Metallic Ni Surface Research Articles

Mechanism of the hydrazine hydrate electrooxidation reaction on metallic Ni electrodes in alkaline media as revealed by electrochemical methods, online DEMS and ex situ XPS

Metallic Ni surface was examined as a catalyst for the hydrazine hydrate oxidation reaction (HHOR) by cyclic voltammetry, chronoamperometry, and online differential electrochemical mass-spectrometry (DEMS). It was found that N2 is the sole gaseous product formed during the HHOR on Ni and 4 electrons are transferred in the process, which corresponds to a complete oxidation of the hydrazine molecule. The mechanism of the HHOR on the Ni surface involves OH− and adsorbed OHads intermediate, the latter improving the rate of the HHOR. Ni catalysts were found to be poisoned during the HHOR, the extent of poisoning depending on the number of surface sites, hydrazine concentration, and the α-Ni(OH)2 coverage. Ex situ X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the species poisoning Ni surface.

Alpha-Nickel Hydroxide Coating of Metallic Nickel for Enhanced Alkaline Hydrogen Evolution.

In this work, alkaline hydrogen evolution reaction (HER) processes of three typical nickel-based electrocatalysts [i. e., Ni, α-Ni(OH)2 , and β-Ni(OH)2 ] were investigated to probe critical factors that determine the activity and durability. The HER activity trend was observed as Ni≫α-Ni(OH)2 >β-Ni(OH)2 , likely attributed to a synergy between metallic Ni and Ni(OH)2 components on the Ni surface and fast water dissociation kinetics on the α-Ni(OH)2 surface. With the HER proceeding, the metallic Ni surface, however, gradually became α-Ni(OH)2 , and α-Ni(OH)2 surface ultimately transformed into β-phase, leading to a dramatic activity decrease of Ni electrodes. Therefore, Ni electrodes were coated with α-Ni(OH)2 nanosheets to slow down the nickel hydroxylation and optimize the surface ratio of Ni(OH)2 to metallic Ni. This simple coating procedure enhanced both activity and durability of Ni electrocatalysts.

What Are the Best Active Sites for CO2 Methanation over Ni/CeO2?

We examined active sites for CO₂ methanation over Ni/CeO₂ catalysts prepared by a wet impregnation method. Four types of Ni/CeO₂ with Ni loadings of 1, 3, 5, and 10 wt % were used in this study, assuming that the Ni sites are well dispersed in the catalysts when changing the Ni loading. According to powder X-ray diffraction and scanning transmission electron microscopy, the low-loading catalysts (1 and 3 wt %) consist mainly of Ni–Ce mixed oxides. The results of temperature-programmed reduction by H₂ suggested that the Ni–Ce mixed oxides under reducing conditions contain oxygen vacancies (Ni–Vₒₓ–Ce). Note that the CO₂ conversion rate was proportional to the Ni loading, which probably means that Ni–Vₒₓ–Ce sites on the Ni–Ce mixed oxides are active in CO₂ conversion. In contrast, when the Ni loading was high (5 and 10 wt %), the catalysts possessed many metallic Ni nanoparticles supported on CeO₂ and Ni–Ce mixed oxides. Because the turnover frequencies of CO methanation for 5 and 10 wt % Ni/CeO₂ were identical, the presence of a metallic Ni surface could be essential for activation in CO methanation. We focused on the fact that the CO₂ conversion rate was not related to the number of oxygen vacancies on CeO₂ (Ce–Vₒₓ–Ce) but was related to the number of the Ni–Vₒₓ–Ce sites. Hence, the formation of Ni–Vₒₓ–Ce sites (CO production via the reverse water-gas shift reaction) and the exposure of metallic Ni sites (methanation of the thus-formed CO) are essential for CO₂ methanation. Although it has been known that oxygen vacant sites on Ni/CeO₂ catalysts are important for the catalytic activity, this study suggested anew that there are two types of the sites, Ce–Vₒₓ–Ce and Ni–Vₒₓ–Ce. Furthermore, it was clarified that the latter oxygen defect is important for CO₂ methanation.

