Ni Active Phase Research Articles

Renewable Diesel Production over Mo-Ni Catalysts Supported on Silica

Nickel catalysts promoted with Mo and supported on silica were studied for renewable diesel production from triglyceride biomass, through the selective deoxygenation process. The catalysts were prepared by wet co-impregnation of the SiO2 with different Ni/(Ni + Mo) atomic ratios (0/0.84/0.91/0.95/0.98/1) and a total metal content equal to 50%. They were characterized by XRD, XPS, N2 physisorption, H2-TPR, and NH3-TPD. Evaluation of the catalysts for the transformation of sunflower oil to renewable (green) diesel took place in a high-pressure semi-batch reactor, under solvent-free conditions. A very small addition of Mo, namely the synergistic Ni/(Ni + Mo) atomic ratio equal to 0.95, proved to be the optimum one for a significant enhancement of the catalytic performance of the metallic Ni/SiO2 catalyst, achieving 98 wt.% renewable diesel production. This promoting action of Mo has been attributed to the significant increase of the metallic Ni active phase surface area, the suitable regulation of surface acidity, the acceleration of the hydro-deoxygenation pathway (HDO), the creation of surface oxygen vacancies, and the diminution of coke formation provoked by Mo addition.

Insights into depolymerization pathways and mechanism of alkali lignin over a Ni1.2–ZrO2/WO3/γ-Al2O3 catalyst

This study performed catalytic depolymerization of alkali lignin over Ni-based catalysts. Effects of different promoters (Zr and W), Ni loadings, reaction temperatures, and the addition of formic acid and catalyst on lignin conversion and products distribution were all investigated. The result showed that the highest oil yield (40.1% (mass)) was obtained at 240 °C over Ni1.2/γ-Al2O3 promoted by Zr and W species. Quantitative analysis indicates that Zr and W species prefer to lignin depolymerization while Ni active phase prefer to hydrodeoxygenation and hydrogenation. The interconversion of products derived from lignin depolymerization was determined by gas chromatography-mass spectrometer, which demonstrated that phenolic compounds were dominant products in all lignin derived bio-oils, wherein the proportion of vanillin was highest (65.7%) at 180 °C, while that of alkyl guaiacols increased with the increase of temperature (from 12.45% at 180 °C to 66.67% at 240 °C). Residual lignin obtained after lignin depolymerization was also investigated for detecting differences on functional groups, wherein the disappearing peaks at 1511 cm−1 (stretching of aromatic rings), 1267, 1215 and 1035 cm−1 (vibrations of guaiacyl and syringyl units) were detected by Fourier transform infrared spectrometry. Additionally, the higher O/C ratio measured by elemental analysis also confirmed that alkali lignin was depolymerized effectively under mild conditions.

Recent progress in the application of Ni-based catalysts for the dry reforming of methane

ABSTRACT Ni-based catalysts are highly efficient in methane-reforming processes. In the particular case of methane reforming in the presence of carbon dioxide, or dry reforming of methane (DRM), it is necessary to modify and control the initial properties of the catalyst to confer on it resistance to carbon deposition in particular, and to sintering of the Ni metal particles. In this regard, catalytic supports and promoters of different natures have been proposed. Likewise, the addition of small amounts of noble metals to avoid oxidation of the Ni active phase during the reforming reaction has been proposed. Catalyst preparation methods have also been identified as being of particular interest, since they can affect the structure of the Ni metal particles. In this review, the thermodynamic and kinetic aspects of the dry reforming of methane reaction are presented first. The most recent developments in synthetic methods (impregnation, sol-gel, co-precipitation, equilibrium deposition filtration, atomic layer deposition, non-thermal glow discharge plasma, multi-bubble sonoluminescence, “core-shell” structure) aimed at maximizing the dispersion and thermal resistance of Ni particles are then discussed and compared. The catalytic supports used to promote dispersion of the active metallic phase, the oxygen-storage capacity, and the metal/support interaction are also described. The review then addresses the fact that both the nature of the support and the addition of promoters and other metallic phases that modify the surface properties can control the interaction between the metal and the support, the electronic density of the active phase, and the degree of Ni reduction. Finally, new lines of research focused on the DRM process to make the reaction conditions milder and favor the process at low temperatures are also summarized.

