Batch sodium-borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition on biopolymers to improve hydrogen storage in the liquid phase by solubility effects

Mestrado Integrado em Engenharia Química Batch sodium-borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Master Thesis by Joana Sofia Oliveira Madureira Performed at Faculdade de Engenharia da Universidade do Porto Supervisor: Dr. Alexandra M. F. Rodrigues Pinto Co-Supervisor: MSc. Maria Josefina F. Ferreira Departamento de Engenharia Química July 2013 Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Acknowledgements I would like to express my grateful thanks to every person that somehow has contributed to the realization of this master thesis: To Professor Alexandra Rodrigues Pinto for the opportunity to develop my work on such an interesting topic and for her supervision and support. To Josefina Ferreira for her co-supervision and everyday assistance, cooperation, helpful suggestions and important advices. To Marina Manic, for her interest and dedication given to me and the work I developed. To Dr. Luís Gales, from IBMC-ICBAS, for his great collaboration in performing the XRD Analysis and for to give me all the necessary information. An appreciation note for his kindness and availability for the time taken to show me the procedures involved in the XRD Analysis. To Professor Fernando Pereira and Salomé Soares for their collaboration in the hydrogen solubility tests. To Professors Margarida Bastos and Adélio Mendes for their availability to help me when I needed. To the workers of some Laboratories of DEQ-FEUP for the laboratory facilities. To Hélder Nunes for his fellowship and support, which were fundamental during the course of this work. To Pedro Magalhães for his English language revision and great suggestions to improve the syntax and readability. To my close friends, that they really know who they are, for their true friendship and support in all the moments we had throughout the last six years. Without them everything would have been undoubtedly more difficult. Last but not least, a special thanks to my parents and family. I will be eternally grateful for the love, understanding, patience, encouragement and support that, especially my mother, sister and grandfathers have given to me during all my life. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Abstract The present thesis aimed at studying the effects of small additions of cellulose based polymers on catalytic sodium borohydride hydrolysis, in order to improve hydrogen storage in the post-reaction liquid phase by solubility effects. The carboxyl-methyl-cellulose (CMC), hydroxyl-ethyl-cellulose (HEC) and methyl-cellulose (MC) biopolymers were tested by their addition to the reactant solution. Three studies were carried out: a reaction kinetics study, preliminary hydrogen solubility study and by-products analysis. The kinetics study enclosed a set of experiments of sodium borohydride hydrolysis in the presence of a reused Ni-Ru catalyst, in which the effects of temperature and biopolymers concentration on the hydrogen gas generation rate, yield and lag time were investigated. The hydrolysis reactions were performed at two different temperatures (25°C and 45°C) in a batch reactor with an internal volume of 369 cm3, made of stainlesssteel with a bottom conical shape, positioned vertically. The polymers were added to the reactant solution (15wt% NaBH4 and 7wt%NaOH) in the two different mass concentrations: 0.05wt% and 0.25wt%. A small addition of CMC and MC polymers gives a very rapid hydrogen generation (when comparing with classic hydrolysis) leading to a complete reaction for the CMC and MC cases at around half of the time. This result is of primordial importance bearing in mind the application of this concept to portable applications. The preliminary solubility study comprised the measurement of conductivity solutions, after and post-reaction. Potential candidates to promote the store of hydrogen in the liquid phase are suggested according to this study. In a very near future, solubility tests will be performed in order to estimate the amount of hydrogen gas that solubilises in each solution and to compare the capability of the post-reactant solutions to store hydrogen in the liquid phase. The crystal structure of the remaining solutions was analyzed by X-Ray Diffraction (XRD) analysis. The XRD analysis results of the crystals by-products structure, from the classic and with the addition of 0.05wt% HEC and 0.05wt% MC hydrolysis, reveals that both of them are a sodium metaborate dehydrate, NaBO2.2H2O. A crystal structure of sodium tetrahydroxoborate (NaB(OH)4) for Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects hydrolysis with 0.05wt% and 0.25wt% of CMC and 0.25wt of HEC and MC polymers was found. Key-words: hydrogen production, sodium borohydride hydrolysis, solubility effects, biopolymers, Ni-Ru based catalyst. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Resumo A presente tese de mestrado investigou a influência da adição de pequenas quantidades de biopolímeros, à base de celulose, na hidrólise catalisada do borohidreto de sódio, realizada em reator por partidas, com o objetivo de possibilitar o armazenamento do hidrogénio produzido na fase liquida remanescente através de efeitos de solubilidade. Através da sua adição à solução reagente foram testados os seguintes biopolímeros: carboxi-metilcelulose (CMC), hidroxi-etilcelulose (HEC) e metilcelulose (MC). Durante o projeto de Tese foram realizados os seguintes estudos: estudo da cinética da reação de hidrólise, estudo preliminar de solubilidade de hidrogénio e análise dos sub-produtos da reação por cristalografia. No estudo cinético foi executado um conjunto de experiências no qual os efeitos da temperatura reacional e da concentração de biopolímeros na taxa de produção de hidrogénio, no rendimento e no tempo de indução da reação de hidrólise do borohidreto de sódio na presença de um catalisador de Ni-Ru foram investigados. As reações de hidrólise ocorreram num reator por partidas posicionado verticalmente, feito de aço inoxidável, com um volume interno de 369 cm3 e uma geometria de fundo cónica, às temperaturas de 25°C e 45°C. Os diferentes biopolímeros foram adicionados à solução reagente (15wt% NaBH4 e 7wt%NaOH) a duas concentrações mássicas diferentes: 0.05wt% e 0.25wt%. Verificou-se que uma pequena adição de CMC e MC tem um impacto elevado na cinética gerando hidrogénio a uma taxa bastante elevada quando comparada com a da hidrólise clássica atingindo-se o fim da reação num tempo significativamente menor. Este é um resultado de extrema importância quando se pensa na aplicação deste conceito (produção e contentorização de hidrogénio em aplicações portáteis). O estudo preliminar de solubilidade de hidrogénio envolveu a determinação da condutividade das soluções antes e após a reação de hidrólise. São sugeridos alguns polímeros candidatos com potencial para promoverem um armazenamento de hidrogénio na fase líquida. Num futuro próximo irá ser realizado um estudo detalhado de solubilidade que permitirá determinar a quantidade de hidrogénio que é capaz de solubilizar em cada solução proposta e comparar a capacidade das soluções pósreação para armazenar hidrogénio. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects A estrutura química dos cristais, obtidos por evaporação da água das soluções sobrenadantes, foi determinada por difração de raios X. A análise dos cristais obtidos revelaram a existência de duas estruturas diferentes: metaborato de sódio dihidratado (NaBO2.2H2O) e tetrahydroxoborato de sódio (NaB(OH)4). Palavras-chave: produção de hidrogénio, hidrólise de borohidreto de sódio, solubilidade de hidrogénio, biopolímeros, catalisador à base de Ni-Ru. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Declaration I declare, on oath, that this work is original and that all the not original contributions were appropriately referred with source identification. Porto, 31st july 2013 ____________________________________________ (Joana Sofia Oliveira Madureira) Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Index i Index Index of figures .................................................................................. iii Index of tables ................................................................................... vii Nomenclature/Glossary ....................................................................... viii 1 Introduction ................................................................................. 1 1.1 Project goals ........................................................................... 