Catalytic decomposition of light alkanes, alkenes and acetylene over Ni/SiO2

The decomposition of different hydrocarbons (CH₄, C₂H₆, C₂H₄, C₂H₂, C₃H₈, and C₃H₆) over Ni (5 wt.%)/SiO₂ catalysts was carried out. The initial rates of decomposition of the hydrocarbons, the kinetic curves of the decomposition and the kinetic curves of the hydrogenation of deposited carbon into methane depended on the types of hydrocarbons. In addition, the catalytic life of the Ni/SiO₂ catalyst was also dependent on the types of hydrocarbons, i.e. the life was longer according to the order, alkanes>alkenesacetylene.

The carbons deposited on the catalyst were characterized by SEM and Raman spectroscopy. The appearances of the deposited carbons were different among alkanes, alkenes, and acetylene, i.e. a zigzag fiber structure from methane, and a rolled fiber structure from alkenes and acetylene. From Raman spectra of the deposited carbons, it was found that the degree of graphitization of deposited carbon was higher in the order, alkanes>alkenes>acetylene. These results suggest that the mechanism of decomposition of hydrocarbons and the growth mechanism of carbon fibers on the catalyst were different among alkanes, alkenes and acetylene.     

TPD and XRD studies of molybdenum nitride and its activity for hydrodenitrogenation of carbazole

The nitrided MoO₃ catalysts formed using two kinds of treatment with either NH₃ or He after nitriding were studied by temperature-programmed desorption and X-ray diffraction analyses. The catalysts were cooled to room temperature in either flowing NH₃ or He (NH or HE catalyst) after nitriding at 773, 973 and 1173 K with NH₃. The activities of the catalysts were determined during the hydrodenitrogenation of carbazole at 573 K and 10.1 MPa total pressure. MoO₂, γ-Mo₂N, and Mo metal were mainly formed in the NH catalysts nitrided at 773, 973 and 1173 K, respectively. Mo oxides and metals in the NH catalysts were nitrided to γ-Mo₂N and β-Mo₂N_0.78 with low crystallinity during TPD. The surface area of the NH and HE catalysts nitrided at 773 K increased to 66 and 59 m² g⁻¹ maximum from 1.1 m² g⁻¹ of fresh MoO₃, respectively, but decreased as the nitriding temperature increased to 973 K and 1173 K. The HE catalysts per surface area were more active than the NH catalysts for both the overall HDN reaction and hydrogenation, and the 1173 K-nitrided catalysts were highest. On the other hand, the NH catalysts were more active than the HE catalysts for C–N hydrogenolysis and the 973 K-nitrided catalyst showed a maximum activity for C–N hydrogenolysis.     

Catalytic conversion of methane and CO2 to synthesis gas over a La2O3-modified SiO2 supported Ni catalyst in fluidized-bed reactor

In this contribution, a commercial spherical SiO₂ was modified with different amounts of La₂O₃, and used as the support of Ni catalysts for autothermal reforming of methane in a fluidized-bed reactor. Nitrogen adsorption, XRD and H₂-TPR analysis indicated that La₂O₃-modified SiO₂ had higher surface area, strengthened interaction between Ni and support, and improved dispersion of Ni. CO₂-TPD found that La₂O₃ increased the alkalescence of SiO₂ and improved the activation of CO₂. Coking reaction (via both temperature-programmed surface reaction of CH₄ (CH₄-TPSR) and pulse decomposition of CH₄) disclosed that La₂O₃ reduced the dehydrogenation ability of Ni. CO₂-TPO, O₂-TPO (followed after CH₄-TPSR) confirmed that only part amount of carbon species derived from methane decomposition could be removed by CO₂, and O₂ in feed played a crucial role for the gasification of the inactive surface carbons. Ni/xLa₂O₃-SiO₂ (x = 10, 15, 30) possessed high activity and excellent stability for autothermal reforming of methane in a fluidized-bed reactor.     

