Reduction Of NiO Research Articles

Efficient synthesis of ethylene glycol from cellulose over Ni–WO3/SBA-15 catalysts

Ni–WO3/SBA-15 catalysts were prepared by an impregnation way and applied to the hydrogenolysis of cellulose in aqueous solution. The effect of nickel and WO3 loading, catalysts reduction temperature and time on cellulose conversion were investigated. Up to 70.7% ethylene glycol yield was obtained over 3%Ni–15%WO3/SBA-15 catalysts. Several physicochemical methods such as XRD, Raman spectroscopy, NH3-TPD, H2-TPR and XPS were used to determine the character of catalysts. Based on these studies, it was found that there was a strong electronic interaction between NiO and WO3, which not only promoted the reduction of WO3, but restrained the reduction of NiO. Together with the experimental results, it can be concluded that the WO3−X, which may be the active species for the selective cleavage of CC bond, was formed after catalysts pretreatment process. Additionally, a possible catalytic mechanism was proposed.

Nickel catalyst auto-reduction during steam reforming of bio-oil model compound acetic acid

Transition metal catalysts widely used in refineries are provided as oxides and require pre-reduction to become activated. The auto-reduction of a NiO/Al2O3 catalyst with acetic acid (HAc) followed by HAc steam reforming was investigated in a packed bed reactor. Effects of temperature and molar steam to carbon ratio (S/C) on reduction kinetics and catalyst performance were analysed. Results showed that a steady steam reforming regime along with complete NiO reduction could be obtained after a coexistence stage of reduction and reforming. A 2D nucleation and nuclei growth model fitted the NiO auto-reduction. The maximum reduction rate constant was attained at S/C = 2. Steam reforming activity of the auto-reduced catalyst was just below that of the H2-reduced catalyst, probably attributed to denser carbon filament formation and larger loss of active Ni. Despite this, a H2 yield of 76.4% of the equilibrium value and HAc conversion of 88.97% were achieved at 750 °C and S/C = 3.

TPR studies on NiO–CGO composites prepared by combustion synthesis

NiO–ceria composites, which are promising candidates as anodes for intermediate temperature solid oxide fuel cells (IT-SOFCs) were prepared by urea combustion synthesis (UCS) method. The UCS method is, in general, a highly suitable synthesis method for the production, at low temperatures, of fine and reactive powders. By means of XRD and SEM-EDX techniques, the structural, microstructural and compositional behavior of the as-prepared powders has been studied. In addition, temperature programmed reduction (TPR) tests were performed to investigate the reducibility of the composites. After reduction of the NiO–CGO as-prepared compositions, the combustion powders exhibit the presence of Ni, the fluorite CGO solid solution that remains stable and NiO is no longer present. The morphology and size of the nanoparticles and aggregates of the as-prepared powders make them reactive at intermediate temperatures (400–800°C). TPR tests show wide overlapping peaks which are associated with the two primary reduction stages; one is related to the surface NiO reduction mechanism and the other to the coexistence of interactions between the NiO–CGO surface and bulk reduction processes. Further, after TPR measurements the resulting products have high phase stability and reproducibility.

Methane reforming with CO2 using nickel catalysts supported on yttria-doped SBA-15 mesoporous materials via sol–gel process

A series of mesoporous yttrium (Y)-containing SBA-15 (mesoporous silica of Santa Barbara Amorphous type material) prepared using a sol–gel method with various Y/Si molar ratios was investigated as the supporting material of nickel (Ni) catalysts for the methane reforming with CO2. The highly ordered hexagonal structure of SBA-15 was well-retained after the incorporation of yttrium at the molar ratio of Y/Si (0.04). The presence of yttrium in the framework of SBA-15 in Ni catalysts effectively enhanced the formation of the Ni metallic particles with small size. It also significantly promoted the reduction of NiO due to the oxygen vacancies on the surface of the yttrium-containing SBA-15 supports, and the high mobility of the surface oxygen species. All yttrium-containing nickel catalysts were effective for the methane reforming with CO2. The supported Ni catalyst with Y/Si molar ratio of 0.04 exhibited the highest activity, which was due to its highly ordered mesoporous structure, large pore diameter and small metallic metal size.

