Synthesis and hydrogen storage properties of carbon nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapor decomposition of acetylene over rare-earth (RE) based _AB2${\mathrm{AB}}_2$ alloy hydride catalysts. The _AB2${\mathrm{AB}}_2$ alloy hydride catalysts have been prepared by hydrogen decrepitation technique and characterized by scanning electron microscopy. The advantage of this novel method of obtaining catalysts has been discussed. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermo gravimetric analysis and Raman spectroscopy. Hydrogen adsorption measurements were carried out on as-prepared and purified MWNT in the temperature range of 143–373 K and pressure range of 10–100 bar using a high pressure hydrogen adsorption setup and the results have been discussed. A maximum hydrogen storage capacity of 3.5 wt% is obtained for purified MWNT prepared with _DyNi2${\mathrm{DyNi}}_2$ alloy hydride catalyst at 143 K and 75 bar.     

Hydrogen storage capacity at high pressure of raw and purified single wall carbon nanotubes produced with a solar reactor

Samples of single wall carbon nanotubes (SWNTs) were prepared using a solar reactor. Graphite targets containing different catalysts (Ni/Co, Ni/Y, Ni/Ce) allowed the synthesis of SWNTs soot in which nanotubes had different diameter distributions. Several consecutive stages of HCl treatment and thermal oxidation in air (HCl protocol) purified the samples. Another protocol involving HNO₃ treatment and H₂O₂ oxidation (HNO₃ protocol) was also used. Isotherms of hydrogen adsorption were volumetrically measured at 253 K under pressures below 6 MPa on raw and treated samples. The highest adsorption capacity (0.7  wt%) was measured on raw soot. HCl protocol clearly increases the BET surface area (_SBET)$(S_{\mathrm{BET}})$ and the microporous volume (_W0(_N2))$(W_0({\mathrm{N}}_2))$ measured by N₂ at 77 K of the treated samples with respect to the as-produced materials, whereas HNO₃ protocol decreases them. A correlation between textural properties and hydrogen storage capacities is discussed.     

Hydrogen release from hydrolysis of borazane on Pt- and Ni-based alloy catalysts

Two types of Pt- and Ni-based alloy catalysts were synthesized and comparatively tested for hydrogen generation from aqueous borazane (ammonia- borane, _BH3NH3)${\mathrm{BH}}_3{\mathrm{NH}}_3)$ solution. The experimental results demonstrated that hydrogen release rates from some of the Pt alloys such as PtRu and PtAu are nearly 9 times higher than those from pure Pt surface, and similarly, most of the Ni alloy catalysts exhibit greatly enhanced catalytic activities than pure Ni catalyst. Particularly, hydrogen release from NiAg-catalyzed _BH3NH3${\mathrm{BH}}_3{\mathrm{NH}}_3$ hydrolysis can complete quickly at room temperature showing a stable hydrogen yield at _H2/_BH3NH3ratio=2.9${\mathrm{H}}_2/{\mathrm{BH}}_3{\mathrm{NH}}_3\mathrm{ratio}=2.9$ (molar ratio), corresponding to 8.7 wt % hydrogen release. Since the Ni alloy catalysts are less costly and highly efficient, it is feasible to use the Ni alloy catalysts for practical hydrogen generation in portable applications.     

Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α$\alpha$-phase and β$\beta$-_Mg2Ni${\mathrm{Mg}}_2\mathrm{Ni}$. It is assumed that the lamellar eutectic structure enhances hydrogenation properties.     

Selective CO oxidation with real methanol reformate over monolithic Pt group catalysts: PEMFC applications

Alumina supported Pt group metal monolithic catalysts were investigated for selective oxidation of CO in hydrogen-rich methanol reforming gas for proton exchange membrane fuel cell (PEMFC) applications. The results are described and discussed in the present paper and show that Pt/γ_Al2O3$\mathrm{Pt}/\gamma {\mathrm{Al}}_2{\mathrm{O}}_3$ was the most promising candidate to selectively oxidize CO from an amount of about 1 vol% to less than 100 ppm. We have investigated the effect of the O₂ to CO feed ratio, the feed concentration of CO, the presence of H₂O and/or CO₂, and the space velocity on the activity, selectivity and stability of Pt/Al₂O₃ monolithic catalysts. Afterwards, the Pt/Al₂O₃ catalyst was scaled up and applied in 5 kW hydrogen producing systems based on methanol steam reforming and autothermal reforming. The hydrogen produced was then used as fuel for an integrated PEMFC.     

