Hydrogen production by catalytic decomposition of selected hydrocarbons and H2O dissociation over CeZrO2 and Ni/CeZrO2

Presented paper deals with the catalytic decomposition of hydrocarbons (methane and toluene) in the aspect of H₂ production and types of obtained carbon deposits. The catalyst used in our studies was nickel supported on ceria–zirconia (Ni/CeZrO₂). The aim of this work was to investigate the reactivity of obtained carbon deposits with H₂O. Both issues are of great importance for determining the mechanisms of carbon deposits formation and their suppression during steam reforming reaction.     

Experimental study of Ni/CeO2 catalytic properties and performance for hydrogen production in sulfur–iodine cycle

The Ni/CeO₂ catalysts with different calcination temperatures have been tested for hydrogen production in sulfur–iodine (SI or IS) cycle. TG-FTIR, BET, XRD, HRTEM and TPR were performed for catalyst characterization. It was found that the Ni²⁺ ions could be inserted into the ceria lattice. This brought about the strong interaction between Ni and CeO₂ and the generation of oxygen vacancies. Perfect crystallites were formed in the catalysts. It was evident that there was a change in particle size and morphology as the calcination temperature increased from 300 to 900 °C. The Ni/CeO₂ catalysts with different calcination temperatures showed better catalytic activity by comparison with blank yield, especially Ni/Ce700. A hypothetic mechanism of HI catalytic decomposition on Ni/CeO₂ has been constructed. The two important reactive sites were assumed for HI catalytic decomposition.     

SOFC anodes for direct oxidation of hydrogen and methane fuels containing H2S

Single cell solid oxide fuel cells using Ni-YSZ and Co-YSZ anodes were tested in H₂, CH₄, H₂S/H₂ and H₂S/CH₄ fuel mixtures. Their performance was found to quickly degrade in dry CH₄ due to carbon deposition and lifting of the anode from the electrolyte. In contrast, hydrogen or methane containing H₂S showed an increase in exchange current densities when compared to H₂,M/YSZ/LSM,air systems (M = Ni or Co) despite having less optimal anode microstructure. Conversion from metal to metal-sulfide in the presence of H₂S produced large, dense metal-sulfide particles surrounded by YSZ, thus decreasing the triple-phase boundary. Furthermore, CoS-based anodes showed phase segregation and densification toward the electrolyte. Despite this, long-term testing at η_a = 0.5 V of the H₂S/CH₄,CoS-YSZ/YSZ/LSM,air system showed no signs of degradation of the anode over a 6-day period. Only after removal of H₂S from the H₂S/CH₄ stream did the CoS-anode reduce back to Co, signifying H₂S is required to maintain the metal-sulfide active anode.     

Mg/SiO2–Al2O3 supported nickel catalysts for the production of naphthenic hydrocarbon fuel by hydro-de-oxygenation of eugenol

Ni–Mg supported on both mesoporous Al₂O₃–SiO₂ (10) and γ-Al₂O₃ with Al₂O₃–SiO₂/Mg = 10 and γ-Al₂O₃/Mg = 10 ratio were prepared via wet impregnation method. The synthesized materials were characterized by XRD, N₂-sorption study, NH₃-TPD/H₂-TPR, FT-IR, DRS-UV and used in hydrodeoxygenation (HDO) of eugenol to produce hydrocarbons in a high pressure reactor. The reactant molecules could be facilitated to yield hydrocarbon fuel by active nickel active sites. The high oxygen removal efficiency of 75% was achieved with C9 hydrocarbon selectivity. Evaluation of catalytic properties such as high hydrogen consumption of 145 μmol/g (H₂-TPR) and enriched surface acidity of 2.35 mmol g⁻¹ (NH₃-TPD) was done. Ni/Mg-SA (10) exhibited the highest catalytic activity of 6.7 × 10⁻⁴ mol g⁻¹ s⁻¹ SRR and 10.27 s⁻¹ TOF respectively. The stability of catalysts was found to be 185 mg/g for Ni/Mg-SA (10) catalyst by TGA analysis. The structure-activity relationship was studied and the product distributions were also discussed in detail.     

Influence of hydrogen on electrochemical behavior of Ni-based superalloy 718

Numerous studies have shown that Ni-based superalloy 718 may be sensitive to hydrogen embrittlement and have highlighted the dominant roles played by the hydrogen solubility and the hydrogen trapping. Samples were hydrogenated by cathodic polarization in molten salts under different conditions to vary the diffusible hydrogen content and to saturate the different hydrogen traps present in the microstructure strengthened by precipitation. Open circuit potential and galvanic coupling measurements were conducted in order to characterize the effect of diffusible and trapped hydrogen on electrochemical behavior and to discuss the possibility of galvanic coupling between zones with different hydrogen contents.     

Anisotropic electrical and abrasion-sensing properties of cement-based composites containing aligned nickel powder

The electrical properties of aligned nickel powder-filled cement-based composites and the abrasion-sensing properties of these composites, as evaluated based on the change in their resistance at varying abrasion depths, were investigated in this paper. Micrograph characterization of nickel powder distribution indicates that the electrical conduction path preferentially forms along the direction of the nickel powder alignment, which leads to increasingly anisotropic electrical properties under greater magnetic field strengths. The level of anisotropy was also determined to be strongly dependent on the nickel powder content. The maximum anisotropic electrical properties were achieved at the percolation threshold content of the nickel powder, which is the critical point for the formation of an effective conductive network. Based on the cement-based composites anisotropic electrical properties and a sectionalized electrode design, the composites filled with aligned nickel powder demonstrated good abrasion-sensing properties, with humidity and temperature self-compensation abilities.     

