Nickel Crystallites Research Articles

Porous nickel based catalysts for partial oxidation of methane to synthesis gas

The phase composition and texture of nickel catalysts supported on ribbon porous nickel with a magnesium oxide underlayer were investigated by X-ray diffraction, low-temperature nitrogen adsorption, and electron microscopy combined with energy dispersive X-ray microanalysis. The MgO underlayer was obtained by impregnation the porous nickel with a Mg(NO 3) 2 solution followed by calcination at 550 °C; nickel was supported additionally by impregnation with a Ni(NO 3) 2 solution followed by calcination at 450 °C. In the supported reduced nickel catalysts (800–900 °C, H 2) the phases of Ni and a solid solution of NiO in MgO were observed; a considerable part of nickel crystallites was epitaxially bound with MgO. The partial oxidation of methane to synthesis gas (800 °С, O 2/C = 0.65, GHSV = 127 L/g h) over porous nickel based catalysts was studied. The activity of initial nickel support decreased considerably during the partial oxidation of methane; the value of methane conversion decreased slightly after 100 h of testing on the supported nickel catalysts.

Catalytic hydrothermal gasification of activated sludge

In this paper, Raney nickel is used as catalyst to gasify activated sludge in near-critical water (NCW) at a temperature range of 320–410°C and a corresponding pressure range of 11.3–28.1MPa in a batch reactor. At 380°C and with 15min reaction time, carbon gasification ratios (CGR) increased with increasing catalyst loading and reached 69% at 1.8g catalyst/g dry sludge. Methane yield also linearly increased with catalyst loading while hydrogen production exhibited a maximum at about 1.5g catalyst/g dry sludge. The gaseous product in this case consisted of 46% hydrogen, 25% methane and 29% carbon dioxide with trace amounts of carbon monoxide. The amount of generated gas as well as its composition did not change significantly after 30min; however, increasing the temperature had a positive effect on the total yield up to 380°C. The addition of sulfur to activated sludge resulted in a significant reduction of the carbon gasification ratio. CGR dropped by about 25% as the sulfur content of the feed increased from 0.8% to 3.0%. The deactivation of the catalyst due to hydrothermal sintering was also examined. Raney nickel significantly lost its activity upon pre-exposure to near-critical water at 380°C for 8min. This loss in activity had a major impact on methane production where methane yield dropped by six folds to a level similar to the catalyst-free gasification reaction and hydrogen yield was decreased by 50%. The decrease in catalytic activity of Raney nickel was likely due to an increase in average nickel crystallite size caused by hydrothermal sintering.

Enhanced oxygen mobility and reactivity for ethanol steam reforming

AbstractThis article describes a strategy for increasing oxygen storage capacity (OSC) of ethanol steam reforming (ESR) catalysts. Sintering and carbon deposition are major defects of nickel‐based catalysts for ESR; tuning oxygen mobility (OM) of CeO2‐based supports can overcome these drawbacks and promote H2 production. We have successfully increased OSC and OM by adding Mg into the lattice of Ni/CeO2 to promote H2 production in ESR. The insertion of Mg into the CeO2 lattice efficiently promotes the reduction of Ce4+ according to X‐ray powder diffraction (XRD) and temperature‐programmed reduction (TPR) analysis. Mg‐modified Ni/CeO2 catalysts have larger OSC and smaller nickel crystallite size compared with bare Ni/CeO2. The optimal Mg addition is 7 mol % (Ni/7MgCe) with the best OM. We also present evidence indicating that Mg addition significantly promotes ethanol conversion and H2 production in ESR, and that Ni/7MgCe yields the best performance due to the high OM of the support. These Mg‐modified catalysts also produce less carbon deposition compared with Ni/CeO2, and the amount of deposited carbon decreases with increasing Mg addition. Ni/7MgCe has the best resistance to carbon deposition owing to the excellent OM. © 2011 American Institute of Chemical Engineers AIChE J, 2012