Catalytic steam reforming of acetic acid for hydrogen production

A variety of supported metal catalysts were tested under conditions of steam reforming of acetic acid (HAc), which was selected as a model compound for pyrolysis oil. The influence of several parameters on catalytic activity and selectivity were examined, including catalyst composition, i.e. nature of the metal and the carrier, reaction temperature and time on stream. The metallic phase of such catalysts was comprised of various metals, such as Pt, Pd, Rh, Ru and Ni, which were supported on metal oxides carriers, such as _Al2O3${\mathrm{Al}}_2{\mathrm{O}}_3$, _La2O3/_Al2O3${\mathrm{La}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$, MgO/_Al2O3$\mathrm{MgO}/{\mathrm{Al}}_2{\mathrm{O}}_3$ and _CeO2/_Al2O3${\mathrm{CeO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$. It was found that Ni-based and Ru-based catalysts present high activity and selectivity toward hydrogen production. Ru catalysts supported on _La2O3/_Al2O3${\mathrm{La}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$ and MgO/_Al2O3$\mathrm{MgO}/{\mathrm{Al}}_2{\mathrm{O}}_3$ carriers, showed good long-term stability as a function of time on stream. However, Ni catalysts were not as stable as Ru catalysts. The amount of carbon deposited on each catalyst was estimated, and it was found that it depends strongly on the nature of the catalyst.     

Hydrogen generation from liquid phase catalytic reforming of sugar solutions using metal-supported catalysts

Most of the hydrogen production processes are designed for large-scale industrial uses and are not suitable for a compact hydrogen device to be used in systems like solid polymer fuel cells. Integrating the reaction step, the gas purification and the heat supply can lead to small-scale hydrogen production systems. The aim of this research is to study the influence of several reaction parameters on hydrogen production using liquid phase reforming of sugar solution over Pt, Pd, and Ni supported on nanostructured supports. It was found that the desired catalytic pathway for _H2${\mathrm{H}}_2$ production involves cleavage of C–C, C–H and O–H bonds that adsorb on the catalyst surface. Thus a good catalyst for production of _H2${\mathrm{H}}_2$ by liquid-phase reforming must facilitate C–C bond cleavage and promote removal of adsorbed CO species by the water–gas shift reaction, but the catalyst must not facilitate C–O bond cleavage and hydrogenation of CO or _CO2${\mathrm{CO}}_2$. Apart from studying various catalysts, a commercial Pt/γ$\gamma$-alumina catalyst was used to study the effect of temperature at three different temperatures of 458, 473 and 493 K. Some of the spent catalysts were characterised using TGA, SEM and XRD to study coke deposition. The amorphous and organised form of coke was found on the surface of the catalyst.     

A review on reforming bio-ethanol for hydrogen production

Bio-ethanol is a prosperous renewable energy carrier mainly produced from biomass fermentation. Reforming of bio-ethanol provides a promising method for hydrogen production from renewable resources. Besides operating conditions, the use of catalysts plays a crucial role in hydrogen production through ethanol reforming. Rh and Ni are so far the best and the most commonly used catalysts for ethanol steam reforming towards hydrogen production. The selection of proper support for catalyst and the methods of catalyst preparation significantly affect the activity of catalysts. In terms of hydrogen production and long-term stability, MgO, ZnO, _CeO2${\mathrm{CeO}}_2$, and _La2O3${\mathrm{La}}_2{\mathrm{O}}_3$ are suitable supports for Rh and Ni due to their basic characteristics, which favor ethanol dehydrogenation but inhibit dehydration. As Rh and Ni are inactive for water gas shift reaction (WGSR), the development of bimetallic catalysts, alloy catalysts, and double-bed reactors is promising to enhance hydrogen production and long-term catalyst stability. Autothermal reforming of bio-ethanol has the advantages of lesser external heat input and long-term stability. Its overall efficiency needs to be further enhanced, as part of the ethanol feedstock is used to provide low-grade thermal energy. Development of millisecond-contact time reactor provides a low-cost and effective way to reform bio-ethanol and hydrocarbons for fuel upgrading. Despite its early R&D stage, bio-ethanol reforming for hydrogen production shows promises for its future fuel cell applications.     

Synthesis and hydrogenation properties of Mg–La–Ni–H system by reactive mechanical alloying

We have synthesized Mg–30 mass%LaNi_2.28 composite material and investigated its hydrogenation behaviour. The reactive mechanical alloying process of the mixture of Mg and LaNi_2.28 was studied. It is found that a composite of _MgH2${\mathrm{MgH}}_2$, _La4H12.9${\mathrm{La}}_4{\mathrm{H}}_{12.9}$ and _Mg2NiH4${\mathrm{Mg}}_2{\mathrm{NiH}}_4$ formed after 80 h ball-milling under 3.0 MPa hydrogen. Scanning electron microscopic analysis indicated that these new phases are distributed homogeneously. This composite shows excellent hydriding properties even at moderate temperature. Under 3.0 MPa hydrogen pressure it absorbed more than 80% of its full capacity in the temperature range of 473–553 K within less than 1 min. The maximum hydrogen absorption capacity at 553 K is 5.4 mass%. The enhanced hydriding properties could be attributed to the fine and uniform particles and a synergeticly catalytic effect generated by mechanical milling.     

