-like sheets of β-Mo 2C and amorphous Ni–Mo carbides.

Low temperature low pressure benzene hydrogenation on Y zeolite-supported carbided molybdenum

Carbided molybdenum catalysts supported on ultrastable Y zeolites were prepared by adsorption of molybdenum hexacarbonyl vapors at 343 K or wet point impregnation with ammonium heptamolybdate solutions, followed in both cases by temperature-programmed carburization under a 20% methane in hydrogen mixture up to 923 K. Nitrogen physical adsorption and X-ray diffraction clearly established that the zeolite structure was preserved during the carburation treatment. Chemical analysis of the catalysts evidenced that very little loss of molybdenum occurred. Despite the fact that all catalysts had similar CO chemisorption capacities, their activity varied widely. For the same silica–alumina ratio (SAR), activity increased with increasing sodium content and, for low sodium contents, with increasing SAR. When catalyst reduction was performed with pure hydrogen, the activity was about one third of the one obtained with the carburation procedure, suggesting that a carbon containing species is the active one in this reaction. Two EPR signals with g ⊥ values 1.96 and 1.98 were observed in the carburized catalysts, that may be attributed to hexa-coordinated Mo 5+. The second one of these signals only appears in the most active catalysts and is attributed to Mo 5+ ions in small carbide or oxycarbide clusters.

Molybdenum carbides and carbonaceous carbons on Mo/Al-FSM16 for methane conversion

The catalytic activity of 12.0 wt.% Mo/Al-FSM16 (Si/Al=30) was related to the Mo carbides and carbonaceous carbons during CH 4 conversion at 973 K. The material had been carburized during the temperature-programmed reaction with 20% CH 4/H 2 at 973 K, with and without purging in He. The temperature-programmed surface reaction and X-ray diffraction analysis showed that the carburized catalysts consisted of Mo carbides (α-Mo 2C 1− x , β-Mo 2C, and η-Mo 3C 2) and carbonaceous carbons (pyrolytic and graphitic carbons). The purging treatments of the carburized catalysts at 973 K for 1 or 2 h lowered the activity, due to a decrease in the carbonaceous carbon releases of C 2H 2, C 2H 4, CO 2, and H 2O and due to the Mo carbides changing to other forms. The CH 4 conversion was responsible for η-Mo 3C 2 conjugation to pyrolytic carbons, which played the role of the intermediate species. The IR study revealed that CH 2 and CH 3 species were formed as the intermediates of the pyrolytic species for CH 4 formation. The pulse experiments of C 2H 4, C 2H 6, C 3H 8, and C 4H 8 showed that hydrogen was readily formed through the adsorbed C 2H 4 and C 3H 8 species as intermediates and that benzene was formed through the adsorbed C 4H 8 species.

Mathematical Description of a Heterogeneous Catalytic Process on the Basis of the Dendrite Model of a Catalyst Pellet

A mathematical model is suggested for a heterogeneous catalytic process. The model is based on the assumption that the spherical surface of a catalyst pellet has a layer in which there are many dendrites that are formed by interconnected macrochannels and microchannels and have a single inlet at the surface. The depth of penetration of fine microchannels into the pellet bulk is assumed to depend on the diameter of the inlet channel. The model is applied to the regeneration of a carburized catalyst.

Study of unsupported and active carbon supported β-Mo 2C prepared from MoCl 5 precursor

We have studied the preparation of unsupported and lignite supported β-Mo 2C using MoCl 5 as precursor. The catalysts were characterized by XRD and TEM techniques and tested in the reaction of propene hydrogenation at room temperature and pressure. Total carburization was observed at 923 K and although incomplete, carburization also occurred at 473 or 723 K. The high temperature supported carbide exhibited a remarkable lifetime in the test reaction (72 h), whereas, the massive carbide rapidly deactivated (<1 h). The low temperature carburized catalysts also rapidly deactivated (10 min). However, in a remarkable way, processing of the used catalysts by H 2 at 723 or 873 K, not only restored the initial activity, but increased their stability with time on stream (3 and >24 h, respectively). The correlation between the degree of carburization, dispersion and catalyst stability is discussed.

Chemisorption of methane on [formula omitted] catalysts and reactivity of the chemisorption products toward hydrogen

The Chemisorption of hydrogen both on bare and carburized Ni SiO 2 catalysts was studied using a low-field magnetic method, infrared spectroscopy, and mass spectrometry. With a freshly reduced and evacuated sample of one of the catalysts, H 2 chemisorption was investigated as a function of the temperature (30 < T < 100 °C). It was found that the slope of the magnetization-volume isotherm decreased with increasing temperature, which does not agree with the theory of superparamagnetism. The smaller slope at more elevated temperatures was ascribed to a more extensive coverage of the smaller nickel particles after admission of the initial H 2 doses. Carburization of the catalysts was established by the decomposition of CH 4 at temperatures from 30 to 300 °C. At low surface coverages the carbon was deposited as Ni 3C strongly affecting the magnetization. At higher surface coverages CH x -complexes without any effect on the magnetization were chemisorbed. After the decomposition of CH 4 the catalysts were evacuated at 250 °C, which was found to result in the conversion of a part of the carbonaceous deposit into methane. Also with the subsequent chemisorption of hydrogen on the carburized catalysts ( T = 30 °C) the reaction between chemisorbed H-atoms and deposited carbon was apparent from the production of CH 4. From a comparison of the magnetization-volume isotherms for H 2 Chemisorption before and after the deposition of small amounts of carbon it was derived that the decomposition of methane preferentially proceeds on small nickel crystallites. Finally it was found that hydrogen was adsorbed not only on bare nickel (with magnetic effect) but also on nickel carbide (without magnetic effect).