Study on Hydrogen Absorption Properties ofTitanium with Surface Modification（表面改質したチタンの水素吸蔵特性に関する研究）

学位取得年月新里2020 年 9 月
恵多

Abstract
In this thesis, we focused on the Titanium (Ti) and hydrogen (H2) system. The kineticsof H2 absorption by Ti is quite slow, then the high temperature of 400 C is required torealize the reaction. It is generally thought that the oxide layer formed on the Ti surface
inhibits the hydrogenation. However, the hydrogenation properties of Ti without oxide
layer has not been reported before, and the effective surface modification technique has
not been proposed yet. In this thesis, the reaction between Ti with fresh surface and H 2
was studied to understand the essential reaction of Ti with H2, and the surface
modification techniques to improve the hydrogenation properties of Ti was also
investigated.
It was clarified that the hydrogenation of Ti with fresh surface occurred even at room
temperature. However, the active Ti is quite sensitive for not only hydrogen but also
oxygen and water. The reactivity with H2 was lost due to oxidation after keeping thesample for 1 day even in the highly purified Argon atmosphere with tiny amount ofoxygen and water.
To make stable active sites for the hydrogenation of Ti, typical catalysts for H2
dissociation were dispersed on the Ti surface. Although nickel, palladium, niobium (V)oxide, and vanadium (V) oxide were effective catalysts for hydrogenation of metals suchas magnesium, significant effects on the hydrogenation were not observed for Ti. Thus, it
was clarified that the influence of surface oxidation on the hydrogenation of Ti was muchlarger than the activation effects of solid additives. Considering the above results, it wasthought that special surface, which can preserve the reactivity with H2 and simultaneously
prevent surface degradation such as the oxide layer formation, is required to improve thehydrogenation properties of Ti.
IWhen Ti powder was ball-milled with organic solvents, such as xylene and acetone,
the surface was effectively modified, and the characteristic surface showed high reactivity
and selectivity for H2 even at room temperature. The results of the characterization for
the Ti modified by organic solvents samples indicated that graphite like structure iseffective for the surface modification of Ti. Thus, the effects of graphite was investigated.
As a result, the Ti with the excellent H2 absorption properties was obtained by ball-millingwith graphite. This sample can absorb H2 at room temperature under 0.1 MPa H2, and thereactivity was kept for more than 7 days although other samples prepared with organic
solvents are inactivated within 1-4 days. From Raman spectrometry and TEM observation
for the graphite-dispersed sample, it was identified that the mixed phase of turbostratic
and amorphous carbons exists on the Ti surface. The mixed carbon phase can prevent theoxidation of Ti by the “hydrophobic adsorbent effects” for water and oxygen, while H2
can pass through the carbon matrix. In addition, carbon-dissolved Ti phase (TiCx) was
found at the interface between Ti and carbons by electron energy loss spectrometry usingtransmission electron microscope. The TiCx phase was also observed in the Ti ball-milledwith acetone, which showed the effects of a function of porous inorganic membrane.
Namely, the carbon matrix and the TiCx layer showed the H2 separation function.
From the experimental works in this thesis, it is clarified that Ti can essentially reactwith H2 at room temperature without any catalyst, suggesting that the Ti surface it active
for the dissociation of H2. It is expected based on the above results that the surface withselective reactivity for H2 without degradation of the active Ti surface by oxygen andwater is effective as surface modification. In fact, formation of the above characteristic
surface was achieved by ball-milling with graphite, and the excellent hydrogenation of Ti
at room temperature without degradation were demonstrated.
II

Contents
1 Introduction………………………………………………………………….…………1
1.1 Sustainable energy………………………………………………………………...1
1.2 Metal-hydrogen systems………………………………………………………......2
1.3 Thermodynamics and kinetics……………………………………………………..8
1.4 Surface modification for metal-hydrogen system………………………………..21
1.5 Titanium-hydrogen system..……………………………………………………...25
References……………………………………………………………………………...27
2 Purpose of this thesis………………………………………………………………….32
3 Experiments…………………………………………………………………………..34
3.1 Sample preparation………………………………………………………………34
3.1.1 Starting materials……………………………………………………………34
3.1.2 Ti with fresh surface…………………………………………………………36
3.1.3 Ti with surface modification…………………………………………………37
3.2 Analysis methods………………………………………………………………...41
3.2.1 Thermogravimetry-differential thermal analysis (TG-DTA)………………...41
3.2.2 Thermal desorption mass spectroscopy (TDMS)……………………………45
3.2.3 Transmission electron microscope (TEM)…………………………………..49
3.2.4 Powder X-ray diffraction (XRD)…………………………………………….53
3.2.5 X-ray photoelectron spectroscopy (XPS)……………………………………60
3.2.6 Raman scattering spectroscopy……………………………………………...65
References……………………………………………………………………………...69

III

4 Results and discussion………………………………………………………………..71
4.1 The reaction between Ti and H2…………………………………………………71
4.2 Surface modification by transition metals and oxides to make active sites…….84
4.3 Surface modification by organic solvents and graphite to prevent the surface
oxidation…………………………………………………………………………91
References……………………………………………………………………………..113
5 Conclusion…………………………………………………………………………..115
Acknowledgements……………………………………………………………...........118
Published papers………………………………………………………………………119IV

1 Introduction
1.1 Sustainable energy
Recently, the problems with the limiting fossil fuel production and global warming are
growing. Global warming is caused by greenhouse gases (GHG) such as CO2 andmethane, and it enhances the risks of climate change, sea-level rise, and so on. In orderto suppress global warming, United Nations Framework Convention on Climate Change(UNFCCC) is found in 1992. In 2015, the 21st conference of the parties (COP21) was
held in Paris. The target of suppressing the temperature rise less than 1.5~2 C has been
decided by Paris agreement in COP21. Then, all of the countries have submitted the aimof GHG emission reduction.1 Thus, the establishment of sustainable energy society isrequired in the world. The aim of Japan on the Paris agreement is to reduce CO2 emission
to 26% by 2030. The proportion of utilization of renewable energy such as wind power,
solar power, and so on, has to be increased in order to reduce the utilization of fossil fuels.
However, the renewable energy is localized, and it has fluctuating output by weather andtime. Therefore, the secondary energy must be necessary to maintain a balance between
demand and supply. Hydrogen is one of the candidates for secondary energy sources
because it can store energy chemically with high gravimetric energy density and can beproduced from any kinds of primary energy. Especially, it is considered that hydrogen issuitable for the storage and transportation of large amount of energy compared torechargeable batteries. Therefore, the technology of hydrogen utilization is required in
the countries having less primary energy like Japan.

