Simultaneous measurements of thermal conductivity and thermal diffusivity of insulators,fluids and conductors using the transient plane source (TPS) technique

Simultaneous measurements of thermal conductivity and thermal diffusivity of composite red-sand bricks, glycerine and mercury have been made at room temperature by the recently developed transient plane source (TPS) technique. This paper describes, in brief, the theory and the experimental conditions for the simultaneous measurements of thermal conductivity and thermal diffusivity of insulators, fluids and metals. The source of heat is a hot disc made out of bifilar spirals. The disc also serves as a sensor of temperature increase in the sample. The measured values of the thermal conductivity and thermal diffusivity of these samples are in agreement with the values reported earlier using other methods. The advantage of the TPS technique is the simplicity of the equipment, simultaneous information on thermal conductivity and thermal diffusivity, and also the applicability of the technique to insulators, fluids and metals.     

Temperature dependence of thermal conductivity and thermal diffusivity of some composites using the transient plane source technique

The recently developed transient plane source (TPS) technique has been applied for the simultaneous measurement of thermal conductivity and thermal diffusivity of two composite materials namely, marble and magnesium oxychloride cement in the range of temperatures from 30 to 150°C. The experimental results of these samples show that there is very slight variation in thermal conductivity and thermal diffusivity of these materials in this range of temperature. An effort has been made to express this variation of thermal conductivity and diffusivity with temperature by a linear relation, in these materials.     

Improved hydrogen desorption properties of ammonia borane by Ni-modified metal-organic frameworks

Ammonia borane (AB) has attracted intensive study because of its low molecular weight and abnormally high gravimetric hydrogen capacity. However, the slow kinetics, irreversibility, and formation of volatile materials (borazine and ammonia) of AB limit its practical application. In this paper, new strategies by doping AB in metal-organic framework MIL-101 (denoted as AB/MIL-101) or in Ni modified MIL-101 (denoted as AB/Ni@MIL-101) are developed for hydrogen storage. In AB/MIL-101 samples, dehydrogenation did not present any induction period and undesirable by-product borazine, and decomposition thermodynamics and kinetics are improved. For AB/Ni@MIL-101, the peak temperature of AB dehydrogenation was shifted to 75 °C, which is the first report of such a big decrease (40 °C) in the decomposition temperature of AB. Furthermore, borazine and ammonia emissions that are harmful for proton exchange membrane fuel cells, were not detected. The interaction between AB and MIL-101 is discussed based on both theoretical calculations and experiments. Results show that Cr-N and B-O bonds have generated in AB/MIL-101 nanocomposites, and the decomposition mechanism of AB has changed.     

High and rapid hydrogen release from thermolysis of ammonia borane near PEM fuel cell operating temperatures

Ammonia borane (NH₃BH₃, AB), containing 19.6 wt % hydrogen, is a promising hydrogen storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles. Our experiments demonstrate the highest H₂ yield (∼14 wt %, 2.15 H₂ equivalent) values obtained by neat AB thermolysis near PEM FC operating temperatures, along with rapid kinetics, without the use of either catalyst or additives. It was also found that only trace amount of ammonia (<10 ppm) is produced during dehydrogenation reaction and spent AB products are polyborazylene-like species, which can be efficiently regenerated using currently demonstrated methods. The results indicate that our proposed method is the most promising one available in the literature to-date for hydrogen storage, and could be used in PEM FC based vehicle applications.     

Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

Ammonia borane (AB, NH₃BH₃) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H₂ yield (11.5 wt% overall H₂ yield, 2.23 H₂ equivalent) at 85 °C, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH₃ (20–30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.     

Improving hydrogen production from the hydrolysis of ammonia borane by using multifunctional catalysts

A novel multifunctional catalytic system has been developed for efficient hydrogen generation through the hydrolysis of ammonia borane. This system combines Pd NPs with acid sites and amines, which are both task-specific functionalities able to destabilize the N → B dative bond. The acidity of the support (zeolites of different structure and SiO₂/Al₂O₃ ratio) used to disperse the Pd NPs causes an increase in the hydrogen production rate. However, the positive effect of incorporating p-phenylenediamine in the catalyst is much more pronounced, causing a two-fold increase in the activity of the catalyst. The combined effect of the different functionalities yields excellent performance in the hydrolysis of ammonia borane, greatly enhancing the activity of the metal-based catalyst and reducing the activation energy of the catalyzed reaction.     

