Instructions for use

Title Feasibility of Utilizing Hydrogen Storage Alloys and Ammonia as Energy Media

Author(s) 安田, 尚人

Citation 北海道大学. 博士(工学) 甲第11443号

Issue Date 2014-03-25

DOI 10.14943/doctoral.k11443

Doc URL http://hdl.handle.net/2115/58055

Type theses (doctoral)

File Information Yasuda_Naoto.pdf

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

https://eprints.lib.hokudai.ac.jp/dspace/about.en.jsp

1

Feasibility of Utilizing Hydrogen Storage Alloys and Ammonia as Energy Media

エネルギーメディアとしての水素吸蔵合金およびアンモニアの利用可能性エネルギーメディアとしての水素吸蔵合金およびアンモニアの利用可能性エネルギーメディアとしての水素吸蔵合金およびアンモニアの利用可能性エネルギーメディアとしての水素吸蔵合金およびアンモニアの利用可能性

Laboratory of Energy Media

Naoto YASUDA

Hokkaido University

2014

2

Contents

Chapter 1 General Introduction ................................................................. 4 1.1 Toward hydrogen economy .................................................................................. 4

1.2 Hydrogen storage methods ................................................................................... 6

1.3 Properties, production and applications of hydrogen storage alloys ............. 10

1.4 Combustion synthesis (CS) in hydrogen atmosphere ...................................... 18

1.5 Composite materials of hydrogen storage alloy ............................................... 20

1.6 Ammonia as a hydrogen medium ...................................................................... 21

1.7 Hydrogen utilization for ironmaking process ................................................... 22

1.8 Purpose of this study ........................................................................................... 24

References .................................................................................................................. 27

Chapter 2 Self-ignition Combustion Synthesis of LaNi5-based Hydrogen Storage Alloy Utilizing Hydrogenation Heat of Metallic Calcium ...................................................................................................... 33

2.1 Introduction ......................................................................................................... 33

2.2 The effect of reaction atmosphere on hydrogenation properties of LaNi5 ..... 36

2.3 Initial activity of LaNi5 synthesized at different hydrogen pressures ............ 49

2.4 Exergy analysis of self-ignition combustion synthesis for producing

rare-earth-based hydrogen storage alloy ................................................................ 63

2.5 Conclusion ............................................................................................................ 82

References .................................................................................................................. 85

Chapter 3 Production of TiFe-based Hydrogen Storage Alloy from Ilmenite via Self-ignition Combustion Synthesis ................................... 88

3.1 Introduction ......................................................................................................... 88

3.2 Experiment .......................................................................................................... 91

3.3 Results and discussion ........................................................................................ 96

3.4 Conclusion .......................................................................................................... 105

References ................................................................................................................ 107

3

Chapter 4 Properties of Metal Hydride Sheet and Its Application to Chemical Heat Pump System ................................................................. 109

4.1 Introduction ....................................................................................................... 109

4.2 Cycle characteristic and thermal conductivity of metal hydride sheet ......... 112

4.3 Application of metal hydride sheet to thermally driven heat pump ............. 123

4.4 Conclusion .......................................................................................................... 140

References ................................................................................................................ 143

Chapter 5 Utilization of Ammonia as Both Hydrogen Medium and Reducing Agent for Ironmaking ............................................................ 145

5.1 Introduction ....................................................................................................... 145

5.2 Reduction and nitriding behavior of hematite with ammonia ...................... 148

5.3 Estimation of the CO2 emission for ammonia ironmaking ........................... 164

5.4 Conclusion .......................................................................................................... 172

References ................................................................................................................ 174

Chapter 6 General Conclusion ............................................................... 176

Acknowledgements ................................................................................. 180

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Chapter 1

General Introduction

1.1 Toward hydrogen economy

Fossil fuels, in the form of coal, oil, and natural gas, have driven the global energy

system since the industrial revolution. However, with increasing the consumption of the

fossil fuels, we face serious problems: one is the depletion and price increasing of the

fossil fuels, and another is the global warming caused by the emission of green house

gases. For sustainable development, production and use of renewable energy should be

increased instead of the fossil fuels. For example, sunlight, solar heat, geothermal heat,

wind power, wave power, and biomass are promising renewable energies. In order to

utilize these renewable energies effectively, they must be converted to a secondary

energy for storage and transportation.

Hydrogen is an ideal candidate as the secondary energy for both transportation and

stationary applications because of the following reasons:

(1) Hydrogen can be produced using various primary energies including renewable

energies.

(2) Storage of hydrogen energy is easier and cheaper than that of electricity.

(3) Hydrogen can be used as green fuel for vehicular application instead of

petroleum.

(4) Hydrogen is directly converted to electricity using fuel cell (FC).

(5) It is clean and environmentally-friendly energy because the product after

extracting energy from the hydrogen is only water.

5

There are three important ingredients for hydrogen utilization; production,

application, and storage and transportation of hydrogen. Hydrogen gas is industrially

produced from natural gas or coal by steam reforming. Furthermore, it can be produced

from water by electrolysis, pyrolysis and photolysis, and it is possible to utilize various

primary energy including the renewable energy. The hydrogen energy is converted to

thermal, mechanical and electric energy using combustor, internal-combustion engine

and fuel cell, respectively.

The storage and transportation of hydrogen is important because its volume energy

density is quite low. There are various hydrogen carriers to store and to transport

hydrogen with high density, which describes in the next section.

6

1.2 Hydrogen storage methods

In particular, hydrogen storage is a key issue in the success and realization of

hydrogen economy [1] because its volume energy density is quite low. Several

hydrogen storage methods have been proposed such as compression, liquefaction,

physisorption, metal hydride, complex hydride, hydrocarbons, and ammonia [2]. Fig.

1-2-1 shows the hydrogen storage densities of various hydrogen storage materials [3].

Fig. 1-2-1 Hydrogen storage densities of various hydrogen storage materials [3].

(1) Compressed gas storage

Presently, compressed gas storage in a high-pressure cylinders are the most

common and the simplest way to store hydrogen. If the cylinders are capable of

withstanding pressure up to 70 MPa with a weight 110 kg, its gravimetric and

Vol

umet

ric

hydr

ogen

den

sity

[kg-

H2m

-3]

Gravimetric hydrogen density [mass%] 2520151050

160

140

120

100

80

60

40

20

0

Density 5 g cm-3 2 g cm-3 1 g cm-3 0.7 g cm-3

TiFeH1.7300 K, 0.15 MPa

LaNi5H6300 K, 0.2 MPa

Mg2NiH2600 K, 0.4 MPa

MgH2620 K, 0.5 MPa

NaBiH4doc. 680 K

LiAlH4doc. 400 K

NaAlH4doc. >520 K

KBH4doc. 580 K

LiBH4doc. 553 K

Liquid hydrogen20.3 K

Al(BH4)3doc. 373 K

NH3 (liq.)b.p. 240 K

CH4 (liq.)b.p. 112 K

Hydrogen physisorbed on carbon(Theoretical value)

C6H12 (liq.) ↔ C6H6 (liq.)

C7H14 (liq.) ↔ C7H8 (liq.)