Coprecipitated iron-containing catalysts (Fe-Al2O3, Fe-Co-Al2O3, Fe-Ni-Al2O3) for methane decomposition at moderate temperatures: Part II. Evolution of the catalysts in reaction

Coprecipitated Fe-Al₂O₃, Fe-Co-Al₂O₃ and Fe-Ni-Al₂O₃ catalysts is shown to be very efficient in carbon deposition during methane decomposition at moderate temperatures (600–650 °C). The carbon capacity of the most efficient bimetallic catalysts containing 50–65 wt.% Fe, 5–10 wt.% Co (or Ni) and 25–40 wt.% Al₂O₃ is found to reach 145 g/g_cat. Most likely, their high efficiency is due to specific crystal structures of the metal particles and formation of optimum particle size distribution. According to the TEM data, catalytic filamentous carbon (CFC) is formed on them as multiwall carbon nanotubes (MWNTs). The phase composition of the catalysts during methane decomposition is studied using a complex of physicochemical methods (XRD, REDD, Mössbauer spectroscopy and EXAFS). Possible mechanisms of the catalyst deactivation are discussed.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.     

Light alkanes CO2 reforming to synthesis gas over Ni based catalysts

The CO₂ reforming of methane and propane has been compared over two different Ni catalysts: one reference Ni/SiO₂ system and a Ni/Mg(Al)O hydrotalcite-derived catalyst, shown previously to display high catalytic stability for long term reforming. By combining the Tapered Element Oscillating Microbalance (TEOM), Temperature Programmed Hydrogenation (TPH), Transmission Electron Microscopy (TEM) and magnetic measurements, the formation of coke and its role on the catalyst activity has been investigated and compared for both hydrocarbons. It was found that Ni/SiO₂ and Ni/Mg(Al)O are both more active for methane reforming than for propane reforming. Coke formation is much more pronounced for propane than for methane over both catalysts. However, for both hydrocarbons a much faster carbon formation is observed over the Ni/SiO₂ catalyst than over the Ni/Mg(Al)O catalyst. The difference in the rates of coke formation for methane and propane is ascribed in the case of propane to partially dehydrogenated C₃ adspecies, which are good coke precursors. The superior stability of the hydrotalcite-derived catalyst is due to the strong interaction of the nickel phase with the support and the capacity of the support to activate CO₂ and channel oxygen to the nickel phase.     

不同前体制备Ni催化剂及其在硫碘制氢中催化分解HI的性能

硫碘制氢是目前最有潜力的大规模低成本制氢方法之一。HI催化分解是硫碘制氢的关键步骤。本文采用4种具有代表性的镍盐前体--硝酸镍[Ni（NO₃）₂]、乙酸镍[Ni（CH₃COO）₂]和两种有机金属络合镍（乙酰丙酮镍[Ni（acac）₂]和亚硝酸三乙二胺合镍（[Ni（en₃）]（NO₂）₂）制备Ni催化剂,考察了不同镍盐前体对制备的Ni催化剂特性的影响规律以及Ni催化剂在HI催化分解中的性能。采用了BET、XRD和TEM等分析测试技术对制备的Ni催化剂进行了表征。研究结果表明,镍盐前体对制备的Ni催化剂的分散度、催化活性以及催化稳定性有重要的影响。以Ni（NO₃）₂和[Ni（en₃）]（NO₂）₂为前体制备的Ni催化剂具有良好的Ni金属分散度,Ni颗粒分散均匀且尺寸较小。由于高温和强腐蚀的反应环境（HI-I₂-H₂O体系）,Ni催化剂经过反应都会出现不同程度的失活。以[Ni（en₃）]（NO₂）₂为前体制备的Ni催化剂表现出最好的催化活性和稳定性,有望成为未来大规模硫碘制氢系统中HI分解的催化剂。     

Simultaneous production of hydrogen and carbon nanostructures by decomposition of propane and cyclohexane over alumina supported binary catalysts

Nano-scale, binary, 4.5 wt.% Fe–0.5 wt.% M (M = Pd, Mo or Ni) catalysts supported on alumina have been shown to be very effective for the decomposition of lower alkanes to produce hydrogen and carbon nanofibers or nanotubes. After pre-reduction at 700 °C, all three binary catalysts exhibited significantly lower propane decomposition temperatures and longer time-on-stream performances than either the non-metallic alumina support or 5 wt.% Fe/Al₂O₃. Catalytic decomposition of propane using all three catalysts yielded only hydrogen, methane, unreacted propane, and carbon nanotubes. Above 475 °C, hydrogen and methane were the only gaseous products. Catalytic decomposition of cyclohexane using the (4.5 wt.% Fe–0.5 wt.% Pd)/Al₂O₃ catalyst produced primarily hydrogen, benzene, and unreacted cyclohexane below 450 °C, but only hydrogen, methane, and carbon nanotubes above 500 °C. The carbon nanotubes exhibited two distinct forms depending on the reaction temperature. Above 600 °C, they were predominantly in form of multi-walled nanotubes with parallel walls in the form of concentric graphene sheets. At or below 500 °C, carbon nanofibers with capped and truncated stacked-cone structure were produced. At 625 °C, decomposition of cyclohexane produced a mixture of the two types of carbon nanostructures.     