Microemulsion vs. Precipitation: Which Is the Best Synthesis of Nickel–Ceria Catalysts for Ethanol Steam Reforming?

Ethanol steam reforming is one of the most promising ways to produce hydrogen from biomass, and the goal of this research is to investigate robust, selective and active catalysts for this reaction. In particular, this work is focused on the effect of the different ceria support preparation methods on the Ni active phase stabilization. Two synthetic approaches were evaluated: precipitation (with urea) and microemulsion. The effects of lanthanum doping were investigated too. All catalysts were characterized using N2-physisorption, temperature programmed reduction (TPR), XRD and SEM, to understand the influence of the synthetic approach on the morphological and structural features and their relationship with catalytic properties. Two synthesis methods gave strongly different features. Catalysts prepared by precipitation showed higher reducibility (which involves higher oxygen mobility) and a more homogeneous Ni particle size distribution. Catalytic tests (at 500 °C for 5 h using severe Gas Hourly Space Velocity conditions) revealed also different behaviors. Though the initial conversion (near complete) and H2 yield (60%, i.e., 3.6 mol H2/mol ethanol) were the same, the catalyst prepared by microemulsion was deactivated much faster. Similar trends were found for La-promoted supports. Catalyst deactivation was mainly related to coke deposition as was shown by SEM of the used samples. Higher reducibility of the catalysts prepared by the precipitation method led to a decrease in coke deposition rate by facilitating the removal of coke precursors, which made them the more stable catalysts of the reaction.

Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration

Effective encapsulation strategies are highly sought-after in heterogeneous catalysis for preparing highly active and stable metal (oxide) nanocatalysts. Herein, we report an optimized Melt Infiltration (MI) procedure to confine nickel(oxide) nanoparticles (NPs) into hierarchical microporous-mesoporous scaffolds. Three SBA-15 silicas were synthesized in order to obtain different degrees of interconnectivity between the main mesopores. The impact of the SBA-15 pore characteristics, i.e., this interconnectivity, also named secondary intra-wall porosity (IWP), on the final nickel (oxide) NPs size and localization has been specifically investigated. Using MI, which consisted in the diffusion of the precursor in the liquid state inside the porosity of the support in the presence of the native surfactant occluding the pores, a selective localization of the NiO NPs inside the IWP was obtained, without large NPs plugging the main mesopores if IWP pores connecting the main mesopores do exist. When IWP – selective localization – occurs for the NPs, they show a size directly depending on the IWP dimensions. The obtained materials were tested, after reduction, in the hydrogenation reactions of cinnamaldehyde and 5-hydroxymethylfurfural. The catalytic results underline the positive effect of IWP - confinement of NPs to obtain and maintain an elevated dispersion of the metallic Ni active phase and to reach a high catalytic activity in hydrogenation under mild reaction conditions.

Steam reforming of ethanol over Ni/MgAl2O4 catalysts

Hydrogen is considered one of the most promising energy vectors in order to match the current energy and environmental issues. Bioethanol steam reforming is a sound opportunity and close to the industrialization considering an integrated biorefinery concept. MgAl2O4 was selected as a stable support, with improved activity, selectivity and stability due to negligible acidity. Increasing the Ni loading from 1.5 to 10 wt% over MgAl2O4 improved the conversion of ethanol as well as the yield of hydrogen, while the carbon deposition and yield of byproducts decreased.Small acidity characterised the samples, attributed exclusively to the Ni active phase. This prevented extensive catalyst coking due to ethylene formation and subsequent polymerisation. Consequently, small coke amount was found on the spent catalysts, mainly amorphous, allowing rather easy regeneration.DRIFT analysis of adsorbed ethanol at variable temperature evidenced the intermediates of reaction and their evolution with temperature, allowing to suggest the main reaction paths. Acetaldehyde was found as intermediate, rapidly evolving to reformate. Among the possible evolution paths of acetaldehyde, the oxidation to acetate and carbonate species (likely stabilised by the support) was preferred with respect to decomposition to methane and CO. This is reflected in the products distribution evidenced through activity testing.

