NiO Crystallites Research Articles

Study of nanostructured Ni/CeO 2 catalysts prepared by combustion synthesis in dry reforming of methane

This work reports the study of several catalysts of Ni–CeO 2 active for dry methane reforming process (CH 4 + CO 2 → 2CO + 2H 2). The use of Ni as active phase is highly preferred, due to its availability, high activity and low cost, although its main lack is the coke formation on the surface of Ni metal particles, resulting in a severe deactivation. Here we report a new synthesis method that allows a simple, effective and fast way to prepare Ni–CeO 2 catalysts, in a wide range of metallic loadings, resulting in all the cases in well-formed NiO crystallites with sizes in the range of 12–18 nm. The use of CeO 2 as a support has been based on its massive use in TWC catalysts formulations, where it is recognized to activate CH 4 and lower hydrocarbon dissociation. Moreover, CeO 2 has been reported to have an intrinsic activity in the CH 4 reforming reaction. Besides the metallic loading, several factors that control the preparation method of the catalyst have been varied, in order to optimize their performance. Most of the catalysts prepared show activity and selectivity values close to thermodynamic ones, maintaining a good stability on long periods of time and severe conditions. Nevertheless, formation of some carbon nano-fibers has been observed, which could result in a drawback for their application at large scale.

Oxidation of polycrystalline Ni studied by spectromicroscopy: Phase separation in the early stages of crystallite growth

Low-energy and photoemission electron microscopy enables the determination of facet planes of polycrystalline surfaces and the study of their chemical composition at the sub-$\ensuremath{\mu}\text{m}$ scale. Using these techniques the early oxidation stages of nickel were studied. After exposing the surface to 20 L of oxygen at 373 K a uniform layer of chemisorbed oxygen was found on all facets. After oxygen exposure at 473--673 K, small NiO crystallites are formed on all facets but not in the vicinity of all grain boundaries. The crystallites are separated by areas of bare Ni without significant oxygen coverage.

Synthesis and coating of silica supported Nickel Oxide particles over ceramic pre-forms through sol–gel derived layered double hydroxides

Silica supported Nickel Oxide fine particles have been synthesized through sol–gel derived Ni–Al Layered Double Hydroxide (LDH) and coated over honeycomb ceramic pre-forms through dip-coating technique. The powder products of supported materials have low crystallinity, negative zeta potential, exhibit high dispersibility and suitable for further processing by coating techniques. The powder X-ray diffraction (XRD) patterns have shown that there is an increase of basal spacing by 3.02 A in acetylacetonate intercalated LDH. The particles of <2 μm size increase with the rise of LDH component in the composite. The particles of NiO structure formed on decomposition of composites have crystallite size <20 nm. Due to the interlayer reduction of NiO crystallites, the unsupported LDH on calcination gives Ni0 particles of size around 4.18 nm. The Scanning Electron Microscopy (SEM) patterns of coated supported suspensions over ceramic substrates show formation of thin, crack free coats with uniform distribution of particles.

Application of cryoeffect to preparation of highly effective hydrodesulfurization catalyst from microemulsion

The expanding use of low-quality crude oil materials in refinery demands the creation of new-generation catalysts for the comprehensive hydrorefinery of hydrocarbon fraction. A technology was developed for the preparation of catalysts based on mesoporous titanium silicate Ti0.03Si0.97O2 with NiO nanocrystals (4.1–4.7 nm) incorporated in pores. Two methods were suggested for preparation of composite catalytic systems: first, the NiO phase was formed directly in pores of titanium silicate from microemulsion; second, NiO crystallites were prepared by cryoprocessing of initial colloid and incorporated as microsuspension in Ti0.03Si0.97O2 pores. In the latter case, the formation of new bonds was found between silicate and NiO, evidencing the formation of a material with new physicochemical properties. The reported results can be applied to the development of mesoporous catalysts for the hydrorefinery of oil fraction.