11.2 Metal-hydrogen systems
To establish the hydrogen utilization systems, the development of related technologies
is strongly required. The metal-hydrogen systems are attractive in the field of various
applications as described below.
Hydrogen storage
Hydrogen as a secondary energy source has the advantages of high abundance, highgravimetric energy density, and so on. However, the volumetric energy density is quite
low because hydrogen is a gaseous state under an ambient temperature. In fact, thevolumetric energy density of H2 at 25 C under 1atm is only 2.69 Wh/L (HHV), which is2000 times smaller than liquid gasoline (even though considering energy loss).2,3
Therefore, several kinds of methods must be developed to compactly store and transport
H2. It can be mainly classified by 3 types of hydrogen storage, which are compression,liquefaction, and solidification. In the case of the compression method, the energy density
is 767 Wh/L at 35 MPa and 1290 Wh/L at 70 MPa, respectively. Here, this estimation iscarried out without considering the weight of the H2 tank although special H2 tank forhigh pressure are necessary. The high pressure H2 storage has the advantages such astemperature control is not necessary, and hydrogen can be supplied into H2 tank rapidly.
This method has been used for H2 storage in hydrogen fuel cell vehicles at 70 MPa.4 Byliquefaction of H2, the energy density is increased to 2330 Wh/L at 253 C under 1 atm.4
However, quite low temperature is necessary for liquefaction, which lead to energy loss.
Japanese company (Kawasaki Heavy Industries, Ltd.) has been developing a ship for
liquid hydrogen transportation from abroad.5 As a third method, hydrogen can be stored
in metals or alloys.6,7 While the volumetric density depends on the metal, hydrogen canbe compactly stored (2936 Wh/L) even at ambient conditions.8–11
2

Heat storage
As a renewable energy system, concentrating solar thermal power (CSP) system has
been developing. In this system, electricity can be generated by turbine powered by solar
heat. However, the amount of energy generation depends on the time and weather. Thus,
heat storage system should be equipped with the solar heat power generation plant inorder to maintain the energy supply round the clock.12 The CSP systems can be classified
in to 4 types, which have different types of the heat concentrator and the receivers asshown in figure 1.1. The operating temperatures of each system are shown in table 1.1.
Molten-salt has been used as conventional heat storage materials for CSP. However, theutilization of molten-salt for high temperature ( 700 C) suffers from the corrosion ofcontainer.12 Thermochemical heat storage is one of the promising systems for hightemperature heat storage because of its chemical stability, high energy density,
theoretically unlimited storage period, and so on.13,14 Metal-hydrogen system has also
been studying for thermochemical heat storage. Although sensible heat and latent heatstorage system cannot control the temperature, the operating temperature of metalhydrogen system can be tuned by the pressure as shown in figure 1.2. The MgH2 system
reported by the group in Max-Planck Institute offers a heat storage material forconcentrating solar thermal energy.15 However, practical use of the Mg−H system islimited by following drawbacks: slow reaction kinetics, requirements of Fe- or Ni-doping,sintering at the high-temperature region, and high operating pressure around 10 MPa for
utilizing around 500 °C.14

3Figure 1.1 Concentrating Solar Power systems cited from IEA Technology Road map.16

Figure 1.2 Dissociation pressure curve of MgH2 suggested by Felderhoff et al.15

Table 1.1 CPS systems and its operating temperatures, the values were cited from reportby NEDO.17

4

Chemical compression
After introducing fuel cell vehicles with high pressure H2 tank in the market in 2014,
the hydrogen refueling station (HRS) is required to enhance the utilization of FCV.
According to the strategic roadmap for hydrogen and fuel cells presented by Japanese
government, the number of hydrogen refueling stations (HRS) will be increased to 320
by 2025, and 900 by 2030.18 However, the number of HRS in May 2020 is still 131.19 In
order to increase the number of HRS, the cost cut is main issue. Japanese government
presented that the cost of components should be decreased from 90 to 50 million yen forcompressor, and 50 to 10 million yen for accumulator by early 2020s, respectively. Tsurui
et al. pointed out that the moving part of mechanical compression is influenced by thehydrogen embrittlement, leading that the construction cost is extremely high to design themoving part with high strength.20 The thermochemical compressor is one of thealternative methods of H2 compression to reduce the cost of hydrogen refueling station
because of its simplicity without moving part, safety and reliability, and the possibility toconsume waste industrial heat.20–22 When the metal hydride are heated under equilibrium
H2 pressure, the hydrogen pressure exponentially increases with increasing temperatureaccording to van’t Hoff relation as explained in next section.23,24 In our group, it wasreported that H2 could be compressed up to 82 MPa by only heating within 300 C as
shown in figure 1.3.25

5

Figure 1.3 Hydrogen compression results by using 2 types of Ti-based alloys pressreleased by Hiroshima University (February, 2016).25
Hydrogen capture
Accidents of hydrogen leakage in closed areas are especially serious due to low ignitionenergy and wide explosion range of hydrogen (4-75 vol.% in air). Thus, hydrogen capturematerials are required to prevent the hydrogen explosion and utilize hydrogen safely. In
Japan, hydrogen explosion occurred in Fukushima Daiichi nuclear power plant
accompanied by the great east Japan earthquake in 2011. According to the report on this
accident, the electricity was totally lost due to the tsunami, then the safety systems were6

not working.26 To prevent hydrogen explosion even without the electricity supply, several
kinds of methods are presented.27,28 The metal-hydrogen system is a promising method
because some metals can absorb hydrogen automatically below the H2 explosion limit.29
The reaction between Mg and H2 has been studied as a hydrogen capture material.30

71.3 Thermodynamics and kinetics
Thermodynamics
Thermodynamics of the reaction between hydrogen and metal is an important factor inorder to understand the hydrogen absorption/desorption properties. Figure 1.4 shows thehydrogen absorption curve of LaNi5, a well-known as hydrogen storage material,
performed at a fixed temperature. When the hydrogen is introduced gradually, hydrogenpressure is increased from 0.0 to 0.2 MPa. As shown by the inserted figure in the figure
1.4, the hydrogen content is slightly increased with increasing pressure. The phenomenon
indicates the formation of solid solution (MHx) in this region. When the pressure exceeds
more than 0.2 MPa, the hydrogen pressure remains constant even though supplyinghydrogen continuously, and so the hydrogen content increases to H/M = 6. The region iscalled the plateau region. The following equilibrium reaction occurs in the plateau region,
LaNi5 + 3H2 ↔ LaNi5 H6 .