High-pressure tank70 MPa, 110 kg

7

volumetric densities are 6% and 30 kg m-3, respectively. However, the volumetric

hydrogen density as such is remarkably lower than the other methods. Moreover,

high-pressure cylinders present a considerable risk; the compression hydrogen is

the most dangerous and complicated part.

(2) Cryogenic hydrogen storage

Condensation into liquid or even solid hydrogen is particularly attractive from

the point of view of increasing the mass per container volume. The density of liquid

hydrogen is 70.8 kg m-3. However, this method faces two challenges: the efficiency

of the liquefaction process and the boil-off of the liquid because the critical

temperature of hydrogen is very low (33.2 K), above which liquid state cannot

exist.

(3) Physisorption

Physisorption, whereby hydrogen molecules interact with surface atoms of

solids, offers great potential as a hydrogen medium. For storage purposes, the

adsorption of hydrogen has been studied on carbon species, whose storage density

is theoretically limited to the density of liquid hydrogen. The monolayer adsorbate

is itself fixed by a weak interaction; therefore, significant physisorption is only

observed at relatively low temperatures.

(4) Metal hydride

Some metals and alloys called metal hydrides or hydrogen storage alloys react

reversibly with hydrogen and enable both hydrogen storage and release at

8

environmental temperatures. They stores hydrogen in the form of hydrides, whose

volumetric hydrogen densities are usually greater than those of compressed or

liquid hydrogen. For example, the volumetric hydrogen density of LaNi5H6 is 1.6

times larger than that of liquid hydrogen.

(5) Complex hydride

Group I, II, and III elements, such as Li, Mg, B, Al, form stable and ionic

compounds with hydrogen. The hydrogen storage densities of these compounds are

quite attractive: LiBH4 contains 18 mass% of hydrogen in gravimetric density and

Al(BH4)3 stores 150 kg m-3 in volumetric density. In contrast, The high reaction

temperature and low kinetics for the hydrogen desorption process are major

problems.

(6) Hydrocarbons

Some hydrocarbons can also be considered as a liquid storage medium for

hydrogen if they can be hydrogenated and dehydrogenated reversibly. Cyclohexane

(C6H12), for example, reversibly desorbs six hydrogen atoms (7.1 mass%) and

forms benzene (C6H6). Recently, toluene (C7H8) ˗ methylcyclohexane (C7H14)

system is discussed in detail.

(7) Ammonia

Ammonia, which has a high hydrogen content, has created considerable

interest in recent years as an attractive hydrogen transport medium [4-7]. The

hydrogen storage density of ammonia is 17.6 mass% and 120 kg/m3-liquid NH3,

9

which are considerably higher than those of most advanced metal hydrides.

Ammonia gas is relatively easy to liquefy (240 K at 101 kPa) as compared with

hydrogen gas (140 K at 101 kPa). Furthermore, there are well-established

production, storage, and transportation technologies already in place since more

than 160 million tons of ammonia are produced per year [8]. In contrast, toxicity is

the major problem for the utilization of ammonia as a hydrogen storage materials.

In this study, the feasibility of utilizing two promising hydrogen media, hydrogen

storage alloy and ammonia, is discussed. The characteristics, applications, and

development challenges of them are described in the following sections.

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1.3 Properties, production and applications of hydrogen storage alloys

1.3.1 Properties of hydrogen storage alloy

1.3.1.1 Metal hydride (MH) and hydrogen storage alloy

Some metals and alloys can react reversibly with a large amount of hydrogen under

certain conditions. They are called metal hydrides (MH), and the reaction can be written

as follow:

Hx

x ∆+=+ MHH2M 2

The prototype MH, so-called hydrogen storage alloy, is composed of two elements

(defined as A and B). The A tends to form a stable hydride such as rare earth, alkaline

earth and IVa grope metals. The B element is often transition metal and forms only

unstable hydrides.

The characteristic of the hydrogen storage alloy is as follows;

(1) The storage of hydrogen in the form of hydrides is the safest and relatively large

volume storage density, compared with high pressure gas tank and liquefied

hydrogen.

(2) It can react with hydrogen of atmospheric pressure under room temperature.

(3) The hydride emits hydrogen easily by heating and/or a reduced pressure.

(4) It has good reversibility of hydrogenation/dehydrogenation with many metallic

hydrides. The reaction is caused with exotherm and endotherm.

From these properties, hydrogen storage alloy have expected to be used not only as

hydrogen storage carrier but also as energy conversion materials. To achieve the

practical use, hydrogen storage alloy should meet the following criteria:

(1) Activation at moderate conditions

(1.3.1)

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(2) High storage capacity

(3) Suitable heat of formation and equilibrium pressure

(4) Wide and flat plateau area and low hysteresis

(5) High thermal conductivity

(6) High stability against O2 and moisture for long cycle life

(7) Low cost and abundant resources

Different types of materials have been investigated for improving above properties,

including LaNi5 and TiFe compounds, Zr- and Ti-based laves phase, Mg2Ni- and

Mg-based materials, and composites [9, 10]. In this study, the production and

applications of LaNi5 and TiFe -based alloy are discussed. These typical hydrogen

storage alloys have been well studied since 1970s because they can easily react with

hydrogen of atmospheric pressure under room temperature, and show good reversibility

of hydrogenation/dehydrogenation.

1.3.1.2 LaNi5 based hydrogen storage alloy

Nickel rare-earth metal compounds, a typical AB5-type hydrogen storage alloys, is

the most studied compound. The parent compound LaNi5 absorbs about 1.4 mass% in

hydrogen storage capacity and is easy to activate at room temperature. The PCT

property shows a flat plateau and low hysteresis [11-17]. The multi-component

La1-xRExNi5-yMy systems have been studied (RE = Ce, Nd and Mm etc., M = Mn, Cr, Fe,

Co, Cu, Al, Sn, Ge and Si) [18-25]. In particular, MmNi5 based alloy was utilized as

electrode material of Ni-MH battery, which has reached the marketing stage since

1990s.

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1.3.1.3 TiFe based hydrogen storage alloy

It is well known that titanium iron (TiFe), a typical AB-type hydrogen storage alloy,

has many advantages such as high cost performance and abundance in resources,

moderate conditions for hydrogenation and dehydrogenation and relatively large

hydrogen storage capacity of 1.8 mass% [26, 27]. However, TiFe has not been used for

practical purposes, mainly because it requires a quite tough activation treatment [28-32].

Further, it requires a cyclic procedure of heating up to 673 K at vacuum, cooling down

to room temperature, and pressurizing hydrogen up to 4 MPa. This procedure must be

repeated over ten times. Even worse, TiFe has dual plateaus in the PCT diagram, where

the equilibrium pressure of β-hydride is different from that of γ-hydride. For this reason,

the difference in pressure for hydrogenating and dehydrogenating is relatively large. To

solve the two problems in hydrogenation properties of TiFe, i.e., the activation difficulty

and dual plateau property, many papers reported the effect of using a third element to

substitute Fe or Ti partially. The multi-component Ti1-xZrxFe1-yMy systems have been

studied (M = Mn, Ni, Cr, Co, and Al etc.) [33-39].