Controllable fabrication of Ni-based catalysts and their enhancement on desorption properties of MgH2

Searching for effective catalysts is one of the most attractive subjects to enhance the hydrogen storage properties, especially the sorption kinetics of the metal-based hydrides. The effect of the Ni morphology (including size and shape) on the desorption performance of MgH2 was studied in the paper, in which a modified wet chemical route was used to control the morphology of Ni. By introducing the as-synthesized Ni-based catalysts into MgH2, a well-distinguished desorption behavior was found, evidencing the existence of the morphology-dependent effect of the catalytic phase. Among them, graphene sheets (GS) supported Ni (Ni6GS4) exhibited the best catalytic performance with a lowered MgH2 onset desorption temperature of 225 °C (the undoped MgH2 initiated to desorb hydrogen at 308 °C) and a saturated hydrogen desorption capacity of 6.74 wt%. The significant enhancement of the hydrogen storage kinetics was ascribed to not only the relative small particle size of the initial Ni active phase but also its well-dispersed distribution state and further refinement during ball milling. In addition, we believe that the good dispersive ability of GS was of particular importance in separating the Ni nanoparticles from each other and preventing aggregating of the MgH2 particles, leading to a desirable cyclic performance. Our results have experimentally shown the influence of shape and size of metal-based catalysts on the MgH2 system, providing guideline or strategy designing nanostructured catalysts with light weight, ultra-fine particle size, well-formed distribution and high activity.

Dry reforming of methane over Ni/Ce0.62Zr0.38O2 catalysts: Effect of Ni loading on the catalytic activity and on H2/CO production

The activity of ceria–zirconia-supported nickel catalysts (Ni/CZ) with various loadings of nickel (2, 4 and 10 wt. %) was studied in the case of low-temperature dry reforming of methane (DRM). XRD, SBET, SEM, TPD-CO2 and thermogravimetry were used to determine the physicochemical properties of the catalysts and of the carbon deposits formed on the surface. It was found that the agglomerates of the Ni-active phase are formed on the surface of the support for high loadings of nickel. The best conversions of CO2 and CH4 and an optimum ratio H2/CO = 1 were obtained for the catalysts with the highest Ni content. It was also found that loading has an influence on the amount of carbon deposits formed in the DRM process.

Synthesis and characterization of Fe–Ni/ɣ-Al2O3 egg-shell catalyst for H2 generation by ammonia decomposition

The Fe–Ni alloyed nanoparticles are a promising alternative to expensive ruthenium-based catalysts for a real-scale application of hydrogen generation by ammonia decomposition. In practical applications, millimeter-sized extrudates are used as catalyst supports, where the spatial distribution of the active phase should match with the type of reaction. In this work, a novel synthesis route was developed for the preparation of a Fe–Ni/ɣ-Al2O3 egg-shell catalyst. Egg-shell is a preferred profile considering the highly endothermic nature of ammonia decomposition reaction. The high viscosity of glycerol, used as a solvent during impregnation, prevents the fast migration of the Fe–Ni active phase solution toward the inner-core of ɣ-Al2O3, giving control over the large capillary pressures. The distribution profiles were analyzed at macroscopic scale through scanning electron microscopy mapping (SEM-EDX) and optical microscopy. A three-dimensional (3-D) reconstruction of the spherical-shaped ɣ-Al2O3 was achieved using X-ray micro tomography and the Fe–Ni egg-shell spatial distribution was inspected throughout the entire volume of the support body. Transmission electron microscopy (TEM) specimen preparation using focused ion bean (FIB) milling allowed to acquire high resolution images of the Ni and Fe nanoparticles on ɣ-Al2O3, which is particularly challenging due to the crystalline nature of this support. Distinct regions of the egg-shell catalyst were analyzed through scanning TEM (STEM) and TEM. The outer-shell region showed the presence of Fe and Ni alloyed nanoparticles with a size of approximately 5nm.. The egg-shell catalyst showed significant higher activity in ammonia decomposition by converting 3 times more ammonia to equilibrium conversion than either egg-white or catalyst with uniform distribution. Moreover, the egg-shell catalyst conversion only dropped 0.05% after 10h of reaction, for a space velocity of 475mlmin−1g−1.