(Equation 1.1)

In this region, the ratio of LaNi5/LaNi5H6 keep changing. After fully conversion of
LaNi5 to LaNi5H6, the hydrogen pressure starts increasing again.
According to the Gibbs’ phase rule, the degrees of freedom (f) can be given by,
𝑓 = 𝑐 − 𝑝 + 2,

(Equation 1.2)

in which c and p are the number of components and phases in the system, respectively. In
the region of solid solution, the number of components and phases are 2. Here, the phaseof solid solution is regarded as the same phase as metal (M). Thus, f can be calculated tobe 2, indicating that the pressure and H content can be chosen freely. In the case of plateau
region, the components and the phases are 2 and 3, respectively (the phases of M and MH2
8are distinguished). Therefore, f is calculated as,
𝑓 = 2 − 3 + 2 = 1.

(Equation 1.3)

This equation indicates that only 1 freedom can be chosen from the temperature, H
content, and the pressure. Thus, the temperature and pressure should be fixed in theplateau region as shown in figure 1.4.
The formation of solid solution can be written as following reaction,𝑥

𝑀 + 2 H2 ↔ 𝑀H𝑥 .

(Equation 1.4)

In the region of solid solution, the chemical potential of hydrogen gas (g) is equal to thatof hydrogen dissolved in  phase () as follows,12

𝜇𝑔 = 𝜇 .

(Equation 1.5)

Based on ideal gas, the chemical potential of the hydrogen molecule ( g) undertemperature (T) and pressure (p) can be written by,𝑝

𝜇g = 𝑘𝑇 ln (𝑝 ) − 𝐸d ,0

(Equation 1.6)

where, k is the Boltzmann constant, p0 is the pressure under the standard condition, and
Ed is the energy for the dissociation of hydrogen molecule. Now, the starting point ofchemical potential is described from the dissociated hydrogen molecule (2H), so that Ed
is included in Equation 1.6. On the other hand, Gibbs free energy of the solid solution
phase (Ga) is given by,𝐺 = 𝐻 − 𝑇𝑆 ,9(Equation 1.7)

in which H and S are enthalpy and entropy of the solid solution phase, respectively. The
enthalpy of the solid solution phase (MHx) is
𝐻a = 𝑥ℎa ,(Equation 1.8)

where, x is the number of the dissolved hydrogen atom and ha is the dissolution enthalpyof 1 atom. The entropy can be separated by the configurational entropy (Sc) and the excess
entropy of solution (Se),𝑆a = 𝑆c + 𝑆e .

(Equation 1.9)

Sc can be given by Boltzmann’s equation using the configuration number (W),𝑆c = 𝑘 ln(𝑊) = 𝑘 ln (𝑟C𝑥)𝑟!

(Equation 1.10)= 𝑘 ln (𝑥!(𝑟−𝑥)!),where, k is the Boltzmann constant, and r is the number of initial sites of metal for thehydrogen atoms. Equation 1.10 can be expressed by Stirling’s approximation as follows,(Equation 1.11)

ln(𝑛!) = 𝑛 ln(𝑛) − 𝑛,
with

𝑟

𝑥

𝑆c = 𝑘𝑟 ln (𝑟−𝑥) − 𝑘𝑥 𝑙𝑛(𝑟−𝑥).

(Equation 1.12)

The excess entropy of solution (Se) can be expressed as follows,(Equation 1.13)

𝑆e = 𝑥𝑠e .

Therefore, by using equation 1.7-9, 1.12, and 1.13, the chemical potential of the solid
solution phase (a) can be given as follows,

10

𝜕𝐺

𝑥

𝜇𝑎 = 𝜕𝑥𝑎 = ℎa − 𝑇𝑠e + 𝑘𝑇 𝑙𝑛 (𝑟−𝑥).

(Equation 1.14)

From equation 1.5, 1.6, and 1.14, the equilibrium condition can be written by,
1
2

𝑝

𝑥

{𝑘𝑇 ln (𝑝 ) − 𝐸d } = ℎa − 𝑇𝑠e + 𝑘𝑇 ln (𝑟−𝑥)0(Equation 1.15)

Because x is quite small (x « r) in the region of solid solution,𝑥ln (𝑟−𝑥) = ln(𝑥) − ln(𝑟).

(Equation 1.16)

Therefore, the following equation is derived from equation 1.15 and 1.16,
1𝑝 2(𝑝 ) = 𝐾s (𝑇) ∙ 𝑥,
0

𝐾s (𝑇) = exp(

12

𝑇𝑠𝑒 −ℎ𝑎 − 𝐸d +𝑘𝑇 ln(𝑟)𝑘𝑇(Equation 1.17)),(Equation 1.18)

Equation 1.17 is known as Sievert’s law, and Ks (T) is called Sievert’s constant. Because
Ks is more than 0, small amount of hydrogen can be dissolved in every material.
At the plateau region, the formation of metal hydride occurs as following reaction,
𝑀 + H2 ↔ 𝑀H2 .

(Equation 1.19)

The Gibbs free energy change (G) of the above reaction can be written by,∆𝐺 = ∆𝐻 − 𝑇∆𝑆(Equation 1.20)

here, H and S are enthalpy change and entropy change for the reaction, respectively.
In the case of hydrogen absorption reaction, Habs and Sabs are given by,∆𝐻abs = 𝐻𝑀H2 − (𝐻𝑀 + 𝐻H2 ) = 𝐻𝑀H2 −𝐻M = ∆𝐻f ,11(Equation 1.21)

(Equation 1.22)

00+ 𝑆H2 ),
∆𝑆abs = 𝑆𝑀H2
− (𝑆𝑀where, Hf is the formation enthalpy of hydride. The entropy of gaseous hydrogen SH2
can be expressed by,𝑝(Equation 1.23)

0
𝑆H2 = 𝑆H2
− R ln(𝑝 ),0where, SH20 is the standard entropy under p0 and R is gas constant. When the hydrogen
absorption and desorption is in the equilibrium (p = peq), Gibbs free energy change forhydrogenation is 0. Therefore, the above equations can be rearranged as follows,∆𝐻abs = 𝑇∆𝑆abs ,(Equation 1.24)𝑝eq

0∆𝐻abs = 𝑇∆𝑆abs+ R𝑇 ln ( 𝑝 ),
0

𝑝eq

∆𝐻abs

0

R𝑇

ln ( 𝑝 ) =

−

0
∆𝑆abs

R

.

