Nickel Crystallites Research Articles

Low-temperature catalytic gasification of sewage sludge-derived volatiles to produce clean H2-rich syngas over a nickel loaded on lignite char

Catalytic gasification (CG) of sewage sludge-derived volatiles (SSDVs) was investigated over a prepared nickel loaded on Loy Yong lignite char (Ni/LYLC) in a two-stage fixed-bed reactor to understand the effects of the catalyst, temperature, and steam on the gas yields and nitrogen transformations. Non-catalytic thermal decomposition of SSDVs below 650 °C is not effective for decomposing the tar and converting the volatile nitrogen species (VNSs) to N2. Ni/LYLC proved to be quite active not only for tar reduction, but also for the conversion of VNSs to N2 at 650 °C. CG of SSDVs over Ni/LYLC produced significant amount of clean H2-rich syngas. CG above 650 °C results in the increase of nickel crystallite size and the deactivation of Ni/LYLC for tar decomposition. The study revealed the possibility of using Ni/LYLC as a potential catalyst for low-temperature CG of sewage sludge to produce clean H2-rich syngas.

The sintering of Ni/Al2O3 methanation catalyst for substitute natural gas production

The sintering behavior of a co-precipitated Ni/Al2O3 methanation catalyst is studied by investigating the effect of treating time, temperature and atmosphere. Fresh and sintered samples are characterized by N2 physisorption, H2 chemisorption, temperature programmed reduction, X-ray diffraction and transmission electron microscopy. A reduction both in total and nickel surface area has been observed, the extent depending on the experimental conditions. Sintering of the studied catalyst, reflected by a significant decrease of nickel surface area, is a combined effect of primary encapsulation of metallic nickel due to the collapse of the support structure and sporadic agglomeration of nickel crystallites. The formation of a Ni2+ doped alumina phase, induced by steam ambience, further accelerates loss of surface nickel atoms. It is found that the sintering rate obeys a simple power law expression, with the apparent activation energy value of 118 kJ/mol. The sintered methanation catalyst suffers considerable decay of CO hydrogenation activity in a simulated industrial atmosphere, which suggests that extraordinarily high temperatures should be avoided as much as possible in the practical operation.

Microstructural and electrochemical analysis of Ni–SiC composite coatings prepared in presence of additives

Ni–SiC composite coatings have been successfully prepared by pulse electrodeposition technique by dispersing SiC nanoparticles into the sulfamate electrolytic bath. Different contents of cetyltrimethyl ammonium bromide (CTAB) and saccharin (SAC) have been used to evaluate the effect of additives concentration on the final properties of deposits. The composite coatings exhibited refined matrix grains and increased microhardness as compared to pure nickel coating. The results revealed that microstructural changes and codeposition of SiC nanoparticles in nickel deposit are influenced by presence of additives. Findings revealed that despite of nickel crystallite grain refinement and increase in vickers microhardness of composite coatings by application of saccharin, electrochemical corrosion property is found to be affected.

A study on electrodeposition of Ni-noncovalnetly treated carbon nanotubes nanocomposite coatings with desirable mechanical and anti-corrosion properties

In this study, we report on the successful incorporation of non-covalently functionalized multi-wall carbon nanotubes (MWCNTs) in electrodeposition bath using a multi-step dissolution to enhance dispersing and untangling. We observed that a saturation concentration exists for CNT dispersion in the electrodeposition solution and consequently an optimum condition for electrodeposition of Ni-MWCNT was obtained at a current density of 4A/dm2, and CNT concentration of 2g/l. X-ray diffraction spectrum revealed a remarkable effect of CNTs on the texture, crystallite size, and internal micro-strain of the coatings. On the basis of nanoindentation and micro-hardness tests, increased hardness and elastic modulus of the coatings were observed which directly depend upon CNT content of the composite coating. Corrosion study results in 3.5% sodium chloride solution showed that corrosion rate decreases with increase in CNT content, though the corrosion rate is directly influenced by nickel crystallite size.