Experimental and thermodynamic investigations regarding the effect of chromium on the carbides population in cast {Ni(bal.)-0.4C-6Ta-xCr} alloys with x varying from 0 to 50 wt%

Involving a particularly strong carbide-former metallic element, the tantalum carbides are potentially very stable at elevated temperatures in term of volume fraction and morphology. The TaC phase represents a major strengthening way to allow cast chromium-rich superalloys resisting mechanical stresses at elevated temperatures. They are exploited in recent high performance cobalt-based superalloys but seemingly not in nickel-chromium refractory alloys. Earlier studies showed that the stability of TaC in Ni-Cr alloys is not so good as in the Co-Cr ones, and they evidenced that chromium carbides may compete with TaC in the formation of the carbides population. A possible way to optimize the presence of TaC in Ni-Cr alloy may consist in rating the chromium content to an ideal value but preliminary knowledge about the TaC dependence on the Cr content is compulsory. The aim of this work is precisely the investigation of the effect of the content in chromium on the appearance and stability of the TaC phase in Ni-Cr alloys, by the means of thermodynamic calculations and real experiments in parallel. A global system Ni(bal.)-xCr-0.4C-6Ta compositions (with x varying from 0 to 50 wt%) was chosen. Thermodynamic calculations were performed to know the theoretic metallurgical states inside the considered x range. These theoretic results being dependent on the suitability of the used database, real experiments of verifications were also carried out for a selection of six alloys (x = 0, 10, 20, 30, 40 and 50 wt%). The alloys prepared by respecting these compositions were cast and isothermally exposed at high temperature (1400 and 1510 K), then subjected to metallographic characterization. For the used database the calculated results showed that no TaC should never appear whatever their Cr content, while TaC were really observed in the as-cast and aged versions of the alloys containing 20 wt%Cr and more, but never alone since chromium carbides were systematically also present. When the Cr content in the alloy is too low, the TaC are rare or even no present. This allowed concluding first that the database used for the calculations needs serious improvements, followed by tests with, as first criteria of quality, a good correspondence with the present experimental results. Second, the presence of Cr in quantity high enough is surprisingly compulsory to obtain TaC carbides in quantity high enough, but it is no possible to avoid the appearance of chromium carbides. Obviously, other ways than Cr adjustments must be found to obtain TaC in nickel-based alloys as the single carbide phase and in quantity high enough to achieve high mechanical properties at high temperature.     

Surface spin polarization in Fe(1 1 0) and Ni(1 1 0)

The magnetism of Ni(1 1 0) and Fe(1 1 0) surfaces was investigated by electron capture spectroscopy. He⁺ and He²⁺ ions impinged on the Fe(1 1 0) and Ni(1 1 0) surfaces under grazing incidence, and the degree of polarization of the light emitted by the neutralized projectiles was analyzed. Our measurements show that in Ni(1 1 0) minority electrons have a higher density of states at the Fermi energy than majority electrons, opposed to the Fe(1 1 0) case. From a comparison of our measurements we estimate the ratio between captured minority and majority electrons in Ni(1 1 0) to be similar as the ratio between captured majority and minority electrons in Fe(1 1 0).     

Impact of Ce-Loading on Ni-catalyst supported over La2O3+ZrO2 in methane reforming with CO2

This paper investigated the effect of doping Ni supported catalysts with different ceria loading. The catalysts (5%Ni+x%Ce/La₂O₃+ZrO₂, where x = 0, 1, 2, 2.5, 3, 5) were synthesized via the wet impregnation technique and tested for methane reforming with carbon dioxide at atmospheric pressure, 700 °C and 42, 000 ml/g_cat.h gas hourly space velocity. The fresh catalysts were subjected to different characterization techniques such as X-ray diffraction, Surface area and pore analysis, H₂-temperature programmed reduction, CO₂-temperature programmed desorption and thermogravimetric analysis (TGA). A fine correlation between characterization results and catalytic activity is found. The results of the reactions indicated that 5%Ni/La₂O₃+ZrO₂ has the lowest conversion which increased with the percentage loading of CeO₂ up to 2.5 wt % and then began to decline. This suggests that 2.5 wt % loading is the optimum for CH₄ and CO₂ conversion. This particular catalyst composition has NiO species that could be reduced easily, as well as dense and wide distribution of all type of basic sites with respect to other catalyst system. The used catalysts were again subjected to TGA and RAMAN analysis where the least carbon deposition and the least deactivation factor was observed for 5%Ni+5%Ce/La₂O₃+ZrO₂ catalysts.     

Catalytic hydrolysis of sodium borohydride solution for hydrogen production using thermal plasma synthesized nickel nanoparticles

In the present study nickel nanoparticles were synthesized by thermal plasma route. In this method we obtained highly crystalline almost spherical nanoparticles with maximum number of particles having size around 30–50 nm. These nanoparticles were thoroughly characterized and employed as a catalyst for hydrogen production using hydrolysis of sodium borohydride (NaBH₄). The effect of initial concentration of NaBH₄, pH and temperature of solution on the rate of hydrogen production was investigated. Nickel nanoparticles exhibits first order reaction with respect to NaBH₄ concentration at elevated temperatures. After hydrolysis, the nickel nanoparticles showed presences of B–O and B–OH species on the nickel surface. The catalyst was found to be stable during 5 sequential cycles of test.