Reduction of NiO to Ni in Nanocrystalline Composite NiO/Ce0.9Gd0.1O2-δ Porous Thin Films: Microstructure Evolution Through in Situ Impedance Spectroscopy

Microstructure stability and electrical properties of Ni(O)–GDC (Ce0.9Gd0.1O2-δ) porous thin films were studied by both in situ X-ray diffraction and in situ impedance spectroscopy during the reduction of NiO into metallic Ni. The initial microstructure of the NiO/GDC porous thin film is composed of particles with diameter ≤10 nm delimitating a porous volume of ∼40%. The thin films microstructure is composed of two types of interpenetrated network, porous and inorganic. The different steps that take place during the reduction for Ni(O)–GDC porous thin films were highlighted and discussed based on the determination of their respective activation energy. The percolation threshold for metallic Ni appears when the Ni content reached a value of ∼30 vol %. Coarsening and coalescence of nickel particles occur at high temperature leading to a decrease in the total film conductivity with time. Finally, films containing 70 vol % of Ni keep a conductivity of ∼1 × 105 S/cm after heat treatment at 400 °C for 1000 min. This study shows that impedance spectroscopy is a useful tool to follow the reduction of NiO into Ni in GDC mesoporous film. It constitutes an aid for defining the experimental conditions (temperature and reducing time), keeping both good electrical and catalytic performances and for studying the evolution of the microstructure when films are maintained under H2 with time.

A comparative study on the catalytic properties of high Ni-loading Ni/SiO2 and low Ni-loading Ni-Ce/SiO2 for CO methanation

Two Ni-based catalysts of 13%Ni/SiO2 (13Ni/Si) and 7%Ni-2%Ce/SiO2 (7Ni-2Ce/Si, by weight) were prepared by the incipient-wetness impregnation method and characterized with N2-sorption, XRD, H2-TPR, FT-IR, TEM, H2-TPD and CO-TPD techniques. It was shown that addition of Ce promoter generated an interaction among NiO, CeO2 and SiO2, which changed chemical environment of Ni-O-Si bond, enhanced the dispersion and reduction of NiO, and increased the active surface area. In particular, a new type of moderately strong CO adsorption sites was formed on the surface of the 7Ni-2Ce/Si catalyst. As a result, the low Ni-loading 7Ni-2Ce/Si catalyst exhibited higher CO adsorption capacity and CO methanation catalytic activity than the high Ni-loading 13Ni/Si. Under the reaction conditions of 1% CO (volume fraction in H2 atmosphere), GHSV of 7000 h−1 and atmospheric pressure, the temperature for complete conversion of CO over the 7Ni-2Ce/Si catalyst was 230°C, being 30°C lower than that found over the high Ni loading 13Ni/Si catalyst.

Initial reduction of the NiO(100) surface in hydrogen

The reduction of NiO in hydrogen, a reaction with many industrial applications, has not received sufficient attention from theoretical standpoint because the complexity of the material properties and the process present considerable computational challenges. We report here the results of a systematic study on the hydrogen reduction of an ideal NiO(100) surface that produces a water molecule and an NiO(100) surface with an oxygen vacancy, using the Hubbard U corrected density functional theory method, with some of the key results verified by the hybrid density functional method. The major findings are: (1) the O vacancy in the NiO(100) surface slab is stabilized in the subsurface layer, although the vacancy is likely to remain on the outermost surface layer because the barrier for O vacancy migration from the surface to the second layer is as high as 3.02 eV; (2) regarding the energetics of hydrogen interaction with the ideal NiO(100) surface, water formation, and concomitant reduction of NiO is favored at higher H coverage even though surface hydrogenation is energetically more favorable than water formation at lower H coverage; (3) kinetically, the pull-off of the surface oxygen atom and simultaneous activation of the nearby Ni atoms play key roles in hydrogen reduction of NiO(100); and (4) a dual role of hydrogen is revealed as both a reactant and a mediator, which reduces the maximum kinetic barrier from 2.41 eV to 1.86 eV.

Fast Electrochemical Synthesis of NdNi5 Hydrogen Storage Alloy in Molten Salt

Abstract NdNi5 hydrogen storage alloy was prepared by direct electrochemical reduction of a solid mixture of Nd2O3–NiO (atomic Nd:Ni = 1:5) in molten LiCl. The cathodic reaction path of the mixed oxide was proposed; it was initiated by the reduction of NiO and promoted by NiNd2O4. The NdNi5 alloy with 1600 ppm oxygen content was successfully synthesized with 50% current efficiency within 1 h. The measured hydrogen capacity of NdNi5 exhibited 6.1 hydrogen atoms per alloy formula unit.

