The hydrogenation of triglycerides using supported alloy catalysts. I. Silica-supported Ni—Ag catalysts

Silica-supported Ni-Ag catalysts with a loading of 2·1·0.6% (w/w) total metal have been prepared using the precursors nickel dimethylglyoxime and silver nitrate by means of a simple impregnation method. The resulting catalysts were activated by calcination at 260°C in air, followed by hydrogen reduction at 450°C. They were then employed for soyabean oil hydrogenation at 1 bar H2 pressure and 160°C in a stirred batch reactor. Characterisation of the catalysts using temperature-programmed reduction and electron microscopy indicated that alloying of nickel and silver had occurred, but metal particle composition, for a given overall composition, varied with metal particle size and smaller metal particles were nickel rich. The hydrogenation activity and selectivity measurements revealed that the catalysts were more active and selective than a commercial nickel catalyst. Furthermore, the specific activities of the alloy catalysts were a maximum for alloys in the range 70–90 at. % Ni. However, the supported alloy catalysts also gave rise to greater trans isomerisation than the commercial catalyst. This is attributed to hydrogen deficiency caused by large triglyceride molecules blocking hydrogen chemisorption on small nickel particles (10–50 Å in diameter), leading to enhanced cis-trans isomerisation.     

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.     

Catalyst preparation and support effects for triglyceride hydrogenation over supported nickel

Nickel catalysts have been prepared by impregnation and precipitation methods on silica and charcoal. Metal loadings were in the range 1–40% w/w Ni on the support. The catalysts were investigated by temperature programmed reduction (TPR) and subsequently reduced in the temperature range 200–500°C. Metal surface areas were measured via H₂ and CO chemisorption and limited studies of the catalysts were carried out using X-ray photon spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) to determine the chemical and physical state of the nickel after reduction. Reduced catalysts were employed for triglyceride (soya bean oil) hydrogenation (1 atmos H₂, 160°C) in a stirred reactor (1000 rpm) and reaction rates (catalyst activity) and product distributions were determined. Charcoal supported catalysts were active and zero valent nickel was detected after reduction. However, surface areas were variable, the performance of such catalysts being affected by pore effects, agglomeration and possibly nickel–charcoal interaction. Silica supported precipitated catalysts were of relatively high activity and metal surface if methanol washed before activation and inactive (irreducible) if only water washed. Although silica supported precipitated catalysts were more active per unit weight of metal than charcoal supported counterparts, they were less active per unit metal area and gave rise to greater trans-acid production. This may be due to triglyceride molecules excluding hydrogen from small nickel crystallites.     

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.     

Abatement of Carbon Monoxide over CeO2–CoO x Catalysts: Effect of Preparation Method

Nanocrystalline TiO₂ with average crystallite sizes 7 and 15 nm were synthesized by the solvothermal method using titanium n-butoxide and 1,4-butanediol as titanium precursor and solvent, respectively. Four transition metals (Ag, Co, Ni, Pt) were supported on the TiO₂ supports by incipient wetness impregnation with metal loadings 1 and 25 atomic%. For any metal type, metal dispersion and CO oxidation activities were higher when the catalysts were prepared on the larger crystallite size TiO₂ for both high and low metal loadings, despite their lower surface areas. The presence of higher amount of Ti³⁺ on the larger crystallite size TiO₂ probably stabilized small metal particles during impregnation, calcination, and reduction steps via stronger metal-support interaction; hence higher CO oxidation activities and lower light-off temperature were obtained.     

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.     

Catalytic activity of Ni-Co supported metals in carbon dioxides methanation

This work deals with the catalytic performance of nickel-cobalt supported on ceria-doped gadolinia (GDC) catalyst in the single and in the simultaneous methanation of carbon monoxide and carbon dioxide. The catalysts have been prepared by impregnation method, starting from metal salts precursors. Samples have been characterized by x-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), hydrogen temperature programmed reduction (TPR-H₂), transmission electronic microscopy (TEM), and scanning electron microscopy (SEM/EDX) technique. The temperature examined for methanation tests ranged from 200°C-600°C. The results show that the prepared and optimized catalysts possess the main characteristics of materials suitable for SOECs (solid oxide electrolyzer cells) applications: high metal content (50% wt/wt with respect to the support), high activity, and high stability. The catalytic performance of bimetallic catalysts highlights that the cobalt does not improve the activity of the nickel catalysts.     