(Equation 1.25)(Equation 1.26)

Here, peq is equilibrium pressure of the reaction. Equation 1.26 is known as van’t Hoff
equation. Thus, the enthalpy and entropy changes of hydrogen absorption can beexperimentally known by the plots of ln (peq/p0) versus 1/T. Figure 1.5 shows pressureconcentration-isotherm curves and the van’t Hoff plot.31

12

Figure 1.4 hydrogen absorption curve of LaNi5 performed at room temperature.

Figure 1.5 (a) Pressure-Concentration-Isotherm curves with different temperatures and(b) the van’t Hoff plot, which figures were presented by ref. [31].31
13

Kinetics
The analyses of reaction kinetics can reveal the reaction rate of the reaction, which canthermodynamically proceed. The hydrogen absorption reaction by a metal proceeds byfollowing steps,(1) physisorption of gaseous H2 molecule on a metal surface,(2) dissociation of H2 molecule into H atoms, and chemisorption of H atoms on metal
surface,(3) surface migration of H atoms into bulk,(4) diffusion of H atom in bulk of metal, and formation of solid solution phase,(5) nucleation of the hydride phase ( phase).
If it is assumed that H2 molecule approaches a metal from the perpendicular direction to
the metal surface, the energy diagram of physisorption U (Z) can be described as onedimensional Lennard-Jones potential, as shown in figure 1.6a. When H2 molecule
approaches a metal surface by distance Zphys from the surface, H2 molecule interact withthe surface through van der Waals attractive force. This interaction is originated in thesmall polarizability of H2 molecule. Therefore, the interaction of physisorption is quite
small with energy in the range of 3~15 kJ/mol H2.32 If the distance Z decreases to lessthan Zphys, the Coulombic repulsive force is generated between H2 and metal. In the caseof physisorption, covalent bond is not formed between H2 molecule and metal surface.
On the other hand, H atom shows different interaction from physisorption. When two H
atoms approach a metal surface, strong attractive interaction originates between H atoms
and metal surface. Because H atoms and the surface atoms interact by chemical bonding,the depths of the potential wells are larger than that of physisorption. This interaction is14

called chemisorption, and the energy of chemisorption is in the range of 500 to 600 kJ/mol
H2.32 Figure 1.6b shows the potential energy in the case of chemisorption. The figure startfrom 431 kJ/mol H2, which is dissociation energy of H2 molecule, because the zero point
is described as the energy of H2 molecule. Therefore, the energy with more than 431
kJ/mol H2 has to be supplied firstly to dissociate H2 molecule in order to form thechemisorption state. However, the temperature more than 1000 C is required todissociate H2 molecule due to large value of the dissociation energy.33 This fact indicatesthat the dissociation could not occur at room temperature. On the other hand, it is known
that the dissociation of H2 molecule is easily observed on a metal surface due to its
catalytic effect. The combination of the figure 1.6a and 1.6b is shown in figure 1.6c.
Molecular hydrogen approaches a metal surface, and then the H2 molecule is dissociatedinto 2H atoms at the point P, which exists near Zphys. In the case of figure 1.6c, thecrossover point P is below E = 0, indicating the spontaneous dissociation of H2 moleculewould happen. Thus, hydrogen can be chemisorbed on a metal surface without supplying
large amount of activation energy. Table 1.2 shows the values of adsorption energy ofhydrogen on metals. The binding energy of H atom and a metal surface (EMe – H) can begiven by,

𝐸Me−H =

𝐸𝑎𝑑 +𝐸d2.

(Equation 1.27)

When molecular H2 is closer than Zphys, the electrons of surface are donated to antibonding
molecular orbitals (σ*) of H2. Then intramolecular bond of H2 is destabilized, and make
Metal-H bond. This indicates that the dissociation ability depends on the electronic
structure of the metal surface. The H2 dissociation ability of light elements, which haveno d-electrons, are low compared with transition metals having d orbital-electron.
15

After the chemisorption, H atoms pass through the sub-surface layer of metal. Figure
1.7 shows a schematic diagram of potential energy from physisorption to diffusion ofhydrogen molecule or atoms. The blue solid and dotted lines are the potential curves
without and with considering reconstruction of surface atoms, respectively. In the case of
potential curve with considering reconstruction of surface atoms, the activation energy
for jumping the sub-surface layer is lower than that without considering reconstruction.
This indicates that the reconstruction of surface atoms is important for permeation of H2
into bulk of metal. Then the chemisorbed H atoms diffuse in the bulk when H atom
obtains the activation energy Ediff. The activation energy for hydrogenation (Ehydr) which
include several steps, can be drawn as figure 1.7. Usually, the step having the highest
activation energy controls the reaction rate and is called as rate-controlling step. In theinitial stage of hydrogen absorption processes, this rate controlling step is chemisorptionof hydrogen, whereas it changes to the hydrogen diffusion with the increase of hydrogenconcentration in the metal.
As a macroscopic view point, the reaction velocity (v) of hydrogen absorption by metal
can be given as follows when the reaction is assumed as first-order reaction,𝑑𝑥

𝑣 = − 𝑑𝑡 = 𝑘𝑥,(Equation 1.28)

here, k is reaction constant and x is the concentration of the reactant. This equation
indicates the reaction kinetics is faster with a larger value of k and x. The reaction constantcan be expressed as,𝐸

𝑘 = 𝑘0 exp(− R𝑇a ),(Equation 1.29)

where, k0 is called the frequency factor, and the term of exponential is known as the16Boltzmann factor, and Ea is the activation energy for the reaction. Ea includes many kindsof activation energy for the reaction steps such as a surface reaction and hydrogen
diffusion as discussed before. The frequency factor k0 represents the frequency ofcollision with reactants. In the case of solid-gas reaction like hydrogen absorption, thefrequency factor can be enhanced by increasing the hydrogen pressure and the number ofactive sites for reaction. It is reported that the hydrogen absorption kinetics of hydrogenstorage materials can be enhanced by increasing the surface area that can be achieved byball-milling.34 The reduction of particle size also affects the hydrogen diffusion. Basically,
the hydrogen diffusion rate can be improved by increasing temperature. On the other hand,
if the particle size is reduced, the high diffusion rate is not necessary to enhance thereaction yields as shown in figure 1.5. The term of exponential represents the probability,
where the reactant has energy Ea at the temperature T. If the rate-controlling step is thehydrogen dissociation on the surface, the activation energy can be decreased by adding asuitable catalyst. Kimura et al. reported that the activation energy for hydrogen absorptionof Mg catalyzed by Nb2O5 was lowered down to 31 kJ/mol H2 from 61 kJ/mol H2 of Mg
without catalyst.35 Figure 1.6 shows the curve of the Boltzmann factor drawn by using
the activation energy of the ball-milled Mg with and without Nb2O5. It can be seen thatthe value of the Boltzmann factor drastically increases when the temperature exceeds overa turning temperature (100 and 250 C for Mg with and without Nb2O5, respectively).
This indicates that the reaction rate is drastically increased from those temperatures. In
addition, it can be understood that the turning temperature is lower with smaller activationenergy as shown in figure 1.6. Therefore, when it is assumed that the frequency factorsof samples are the almost same value, the activation energy can be relatively comparedby measuring the onset temperature of the reaction.
17