Reduction Kinetics of Nickel Species Supported on Silica

The reaction mechanism of reduction processes of NiO supported on silica has been clarified by means of the time-resolved dispersive XAFS (DXAFS) technique. The reduction of NiO shows two-phase changes. The first reaction corresponds to the reduction of NiO to the metallic Ni particle, and the sintering of the formed Ni particles occurs in the second step. The reduction process is composed of the surface reduction of the NiO particle and the successive oxygen diffusion from the particle core to the surface. The first-order kinetics suggests that the latter is the rate-determining step. The conditional rate constants indicate that the diffusion of oxygen atoms is relatively faster for the lower Ni loading.

Effect of Indium–Tin-Oxide Schottky Contact on the Resistance Switching of NiO Film

The effect of electrode materials on resistance switching was evaluated on the Pt/NiO/electrode (EL) structures where the EL contacts were Pt, Al, and indium–tin-oxide (ITO). It was confirmed that ohmic Pt contact needs to induce the effective electric field for resistance switching across the NiO film. For the Pt/NiO/Al structure, the barrier height of the Al Schottky contact was measured as 0.66 eV and no resistance switching was observed owing to a large voltage drop at the rectifying interface induced by the reduction of NiO resulting from the formation of Al oxide. In the ITO (EL)/NiO/Pt structure, the barrier height of the Schottky contact between ITO and NiO was about 0.52 eV and it did not show any resistance switching, either. Through the depth-profile study by X-ray photoelectron spectroscopy, chemical reactions at the interface ITO/NiO was identified to be not too much evolved compared with that of NiO/Al, which might due to be abundant oxygen on the ITO surface. Such Schottky barrier heights 0.52–0.66 eV were considered too high to induce a sufficient electric field in the NiO film causing the resistance switching.

CH4–CO2 reforming over Ni-substituted barium hexaaluminate catalysts

A series of Ni-substituted barium hexaaluminate catalysts, Ba0.75NiyAl12−yO19−δ (y=0.4, 0.6 and 1.0), were tested for CO2 reforming of CH4 at temperatures between 200 and 900°C. Temperature programmed surface reaction results show that the reaction lights-off in a temperature range between 448 and 503°C with a consistent decrease in light-off temperature with increasing Ni substitution. Isothermal runs performed at 900°C show near equilibrium conversion and stable product concentrations for 18h on all catalysts. Temperature programmed oxidation of the used catalysts show that the amount of carbon deposited on the catalyst increases with Ni substitution. High resolution XRD of the used Ba0.75Ni0.4Al11.6O19−δ catalyst shows a statistically significant contraction of the unit cell which is the result of NiO reduction from the lattice. XRD of the used catalyst also confirms the presence of graphitic carbon. XPS and ICP measurements of the as prepared catalysts show that lower levels of Ni substitution result in an increasing proportion of Ba at the surface.

Electrochemical reduction of NiO in a composite electrode

Electrochemical reduction of NiO in a composite electrode along with 8mol% Y2O3 stabilized zirconia (8YSZ) is studied. Voltage sweeps are performed on cells with a counter electrode made from Pt or the composite of (La0.75Sr0.25)0.95MnO3±δ (LSM25) and 8YSZ, but the composite electrode gives a lower overpotential. Microstructures of NiO with different reduction degrees are shown. Electrochemical impedance spectroscopy is carried out during the reduction process. Electrochemical reduction of NiO may need an induction period. When NiO is reduced at a constant voltage the current initially increases, hereafter a decay is observed. The kinetics of electrochemical reduction of NiO can be described by the Avrami equation. In performed experiments the exponent of the Avrami equation is in the range of 0.5–0.7 and the overall rate constant varies from 1.19 to 7.73×10−3 and increases with temperature and in particular with the applied voltage. The maximum reaction rate is obtained when 2–11% NiO is reduced. The increasing resistance in NiO reduction can be exclusively ascribed to one arc in impedance spectra.