Partial oxidation of methane to synthesis gas over iridium–nickel bimetallic catalysts

Partial oxidation of methane to synthesis gas was carried out using supported iridium–nickel bimetallic catalysts, in order to reduce loading levels of iridium and nickel, and to avoid carbon deposition on nickel-based catalysts by adding iridium. The performance of supported iridium–nickel bimetallic catalysts in synthesis gas formation depended strongly upon the support materials. La₂O₃ gave the best performance among the support materials tested. Ir(0.25 wt%)–Ni(0.5 wt%)/La₂O₃ afforded 36% conversion of methane (CH₄/O₂=5) to give CO and H₂ with the selectivities of above 90% at 800°C, and those at 600°C were 25.3% conversion of methane and CO and H₂ selectivities of about 80%, respectively. Reduced monometallic Ir(0.25 wt%)/La₂O₃ and Ni(0.5 wt%)/La₂O₃ catalysts did not produce synthesis gas at 600°C. A higher conversion of methane was obtained by synergistic effects. The product concentrations of CO, H₂, and CO₂, and CH₄ conversion were maintained in high values, even increasing the space velocity of feed gas over Ir–Ni/La₂O₃ catalyst, indicating that rapid reaction takes place. As a by-product, a small amount of carbon deposition was observed, but carbon formation decreased with increasing the space velocity. On the other hand, with reduced monometallic Ni(10 wt%)/La₂O₃ catalyst, yield of synthesis gas and carbon decreased with increasing the space velocity.     

Decomposition of methane over Ni catalysts supported on carbon fibers formed from different hydrocarbons

In order to get pure hydrogen without CO and CO₂, the decomposition of methane into hydrogen and carbon fibers (CF) over Ni/carbon catalysts has been investigated. The reason for the use of carbon materials as supports is to avoid a costly elimination treatment of the catalyst from the formed CF. The Ni/carbon catalysts prepared by the impregnation of various carbon materials with Ni(NO₃)₂ dissolved in acetone, followed by reduction in hydrogen at 573 K, showed better catalytic performance in the decomposition of methane than those prepared by the impregnation with aqueous Ni(NO₃)₂. The Ni(40 wt%)/CF(from 1-C₄H₈) showed the highest catalytic performance giving a C/Ni value (moles of deposited carbon per mole of Ni on the catalyst) of 1920 until complete deactivation of the catalyst. SEM and TEM images of the CF formed from methane indicated their thickness to be ≈10-150 nm with the same size of Ni particles at their tips at the early stage of the decomposition of methane, but the thickness changed to ≈40-100 nm at the final stage of decomposition. An estimate of the average size of Ni crystallites from XRD measurement suggested that the carbons deposited from methane on various Ni/CF would modify the size of Ni crystallites during the reaction. It is suggested that ≈20 nm Ni crystallites are most active for the growth of carbon nanofibers.     

Low-temperature dry reforming of methane tuned by chemical speciations of active sites on the SiO2 and γ-Al2O3 supported Ni and Ni-Ce catalysts

The cognition of active sites in the Ni-based catalysts plays a vital role and remains a huge challenge in improving catalytic performance of low temperature CO₂ dry reforming of methane (LTDRM). In this work, typical catalysts of SiO₂ and γ-Al₂O₃ supported Ni and Ni-Ce were designed and prepared. Importantly, the difference in the chemical speciations of active sites on the Ni-based catalysts is revealed by advanced characterizations and further estimates respective catalytic performance for LTDRM. Results show that larger[Ni_n⁰] particles mixed with [Ni-O-Si_n]) species on the Ni/SiO₂(R) make CH₄ excessive decomposition, leading to poor activity and stability. Once the Ce species is doped, however, superior activity (59.0% CH₄ and 59.8% CO₂ conversions), stability and high H₂/CO ratio (0.96) at 600?℃ can be achieved on the Ni-Ce/SiO₂(R), in comparison with other catalysts and even reported studies. The improved performance can be ascribed to the formation of integral ([Ni_n⁰]-[Ce^III-□-Ce^III]) species on the Ni-Ce/SiO₂(R) catalyst, containing highly dispersed [Ni_n⁰] particles and rich oxygen vacancies, which can synergistically establish a new stable balance between gasification of carbon species and CO₂ dissociation. With respect to Ni-Ce/γ-Al₂O₃(R), the Ni and Ce precursors are easily captured by extra-framework Aln-OH groups and further form stable isolated ([Ni_n⁰]-[Ni-O-Al_n]) and [Ce^III-O-Al_n] species. In such a case, both of them preferentially accelerate CO₂ adsorption and dissociation, causing more carbon deposition due to the disproportionation of superfluous CO product. This deep distinguishment of chemical speciations of active sites can guide us to further develop new efficient Ni-based catalysts for LTDRM in the future.     