Effects of the ambient temperature and initial diameter in droplet combustion

Results are presented from transient numerical simulations of combustion of an isolated heptane (n-_C7H16${\mathrm{C}}_7{\mathrm{H}}_{16}$) droplet in nearly quiescent ambient air. The focus is on the effects of environment temperature and droplet size on the combustion characteristics. Droplet sizes range from 10 μm to 1 mm, environment temperatures range from 800 K to 1600 K, and pressure is 1 atm.     

Hydrogen production via hydrolysis reaction from ball-milled Mg-based materials

Hydrogen generation by hydrolysis of Mg and MgH₂${}_2$ has been investigated in pure water and 1 M KCl. It has been found that hydrolysis reaction of Mg and Mg–Ni composite, both obtained by high-energy ball milling, is faster and extensive when they are immersed in 1 M KCl. In contrast, milled Mg and Mg–Ni composite in pure water, MgH₂${}_2$ and MgH₂${}_2$–Ni composites in pure water and in 1 M KCl show low yield and reactivity. Hydrolysis kinetics and yield are maximum with Mg–10 at% Ni composite milled for 30 min, so reaction is fully completed within an hour in the presence of chloride ions. It is related to the creation of micro-galvanic cells between Mg and dispersed Ni elements, accentuating greatly Mg corrosion in highly conductive aqueous media. A significant increase of the H₂${}_2$ production is also observed with 30 min milled Mg sample, likely because of the accentuation in the pitting corrosion resulting from the creation of numerous defects and fresh surfaces through the milling process. On the other hand, intensive ball milling of pure magnesium has no effect on the Mg reactivity in pure water. Ball milling effect is likely masked by the significant Mg passivation in pure water. A correlation is established between the conversion yield of ball-milled MgH₂${}_2$ powder in pure water and its effective surface area, which is increased by the milling process. Ni addition has no effect on the hydrolysis reaction in nonconductive media (i.e. pure water) and with nonconductive material (i.e. MgH₂${}_2$).     

Application of nickel–lanthanum composite oxide on the steam reforming of ethanol to produce hydrogen

Nickel–lanthanum composite oxides, LaNiO_x, were used for steam reforming of ethanol (SRE). The composite oxides (with 3:1, 1:1, 1:3 M ratios, assigned as 3La–1Ni, 1La–1Ni, and 1La–3Ni, respectively) were prepared by co-precipitation-oxidation (PO) and assisted with ultrasonic irradiation (240 W). Meanwhile, the as-prepared 1La–1Ni sample was further calcined at 300 and 700 °C for 2 h (assigned as C300 and C700). All samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and temperature programmed reduction (TPR). Catalytic activities towards the SRE reaction were tested from 300 to 450 °C in a fixed-bed reactor. The study focused on deriving optimized composition of composites and compared the effect on the reduction pretreatment under 200 °C [assigned with (H)]. The results indicated that the ethanol conversion reached completion around 325 °C for the 1La–1Ni(H) sample while it required 400 °C for the 1La–1Ni sample with a minor CO distribution for both samples under an H₂O/EtOH molar ratio of 13 and 22,000 h⁻¹ GHSV. The yield of hydrogen (YH₂$Y_{{\text{H}}_2}$) approached 5.2 around 375 °C for the 1La–1Ni(H) sample.     

Ti–Ni alloys as MH electrodes in Ni–MH accumulators

In hydrogen solid–gas reaction at 300 K and 1 bar, the hydrogen content for Ti_3.87Ni_1.73Fe_0.7O_x (0.2≤ × ≤0.8) alloys was in range 1.93–0.05 (Cwt.H,%), and discharge capacity of 360–235 A h/kg was achieved accordingly. The ΔHH₂$\Delta H_{{\text{H}}_2}$ and ΔSH₂$\Delta S_{{\text{H}}_2}$ values of −32.29 kJ mol⁻¹ and −111.04 J mol⁻¹ K⁻¹, respectively, for Ti_3.87Ni_1.73Fe_0.7O_0.5 alloy were obtained using experimental PCT relations, where hysteresis effect was only slightly visible. The half-cell potentials (vs. Hg/HgO) of metal hydride (MH) electrodes based on Ti_3.87Ni_1.73Fe_0.7O_x (0.2≤ ×≤ 0.8) alloys were calculated.