Activities of Ni-based nano catalysts for CO2–CH4 reforming prepared by polyol process

Nano nickel catalysts supported over nano sized oxide powders (CeO2, ZrO2 and Al2O3) were prepared by the polyol method in ethylene glycol (EG) medium with polyvinylpyrrolidone (PVP) as a nucleation-protective agent. The activities of the prepared catalysts were evaluated for CO2 reforming of methane in the temperature range 500–800°C. For comparison purpose, another series of Ni based catalysts was prepared by conventional impregnation method. Numerous techniques, such as N2 adsorption–desorption isotherm, temperature-programmed reduction (H2-TPR), temperature-programmed desorption (CO2-TPD), X-ray diffraction (XRD), thermogravimetric analysis (TGA), high resolution transmission electron microscopy (HRTEM), and pulse chemisorption were applied for characterization of fresh and spent catalysts. The results of catalyst testing and characterization revealed that the catalysts synthesized by polyol method have superior catalytic performance (activity and stability) than those prepared by impregnation method. The better performance of polyol catalysts could be attributed to stronger metal to support interaction and presence of evenly distributed nano nickel particles in these catalysts; since strong metal to support interaction resists sintering and small nickel crystallite size hinders coking.

Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous nickel–phosphorus–alumina aerogel catalyst

A mesoporous nickel–phosphorus–alumina aerogel catalyst (NPAA) was prepared by a single-step epoxide-driven sol–gel method and a subsequent supercritical CO2 drying method for use in the hydrogen production by steam reforming of liquefied natural gas (LNG). In order to investigate the effect of drying method of nickel–phosphorus–alumina catalysts on their physicochemical properties and catalytic activities, a mesoporous nickel–phosphorus–alumina xerogel catalyst (NPAX) was also prepared by a single-step epoxide-driven sol–gel method and a subsequent evaporative drying method for comparison purpose. It was found that supercritical CO2 drying method was effective for enhancing textural properties of NPAA catalyst. Although both NPAX and NPAA catalysts retained surface nickel aluminate phase, NPAA catalyst showed stronger metal-support interaction than NPAX catalyst. XRD patterns of reduced NPAX and NPAA catalysts revealed that NPAA catalyst retained smaller metallic nickel crystallite than NPAX catalysts. It was also observed that the reduced NPAA catalyst exhibited high nickel dispersion, large amount of strong hydrogen-binding sites, and large amount of methane adsorption compared to the reduced NPAX catalyst. In the steam reforming of LNG, NPAA catalyst with high affinity toward methane showed a better catalytic performance than NPAX catalyst.

Ni/C nanostructures: Impregnating-method preparation, textural and structural features, and catalytic property for the hydrogen production

Abstract

Influence of multiple electroless nickel coatings on beech wood: preparation and characterization

Surface modification of beech wood layers was first carried out by rinsing them in distilled water, followed by surface activation with nickel metallic as a catalyst that is prepared on beech wood surface by reduction of Ni+2 ions using as a reduction agent at pH 11. Finally, the nickel was deposited by electroless nickel coating in many baths separated by drying at 100 °C during 24 h. Both texture and structural properties of nickel deposited on beech wood were characterized using both scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The SEM results show that the nickel film thickness prepared on beech wood surface after four baths is within 2.9–3 μm. The XRD patterns revelated that nickel deposited on beech wood has crystallite size between 83.37 and 149.95 Å and the results indicate that the nickel crystallites grow rapidly with increasing number of deposition run.

Bimetallic MNi/Al2O3-La catalysts (M = Pt, Cu) for acetone steam reforming: Role of M on catalyst structure and activity

The effect of Pt and Cu addition on the behaviour of Ni catalysts supported on Al2O3 modified with La for acetone steam reforming (as model molecule of bio-oil) was studied. Catalytic activity was found to depend strongly on the bimetallic formulation, achieving the bimetallic PtNi catalyst almost complete gasification of the acetone. The characterization results achieved with XRD, TPR and XPS showed differences in surface metal concentrations which depend on the presence of Pt or Cu in catalysts composition. The decrease in activity observed for the bimetallic CuNi catalyst is associated with changes in the acetone reaction mechanism due to the presence of metallic copper since formation of CuNi alloy was not formed in this catalyst. In the case of bimetallic PtNi catalyst, a lower carbon deposition and a higher thermal stability of the Ni crystallites under reaction conditions was observed with respect to monometallic Ni reference catalyst. It is suggested that the better dispersion of metallic nickel crystallites together with the enhanced H-atom mobility on Pt are responsible of the better catalytic behaviour of the bimetallic PtNi formulation in the steam reforming of acetone.

