Methane reforming reaction with carbon dioxide over SBA-15 supported Ni-Mo bimetallic catalysts

A series of mesoporous molecular sieves SBA-15 supported Ni-Mo bimetallic catalysts (xMo1Ni, Ni = 12 wt.%, Mo/Ni atomic ratio = x, x = 0, 0.3, 0.5, 0.7) were prepared using co-impregnation method for carbon dioxide reforming of methane. The catalytic performance of these catalysts was investigated at 800 °C, atmospheric pressure, GHSV of 4000 ml·g_cat^− 1·h^− 1 and a V(CH₄)/(CO₂) ratio of 1 without dilute gas. The result indicated that the Ni-Mo bimetallic catalysts had a little lower initial activity compared with Ni monometallic catalyst, but it kept very stable performance under the reaction conditions. In addition, the Ni-Mo bimetallic catalyst with Mo/Ni atomic ratio of 0.5 showed high activity, superior stability and the lowest carbon deposition rate (0.00073g_c·g_cat^− 1·h^− 1) in 600-h time on stream. The catalysts were characterized by power X-ray diffraction, N₂-physisorption, H₂-TPR, CO₂-TPD, TG and TEM. The results indicate that the Ni-Mo bimetallic catalysts have smaller metal particle, higher metal dispersion, stronger basicity, metal-support interaction and Mo₂C species. It is concluded that Mo species in the Ni-Mo bimetallic catalysts play important roles in reducing effectively the amount of carbon deposition, especially the amount of shell-like carbon deposition.     

Long-term electrolytic hydrogen permeation in nickel and the effect of vanadium species addition

Nickel cathodes have been found to become deactivated under long-term polarization in the H₂ evolution region during alkaline water electrolysis. The cause of deactivation was examined using steady state polarization and measurement of hydrogen permeation through nickel foil in 8 mol/l KOH at 70 °C and 100 mA/cm². The long-term (over 50 h) permeation behaviour was explained by formation and growth of a nickel hydride phase. The rise in hydrogen overpotential was ascribed to an increase of the hydrogen surface coverage on the newly formed hydride. The effect of an electrolyte additive (a vanadium salt) on the hydrogen overpotential and permeation rate was also investigated. Upon addition of dissolved V₂O₅, the permeation rate was found to increase quickly and then slowly decrease to a steady value close to that measured for hydride-free nickel. Meanwhile, the hydrogen overpotential was observed to recover back to nearly its initial value for fresh nickel. The exhibited behaviour was attributed to decomposition of the hydride phase, after deposition of a vanadium-bearing compound. The prolonged contact between Ni and V was proposed as the main reason for hydride decomposition. The addition of more vanadium had no further result on the hydrogen overpotential.     

Morphology and phase composition of as-deposited and recrystallized Ni-Mo-O powders

The Ni-Mo-O alloy powders were electrodeposited from ammonium sulfate containing electrolytes for different Ni/Mo ions concentration ratios. Electrodeposition was investigated by the polarization measurements. The morphology, chemical composition and phase composition of these powders were investigated using SEM, EDS, AAS and XRD analysis. The EDS and AAS analysis showed that the powder composition depends on the Ni/Mo ions concentration ratio, with the Ni/Mo metals ratio in the powders being the same as the one in the solution. The as-deposited alloy powders were nanocrystalline, while after stepwise annealing at 300, 400, 500 and 600 °C for 2 h in N₂ atmosphere their crystallinity became more and more pronounced, with the dimension of crystallites increasing with the increase of the annealing temperature. Already after the annealing at 300 °C the presence of two phases, MoO₃ and NiMoO₄, was identified by XRD, while SEM analysis showed that the surfaces of powder particles were partially recrystallized. With the increase of the annealing temperature the amount of the NiMoO₄ was found to increase in all powders. For the powder electrodeposited from the solution with the highest Ni/Mo ratio (1/0.3) NiMoO₄ phase was detected together with a small amount of Ni₄Mo phase, indicating phase transition of MoO₃ into NiMoO₄ (and probably Ni₄Mo) at the temperature of 600 °C.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

Eggshell-membrane-templated synthesis of hierarchically-ordered NiO–Ce0.8Gd0.2O1.9 composite powders and their electrochemical performances as SOFC anodes

Hierarchically-ordered NiO-Ce_0.8Gd_0.2O_1.9 (GDC) composite anode powders were synthesized using eggshell membranes as biotemplates. The morphology of the as-synthesized powders depended on the kind of Ni precursor and the use of EDTA as a chelating agent. Hierarchically-ordered anode powders were obtained from Ni chloride and Ni acetate precursors with EDTA. The Ni–GDC anode synthesized from Ni chloride precursor with EDTA exhibited the lowest polarization resistance at 800 °C and an activation energy of 0.01 Ω cm² and 0.74 eV, respectively, in humidified H₂. In accordance with the polarization resistance results, the 0.5-mm thick GDC electrolyte-supported single cell with the Ni–GDC anode synthesized from Ni chloride precursor with EDTA showed a maximum power density of 0.34 W cm⁻² at 800 °C with humidified H₂ fuel.     