Preparation and structure development of NiO/YSZ nanocomposite powders by urea hydrolysis

NiO/YSZ composite powders, with various NiO contents, have been prepared by the urea hydrolysis method. The crystallization behavior and microstructure of composite powders has been studied in detail, using differential scanning calorimetry analysis, X-ray diffraction, and transmission electron microscope. The results indicated that the actual NiO content of the NiO/YSZ powders largely deviated from the nominal value, and finally reached a saturated value. The NiO addition would retard the crystallization of NiO/YSZ composite. When the calcination temperature was increased, the NiO crystallites first precipitated at around 500 °C, and then the YSZ phase presented at about 600 °C. The calcined powders consist of NiO/YSZ nanocomposite particles, which are comprised of nano-sized NiO and YSZ crystals. In addition, with the aid of H 2 plasma treatment, it is easier to distinguish the Ni and YSZ phases of Ni/YSZ cermets after sintering and subsequent reduction. This could reveal that such Ni/YSZ cermets exhibited a uniform microstructure that has fine Ni particles homogeneously dispersed within the YSZ matrix. As the NiO content was increased, the size and density number of the Ni phase within an YSZ matrix was increased.

Synthesis of nanocomposite nickel oxide/yttrium-stabilized zirconia (NiO/YSZ) powders for anodes of solid oxide fuel cells (SOFCs) via microwave-assisted complex-gel auto-combustion

Nanocomposite NiO/YSZ powders for high performance anodes of SOFCs have been synthesized via a microwave-assisted complex-gel auto-combustion approach using nitrates of Ni 2+, Zr 4+ and Y 3+, citric acid (CA) and ethyl glycol as starting materials. The complexing conditions including pH and molar ratio of CA to the metal ions in the aqueous solutions have been quantitatively analyzed and optimized to realize their complete chelating into the CA-based gel network. The processing features and the microstructural characteristics of NiO/YSZ phases formed during auto-combustion of gels have been investigated by FT-IR, TG-DSC, XRD, SEM, SAED and HRTEM. It has been found that the nanoscale composite powders of NiO/YSZ, directly obtained through the in situ auto-combustion reactions within the gels, are composed of loosely agglomerated particles with sizes of ∼200 nm, while these particles themselves are the aggregates of finer NiO and YSZ crystallites of ∼15 nm in size. Moreover, the anodes prepared with such nanoscale composite powders have been manifested to possess much better electrochemical properties than the ones obtained by normal NiO/YSZ powder (∼2.5 μm) according to their measurements of AC impedance and temperature programmed reduction behavior due to the highly enhanced three-dimensional transport network of oxide ions and triple-phase boundaries for surface reactions.

Organized mesoporous silico-nickelates (OMSiNi) and silico-lanthano-nickelates (OMSiLaNi): Crystallogenesis vs. morphogenesis and microporosity vs. pore anisotropy

In this work organized mesoporous silico-nickelates (OMSiNi) and silico-lanthano-nickelates (OMSiLaNi) have been studied. The synthesis took place in one step using of poly-acrylic acid (Pac), complexed with cetyl-trimethyl-ammonium-bromide (C 16TAB) towards a mesostructured flexible backbone on which hydrolysis of tetra-ethyl-ortho-silicate (TEOS) and Ni(NO 3) 2, or Ni(NO 3) 2 + La(NO 3) 3, takes place at increasing pH values. The loading of nickel up to 10%, and nickel plus lanthanum up to 5% each, increases with the pH values (=5.5, 7.5, 9.5) where the materials were isolated and the same effect has the increase of the temperature of the preparation bath from RT to 323 K. The surface area of the OMSiNi and OMSiLaNi solids, containing the lower amounts of metals, was determined in the range ∼1100 and ∼970 m 2/g, respectively, and drops as the Ni and/or the Ni + La loading increases. The OMSiNi materials exhibits organized mesoporosity of MCM-41 type in nanometer scale (XRD) and remarkable morphologenesis of the Ni-doped silicate mass in micrometer scale (SEM). The introduction of La in the group OMSiLaNi results in gradual deterioration of organized porosity and destruction of morphogenesis. Both OMSiNi and OMSiLaNi solids contain NiO crystallites of 3–6 nm size (XRD) while the samples with high La content prepared at 323 K show extensive crystallogenesis of perfect NiO crystals of micrometer size (SEM). The pore anisotropy b was determined in the range 7 < b < 56,000 and is lower in the samples prepared at higher temperature and containing higher amounts of metal(s). The % microporosity of the solids is related to the log b by a relationship log b = log b 0 − k(% micro) where k is a parameter related inversely to the width of the micropore range of distribution. An explanation of this is proposed based on the assumption that the introduction of micropores, in conjunction with the width of their distribution, result in numerous interruptions of mesopore channels and as a result b decreases exponentially.