Non-thermal plasma assisted synthesis and physicochemical characterizations of Co and Cu doped Ni/Al2O3 nanocatalysts used for dry reforming of methane

Ni/Al2O3 nanocatalysts doped with Co and Cu were prepared by co-impregnation and modified by non-thermal plasma. The nanocatalysts were characterized by XRD, FESEM, TEM, EDX dot-mapping, BET, FTIR, TGA-DTG, and XPS analysis. According to XRD and XPS results, good interaction between active phase and support can be observed in both Ni–Co/Al2O3 and Ni–Cu/Al2O3 nanocatalysts. A uniform morphology, high surface area, and well dispersed particles of active sites in Ni–Co/Al2O3 nanocatalyst were observed that shows the effect of cobalt in controlling Ni ensemble size. In contrast Ni–Cu/Al2O3 nanocatalyst had no homogenous dispersion of active phase due to sintering of copper particles. The activity measurements illustrated better Ni–Co/Al2O3 nanocatalyst activity in comparison to Ni/Al2O3 and Ni–Cu/Al2O3 in terms of CH4 and CO2 conversion. H2 and CO yield were higher for Ni–Co/Al2O3 and higher H2/Co ratio was obtained as well. Whereas Ni/Al2O3 and Ni–Co/Al2O3 did not experience deactivation, Ni–Cu/Al2O3 suffered from activity loss by ca. 22% and 16% for CH4 and CO2 conversion, respectively. Sintering most likely happened in Ni–Cu/Al2O3 nanocatalyst due to high temperature of calcination while cobalt by controlling the size of Ni particles, alternated the size of active sites to a size range in which carbon formation was suppressed. Ni/Al ratio from XPS analysis which signifies Ni dispersion on alumina support was 5.15, 9.16, and 6.35 for Ni/Al2O3, Ni–Co/Al2O3, and Ni–Cu/Al2O3 nanocatalysts respectively. The highest ratio of Ni/Al was for Ni–Co/Al2O3 nanocatalyst that shows the best coverage of support by Ni active phase in this nanocatalyst.

Oxygen vacancy formation on the Ni/Ce0.75Zr0.25O2(111) surface. A DFT+U study

Ni/CeO2–ZrO2 solid solutions are well known by their promissory catalytic role in hydrogen production processes such as reforming and partial oxidation of hydrocarbons. CeO2–ZrO2 support can effectively disperse Ni particles and donate oxygen promoting Ni active phase resistance to poisoning by carbon deposition. Thus, in this work we study the influence of Ni on oxygen vacancy formation on the Ce0.75Zr0.25O2(111) surface, by using the density functional theory (DFT) approach with the Hubbard’s correction (U) for Ce(4f) electrons. We performed DFT+U energetic calculations on Ni/Ce0.75Zr0.25O2 (Ni is adsorbed over an O–O bridge site) and Ni–Ce0.75Zr0.25O2 (Ni is inserted into the slab) systems, with an O-vacancy. Moreover, we analyzed the electronic structures of these solids by drawing their spin polarized density of states (DOS) plots. Our results indicate that Ni insertion between two O subsurface layers facilitates Ce0.75Zr0.25O2 mixed oxide reduction and enhances its ionic mobility and oxygen donation properties.