13

1.3.2 Production of hydrogen storage alloy and its problems

In general, hydrogen storage alloys are conventionally produced by using a melting

method that requires several time- and energy-consuming processes such as heat

treatment, pulverization, and activation treatment. In the heat treatment, the product is

kept at high temperature for long time for homogenization. The post-treatment of

pulverization and activation is also necessary for increasing the surface area of the

product and for improving the reactivity with hydrogen. In particular, the activation

treatment needs a repeated procedure of hydrogenation and dehydrogenation by heating

and vacuuming, which is the most energy consuming among the processes [30, 40] As

mentioned above, TiFe alloy requires a quite tough activation treatment. In contrast,

LaNi5 is easy to activate at room temperature. However, the initial activity at hydrogen

pressure lower than 1.0 MPa is much more important for the practical use from a

Japanese regulation on the use of high pressure gas. In general, an activation treatment

required excess hydrogen pressure than equilibrium hydrogen pressure of hydrogen

storage alloys [41]. The newly developed alloy is strongly required to store as much

hydrogen as possible smoothly at lower pressure close to an equilibrium pressure.

According to literature research using a major database, for producing

rare-earth-based materials, reduction-diffusion (RD) process has been proposed based

on the reduction of rare-earth oxide by calcium [42]. Itagaki et al. directly produced a

nickel-mischmetal compound by using the RD process [43, 44], in which the raw

materials were maintained at around 1273 K for as long as several hours because of the

slow solid diffusion whereas a pulverization treatment is not required because rare-earth

oxide was directly reduced to powders by metallic calcium or calcium hydride. This

paper elucidates a new production route of a rare-earth based alloy using the reduction

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of rare-earth oxide by calcium; however, the RD process still requires heat treatment at

around 1273 K for several hours.

Especially for TiFe production, alloying and homogenization is a time- and

energy-consuming process because the melting temperature of Ti is considerably high

(1941 K). Moreover, metallic Ti production involves many processes. Ilmenite, which

contains both Ti and Fe, is one of the common Ti ores. In metallic Ti production, iron in

the ilmenite is first removed by an upgrading process such as the Becher process [45].

Subsequently, the titanium oxide is chlorinated to produce titanium tetrachloride and is

then reduced by metallic magnesium at about 1273 K (Kroll process) [46]. In overall

TiFe alloy production process, iron is removed from ilmenite once to produce titanium

oxide and added again during alloying stage. Many researchers have used ilmenite as an

attractive raw material to produce TiFe-based alloys. Saito et al. reported that metallic

Fe–Ti–Si alloys were successfully produced from ilmenite ore by the direct reduction

process using Si as reducing agent and additional element [47]. Molten salt electrolysis

is another method of synthesizing intermetallic compounds [48-50]. Meng et al.

succeeded in synthesizing TiFe by electrochemical synthesis in molten calcium chloride

from solid ilmenite [51]. Xionggang et al. synthesized TiFe alloy by solid oxide

membrane (SOM) electrolysis method from natural ilmenite and revealed the

mechanism involved [52]. On the other hand, research for TiFe production from

ilmenite by means of thermal-chemical processes has been limited.

Therefore, the challenges for the production of hydrogen storage alloys were

summarized as follows:

(1) Hydrogen storage alloys are conventionally produced by using a melting method

that requires several time- and energy-consuming processes such as heat treatment,

15

pulverization, and activation treatment.

(2) Reduction-diffusion (RD) process is a new production route of a rare-earth based

alloy using the reduction of rare-earth oxide by calcium; however, it still requires

heat treatment at around 1273 K for several hours.

(3) Especially for TiFe production, alloying and homogenization is a time- and

energy-consuming process because the melting temperature of Ti is considerably

high (1941 K). Furthermore, in overall production process, iron is removed from

ilmenite once to produce titanium oxide and added again during alloying stage.

16

1.3.3 Applications of hydrogen storage alloy and its problems

An electrical power demand and demand gap between daytime and nighttime are

increasing. To solve those problems, the utilization of hydrogen storage alloy for energy

storage was proposed [53, 54]. The hydrogen storage alloy is expected to be used not

only as a hydrogen storage carrier but also as an energy conversion materials. The

applications of hydrogen storage alloy have been much extended, for example, Ni-MH

battery [55, 56], heat storage [57], purification of hydrogen gas [58, 59], and chemical

heat pump [60]. The thermally-driven metal hydride heat pump (MHHP) system is a

promising candidate to utilize low-grade heat, because it has no mechanical moving

parts and uses an environmentally friendly working fluid. Moreover, the system can be

applied in wide operating-temperature ranges by varying the composition of alloy.

After numerous cycles of hydrogen absorption/desorption, grain sizes of metal

hydride (MH) including the hydrogen storage alloy decrease to about 1 µm because of

volume expansion which brings large microstress and leads to pulverization. Reduced

grain size has some negative effects on the heat transfer of MH beds and the walls of

reactors. A packed-bed reactor filled with MH powder has insufficient heat transfer

because of its low thermal conductivity and high thermal contact resistance to the wall

of the reactor. In contrast, the intrinsic reaction kinetics of MHs is relatively fast. Thus,

the reaction rate of hydrogen absorption/desorption is limited mainly by heat transfer.

A large amount of stress, on the other hand, is generated on the walls of vessels by

volume expansion of MH during hydrogen absorption. Several investigations have been

conducted on the expansion characteristic of MH and on its effects on reaction vessels

[61-63]. Nasako et al. reported a stress accumulation mechanism in which fine powder

generated by pulverization over an increasing number of cycles fell to the bottom of the

17

vessel where it gradually increased the packing fraction [61]. Qin et al. investigated the

pulverization and expansion characteristics of La0.6Y0.4Ni4.8Mn0.2, and concluded that

the packing fraction should not exceed 35 vol% for the case of hydrogen contents

around 1.0 mol-H/mol-alloy [63]. However, the effective thermal conductivity of an

MH bed must decrease with decreasing the packing fraction.

In order to solve these problems, it is important to improve the heat transfer of the

MH bed, to prevent sediment accumulation of finely powdered MH onto the bottom of

the reactor, and to control the packing fraction of the MH bed.

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1.4 Combustion synthesis (CS) in hydrogen atmosphere

In this thesis, a kind of powder technology, called combustion synthesis (CS) or

self-propagation high-temperature synthesis (SHS), is applied to the production process

of hydrogen storage alloys. The CS based on self-propagation mode was established by

Merzhanov et al. [64, 65] and has been used to synthesis various materials including

ceramics, intermetallics and composite. This process utilizes highly exothermic reaction

between two or more species (including gas species) which propagates as a combustion

wave through a mixture of raw material powders. The CS has many advantages such as

shortening the processing time, boosting energy efficiency and getting the product of

high pure.

In 1997, hydriding combustion synthesis (HCS) that was based on the

self-propagation mode was proposed for direct production of metal hydrides [66, 67]. In,

this process, the CS is applied in hydrogen atmosphere in order to utilize exothermic

reactions not only between raw material powders, but also between the powders and

hydrogen. One example is direct production of Mg2NiH4 without activation treatment

[68-73]. Saita et al. [74] reported that HCS using excess Ti powder activated TiFe more

easily when compared to the conventional melting method. The products exhibited good

cycling property; however, this method was not suitable for the mass production of TiFe

because the excess Ti was needed as the heat source. Thus, the CS based on self-ignition

mode was applied to produce TiFe alloy, where the mixed powders of the desired

compositions are uniformly heated up to the ignition point without using excess Ti. In

this study, we call this method self-ignition combustion synthesis (SICS). Recently,

Wakabayashi et al. [75-78] reported the use of self-ignition combustion synthesis (SICS)

to produce TiFe-based alloys; in this method, the mixed powders were uniformly heated

19

up to the ignition point in pressurized hydrogen atmosphere. The product was also

directly synthesized with the help of the exothermic reaction of titanium hydrogenation.