Porous nickel-based catalysts for combined steam and carbon dioxide reforming of methane

X-ray diffraction, low-temperature nitrogen adsorption, and electron microscopy combined with energy dispersive X-ray microanalysis were used for studying the effect of synthesis conditions on the phase composition and texture of a porous nickel ribbon with a MgO underlayer and of nickel catalysts supported on it. The MgO underlayer was prepared by impregnation of a nickel ribbon with a Mg(NO3)2 solution and subsequent calcination at 550°C in air or H2. Catalysts were synthesized by additional supporting of nickel onto supports by its impregnation with a solution of Ni(NO3)2 or a mixture of solutions Ni(NO3)2 and Mg(NO3)2. In supported reduced catalysts (750–900°C, H2), the phase of nickel and the solid solution of NiO in MgO were observed. A considerable part of nickel crystallites was epitaxially bound with MgO. The nickel-based catalysts were tested in combined steam and carbon dioxide reforming of methane to synthesis gas (750°C, CH4/CO2/H2O/N2=35/23/39/3, GHSV=62.5L/gh). The catalysts with the MgO underlayer completely covering the nickel ribbon showed stable activity throughout the test period (18h). After the reaction, in these catalysts on the large crystallites of nickel ribbon, dispersed nickel crystallites (5–10nm) epitaxially bound with MgO appeared; no carbon deposits were found in them. The resistance of these catalysts to carbonization was attributed both to the formation of nickel crystallites epitaxially bound with MgO and to the formation of the MgO underlayer that covers the nickel ribbon and prevents its contact with the reaction medium.

Preparation of highly active nickel catalysts supported on mesoporous nanocrystalline γ-Al2O3 for CO2 methanation

In this study, a group of Ni catalysts supported on mesoporous nanocrystalline γ-Al2O3 with high surface area and with different Ni contents was prepared and employed in carbon dioxide methanation reaction. The obtained results showed increasing in nickel content from 10 to 25wt.% decreased the specific surface area of catalysts from 177 to 130m2/g and increased the nickel crystallite size from 12 to 24nm. In addition, increasing in nickel content increased the reducibility of catalyst. The catalytic results revealed that the catalyst with 20wt.% of Ni possessed high activity and stability in CO2 methanation reaction.

Influence of the Organic Additions on Structure, Corrosion Resistance and Tribological Properties of Ni/Al<sub>2</sub>O<sub>3 </sub>Composite Coatings

Composite Ni/Al2O3 coatings were electrochemically deposited from a Watts bath modified with the organic additions of dioctyl sulphosuccinate sodium salt C20H37NaO7S–(DSS) and 2,3-dihydroxy-1,2 benzisothiazol-3-one 1,1-dioxide C7H5NO3–(LSA). The effect of different amount (50 and 100 g/l) of Al2O3 powder and organic additions on microstructure, microhardness, corrosion resistance and tribological properties was investigated. The coatings were examined by optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) techniques. The electrochemical behaviour of the coatings in corrosive solutions (1 M NaCl and 1 M Na2SO4) was investigated by potentiodynamic polarisation. The tribological properties were investigated by the Taber abrasive wear test, a standard test often applied in industrial practice. The results show that Al2O3 particles are uniformly distributed in the composite coatings compared with coatings without organic additions. The addition of organic compounds also reduced the size of the forming nickel crystallites and improved the tribological and corrosion properties of coatings containing the dispersed hard particles of Al2O3 added in an amount of 50 g/l and the addition of organic LSA and DSS compounds.

High-loaded nickel–alumina catalyst for direct CO2 hydrogenation into synthetic natural gas (SNG)

The methanation of carbon dioxide was carried out over a high-loaded Ni–Al mixed oxide catalyst (ca. molar ratio Ni/Al=5), prepared by conventional coprecipitation of the metal precursors. This route makes possible to obtain multimetallic mixed oxides upon calcination with high metal loading and high surface area. X-ray powder diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscope (TEM) and thermogravimetric analysis were used for studies of the precursors and catalysts. Calcined samples were activated under H2 at 500°C, leading to the formation of small metallic nickel particles (ca. 6nm) dispersed on a high surface area Ni(Al)Ox matrix. Activity tests were conducted using different H2/CO2 molar ratios (3–5), WHSV (0.2–1.0molCO2/(gcath), temperatures (250–500°C), and pressures (5–20bar). Despite the high nickel loading (ca. 70wt.%), which is theoretically thought to be counter-productive for the nickel-based catalyst performance, our Ni–Al activated catalyst exhibited high CO2 conversion, and rendered a CH4 selectivity very close to 1. This is originated by the formation of small metallic nickel crystallites (ca. 6nm) dispersed over NiO–alumina upon partial reduction of the mixed oxide. The catalyst experiences complete reduction during reaction, which slightly increases the Ni crystallite size, but preserves the high activity in ca. 500h lifetime tests even at high space velocity.