Effect of B and Sn on Ni catalysts supported on pure- and on WO3/MoO3-modified zirconias for direct CH4 conversion to H2

Methane cracking for hydrogen production was studied over monometallic/bimetallic Ni catalysts supported on pure or WO3/MoO3-modified zirconias. The catalysts were reduced easily with methane at 773K, indicating that regeneration studies can be performed without using hydrogen. Ni–B and Ni–Sn bimetallic catalysts presented different behaviors when exposed to methane at 773K. The former, because of its low thermal stability, behaved as a well dispersed phase with small nickel crystallites, while the latter behaved as a bimetallic catalyst. For all catalysts, except for Ni–Sn, the metallic particles, after 8h of methane exposure at 773K, were mostly concentrated at the tip of the carbon filaments, whereas zirconia surface was found to be free from these filaments. Additionally, our results suggest that the presence of Sn does not improve catalytic performance for hydrogen production by methane cracking, considering the catalytic systems under study. Rather, adding Sn seems to inhibit access to Ni sites, which are necessary for the C–H bond activation of methane and then for the nucleation of carbon species.

Pulse Electrodeposition and Characterization of Ni-Si3N4Composite Coatings

<TEX>$Ni-Si_3N_4$</TEX> nano-composite coatings were prepared by pulse current (PC) electrodeposition and direct current (DC) electrodeposition techniques. The micro-structure of the coatings was characterized by scanning electron microscopy (SEM), vickers microhardness, X-Ray Diffraction (XRD) and wear-friction tests. The results showed that the micro-structure and wear performance of the coatings were affected by the electrodeposition techniques. Pulse current electrodeposited <TEX>$Ni-Si_3N_4$</TEX> composite coatings exhibited higher microhardness, smooth surface, and better wear resistance properties as compared to coatings prepared under DC condition. The <TEX>$Ni-Si_3N_4$</TEX> composite coatings prepared at 50 Hz pulse frequency with 10% duty cycles has shown higher codeposition of nano-particles. Consequently, increased microhardness and less plastic deformations occurred in coatings during sliding wear test. The XRD patterns revealed that the increased pulse frequencies changed the preferred (100) nickel crystallite orientations into mixed (111) and (100) orientations.

Preparation and Characterization of Nanostructured Ni-TiN Composite Films

Ni-TiN nanocomposite films were produced from a Ni plating bath containing TiN nanoparticles by using dc electroplating method. The structure and surface morphology of Ni-TiN composite coatings were analyzed by atom force microscope, X-ray diffraction, and transmission electron microscopy. Meanwhile, the anti-corrosion properties, hardness and thermostability of Ni-TiN nanocomposite films were also investigated and compared with the traditional polycrystalline Ni coatings. The results show that, compared with the traditional polycrystalline Ni film, Ni-TiN nanocomposite coatings display much better corrosion resistance, higher film hardness, and thermal stability. In addition, the hardness of Ni-TiN nanocomposite coatings decreases slightly with the increase of electroplating current density, which may be due to the synergism of hydrogen evolution and faster nucleation/growth rate of nickel crystallites.

Nanomechanical properties of functionally graded composite coatings: Electrodeposited nickel dispersions containing silicon micro- and nanoparticles