The results showed that the SICS in pressurized hydrogen is considerably attractive as it

has a relative short operating time, consumes less energy because of the efficient

utilization of the reaction heat and improves activation behavior of the products, in

comparison to the conventional melting method. These results demonstrated the

significant benefits of SICS on Mg-based and Ti-based alloys. However, no papers have

reported on nickel rare-earth-based alloys of SICS from technical difficulties; in general,

the fine powders of rare-earth metal are easily oxidized or hydroxylated in air.

As mentioned in section 1.3.2, for producing rare-earth-based materials,

reduction-diffusion (RD) process has been proposed based on the reduction of rare-earth

oxide by calcium [79]. As far as we know, thus far, a combination of the SICS and the

RD process has never been reported in spite of its engineering significance. Therefore,

the purpose of this study is to synthesize hydrogen storage alloy by SICS using metallic

oxide powders as raw materials and calcium grains as both the reducing agent and the

heat source; the SICS effectively utilizes the exothermic reaction due to the

hydrogenation of calcium as the heat source without a heat treatment for solid diffusion.

In chapter 2, the LaNi5 alloy was produced by the SICS using Ni and La2O3 powders as

raw materials and calcium grains as both the reducing agent and the heat source. In

chapter 3, a new TiFe hydrogen storage alloy production route starting from ilmenite

was proposed by a three step heat treatments, i.e. a combination of roasting, hydrogen

reduction, and the SICS.

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1.5 Composite materials of hydrogen storage alloy

As mentioned in section 1.3.3, After numerous cycles of hydrogen absorption/

desorption, grain sizes of MH powder decrease to about 1 µm. The reduced grain size

has some negative effects on the heat transfer of MH beds and the walls of reactors.

A packed-bed reactor filled with MH powder has insufficient heat transfer because

of its low thermal conductivity and high thermal contact resistance to the wall of the

reactor. Thus, various composite materials have been proposed to increase the thermal

conductivity of the reaction bed. For example, Ron et al. tested porous metal matrix

hydride compacts of MH powder with aluminum [80, 81], Groll et al. evaluated the heat

transfer characteristics of expanded natural graphite/MH compacts [82, 83], and Gawlik

et al. measured the thermal conductivity of copper-coated MHs [84]. A large amount of

stress, on the other hand, is generated on the walls of vessels by volume expansion of

MH during hydrogen absorption. Considering the volume expansion, the packing

fraction should not exceed 35 vol% for the case of hydrogen contents around 1.0

mol-H/mol-alloy [63]. However, the effective thermal conductivity of an MH bed must

decrease with decreasing the packing fraction.

In order to solve these problems, it is important to improve the heat transfer of the

MH bed, to prevent sediment accumulation of finely powdered MH onto the bottom of

the reactor, and to control the packing fraction of the MH bed. In chapter 4, a sheet-like

composite formed using MH powder, aramid pulp (AP), and carbon fiber (CF) was

developed. We called it “Metal hydride sheet (MHS)”. The thermal conductivity and

cycle characteristic of MHS were experimentally investigated. The MHS was applied to

a MHHP system to evaluate the effect of the use of MHS on the system performances.

21

1.6 Ammonia as a hydrogen medium

Ammonia, which has a high hydrogen content, has created considerable interest in

recent years as an attractive hydrogen transport medium [4-7]. The storage density of

ammonia is 17.6 mass% and 120 kg/m3-liquid NH3, which are considerably higher than

those of most advanced metal hydrides. Ammonia gas is relatively easy to liquefy (240

K at 101 kPa) as compared with hydrogen gas (140 K at 101 kPa). Furthermore, there

are well-established production, storage, and transportation technologies already in

place since more than 160 million tons of ammonia are produced per year [85].

Ammonia is primary produced from natural gas or coal using the Haber–Bosch process,

in which nitrogen reacts with hydrogen over an iron oxide catalyst [86]. Recently,

Kitano et al. reported a novel catalyst for ammonia synthesis using a stable electride

[87]. Solid-state electrochemical synthesis is also a promising method for ammonia

synthesis from various hydrogen sources such as H2, H2O, CH4, and C2H6 [88].

To utilize ammonia as a source of hydrogen, catalytic ammonia decomposition is a

key step. Many catalysts have been studied, such as Ru, Ni, Ir, Pt, and Fe etc [89-92]. In

order to produce high purity hydrogen from ammonia, membrane reactor with a

combination of Pd membrane for hydrogen separation and catalyst for the ammonia

decomposition [93, 94] was proposed. The direct use of ammonia as an energy source

for solid state fuel cells [95] and automobile fuels [5] were also discussed.

22

1.7 Hydrogen utilization for ironmaking process

The reduction of CO2 emissions is still a common subject in the ironmaking

industry, although advanced energy-saving technologies are already underway.

Ironmaking blast furnaces need approximately 500 kg of carbon as the fuel, packing

materials, and a reducing agent to produce one ton of pig iron. In Japan, the "CO2

Ultimate Reduction in Steelmaking process by Innovative technology for cool Earth 50

(CURSE50)" project has been in progress since 2008 to reduce CO2 emissions from

blast furnaces by approximately 30% [96]. There are two methods for reducing CO2:

one is to decrease CO2 emissions from blast furnaces and another is to capture, separate,

and recover CO2. In first category, the introduction of hydrogen into the blast furnace

has been discussed because hydrogen is a carbon-free reducing agent. Application of

hydrogen is expected to improve the reduction rate of iron ore. However, the hydrogen

used is derived from a fossil fuel since coke oven gas (COG) will be utilized as the

hydrogen source.

On the other hand, MIDREX process has been developed for producing direct

reduced iron (DRI) [97]. The process reduces iron ore using a reforming gas, which

consists of CO and H2, made from natural gas. The DRI is used mainly as the raw

material for electrical furnace, as a clean iron source substitute for scrap iron. The

MIDREX process emits less CO2 than other process using coal because it utilizes not

only carbon but also hydrogen generated by natural gas reforming as reducing agent and

heat source.

In either case, if hydrogen is produced from non-fossil fuels, replacing carbon with

hydrogen in the ironmaking process would be a more effective way of reducing CO2

emissions. These hydrogen must be transported from outside of the ironworks. However,

23

hydrogen gas is not suitable for transportation owing to its low volume energy density.

Thus, hydrogen transportation is one of key issues in the realization of hydrogen

utilization for the iromaking industry.

Therefore, in chapter 5, the feasibility of ammonia as reducing agent for

ironmaking process was investigated. Although the ammonia is primary produced using

hydrogen from natural gas or coal, the hydrogen can be produced from water by

electrolysis, pyrolysis and photolysis, and it is possible to utilize various primary energy

including the renewable energy.