3 1.2 Thesis organization .................................................................... 4 2 State of the Art ............................................................................. 5 2.1 Hydrogen as a energy carrier ........................................................ 5 2.2 Sodium Borohydride as energy/hydrogen carrier ................................. 7 2.3 Polymer Electrolyte Membrane Fuel Cell ......................................... 10 2.4 Hydrogen gas solubility .............................................................. 12 3 Experimental Procedure and Technical Description .................................. 19 3.1 Reaction kinetics study .............................................................. 19 3.1.1 Materials ......................................................................... 19 3.1.2 Experimental setup ............................................................. 20 3.1.3 Experimental procedure ....................................................... 22 3.1.4 Preliminary studies ............................................................. 23 3.1.5 Effect of biopolymers concentration and reaction temperature ......... 24 3.2 Catalyst activity study ............................................................... 25 3.3 Preliminary hydrogen solubility study ............................................. 25 3.4 Reaction products analysis .......................................................... 25 4 Results and Discussion..................................................................... 27 4.1 Reaction kinetics study .............................................................. 27 4.1.1 Effect of biopolymers concentration and reaction temperature ......... 27 Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Nomenclature/Glossary viii Nomenclature/Glossary dP/dt linear region slope bar.s-1 KH Henry’s constant MPa, bar-1 mcat catalyst mass g mNaBH4 mass of sodium borohydride in the reactant solution g MMH20 molecular mass of water g.mol-1 MMNaBH4 molecular mass of sodium borohydride g.mol-1 msolution mass of reactant solution injected g nair number of moles of air produced mol P(air + H2)experimental absolute pressure of the gas produced bar Patmospheric pressure inside of laboratory bar P(air + H2)relative Relative pressure of the gas produced bar Pg gas pressure Q hydrogen flow rate L(H2)min-1gcat-1 R Ideal gas constant cm3.bar.mol-1.K-1 T temperature °C, K Vinitial available volume inside reactor for gas production cm3 Vfinal volume occupied by the gas after reaction cm3 Vfree Volume occupied by the hydrogen injected cm3 Vm the molar volume of an ideal gas at STP conditions 22,446 L.mol-1 Vr reactor internal volume cm3 x hydration factor y gas concentration in liquid (molar ratio) ρH2O density of water g.cm-3 ρcat catalyst density g.cm-3 ρsolution density of reactant solution injected g.cm-3 Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Nomenclature/Glossary ix Acronyms CMC carboxyl-methyl-cellulose FCs fuel cells systems HEC hydroxyl-ethyl-cellulose ILs ionic liquids LPOPS Less polar organic polymeric solution MC methyl-cellulose PEMFC polymer electrolyte membrane fuel cell XRD X-Ray Diffraction Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Introduction 1 1 Introduction The mankind’s constant progress during the last three centuries drove progressively to an excessive and uncontrolled use of fossil fuels. The supply of fossil fuels is limited and the development of alternative and clean renewable energy carriers is of most importance [1]. Currently, the society still consumes large amounts of fossil fuels, although this large consumption is clearly a concern. The world’s population has been focused on finding solutions for a secure energy future. In this context, hydrogen arises as an alternative and clean renewable energy vector mainly for the transportation sector and other portable applications [1]. The hydrogen as an energy carrier is particularly interesting for Polymer Electrolyte Membrane Fuel Cell (PEMFC) technologies in which the chemical potential energy of the gas is directly converted into electric energy [2]. Unfortunately, molecular hydrogen is not available in large quantities, it must be synthesized from other energy sources before energy can be transferred to the end user. Moreover it has a very low volumetric energy density, so it is very difficult to store a sufficient amount of hydrogen in a small and light enough vessel [3]. This problem is the main impeditive for the establishment of a sustainable hydrogenbased energy especially for portable applications. Therefore, it is important to find and develop efficient and safe methods of hydrogen production and storage [1]. One of the promising hydrogen storage methods relies on liquid-phase chemical hydrogen storage materials. Chemical hydrogen storage, which involves storing of hydrogen in the form of chemical bonds, has received great attention and there are a number of materials with high hydrogen content that are being explored. For many years, solid-state hydrogen storage materials were considered as promising chemical hydrogen materials due to their appealing characteristics. However, their disadvantages, like the high temperature required to desorb hydrogen, slow hydrogen release kinetics as well as heat dissipation issues and the high system weight, led to a search for efficient and safe liquid phase hydrogen storage materials [4]. Aqueous sodium borohydride, ammonia borane, hydrazine, hydrazine borane and formic acid have been referred as promising liquid-phase chemical hydrogen storage materials. The use of these compounds provides high gravimetric and Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Introduction 2 volumetric densities, low potential risk and low capital investment in hydrogen storage [4]. The present project focuses on sodium borohydride as a material system for hydrogen production and storage and on its catalyzed hydrolysis reaction. When sodium borohydride (NaBH4) reacts with water (H2O) at room temperature in presence of a catalyst, a theoretical hydrogen (H2) content of 10.8wt% can be released and the by-product sodium metaborate is formed (Na.BO2.xH2O), through the following exothermic reaction [2]: Ideal hydrolysis is attained for hydration factor x = 0, where two moles of water are required to react with one mole of solid sodium borohydride to release four moles of hydrogen. In practice, an excess of water is necessary but lowers the theoretical gravimetric hydrogen storage capacity of sodium borohydride [4]. The potential application of sodium borohydride as hydrogen carrier for portable applications is dependent not only on an easily controllable catalysed hydrolysis reaction at significant rates, to comply with fuel cell feeding, but also on the increase of the gravimetric density, which is of utmost importance [5]. Therefore, this technology of hydrogen production and storage is not yet economically feasible. One of the biggest challenges is to find a way to produce and simultaneously store molecular hydrogen in the liquid phase. It is known that molecular hydrogen is a strongly non polar compound, not soluble in water and in almost all the liquid solvents at near ambient conditions. Due to this fact one possibility is to change de polarity of the reaction medium, in order to lowering the polarity/conductivity of the post-reaction solution, and make possible the storage of molecular hydrogen in the liquid phase by solubility effects. A way to improve the affinity of hydrogen storage in the liquid phase is to increase the non-polar hydrogen-electrolyte interactions by adding small portions of compounds to the reactant solution. These compounds must be soluble in water and slightly polar. For the first time, the addition of biopolymers to the reactant solution of sodium borohydride was explored through a study entitled “Effects of the addition of an organic polymer on the hydrolysis of sodium tetrahydroborate in batch reactors” [2]. The capability of storing hydrogen gas by its dissolution on the liquid by-product was investigated. That experimental study showed that at moderate pressures, up to Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Introduction 3 2.7 MPa, both the generation and storage of molecular hydrogen slightly increased by small additions of the organic polymer – carboxymethyl cellulose (CMC)[2]. It is important to continue the study of this new route to improve simultaneous hydrogen production and storage in the liquid phase, via batch hydrolysis of sodium borohydride. It is expected that other polymer compounds, also based on cellulose could change the polarity of the reaction medium and contribute to the hydrogen capture in their molecular cavities, increasing the yields and rates of sodium borohydride catalysed hydrolysis, with higher gravimetric and volumetric hydrogen densities. In the present thesis, studies on the effects of small additions of cellulose based polymers on sodium borohydride hydrolysis, in the presence of a Ni-Ru based catalyst are reported. The biopolymers tested were: carboxyl-methyl-cellulose (CMC), hydroxyl-ethyl-cellulose (HEC) and methyl-cellulose (MC). 