Catalytic filamentous carbon as supports for nickel catalysts

Ni catalysts supported on catalytic filamentous carbon (CFC) were studied in the model reaction of methane decomposition at 525 °C. The supports (CFC) were synthesized by decomposition of methane over metal catalysts (Ni, Ni-Cu, Co and Fe-Co-alumina) at 500-675 °C. The yield of secondary carbon was shown to reach 224 g/g_Ni on the Ni/CFC (Ni-Cu, 625 °C) catalyst. The stability and activity of the Ni/CFC catalysts for deposition of the secondary carbon at 525 °C depend both on textural properties of the support and on the surface structure of the CFC filaments. It seems that highly porous carbon supports are more suitable for development of Ni/CFC catalyst for methane decomposition. The catalytic properties of the supported Ni/CFC systems may be accounted for by generation of weak dispersive interactions between specifically shaped Ni crystallites 30-70 nm in size and basal planes on the surface of the carbon filament.     

Characterization of nickel catalysts by chemisorption techniques,x-ray diffraction and magnetic measurements: Effects of support,precursor and hydrogen pretreatment

Two series of supported nickel catalysts were prepared by using alumina, silica and titania as supports and nickel nitrate or nickel chloride as impregnating salts. The catalysts, prereduced with hydrogen in the range 300–700°C, were characterized by adsorption of hydrogen and oxygen, X-ray diffraction (XRD) and magnetic methods. Strong effects of the nature of the support, of the nickel salt precursor and, in a few instances, of the reduction temperature on the adsorptive and textural properties of nickel catalysts were observed. For the series prepared from nickel nitrate, alumina support gave the highest dispersions of nickel, which varied only slightly with the reduction temperature, whereas the dispersion of titania-supported catalysts decreased significantly when the reduction temperature was increased. In contrast, the series prepared with nickel chloride always exhibited low metal dispersions which were nearly independent of the nature of the support and the reduction temperature. A strong decrease in hydrogen adsorption was observed on all samples prepared from nickel chloride. This decrease was recorded, for the nitrate preparation series, only on Ni/TiO₂ reduced at 500 and 700°C and on Ni/SiO₂ reduced at 700°C, which, in this instance, may be related to a strong metal-support interaction. On the other hand, oxygen chemisorption took place on all catalysts, allowing the determination of their metallic dispersion. Nickel crystallite sizes calculated from oxygen chemisorption were in good agreement with those determined from XRD and magnetic measurements, provided that the adsorption stoichiometry O/Ni_s=2 is assumed for typical catalysts whereas O/Ni_s = 1 should be applied to Ni/TiO₂ under the strong metal-support interaction state.     

Partial oxidation of methane to syngas over Ni/MgO, Ni/CaO and Ni/CeO2

Partial oxidation of methane to syngas at atmospheric pressure and 750°C was examined over Ni/MgO, Ni/CaO and Ni/CeO₂ catalysts with nickel loading of 13 wt%. All catalysts had similar high conversion of methane and high selectivity to syngas, which nearly approached the values predicted by thermodynamic equilibrium. However, only Ni/MgO showed high resistance to carbon deposition under thermodynamically severe conditions (CH₄/O₂ = 2.5, a higher CH₄ to O₂ ratio than the stoichiometric ratio). Its catalytic activity remained stable during 100 h of reaction, with no detectable carbon deposition. The oxidation of carbon deposited from pure CH₄ decomposition and from pure CO disproportionation was investigated by in situ TPO-MS study which showed that both were effectively inhibited over Ni/MgO. In addition, the catalysts were characterized by TPR, XRD and XPS. It was revealed that the excellent performance of Ni/MgO resulted from the formation of an ideal solid solution between NiO and MgO. This revised version was published online in August 2006 with corrections to the Cover Date.     

From Al2O3-supported Ni(II)-ethylenediamine Complexes to CO Hydrogenation Catalysts: Importance of the Hydrogen Post-treatment Evidenced by XPS

Ni (7 wt%)/Al₂O₃ catalysts prepared by decomposition of Ni(II)-ethylenediamine complexes in inert atmosphere initially contain a mixture of metallic and oxidized nickel. X-ray photoelectron spectroscopy shows that after a hydrogen treatment at 500 °C, the system contains more metallic nickel than catalysts prepared from the usual precursor, nickel nitrate. Carbonaceous species resulting from the partial oxidation of ethylenediamine are also eliminated. The catalyst post-treated in hydrogen exhibits a high metallic surface area accessible to reactants and is able to catalyze CO methanation.     