Figure 1.6 Potential diagram of (a) physisorption, (b) chemisorption, and (c) combined
schematic of physisorption and chemisorption.32

Figure 1.7 Hydrogen migration and diffusion into the bulk of metal. Esol is heat of solution,
Ediff is activation energy for bulk diffusion, Ess is activation energy for desorption fromthe H subsurface sites, Ehydr is activation energy for hydrogenation, EH is activation energy
for desorption from the chemisorbed state, EH2 is adsorption energy of the physisorbed
H2, and Hd is heat of dissociation of H2 molecule. This figure is drawn by referring toprevious paper reported by Christmann.32

18

Figure 1.8 Scheme of reaction yields with different particle sizes. The reaction yields canbe improved by the reduction of particle size even the same diffusion rate.

Intensity (arb. unit)

exp (-Ea/RT)

Ea = 61 kJ/mol H2
Ea = 38 kJ/mol H2

0

200

400
600
800
Temperature (C)

1000

Figure 1.9 The curve of exp (Ea/RT) versus T. The activation energy of Mg with andwithout catalyst, which was measured by Kimura et al., were used to draw this figure.35

19

Table 1.2 The adsorption energy (Ead, 0) at the initial state and binding energy between
metal and a hydrogen atom (EMe-H) presented by Christmann.32

20

1.4 Surface modification for metal-hydrogen system
In metal-hydrogen systems, hydrogen is absorbed through the metal surface. Therefore,
the surface of the metal should be active for hydrogen absorption. To produce an active
surface, several methods of surface modification have been studied. In simple
understanding is d-electrons of transition metal can effectively dissociate molecular H2
as mentioned in section 1.3. However, light elements such as Mg do not have d-electrons,
indicating that it is difficult to dissociate H2 molecule into atoms on the surface of theseelements. It is reported that transition metals can catalyze the hydrogen dissociation forhydrogen absorption of Mg. On the other hand, although transition metals have thedissociation ability for H2, the stable oxide layer formed on the surface of metals inhibits
the permeation of hydrogen into the bulk. Therefore, initial activation is required togenerate a fresh surface.
Surface modification of light element
Metal hydrides composed of light elements such as LiH, NaH, MgH2, and CaH2, havea high gravimetric hydrogen density. However, high temperature is required to desorb
hydrogen because those metal hydrides are quite stable. Among them, MgH2 canreversibly react with hydrogen at relatively low temperature as follows,36
MgH2  Mg + H2.

(Equation 1.30)

However, the kinetics of hydrogen absorption/desorption reaction is quite slow and hightemperature of more than 300 C is required to realize the above reaction.37 It is causedby the less surface reactivity of Mg itself for H2. In order to improve the kinetics, manykinds of additives, which have a high dissociation ability towards H2, have been
investigated to find suitable catalyst since 1970s.38 Especially, several kinds of transition21

metals have been investigated as catalysts.37,39 Furthermore, the catalytic effects of metaloxides such as TiO2, V2O5, Cr2O3 etc. have been also reported.40,41 Barkhordarian et al.
presented that Nb2O5 has remarkable catalytic effects for the hydrogen absorption kineticsof Mg compared to other metal oxides.42 Kimura et al. reported that Mg catalyzed by
Nb2O5 absorbs hydrogen even at 60 C.35 In addition to transition metals and their oxides,
there are numerous reports on the catalytic effects of chemical compounds, such as
halides, carbides, and hydrides.43–45 Ma et al. investigated the hydrogen storage propertiesof MgH2 with titanium compounds (TiF3, TiCl3, TiO2, TiN, and TiH2) systematically.
Although Mg without any catalysts did not absorb hydrogen under 2 MPa H2 at 150 C,
the composites of Mg and titanium compounds absorb hydrogen at the same conditions.46
As another approach for the improvement of hydrogen absorption kinetics of Mg, theeffects of organic materials during ball-milling of Mg have been investigated.47,48
Imamura et al. investigated the effects of several kinds of organic additives for thehydrogen absorption/desorption reaction of Mg systems.47,49 Especially, they have studied
the composite of Mg and graphite ball-milled with organic solvents. The solid-phasereaction between magnesium and graphite can be controlled by organic solvents, and the
additives can prevent the excessive breakage of graphite.49
Surface Modification of transition metal
Titanium-based alloys are one of the promising hydrogen storage materials because of
their low cost compared to conventional alloy (LaNi5). Among the titanium alloys, TiFe
is attracting because of its suitable thermodynamic properties for hydrogen
absorption/desorption reactions at room temperature. However, high temperature (400
C) and high pressure ( 6 MPa) are required to activate the pristine TiFe, while LaNi5
can be easily activated even at 20 C under 5 MPa H2. Schlapbach et al. have investigated22the surface state of LaNi5 and TiFe by X-ray photoelectron spectroscopy (XPS) and Auger
electron spectroscopy (AES). Their results showed that the surface of both non-activatedalloys were covered by La2O3 and TiO2, respectively as shown by the schematic in figure
1.7. On the other hand, Fe existing on the TiFe surface can also be oxidized easily, while
Ni on the LaNi5 surface remains as metallic state. Usually, the transition metals are known
to have the ability of H2 dissociation nevertheless lose this property upon oxidation.
Therefore, they concluded that Ni can catalyze hydrogen absorption of LaNi5, while Fe
cannot catalyze hydrogen absorption in case of TiFe. Schlapbach et al. also reported thatthe precipitation of metallic Fe occurred at high temperature (200-400 C), and then TiFe
becomes active for H2.50,51
In order to overcome the drawback of TiFe, several studies of surface modification
have been reported. Fresh surface can be created by high energy ball-milling. Aoki et al.
have reported that the ball-milling effect under inert gas for initial activation of TiFe,
which could absorb hydrogen even at 10 C under 2 MPa H2.52,53 In addition, Chiang etal. performed the ball-milling of TiFe under 0.5 MPa H2 and hydrogen absorption
occurred during ball-milling.54 However, the ball-milled sample was inactivated after
exposing to air and then the reactivation was required. As another approach, the partial
substitution of Ti or Fe in TiFe by third elements has been widely investigated in order toobtain the Ti1-xFe1-yAxBy alloys, which could be more easily activated. Bounudina et al.
reported that TiFeNix could absorb H2 at room temperature under 2 MPa H2 even after
exposing in air. Lee et al. have systematically investigated the effect of partial substitution
of Fe in TiFe by 3d metals (Cr, Mn, Co, Ni), then these alloys absorbed hydrogen at room
temperature without activation.55,56 For the substitution of Ti in TiFe, there is a report on
the effects of zirconium substitution, which could be activated at 400 C under 1 MPa H223although it was not so effective compared to the substitution of Fe.57 In addition, Mizuno
et al. have investigated the effect of Ti concentration in TiFex.58 The stable phases of TiFe are TiFe and TiFe2, and the TiFe phase has a composition range of 0.98  Ti/Fe  1.08
at room temperature. They revealed that Ti1.2Fe can be hydrogenated at room temperature
under 3 MPa without activation, whereas the hydrogenation kinetics of Ti1.05Fe at thesame condition was quite slow. As an interesting report, Emami et al. succeeded in theactivation of TiFe by ball-milling with an organic solvent. In addition, the TiFe sample
could be easily activated by the only evacuation at 150 C for 2 h even though after
exposing in air for 1 month.59 However, the activation mechanism is not well understood
until now.