1.1 Project goals The objective of the present research work was to investigate the influence of small additions of based cellulose polymers in the kinetics reaction of catalytic hydrolysis of stabilized aqueous solutions of sodium borohydride, in a batch reactor, bearing in mind the possibility of hydrogen storage in the liquid phase. In order to achieve this goal, experiments of hydrolysis of sodium borohydride, in the presence of a reused nickel-ruthenium based catalyst, were carried out under pressure up to 2.5 MPa. Hydroxyl-ethyl-cellulose, carboxyl-methylcellulose and methyl-cellulose polymers, through small additions to the reactant solution were tested. The influence of the biopolymer concentration and reaction temperature on the reaction rate and yield was investigated. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Introduction 4 1.2 Thesis organization The thesis is divided in 7 main chapters, next summarized. Chapter 1, Introduction, introduces the reader to the research topic and presents the main objective. In the Chapter 2, State of the art, the potential of hydrogen as an energy carrier and sodium borohydride as hydrogen carrier are described. A review of ruthenium based catalysts on sodium borohydride hydrolysis and a general introduction to polymer electrolyte membrane fuel cells are also reported. Finally, the concept of hydrogen solubility is introduced as well as the chemical compounds that may enhance its solubility on the hydrolysis liquid by-product. Chapter 3, Experimental Procedure and Technical Description is focused on the methodology and materials used to produce hydrogen via batch sodium borohydride hydrolysis. Chapter 4, Results and Discussion is devoted to presentation and discussion of the results, regarding the impact of the small additions of biopolymers on the kinetics reaction and on hydrogen solubility. Chapter 5, Conclusion, presents the main conclusions of the developed work. Chapter 6, Project assessment, gives an overall judgment about the work developed. Chapter 7, References, is the list of the bibliographic references used. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 5 2 State of the Art This chapter presents firstly, the potential of the use of hydrogen and sodium borohydride as energy carriers. A review of ruthenium based catalysts used in sodium borohydride hydrolysis and a general introduction to polymer electrolyte membrane fuel cells are also reported. Finally, the concept of hydrogen solubility is recalled and some chemical compounds that may enhance its solubility on the hydrolysis byproducts are proposed. 2.1 Hydrogen as a energy carrier Hydrogen is an energy carrier, an indirect source of energy, not a resource itself. In spite of being the simplest and the most abundant atom both on the Universe and in Earth, it does not exist in its free form. It must be synthesized from other energy sources, and then used to transfer energy to another use [3]. As an energy carrier, hydrogen has high potential with an energy density of 142 MJ/kg, around three times higher than that of petroleum, 47 MJ/kg [6]. Moreover it presents minimal pollution characteristics, combustibility in internal combustion engines, feasibility of electrochemical combination with an oxidant in fuel cells and mass transportability comparable to that of petroleum [3]. On the other hand, hydrogen has a very low volumetric storage density which is not yet suitable for the establishment of a sustainable hydrogen-based energy future [3]. According to the U.S. Department of Energy, the current hydrogen storage technologies involve compressed hydrogen gas tanks, liquid hydrogen tanks, cryogenic compressed hydrogen, metal hydrides, high-surface-area adsorbents, and chemical hydrogen storage materials [7]. The most traditional storage system is the high-pressure gas cylinders. This type of storage operates at pressures between 300 and 700 bar. This higher pressure level requires material and design improvements in order to ensure tank integrity. Advances in compression technologies are also required to improve efficiencies and reduce the cost of producing high-pressure hydrogen [7]. Liquid hydrogen tanks can store more hydrogen in a given volume than compressed gas tanks. The volumetric capacity of liquid hydrogen is 0.070 kg/L, Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 6 compared to 0.030 kg/L for 700 bar gas tanks. However, this technology presents some drawbacks such as: hydrogen boil-off, the energy required for hydrogen liquefaction, volume, weight, and tank cost. New approaches able to lower these energy requirements and consequently the cost of liquefaction are needed [7]. The metal hydrides have higher volumetric density storage than pressurized hydrogen or liquid hydrogen. A further important feature of metal hydrides is the reversibility provided by almost all of the metal hydrides. As main limitations, they present low hydrogen gravimetric capacity, slow uptake, low stability and high cost [7]. Hydrogen adsorption on solids with large surface areas includes a range of carbon-based materials such as carbon nanotubes, aerogels, nanofibers, as well as metal-organic frameworks, conducting polymers, and clathrates. This is a new field on hydrogen storage that may be promising. For hydrogen storage capacity, a value of 4 wt% at 78K and of approximately 1 wt% at room temperature, via adsorptive mechanism has been reported. However, due to the highly porous nature of solids, volumetric capacity may still be an important issue to continuing exploring and improve [7]. Chemical storage materials have been investigated extensively as a primary hydrogen storage medium due their very high volumetric hydrogen storage density, ability to produce hydrogen on demand through hydrolysis reaction and stability during long periods of storage without usage [3]. Compared with other technologies, the chemical hydrides can store hydrogen at much milder ambient conditions and relatively low pressure. However, these hydrolysis reactions are not easily reversible on-board a vehicle. Hence, the by-products must be removed from the vehicle and regenerated off-board [7]. The present reached gravimetric and volumetric efficiencies for these six storage technologies are compared in Figure 1, as well as, the FreedomCAR targets for automotive hydrogen storage systems in 2010 and 2015, published by US Department Of Energy (DOE) [7]. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 7 2.2 Sodium Borohydride as energy/hydrogen carrier Sodium borohydride, also known as sodium tetrahydroborate is not a new compound: it has been discovered in 1940s by Prof. H.C. Brown, Nobel Laureate in Chemistry in 1979. It is especially known as a reducing agent and it is widely used in organic chemistry. In industry, it is mainly used as reducing agent in the manufacture of pharmaceuticals and a bleaching agent in the manufacture of paper [8]. The year of 2000 marked a new beginning in the sodium borohydride history with the work of Amendola et al.: “a safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst”. After that date, intensive research has been focused on sodium borohydride as a promising hydrogen carrier [2]. Nevertheless, the U.S. Department of Energy (U.S. DOE) recommended a nogo for sodium borohydride for vehicular applications due the non-compliance of the targets established for 2007 [9]. The U.S. DOE decision was based on the following observations: too low effective gravimetric hydrogen storage capacities, inefficiency of hydrolysis by-product recycling and cost [10]. Sodium borohydride did not meet the 2007 target from U.S. DOE but according to the intense published literature it has definitively potential for portable applications, or at least niche applications [9]. Hence, it is expected that sodium borohydride can meet the 2015 target: a gravimetric hydrogen storage capacity of 9 Figure 1 - Status of hydrogen storage technologies: gravimetric and volumetric efficiencies [7]. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 8 wt% [2]. In fact, sodium borohydride, for ideal hydrolysis (x=0) presents a high theoretical gravimetric storage capacity (10.8 wt%) [4]. When stored in aqueous solutions, sodium borohydride undergoes selfhydrolysis. Schlesinger et al. [11] discovered that it is possible to form a highly stable aqueous solution of sodium borohydride by dissolving it in basic solution. The hydrolysis reaction can then be initiated on demand by bringing the solution into contact with an heterogeneous catalyst, making the release of hydrogen very easy to control [6]. Hence, the catalytic hydrolysis of stabilized aqueous solutions of sodium borohydride occurs through the following exothermic reaction [2]: The standard state enthalpy change of the above reaction at room temperature is -217 kJ/mol [3]. Ideal hydrolysis is attained for hydration factor x = 0, where two moles of water are required to react with one mole of solid NaBH4 to release four moles of hydrogen. In practice, an excess of water is necessary due to the fact that the solid by-product sodium metaborate (NaBO2.xH2O), can exist with varying degrees of hydration; also the solubility of sodium borohydride and byproducts in water are low: 55g NaBH4/100g H2O at 25 ºC and 28g NaBO2/100g H2O at 25 ºC, respectively [3][9]. However, excess of water lowers the theoretical gravimetric hydrogen storage capacity of sodium borohydride [4]. Some issues related to sodium borohydride hydrolysis have been intensively studied such as: concentration of inhibitor, usually sodium hydroxide, concentration of sodium borohydride, reaction temperature and suitable catalysts. In fact these parameters have a significant influence on the hydrolysis reaction [12]. Amendola and co-workers [13] investigated the influence of sodium hydroxide concentration in sodium borohydride solutions on the hydrogen generation rate, using a Ruthenium catalyst. They reported that for a given weight percent of sodium borohydride, hydrogen generation rates decreased with the increasing of the weight percent of sodium hydroxide (from 1wt% to 10 wt%). Ingersoll et al. [14] reported similar results after their experiments with a (Ni– Co–B) catalyst: the hydrogen generation rate increased for lower sodium hydroxide concentrations and then decreased after reaching a maximum at 15wt% of sodium hydroxide. Minkina et al. [15] investigated the effects of temperature and sodium hydroxide concentration on the degradation of concentrated sodium borohydride Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 15 Other chemical compounds, such as cyclodextrins and ionic liquids, due to their particular properties have become interesting for different application fields, namely for hydrogen. Cyclodextrins are a family of cyclic oligosaccharides formed by glucopyranose units linked by α-(1,4) bonds. They are also known as cycloamyloses, cyclomaltoses and Schardinger dextrins. These polymers were first discovered by Villiers in 1891 and are produced through the intramolecular transglycosylation reaction from degradation of starch by an enzyme named glucanotransferase (CGTase) [30][31]. The most common are α-, β-, and γ-Cyclodextrins, which are formed by six, seven, and eight glucose units, respectively [30]. The major interest in cyclodextrins lies in their capability to form inclusion complexes (host-guest complexes) with several compounds by a molecular complexation. Due to this ability, cyclodextrins can modify significantly the properties of the materials. They actually enhance the solubility of highly insoluble guests [30][31]. Because of the presence of a lipophilic cavity that provides a microenvironment into which appropriately sized non-polar moieties can enter to form reversible inclusion complexes [30]. In aqueous solution, water molecules from the cavity are displaced by more hydrophobic guest molecules present in the solution. A non polar-non polar association is formed as well as a more stable lower energy state [30]. No covalent bonds are broken or formed during formation of the complex. Complex formation is a dimensional fit between host cavity and guest molecule [30][31]. Figure 5 and table 1 show the chemical structure of the three types of cyclodextrins and some of their properties, respectively. Figure 4 - Chemical structures: a)Carboxymethyl cellulose b) hydroxyethyl cellulose; c) methyl cellulose [48] b) a) c) Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 16 Figure 5 - Cyclodextrins chemical structures [32]. Table 1 – Cyclodextrins properties [30]. Property α-Cyclodextrin β-Cyclodextrin γ-Cyclodextrin Number of glucopyranose units 6 7 8 Molecular weight (g/mol) 972 1135 1297 Solubility in water at 25°C (%,w/v) 14.5 1.85 23.2 Outer diameter (Å) 14.6 15.4 17.5 Cavity diameter (Å) 4.7-5.3 6.0-6.5 7.5-8.3 Height of torus (Å) 7.9 7.9 7.9 Cavity volume (Å3) 174 262 427 Ionic liquids (ILs) are salts with organic ions that are liquids at room temperature. Their properties could be tailored for specific applications by adjusting the structure and species of cations and/or anions. In fact, the number of possible combinations of cations and anions is uncountable with vast and still unexplored possibilities [33]. The ionic liquids are of interest because they are considered environmentally friendly or green media due to their favourable properties such as chemical and thermal stability, negligible vapour pressure, non-flammability and high ionic conductivity [34]. Moreover, their ionic bonds induce a miscibility in a large concentration range with polar compounds, and the cations chains determine their solubility in less polar compounds [34]. Currently ILs are under intensive investigation as alternative solvents for biphasic catalysis and a number of reviews on this subject are available [35]. Catalytic applications involving gaseous substrates, in particular hydroformylation and hydrogenation, have been well developed. The solubility of Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 17 gases in ionic liquids is an important factor in catalytic reactions, where the gas is the substrate. It has been suggested that increased reaction rates, in biphasic hydrogenation reactions in ionic liquids, could be due to high solubility of hydrogen in the ionic liquid [35]. The most used techniques to measure the solubility of gases in ILs are the same to measure the solubility of gases in conventional liquids (for example the volumetric or saturation method). Due to ILs negligible vapour pressure it is also possible to determine the gas solubility through the methods used in solubility of gases in solids, such as gravimetric and oscillation method. Other methods such as spectroscopic and chromatographic methods have been occasionally used. Table 2 shows the Henry’s constant for the solubility of hydrogen in some of the most studied ionic liquids: 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]). Figure 6 puts in evidence their ions chemical structure. Table 2 – Values of Henry’s constant for the hydrogen solubility in ionic liquids. Ionic Liquid Henry’s constant KH / MPa Temperature °C Pressure MPa Method Authors [bmim][PF6] 118 104 40 60 <9 Static Kumelan et al. [36] [hmim][Tf2N] 86,3 20 <10 Kumelan et al. [37] [bmim][BF4] 203,6 25 <0,1 Isochoric saturation Jacquemin et al. [38] 239,1 40 [bmim][PF6] 244,5 20 <0,1 Jacquemin et al. [39] 187,8 45 Figure 6 - Cations and anions chemical structures [40]. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects State of the Art 18 Taking into account the expected impact of polymers addition on sodium borohydride hydrolysis, it is expected that the ionic liquids listed in Table 2, when added to the reactant solution also enhance the solubilisation of hydrogen in the liquid phase. It is suggested that the most electronegative elements such as fluorine and nitrogen, present in the ILs chemical structures may establish chemical bonds with the produced hydrogen. Some ILs have been already investigated and tested as solvents in reactions of ammonia borane dehydrogenation and as catalysts in the sodium borohydride hydrolysis for hydrogen production [41][34]. Himmelberger et al. [41], reported that imidazolium based ionic liquids tested provide advantageous media for ammonia borane dehydrogenation in which both the extent and rate of hydrogen release are significantly increased. Their low solvent volatility, the high extent of their hydrogen release, the tunability of both their hydrogen materials-weight-percent and release rates, and their product control attained by either trapping or suppressing unwanted volatile side products, make ammonia borane/ionic liquid based systems appellative candidates for hydrogen storage applications. On the other hand, Chinnappan et al .