A Comparative Study of Hydroxyapatite- and Alumina-Based Catalysts in Dry Reforming of Methane

The dry reforming of methane over hydroxyapatite- and alumina/magnesia (commercial Pural MG 30)-supported nickel catalysts was investigated. The catalytic performance of the catalysts prepared with fresh supports highly depended on the basicity, the metal-support interaction, and the metal particle size. Calcination of the supports at 1200 °C for 5 h made the catalysts less active because of specific surface area reduction and basicity destruction. However, this treatment allowed avoiding any further catalyst deactivation by thermal sintering and maintained excellent catalytic stability over 300 h of time-on-stream. These tests under simulated industrial conditions (high contact time and long time-on-stream) showed the competitiveness of the prepared catalysts in this important catalytic process.     

Pt-Al2O3 Cryogel with High Thermal Stability for Catalytic Combustion

The cryogel catalyst of platinum on alumina was prepared from aluminum sec-butoxide and H₂PtCl₆ through the sol-gel technique and subsequent freeze drying. The cryogel catalyst showed higher thermal stability of platinum than the corresponding xerogel or impregnation catalysts, which was ascribed to the more intimately developed platinum-alumina interaction accompanied by the encapsulation of the metal into the alumina cryogel. It was also shown that platinum accessibility was higher on the cryogel than on the xerogel despite the higher thermal stability of the metal on the formed than on the latter. For the VOC combustion, the cryogel exhibited higher activity than the xerogel and impregnation catalysts. Also for the methane combustion the cryogel showed higher activity, although it showed lower activity than the impregnation catalysts above 600 °C. By the addition of ceria as an additive to the cryogel catalyst, the CH₄ combustion activity was improved especially in the temperature region above 600 °C.     

Dry reforming of methane over Ni/SBA-15 catalysts prepared by homogeneous precipitation method

Ni/SBA-15 catalyst was prepared by homogeneous precipitation method (Ni-HP) and used for dry reforming of methane (DRM). The related characterization results indicated that the Ni particles were highly dispersed with a size range of 2-5 nm. Compared with Ni/SBA-15 catalyst prepared by impregnation (Ni-IM), the reduction temperature of Ni-HP obtained from H2-TPR was greatly improved, suggesting the stronger metal-support interaction. After reacting at 700 °C for 100 h, the CH₄ conversion of DRM over Ni-HP catalyst slightly decreased from 74.5% to 73.8%. While, for the Ni-IM catalyst, the CH₄ conversion dropped from 61.7% to 37.3%. Furthermore, the average particle size of Ni-HP was 3.7 nm and 4.7 nm before and after the long-time stability test, respectively, ascribed to the good anti-sintering property. Although a certain amount of coke was produced, mainly with disorder filamentous carbon of base-growth, the Ni/SBA-15 prepared by homogeneous precipitation exhibited excellent catalytic activity and stability.     

Nickel Oxide Based Supported Catalysts for the In-situ Reactions of Methanation and Desulfurization in the Removal of Sour Gases from Simulated Natural Gas

Supported nickel oxide based catalysts were prepared by wetness impregnation method for the in-situ reactions of H₂S desulfurization and CO₂ methanation from ambient temperature up to 300 °C. Fe/Co/Ni (10:30:60)–Al₂O₃ and Pr/Co/Ni (5:35:60)–Al₂O₃ catalysts were revealed as the most potential catalysts, which yielded 2.9% and 6.1% of CH₄ at reaction temperature of 300 °C, respectively. From XPS, Ni₂O₃ and Fe₃O₄ were suggested as the surface active components on the Fe/Co/Ni (10:30:60)–Al₂O₃ catalyst, while Ni₂O₃ and Co₃O₄ on the Pr/Co/Ni (5:35:60)–Al₂O₃ catalyst.     

Characterization of aerogel Ni/Al2O3 catalysts and investigation on their stability for CH4-CO2 reforming in a fluidized bed

The CH₄-CO₂ reforming was investigated in a fluidized bed reactor using nano-sized aerogel Ni/Al₂O₃ catalysts, which were prepared via a sol–gel method combined with a supercritical drying process. The catalysts were characterized with BET, XRD, H₂-TPR and H₂-TPD techniques. Compared with the impregnation catalyst, aerogel catalysts exhibited higher specific surface areas, lower bulk density, smaller Ni particle sizes, stronger metal-support interaction and higher Ni dispersion degrees. All tested aerogel catalysts showed better catalytic activities and stability than the impregnation catalyst. Their catalytic stability tested during 48 h reforming was dependent on their Ni loadings. Characterizations of spent catalysts indicated that only limited graphitic carbon formed on the aerogel catalyst, while massive graphitic carbon with filamentous morphology was observed for the impregnation catalyst, leading to significant catalytic activity degradation. An aerogel catalyst containing 10% Ni showed the best catalytic stability and the lowest rate of carbon deposition among the aerogel catalysts due to its small Ni particle size and strong metal-support interaction.     