24

1.8 Purpose of this study

Hydrogen is an ideal candidate as secondary energy for both transportation and

stationary applications because it can be produced using various primary energies

including renewable energies and the storage of hydrogen energy is easier and cheaper

than that of electricity. The hydrogen storage is a key issue in the success and realization

of hydrogen economy because the volume energy density of hydrogen gas is quite low.

The hydrogen storage alloys show relatively high volumetric hydrogen densities and

enable both hydrogen absorption and desorption at environmental temperatures. They

are expected to be used not only as a hydrogen storage carrier, but also as an energy

conversion materials. Ammonia has created considerable interest in recent years as an

attractive hydrogen transport medium. The hydrogen storage density of ammonia is 17.6

mass% and 120 kg/m3-liquid NH3. Ammonia gas is relatively easy to liquefy as

compared with hydrogen gas. Furthermore, there are well-established production,

storage, and transportation technologies already in place.

However, the following problems discourage the practical use of hydrogen and/or

hydrogen storage materials as energy media.

(1) Hydrogen storage alloys are conventionally produced by using a melting method

that requires several time- and energy-consuming processes such as heat treatment,

pulverization, and activation treatment.

(2) A packed-bed reactor filled with the powder of hydrogen storage alloys has

insufficient heat transfer because of their low thermal conductivities and high

thermal contact resistance to the wall of the reactor. Moreover, a large amount of

stress is generated on the walls of the reactor by volume expansion of MH during

25

hydrogen absorption.

(3) Application of hydrogen to ironmaking process is expected to reduce CO2 emission

and to improve the reduction rate of iron ore. However, the hydrogen used is

derived from fossil fuels since coke oven gas (COG) or natural gas are utilized as

the hydrogen source. If hydrogen is produced from non-fossil fuels, replacing

carbon with hydrogen in the ironmaking process would be a more effective way of

reducing CO2 emissions. The hydrogen must be transported from outside of the

ironworks. However, hydrogen gas is not suitable for transportation owing to its

low volume energy density.

To overcome these challenges, the following solutions are proposed in this thesis.

(1) Self-ignition combustion synthesis (SICS) of hydrogen storage alloys utilizing

hydrogenation heat of metallic calcium (Chapters 2 and 3)

(2) Application of metal hydride sheet (MHS), which formed using MH powder,

aramid pulp, and carbon fiber, to metal hydride heat pump (MHHP) system

(Chapter 4)

(3) Utilization of ammonia as reducing agent for ironmaking process (Chapter 5)

This thesis includes six chapters:

Chapter 1 presents a general introduction of this study.

Chapter 2 describes SICS of LaNi5 alloy using Ni and La2O3 powders as raw

materials and calcium grains as both the reducing agent and the heat source. In

experiment, the effects of reaction atmosphere and pressure during SICS on the ignition

temperature and the hydrogenation properties of the products were comparatively

26

studied. Furthermore, the required primary energy and total exergy loss (EXL) in the

proposed system based on SICS were evaluated in comparison with that in a

conventional system during the production of 1 kg of LaNi5 alloy.

Chapter 3 describes the production of TiFe hydrogen storage alloy from ilmenite by

a combination of roasting, hydrogen reduction, and SICS. In experiment, the effect of

reaction atmospheres during SICS on hydrogenation properties was investigated.

Chapter 4 describes the thermal and hydrogenation properties of a MHS consisting

of MH powder, aramid pulp, and carbon fiber, and its application in a MHHP system. In

the experiments, the effect of the use of MHS on the system performances was

investigated, in which the MHSs were used to replace part of the MH powder to

improve the heat exchange performance.

In Chapter 5, the feasibility of using ammonia as reducing agent for ironmaking

process is discussed. In experiments, the reduction and nitriding behavior of hematite

with ammonia were examined. Furthermore, the CO2 emission of ammonia-based

ironmaking process was estimated based on the mass and heat balance.

Chapter 6 summarizes the results of this thesis as a general conclusion.

27

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33

Chapter 2

Self-ignition Combustion Synthesis of LaNi5-based

Hydrogen Storage Alloy Utilizing Hydrogenation Heat of

Metallic Calcium

2.1 Introduction

Nickel rare-earth based hydrogen storage alloys have been extensively studied

owing to their good hydrogenation properties [1, 2] and potential applications such as

hydrogen storage [3, 4], chemical heat pumps [5], hydrogen purification [6], and

Ni-metal hydride batteries [7, 8]. In general, hydrogen storage alloys are conventionally

produced by using a melting method that requires several time- and energy-consuming

processes such as heat treatment, pulverization, and activation treatment. In the heat

treatment, the product is kept at high temperature for long time for homogenization. The

following pulverization and activation treatment are a must for increasing the surface

area of the product and for improving the reactivity with hydrogen. In particular, the

activation treatment needs a repeated procedure of hydrogenation and dehydrogenation

by heating and vacuuming, which is the most energy consuming among the processes [9,

10].

Wakabayashi et al. [11-13] reported self-ignition combustion synthesis (SICS) in

hydrogen atmosphere to produce TiFe-based alloys; in this method, powders mixed in a

desired molar ratio were uniformly heated up to the ignition point in pressurized

hydrogen atmosphere. The product was directly synthesized with the help of the

exothermic reaction of titanium hydrogenation. The results showed that the SICS in

34

pressurized hydrogen is considerably attractive as it has a relative short operating time,

consumes relatively less energy because of the efficient utilization of the reaction heat

and improves activation behavior of the products, in comparison to the conventional

melting method. However, no papers have reported on nickel rare-earth-based alloys of

SICS from technical difficulties; in general, the fine powders of rare-earth metal are

easily oxidized or hydroxylated in air. LaNi5 is one of the most commercially produced

hydrogen storage alloys in addition to Mg2Ni and TiFe.

For producing rare-earth-based materials, reduction-diffusion (RD) process has

been proposed based on the reduction of rare-earth oxide by calcium [14]. Itagaki et al.

directly produced a nickel-mischmetal compound directly by using the RD process [15,

16], in which the raw materials were maintained at around 1273 K for as long as several

hours because of the slow solid diffusion whereas a pulverization treatment is not

required because rare-earth oxide was directly reduced to powders by metallic calcium

or calcium hydride. This paper elucidates a new production route of a rare-earth based

alloy using the reduction of rare-earth oxide by calcium; however, the RD process is

still time- and energy-consuming process. As far as we know, thus far, a combination of

the SICS and the RD process has never been reported in spite of its engineering

significance. Therefore, the purpose of this study is to synthesize LaNi5 alloy by SICS

using Ni and La2O3 powders as raw materials and calcium grains as both the reducing

agent and the heat source. The LaNi5 alloy is expected to produce without a heat

treatment for solid diffusion by the SICS which effectively utilizes the exothermic

reaction due to the hydrogenation of calcium.

In section 2.2, the effects of reaction atmosphere during SICS on the ignition

temperature and the hydrogenation properties of the products were mainly examined. In

35

section 2.3, the effect of hydrogen pressure during SICS on the ignition temperature and

the initial activity of the products was discussed. In section 2.4, the required primary

energy and total exergy loss (EXL) in the proposed system based on SICS were

evaluated in comparison with that in a conventional system during the production of 1

kg of LaNi5 alloy.