High and rapid hydrogen release from thermolysis of ammonia borane near PEM fuel cell operating temperatures: Effect of quartz wool

Ammonia borane (NH₃BH₃, AB), containing 19.6 wt% hydrogen, is a promising hydrogen storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles. We recently demonstrated that using quartz wool, the highest H₂ yield (2.1–2.3H₂ equivalent) values were obtained by neat AB thermolysis near PEM FC operating temperatures, along with rapid kinetics, without the use of either catalyst or chemical additives. It was found that quartz wool minimizes sample expansion and facilitates the production of diamoniate of diborane (DADB), which is a key intermediate for the release of hydrogen from AB. It was also found that only trace amount of ammonia (<10 ppm) is produced during dehydrogenation reaction and spent AB products are found to be polyborazylene-like species, which can be efficiently regenerated using currently demonstrated methods. The results indicate that our proposed method is the most promising one available in the literature to-date for hydrogen storage, and could be used in PEM FC based vehicle applications.     

Water-promoted,safe and scalable preparation of ammonia borane

Ammonia borane (AB) has been synthesized in very high purity (>99%) from sodium borohydride and ammonium sulfate under ambient conditions in tetrahydrofuran (THF) at 1 M concentration using water as the promoter. Less than quantitative yields of AB are obtained due to a competing hydrolysis of the primary metathesis product ammonium borohydride. The solvent THF can be substituted with a greener alternative, 2-methylTHF, with no loss in yield or purity and complete recyclability.     

Temperature dependence of thermal conductivities and thermal diffusivities of composites using transient hot-strip method

Simultaneous measurements of the thermal conductivity and thermal diffusivity of four composite building materials, namely black marble stone, normal building stone, red sand brick and cement brick have been made at room temperature using the transient hot-strip method. The power supplied to these samples is quite large compared with that for loose building materials owing to their composite (brick) nature. The temperature dependence of thermal conductivity and thermal diffusivity of these materials has also been investigated in a temperature range from 10 to 60°C. Temperature stability is assured within 0.1°C by using a platinum resistance thermometer in the chamber containing samples and a digital voltmeter to record any variation in resistance. The results of these experiments show that there is a very slight variation in thermal conductivity and thermal diffusivity of these materials in this temperature range.     

Study of kinetics and thermal decomposition of ammonia borane in presence of silicon nanoparticles

Ammonia borane (AB, NH₃BH₃) is a promising material by virtue of its high gravimetric hydrogen storage capacity of 19.6 wt%. Hydrogen release from AB initiates at around 100 °C and as such is compatible to meet the present-day requirements of a PEM fuel cell. The thermal decomposition of AB is a complex process involving several reactions. Major issues include poor reaction kinetics, leading to delayed commencement of hydrogen generation i.e. long induction period, and the small amount of hydrogen released at optimal temperature. In the current paper the thermal decomposition of AB is studied at different temperatures. Further the effect of Si nanoparticles on the induction period and kinetics as well as the gas release reaction is studied in detail using different characterization techniques. It was found that the induction period reduced and the amount of gas released increased as a result of Si nanoparticle addition. This was facilitated by a reduction in the activation energy of decomposition and improved kinetics with the addition of silicon nanoparticles.     

Hydrogen release by thermolysis of ammonia borane NH3BH3 and then hydrolysis of its by-product [BNHx]

Ammonia borane (AB) is one of the most attractive hydrides owing to its high hydrogen density (19.5 wt%). Stored hydrogen can be released by thermolysis or catalyzed hydrolysis, both routes having advantages and issues. The present study has envisaged for the first time the combination of thermolysis and hydrolysis, AB being first thermolyzed and then the solid by-product believed to be polyaminoborane [NH₂BH₂]_n (PAB) being hydrolyzed. Herein we report that: (i) the combination is feasible, (ii) PAB hydrolyzes in the presence of a metal catalyst (Ru) at 40 °C, (iii) a total of 3 equiv. H₂ is released from AB and PAB-H₂O, (iv) high hydrogen generation rates can be obtained through hydrolysis, and (v) the by-products stemming from the PAB hydrolysis are ammonium borates. The following reactions may be proposed: AB → PAB + H₂ and PAB + xH₂O → 2H₂ + ammonium borates. All of these aspects as well as the advantages and issues of the combination of AB thermolysis and PAB hydrolysis are discussed.     