Functionally graded composite coatings constitute a class of materials which are mostly used for mechanical and tribological applications. Among these materials, nickel metal deposits with incorporation of SiC particles have excellent mechanical properties due to nickel metal and good tribological properties due to the SiC particles. In this work, nickel coatings containing different sizes of SiC particles, nanoparticles and microparticles (10 nm to 5 μm), were electrodeposited from an additive-free sulfate bath containing nickel ions and SiC particles. The material properties of the coatings were compared to nickel coatings containing microparticles (5 μm). The effect of current density, SiC content in the bath, and electroactive species concentration on the codeposition of SiC were studied. Afterwards, the effect of particle size and codeposition percentage of SiC particles on the nanomechanical properties on the morphology and structure of the electrodeposits were investigated. The coatings were analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD), nanoidentation and lateral force microscopy (LFM). The Ni–SiC electrocomposites, prepared at optimum conditions, exhibited improved nanomechanical properties in comparison to pure nickel electrodeposits. The improved properties of the composite coatings are associated to structural modifications of the nickel crystallites as well as the morphology of the electrodeposited layers. The improved nanomechanical properties of electrocomposites containing nanosized SiC particles, as compared to electrocomposites containing micron-sized SiC particles, is attributed to the increasing values of the density of embedded SiC particles with decreasing particle size and the mechanism of embedment of the SiC particles.

Demonstration and Characterization of Ni/Mg/K/AD90 Used for Pilot-Scale Conditioning of Biomass-Derived Syngas

A fluidizable, Ni/Mg/K/AD90(90% α-Al2O3) catalyst was evaluated in a pilot-scale study with oak-derived syngas for tar and methane reforming at 900 °C. The catalyst was operated in a semi-batch mode for ten individual reaction cycles, each separated by a regeneration protocol consisting of steaming followed by H2 reduction. During the experiment, subsequent reaction cycles showed a decrease in activity as indicated by decreased initial methane conversion. During each reaction cycle, H2S poisoning was the dominant deactivation mechanism, while coking was deemed to be minor in comparison. Catalyst characterization by XRD and TPR suggest that the loss of activity for subsequent reaction cycles coincides with the formation of a NiAl2O4 species that is not fully reduced under process conditions, which in turn results in a decreased number of potential metallic nickel (Ni0) sites available for hydrocarbon steam reforming. An increase in nickel crystallite size with time-on-stream was also observed using XRD, indicating that sintering may also play a role in loss of catalyst activity, although this is not considered the primary deactivation mechanism because activity loss was observed even when nickel crystallite sizes remained nearly constant.

Sulfidation of an aluminum-nickel-molybdenum catalyst and its activity in thiophene hydrodesulfurization

We consider the interaction of H2S with the molybdenum component of the catalyst using an Mo7O24H12 cluster as an example. At 200–300°C, SH groups substitute for OH groups of the molybdenum component. Intense reduction of the molybdenum component starts at 300°C. The reaction of hydrogen with the molybdenum component at 300°C generates Mo-H hydride groups, Mo5+ ions, and anionic vacancies; these vacancies are filled in by sulfur anions produced by the dissociation of hydrosulfuric acid. An NiMoO4 crystal phase is reduced and completely sulfided at 300°C. Mo7O24H12 clusters are sulfided only partially; complete substitution by sulfur does not occur in them even at 600–650°C. We consider the thiophene hydrodesulfurization scheme on molybdenum oxysulfide in the presence of nickel, as well as the scheme of this reaction on MoS2 in the absence of nickel. We conclude that hydrodesulfurization and hydriding occur at the same sites. Catalytically active surface sites in Mo7OxSy + Ni, MoS2 + Ni, and MoS2 structures are alike. A common feature of these structures is the existence of Mo-H− and MoSH− basic sites. The difference between them lies in the nature of proton-donor sites: in MoS2, proton donors are Mo-S-H+ groups, whereas in nickel-containing structures, nickel crystallites reacting with hydrogen are generators of protons and hydride ions.