Characterization of electrodeposited powders of the system Ni-Mo-O

The electrodeposition of the Ni-Mo-O alloy powders from ammonium chloride containing electrolytes for different Ni/Mo ions concentration ratios was investigated by the polarization measurements. The morphology, chemical composition and phase composition of the electrodeposited Ni-Mo-O alloy powders were investigated using DSC, TGA, SEM, EDS and XRD analysis. According to the EDS results, the powder composition depends on the Ni/Mo ions concentration ratio. However, some deviations in the composition of the as-deposited powders, as the result of the position of the EDS analysis, revealed that the composition of such powders is not homogeneous. The as-deposited alloy powders were nanocrystalline showing only one broad XRD peak around 44°. Their morphology was found to depend on the Ni/Mo ions concentration ratio. After annealing of the Ni-Mo-O powders for 2 h in N₂ atmosphere at 600 °C, well-defined crystals for all powders were detected by SEM. The presence of the NiMoO₄ phase only or NiMoO₄ and MoO₃ phases in these powders (XRD), depending on the Ni/Mo ions concentration ratio, was identified. The additional EDS analysis on different recrystallized grains revealed the influence of the annealing on a composition of the alloy powders.     

Electrochemical corrosion characterization of Al-Ni alloys in a dilute sodium chloride solution

The aim of this study is to characterize the electrochemical corrosion resistance of Al-Ni alloy samples which were directionally solidified under upward unsteady state heat flow conditions. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques were used to analyze the corrosion resistance in a dilute 0.05 M NaCl solution at 25 °C. Equivalent circuit analyses were also conducted. It was found that microstructural features such as the dendritic arrangement and the distribution of Al₃Ni intermetallic particles have important roles on both the resulting pitting potential and the general corrosion resistance of Al-Ni alloys.     

Kinetics of the carbothermal synthesis of Mo carbide catalyst supported on various semiconductor oxides

Molybdenum carbide synthesized by temperature-programmed carburization of MoO₃ supported on various semiconductor oxides (10 wt.% Mo) with a H₂/C₃H₈ mixture have been characterized and evaluated for Fischer-Tropsch synthesis. The carburization reaction appeared to be a 2-stage process involving formation of intermediate oxycarbide phase, which was further carburized to the metal carbide form. Both α-MoC_1 − x and β-MoC_1 − x phases were detected in all Mo carbide catalysts and MoO₃ was converted completely to molybdenum carbide during carburization. The carburization rate depended on the C₃H₈ composition in the feed and attained an optimum at a H₂:C₃H₈ ratio = 5 for all four supports (Al₂O₃, TiO₂, SiO₂, and ZrO₂). Carbide formation rate increased with Mo loading although it reached a ‘plateau’ at Mo loading beyond 15 wt.% Mo. The existence of a compensation effect and isokinetic relationship for both oxycarbide and carbide phases suggested that the conversion of Mo oxide to oxycarbide and oxycarbide to carbide phase was governed by the same topotactic mechanism. Mo carbide catalysts were evaluated for CO hydrogenation activity and Fischer-Tropsch specific reaction rate decreased in the order; MoC_1 − x/TiO₂ > MoC_1 − x/SiO₂ > MoC_1 − x/ZrO₂ > MoC_1 − x/Al₂O₃ parallel to the trend for Mo carbide production rate.     

Enhancement of oxygen reduction by incorporation of heteropolytungstate into the electrocatalytic ink of carbon supported platinum nanoparticles