Dispersion of Nano Nickel Particles over SBA-15 Modified by Carbon Films on Pore Walls

Mesoporous SBA-15 was prepared by using P123 as a template. The precursor with the template was calcined in an inert atmosphere so that carbon films might be formed in pores of SBA-15 due to the decomposition of template. The SBA-15C thus formed contained 3% C and exhibited similar pore structures as the SBA-15. Both SBA-15 and SBA-15C were used to support 20% nickel (by weight) via impregnation. It was found that doping with carbon films enhanced the dispersion of supported nickel. However, calcination at high temperatures before the reduction had a negative effect on the dispersion of nickel. The un-calcined 20%Ni/SBA-15C after the reduction in H₂ at 673 K exhibited the highest dispersion of nickel (42%) and smallest average particle size of about 2.4 nm, in the catalysts studied in this work. It was also the most active catalyst for the hydrogenation of toluene to methyl cyclohexane. Conversion of toluene could be detected even at room temperature and atmospheric pressure for the catalyst in a fix-bed reactor, and 100% conversion of toluene was reached when temperature was raised to 358 K.     

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.     

Bimetallic PtAu cluster-derived catalysts for the selective catalytic reduction of NO by propylene

The selective catalytic reduction of nitric oxide by propylene in the presence of excess oxygen has been investigated over a series of silica supported Pt and PtAu catalysts. Cluster-derived catalysts were prepared by wet impregnation using a Pt₂Au₄(CC^tBu)₈ organometallic cluster precursor, and compared to catalysts obtained by incipient wetness impregnation using individual Pt and Au salts. The addition of Au by co-impregnation resulted only to a small shift (20–25 °C) in the light-off of propylene towards higher temperatures. A markedly different catalytic behavior was observed in the case of cluster-derived catalysts, where a 150 °C delay was observed both in the temperature of maximum NO reduction and the light-off of propylene in the presence and in the absence of NO. Most importantly, the selectivity towards N₂ increased by 50% compared to the monometallic Pt catalysts. The results of characterization studies indicate that the monometallic Pt/SiO₂ and the co-impregnated PtAu/SiO₂ samples have similar metal particle sizes. A more narrow particle size distribution was observed with the cluster-derived bimetallic sample.     

Surface Modification of Pd/α-Si3N4 Catalysts Through the Solvent Used During Synthesis. Implications on the CO Chemisorption Properties and Catalytic Performances

Palladium catalysts supported on α-Si₃N₄ were prepared by impregnation with Pd(II)-acetate dissolved either in toluene or in water. The mean metal particle size of ~0.5 wt% Pd catalysts was similar (~5 nm) and independent of the way of preparation. Nevertheless, the two catalysts present very different chemisorption behaviour chemisorptive and catalytic properties. Fourier transformed infrared (FTIR) spectra of adsorbed CO at different temperatures (ranging from room temperature to 300 °C) show a very different behaviour for both catalysts. While the CO adsorption states on the Pd/α-Si₃N₄ prepared in toluene are very similar to those generally measured for silica and/or alumina supported palladium catalysts, CO chemisorbs less strongly on Pd/α-Si₃N₄ prepared in water and on different adsorption sites. The Pd/α-Si₃N₄ catalyst obtained by aqueous impregnation is much less efficient for the methane total oxidation. It is less active and less stable: it deactivates strongly after 3 h on stream at 650 °C. The two catalysts present about the same activity for the 1,3-butadiene hydrogenation after stabilisation at 20 °C. But, the catalyst prepared in water shows a much better selectivity to butenes. The results are discussed in terms of the possible migration of silicon atoms from the silicon nitride support to the surface of the palladium particles, when the catalyst is prepared in water. This is not the case when prepared in an organic solvent.     

In situ EXAFS study of the nucleation and crystal growth of Ni particles on SiO2 support

Ni/SiO₂ catalysts were prepared by wetness impregnation method (INi) and by a new two-step method (ENien_2.5/600+INi_2.5). The two-step method consists of formation of chemical glue, “nickel nuclei”, having strong metal support interaction with SiO₂ and preparation of the “nickel reservoir” which has weak interaction with support. The nucleation and growth behaviors of such Ni/SiO₂ catalysts in the preparation stage were continuously monitored by in situ EXAFS spectroscopy. The in situ EXAFS spectra of INi catalyst began to change after 320°C and bulk NiO like structure was formed at 500°C heat treatment. The Ni/SiO₂ prepared with the two-step method had larger coordination number of Ni with silica support in the initial stage of in situ monitoring than that of INi catalyst, which showed the coexistence of Ni nuclei and Ni reservoir. The ENien_2.5/600+INi_2.5 had smaller particles of Ni compared with INi after high temperature calcination. The formation of small Ni particles could be attributed to the strong ion-support interaction of nickel nuclei with nickel reservoir in ENien_2.5/600+INi_2.5 catalyst. Slight particle growth occurred after 370°C calcination in Ni/SiO₂ prepared by two-step method.     