Self cleaning behaviour of Ni/nano-TiO2 metal matrix composites

Titanium dioxide (TiO2) nano-particles were immobilized in nickel matrix coatings using the direct current electrodeposition technique. The influence of nickel surface morphology and crystallographic orientation on the photocatalytic activity and photo-induced hydrophilicity of the composite coatings were investigated under UVA illumination. The overall self-cleaning character of the composites was improved following the titania codeposition, as both hydrophilic and photocatalytic effects are present. Simultaneous optimization of the two functionalities was possible via tuning the titania percentage, indicating that the corresponding properties of the composite coatings are associated also to the induced crystalline nickel orientation and the nickel grain size. In particular, the data indicate that both enhanced photocatalytic activity and photo-induced hydrophilicity are achieved by increasing amounts of titania nano-particles embedded in the Ni matrix in combination with reduced grain size of nickel crystallites that are oriented towards the [110] axis.

Selective Hydrogenation of Maleic Anhydride to Succinic Anhydride in Liquid Phase over Ni-Cu/Al<SUB>2</SUB>O<SUB>3</SUB> Catalyst

A series of Ni-Cu/Al2O3 catalyst samples were prepared by the wetness impregnation method and characterized by N2 physisorption, H2 temperature-programmed reduction, H2 temperature-programmed desorption, X-ray diffraction, and CO temperature-programmed surface reaction. Their catalytic performance in the liquid phase hydrogenation of maleic anhydride was investigated. The results indicated that the particle size of nickel crystallites was reduced, and the nickel species was well dispersed. Then, the C=C hydrogenation activity was significantly improved when the catalyst was promoted by Cu. However, due to the interaction between Ni and Cu, the C=O hydrogenation was retarded. The combination of the above two factors led to high selectivity for succinic anhydride over 15%Ni-7%Cu/Al2O3 catalyst. The conversion of maleic anhydride and selectivity for succinic anhydride reached 100% under the reaction conditions of 5 MPa hydrogen pressure and 210 oC for 40 min.

On the Predictions of Carbon Deposition on the Nickel Anode of a SOFC and Its Impact on Open-Circuit Conditions

Previous thermodynamic analyses of carbon formation in SOFCs assumed that graphite could be used to represent the properties of carbon formed in the anode. It is generally observed, however, that catalytically grown carbon nanofibers (CNF) are more likely to form in the SOFC anode with nickel catalysts. The energetic and entropic properties of CNF are different from those of graphite. We compare equilibrium results based on thermochemical properties for graphite, to new results based on a previously reported value of an empirically determined Gibbs free energy for carbon fibers grown on a nickel support (with fitted values of H°CNF = 54.46 kJ/mol and S°CNF = 68.90 J/mol/K for a nickel crystal size of 5.4 nm). There is little difference in predictions of carbon formation under open-circuit conditions between the two carbon types for methane mixtures, with graphite predicted to form at lower temperatures than CNF. There is a much bigger difference in predictions for methanol mixtures, especially at low steam-carbon ratios. The differences for propane are even more pronounced, and the improved predictions assuming CNF are in closer agreement with past observations. We show a strong dependence of CNF formation and “coking threshold” on nickel crystallite size, supporting previous reports that the nickel particle size is a dominating parameter for controlling filament growth. If both carbon types are included in the calculations, only the thermodynamically favored form (i.e., the type having the lowest formation energy) exists. Predicted Nernst potentials are more-or-less independent of the carbon type and in agreement with measured open-circuit voltages.

Ni–Fe catalysts derived from hydrotalcite-like precursors for hydrogen production by ethanol steam reforming