36

2.2 The effect of reaction atmosphere on hydrogenation properties of LaNi5 2.2.1 Objective of section 2.2

The SICS in hydrogen atmosphere is considerably attractive as it has a short

operating time, consumes relatively less energy because of the efficient utilization of the

reaction heat and improves activation behavior of the products, in comparison to the

conventional melting method. The significant benefits of SICS on Ti-based alloys have

been demonstrated [12]. On the other hand, for producing rare-earth-based materials,

reduction-diffusion (RD) process has been proposed based on the reduction of rare-earth

oxide by metallic calcium [17]. As far as we know, thus far, a combination of the SICS

and the RD process has never been reported in spite of its engineering significance.

Therefore, the purpose of this study is to synthesize LaNi5 alloy by SICS using Ni and

La2O3 powders as raw materials and calcium grains as both the reducing agent and the

heat source. In the experiments, the effect of atmosphere during the SICS on the

properties of the products was mainly examined.

37

2.2.2 Experiment

Fig. 2-2-1 shows the schematic diagram of the experimental apparatus. The

chamber (size; φ350 mm × 350 mm) with water cooling had a reactor and a graphite

heater. The reactor had two R-type thermocouples for controlling the temperature inside

the reactor and for measuring the sample temperature. The graphite heater completely

covered the sample in order to carry out the combustion synthesis of LaNi5 on the

self-ignition mode; the raw materials were uniformly heated up to 1773 K under a high

pressure of up to 1.0 MPa in a hydrogen or an argon atmosphere. The pressure in the

reactor was also maintained at constant value by using the on–off controller. A heating

electric power of 7.5 kW was sufficiently large for the uniform heating of the sample to

1773 K.

La2O3 and Ni powders used had both 99.9 % in purity and were less than 45 µm

and 3-5 µm in size, respectively. The reagents were obtained from Kojundo Chemical

Laboratory Company. Small chips of metallic calcium, purchased from Nirako

Company, had 95 % in purity and were 1-3 mm in size. In the experiments, 40 g of the

reagents was first mixed in the molar ratio of La2O3:Ni:Ca = 1:10:6, where the amount

of Ca was twice as much as the minimum quantity required for the reduction of La2O3,

for accelerating the reaction. Then, when each sample was placed in a carbon crucible

(dimensions: 45 mm × 90 mm × 35 mm), the crucible was carefully covered by a carbon

sheet with thickness of 1.0 mm in order to prevent the sample from sticking between the

crucible and the product. An R-type thermocouple placed inside a protective alumina

tube with outer diameter of 6.0 mm was introduced into the center of the sample.

Table 2-2-1 shows the heating conditions for the self-ignition mode during

combustion synthesis. The furnace was evacuated at a pressure of 20 Pa using a rotary

38

pump and was replaced by 99.999 % pure argon. After repeating the procedure of

vacuuming and replacing to achieve substitution four times, we finally charged

hydrogen or argon to a desired pressure. In the experiments, samples were uniformly

heated by using the graphite heater at the rate of 7.5 kW, and temperature changes in the

sample were monitored. As soon as an exothermic reaction was observed, the heating of

the samples was immediately stopped and the samples were naturally cooled in the

same atmosphere of pressurized hydrogen or normal argon. Finally, the products were

recovered from the furnace. The products were polished in order to remove residue of

carbon sheet from their surface and crushed using tungsten mortar. The product

powders obtained were washed with an aqueous solution of 5 mass% acetic acid in

order to remove the residue of calcium and byproduct of calcium oxide, and were kept

in a desiccator in air at 353 K for 12 h. The crystalline phases of products were

identified by X-ray diffraction (XRD), and the surfaces of the products were observed

using a scanning electron microscope (SEM).

Table 2-2-1 Heating conditions for self-ignition mode during combustion synthesis.

Shortly after the washing treatment, the pressure-composition-isotherm (PCT)

property of the products was evaluated using Sievert’s method by employing a

commercially available reactor (Suzuki Shokan Co., Ltd.) after four cycles of

hydrogenation/dehydrogenation. For measuring the initial hydrogenation kinetics, the

Heating rate Pressure(kW) (MPa)

1 7.5 Argon 0.12 7.5 Hydrogen 1.0

AtmosphereRun No.

39

reactor was first evacuated for at least 1 h at room temperature by using a

turbo-molecular pump, and then was introduced by hydrogen with a pressure of 4.1

MPa at 298 K.

Fig. 2-2-1 Schematic diagram of Self-Ignition Combustion Synthesis (SICS) reactor,

in which well-mixed powders of La2O3, Ni and Ca were uniformly heated by a

graphite heater at argon or hydrogen atmosphere.

P

Gas inflow

Gas outflow

Pressure gauge for controlling pressure

Carbon crucible(80 ×150 × 45 mm)

Carbon heater

Sample Insulator

Chamber with water-cooling (Inner volume : 35litter)

Thermocouple for controlling furnace temperature

Thermocouple for measuring sample temperature

Top view

Side view

40

2.2.3 Results and discussion

Fig. 2-2-2 shows the temperature changes in the sample with time during the SICS

experiments in an argon atmosphere of 0.1 MPa (Run 1) and a hydrogen atmosphere of

1.0 MPa (Run 2). Note that the temperature changes in the sample strongly depended on

the atmosphere because the thermal conductivity of hydrogen is one digit larger than

that of argon.

In Run 1, the temperature of the sample increased at around 1100 K, close to the

melting temperature of calcium (1115 K). Shortly after the exothermic reaction was

observed, the electric heating was stopped, and then each sample was naturally cooled

in the furnace. The melting of calcium increased contact area between calcium and the

other raw materials, accelerating the following exothermic reactions:

Ca (l) + 5 Ni (s) → CaNi5 (s)

3Ca (l) + La2O3 (s) → 2 La (l) + 3CaO(s)

CaNi5 (s) + La (l) → LaNi5 (s, l) + Ca (l)

The reaction mechanism of SICS in argon atmosphere was similar to the RD process.

Ohtsuka and Tanabe et al. particularly reported that the formation mechanism of LaNi5

in the RD process, CaNi5 compound is generated as an intermediate product, and that

the melting of calcium enhances the growth rate significantly [18, 19]. The overall

reaction in Run 1 can be expressed by the following equation;

1/2La2O3 + 5Ni + 3/2Ca = LaNi5 + 3/2CaO + 214 kJ

Assuming that the ignition temperature was 1115 K, we estimated the adiabatic

temperature of the reaction to be 1598 K; the value is melting point of LaNi5 [20, 21].

Meanwhile, the temperature of the sample in Run 2 increased sharply at around 600

K and jumped to 1238 K. As soon as the exothermic reaction occurred, the electric

(2.2.4)

(2.2.3)

(2.2.1)

(2.2.2)

41

heating of the sample was turned off. Then, each sample was naturally cooled in the

furnace. The ignition was triggered by the hydrogenation of calcium, accelerating the

reduction of La2O3 by CaH2 and the synthesis of LaNi5. The hydrogenation of calcium

was accompanied by the considerable exothermic heat of formation.