Understanding of redox behavior of Ni–YSZ cermets

The oxidation of Ni–YSZ cermet as well the reduction of re-oxidized Ni–YSZ cermet was investigated by using temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR) and scanning electron microscope (SEM). The scanning electron microscope (SEM) photographs and temperature-programmed reduction (TPR) profiles indicated that the sintering of smaller nickel oxide crystallites to larger aggregates occurred concurrently with the formation of smaller nickel oxide crystallites from the oxidation of nickel at 800 °C, and the sintering of smaller nickel oxide crystallites at 600 °C was slower than that at 800 °C. The SEM results showed that each Ni particle was separated into a lot of smaller NiO particles during oxidation. The TPO profiles showed that two kinds of nickel particles exist in the anode reduced at 800 and 600 °C, one with high activity towards oxidation for the nickel crystallites directly from reduction, and another one with low activity towards oxidation for the sintered nickel particles. The Ni–YSZ anodes reduced at higher temperature showed higher re-oxidation temperature than the one reduced at lower temperature because of the accelerated passivating and sintering of the smaller nickel particles at higher temperature. The re-oxidation profiles were almost unchanged during redox cycling at 600 °C, whereas the re-oxidation peak temperature decreased during redox cycling at 800 °C, indicating that the primary nickel grains split to smaller ones upon cyclic reduction at higher temperature.

Comparison between Ni–Rh/gadolinia doped ceria catalysts in reforming of propane for anode implementations in intermediate solid oxide fuel cells

Steam and autothermal reforming of propane over Ni–Rh/GDC catalysts prepared by coprecipitation and by Pechini method were investigated in the temperature range 873–1073 K. The weight ratio for Ni, Rh and Ce 0.8Gd 0.2O 2 (45:5:50) and the operating temperatures were chosen in order to gain propaedeutical information on fuel reactivity under typical intermediate solid oxide fuel cell (IT-SOFC) operating conditions. The Pechini synthesis allows to obtain catalysts with lower surface area, smaller nickel crystallites and a bimodal distribution of rhodium in comparison to the coprecipitation method. Despite the different methods of synthesis lead to catalysts with different morphological and structural properties, the activity of catalysts is quite similar. At reaction temperature higher than 973 K, under both steam reforming (SR) and autothermal reforming (ATR), the catalysts show high propane conversion and syngas (H 2 + CO) productivity. Deactivation of catalysts was observed at 873 and 973 K under SR conditions due to coke formation. In ATR, coke formation was almost completely depressed and the catalysts resulted to be very stable even at low reaction temperature (873 K). In SR coke formation occurs with higher rate on the catalyst having higher Ni dispersion, probably since propane cracking reaction is the pre-eminent phenomenon in promoting coke formation.

Temperature influence on the global and local structure of the chromia supported nickel catalysts

The changes induced by the heat treatment on the global and the local structures of the supported nickel catalysts are investigated. The global structure parameters were obtained by processing the X-ray diffraction patterns of the samples using the Scherrer formula. The local structure parameters were determined by X-ray absorption spectra processing. Both types of measurements were carried out using synchrotron radiation. The nickel catalysts supported on chromia were prepared by coprecipitation method. The nickel/chromium atomic percent was 70/30. After preparation, the samples were thermally treated for three hours at 350, 650 and 950°C. A strong correlation between previous catalytic activity information and the global and local structure is evidenced. Also the correlation between the nickel crystallite size and the active metal-oxide support interaction is discussed.

Oxidation kinetics of nickel nano crystallites obtained by controlled thermolysis of diaquabis(ethylenediamine)nickel(II) nitrate

The metal complex, [Ni(en)2(H2O)2](NO3)2 (en = ethylenediamine), was decomposed in a static furnace at 200 °C by autogenous decomposition to obtain phase pure metallic nickel nanocrystallites. The nickel metal thus obtained was studied by XRD, IR spectra, SEM and CHN analysis. The nickel crystallites are in the nanometer range as indicated by XRD studies. The IR spectral studies and CHN analyses show that the surface is covered with a nitrogen containing species. Thermogravimetric mass gain shows that the product purity is high (93%). The formed nickel is stable and resistant to oxidation up to 350 °C probably due to the coverage of nitrogen containing species. Activation energy for the oxidation of the prepared nickel nanocrystallites was determined by non-isothermal methods and was found to depend on the conversion ratio. The oxidation kinetics of the nickel crystallites obeyed a Johnson–Mehl–Avrami mechanism probably due to the special morphology and crystallite strain present on the metal.