Nanocrystalline, mesoporous NiO/Ce0.9Gd0.1O2−δ thin films with tuned microstructures and electrical properties: in situ characterization of electrical responses during the reduction of NiO

Crystalline, mesoporous NiO/GDC thin films with thicknesses ranging from 50 nm to 250 nm were synthesized through templated sol–gel chemistry coupled with the dip-coating process and heat-treatment in air. The thin films' microstructure is composed of two interpenetrated networks made of mesopores and inorganic components. Efficient coupling between the temperature and the NiO volume fraction (vol%) allows tuning of both the pore size with dimensions ranging from macro- to meso-size and the NiO or GDC crystallite size with a diameter below 10 nm. X-ray diffraction and impedance spectroscopy performed in 10% H2 in Ar allowed the in situ study of the reduction process of NiO to metallic Ni. Coalescence of Ni particles generated by the reduction step and percolation phenomena controls the resulting conductivity of the final materials. Thin films with an initial content of 50 vol% of NiO exhibit lower electrical properties compared to those with an initial content of 70 vol% NiO. Electrical properties in a reducing atmosphere were also studied as a function of microstructure such as the pore dimension and the thickness of the pore wall. Excellent electrical properties are obtained for Ni70/GDC30-porous thin films synthesized with the block-co-polymer PS40-PEO36 that have a final conductivity of 9 x 104 S m−1 at 500 °C. These mesostructured nanocomposite thin films exhibit a connected pore network that ensures good gas diffusion and good particle–particle contact for GDC and Ni, which gives satisfactory electrical properties. These films have all the attributes to be used as anodes in micro-SOFCs.

Novel Process for Solid State Reduction of Metal Oxides and Hydroxides

Recently the reductive expansion synthesis (RES) method was introduced as a means to create nano- and sub-micron metal particles and alloys by rapid heating of physical mixtures of urea with a metal nitrate. In the present work the generality of the RES method was demonstrated by creating metal micron and sub-micron particles from oxide and hydroxide precursors, and outlining the impact of temperature, precursor ratio, and gas flow rate on the product. For example, precursor selection impacted the temperature required for complete reduction, the amount of carbon present, and the size of the metal particles. For complete NiO reduction to micron scale particles, high urea content and a high temperature [ca. 1073 K (800 °C)] were required. In contrast, Ni(OH)2 was reduced to metal at far lower temperatures. Moreover, the Ni particles formed from NiOH were sub-micron (ca. 200 nm) in size and carbon encapsulated. Other parameter variations had a similarly significant impact. Indeed, the reciprocal relationship between inert gas flow rate and the extent of reduction supports the supposition that the primary mechanism of reduced metal particle formation is the reduction of metal oxide particles by gases produced by urea decomposition. Collectively these and other findings indicate the RES method can be manipulated to create a range of micron and sub-micron reduced metal particle architectures appropriate for different applications.

Nickel–Iron Anode Substrate for Smart Solid Oxide Fuel Cells with a Self‐Protecting Function Against Reoxidation

AbstractDevelopment of highly reliable solid oxide fuel cells (SOFCs) is strongly requested, and the introduction of a self‐protecting function is an ideal approach to increase the reliability of SOFCs. A highly porous (>33%) Ni–Fe metal substrate, which has well‐developed nanopores, is prepared by reduction of NiO–Fe2O3. In an oxidizing atmosphere, a thin layer of Fe2O3 forms on the surface of the substrate. As a result, the porous morphology changes at the surface and becomes denser. This morphological change occurs only at the surface and prevents oxidation of Ni in the bulk of the substrate. Furthermore, the surface morphology returns to its original state following reduction. Therefore, despite the fact that Ni is readily oxidized, Ni metal phase is sustained in the Ni–Fe bimetallic alloy substrate even after 480 h oxidation in air. The cell power density is also stably sustained after a few reduction–reoxidation cycles. Here, we report that Ni–Fe bimetal alloy substrate exhibits a self‐protecting function against reoxidation of the substrate, which would otherwise lead to a permanent failure of the cell.

Toward a general theory of heterogeneous reactions

The thermochemical approach, using the quasi-equilibrium methodology of kinetic analysis and the concepts of equimolar and isobaric reaction modes (regimes), has been used to prove the common mechanism of the representative heterogeneous reactions: decompositions of CaCO3 and of Ag2O, reduction of NiO by H2, and the catalytic oxidation of CO and H2 on PtO2. All these rate processes have been analyzed using the congruent dissociative vaporization of the participating solid reactants or catalysts and are described by similar mechanistic schemes. The possible role of atomic oxygen evolution in oxidation catalysis is discussed. Quantitative proofs of the common mechanism of these reactions were provided by agreement between calculated reaction enthalpies and the Arrhenius E parameters, the retardation effect of gaseous products on the reaction rates, and the accordance between experimental and theoretical ratios of isobaric to equimolar values of the E parameter. In conclusion, milestones in the history of the thermochemical approach during the last century are discussed.