Ca + H2 = CaH2 + 177 kJ

The intermediate products of the SICS in the hydrogen atmosphere are discussed in the

next section. When the amount of calcium was twice as the chemical equivalent value,

the overall reaction in Run 2 was expressed by the following equation:

1/2La2O3 + 5Ni + 3Ca +3/2H2 = LaNi5 + 3/2CaO + 3/2CaH2 + 479 kJ

Assuming that the ignition temperature was 600 K, the adiabatic temperature of the

reaction was as high as 1598 K. Under the assumption that the amount of calcium was

equal to the chemical equivalent value, the adiabatic temperature was 1419 K. Excess

calcium served not only as the reducing agent of La2O3, but also as the heat source due

to hydrogenation.

It should be noted that the hydrogen atmosphere reduced the ignition temperature

from 1100 to 600 K as compared to argon. This fact implies different trigger reactions—

hydrogenation in the case of the hydrogen atmosphere and melting reactions of calcium

in the case of argon atmosphere. The time taken for the ignition was 200 s, and the

furnace temperature reached around 900 K at during this time. The results revealed that

the SICS process is more attractive than the RD process, because the RD process

requires heating at 1273 K for at least several hours.

(2.2.5)

(2.2.6)

42

Fig. 2-2-2 Changes in sample temperature with time during SICS experiments at

hydrogen atmosphere of 1.0 MPa (Run 1) and at argon atmosphere of 0.1 MPa

(Run 2). Note that thermal conductivity of hydrogen is larger than that of argon

and the ignition point at Run 2 was much lower than that at Run 1, implying

different trigger reactions.

Fig. 2-2-3 shows the XRD patterns of the commercially available LaNi5 reagent

and the SICSed products after the washing treatment. The major peaks of the SICSed

products were exactly indexed to the LaNi5 phase, although, the peak, observed at 45º

suggested the existence of un-reacted nickel. The ratio of un-reacted nickel will

decrease when optimizing the experimental conditions such as the amount of reducing

agent, heating rate and hydrogen pressure are optimized.

Fig. 2-2-4 shows the SEM images of the products SICSed before and after the

washing treatment. Calcium oxide and calcium on the grain surface of the alloy were

removed by washing with 5 mass% acetic acid and distilled water in Runs 1 and 2.

During the washing treatment, calcium oxide and calcium reacted vigorously with acetic

acid aqueous solution to be soluble in it, and then the product crumbled to less than 50

µm in size without pulverization.

300

500

700

900

1100

1300

1500

0 0.2 0.4 0.6 0.8

Run 1

Run 2

Time (ks)

Tem

pera

ture

(K)

Heater offHeater off

Ignition Point

I.P.

43

Fig. 2-2-3 XRD patterns of commercially available LaNi5 reagent and the SICSed

products after washing treatment, showing successful synthesizes of LaNi5 phase

through SICS process.

Fig. 2-2-4 SEM images of the SICSed products at: (1-a) Run 1 before washing;

(1-b) Run 1 after washing; (2-a) Run 2 before washing; (2-b) Run 2 after washing.

CaO and Ca on the grain surface were removed by washing the sample with

distilled water of 5 mass% acetic acid.

20 30 40 50 60 70 802θ (degree)

Inte

nsit

y(a

.u.)

NiLaNi5

LaNi5 reagent

Run 1

Run 2

10 µµµµm

10 µµµµm

10 µµµµm

10 µµµµm

(1-a)

(1-b)

(2-a)

(2-b)

44

Fig. 2-2-5 shows the hydriding curves of the products obtained in different

atmospheres, which were measured by using Sievert’s method at temperature of 298K

and initial hydrogen pressure of 4.1 MPa. We found that the products obtained in Run 2

had only one-third in full charge time, in comparison to the product obtained in Run 1.

The initial activity was probably improved by hydrogen absorbed during the cooling

period. In fact, many nanofissures were observed within the product due to the release

of absorbed hydrogen [22] when the reactor was evacuated by a turbo-molecular pump

or was heated up before the initial hydrogenation. As a result, the product had a large

surface area of LaNi5 phase, which was not poisoned by air. This was the reason why

the hydrogenation proceeded very quickly. The initial activity of the product obtained

by the SICS is discussed in detail in the next section. The product obtained in Run 2

showed 1.54 mass% in hydrogen storage capacity, as the same as the value of

commercially-available LaNi5. Meanwhile, the product obtained in Run 1 stored

hydrogen as much as 1.33 mass%; this value is corresponds to 86% of the value

commercially-available LaNi5. In conclusion, the high-pressure hydrogen atmosphere in

SICS is considerably more attractive than the normal argon atmosphere because of the

improvement in the purity and the kinetics of the product.

Fig. 2-2-6 shows three PCT curves of a reagent of LaNi5 with 99.9% in purity and

two products obtained at 298K; these curves were measured by Sievert’s method. The

equilibrium pressure of the products was the same as that of the reagent. The peak

positions in XRD patterns of the products were also exactly the same as those of the

reagent. The product obtained in Run 2 was larger in effective hydrogen storage

capacity than the reagent. This was probably caused by higher crystallinity of the

product.

45

Fig. 2-2-5 Hydriding curves of SICSed LaNi5 at different atmospheres, measured

by Sievert’s method at 298 K in temperature and 4.1 MPa in initial hydrogen

pressure. Here, the products SICSed at Run 2 had only one-third in full charge

time, in comparison to the product SICSed at Run 1, showing 1.54 mass% in

hydrogen storage capacity, as the same as that of a reagent of LaNi5.

Fig. 2-2-6 Three PCT curves of a reagent of LaNi5 with 99.9% in purity and two

products of SICSed LaNi5 at 298K, measured by Sievert’s method, in which the

equilibrium pressure of the SICSed products were the same as that of the reagent.

0

0.4

0.8

1.2

1.6

0 10 20 30 40Time (ks)

Sto

red

hydr

ogen

(mas

s%)

Run 2

Run 1

Initial Hydrogen Pressure: 4.1 MPaTemperature: 298 KSample weight: 1.0 g

0.01

0.1

1

10

0 0.4 0.8 1.2 1.6Stored hydrogen (mass%)

Pre

ssur

e (M

Pa)

LaNi5 (regent)

Run 1-Ar

Run 2-H2

Reagent

Run 1

Run 2

Temperature: 298 KSample weight: 1.0 g

46

Fig.2-2-7 shows the temperature dependences of the PCT curves. From the PCT

curves, we could draw the van’t Hoff plot on the temperature dependences of the

hydrogenation and dehydrogenation plateau pressures of the products at the half of the

hydrogen storage capacities. Data plotted at both pressures showed good linearity with

good correlation coefficients according to least mean square approximation (see

Fig.2-2-7). Table 2-2-2 shows the reaction heat ∆H of a reagent and products of LaNi5.

They were calculated from the equation: lnP = ∆H/(RT) − ∆S/R. The value of ∆H of the

product obtained by SICS agreed well with that of a reagent of LaNi5.

Fig. 2-2-7 Van’t hoff plots for a reagent and products of SICSed LaNi5, here we can

calculate the reaction heat ∆∆∆∆H and entropy ∆∆∆∆S easily using the following equation:

lnP = ∆∆∆∆H/(RT) − ∆∆∆∆S/R.