Combination of CO 2 reforming and partial oxidation of methane to produce syngas over Ni/SiO 2 prepared with nickel citrate precursor

The reaction of combination of CO 2 reforming and partial oxidation of methane to produce syngas (CRPOM) was tested over Ni/SiO 2 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 H 2-temperature-programmed reduction (H 2-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/SiO 2 prepared with the nickel citrate precursor exhibited good catalytic performances for its highly dispersed metallic nickel derived from the nickel silicate species.

Surface roughening and transport properties in the growth of nano-structured nickel electrodeposits on ITO substrate

We here show that indium tin oxide (ITO)-coated glass can be an interesting substrate for the electrodeposition of separated nickel crystallites growing in the face-centred cubic (fcc) structure. The resulting bi-modal film surface morphology corresponds to the formation of vertical crystallites whose size can reach 400 nm mean height. The topography study confirms the occurrence of both the lateral and vertical growth orientations as the growth exponent coefficient always remains β > 0.50, which is characteristic of such an effect. We also show that the model of link between the topography and surface transport properties is relevant here, which gives it a more general validity. The surface diffusion of the Ni ad-atoms on ITO range in the interval 10 −10 < D s < 10 −9 cm 2 s −1 that is higher than the values obtained on metal substrates. The increase of J favours the vertical growth as the surface diffusion length, Λ s , decreases with the increase of the crystallites size.

Methane decomposition over Ni–MgO–Al 2O 3 catalysts

Studies of methane decomposition were performed over nickel alumina catalysts modified with magnesia. The catalysts were obtained by the co-precipitation method. The properties of catalysts were investigated by the temperature programmed reduction (TPR) and hydrogen desorption (TPD) methods. Methane decomposition was studied by the thermogravimetric and transient temperature programmed reaction methods. The nature of carbon deposits formed under different reaction conditions was investigated by the temperature programmed oxidation method (TPO) and high resolution transmission electron microscopy (HRTEM). An introduction of magnesium to the preparation mixture of catalysts led to the formation of smaller nickel crystallites with stronger adsorption sites. It was found out that the activity of catalysts and the properties of carbon deposits were related to the catalysts composition and the reaction temperature. Carbon filaments with fishbone-like packed graphitic layers were formed at 500 °C, while the multi-walled nanotubes were observed after reaction performed at 700 °C. The increase of magnesium content led to the increase of the rate of methane decomposition at the initial stages of the reaction. Deactivation of the catalysts was related to the insufficient removal of carbon species from the surface of nickel crystallites and the formation of stable, graphitic carbon deposits covering the surface of metal.

Structural and magnetic properties of inverse opal photonic crystals studied by x-ray diffraction, scanning electron microscopy, and small-angle neutron scattering

The structural and magnetic properties of nickel inverse opal photonic crystal have been studied by complementary experimental techniques, including scanning electron microscopy, wide-angle and small-angle diffraction of synchrotron radiation, and polarized neutrons. The sample was fabricated by electrochemical deposition of nickel in voids in a colloidal crystal film made of 450 nm polystyrene microspheres followed by their dissolving in toluene. The microradian small-angle diffraction of synchrotron radiation was used to reveal the opal-like large-scale ordering proving its tendency to the face-centered-cubic fcc structure with the lattice constant of 65010 nm. The wide-angle x-ray powder diffraction has shown that nanosize fcc nickel crystallites, which form an inverse opal framework, have some texture prescribed by principal directions in inverse opal on a macroscale, thus showing that the atomic and macroscopic structures are correlated. The polarized small-angle neutron scattering is used on the extreme limit of its ability to detect the transformation of the magnetic structure under applied field. Different contributions to the neutron scattering have been analyzed: the nonmagnetic nuclear one, the pure magnetic one, and the nuclear-magnetic interference. The latter in the diffraction pattern shows the degree of the spatial correlation between the magnetic and nuclear reflecting planes and gives the pattern behavior of the reversal magnetization process for these planes. The field dependence of pure magnetic contribution shows that the three-dimensional geometrical shape of the structure presumably leads to a complex distribution of the magnetization in the sample.