Nickel oxide redox processes with oxide ion conductor-supported nickel oxide in dry and humidified methane: Effect of oxide ion conductors on induction period in nickel oxide reduction and subsequent hydrogen production

NiO redox processes and hydrogen production from dry and humidified methane were investigated by chemical looping combustion (CLC) and chemical looping reforming (CLR) using NiO as an oxygen carrier. To achieve a lower operating temperature in the NiO reduction process, oxide ion conductors such as gadolinia doped ceria (GDC) and strontium doped lanthanum aluminate (LSA) were investigated as the support materials of NiO. Hydrogen rich gas products were obtained by the CLC and CLR processes but a prolonged induction period for the NiO reduction by methane (i.e., induction period for CO2 formation from methane) was observed at relatively low operating temperatures in both processes. NiO/GDC and NiO/LSA lead to a decrease in the induction period of NiO reduction by comparison with the reference NiO/ZrO2. Upon repeating the NiO reduction–oxidation cycles, we also found that the induction period for NiO/GDC became shorter with an increase in the redox cycle number. However, for NiO/LSA this time became longer, which may result from the strong interaction between Ni and the support materials. This NiO reduction behavior on the oxide ion conductors is discussed together with the autocatalytic reaction mechanism for NiO reduction, which provides new insights into the efficient use of lattice oxygen in CLC and CLR.

Microstructure Modification of NiO/YSZ Anode in Solid Oxide Fuel Cell by Particle Size Gradation of YSZ

The idealized microstructure of NiO/YSZ anode of solid oxide fuel cell (SOFC) is that YSZ structures the bone of the anode and NiO is segregated by YSZ to keep Ni from congregation. To obtain this microstructure, the mixture of coarse YSZ and fine YSZ was used. The effect of the particle size gradation of YSZ on the microstructure of anode was studied. The results show that the homogenous microstructure can be obtained when the mass ratio of the coarse YSZ (d50＝3.38µm) to the fine YSZ (d50＝0.4µm) is 7:3. The pre-sintering temperature has obvious effect on the porosity of the NiO/YSZ. The bending strength of NiO/YSZ and Ni/YSZ are 123MPa and 85MPa, respectively. The reduction of NiO to Ni has less effect on the bending strength for 40vol%Ni/YSZ (7/3).

Direct and Highly Efficient Reduction of NiO into Ni with Cellulose under Hydrothermal Conditions

A direct reduction of NiO into Ni was investigated by using cellulose as a reducing agent under mild hydrothermal conditions. The results showed that NiO can be reduced completely to Ni at 250 °C in the presence of NaOH. Meanwhile, cellulose was converted into value-added chemicals, such as lactic acid and acetic acid. A possible mechanism for reduction of NiO into Ni with cellulose under alkaline hydrothermal conditions was proposed. The method provides a comparative green and efficient conversion of NiO into Ni without an expensive and toxic reducing agent at low temperatures.

Pt-promoted α-Al2O3-supported Ni catalysts: Effect of preparation conditions on oxi-reduction and catalytic properties for hydrogen production by steam reforming of methane

The effect of Pt addition on the oxi-reduction properties of α-Al2O3-supported Ni catalysts, with different degrees of interaction between NiO and the α-Al2O3 support, was studied using atmospheres of H2, H2/H2O, and CH4/H2O. The effect of Pt promotion on the reduction of NiO with H2 was significant for NiO species that interacted more strongly with the alumina surface, but was much lower when a NiAl2O4-like bulk phase was formed. For samples activated with H2, although metal dispersion decreased with increasing Pt content, the activity was maintained constant by the presence of Pt sites. For samples activated with a CH4/H2O mixture, the activity increased with increasing Pt content, due to the higher reducibility of Ni in the Pt-promoted catalysts. The Pt promotion effect was stable; there was no important decrease in the influence of Pt on NiO reduction, even after high temperature re-oxidation of the catalysts.