On the Electrochemical Reduction of β-Ni(OH)2 to Metallic Nickel

Nickel (Ni) and its (oxy)hydroxide species are an important class of materials that attract significant attention from the scientific community due to their application in alkaline electrochemical energy technologies. Despite their increasing importance, Ni electrochemistry and electrocatalysis are not very well understood. In this article, we contribute to the understanding of the interfacial electrochemical phenomena occurring at polycrystalline Ni electrodes by conducting the first ever systematic analysis of the electrochemical reduction of β-Ni(OH)2. The procedure for the reduction of surface β-Ni(OH)2 is as follows. First, a cyclic voltammetry (CV) measurement is performed in the potential region of β-Ni(OH)2 formation to grow a reproducible hydroxide layer, followed by a linear sweep voltammetry (LSV) measurement conducted between an upper limit potential of Eu = 0.00 V and a lower limit potential (El) ranging between −0.15 V and −0.60 V. The LSV measurement is carried out using very low potential scan rates (s = 0.10, 0.20, 0.50, and 1.0 mV s−1). The analysis of the electrochemical reducibility of the surface β-Ni(OH)2 layer is accomplished by comparing the CV transient of a freshly polished Ni electrode and the CV transient recorded after the LSV experiment that is designed to reduce the β-Ni(OH)2 layer. The influence of different experimental parameters of the above-described procedure is analyzed independently, namely, (i) the lower potential limit of the LSV measurement, (ii) the potential scan rate of the LSV measurement, (iii) the direction of the LSV measurement (anodic or cathodic), (iv) the thickness of the surface layer of β-Ni(OH)2, and (v) the cathodic polarization of the electrode for different polarization times. Our findings demonstrate that, contrary to an existing belief, the reduction of β-Ni(OH)2 can be achieved electrochemically. Features characteristic of the formation and reduction of α-Ni(OH)2 are observed in the CV transients acquired after employing the procedure for the electrochemical reduction of β-Ni(OH)2, indicative that a metallic Ni surface is regenerated. The analysis of the charge density value of the anodic peak in the CV transient also points to the electrochemical reduction of β-Ni(OH)2 to metallic Ni. These findings lead to an updated version of the Bode diagram and might have significant implications for alkaline electrochemical energy technologies using Ni-based materials.

Self-limiting processes in thermal atomic layer etching of nickel by hexafluoroacetylacetone

In thermal atomic layer etching (ALE) of Ni, a thin oxidized Ni layer is removed by a hexafluoroacetylacetone (hfacH) etchant gas at an elevated surface temperature, and etching ceases when a metallic Ni surface appears (self-limiting step). However, atomistic details of the self-limiting step was not well understood. With periodic density-functional-theory calculations, it is found that hfacH molecules barrierlessly adsorb and tend to decompose on a metallic Ni surface, in contrast to the case of a NiO surface, where they can form volatile Ni(hfac)2. Our results clarify the origin of the self-limiting process in the thermal ALE.

Electrochemical study of S and Pb adsorption on Ni in aqueous acidic media

The adsorption competition of lead and sulfur on nickel has been explored in acidic aqueous media at room temperature by examining the electrochemical behavior of Ni. A separate PbO or K2S4O6 inoculation resulted in significant inhibition and activation of Ni corrosion, respectively. The investigation of the combined effects of Pb and S revealed that S mostly dominates over Pb for adsorption on Ni. When added in sequence and in different proportions, the kinetic evidence indicates neither species fully replaces the other on a pure metallic Ni surface, i.e., some permanent effect of Pb was observed even after S was added.

Enhanced Coke Resistance and Antioxidation Stability of γ‐Alumina‐Supported Nickel‐Based Catalysts via Decoration with Lanthanum for Propane Pre‐Reforming

AbstractLa‐doped mesoporous γ‐alumina supported Ni oxides (xLa@NiO/γ‐MA) were prepared through one‐pot hydrolysis of inorganic salts, without the need for surfactants as in conventional practice. The catalysts were studied in the pre‐reforming of propane at a low steam to carbon molar ratio of 0.7. The results showed that the addition of La could promote the uniform dispersion of Ni species with reduced crystallite sizes, improving the catalytic properties. The 10La@Ni/γ‐MA catalyst (10 wt % La) exhibited the highest catalytic activity and excellent stability without carbon deposition for 150 hrs time‐on‐stream at 350 °C. This could be attributed to the presence of ultrafine Ni nanoparticles with an average metal particle size of 2.3 nm. Meanwhile, the doped La can donate electrons to the metallic Ni surface, preventing it from being oxidized. A minimum La/Ni atomic ratio of 1/11 (10 wt % La) was found to maintain the Ni antioxidation stability.

Ni/La2O3 Catalysts for Dry Reforming of Methane: Insights into the Factors Improving the Catalytic Performance