Combined reforming of methane over co-precipitated Ni–CeO 2, Ni–ZrO 2 and Ni–Ce 0.8Zr 0.2O 2 catalysts to produce synthesis gas for gas to liquid (GTL) process

Ni–CeO 2, Ni–ZrO 2 and Ni–Ce 0.8Zr 0.2O 2 catalysts have been prepared by a co-precipitation method to develop catalysts suitable for synthesis gas production for gas to liquid (GTL) process. A conventional impregnation method was also employed to prepare Ni/CeO 2, Ni/ZrO 2 and Ni/Ce 0.8Zr 0.2O 2 catalysts to compare the impregnated catalysts with the co-precipitated ones. Ni content was fixed at 15% for both cases. It has been confirmed that the co-precipitated Ni–CeO 2 and Ni–Ce 0.8Zr 0.2O 2 catalysts exhibited relatively high activity as well as stability, while the impregnated Ni/CeO 2 and Ni/Ce 0.8Zr 0.2O 2 catalysts slowly deactivated with time on stream at 800 °C. At the temperature range from 700 to 750 °C, co-precipitated Ni–Ce 0.8Zr 0.2O 2 catalyst showed higher CH 4 and CO 2 conversion than Ni–CeO 2 catalyst. The enhanced catalytic activity and stability of the co-precipitated Ni–Ce 0.8Zr 0.2O 2 catalyst is due to the combination of nano-crystalline nature of cubic Ce 0.8Zr 0.2O 2 support and finely dispersed nano-sized NiO crystallites resulting in intimate contact between Ni and support, better Ni dispersion, and enhanced oxygen transfer during the reaction.

Ce and La modification of mesoporous Cu–Ni/SBA-15 catalysts for hydrogen production through ethanol steam reforming

A series of Cu–Ni catalysts supported on Ce- or La-modified SBA-15 was prepared and tested on ethanol steam reforming for hydrogen production. Samples were characterized by TGA, XRD, ICP-AES, N 2-physisorption, TPR and TEM. XRD analyses revealed different tendencies in the dispersion of Ce and La on the SBA-15 mesoporous support. Bulk CeO 2 particles together with large NiO crystallites found on CuNi/CeO 2/SBA-15 catalysts were responsible for their lower metallic dispersion and hydrogen production in comparison to CuNi/SBA-15 sample. At high La loadings (10–20%), the development of a La 2O 3 layer over the SBA-15 support strengthened metal-support interaction and enhanced metallic dispersion, achieving an improvement in the CuNi/La 2O 3/SBA-15 catalytic performance. Particularly, the incorporation of 20 wt% of La led to a significant enhancement of hydrogen selectivity (from 77.2 to 85.4 mol%) and, mainly, an important reduction in coke deposition (from 42.3 to 26.3 w/w% after 3 h of time on stream).

Hydrogen production at low temperature from methane on cerium and nickel based mixed oxides

Partial oxidation of methane (POM) to synthesis gas, CO + H 2, was investigated between 50 °C and 700 °C as a function of metal loading over cerium and nickel based CeNi X O Y catalysts. At high temperature, 700 °C, a H 2 yield of 100% is obtained. At 200 °C, an interesting yield of 34% is reached with a stable methane conversion of 70% and a H 2 selectivity of 49%, only when the solid is previously in situ treated in H 2 at 200 °C. Activated in H 2 CeNi x O Y solids are hydrogen reservoirs. Different physico-chemical techniques were used to characterize the structural, dispersion and reduction properties of the solids. Depending on composition, a solid solution and/or a highly dispersed nickel oxide on ceria can be obtained. Ion sputtering followed by XPS analysis was very useful for estimating the size of small NiO crystallites present in the compounds. It is found that active nickel species belong to small clusters and/or to solid solution. Correlations among species present in solid and catalytic performances are discussed. An active site based on formation of anionic vacancies and a mechanism involving a heterolytic abstraction of a hydride species from methane are proposed.