Nafion stabilized inks of Vulcan XC-72 supported platinum (20 wt.%) nanoparticles (Pt/XC-72) were utilized to produce electrocatalytic films on glassy carbon. The catalysts were modified (activated) with phosphododecatungstic acid H₃PW₁₂O₄₀ (PW₁₂). Comparison was made to bare (PW₁₂-free) electrocatalytic films. Electroreduction of dioxygen was studied at 25 °C in 0.5 mol dm⁻³ H₂SO₄ electrolyte using rotating disk voltammetry. For the same loading of platinum (≈95 μg cm⁻²) and for the approximately identical distribution of the catalyst, the reduction of oxygen at a glassy carbon electrode modified with the ink containing PW₁₂ proceeded at ca. 30-60 mV more positive potential (depending on the PW₁₂ content), and the system was characterized by a higher kinetic parameter (rate of heterogeneous electron transfer), when compared to the PW₁₂-free electrocatalyst. Gas diffusion electrodes with Pt/XC-72 supported on carbon paper (Pt loading 1 mg cm⁻²) were also tested. Under the same experimental conditions, while the exchange current density and the total resistance contribution to polarization components, computed from the galvanostatic polarization curves were found to be clearly higher and lower, respectively, for the ink modified with PW₁₂ relative to the unmodified system. The results demonstrate that addition of heteropolytungstatic acid (together with Nafion) enhances the electrocatalytic activity of platinum towards reduction of oxygen.     

Electrodeposition of amorphous/nanocrystalline and polycrystalline Ni-Mo alloys from pyrophosphate baths

Pyrophosphate plating bath was found to be a good alternative to citrate bath for deposition of Ni-Mo amorphous alloys. The addition of wetting agents such as 2-butyne-1,4-diol and rokafenol N-10 to the pyrophosphate bath resulted in the removal of bumps, spheres and cracks from the Ni-Mo alloy surface. The plated alloy layers adhered well to Cu-Zn brass and steel, were of thickness from a fraction to tens of micrometers and the molybdenum content was independent of the distance from the support. An increase in the concentration of the molybdate ion in the bath leads to an increase in the amount of Mo in the alloys up to 33-35 at.% and to a decrease in the deposition rate. These changes and the influence of pH are discussed in the paper. The atom arrangement in the alloys changes from (2 2 0) preferred for pure-nickel deposition to (1 1 1) for content of Mo higher than 15 at.%. For 20 and more at.% of Mo the structure of the alloy is amorphous like. An analysis of SEM and STM micrographs obtained indicates that contrary to the Ni-W alloy the “amorphous” phase is made of circa 10-50 nm in diameter objects and not by long needles perpendicular to the substrate.     

A direct electrochemical route from oxide precursors to the terbium-nickel intermetallic compound TbNi5

Successful direct electrochemical reduction of mixed powders of terbium oxide (Tb₄O₇) and nickel oxide (NiO) to the intermetallic compound, TbNi₅, is demonstrated in molten CaCl₂ at 850 °C by constant voltage (2.4-3.2 V) electrolysis. The reduction mechanism was investigated by cyclic voltammetry using a molybdenum cavity electrode in conjunction with characterisations of the products from both constant voltage and potentiostatic electrolysis under different conditions by XRD, SEM and EDX. It was found that the reduction started from NiO to Ni, followed by that of Tb₂O₃ (resulting from Tb₄O₇ decomposition) on the pre-formed Ni to form the intermetallic compound. The reduction speed increased with increasing the cell voltage, but the speed gain was counterbalanced by decreased current efficiency and increased electric energy consumption. At 2.4 V, the current efficiency reached 63.2%, and the energy consumption by electrolysis was as low as 3.2 kWh/kg TbNi₅ when the oxide phase was converted fully to the metal phase (XRD) in 4 h. The oxygen level in the produced TbNi₅ could readily reach 1800 ppm by electrolysis at 3.2 V for 12 h with the energy consumption being 18.9 kWh/kg TbNi₅.     

Nickel and cobalt promoted tungsten and molybdenum sulfide mesoporous catalysts for hydrodesulfurization

High-performance hydrodesulfurization (HDS) catalysts were prepared by incipient wetness impregnation of Ni-Mo(W) and Co-Mo(W) species over siliceous MCM-41 doped with zirconium. Catalysts with W and Mo loadings of 20 and 11 wt%, respectively, and with a Ni or Co loading of 5 wt%, were prepared. As a reference, a nickel-tungsten catalyst supported on a commercial γ-Al₂O₃ with a 5 and 20 wt% metal loadings, respectively has also been prepared. HDS reaction of dibenzothiophene (DBT) under 3.0 MPa of total pressure and with hourly space velocity (WHSV) of 28 h⁻¹ was used to evaluate the activity of these sulfided catalysts. All the catalysts displayed a very good performance in the temperature range of 300-340 °C, with conversions between 49.0% and 92.6%. The Ni promoted catalysts displayed better performances than those of Co promoted catalysts in the HDS of DBT. On the other hand they show different selectivity to hydrogenation, thus, in Ni promoted catalysts, the hydrogenation (HYD) reaction contributes more to the conversion of DBT than Co promoted catalysts where the direct desulfurization (DDS) reaction is more important. The performance of this set of catalysts is similar to that observed with a Ni5W20-Al₂O₃ catalyst in the same range of temperature (300-340 °C). However, the selectivity to the HYD product, CHB, observed with nickel promoted catalysts (Ni5-Mo11 and Ni5-W20) is higher than that found for Ni5W20-Al₂O₃ catalyst probably due to a higher superficial area of the MCM-support and to the presence on the surface of zirconium species, which leading to a better dispersion and lower stacking of the active phases.     

