O papel da pós-graduação na formação do químico

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Hydrogen generation by borohydrides: critical issues for portable applications

High volumetric and gravimetric efficiency are key to potential hydrogen energy carriers. Sodium borohydride emerges as such potentiality and a storage capacity well within DOE targets for 2015. Limitations exist due to the fact that hydrolysis is restricted by available water and due to the lack of low cost re-usable catalysts. An extensive amount of work has been done in our laboratories on Ni and Ru based catalysts, including synthesis and characterization and solutions have been found for durability, stability and reutilization under operating conditions in small volume batch reactors. Results showed that the Langmuir-Hinshelwood model described fairly well the reaction kinetics for all tested temperatures up to 60ºC and up to reactant exhaustion. In this work, issues such as self-hydrolysis, stability of solutions for storage, water management, some aspects of the catalyzed hydrolysis as well as gas conditioning are studied in order to associate a storage solution with sodium borohydride to a low power air breathing cathode PEM fuel cell

On-demand hydrogen generation by hydrolysis of sodium borohydride in batch reactors: effect of the buffer pressure

A study was undertaken in order to investigate the potential of hydrogen generation by hydrolysis of sodium borohydride in batch reactors, operating at moderate pressures, in the presence of a reused nickel-ruthenium based catalyst, to feed on-demand a proton exchange membrane fuel cell. The effect of the buffer pressure is explored and hydrogen generation rates are evaluated by changing catalyst amount, operating pressure and successive refuelin

Impact of the reactor bottom shape on the solid sodium borohydride hydrolysis for hydrogen generation

Sodium borohydride (NaBH4) is a chemical hydride that produces hydrogen (H2) ‘on-demand’ through the reaction with water, and exhibits high gravimetric hydrogen storage capacity (10.8 wt.%). NaBH4 has been appointed as an efficient energy/hydrogen carrier for use with fuel cells [1-6]. Unfortunately, problems also exist with NaBH4 hydrolysis: H2 production rates are not sufficiently fast, reaction completion is not always reachable and effective gravimetric (and volumetric) H2 storage capacity is far from the theoretical value. The present study reports original experimental work on generation of hydrogen, by hydrolysis of solid sodium borohydride with stoichiometric amount of distilled water (H2O/NaBH4: 2, 3 and 4 mol/mol), in the presence of a powder unsupported Ni-Ru based catalyst, reused about 320 times. The experiments, performed in two batch reactors with equal internal volume but with different bottom shapes (flat and conical), reveal - for the conical bottom shape with any excess of water - 8.1 H2 wt% and 92 kg H2/m3 (materials-only basis), and a H2 rate of 87.4 L(H2) min-1g-1 catalyst. The role of reactor bottom geometry on the solid NaBH4 hydrolysis - with any excess of water - is, as the authors are aware, for the first time here referred

The effect of NaOH on the kinetics of Hydrogen production from sodium borohydride using Ni-based catalysts doped with Ruthenium

Previous work by the authors has demonstrated a high rate and high yield hydrolysis of sodium borohydride in the presence of a Ni-Ru catalyst synthesized by wet chemistry. The catalyst has been fully characterized and utilized more than 300 times, exhibiting high stability and durability. In this work, the effect of temperature on the reaction rate was studied and the activation energy of the process estimated for temperatures up to 65 ºC. Typical data in the form of an Arrhenius type relationship showed two slope regions suggesting a change of mechanism that lead to a more accentuated role of ruthenium for temperatures higher than 45 ºC. This effect is maintained with increasing Ru doping of the catalyst. The effect of NaOH as a stabilizer was also studied and the role of Ni and Ru in the hydrolysis of sodium borohydride is under study on the basis of data obtained for various Ru concentrations and data for 100% Ni and 100% Ru as catalysts. An increase in the concentration of Ruthenium in the catalyst allowed effective utilization of the catalyst without the need for the stabilizer, minimizing the induction reaction time. Excellent catalytic activity and catalyst minimal deactivation for sodium borohydride hydrolysis are characteristic of the series of Ni-Ru catalyst synthesized by wet chemistry used in this work

Ductility and durability of strain hardening cementitious composites in the marine environment

Modern structures are being exposed to severe environments and the lack of durability is one of the most serious problems in concrete infrastructures. Structural concrete exposed to marine environment deserves special attention as the sea salts chemically react with the cement matrix and the steel reinforcement which results in loss of strength, cracking, spalling, etc. The challenges of Civil Engineering, especially within the structures in extreme environments, pose considerable expectations with regards to the development of fibre reinforced materials for the development of more resistant and durable solutions. In the present work, the behaviour of an Engineered Cementitious Composite (ECC) was studied. All the specimens prepared were cured in 4 types of environments: exposed to air (20ºC of temperature and 60% of humidity) immersed in tap water, immersed in salted water and immersed in seawater, all at an average temperature of 18ºC. A series of experiments, including compressive and direct tension tests were carried out to characterize the mechanical properties of the ECC materials while exposed to different environments. The most important characteristic of ECC, which include multiple-cracking behaviour at increasing tensile strains when subjected to increasing tensile loading, was confirmed in all types of curing environments. In all cases the cementitious composites performed well with regards to the strain hardening behaviour typically observed in these materials, although the cracking processes have shown different characteristics. Due to the ability of the material to control crack opening below extremely low values, typically under 100 µm, the durability of structures can be significantly improved when ECC materials are used in the in marine environments. It was shown also that the salted water does not represent well the effect of seawater while characterising ECC mechanical characteristics in the laboratory.The authors acknowledge the material suppliers Secil, Civitest, Sika, Saint-Gobain (Fibraflex) and Eurocálcio – Calcários e Inertes SA for providing the materials used in this study