AbstractTo understand the structure‐reactivity relationship of Ni/La2O3, and eventually get more applicable catalysts for DRM, glycine nitrate combustion (GNC), precipitation (PP) and thermal decomposition (TD) methods have been used to prepare La2O3 supports. Although all the supports possess a hexagonal La2O3 phase, their bulk and surface properties are significantly changed. By using them as supports, the interactions between NiO/Ni and La2O3 are varied, thus achieving Ni/La2O3 catalysts with different activity, stability and anti‐coking ability, which follow the order of 5Ni/La2O3‐GNC>5Ni/La2O3‐PP>5Ni/La2O3‐TD. On La2O3 having a higher surface area, a catalyst with a higher active metallic Ni surface area can be achieved. Therefore, the interfaces between Ni and La2O2CO3 can be enlarged, which effectively facilitates the reaction between carbon deposits and the La2O2CO3 formed during the DRM, thus preventing the accumulation of both and keeping the catalyst surface clean, active and stable. In addition, the amount of surface alkaline and active oxygen sites of the reduced catalysts obey the order of 5Ni/La2O3‐GNC>5Ni/La2O3‐PP>5Ni/La2O3‐TD, which is well consistent with the reaction performance. Therefore, these two factors are also believed to be critical to decide the reaction performance. It is concluded that Ni/La2O3 catalysts with high activity, stability and potent anti‐coking ability for DRM can be achieved by preparing catalysts with high Ni dispersion.

Surface titration of supported Ni catalysts by O2-pulse thermal analysis

A new method for determining the Ni0 metal surface area of MgO supported catalysts is proposed. Different series of MgO supported Ni catalysts were prepared and titrated by chemical adsorption of oxygen inside an STA-MS setup. Simultaneous recording of the mass gain upon oxidation, the enthalpy of formation of the oxide and the evolved gas allowed a precise quantification of the metallic Ni surface area. In addition, any uncertainties linked with the complex Ni oxide stoichiometry could be ruled out. A comparison of the newly developed oxygen pulse method with already established techniques using H2 or N2O confirmed the applicability of the method and further revealed its surface sensitivity and accuracy with respect to MgO supported Ni catalysts.

Methane dry reforming over Ni/Mg-Al-O: On the significant promotional effects of rare earth Ce and Nd metal oxides

In this study, with the purpose to obtain more efficient catalysts for methane dry reforming, an Mg-Al-O support having excess MgO and higher alkalinity than pure MgAl2O4 spinel support have been designed and used to prepare Ni/Mg-Al-O catalysts. XRD has demonstrated that this support consists of both MgO and MgAl2O4 spinel phases. As a result, Ni/Mg-Al-O displays better reaction performance than Ni/MgAl2O4 and Ni/Al2O3 catalysts. With the promotion by Ce and Nd oxides, the Ni dispersion and the surface active oxygen amount can be significantly improved, which results in more active and stable catalysts with also significantly improved coking resistance. It is revealed that the active metallic Ni surface area, surface alkalinity and active oxygen abundance are the major factors deciding the reaction performance of the Ni/Mg-Al-O catalysts. Because of the optimal concerted action of these factors, Ni-Ce/Mg-Al-O, the sample promoted by ceria, displays the best reaction performance among all the catalysts.

NiO/Nb2O5/C Hydrazine Electrooxidation Catalysts for Anion Exchange Membrane Fuel Cells

NiO/Nb2O5/C (8:1), (4:1), (2:1), NiO/C, and Ni/C catalysts for hydrazine electrooxidation were synthesized by an evaporation drying method followed by thermal annealing. Prepared catalysts were characterized by X-ray diffraction (XRD), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectrometry (EDS), and X-ray absorption fine structure (XAFS). Catalytic activity, durability, and selectivity in the reaction of hydrazine electrooxidation were evaluated in alkaline media. The highest catalytic activity in mentioned above reaction was found for Ni/C, followed by: NiO/Nb2O5/C (8:1), NiO/Nb2O5/C (4:1). NiO/Nb2O5/C (2:1) whiles NiO/C has almost no activity for hydrazine oxidation. NiO/Nb2O5/C (8:1) and (4:1) had a highest stability during electrooxidation of hydrazine at 60°C. It was explained by oxygen defect of NiO in NiO/Nb2O5/C from XAFS analysis. The selectivity hydrazine electrooxidation as measured by ammonia production resulted in observation that metallic Ni surface facilitates N-N bond breaking of hydrazine, which was confirmed by density functional theory (DFT) calculations.

High-Temperature Stable Ni Nanoparticles for the Dry Reforming of Methane

Dry reforming of methane (DRM) has been studied for many years as an attractive option to produce synthesis gas. However, catalyst deactivation by coking over nonprecious-metal catalysts still remains unresolved. Here, we study the influence of structural and compositional properties of nickel catalysts on the catalytic performance and coking propensity in the DRM. A series of bulk catalysts with different Ni contents was synthesized by calcination of hydrotalcite-like precursors NixMg0.67–xAl0.33(OH)2(CO3)0.17·mH2O prepared by constant-pH coprecipitation. The obtained Ni/MgAl oxide catalysts contain Ni nanoparticles with diameters between 7 and 20 nm. High-resolution transmission electron microscopy (HR-TEM) revealed a nickel aluminate overgrowth on the Ni particles, which could be confirmed by Fourier transform infrared (FTIR) spectroscopy. In particular, catalysts with low Ni contents (5 mol %) exhibit predominantly oxidic surfaces dominated by Ni2+ and additionally some isolated Ni0 sites. These properties, which are determined by the overgrowth, effectively diminish the formation of coke during the DRM, while the activity is preserved. A large (TEM) and dynamic (microcalorimetry) metallic Ni surface at high Ni contents (50 mol %) causes significant coke formation during the DRM.