Hydrogen production from the methanolysis of ammonia borane by Pd-Co/Al2O3 coated monolithic catalyst

The aim of this study is to enable high hydrogen production yield from catalytic methanolysis of ammonia borane (AB) in the presence of a cordierite type ceramic monolithic. The monolithic channel surfaces were coated with Al₂O₃ by wash-coating method and then this layer was impregnated with 1 wt%Pd-2 wt%Co bimetallic catalyst. SEM-EDX and multi-point BET analysis were used in order to characterize the catalyst. The experimental studies were conducted in a continuous flow type reactor, which was used for the first time in this study. The reactions were carried on low temperature (40 °C), and with various AB feed concentrations and flow rates. It was found that the highest hydrogen production yield (88.5%) was obtained from AB flow rate of 3.3 mL/min, and AB feed concentration of 0.1 wt%. It was concluded that Pd-Co/Al₂O₃ coated monolithic, which is a stable, active and low-cost catalyst, was a very promising catalyst for on-board hydrogen production from the methanolysis of ammonia borane.     

Mechanistic insight into the promoting effect of magnesium nickel hydride on the dehydrogenation of ammonia borane

In this paper, we report an in-depth study of the post-milled 4AB/Mg₂NiH₄ sample, with a special focus on the promoting mechanism of Mg₂NiH₄ on the dehydrogenation of AB. A combination of X-ray diffraction (XRD), Fourier transformation infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) characterizations, together with selective isotopic labelling and other designed experiments, revealed that AB and Mg₂NiH₄ react with each other from the starting phase of the dehydrogenation process, which eventually results in the formation of MgNiBNH complexes. On the other hand, it was found that the reaction between AB and Mg₂NiH₄ cannot proceed directly, but requires phase transition of normal AB to its mobile phase AB* to occur first. Hence, the promoting mechanism of Mg₂NiH₄ on the dehydrogenation of AB is attributed to its promoting effect on phase transition of normal AB to AB* under mild conditions and in particular its chemical modification of AB with Mg and Ni.     

Kinetic model analysis and mechanistic correlation of ammonia borane thermolysis under dynamic heating conditions

Solid state decomposition of ammonia borane has been extensively reported to follow the Avrami-Erofe'ev equation. In this work, thermal analysis of ammonia borane is carried out under dynamic heating conditions under heating rate of 1–5 °C/min. The experimentally derived kinetic data is fitted against various solid state decomposition and autocatalytic kinetic models. It has been observed that the two steps of ammonia borane decomposition reactions follow different kinetics. The first step is more likely to be associated with the homogeneous autocatalytic reactions where the second part demonstrates the traditional nucleation growth kinetics.     

Effect of maleic acid on onset temperature and H2 release kinetics for thermal dehydrogenation of ammonia borane

Ammonia borane (AB, NH₃BH₃) has received great attention as an attractive hydrogen storage candidate because it has high hydrogen contents and releases hydrogen under mild operating conditions. Despite the favorable properties, AB thermolysis has several drawbacks such as long induction period, slow kinetics, and relatively high onset temperature, compared to hydrolysis approach. In this study, hydrogen release properties from AB were investigated in the addition of maleic acid (C₄H₄O₄, MA). Using thermogravimetric analysis, temperature programmed reaction with mass spectrometry, and FTIR analyses, the solid and gaseous products generated by thermolysis of the AB-MA mixture were characterized to understand the reaction mechanism. It was found that with the addition of MA, hydrogen yield and release kinetics were enhanced, while the onset temperature reduced significantly to ~60 °C. It is likely that the hydrolysis between O–H bonds in MA and B–H bonds in AB was initiated, and the heat released from the hydrolysis triggers the thermolysis of AB. It was also confirmed that a combination of the two additives (MA and boric acid) enables a further increase of H₂ yield while the onset temperature remains at ~60 °C. Our results suggest that MA is a promising additive to improve AB dehydrogenation.     