[34], demonstrated that 1,1’-hexane-1,6diylbis (3-methylpyridinium) tetrachloronickelate (II) is an efficient catalyst for the hydrogen generation from sodium borohydride showed by it excellent catalytic activity results. To conclude the above considerations, and bear in mind the particular properties of cyclodextrins and some ILs, it can be stated that those compounds have strong capabilities of contributing to hydrogen storage in post-reaction solutions by solubility effects at moderate pressures (up to 2.5 MPa). Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Experimental Procedure and Technical Description 19 3 Experimental Procedure and Technical Description The materials, experimental setup and experimental procedure used in the kinetic study of the catalytic hydrolysis of sodium borohydride solutions are thoroughly described in this section. The techniques used for the analysis of the reaction by-products are also presented. 3.1 Reaction kinetics study The kinetics reaction study involved a set of experimental work allowing the analysis of the effect of biopolymer (CMC, HEC and MC) concentration, reaction temperature and catalyst activity, on hydrogen generation rates, yields, the reaction induction time and on the gravimetric and volumetric hydrogen storage capacities. 3.1.1 Materials The materials used for the preparation of the reactant solutions are mentioned below: - Sodium borohydride white powder was provided by Panreac (123314.1608) and stored in a dessicator until use. In all the experiments it was used dissolved in water forming aqueous solutions. - Sodium hydroxide pellets (98% purity) was supplied by EKA (No.: 1310.73.2) and it was used as a hydrolysis inhibitor. - Methyl-cellulose microgranular powder was provided by SIGMA-ALDRICH (CAS 9000-11-7). - Hydroxyethyl cellulose and Methyl cellulose powders were already available in the laboratory. - Nickel based bimetallic unsupported catalyst, in the form of a finely divided black powder was synthesized at LNEG – Laboratório Nacional de Energia e Geologia, Fuel Cells and Hydrogen Unit, Lisbon – Portugal, in November 2012. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Experimental Procedure and Technical Description 20 3.1.2 Experimental setup The experimental setup for the reaction kinetics study comprised a batch reactor where hydrogen was produced, a demonstration prototype with a PEM fuel cell and a data acquisition system. The batch reactor made of stainless steel AISI 316L, with wall width equal to 10 mm, has a conical bottom geometry with an internal volume of 369 cm3.The reactor bottom configuration – conical – enables non dispersible effects between the contacting powdered catalyst and the injected reactant solution [3]. The reactor design allows the possibility of following the reaction temperature at two different locations inside it by using two thermocouples. One of the thermocouples is used to monitor the increase of temperature in the core of the catalyzed exothermic hydrolysis reaction, and the second one, measures the temperature of molecular hydrogen in the free volume of the container opened for the generated gas. The temperature of the reactor medium and the rate of hydrogen generation were recorded with a data acquisition system using Labview software. The gas pressure inside the reactor was followed with an adequate pressure transducer calibrated from 0-40 bar gauge and also by a Bourdon manometer (from 060 bar gauge), until attaining a constant pressure inside the reactor. The hydrogen generated was supplied to a PEMFC single cell placed inside of a didactic demonstration prototype named “MicroBoro Bus”, which is a bus like shaped mobile platform [42]. The developed low power PEM fuel cell uses an air breathing cathode and humidified hydrogen produced by the borohydride reactor. Natural convection to air feed the cathode is used when the platform is located on a stand and the energy conversion required is just the one needed to make the wheels of the platform bus turn [3][42]. In figures 7 and 8 it is possible to see a photograph of the experimental rig as well as the reactor bottom configuration. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Experimental Procedure and Technical Description 21 a) b) c) c) d) e) Figure 7 –Photograph of the experimental rig: a) batch reactor; b) “MicroBoro Bus” with a PEM fuel cell inside; c) data acquisition system; d) refrigeration unit; e) thermostatic bath. Figure 8 – Reactor batch design: a) external view of the batch reactor closed; b) schematic view of the batch reactor inside with mention of the internal available free volume [3]; c) view of the batch reactor inside. a) b) c) d) c) Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Experimental Procedure and Technical Description 22 3.1.3 Experimental procedure Two different types of reactant solutions were prepared to study the kinetics of catalysed sodium borohydride hydrolysis for simultaneous hydrogen generation and storage: classic solutions of sodium borohydride and Less Polar Organic Polymeric Solutions. The first solutions were prepared by adding the appropriate amount of sodium borohydride to a certain volume of aqueous solution of the inhibitor sodium hydroxide. The second type of solutions was prepared by adding the appropriate amount of sodium borohydride to a certain volume of aqueous solution of the inhibitor sodium hydroxide plus small portions of cellulose based polymers. In order to achieve a high energy density and pressures up 2.5 MPa, which would be advantageous for hydrogen storage in the liquid phase by solubility effects, a sodium borohydride constant concentration of 15wt% by weight was found suitable for all the experiments and all of them occurred in excess of water as required. This value was chosen based on previous studies briefly referred in the Chapter 2. The concentration of sodium hydroxide and mass ratio catalyst/sodium borohydride (g/g) were also held constant. In all the experiments, the selected values of these two parameters were defined after the performance of preliminary studies (section 3.1.4). The preparation of test solutions involved a continuous magnetic stirring. Firstly, sodium hydroxide pellets were added to the distilled water. After complete dissolution, the specific biopolymer was added and lastly, sodium borohydride was completely dissolved in the solution. Due to the fact that methyl cellulose polymer is not soluble in water at room temperature, special care was given on its dissolution. A dissolving method of Sigma-Aldrich product information sheet of methyl cellulose was followed [43]: - Heat about 1/3 of the required volume of water to at least 80°C; - Add the methyl cellulose powder to the hot water with agitation; - Agitate the mixture until the particles are thoroughly wetted and evenly dispersed; - For complete solubilisation, the remainder of the water is then added as cold water or ice to lower the temperature of the dispersion. Solution should be cooled to 0-5 °C for 20-40 minutes; - Continuous agitation for at least 30 minutes after the proper temperature is reached [43]. For each experiment, a proper quantity of catalyst measured in an analytical balance, was previously stored in the bottom of the reactor (approximately 0.6 g). Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Experimental Procedure and Technical Description 23 After perfectly sealing the batch reactor, 20 cm3 of reactant solution were rapidly injected into the reactor by using a syringe with a long needle to guarantee that the reactant solution is delivered very close to the catalyst powder. All the experiments were done with one single injection of reactant solution and with reaction temperature control, by using a thermostatic bath, coupled with a refrigeration unit. After each reaction, the remaining solution in the reactor was treated in two centrifuges, firstly at 4000 rpm and then after at 14000 rpm, for the complete separation of the liquid by-product from the solid Ni-Ru based catalyst. The byproduct was collected for posterior hydrogen solubility tests and X-Ray Diffraction studies. The catalyst, after each experiment, was recovered for reutilization. It was washed four times with distilled water, before the complete physical separation from distilled water by gravitational sedimentation. Thereafter it was dried in an oven at 80 ºC for one hour. The calculus involved in the determination of hydrogen rate, yield and gravimetric and volumetric hydrogen density for each reaction is described in the appendix B. 3.1.4 Preliminary studies Preliminary studies were carried out to select the most appropriate sodium hydroxide concentration and a suitable mass ratio catalyst/sodium borohydride. Firstly, for the selection of the most appropriate inhibitor concentration, two classic hydrolysis with 15wt% of sodium borohydride, were performed at room temperature, for two different sodium hydroxide concentrations, 3wt% and 7wt%. These two values of concentration were chosen according to the previous studies mentioned in Chapter 2. For each reaction, 20 cm3 of reactant solution was injected into the batch reactor and a ratio catalyst/sodium borohydride of 0,2 g/g was used. Based on the results obtained, presented in Appendix C, the value of 7wt% for the inhibitor concentration was chosen and was used in all the experiments described in the following sections. The most suitable ratio catalyst/sodium borohydride was selected through the execution of three classic hydrolysis reactions, with different mass ratio catalyst/sodium borohydride: 0.1 g/g, 0.2 g/g and 0.4 g/g. In this preliminary study, 5 cm3 of reactant solution, with 10wt% of sodium borohydride and 7wt% of inhibitor, Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Experimental Procedure and Technical Description 24 was injected into the batch reactor (instead of 20 cm3 of a reactant solution of 15wt% sodium borohydride, justified by economical purposes). The reactions were performed at 45°C which is the optimal operation temperature for the synthesis of the Ni-Ru catalyst. A mass ratio catalyst/sodium borohydride of 0.2 g/g was found suitable (Appendix C) and was used in all the experimental tests described in the following sections. 3.1.5 Effect of biopolymers concentration and reaction temperature The effect of small additions of polymers on hydrogen generation rates and yields were investigate by performing the following experiments: -Classic hydrolysis: 15wt% NaBH4, 7wt% NaOH, 78wt% H2O; -Less Polar Organic Polymeric Solutions Hydrolysis: 15wt% NaBH4, 7wt% NaOH, with the range of polymer concentrations shown in the Table 3, and remaining percentage of H2O. Table 3 – Polymers concentration (weight percent) HEC CMC MC Weight percent (wt %) 0.25 0.05 To explore the effect of reaction the temperature on hydrogen generation rate and yields, the experiments described above, were carried out at two different temperatures: 25°C and 45°C. The first one, close to room temperature, is the ideal temperature for the sodium borohydride hydrolysis to become a real option for producing hydrogen for PEMFC. The second one is referred as the optimal operation temperature for the synthesis of the Ni-Ru catalyst. In these experiments the Ni-Ru based powdered catalyst was reused between 14 and 35 times. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 31 evolution. For this polymer concentration, the CMC polymer enhances strongly the reaction rate reaching a yield above 100%. It seems that for this condition the catalyst is active enough to break the CH bonds in polymers chains, which allows the generation of additional hydrogen gas. At 45°C, the results are surprising. The kinetics of the classic hydrolysis is, as expected, enhanced by the higher temperature, but the evolution curve still has two rate stages. The experiments with the smaller concentration of polymers (Figure 11) show a positive impact of the addition of polymer in the generation rates. One should note the remarkable value of 203 L(H2) min-1 gcat-1 for the CMC case. The curves obtained with the higher polymer concentration show a poorer performance namely for the MC and HEC cases showing a very slow gas evolution after an initial relatively high generation rate. The CMC polymer seems to be the better choice for a higher generation rate and a rapid complete hydrolysis but, as is evident when comparing the curves in Figures 9 to 12, a very small addition of this polymer at room temperature is sufficient to achieve good hydrolysis performances. The hydrogen generation with 0.25wt% of HEC polymer is very slow: at 25°C the gas pressure attained the theoretical predicted value after only approximately 8 hours (30000 seconds), (Figure D.4, Appendix D); at 45°C, the final plateau was reached only after 50 hours (180000 seconds), (Figure D.6, Appendix D). The remaining solution, inside the reactor, after these reactions, has the appearance shown in Figures 13 and 14: Figure 13 - Remaining solution after hydrolysis reaction with 0.25wt% HEC polymer (15wt% NaBH4 and 7wt% NaOH) at 25°C. The catalyst, in the proportion Ni-Ru catalyst/NaBH4: 0.2 g/g was reused 15 times. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 32 It seems that the HEC polymer addition to the reactant solution in a concentration of 0.25wt% forms a “sponge” that may act as a barrier for the hydrogen liberation from solution. On the other hand, the reaction with 0.05wt% of HEC at 45°C presents a better performance than the reaction at 25°C (Figures D.3 and D.5 in Appendix D). Apparently, the increase of temperature for lower concentrations of HEC polymers enhances the reaction performance. The reaction lag time, which is defined as the time required to observe the hydrogen formation, is similar for all the performed experiments. This means that the catalyst reutilization does not affect the lag time (it should be noted that the catalyst was reused among each experiment, between eleven and thirty five times). Curiously, the reactions with HEC polymers present the lower lag time values. It seems that the addition of this polymer is favourable for the beginning of reaction. However, as referred above, probably while the reactants are consumed the solution forms a sponge that slows the hydrogen production rate. The gravimetric and volumetric hydrogen densities obtained in the present work are much lower than the FreedomCAR requirements for hydrogen storage systems established by the US Department of Energy (DOE) – see Table 5, next presented: Figure 14 - Remaining solution after hydrolysis reaction with 0.25wt% HEC polymer (15wt% NaBH4 and 7wt% NaOH) at 45°C. The catalyst, in the proportion Ni-Ru catalyst/NaBH4: 0.2 g/g was reused 27 times. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 33 Table 5 - The gravimetric and volumetric hydrogen densities values publish by US Department Of Energy (DOE) and the values obtained in the present work. The FreedomCAR targets Present work 2007 2010 2015 Gravimetric H2 density (wt%) 4.5 6.0 9.0 2.5-3.2 Volumetric H2 density (kg H2/m3) 36 45 81 27-35 However these specifications are based on system mass and volume, i.e., not only the storage material itself but also the reactors, tanks, valves, and all auxiliary equipment (which usually are call hardware) and the predicted values for gravimetric hydrogen density and volumetric hydrogen density achieved in the present work are based only on the storage material itself. The Appendix B presents the calculus involved in the determination of gravimetric and volumetric hydrogen densities. Table 6 shows the gravimetric and volumetric hydrogen densities and reaction rate obtained in the hydrolysis with addition of 0.25wt% and 0.05wt% CMC polymer performed in the present work and the values published by Ferreira et al. [2]. Table 6 – Comparison between the results obtained from the hydrolysis with CMC addition in the work published by Ferreira et al.