Unsupported nickel catalysts for methane catalytic decomposition into pure hydrogen

Catalytic decomposition of methane to pure hydrogen is a reaction of crucial importance for clean energy, if the problem of catalyst separation is solved and the carbon material has an increased commercial value. Unsupported nickel catalysts were synthesized by fusion method. The catalyst derived from nickel nitrate forms heterogeneous octahedral NiO, whereas the nickel hydroxide precursor results in catalyst containing sponge‐like NiO with folding lamellar structure of high porosity. The catalysts reactivity test was conducted with a fixed bed system at 1073 K. The catalyst subjected to hydrogen prereduction proved to be inactive. However, the methane prereduction was found to produce some coke to disperse the Ni particles and thus activated the catalyst. It was found that the higher concentration of methane resulted in a better methane conversion, but a higher deactivation rate. Carbon growth models were formulated to explain the formation of different types of carbon over Ni catalyst. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2907–2917, 2014     

Non-metal Catalysts for Dioxygen Reduction in an Acidic Electrolyte

Active non-metal catalysts for the Oxygen Reduction Reaction (ORR) were prepared by decomposition of acetonitrile vapor at 900°C over a pure alumina support, and supports containing 2 wt% Fe or 2 wt% Ni on alumina. The exposed alumina and metal in the samples were subsequently washed away with HF acid to purify the solid carbon material. The sample prepared with iron was the most active sample for the ORR, with only 100 mV greater overpotential than a commercial 20 wt% Pt / Vulcan Carbon catalyst. However, nitrogen-containing carbon deposited on pure alumina (which contained less than 1 ppm metal contamination) was also quite active, demonstrating that platinum or iron is not required for ORR activity. Characterization by XPS and TEM revealed that the more active samples had nanostructured carbon with more edge plane exposure than the less active tube structures formed from the nickel sample.     

Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.     

Formation of multi-walled carbon nanotubes by Ni-catalyzed decomposition of methane at 600–750 °C

Ni-catalyzed decomposition of methane at high temperatures was examined by using a thermogravimetric apparatus. The catalyst (10 wt.% Ni supported on spherical alumina) gave quite a high carbon nanotube (CNT) yield at the temperatures below 680 °C. At > 700 °C, however, carbon formation rate decreased with increasing the reaction temperature. Temperature-programmed reaction also showed the maximum CNT growth rate at ~ 690 °C. This result ruled out the possibility that the apparent negative activation energy is caused by sintering of Ni particles. Detailed examination on the kinetic expression led us to a conclusion that the dissolution of carbon atoms formed by dissociation of methane into bulk of the nickel particles is the rate-determining step at high temperatures, while methane adsorption is the rate-determining step at lower temperatures. This idea also explains the fact that the carbon yield drastically decreased at high temperatures. The CNTs formed at these temperatures had thinner walls than those formed at lower temperatures. The latter fact also supports the idea that the solubility of carbon in the nickel particles decreases at high temperatures.     

Studies on the promotion of nickel—alumina coprecipitated catalysts: II. Lanthanum oxide

Two series of lanthanum promoted nickel—alumina catalysts have been prepared by coprecipitation of the metal nitrates, using potassium carbonate. The molar ratio between nickel and the sum of aluminium and lanthanum was kept constant at 2.5 or 9.0 within each series. The calcination and reduction of these samples were studied by thermogravimetry and their structures before and after calcination and reduction were examined by X-ray diffraction. The methanation activities of the final catalysts were determined by differential scanning calorimetry. The results showed clearly that the methanation of carbon monoxide over nickel—alumina catalysts is enhanced by the presence of La₂O₃. With low percentages of lanthanum, the promoter is built into the precursor structure during the coprecipitation process. This is a meta-stable situation; phase separation occurs during hydrothermal treatment. In both series there was an optimum amount of lanthanum at which the activity per gram of nickel reached a maximum. The optimum specific activity of a lanthanum promoted nickel—alumina catalyst was twice as large as that of the unpromoted material. Above these optimum values, the activity per gram of nickel decreased because of two effects: an increase in the nickel particle sizes and an increase in the amounts of potassium remaining from the precipitation step. Alumina is needed to stabilize the nickel crystallites against sintering. The promoting action of La₂O₃ is slightly higher after reduction at 400°C than after reduction at 600°C. Lanthanum increased the amount of carbon monoxide which was adsorbed slowly; the amount of carbon monoxide which was rapidly adsorbed, however, was not altered. The increase in activity was accompanied by an increase in the apparent activation energy.     