Nickel–iron mixed oxides derived from reevesite, a hydrotalcite-type compound, were tested in steam reforming of ethanol for hydrogen production. The influence of iron content (Ni/Fe ranging from 3 to 1) and the calcination temperature of the catalyst precursor (773 and 1073K) on the catalytic performance were investigated. Both parameters were essential to optimize the reforming performance. Increasing the amount of iron in the reevesite precursors affected both the chemical and activity properties of the derived mixed oxide catalysts. Iron displays a positive role in nickel-based catalysts due to the enhancement of catalytic activity and hydrogen selectivity induced by the improved dispersion of nickel and the alleviation in carbon deposition. The calcination temperature led to variations in phase composition consisting of Ni(Fe)Ox solid solution and NiFe2O4, which affected the final size and dispersion of nickel species formed during the reaction. The best catalyst, with a Ni/Fe ratio of 1 and calcined at 773K, rendered high and stable hydrogen and carbon dioxide selectivity of up to ca. 60% and 40%, respectively, low methane content, and consisted of a Ni(Fe)Ox+NiFe2O4 mixture with high surface area and small Ni0 crystallites. A higher percentage of crystalline NiFe2O4 attained at high calcination temperature (1073K) associated with a lower carbon deposition resistance and probably Ni0 sintering brings about lower activity and fast deactivation. The improved performance over catalysts calcined at lower temperature and with lower Ni/Fe ratio is motivated by the effect of iron on the structural and electronic properties of the mixed oxides, thus inducing a slow formation of metallic nickel particles and coke deposits. Features like high surface area, higher iron content, lower reducibility of nickel species and small nickel crystallite size well dispersed on the surface of the catalyst with high iron content lead to a higher activity in ethanol dehydrogenation, acetaldehyde decarbonylation and reforming, and WGS.

Partial oxidation of methane to syngas on bulk NiAl2O4 catalyst. Comparison with alumina supported nickel, platinum and rhodium catalysts

In this work the catalytic performance of NiAl2O4 catalyst in the partial oxidation of methane (CH4(1)/O2(0.5)/N2(8.5) and 4800mLCH4g−1h−1) was investigated. This bulk catalyst resulted more active (70% conversion at 625°C) than alumina supported nickel catalysts prepared by conventional impregnation (11%Ni/Al2O3-30% conversion at 625°C and 14%Ni/Al2O3-65% conversion at 625°C) and a commercial platinum catalyst (1%Pt/Al2O3-30% conversion at 625°C). Further, the behaviour of NiAl2O4 was comparable to that of a commercial rhodium catalyst (1%Rh/Al2O3-75% conversion at 625°C). Fresh and spent catalysts were characterised by N2-physisorption, H2-TPR, UV-vis-DRS, WDXRF, XRD, TGA-MS, TEM and Raman spectroscopy. After catalyst reduction CH4 conversion over bulk NiAl2O4 was associated with the formation of large amounts of metallic nickel crystallites with a relatively small size. Although significant coking was noticed, conversion and selectivity to H2/CO were not significantly affected after prolonged time on stream.

Synthesis gas production from catalytic gasification of waste biomass using nickel-loaded brown coal char

This paper presents an experimental research concerning the catalytic gasification of waste biomass to synthesis gas using Ni-loaded brown coal char (Ni/BCC). The attention is focused on the catalytic conditions for enhancing the synthesis gas production, improving its composition and extending the catalyst lifetime. The aim is achieved by means of the characterization of Ni/BCC and the product gas analysis, through gasification experiments of a woody biomass and an animal-waste-derived biomass both in a fixed bed reactor and a fluidized bed reactor at various catalytic temperatures and steam/biomass–carbon (S/C) ratios. Effects of the pyrolysis temperature and the steam treatment on the nickel crystallite size are analyzed and used as the reference of choosing optimum gasification conditions. With brown coal char as support material, Ni/BCC is found as an excellent catalyst even at low temperature of about 650°C and shows a good resistance ability of coke formation, compared to non-catalyst and Ni/Al2O3. More than 25h continuous operation is also performed in a 1kg/h Internally Circulating Fluidized-bed Gasifier (ICFG) to assess the lifetime of Ni/BCC. Smaller S/C ratio at the appropriate temperature is suggested to lower the coal char gasification, thereby reducing the degradation of the support material.

The role of CeO2–ZrO2 distribution on the Ni/MgAl2O4 catalyst during the combined steam and CO2 reforming of methane

The effect of the distribution of CeO2–ZrO2 on Ni/MgAl2O4 catalyst on the catalytic performance during the combined steam and carbon dioxide reforming of CH4 (CSCR) was investigated on two different catalysts prepared using a sequential impregnation and co-precipitation of Ni and CeO2–ZrO2 components. The CeO2–ZrO2 plays an important role in the CO2 conversion by enhancing a facile activation of CO2 with an adjacent contact with nickel crystallites. The CSCR catalysts prepared by the co-precipitation of nickel and CeO2–ZrO2 on MgAl2O4 support shows a catalytic activity superior to the catalyst prepared by sequential impregnation of nickel on the MgAl2O4 support with CeO2–ZrO2 due to the homogeneous distribution of CeO2–ZrO2 with the formation of small nickel crystallites and a large amount of active sites for CO2 adsorption.