[2] and in the present work. Ferreira et al. Present work NaBH4 (wt%) 10 15 NaOH (wt%) 7 CMC polymer (wt%) 0.25 0.25 0.05 Temperature (°C) 45 Catalyst reutilization 280 30 28 mass ratio catalyst/sodium borohydride (g/g) 0.25 0.20 Hydration factor (x) 16 10 Gravimetric H2 density (wt%) 2.0 3.1 Volumetric H2 density (kg H2/m3) 22 34 dP/dt slope (bar/s) 0.11 3.57 6.21 Hydrogen rate (L(H2) min-1 gcat-1) 2.34 117 203 Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 34 As can be seen in the Table 6, the results obtained for the hydrolysis carried out in the present work are much higher than the values published by Ferreira et al. [2]. However, it is important to note that although the catalyst used in both works has the same chemical properties (Ni-Ru), the number of reutilizations is much different, which has an important role in the performance of the hydrolysis. Also the sodium borohydride concentration presents a great contribution. In fact the increase of its concentration value enhanced the hydrolysis reaction hydrodynamics as stated in Chapter 2. On the other hand, the gravimetric and volumetric energy densities achievable from hydrolysis of chemical hydrides depend in large measure on the amount of water required for the process. Experimental studies revealed that if the excess hydration factor (x) is less that 2 (x < 2), unreacted hydride remains in solution, but if x > 2 there is enough water for complete reaction and hydration of the borate [3]. However, chemical hydrides have the potential to meet the hydrogen storage targets only if the water consumption is minimized. In fact, the reduction of the excess hydration factor (x) from 16 to 10 leads to an increase of gravimetric and volumetric densities, as can be checked in Table 6. The Table A.1 (Appendix A) shows a review of Ruthenium based catalyst for sodium borohydride hydrolysis since the year 2000. Comparing the values of hydrogen rate obtained in the present work with other values published elsewhere, it is possible to say that, in general, Ni-Ru based catalyst presents a great performance. All the present values of hydrogen rate (L(H2) min-1 gcat-1) are higher than the published by the authors referred in the Table A.1. However it is worth to mention that the experimental conditions are not the same. 4.2 Catalytic activity study A set of classic hydrolysis experiments was performed, at different temperatures, using fixed concentrations of sodium borohydride and sodium hydroxide as well as a constant ratio catalyst/sodium borohydride (15wt%, 7wt% and 0.2 g/g). Those experiments allowed the study of the Ni-Ru catalyst activity by the determination of its activation energy. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 35 0 0,5 1 1,5 2 2,5 3 0100 200 300 400 Pressure / bar Time / s 15°C 25°C 35°C 45°C 55°C 65°C Figure 15 shows the rate of hydrogen generation, in terms of hydrogen pressure as a function of time, for a range temperature between 15°C and 65°C. In Figure 15, it is possible to verify the increasing slope values on the linear region of the plots, for increasing values of the reaction temperature. In fact the reaction rate is strongly influenced by the reaction temperature. During the course of hydrolysis reaction, the reaction rate remains approximately constant while sodium borohydride and water are consumed. The reaction rate follows a linear variation with time until the pressure plateau is reached, indicating the end of reaction. These results allow to confirm that the studied sodium borohydride hydrolysis presents a zero-order kinetics related to the temperature as stated by Liu et al. [12]. In order to estimate the catalyst activation energy of the used Ni-Ru catalyst, the results were represented in an Arrhenius plot, as shown in Figure 16. An activation energy of 104 kJ/mol was determined by the slope of Arrhenius plot. Figure 15 – Hydrogen generation rate as a function of time for catalyzed hydrolysis of sodium borohydride (5cm3 of reactant solution with 15wt% NaBH4 and 7wt% NaOH) at different temperatures. The catalyst, in the proportion Ni-Ru catalyst/NaBH4: 0.2 g/g was reused between 38 and 43 times. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 36 -8 -7 -6 -5 -4 -3 -2 -1 0 0,0029 0,003 0,0031 0,0032 0,0033 0,0034 0,0035 Ln(k) 1/T /K-1 Regarding to the information about the optimal sodium borohydride hydrolysis operation temperature for the used Ni-Ru based catalyst, which is 45°C and the highest slope value for 45°C, it was also determined the catalyst activation energy for the temperature range between 45°C and 65°C. In fact, for this latter temperatures interval, the catalyst presents an activation energy of 65 kJ/mol, which is considerable lower than the determined for all the studied temperature range, patent in Figure 16. The Table 7 shows a comparison between the activation energy value obtained in the present work thesis and the values available in the open literature for ruthenium based catalyst. Table 7 - Comparison of the activation energy between the catalyst used in the present work with data published on literature. Authors /Year Catalyst Temperature °C Energy activation (kJ/mol) Amendola et al. 2000 Ru/Resin 25-55 47 Ru/IRA400 0-40 56 Shang et al. / 2006 Ru/C 26-60 37 This work Ni-Ru 15-65 104 This work Ni-Ru 45-65 65 Figure 16 – Arrhenius plot obtained from the kinetic data of hydrolysis reaction performed at temperatures from 15°C to 65°C. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 37 4.3 Preliminary Hydrogen solubility study The polymers addition to the reactant solution makes it a less Polar Organic Polymeric Solution (lPOPS) and causes a change in the overall conductivity of the remaining solution inside the reactor. In the Table 8, the pH and conductivity values for all the solutions tested, measured before and after the reaction completion are listed. Table 8 - Values of pH and conductivity of the reactant solutions before and after reaction completion Before reaction After reaction pH Condutivity (mS/cm) pH Condutivity (mS/cm) Classic Hydrolysis 45°C 11.37 96.8 11.88 86.1 0.05wt% HEC 45°C 11.63 117.5 11.73 54.9 0.25wt% HEC 25°C 11.63 124.8 11.58 85.2 0.05wt% CMC 45°C 11.47 145.1 11.67 53.2 0.25wt% CMC 45°C 11.45 ------- 11.60 112.4 0.05wt% MC 45°C 11.58 109.1 11.71 105.9 0.25wt% MC 25°C 11.75 ------- ------- 96.4 It is well known that the conductivity of an aqueous solution measures its ability to conduct an electric current and is highly dependent on the concentration of dissolved salts. As can be seen from the values conductivity displayed, the use of the small addition of only 0.05 wt% of HEC and CMC at the higher temperature reveals a strong decrease on the conductivity after reaction making these two polymers as good candidates for the storage of H2 (a non polar molecule) in the liquid phase. The Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 38 presence of the polymers probably changes the polarity of the final solution due to the presence of the C-H groups. In a very near future and after solving some experimental difficulties, solubility tests will be performed, in which the remaining solutions after sodium borohydride hydrolysis will contact with pure hydrogen, pressurized at different pressures, up to 2.5 MPa. This study will allow to estimate the amount of hydrogen gas that solubilises in each solution and to compare the capability of the postreactant solutions to store hydrogen in the liquid phase. From the experiments reported and recalling the reaction yields showed in Table 4 and the complete generation curves presented in Appendix D, the following remaining solutions should be also tested: at 25 oC HEC 0.05wt% and MC 0.25wt%; at 45 oC HEC 0.25wt% and MC 0.25wt%. It should be noted that in these experiments the gas generation was very slow after an initial rapid generation rate, suggesting that the H2 formed and dissolved in the solution was slowly released. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 39 4.4 Reaction products analysis The by-products crystals of all the reactions performed at 25°C were obtained by slow evaporation of a water solution at environmental conditions in the laboratory. The crystals (pictures shown in Figure 17) were analyzed by X-Ray Diffractometry. Figure 17 – By-products crystals pictures of the reactions performed at 25°C, obtained by slow evaporation of a water solution at environmental conditions in the laboratory. Classic hydrolysis 0.05wt% CMC 0.25wt% CMC 0.05wt% HEC 0.25wt% HEC 0.05wt% MC 0.25wt% MC Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Results and Discussion 40 The XRD analysis of the crystals by-products structure, from the classic and with the addition of 0.05wt% HEC and 0.05wt% MC hydrolysis, reveals that both of them are a sodium metaborate dehydrate, NaBO2.2H2O. The boron atoms are in a triangular configuration with three oxygen atoms, with B-O bond lengths between 1.27 and 1.29 Å. The BO3 triangular arrangement was already found in the crystal structure of orthorhombic [44] and monoclinic [45] metaboric acid although in the title crystal structure the oxygen atoms are bound to just one B atom. The BO3 triangles form layers 3.238 Å apart that sandwich a layer of sodium and water molecules. For each boron atom there are three bound oxygen atoms and one oxygen of water. Half of the sodium atoms have their valence strength distributed between six Na-O bonds and the other half between seven Na-O bonds (as already determined in other sodium metaborate structures [46]). The water molecules are involved in a net of interactions with the sodium atoms. Figure 18 shows two different views of the crystal structure of sodium metaborate dehydrate. The crystal structure of by-products resultant from the sodium borohydride hydrolysis reactions with addition of 0.25wt% of all the tested polymers and of 0.05wt% of CMC reveals that the by-product is a sodium tetrahydroxoborate NaB(OH)4. The boron atoms are in a tetrahedral configuration with four oxygen atoms, with B-O bond lengths within the interval 1.465 and 1.487 Å. The B-O interatomic distances are around the expected value for a tetrahedral configuration 1.475 Å, which is appreciable higher than observed for triangular configurations a) b) Figure 18 Views of crystal structure of sodium metaborate dehydrate (obtained from the classic hydrolysis and hydrolysis with addition of 0.05wt% HEC and 0.05wt% MC, at 25°C). Na in violet, O in red, B in pink and H in white: a) view along the a-axis of the crystal structure showing stacking layers formed by BO3 triangles and by Na and water molecules; b) view along the c-axis of the crystal structure showing the BO3 triangular arrangement and the Na bound network. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects References 47 7 References [1] [1] M.J.F. Ferreira, L. Gales, V.R. Fernandes, C.M. Rangel, A.M.F.R. Pinto, “Alkali free hydrolysis of sodium borohydride for hydrogen generation under pressure” International Journal of Hydrogen Energy 35 (2010) 9869– 9878. [2] M.J.F. Ferreira, V.R. Fernandes, L. Gales, C.M. Rangel, A.M.F.R. Pinto, “Effects of the addition of an organic polymer on the hydrolysis of sodium tetrahydroborate in batch reactors” International Journal of Hydrogen Energy 35 (2010) 11456–11469. [3] "Catalytic generation and storage of hydrogen from hydrolysis of sodium borohydride under pressure" M.J.F. Ferreira, Master Thesis, Faculty of Engineering from University of Porto (2009). [4] M. Yadav, Q. Xu, “Liquid-phase chemical hydrogen storage materials” Energy & Environmental Science 5 (2012) 9698-9725. [5] V.R. Fernandes, A. Esteves, M.J.F. Ferreira, A.M.F.R. Pinto, C.M. Rangel, “Generation of Hydrogen from Chemical Hydrides under Pressure up to 70 Bar", 3rd International Seminar: Hydrogen Energy and Sustainability - Advances in Fuel Cells and Hydrogen Workshop, Torres Vedras, Portugal, 2930 April (2010). [6] S.S. Muir, X. Yao, “Progress in sodium borohydride as a hydrogen storage material: Development of hydrolysis catalysts and reaction systems” International Journal of Hydrogen Energy 36 (2011) 5983–5997. [7] “U.S. Department of Energy: Fuel Cells Technologies Office" -Available from: http://www.eere.energy.gov/. [8] U.B. Demirci, P. Miele, “Sodium tetrahydroborate as energy/hydrogen carrier, its history” Comptes Rendus Chimie 12 (2009) 943–950. [9] "On-Board Hydrogen production technologies (reforming , chemical hydrides , ammonia borane )”C.M. Rangel, V.R. Fernandes, T. Sousa, P.R. Pinto, Technology Watch Report, ISQ (2011). [10] U.B. Demirci, O. Akdim, P. 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Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Appendix A 51 Appendix A – Review of ruthenium based catalyst Table A.1 - Data on Ruthenium based catalyst for sodium borohydride hydrolysis since the year 2000[6]. Year Authors Catalyst Activity (L(H2)/min/ gcat) Activity (L(H2)/min/ gmetal) NaBH4 wt% NaOH wt% T (ºC) 2000 Amendola et al. Ru/IRA-400 0.189 3.80 20 10 25 2005 Krishnan et al. PtRu/LiCoO2 2.400 24.0 5 5 25 2005 Ozkar et al. Ru nanoclusters 96.80 96.8 0.75 0 25 2006 Zahmakiran et al. Ru nanoclusters 0.004 0.004 0.57 10 25 2007 J. Zhang et al. Ru/C 0.770 12.9 1 3.75 25 2008 Dermici et al. Ru-Pt/TiO2 0.150 15.2 2 4 20 2008 Park et al. Ru-Fe-Co/AC 5.030 41.7 10 4 25 2008 Z. Liu et al. Ru/LiCoO2 2.700 270 10 5 25 2008 Hsueh et al. Ru/IR-120 0.132 13.2 5 1 25 2009 Zahmakiran et al. Ru nanoclusters / zeolite 0.130 16.1 1.1 5 25 2009 Chen et al. Ru/PS 0.216 5.40 1 1 Not state d 2010 Liang et al. Ru/C (graphite) 0.969 32.3 10 5 30 Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Appendix B 52 Appendix B – Hydrogen rate, yield and gravimetric and volumetric hydrogen densities determination 1) Hydrogen Reaction rate determination The hydrogen generation rate Q (L(H2)min-1gcat-1) was determined taking into account the ideal gas Law: Where: 2) Reaction yield determination The hydrogen yield in all the reactions was determined by the following equation: Where n(H2)experimental corresponds to the number of moles of hydrogen produced and n(H2)theoretical is the theoretical amount of hydrogen produced assuming 100 % conversion of sodium borohydride by applying the ideal gas law to the final volume of gas inside the batch reactor. The reactor was not initially evacuated, so the air is taken into account in the calculations. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Appendix B 53 3) Gravimetric and volumetric hydrogen densities determination The gravimetric capacity or specific energy is a measure of the usable energy per kilogram of system mass. In practical terms, this quantity is usually expressed as a weight percentage of hydrogen. Volumetric capacity or energy density is the useful energy per litre of system volume. The gravimetric and volumetric hydrogen densities were determined by the following equations: Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Appendix C 54 0 5 10 15 20 25 30 0500 1000 1500 2000 Pressure / bar Time / s 7wt% NaOH 3wt% NaOH Appendix C - Preliminary studies In order to select the most adequate sodium hydroxide concentration for all the hydrolysis experiments of the present project, two experiments were made at constant temperature, 25°C, for two sodium hydroxide concentrations, 3wt% and 7wt%.The influence of sodium hydroxide concentration on the classic hydrolysis is put in evidence in Figure C.1. As can be seen in the Figure C.1, for the higher value of sodium hydroxide concentration, the reaction rate is significantly enhanced. This result is according to the previous studies mentioned in the Chapter 2. The reaction with 3wt% of sodium hydroxide attained completion after 500 seconds and a yield of 97% was found. On the other hand the reaction, with 7wt% of sodium hydroxide, attained completion after 400 seconds and a yield of 100% was found. Hence, the value of 7wt% sodium hydroxide was chosen for all the experiments carried out. Figure C.1 - Hydrogen generation rate as a function of time for classic hydrolysis of sodium borohydride (15wt%) at two different sodium hydroxide concentrations, with a ratio catalyst/sodium borohydride of 0.2g/g. The reactions, for 20 cm3 of reactant solution were performed at 25°C.The catalyst was reused 11 and 12 times. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Appendix C 55 0 1 2 3 4 5 0100 200 300 400 500 Pressure / bar Time / s 0.1 g/g 0.2 g/g 0.4 g/g The most suitable ratio catalyst/sodium borohydride was defined after the execution of three classic hydrolysis reactions at 45°C with different mass ratio catalyst/sodium borohydride: 0.1 g/g, 0.2 g/g and 0.4 g/g. Figure C.2 shows the plot of the reaction rate with different ratios catalyst/sodium borohydride. According to the Figure C.2, the hydrolysis reaction under study presents a better performance with the increasing of ratio catalyst/sodium borohydride. However, the value of 0.2 g/g was defined as suitable for the experiments, according to the total amount of catalyst available for present work. Figure C.2 - Effect of ratio catalyst/sodium borohydride on the hydrogen generation rate via classic hydrolysis of sodium borohydride (10wt% NaBH4 and 7wt% NaOH). The reactions, for 5 cm3 of reactant solution were performed at 45°C and stabilized with 7wt% of sodium hydroxide.The catalyst was reused 8, 9 and 10 times. Batch sodium borohydride hydrolysis in the presence of nickel-ruthenium based catalyst: Addition of biopolymers to improve hydrogen storage in the liquid phase by solubility effects Appendix D 56 0 20 40 60 80 100 120 0 5 10 15 20 25 30 0200 400 600 800 1000 1200 1400 1600 1800 Temperature /°C Pressure / bar Time / s Classic hydrolysis Theoretical pressure T bottom T top 0 20 40 60 80 100 120 140 0 5 10 15 20 25 30 01000 2000 3000 4000 Temperature /°C Pressure / bar Time / s Classic hydrolysis Theoretical pressure T bottom T top Appendix D – Hydrogen generation and temperatures profiles  Classic hydrolysis: Figure D.1 - Hydrogen generation rate as a function of time and temperatures profile for classic hydrolysis of sodium borohydride (20cm3 of reactant solution with 15wt% NaBH4 and 7wt% NaOH) at 25°C. The catalyst, in the proportion Ni-Ru catalyst/NaBH4: 0.2 g/g was reused 11 times. Figure D.2 - Hydrogen generation rate as a function of time and temperatures profile for classic hydrolysis of sodium borohydride (20cm3 of reactant solution with 15wt% NaBH4 and 7wt% NaOH) at 45°C. The catalyst, in the proportion Ni-Ru catalyst/NaBH4: 0.2 g/g was reused 35 times.