Figure 1.10 Scheme of illustration for surface segregation of alloys

241.5 Titanium-hydrogen system
Many researchers have investigated the thermodynamic properties of the Ti-H system
to describe the phase boundaries by experimental or theoretical analyses.60–63 Titanium
can react with hydrogen, and it forms hydride with a various compositions which areclassified by 4 kinds of phases, i.e. , , , and  phase as shown in figure 1.11.62 Figure
1.12 shows PCI curves measured by Haag et al.60 at different temperatures, where 300 C
is the lowest temperature so far, although there are results at 250 C also which were
measured by electrochemical technique.64 The PCI curves indicated that Ti ( phase) was
directly transformed into  phase at 300 C, while the transformation occurred through phase at 400 C. However, quite long time is needed to reach the equilibrium pressure
due to slow kinetics at low temperature. TiH2 has quite low dissociation pressure (1017
Pa at 25 C) due to large enthalpy change (TiH2, 142 kJ/mol H261). It is known thathydrogen absorption of Ti is inhibited by surface oxide layer at low temperature.
Hadjixenophontos et al. investigated the role of surface oxides on hydrogen absorption
kinetics of titanium by using Ti thin film courted by various thickness of TiO2. They
mentioned that the rate-controlling step of the hydrogenation by Ti is the H diffusion
through the surface oxide layer. They succeeded the room-temperature hydrogenation of
Ti coated by Pd film. Since Pd is well known as a catalyst for H2 dissociation, it is still
unknown that Ti itself is active or not for H2.

25

Figure 1.11 Phase diagram of Ti-H presented by San-Martin et al.62

Figure 1.12 PCI curves with different temperatures measured by Haag et al.60
26

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31

2 Purpose of this thesis
Various types of metal-hydrogen systems have been studied to establish the hydrogenenergy society. However, most of research devoted in the practical applications forhydrogen storage so far while the fundamental research is imperative for understanding
the interaction between the metal and hydrogen. The fundamental research on Tihydrogen system is not enough carried out so far, because Ti is not suitable as a hydrogenstorage material due to high thermodynamic stability of hydride. On the other hand, fromdifferent point of view, Ti is a potential thermochemical heat storage material for hightemperature as well as a hydrogen capture material for accidents such as hydrogen leakage.
However, it is generally known that the reaction between Ti and H2 based on thethermodynamic properties is difficult due to existence of stable oxide (TiO2) layer on thesurface, and thermal activation at several hundred C is required to realize the reaction.
In addition, the effective surface modification for hydrogen absorption by Ti is notreported. To utilize Ti as a metal-hydrogen system described in Introduction part, thedetailed mechanism of hydrogen absorption by Ti and the effective surface modification
should be understood.
In this thesis, the essential hydrogen absorption properties of Ti and its potential surface
modification processes to control the reaction are discussed based on results of following
experiments.
(1) The reaction between Ti and H2
In principle, Ti should be active for hydrogen without catalysts, since the hydrogen
absorption is exothermic reaction and H2 can be dissociated easily by d-orbital electronsin Ti. However, the hydrogenation of Ti itself at the temperature less than 100 C has not
32

reported yet. In this work, to understand the essential hydrogen absorption properties, Ti
with fresh surface is prepared and the hydrogen absorption properties are investigated.
(2) Surface modification by transition metals and oxides to make active sites
It is well known that transition metals can provide an active site to enhance the reaction
with H2. In this work, surface modification effects of typical transition metals and oxides
were investigated, where the additives were chosen by referring research on Magnesium(Mg)-hydrogen system.
(3) Surface modification by organic solvents and graphite to prevent the surface oxidation
As different approach from the above, we also focused on surface modificationby using organic materials. It is expected that the Ti surface is modified by reactions
with organic materials to realize active and selective hydrogen absorption. In fact,
several papers of research on Ti alloys ball-milled with organic solvents for surface
modification were reported. In this work, the effects of organic solvents and graphite
for surface modification of Ti were investigated.

33

3 Experiments
3.1 Sample preparation
3.1.1 Starting materials
The starting materials are listed in table 3.1. Nickel (Ni), palladium (Pd), niobium oxide(Nb2O5), and vanadium oxide (V2O5) were selected as solid additives, which are wellknown as hydrogen dissociation catalysts for Mg/MgH2 systems.1,2 As another surface
modification technique, ball-milling in organic solvents (wet-milling) was also performed.
The sample preparation by wet milling has been already reported in Mg/MgH2 systems
and TiFe/TiFeH2 systems.3,4 In this thesis, seven different kinds of organic solvents were
investigated for the surface modification of Ti. In addition to above organic solvents,
graphite was also investigated as an organic material. Graphite was considered as theeffective additive for hydrogenation properties of Ti after the surface modification effectsof organic solvents were investigated.