Metal organic framework derived Ni/CeO2 catalyst with highly dispersed ultra-fine Ni nanoparticles: Impregnation synthesis and the application in CO2 methanation

CO₂ methanation is a promising strategy to convert the greenhouse gas into synthetic natural gas, which is known as a clean fuel with higher energy density. Ni/CeO₂ is one of the active catalysts for CO₂ methanation reaction. However, Ni/CeO₂ with conventional metal/support structure suffers from the problem of nanocrystal coarsening during high-temperature reaction and thus decreasing catalytic activity. In this work, a Ni/CeO₂ catalyst with novel structure was designed and synthesized through impregnation of Ce-based metal organic framework (MOF) with Ni precursor followed by calcination process. Due to the confinement effect of ultra-small pores derived from the MOF, Ni/CeO₂ catalysts with ultrafine Ni nanoparticles, high dispersion and good thermostability were resulted. Among all the samples, Ni/CeO₂ calcined at 600 °C showed the best catalytic performance due to the highest amount of oxygen vacancies. This work demonstrates a facile way to synthesize a broad range of ultra-fine metal/metal oxide nanocomposite catalysts with high catalytic activity and good stability for various applications.     

Influence of Catalyst Pretreatments on Propane Oxidation Over Ru/γ-Al2O3

The influence of different treatments (in H₂ or in O₂ at 250 or 600 °C) of alumina supported Ru catalysts on the total oxidation of propane was investigated. Ruthenium catalysts were prepared using RuCl₃ as metal precursor and characterized by H₂ chemisorption, O₂ uptake, BET, XRD and TEM. The presence of chloride on the catalyst surface was found to exert an inhibiting effect on the activity of Ru. The reduced Ru/γ-Al₂O₃ catalysts after partial removing chlorine ions were more active than the same samples oxidized at 250 °C. The higher activity of the reduced Ru/γ-Al₂O₃ catalysts was attributed to the presence of a large amount of active sites on small Ru _x O _y clusters without well defined stoichiometry or on a poorly ordered layer of a ruthenium oxide on the larger Ru particles. The formation of highly dispersed, but in some extent crystallized RuO₂ phase in catalysts oxidized at 250 °C, leads to slightly lower activity of the Ru phase. Strong decline of the activity was found for catalysts oxidized at 600 °C. At this temperature, the Ru particles were completely oxidized to well-crystallized RuO₂ oxide, and the mean crystallite size of the Ru oxide phase was much higher (9–25 nm) than that of after oxidation at 250 °C (~4 nm). The effect of the regeneration treatment in H₂ on the activity of the Ru/γ-Al₂O₃ catalysts was also studied. The active ruthenium species for propane oxidation were discussed based on the catalytic and characterization data both before and after activity tests.     

Effects of metal support interaction on dry reforming of methane over Ni/Ce-Al2O3 catalysts

Dry (CO₂) reforming of methane is conducted over two newly synthesized Ni20/Ce-γAl₂O₃ and Ni20/Ce-meso-Al₂O₃ catalysts. The x-ray diffraction (XRD) patterns indicated that Ni20/Ce-meso-Al₂O₃ exhibits a better dispersion of nickel, while Ni20/Ce-γAl₂O₃ has larger amounts of nickel crystallites. The temperature programmed desorption (TPD) kinetics analysis indicated that Ni20/Ce-meso-Al₂O₃ had a lesser metal-support interaction than the Ni20/Ce-γAl₂O₃. The thermal gravimetric analysis (TGA) indicated that the incorporation of ceria into the Al₂O₃ matrix helps to stabilize Ni20/Ce-meso-Al₂O₃ during dry reforming of methane. The temperature programmed reduction (TPR) indicated that the synthesized catalysts were sufficiently reducible below 750 °C. A fixed bed reactor evaluation (at 750 °C) showed that both catalysts can facilitate methane reforming to syngas with minimal coking throughout the 30 hours time-on-stream (TOS). However, Ni20/Ce-meso-Al₂O₃ is more promising in terms of prolonged stability for dry reforming applications. Moreover, the syngas yield for Ni20/Ce-γAl₂O₃ is close to equilibrium prediction during the first 1 hour of reaction time.