CO2 reforming of methane to syngas over highly active and stable Pt/MgO catalysts

Pt/MgO catalysts were prepared by wet impregnation. At 800 °C and atmospheric pressure, Pt/MgO catalysts exhibited a high stability at high gas hourly space velocity of 36,000 ml/g h with a CH₄/CO₂ ratio of 1.0. During 72 h time on stream, the conversion of CH₄ and CO₂ remained almost constant, at about 88% and 90%, respectively. There was no loss of Pt. After reaction, the XRD peaks of MgO became broader, indicating amorphization of MgO, which was supported by TEM results. XPS indicated that the reforming reaction had little influence on Pt. CO₂-TPSR results showed that some carbon deposition occurred under stoichiometric feed of CH₄ and CO₂, but it did not result in the deactivation of the catalyst. The deposited carbon came mainly from the decomposition of methane.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).     

CO hydrogenation over a rhodium vanadate catalyst: Reduction and regeneration of RhVO4 on SiO2

The hydrogenation of CO over an Rh vanadate (RhVO₄) catalyst supported on SiO₂ (RhVO₄/SiO₂) has been investigated after H₂ reduction at 500°C, and the results are compared with those of vanadia-promoted (V₂O₅–Rh/SiO₂) and unpromoted Rh/SiO₂ catalysts. The mean size of Rh particles, which were dispersed by the decomposition of RhVO₄ after the H₂ reduction, was smaller (41 Å) than those (91–101 Å) of V₂O₅–Rh/SiO₂ and Rh/SiO₂ catalysts. The RhVO₄/SiO₂ catalyst showed higher activity and selectivity to C₂ oxygenates than the unpromoted Rh/SiO₂ catalyst after the H₂ pretreatment. The CO conversion of the RhVO₄/SiO₂ catalyst was much higher than that of V₂O₅–Rh/SiO₂ catalyst, and the yield of C₂ oxygenates increased. We also found that the RhVO₄/SiO₂ catalyst can be regenerated by calcination or O₂ treatment at high temperature after the reaction.     

Syngas conversion using RhVO4 and Rh2MnO4 catalysts: Regeneration and redispersion of Rh metal by calcination and reduction treatments

The hydrogenation of CO over mixed oxides (RhVO₄, Rh₂MnO₄) supported on SiO₂ has been studied after H₂ reduction at 300°C and at 500°C, and the results compared with those of unpromoted Rh/SiO₂ catalysts. Rh was more highly dispersed (40 Å) after the decomposition of RhVO₄ by the H₂ reduction than those of Rh₂MnO₄/SiO₂ and unpromoted Rh/SiO₂ catalysts. The activity and the selectivity to C₂ oxygenates of the mixed-oxide catalysts after the H₂ reduction were higher than those of the unpromoted Rh/SiO₂ catalysts, but the activity of the RhVO₄/SiO₂ catalyst increased more dramatically after the decomposition by the H₂ reduction at 300°C, and hence the yield of C₂ oxygenates increased. These results suggest that a strong metal–oxide interaction (SMOI) was induced by the decomposition of the mixed oxides after the H₂ reduction. The catalytic activity and selectivity were reproduced repeatedly by the calcination and reduction treatments of the spent (used) catalyst because of the regeneration of RhVO₄ and redispersion of Rh metal.     