34

Table 3.1 Information of Starting materials

353.1.2 Ti with fresh surface
In this work, the reaction between Ti with fresh surface (TiFresh) and H2 was studied
prior to the investigation of surface modification effects. It is well-known that the surfaceof Ti is usually covered by the TiO2 surface layer. Therefore, the oxide layer should be
removed in order to know the essential hydrogen absorption properties of Ti. Wang et al.
reported that surface oxide layer on TiH2 can be reduced during hydrogen desorption of
the hydride.5,6 According to the reports, it is expected that TiFresh can be prepared by thedehydrogenation of TiH2 without air contamination. TiH2 was put into a reactor in glovebox (Miwa MFG, MDB-2BL, O2  5ppm, H2O  2ppm). After sealing the reactor, it was
taken out from glove box. Further, the reactor was connected to a Sievert-type systemequipped with rotary pump and turbomolecular pump (figure 3.1). The reactor was heated
up to 580 C under dynamic vacuum conditions using rotary pump and turbomolecularpump for 4 h. Here, the temperature of heat-treatment was determined by the hydrogen
desorption profile, which will be explained in results and discussion part. After heating,the reactor was transferred into glove box to investigate hydrogenation properties. All theoperations were carried out without exposing the sample to air to minimize influence of
air contamination.

36

3.1.3 Ti with surface modification
Ball-milling techniqueBall-milling is a technique for mixing and pulverization of sample by adding thephysical energy to the sample. It is commonly used for mechanical alloying, dispersionof catalyst, particle size reduction, and so on.7–10 In this work, planetary ball-milling (PM)apparatus (Fritsch P7) was used for mixing Ti (or TiH2) and additives. In the case of the
PM method, the vessel and the disk are rotated simultaneously as shown in figure 3.2.
Therefore, centrifugal force is applied to the sample and balls. Two types of ball-milling
vessels were used for sample preparation (figure 3.3). Chromium (Cr) steel vessel of 30
cm3 inner volume with quick connector (Swagelok) was used for the ball-milling of TiH2
and solid additives (figure 3.3a). This vessel was sealed by O-ring (Viton, fluorine
rubber). However, this O-ring cannot be used for ball-milling with organic solvents due
to possibility of corrosion by organic solvents. Therefore, another type of the vessel,
which can be sealed by Copper (Cu) gasket, was used for wet-milling. (Figure 3.3b). Here,the temperature of heat-treatment was determined by the hydrogen desorption profile,which will be explained in results and discussion part.
Ti ball-milled with solid additives
Titanium hydride and solid additives (total 300 mg) were mixed with molar ratio of
100:1. The mixture and steel balls (SUJ-2, 7 mm in diameter) were put into the ballmilling vessel. After sealing the vessel, the inside gas was exchanged to 1 MPa H 2 byusing the Sievert-type system (figure 3.1). Then the sample was ball-milled for 20 h at
370 rpm under 1 MPa H2. To avoid temperature increase in the vessel, the ball-milling
was stopped for 30 min after every 1 h milling. After the ball-milling, the sample was37

taken out from the vessel, and then it was transferred into the reactor for heat-treatment.
In order to prepare the dehydrogenated TiH2 with each solid additive, the samples were
heated up to 550 C under dynamic vacuum condition for 4 h using the gas system.
Ti ball-milled with organic solvents and graphite
Ti and graphite were mixed with 1:0.5 molar ratio. Total 300 mg of the mixture and 20
zirconium dioxide (ZrO2) balls (8 mm in diameter) were put into the ball-milling vessel(30 cm3 inner volume) with Cu gasket type. The sample was ball-milled for 3 h at 200
rpm under 0.1 MPa Ar. The ball-milling was stopped for 1 min after every 10 min milling.
Here, the Ti ball-milled with graphite is denoted as TiGraphite in this work. Titanium (300mg) and 20 ZrO2 balls were put into the ball-milling vessel. Approximately 20 wt.%
organic solvent was added using Pasteur pipette. Here, organic solvent was partiallyvaporized during the operation, thus it was difficult to add 20 wt.% of organic solvent
accurately. Considering this problem, about 70 mg of organic solvent was added. After
that, the sample was ball-milled with the same conditions of TiGraphite. Here, the volatile
by-products were observed in a temperature range of 200 to 300 C during TG
measurement for Ti ball-milled with acetone and 4-methyl-2-pentanone, which were notrelated to hydrogenation properties. Therefore, after ball-milling of Ti with each organic
solvent, the samples were vacuumed at 300 C for 2 h to remove the residual solvents andvolatile by-products. The prepared Ti samples in each organic solvents are called asTiorganic material in this thesis.

38

Figure 3.1 Image of the Sievert-type system used for heat-treatment.

Figure 3.2 Planetary ball-milling39

Figure 3.3 Ball-milling vessels sealed by (a) O-ring and (b) Cu gasket.

40

3-2 Analysis methods
3.2.1 Thermogravimetry-differential thermal analysis (TG-DTA)
Principle
Thermogravimetry (TG) is a technique for analyzing weight change of sample inheating process. A sample pan and an empty reference pan are put on the arms attachedwith thermocouples (figure 3.4). Both the arms are adjusted to the same height byelectromagnet. The heights of both arms are monitored. If weight loss occurs during
heating, the height of arm with the sample increases. Then, the current of electromagnet
is decreased to adjust the height to original position. Since the change of the current is
proportional to the weight loss/gain of the sample, the weight change can be calculatedby the value of current. At the same time, the temperature difference between both pansis measured by the thermocouples attached to the arms. The temperature of sample panin comparison to reference pan is increased during an exothermic reaction while it isdecreased during an endothermic reaction, thus enabling to estimate the nature of reaction
by this temperature difference. This technique is known as differential thermal analysis(DTA). The TG-DTA curves of a standard sample i.e. Ca(COO)2H2O are shown in figure3.5. This measurement was carried out under Ar flow up to 900 C at 10 C/min heatingrate. The x-axis is temperature, and y-axis is weight loss (wt.%) for TG and heat flow
(arb. unit) for DTA. Three peaks were observed in DTA curve with weight loss of 12.2,
31.2, and 60.8 wt.% at 169, 514, and 733 C, respectively. These weight changes arecorresponding to the reactions reported in previous literature.11
Ca(COO)2H2O → Ca(COO)2 + H2O↑ (12.3 wt.%)(Equation 3.2)

Ca(COO)2 → CaCO3 + CO↑ (31.5 wt.%)(Equation 3.3)

CaCO3 → CaO + CO2↑ (61.6 wt.%)(Equation 3.4)

41

In addition, it can be decided by DTA curves whether the reaction is exothermic orendothermic. In order to understand the details of reaction, TG-DTA measurement is often
combined with thermal desorption mass spectrometry as described in next section.
ProcedureIn this study, TG-DTA apparatus (Rigaku, TG8120) used to investigate thehydrogenation and dehydrogenation temperature of the Ti samples. The TG-DTA
apparatus was located in glove box (Miwa MFG, MDB-2BL, H2O  0.2ppm) as shownin figure 3.6. When Ti absorbs hydrogen, the weight of sample increases according to thefollowing reaction:
Ti + H2  TiH2 (4.2 wt.%).