Highly active and stable Ni/Y2Zr2O7 catalysts for methane steam reforming: On the nature and effective preparation method of the pyrochlore support

In this study, Y2Zr2O7 pyrochlore supports were synthesized by co-precipitation (Y2Zr2O7-CP), glycine-nitrate combustion (Y2Zr2O7-GNC) and hydrothermal (Y2Zr2O7-HT) methods, which were used to support 10% Ni to prepare catalysts for methane steam reforming for hydrogen production. It is revealed that the three supports consist solely of pyrochlore phase, but have different morphologies, pore structures and surface areas. While Y2Zr2O7-HT support possesses the highest surface area, Y2Zr2O7-CP has the lowest one. As supports for Ni, H2-TPR demonstrates that the Ni active species has stronger interaction with Y2Zr2O7-GNC than with other two supports. Therefore, much smaller Ni crystallite sizes with higher metallic active surface area have been achieved. In addition, the surface of this catalyst is enriched with Ni species, as evidenced by the XPS results. As a consequence, Ni/Y2Zr2O7-GNC exhibits the highest activity and stability, and the most potent coke resistance among all the catalysts, for which no evident coking and deactivation was observed even after testing at 800 °C under 20 atm for 200 h. It is concluded that the active metallic Ni surface area is the determining factor for the activity and coke-resistance of the catalyst.

In situ XPS study of self-sustained oscillations in catalytic oxidation of propane over nickel

Self-sustained oscillations in the propane oxidation over a nickel foil were studied in situ with the use of ambient-pressure X-ray photoelectron spectroscopy (XPS) coupled with on-line mass-spectrometry and gas chromatography (GC). Regular oscillations of a relaxation type were observed at 0.5mbar in the temperature range of 600–750°C in oxygen-deficient conditions. CO, CO2, H2, H2O, and propylene were detected as products. CO selectivity in active half-periods achieved 98% decreasing to 40–60% in inactive half-periods. It has been found that the chemical state of the catalyst drastically changes together with the oscillations of the catalytic activity. According to the Ni2p and O1s core-level spectra measured in situ, the active catalyst surface is represented by metallic nickel, whereas it is covered with a layer of NiO with a thickness of at least 3nm during the inactive half-periods. It means that the oscillations in the propane oxidation over nickel are originated from the reversible bulk oxidation of Ni to NiO. We suggest that the propane oxidation over the metallic Ni surface occurs via the Langmuir–Hinshelwood mechanism, whereas the Mars–van Krevelen mechanism prevails when the reaction proceeds over NiO. The switching between the metallic surface and the oxide shows a significant change in the catalytic activity. According to GC measurements the activity of metallic nickel is approximately 40-fold higher than that of NiO.

The influence of B 4C and SiC additions on the morphological, physical, chemical and corrosion properties of Ni coatings

Microhardness, internal stresses, Scanning Electron Microscopy (SEM) and Electrochemical Impedance Spectroscopy (EIS) measurements were used to compare nickel matrix composite coatings — NiB 4C and NiSiC — with a pure nickel coating. The influence of incorporated carbides on Ni electro-co-deposition, the surface morphology, physical and chemical strength and corrosion ability of Ni coatings has been studied. Ni coating properties differ from those of NiSiC and even more from NiB 4C properties. Their microhardness is lower than that of NiSiC coatings but their corrosion resistance is twice higher than that of NiB 4C coating. The correlation between physical properties of the surface of incorporated carbides and coating surface properties exists: due to the conductive B 4C the activity of the surface increased and its stability decreased. As a result, the corrosion resistance ability of hydrophobic metallic Ni surface decreases, hard and corrosion resistant compact matrix is formed with non-conductive SiC particles. High temperature does not change the integrity of Ni composites.

Formation of carbonaceous deposits on metallic Ni from methane-water vapor mixtures

The influence of water vapor on the formation of carbonaceous deposits formed in the course of CH 4-H 2O reaction on metallic Ni was studied at 1000 K. It was found that a very low concentration of water vapor ( x H 2 O = 0.05) increased the quantity of carbon deposits on the metallic Ni-surface. Only with higher contents of water in the reaction gas mixture an inhibitory effect of this component was established. The reaction rate of C-formation varied with contact time showed a minimum. The reaction mechanism of C-deposition on metallic Ni-surface in the course of CH 4-H 2O reaction is proposed.