TPD study and carbazole hydrodenitrogenation activity of nitrided molybdena–alumina

The relationship between the activity and surface molybdenum species of nitrided 12.5% MoO₃/Al₂O₃ was studied in the hydrodenitrogenation (HDN) of carbazole at 573 K and 10.1 MPa total pressure. The surface molybdenum species were determined by the desorption of nitrogen gas during TPD. The surface area of NH₃-cooled Mo/Al₂O₃ nitrided at 773 and 1173 K was decreased by 8% and 61% from 245 m² g⁻¹ of the fresh MoO₃/Al₂O₃, respectively. The NH₃-cooled Mo/Al₂O₃ catalysts had slightly higher surface area than the He-cooled catalysts. The HDN rate increased with increasing nitriding temperature in the HDN of carbazole on the nitride catalysts. The NH₃-cooled Mo/Al₂O₃ catalysts nitrided at 1173 K were the most active in carbazole HDN and the He-cooled catalyst nitrided at 773 K was the least.     

A study of the catalytic activity of cobalt–zinc manganites for the reduction of NO by hydrocarbons

In this work the catalytic behaviour of pure zinc manganite, ZnMn₂O₄, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N₂O decomposition as well as the reduction of N₂O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973 K for 24 h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N₂ and CO₂ was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873 K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773 K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773 K and disaggregate, by reduction of the spinel framework Mn³⁺ cations to Mn²⁺, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work.     

Ni/Al_2O_3催化剂的CO_2-TPD表征

Ni/Al_2O_3催化剂是甲烷二氧化碳重整反应制取合成气研究最多、最具应用潜力的一种催化剂。通过对催化剂进行CO_2-TPD研究,考察还原态Ni/Al_2O_3催化剂的CO_2脱附特性。结果表明,浸渍法制备的Ni/Al_2O_3催化剂CO_2脱附曲线呈现双峰,分别在(60~65)℃和(350~380)℃出现高低温两个活性位;高温CO_2吸附量为3.0 cm~3·g~(-1),低温CO_2吸附量为24.0 cm~3·g~(-1)。催化剂的CO_2吸附量与其Ni含量无关。考察选用不同载体的CO_2脱附行为,发现以Al_2O_3为载体的催化剂CO_2吸附量是MgO和SiO_2为载体催化剂的2~4倍,以TiO_2为载体的催化剂几乎不吸附CO_2。     

Stability of Ni/SiO2-ZrO2 catalysts towards steaming and coking in the dry reforming of methane with carbon dioxide

Ni/SiO₂-ZrO₂ catalysts with Ni loadings of 1 to 13 wt-% were prepared, characterized by elemental analysis, X-ray diffraction, N₂ sorption, temperature programmed oxidation, temperature programmed reduction, and tested for their activity and stability in the dry reforming of methane with carbon dioxide at 850 °C, gas hourly space velocity of 6000 and 1800 h^–1 and atmospheric pressure. The SiO₂-ZrO₂ support as obtained through a simple and efficient sol-gel synthesis is highly porous (A_BET = 90 m²·g^–1, d_P = 4.4 nm) with a homogeneously distributed Si-content of 3 wt-%. No loss of Si or formation of monoclinic ZrO₂, even after steaming at 850 °C for 160 h, was detectable. The catalyst with 5 wt-% Ni loading in its fully reduced state is stable over 15?h on-stream in the dry reforming reaction. If the catalyst was not fully reduced, a reduction during the early stages of dry reforming is accompanied by the deposition of up to 44 mg·g^–1carbon as shown by experiments in a magnetic suspension balance. Rapid coking occurs for increased residence times and times-on-stream starting at 50 h. The Ni loading of 5 wt-% on SiO₂-ZrO₂ was shown to provide an optimal balance between activity and coking tendency.     

Partial oxidation of alkanes to oxygenates in supercritical carbon dioxide

The catalytic partial oxidation of propane in supercritical carbon dioxide has been investigated in a stirred batch reactor. Various metals (oxides) have been used as supported catalysts with respect to their activity and selectivity for the formation of oxygenates. The reactions run with a 1:2.3–2.9:68–108 molar ratio of propane:synthetic air:CO₂ at 453–573 K and 80–100 bar. Using a precipitated 2.4 wt.% Co₃O₄–SiO₂ catalyst at 573 K, a total oxygenate (i.e. acetic acid, acetone, acetaldehyde, methanol) selectivity of 59% and a propene selectivity of 21% were obtained at a propane conversion of 12 mol%. The same catalyst has been used to investigate the influence of the supercritical conditions and initial feed composition on the reaction, varying the density of CO₂ and the concentration of synthetic air, respectively.