Dependence of electrocatalytic activity on film thickness for the hydrogen evolution reaction of Pd overlayers on Au(1 1 1)

The hydrogen evolution reaction has been studied for ultrathin Pd overlayers of various thickness on Au(1 1 1) in 0.1 M H₂SO₄. A clear correlation of the electrocatalytic activity as expressed by the exchange current density and the binding energy of adsorbed hydrogen has been found. While hydrogen is bound strongest on the second Pd monolayer (ML), the respective catalytic activity is poorest for all the surface structures under study. The exchange current density increases in the order 2 ML Pd < 1 ML Pd < bulk Pd (more than 2 ML). The electronic ligand effect, a geometric effect due to pseudomorphic growth and the surface defect density belong to the most crucial parameters in relations between structure of the electrode surface and its electrocatalytic activity. The experimental results are supported by an excellent agreement with theoretical predictions.     

Synthesis of twisted carbon fibres comprised of four intertwined helical strands

NiSO₄/Al₂O₃ was reacted with H₂ and C₂H₂ in a fluidised-bed reactor at 650 °C to form Ni/Al₂O₃ and H₂S. The Ni catalyst became coated with a moderately graphitised (0.6 I_D/I_G ratio) carbonaceous product containing twisted carbon fibres, 200-500 nm in diameter and up to 27 μm in length. The fibres are comprised of four intertwined helical strands and are a unique carbon morphology. Their growth pattern and surface morphology is attributed to differing carbon extrusion speeds from the faces of the polycrystalline Ni catalyst particles that have been modified by H₂S.     

Influence of the micro-addition of Mo on glass forming ability and corrosion resistance of Cu-based bulk metallic glasses

(Cu₄₇Zr₁₁Ti₃₄Ni₈)_100−xMo_x bulk metallic glasses (BMGs) with x = 0, 1 and 2 at.% and a bulk metallic glass matrix composite with x = 5 at.% were successfully prepared by water-cooled copper mold casting. The effect of the addition of a small amount of Mo on the glass forming ability (GFA), thermal properties of the base alloy (i.e. x = 0) were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analyzer (DTA). It is found that the addition of appropriate amount of Mo can enhance the GFA of the Cu-based BMG, as indicated by the increase in the reduced glass transition temperature T_rg (=T_g/T_l) and the parameter γ (=T_x/(T_g + T_l)) with the increase of Mo. On the other hand, the corrosion resistance of the Cu-based BMGs with different Mo contents was examined by electrochemical polarization and weight loss measurement in 1 mol/L H₂SO₄ and 1 mol/L NaOH solutions, respectively. It is found that the corrosion resistance of Cu-based BMGs increased with increasing Mo content with the lowest corrosion rate of (0.9 ± 0.2) × 10⁻³ mm/year in 1 mol/L H₂SO₄ solution and (0.3 ± 0.1) × 10⁻³ mm/year in 1 mol/L NaOH solution, respectively, for the BMG containing 2 at.% Mo. X-ray photoelectron spectroscopy (XPS) results revealed that the improvement of corrosion resistance of Cu-based BMG containing appropriate amount of Mo originated from the enrichment of ZrO₂ and TiO₂, but depletion in Cu- or Ni-oxides in the passive films formed during electrochemical polarization. Finally, the galvanostatic-step measurement was performed to investigate the kinetics of the formation of passive films on the BMG surfaces. It is demonstrated that the addition of an appropriate amount of Mo can effectively improve the stability and uniformity of the passive film. The role of Mo addition on the glass forming ability and corrosion behavior is discussed.     