Carbon dioxide reforming of methane over nickel catalysts supported on mesoporous MgO

In this paper, ordered mesoporous MgO nanocrystals [MgO(M)] were synthesized, and the nickel catalysts supported on MgO(M) were facilely prepared by impregnation method. The obtained Ni/MgO(M) catalysts with advantageous textural properties were investigated as the catalysts for the carbon dioxide reforming of methane reaction. It was found that compared with the Ni/MgO(C) catalyst [MgO(C): commercial MgO], the mesoporous pore structure of MgO(M) could effectively limit the growth of the activity metal, and the Ni/MgO(M) catalysts showed high catalytic activities as well as long catalytic stabilities toward this reaction. The results showed that the conversions of CH₄ and CO₂ were only decreased <5 % after 100 h of reaction at 650 °C. The improved catalytic performance was suggested to be closely associated with both the advantageous structural properties, such as large specific surface area, uniform pore size, and the “confinement effect” of the mesoporous matrixes contributed to stabilize the Ni active sites during the reaction. The carbon species deposited on the spent Ni/MgO(M) catalyst were analysized by TG and Raman, and the results exhibited that the carbon species after 100 h of reaction were mainly active carbon species.     

Production of COX Free Hydrogen by Catalytic Decomposition of Methane Over Ni/HY Catalysts

In this present paper, we report catalytic decomposition of methane over Ni/HY catalysts, with varying Ni loading at 550 °C and atmospheric pressure. The relationships between catalyst performance and characterization of the fresh and used form of catalysts are discussed from the data obtained by scanning electron microscopy, X-ray diffraction analysis, temperature programmed reduction, O₂ pulse chemisorption and carbon elemental analyses. It is observed that, the catalytic activity of Ni/HY catalysts is high at initial stages and gradually decreased with time and finally deactivated completely. The yield of hydrogen and carbon nanofibers is strongly dependent on Ni loading. It is found that 20 wt% Ni/HY catalyst showed higher hydrogen yield over the other loadings.     

Effect of calcination temperature on catalyst reducibility and hydrogenation reactivity in rice husk ash–alumina supported nickel systems

Nickel catalysts supported on rice husk ash–alumina (Ni/RHA–Al₂O₃) were prepared by an incipient wetness impregnation method. Characterization included TGA, DSC, TPR, XRD, and BET. Results show that the decomposition of the nickel compound to nickel oxide was complete above 500 °C. The TPR analysis revealed a strong interaction between nickel and support, and a decrease in reducibility of NiO with increasing calcination temperature. The XRD analysis of Ni/RHA–Al₂O₃ catalyst precursors demonstrated the presence of spinel. It also showed that the size of crystallites in the supported NiO first decreased with increase in calcination temperature up to 700 °C, and then increased due to phase transformation of nickel oxide to spinel. The pores are mesopores and their meshy surface structure was not affected by calcination temperature in the range investigated. The catalytic activity was tested by CO₂ hydrogenation with an H₂/CO₂ ratio of 4/1 at 500 °C. The CO₂ conversion and CH₄ yield for CO₂ hydrogenation over 15 wt% Ni/RHA–Al₂O₃ catalyst were almost independent of calcination and reduction temperatures. Copyright © 2004 Society of Chemical Industry     

Chain-Length Distributions Obtained over Nickel(II)-Exchanged or Impregnated Silica–Alumina Catalysts for the Oligomerization of Lower Alkenes

In the oligomerization of ethene, the chain-length distributions obtained over heterogeneous nickel catalysts at a low reaction temperature (ca. 110 °C) depend on the acidity of the silica–alumina support, the nickel loading and the olefin feed. Most significantly, ion-exchanged catalysts based on a silica–alumina of high acidity (ca. 0.7 mass% Ni), and those based on a highly-exchanged medium-acidity support (ca. 1.6% Ni), show a distinct superimposed activity for the dimerization of the intermediate butenes. This causes a desirable shift to higher hydrocarbon products, and it is possible to obtain 41 mass% C₁₀₊ oligomers, while the corresponding value predicted from Schulz–Flory statistics is only 24 mass%. No such deviation from Schulz–Flory statistics is observed in propene oligomerization.     