Co/ZnO and Ni/ZnO catalysts for hydrogen production by bioethanol steam reforming. Influence of ZnO support morphology on the catalytic properties of Co and Ni active phases

Renewable hydrogen production from steam reforming of bioethanol is an interesting approach to produce sustainable hydrogen. However, simultaneous competitive reactions can occur, decreasing the hydrogen production yield. To overcome this problem, modifications in the steam reforming catalysts are being studied. Ni and Co active phases supported over modified ZnO have been widely studied in hydrogen production from steam reforming of bioethanol. However, the influence of the morphology and particle size of ZnO supports on the catalytic behaviour of the supported Ni and Co has not been reported. In the present work, we show how the morphology, shape, and size of ZnO support particles can control the impregnation process of the metal active centres, which manages the properties of active metallic particles. It has been found that nanorod particles of ZnO, obtained by calcination of Zn acetate, favour the metal-support interactions, decreasing the metallic particle sizes and avoiding metal (Co or Ni) sinterization during the calcination of metal precursors. Small metallic particle sizes lead to high values of active metal exposure surface, increasing the bioethanol conversion and hydrogen production.

Hydrogen production by thermocatalytic decomposition of methane over Ni-Al and Ni-Cu-Al catalysts: Effect of calcination temperature

Thermo catalytic decomposition of methane using Ni-Al and Ni-Cu-Al catalyst prepared by fusion of the corresponding nitrates is studied. The effects of catalyst calcination temperature on the hydrogen yields and the characteristics of the carbon obtained are studied. The role of copper has been also analyzed. Whatever the calcination temperature, all the catalysts show a high and almost constant hydrogen yield without catalyst deactivation after 8 h on stream, which confirms the good performance of this kind of catalysts. The presence of copper enhances the hydrogen production and the best results were obtained using catalysts calcined at 600 °C. Cu has a strong influence on the dispersion of Ni in the catalysts and inhibits NiO from the formation of nickel aluminate even at high calcinations temperatures, which facilitates the formation of the metallic Ni active phase during the subsequent catalyst reduction step. All catalysts tested promote the formation of very long filaments of carbon a few tens of nanometers in diameter and some micrometers long. The structural properties of these carbon filaments highly depend on the presence of Cu:Ni-Cu-Al catalysts promote the formation of a well-ordered graphitic carbon while Ni-Al catalysts enhance the formation of a rather turbostratic carbon.

Effect of potassium content in the activity of K-promoted Ni/Al 2O 3 catalysts for the dry reforming of methane

In this paper the effect of the potassium content in the structure and properties of the Ni active phase and in the activity and selectivity of the NiK/Al 2O 3 catalysts for dry reforming of methane has been studied. The following characterization techniques were used: SEM, TEM, temperature programmed reduction (TPR-H 2) and reaction (TPR-CH 4), temperature programmed oxidation (TPO) and XAFS. The reforming of methane with CO 2 was carried out at 973 K using 0.18 g of catalyst and a mixture CH 4:CO 2 (50:50, 60 ml/min, space velocity of 22,500 h −1). Catalytic tests for 6 and 24 h have been developed. TPR-H 2 and XAFS results reveal that potassium does not modify the arrangement of Ni atoms although facilitates the reduction of nickel species by H 2 due to the modification of the interaction between metallic species and the alumina support. The activity data indicate that the addition of a low amount of potassium (0.2 wt.% K 2O) allows to obtain a catalyst with an acceptably high activity (over 63% methane conversion, close to thermodynamic equilibrium) and very low coke deposition (below 30 mg C/g cat. during 6 h reaction). Independently of the amount of potassium, the catalytic activity remains almost constant during at least 24 h.