Rapid synthesis and morphology control of nickel powders via a microwave-assisted chemical reduction method

Nickel powders with different morphologies, such as sphere, chain, and thorn-like nickel wire have been successfully synthesized by hydrazine reduction of NiCl2 in the presence of polyvinyl pyrrolidone (PVP) by using microwave-assisted chemical reduction method. The obtained products were characterized by XRD, SEM, TEM, and SAED techniques. Characterization by TEM and SAED showed that as-produced nickel powders were formed by self-assembly and aggregation of nanometer-sized nickel crystallites. PVP served as the structure directing agents and plays a key role for the morphology transformation of the nickel powders. In addition, compared with conventional heating method, microwave irradiation heating was in favor of the formation of nickel wires. The resultant nickel wires showed excellent electrical properties in the nickel wires/epoxy composite.

The state of nickel in the silver modified NiMg/SiO2 vegetable oil hydrogenation catalysts

Two series of silver modified Ni-Mg materials were synthesized by precipitation-deposition on SiO2 support derived from two silica sources: diatomite activated at 800°C (Series a; Mg/Ni = 0.1 and SiO2/Ni = 1.07) and synthetic water glass (Series b; Mg/Ni = 0.1 and SiO2/Ni = 1.15). The modification with silver was made at three molar Ag/Ni ratios, namely 0.0025, 0.025, and 0.1. The effects of the source of the silica support and the silver presence and content on the nickel state in the silver modified reduced-passivated NiMg/SiO2 precursors of the vegetable oil hydrogenation catalyst were established by X-ray diffraction and X-ray photoelectron spectroscopy techniques. The passivation procedure was applied in order to protect the metallic nickel particles from further oxidation. The crystallization of the formed nickel hydrosilicate phases depends on the source of the silica support, more expressed in the diatomite supported samples. It was shown that the silver modification of the NiMg/SiO2 precursors enhances the reduction of the nickel hydrosilicates accompanied by formation of relatively smaller metallic nickel particles, more pronounced in the water glass supported precursors. The increase of the silver content in the water glass deposited samples is responsible for the metallic nickel dispersion increase. The higher content of the Ni0 particles on the surface of the diatomite deposited samples is in accordance with the higher stability of the larger metallic nickel crystallites to oxidation during the passivation step. On contrary, higher dispersed Ni0 particles on the surface of the water glass supported samples are more susceptible to the oxida

Hydrogenation of chloronitrobenzene to chloroaniline over Ni/TiO2 catalysts prepared by sol–gel method

Abstract Ni/TiO2 catalysts were prepared by the sol–gel method, and the effect of calcination and reduction temperatures on their physicochemical properties was investigated. The increase of the calcination temperature decreases the surface area and enhances the interaction between nickel species and support, which results in the decrease of the available nickel active sites and the catalyst performance. With raising the reduction temperature, the catalyst performance also declines due to the enhanced interaction between the nickel species and TiOx and the serious sintering of metallic nickel crystallites. The suitable calcination and reduction temperatures are 673 and 623 K, respectively. Under 348 K and 1.0 MPa, o-chloronitrobenzene conversion and o-chloroaniline selectivity over the optimum Ni/TiO2 catalyst exceed 99%; moreover, the catalyst was not deactivated during 9 times recycles. The reasons for the catalyst deactivation were also investigated. It has been found that the main reason is the surface coverage and the pore blockage with bulky molecular species rather than the sintering of the metallic nickel crystallites. In addition, the hydrogenation activity of chloronitrobenzene (CNB) isomers decreases in the sequence of m-CNB, o-CNB and p-CNB. This sequence might be related to the electronic and steric factors of Cl substitute and the catalyst property.