Development of Methane Decomposition Catalysts for COx Free Hydrogen

Now-a-days, catalytic decomposition of methane (CDM) into hydrogen and carbon is a promising technique for production of fuel cell grade hydrogen. The Ni based catalysts seems promising particularly for the production of COx free H2 by methane decomposition process. The CDM activity and longevity of the Ni based catalysts are mainly influenced by the amount of Ni and type of support material. In this paper the CDM activity results are correlated with NiO crystallite size, Ni metal surface area and acidity of the catalysts. In case of bimetallic catalysts addition of Cu to Ni catalysts lead to enhance the CDM activity at higher temperature thus resulting in the increased concentration of hydrogen in the outlet stream. Finally, some of the carbon-based catalysts are studied for methane decomposition activity at higher temperature. The surface changes over carbon catalysts with methane decomposition are studied using various characterization techniques.

A highly effective and stable nano-sized Ni/MgO–Al 2O 3 catalyst for gas to liquids (GTL) process

Highly active and stable nano-sized Ni catalysts supported on MgO–Al 2O 3 prepared from hydrotalcite-like materials have been designed for the combined steam and carbon dioxide reforming of methane (CSCRM), which is a useful process to adjust the H 2/CO ratio for Fischer–Tropsch process. Ni/MgO–Al 2O 3 exhibits remarkable coke resistance, while commercial Ni/MgAl 2O 4 catalyst shows considerable coke deposition during the target reaction. A strong metal to support interaction (SMSI) of Ni/MgO–Al 2O 3 enhances coke resistance. The change of the surface area and NiO crystallite size with varying the pre-calcination temperature of support was investigated in relation to the coke resistance. It has been concluded that highly dispersed Ni metal pre-calcined at 800 °C shows good coke resistance and high activity. As a consequence, Ni/MgO–Al 2O 3 catalyst will be a promising catalyst in CSCRM for the GTL process.

Measuring Location, Size, Distribution, and Loading of NiO Crystallites in Individual SBA-15 Pores by Electron Tomography

By the combination of electron tomography with image segmentation, the properties of 299 NiO crystallites contained in 6 SBA-15 pores were studied. A statistical analysis of the particle size showed that crystallites between 2 and 6 nm were present with a distribution maximum at 3 and 4 nm, for the number-weighted and volume-weighted curves, respectively. Interparticle distances between nearest neighbors were 1-3 nm with very few isolated crystallites. In the examined pores, a local loading twice the applied average of 24 wt % NiO was found. This suggests that a very high local loading combined with a high dispersion is achievable.

Combined reforming of methane over supported Ni catalysts

Various supported Ni catalysts have been applied for combined steam and carbon dioxide reforming of methane to produce synthesis gas (H2/CO = 2). Highly active and stable nano-sized Ni/Mgo–Al2O3 catalyst has been successfully developed for the target reaction. The high activity and stability is due to beneficial effects of MgO such as enhanced steam adsorption, basic property, nano-sized NiO crystallite size and strong interaction between Ni and support.

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/SiO 2 > NiO/HZSM-5 > NiO/CeO 2 > NiO/Al 2O 3 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 Al 2O 3 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/Al 2O 3 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.

SOFC 음극 제조를 위한 NiO가 코팅된 YSZ 분말의 합성

NiO-coated YSZ powder was prepared using heterogeneous precipitation of Ni hydroxides on YSZ particle surface and high energy milling. The powders were characterized by TG/DTA, XRD, XPS, and SEM. Amorphous Ni precipitate completely decomposed into NiO at 500℃ and the growth of NiO crystallites was constrained by the core particles. Nanocrystalline NiO-coated YSZ core-shell structure powder could be obtained after calcination at 800℃ for 2 h. A core-shell powder compact, due to high sinterability, showed a near theoretical density at 1350℃. After reduction at 900℃, interpenetrating Ni-YSZ microstructure with very uniformly distributed fine Ni and YSZ grains and pores was observed. In contrast, the mechanically mixed oxide sample showed less uniform distribution of pores and larger discontinuous Ni particles as compared with the core-shell samples.