Methanolysis of ammonia borane catalyzed by magnetically isolable RHODIUM(0) nanoparticles

This article reports the preparation and employment of rhodium (0) nanoparticles (Rh⁰NPs) on the surface of magnetite nanospheres, denoted as Rh⁰@Fe₃O₄, as magnetically isolable nanocatalyst in the methanolysis of ammonia borane (MAB). The monodispersed Fe₃O₄ nanospheres are fabricated by a simple technique and used as nanosupport for Rh⁰NPs which are well stabilized and homogeneously distributed on the surface of nanospheres with a mean particle size of 2.8 ± 0.5 nm. The as-synthesized Rh⁰@Fe₃O₄ has a remarkable TOF value of 184 min⁻¹ in the MAB to produce H₂ gas in RT. Most of all, Rh⁰@Fe₃O₄ nanocatalyst can be reused, evolving 3.0 mol of H₂ gas for a mole of AB, keeping 100% of its initial activity even in the fourth reuse of MAB at 25 °C. Recovery of the Rh⁰@Fe₃O₄ nanocatalyst can be accomplished by simply approaching an external magnet, which eliminates many laborious catalyst removal steps in catalytic reactions. Reported are the outcomes of kinetic investigation, done by altering the concentration of substrate and catalyst together with temperature. Kinetic studies reveal that the catalytic MAB shows dependence on the concentration of reactants and temperature.     

Thermal conductivity and its temperature dependence in selected steel samples

The transient plane source (TPS) technique was employed to measure experimentally the thermal conductivity of three selected steel samples. Measured values of thermal conductivities are critically analysed, taking into account factors such as the composition and the nature of the alloying elements. An effort is made to determine the temperature dependence of thermal conductivity of these samples from room temperature to 300°C. Results of the present measurements are also discussed in the light of the values of thermal conductivity on equivalent steels measured by other workers.     

In situ synthesis of dendrimer-encapsulated palladium(0) nanoparticles as catalysts for hydrogen production from the methanolysis of ammonia borane

Addressed herein is the in situ synthesis of a PAMAM dendrimer-encapsulated palladium(0) NPs (Pd(0)/Dnd) during the methanolysis of ammonia borane (AB) and the catalytic performance of the yielded Pd(0)/Dnd nanocatalysts in hydrogen production from the methanolysis of AB under ambient conditions. A two-step procedure that includes the impregnation of Pd(II) ions via their coordination to –NH₂ groups of the dendrimer and then reduction of Pd(II) ions into the dendrimer-encapsulated Pd(0) NPs by AB during the methanolysis reaction was followed for the synthesis of Pd(0)/Dnd nanocatalysts. However, apart from the existing reports on the synthesis of dendrimer-encapsulated metal NPs, the present study includes for the first time the examination of effect of generation size (G4-G6), core type (ethylene diamine (E) or Jeffamine (P)) and terminal groups (-NH₂, –COOH and –OH) of a PAMAM dendrimer on the stability, particle size, morphology and catalytic activity of metal NPs. After finding the optimum Pd(0)/Dnd catalysts considering all these effects, a detailed kinetic study comprising the effect of catalyst and AB concentrations as well as temperature was conducted by monitoring the hydrogen production from the methanolysis of AB. The best catalytic activity in the methanolysis of AB was obtained by using a PAMAM dendrimer with generation G6, amine terminal groups and Jeffamine core (P6.NH₂) encapsulated Pd(0) NPs, providing the highest initial turnover frequency (TOF) of 55.8 mol H₂.mol Pd⁻¹.min⁻¹ and apparent activation energy (Ea^app) of 48 ± 3 kJ.mol⁻¹ at room temperature.     

Zn-MOF assisted dehydrogenation of ammonia borane: Enhanced kinetics and clean hydrogen generation

We report controllable and enhanced hydrogen release kinetics at reduced temperatures in ammonia borane (AB) catalyzed by Zn-MOF-74. AB is loaded into the unsaturated Zn-metal coordinated one-dimensional hexagonal open nanopores of MOF-74 (ABMOF) via solution infiltration. The ABMOF system provides clean hydrogen by suppressing the release of detrimental volatile byproducts such as ammonia, borazine and diborane. These byproducts prevent the direct use of AB as a hydrogen source for polymer electrolyte membrane fuel cell applications. The H₂ release temperature, kinetics, and byproduct generation are dependent on the amount of AB loading. We show that nanoconfinement of AB and its interaction with the active Zn-metal centers in MOF are important in promoting efficient and clean hydrogen generation.