Study of structural and electrochemical characteristics of Co-based hypo-hyper d-electrocatalysts for hydrogen evolution

Structural and electrochemical characteristics of hypo-hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied. The basic catalytic material was prepared of 10% amorphous Co (grain size <2 nm), 18% amorphous TiO₂ and Vulcan XC-72, by sol-gel procedure. A number of modifications were applied aimed at improving the materials performances: (i) TiO₂ was transformed into anatase by heating at 480 °C for 1 h, (ii) multiwalled carbon nanotubes (MWCNT) were used as a catalyst support instead of Vulcan XC-72 and (iii) Mo was added to Co phase in a quantity of 25 at.% (Mo:Co = 1:3).Both, material's intrinsic catalytic activity and surface area were affected by these modifications. As a result, the electrocatalytic activity for hydrogen evolution was improved, e.g. transformation of TiO₂ into anatase form lowers the HER overpotential (η) for 15 mV at 60 mA cm⁻². Introduction of MWCNTs lowered η for 30 mV, while addition of Mo to metallic phase for 40 mV.The complete modification of all three catalyst's components (10% MoCo₃ + 18% anatase + MWCNTs) was the most effective with 60 mV decrease of overpotential.Characterization was made by XRD, SEM, IR and XPS methods. Surface area was measured by means of cyclic voltammetry.     

Co-deposition of Al-Cr-Ni alloys using constant potential and potential pulse techniques in AlCl3-NaCl-KCl molten salt

To improve the oxidation resistance of TiAl intermetallic compound under high temperature condition, cathodic co-deposition of Al-Cr and Al-Ni alloy was carried out by constant potential control or potential pulse control in AlCl₃-NaCl-KCl molten salt containing CrCl₂ and/or NiCl₂ at 423 K. Cathodic reduction of Ni and Cr starts at potential of 0.8 and 0.15 V versus Al/Al³⁺ in the molten salt, respectively. The co-deposition of Al, Cr, and Ni occurred at potentials more negative than −0.1 V to form a mixture of intermetallic compounds of Cr₂Al, Ni₃Al, and Al₃Ni. Concentration of Cr in the deposit was enhanced to 43 at% at −0.1 V; however, concentration of Ni in the deposit was 6 at% at the same potential. The concentration of Ni further decreased with more negative potential to 1 at% at −0.4 V. The potential pulse technique enhanced the Ni concentration in the deposit to about 30 at%, due to anodic dissolution of Al content from the deposit at the higher side of potential on the potential pulse electrolysis.     

Effect of intermetallic Ti2Ni on the electrochemistry of TiCODE-12 in hydrochloric acid

The effect of heat treatment on the electrochemistry of TiCode-12 (Ti—0.8 Ni—0.3 Mo) in 1 N HCl has been examined in this study. Specimens, commercially processed in the mill-annealed condition, were subjected to heat treatment at 600°C for varying times. This results in a β å α + Ti₂Ni phase transition, with the amount of intermetallic Ti₂Ni formed being proportional to the duration of the heat treatment. It is shown that Ti₂Ni is a more effective cathode for hydrogen evolution than α-Ti, and alloy passivity resulted from galvanic coupling between intermetallic Ti₂Ni and the metal matrix. A comparison is made of the anodic polarization characteristics of TiCode-12 subjected to different heat treatment durations.     

New NiFe2−xCrxO4 spinel films for O2 evolution in alkaline solutions

Spinel-type ternary ferrites with composition NiFe_2−xCr_xO₄ (0 ≤ x ≤ 1) were synthesized by a precipitation method and their physicochemical and electrocatalytic properties have been investigated using IR, XRD, BET surface area, XPS, impedance and Tafel polarization techniques. The study indicated that substitution of Cr from 0.2 to 1.0 mol in the spinel matrix increased the apparent electrocatalytic activity of the base oxide towards the O₂ evolution reaction in 1 M KOH at 25 °C. The apparent electrocatalytic activity of the oxide with 0.8-1.0 mol Cr was found to be the greatest among the present series of oxides investigated. It is noteworthy that the electrocatalytic activity of the oxide with x = 0.8-1.0 was also greater than those of other spinel/perovskite O₂ evolving electrocatalysts reported in literature.     

Investigation of the anodic behavior of Al current collector in room temperature ionic liquid electrolytes

We have investigated the anodic behaviors of aluminum as a cathodic current collector for lithium ion batteries in several kinds of room temperature ionic liquids (RTILs) and EC + DMC solutions containing LiN(CF₃SO₂)₂ by cyclic voltammetry (CV), chronoamperometry (CA), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry and chronoamperometry data showed that the current density for aluminum foil with the RTIL electrolytes was less than that of aluminum foil in the EC + DMC solutions. Besides, much corrosion pits appeared on the aluminum foil surface after the electrochemical measurement in the EC + DMC solutions, while they were not observed on the aluminum foil with the RTIL electrolytes, suggesting that aluminum current collector was stable in the RTIL electrolytes. Further research by EDX and XPS analysis revealed that a good passivating film composed mainly of the products from the oxidation between aluminum and the anions of the RTIL electrolytes on the aluminum foil surface after the anodic polarization which suppressed the aluminum corrosion.