Nano-sized Ni particles on hollow alumina ball: Catalysts for hydrogen production

Nickel nanoparticulate catalysts supported on hollow Al2O3 ball (<3μm in diameter) have been prepared by spraying a mixed solution of nickel and aluminum nitrates in atmospheric pressure plasma. A wall of each ball (ca. several 10nm in thickness) consisted of uniformly dispersed oxide bricks of ca. 30nm sizes, which were identified as densely packed NiO, NiAl2O4 and Al2O3 crystallites. Highly dispersed Ni particles (<10nm in diameter) have been formed on the ball surfaces by H2 reduction of NiO crystallites, surrounded by the nano-sized oxide bricks of NiAl2O4 and Al2O3. A catalytic activity of the Ni catalyst thus prepared for methane steam reforming reaction was tested from a viewpoint of the application to fuel processors for polymer electrolyte fuel cells (PEFCs). The results showed the catalyst possessed superior performances for the reaction in the activity and the resistance to coke deposition even under a low H2O/CH4 molar ratio (S/C<2) condition.

Synthesis and high performance of ceria supported nickel catalysts for hydrodechlorination reaction

Catalysts containing 10 wt% Ni supported on CeO 2 were prepared by two ways, namely, co-precipitation method using nickel nitrate precursor and impregnation method using nickel nitrate and nickel acetylacetonate as two separate precursors. The catalysts were characterized by pulse chemisorption of H 2, X-ray diffraction, and temperature programmed reduction (TPR) techniques and evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene to benzene in a fixed-bed down-flow glass reactor at 573 K under normal atmospheric pressure. The hydrogen uptake values were used to determine the catalyst properties of Ni/CeO 2 like dispersion, metal area, and particle size. Among the two preparatory routes, co-precipitation method gave better catalytic performance in terms of hydrogenation activity, benzene selectivity, and coking resistivity than impregnated Ni/CeO 2 catalysts. This may be attributed to high dispersion of smaller NiO crystallites and the appearance of the second reduction peak at a higher temperature (578 K) in TPR profile with co-precipitated Ni/CeO 2 catalyst. This indicates that a strong interaction may take place between the NiO crystallites and CeO 2 on the surface of co-precipitated Ni/CeO 2 catalyst. Contrary to general expectation that the large Ni particles are preferable for HDC reaction, it is observed that smaller metal particles with high dispersion, as in the case of co-precipitated Ni/CeO 2 catalyst, promotes better catalyst with longer life.

Hydrogen Production by Gasification of Cellulose over Ni Catalysts Supported on Zeolites

Hydrogen production by cellulose gasification over Ni catalysts was investigated. The catalytic activity and the species of products were affected by the type of supports. The use of simple metal oxides as supports resulted in the formation of large quantities of dark-colored tar but avoided carbon deposition, whereas the use of zeolites as supports inhibited tar formation but promoted carbon deposition. The loading of Ce on zeolite enhanced the rate of gasification and partially inhibited carbon deposition. On the other hand, in most cases, higher reaction temperatures led to a greater quantity of hydrogen being formed, a higher rate of gasification, and less tar formation or carbon deposition. In this reaction, metallic Ni is likely to be an active site and there is a structure sensitivity of Ni and NiO crystallite: larger Ni crystallite favored hydrogen formation and inhibited tar formation, while larger NiO crystallite inhibited hydrogen formation and tar decomposition. Our results indicate that Ni/Ce/H-ZSM-5 catalyst is a promising candidate for this reaction, despite being slightly less effective than Rh/Ce/SiO2 catalyst, since Ni catalysts are cheaper.

Redox cycling of Ni–YSZ anode investigated by TPR technique

Effect of redox cycling on a Ni–YSZ anode prepared from 50 wt.% NiO and 50 wt.% YSZ was investigated by using temperature-programmed reduction (TPR), XRD and SEM techniques. XRD results showed that NiO was formed during re-oxidation. Both the XRD and TPR results depicted that the conversion of nickel to NiO depended on the re-oxidation temperature. The oxidation of Ni to NiO occurred quickly in the initial several minutes and then reached a quasi equilibrium. The TPR profiles tracing the redox cycling showed that it brought continuous changes in the NiO micro-structure at 800 °C, whereas at 600 °C it had only little effects on the reduction of NiO. Re-oxidation resulted in the formation of spongy aggregates of NiO crystallites. Redox cycling at 800 °C led to a continuous decrease in the primary crystallite size of NiO and a high dispersion of the Ni particles. A continuous expansion of the slice sample was observed in both of the oxidized and reduced states during the redox cycling at 800 °C, whereas this process did not occur during the redox cycling at 600 °C.