Synthesis of nickel catalysts supported on Zr-doped ordered mesoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and their catalytic performance for low-temperature CO<Subscript>2</Subscript> reforming of CH<Subscript>4</Subscript>

A series of Zr-doped ordered mesoporous Al₂O₃ with various Zr contents were synthesized by evaporation-induced self-assembly strategy and the Ni-based catalysts supported on these Al₂O₃ materials were prepared by impregnation method. These catalysts with large specific surface area, big pore volume, uniform pore size possess excellent catalytic performance for the low-temperature carbon dioxide reforming of methane. The activities of these catalysts were tested in carbon dioxide reforming of methane reaction with temperature increasing from 500 to 650?°C and the stabilities of these catalysts were evaluated for long time reaction at 650?°C. It was found that when Zr/(Zr?+?Al) molar ratio?=?0.5%, the Ni/0.5ZrO₂–Al₂O₃ catalyst showed the highest activity, and exhibited superior stabilization compared to the Ni-based catalyst supported on traditional ordered mesoporous Al₂O₃. The “confinement effect” from mesoporous channels of alumina matrix is helpful to stabilize the Ni nanoparticles. As a promoter, Zr could stabilize the ordered mesoporous framework by reacting with Al₂O₃ to form ZrO₂–Al₂O₃ solid solution. Since ZrO₂ enhances the dissociation of carbon dioxide, more oxygen intermediates are given to remove the carbon formed during the reaction.     

Nano‐engineered nickel catalysts supported on 4‐channel α‐Al2O3 hollow fibers for dry reforming of methane

A nickel (Ni) nanoparticle catalyst, supported on 4‐channel α‐Al₂O₃ hollow fibers, was synthesized by atomic layer deposition (ALD). Highly dispersed Ni nanoparticles were successfully deposited on the outside surfaces and the inside porous structures of hollow fibers. The catalyst was employed to catalyze the dry reforming of methane (DRM) reaction and showed a methane reforming rate of 2040 Lh^?1gNi^?1 at 800°C. NiAl₂O₄ spinel was formed when Ni nanoparticles were deposited on alpha‐alumina substrates by ALD, which enhanced the Ni‐support interaction. Different cycles (two, five, and ten) of Al₂O₃ ALD films were applied on the Ni/hollow fiber catalysts to further improve the interaction between the Ni nanoparticles and the hollow fiber support. Both the catalyst activity and stability were improved with the deposition of Al₂O₃ ALD films. Among the Al₂O₃ ALD coated catalysts, the catalyst with five cycles of Al₂O₃ ALD showed the best performance. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2625–2631, 2018     

Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 prepared with nickel citrate precursor

The reaction of combination of CO₂ reforming and partial oxidation of methane to produce syngas (CRPOM) was tested over Ni/SiO₂ catalysts which were prepared via incipient-wetness impregnation using precursors of nickel citrate and nickel nitrate. The catalysts were characterized by X-ray powder diffraction analysis (XRD) and H₂-temperature-programmed reduction (H₂-TPR) techniques. It was shown that the nickel citrate precursor strengthened interaction between NiO and support to form nickel silicate like species which could be reduced to produce small crystallites of metallic nickel at high temperatures. The Ni/SiO₂ prepared with the nickel citrate precursor exhibited good catalytic performances for its highly dispersed metallic nickel derived from the nickel silicate species.     