0.01

0.1

1

3.1 3.3 3.5 3.7

Regent

Run 1

Run 2

Inverse of temperature (K-1)

Pres

sure

(M

Pa)

(×10-3)

Reagent

Run 1

Run 2

HydrogenationR2 = 0.9995-0.9999

DehydrogenationR2 = 0.9999-1.000

47

Table 2-2-2 Reaction heat ∆∆∆∆H of a reagent and products of LaNi5.

Sample Name

Reaction Heat

[kJ/mol-H2]

Absorption Desorption

Reagent -31.9 32.5

Run 1 -32.4 32.0

Run 2 -32.1 31.9

48

2.2.4 Summary

In this section, the self-ignition combustion synthesis (SICS) of LaNi5 hydrogen

storage alloys in high-pressure hydrogen and normal argon atmospheres from La2O3, Ni,

and Ca was comparatively studied and the following conclusions were derived.

1. In both atmospheres, LaNi5 alloy was successfully produced by the SICS, whereas

the ignition temperature was quite different; only 600 K caused by Ca + H2 → CaH2

in the case of the hydrogen atmosphere as compared to 1100 K caused by melting

reaction of calcium in the argon atmosphere. During the SICS in the hydrogen

atmosphere, excess calcium served not only as the reducing agent of La2O3 but also

as the heat source due to the hydrogenation of calcium.

2. The hydrogenation time of the product obtained in hydrogen atmosphere was only

one-third in comparison to the product obtained in argon atmosphere. This

demonstrated that the products were well-activated in hydrogen atmosphere.

3. The product obtained in hydrogen showed 1.54 mass% in hydrogen storage capacity

and 0.17 MPa in equilibrium dissociation pressure; they were as the same property

as commercially-available product.

49

2.3 Initial activity of LaNi5 synthesized at different hydrogen pressures

2.3.1 Objective of section 2.3

In the preceding section, we proposed that SICS utilizing the hydrogenation heat of

metallic calcium for the production of nickel rare-earth-based alloys such as LaNi5 [23].

The proposed method resulted in the production of a hydrogen storage alloy of LaNi5

using nickel and La2O3 powders as raw materials and calcium grains as both the

reducing agent and the heat source. The SICS product obtained in hydrogen improved

the initial hydrogenation properties in comparison to the product obtained in argon at a

temperature of 298K and an initial hydrogen pressure of 4.1 MPa. However, the initial

activity at hydrogen pressures lower than 1.0 MPa is much more important for practical

use owing to the Japanese regulation of the use of high-pressure gas. In general, the

activation treatment requires more hydrogen pressure than the equilibrium hydrogen

pressure of hydrogen storage alloys [24]. The newly developed alloy is strongly

required to store as much hydrogen as possible at pressures closer to the equilibrium

pressure. However, no papers have reported on such optimization from the kinetics and

equilibrium theory perspectives.

Therefore, the purpose of this section is to synthesize a LaNi5 alloy by SICS at

different hydrogen pressures and to evaluate the hydrogenation properties, in which

initial activation behavior at less than 1.0 MPa in hydrogen pressure was mainly

investigated. The effect of hydrogen pressure during SICS on the ignition temperature

and particle size distribution were also examined in the experiments. The results

obtained will support the possibility of a new LaNi5 production process based on SICS

for practical use.

50

2.3.2 Experiment

A detailed explanation of both the apparatus and the preparation of samples is

provided in section 2.2.2 [12, 23]; only the framework of the experiments is described

here. La2O3 and Ni powders used were both 99.9 % pure and were less than 45 µm and

3-5 µm in size, respectively. Small chips of metallic calcium were 99.5 % pure and were

1-3 mm in size. In the experiments, 40 g of the reagents were first mixed in a molar

ratio of La2O3:Ni:Ca = 1:10:6, where the amount of Ca was twice as much as the

minimum quantity required for the reduction of La2O3, for accelerating the reaction.

Then, each sample was placed in a carbon crucible, which was carefully covered by a

carbon sheet to prevent the sample from sticking between the crucible and the product.

An R-type thermocouple placed inside a protective alumina tube was introduced into

the center of the sample.

Table 2-3-1 summarizes the heating conditions during combustion synthesis. The

furnace was evacuated at a pressure of 20 Pa using a rotary pump and was replaced by

99.999 % pure argon. After repeating the procedure of vacuuming and replacing to

achieve substitution four times, we finally charged hydrogen to the desired pressure. In

the experiments, samples were uniformly heated by using the graphite heater at an

electric power of 7.5 kW, and changes in the sample temperature were monitored. As

soon as an exothermic reaction was observed, the heating of the samples was

immediately stopped and the samples were naturally cooled in the same atmosphere for

2 h. The products were polished in order to remove the carbon sheet from the surface of

them. The bulk product was first crushed to less than 2.8 mm in size using a tungsten

mortar. Then, the product powders obtained were washed with an aqueous solution of 5

mass% acetic acid in order to remove the residue of calcium and the by-product of

51

calcium oxide and were finally dried in a desiccator in air at 353 K for 12 h.

The crystalline phases of products were identified by X-ray diffraction (XRD), and

the surfaces of the products were observed using a scanning electron microscope (SEM).

The specific surface areas of the products were also determined by the

Brunauer-Emmett-Teller (BET) method. The particle-size distributions of products were

measured by the laser diffraction scattering method.

Shortly after the washing treatment, the products were placed in two stainless steel

reactors for the measurement of the pressure-composition-isotherm (PCT) property and

the initial hydrogenation kinetics, which were measured using Sievert’s method. It

should be noted that each measurement was performed using different samples after the

washing treatment. The PCT properties of the products were evaluated after repeating

the procedure of vacuuming/pressurizing with 4.1 MPa of hydrogen, five times; these

experimental conditions were sufficient to activate and to stabilize the LaNi5 alloy. For

the measurement of the initial hydrogenation kinetics, the other reactor filled with

products was first evacuated for more than 1 h at 298 K using a turbo-molecular pump,

and then hydrogen, with a pressure of 0.95 MPa, was introduced.

Table 2-3-1 Heating conditions for self-ignition mode during combustion synthesis.

Thermal Power Hydrogen Pressure[kW] [MPa]

1 7.5 0.12 7.5 0.53 7.5 1.0

Run No.

52

2.3.3 Results and discussion

Fig. 2-3-1 shows the changes in the sample temperatures with time and their

quadratic differential values during SICS experiments at different hydrogen pressures.

The temperature of all samples jumped at around 0.2-0.3 ks owing to a exothermic

reaction. As soon as the exothermic reaction occurred, the electric heating of the sample

was turned off, and then, each sample was naturally cooled in the furnace. The ignition

was triggered by the hydrogenation of calcium (Ca +H2 → CaH2), accelerating the

reduction of La2O3 by CaH2 and the synthesis of LaNi5, as mentioned in the section

2.2.3. The rapid reaction kinetics of the hydrogenation can be explained as follows. The

surface of metallic Ca was covered by thick layer of CaO and/or Ca(OH)2 which

inhibited the hydrogenation reaction. When H atom dissociated on surface reached to

metallic Ca, the hydrogenation was immediately caused with the formation heat and

volume expansion. The reaction heat and highly reactive surface generated by the

volume change accelerated the hydrogenation. This is similar to the activation

mechanism of hydrogen storage alloys such as LaNi5 and TiFe [25-29]. For example,

the surface of LaNi5 exposed to air is cover