Effect of calcination temperature of mesoporous nickel–alumina catalysts on their catalytic performance in hydrogen production by steam reforming of liquefied natural gas (LNG)

Mesoporous nickel–alumina (Ni–A-NS) catalysts prepared by a non-ionic surfactant-templating method were calcined at various temperatures for use in hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of calcination temperature of nickel–alumina catalysts on their physicochemical properties and catalytic activity for steam reforming of LNG was investigated. Nickel oxide species were finely dispersed on the surface of Ni–A-NS catalysts through the formation of nickel aluminate phase. Reducibility, nickel surface area, and nickel dispersion of Ni–A-NS catalysts decreased with increasing calcination temperature. In the steam reforming of LNG, both LNG conversion and hydrogen composition in dry gas decreased with increasing calcination temperature of Ni–A-NS catalysts. Nickel surface area and reducibility of Ni–A-NS catalysts were well correlated with catalytic performance of the catalysts. Among the catalysts tested, Ni–A-NS700 (nickel–alumina catalyst calcined at 700 °C) with the highest nickel surface area and the highest reducibility exhibited the best catalytic performance.     

Pore structure control in Ni/SiO2 catalysts with both macropores and mesopores

Ni/SiO₂ catalysts with bimodal pore structure were prepared by the sol–gel method of silicon tetraethoxide and nickel nitrate in the presence of poly(ethylene oxide) (PEO) and urea. The presence of PEO was effective to induce phase separation during the sol–gel process, and through-macropores interconnected three-dimensionally were formed by fixing transitional structure of the phase separation. After gelation, the as-prepared wet gel was aged at 80 °C for decomposition of urea in order to increase pH of the solution within the gel. This pH increase led both the ripening of silica gel network through dissolution–reprecipitation and homogeneous deposition of nickel hydroxide. As a result, Ni could be dispersed highly in the silica network. In addition to the high dispersion of Ni, thus prepared Ni/SiO₂ has typical bimodal pore structure with size-controllable macropores and mesopores. The bimodal porous Ni/SiO₂ also had high thermal stability and showed steady catalytic activity in CO₂-reforming of methane at 700 °C.     

Effect of preparation variables on the activity of Ni-W/Al2O3 hydrotreating catalysts

Studies have been carried out on the influence of preparation parameters on the activity of nickel-tungsten (Ni-W) catalysts, used to promote the hydrotreatment of model compounds representative of nitrogen, sulphur, oxygen and aromatic containing compounds which typically occur in coal derived liquids. The preparation variables considered for optimisation were the pH of the impregnating solution, the impregnation order, the starting Ni salts, the Ni/Ni + W ratio, the drying conditions and the calcination temperature. It was found that changes in pH, within the range where binding of the starting ions to alumina takes place, is not a significant variable influencing the activity. The use of nickel nitrate as a precursor yields better catalysts. If W is impregnated first it is redistributed during subsequent nickel impregnation, and better catalysts are obtained when Ni is impregnated first. The optimum Ni/Ni + W ratio is between 0.3 and 0.4, based on the total active metals present on the catalyst. The hydrodesulphurisation hydrodeoxygenation and hydrogenation activities decrease with increase in the drying temperature; the reverse is true for hydrodenitrogenation. An optimum activity is obtained when drying at 100°C in a closed oven. The optimum calcination temperature was found to be 500°C.     

MCM-41 supported PdNi catalysts for dry reforming of methane

A series of bimetallic PdNi catalysts supported on mesoporous MCM-41 with different Ni content (Ni/Si ratio of 0.2–0.4) was synthesized. The effect of Pd addition to Ni-containing catalysts as well as the effect of the Ni content on the surface and catalytic properties of the catalysts was studied. The samples were characterized using various techniques, such as energy-dispersive X-ray spectroscopy, N₂ adsorption–desorption isotherms, X-ray diffraction, thermogravimetric and differential analyses, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and temperature-programmed reduction. Reforming of methane with carbon dioxide was used as a test reaction. The results indicated that the addition of a small amount of Pd (0.5%) to Ni-containing catalysts leads to formation of small nano-sized, easy reducible NiO particles. Agglomeration of NiO as well as of metallic nickel phase over PdNi samples increased with increasing the Ni content. Formation of filamentous carbon over surface of spent monometallic Ni and bimetallic PdNi catalyst was observed. In spite of filamentous carbon deposition, the catalytic activity and stability of bimetallic PdNi catalysts are higher than those of monometallic Ni one. Within bimetallic system, the PdNi catalyst with Ni/Si ratio of 0.3 revealed the best performance and stability caused by presence of small nickel particles well dispersed on the catalyst surface.