(Equation 3.5)

Therefore, the hydrogenation temperature can be measured by using TG-DTA apparatus
under H2 flow. Hydrogen absorption measurement was carried out by following processes.
The Ti sample was heated from room temperature to 40 C under Ar flow, then the carriergas was switched to H2 at 40 C. After switching the carrier gas, the temperature was kept
for 10 min to investigate the hydrogen absorption at around room temperature. After
keeping the temperature for 10 min, the sample was heated up to 400 C. In this work,
the reactivity of each sample with H2 was evaluated by comparing the onset temperaturesof weight gain. Here, isotherm measurement is required for quantitative analysis ofhydrogen absorption kinetics. However, the temperature immediately increases when Ti
absorbs hydrogen due to the large hydrogenation enthalpy change.12 Therefore, it isdifficult to analyze the reaction kinetics quantitatively by this experiment. For the analysisof the hydrogen desorption properties of the TiH2 samples, sample was heated up to 650
C under Ar flow. Desorbed gas during heating was analyzed by thermal desorption mass42

spectroscopy connected with TG-DTA apparatus.

Figure 3.4 Reactor of TG-DTA apparatus0

TG curve (wt. %)12.2 wt.%-20
31.2 wt.%-30

endothermic
reaction-40

DTA curve (arb. unit)

-10

-50
60.8 wt.%

-60

200

400
600
Tempearture (C)

800

Figure 3.5 TG-DTA curves of Ca(COO)2H2O performed under Ar flow at 10 C/min.

43

Figure 3.6 A picture of TG-DTA apparatus located in glove box.

44

3.2.2 Thermal desorption mass spectroscopy (TDMS)
Principle
Mass spectroscopy (MS) is a technique for the identification of molecular mass. By
combining MS with TG-DTA measurements, the released gases as well as the reaction
temperature and the reaction heat can be determined. The MS measurement is mainlycomposed of the processes in the ionization of gas, separation, and finally detection of
the ionized molecules. There are several kinds of ionization methods. The mostconventional method is electron impact ionization (EI method). In the case of EI method,
the gases are ionized by bombarding with electrons having energy in the range of 10~70
eV.13 Here, the gases ionized by EI method are sometimes split into fragment ions due toexcess internal energy. After ionization, these gases are accelerated into magnetic field
for separation. When the velocity of ion is v, the mass of ion is m, and the valence is z,
the kinetic energy of ion can be given by,1
2

𝑚𝑣 2 = 𝑧𝑒𝑉,

(Equation 3.6)

where, V is the acceleration voltage and e is the elementary electrical charge. The ionizedgases are deflected by the magnetic field (B), and follow a circular motion due to theLorentz force (B zev) imposed perpendicularly to the traveling direction as shown infigure 3.7. If the radius of the circle is r, the ionized gases feel a centrifugal force equals
to mv2/r, which is balanced with the Lorentz force,

mv2/r = B zev.

(Equation 3.7)

From the equation 3.6 and 3.7, following equation is derived.

m/z = er2B2/2 V.
45(Equation 3.8)

Therefore, the molecules with m/z can be separated by scanning B or V. Here, the valence(z) is usually 1 or 2.
In this work, quadruple mass spectrometer was used, which has advantages of the fastscanning and compactness, although the detection resolution is slightly low comparedwith other spectrometers.13 The quadruple mass spectrometer is composed of two twinelectrodes as shown in figure 3.8. The modulated voltage Vm is applied to the electrodes
as follows,

Vm = U + V cos t,(Equation 3.9)

where, U is direct current (D.C.) voltage and V cos t is high frequency voltage ofalternating current (A.C.). When ionized molecules pass into the two twin electrodes, they
are vibrated by the high frequency voltage. Although most of the molecules collide withthe electrodes, only the ionized molecular gases with specific m/z ratio can reach thedetector. If the frequency of the A.C. and U/V are constant, the molecules satisfying thefollowing equation can reach the detector,

m/z = 0.14 V/f 2 r02,with𝜔

𝑓 = 2𝜋.

(Equation 3.10)(Equation 3.11)

Therefore, the molecules with different m/z can be separated by scanning V. Figure 3.9
represents the TDMS curves of Ca(COO)2H2O simultaneously performed with TG-DTA
measurement. The m/z = 18, 28, 44 were detected, which are corresponding to H2O, CO,and CO2, respectively. The desorbed gases are consistent to the reactions equation 3.23.4. Thus, the thermal properties of sample and desorbed gases can be investigated by
TG-DTA-TDMS.
46

Procedure
The thermal desorption mass spectroscopy (Anelva, M-QA200TS) was used for
identification of desorbed gas during TG-DTA measurements. The equipment isconnected to the TG-DTA equipment by capillary as shown in figure 3.4. The
spectrometer was dynamically vacuumed below 103 Pa by turbomolecular pump androtary pump. High purity Ar gas was flowed as a carrier gas.

Figure 3.7 Mass spectrometer of magnetic field typeFigure 3.8 Quadruple spectrometer

47

-10

12.2 wt.%-20

513 C-30

31.2 wt.%-40-50

endothermic
reaction

166 C60.8 wt.%-60

Intensity (arb. units)731 CDTA curve (arb. unit)

TG curve (wt. %)0m/z = 18m/z = 28m/z = 44

200

400
600
Temperature (C)

800

Figure 3.9 TG-DTA-MS curves of Ca(COO)2H2O performed under 0.1 MPa Ar flow at
10 C/min heating rate.

48

3.2.3 Transmission electron microscope (TEM)
Principle
TEM is a microscope, which possesses the high resolution in the order of 0.1 nm. By
combining with other spectroscopy, such as energy dispersive spectroscopy (EDS) andelectron energy loss spectroscopy (EELS), TEM can be utilized to understand the
distribution of elements and the chemical state as well as the morphology. TEM iscomposed of 3 parts namely, electron gun, electron lens, and vacuum system.
Apparatus
1) Electron gun
Thermal electron gun (TEG) and field emission gun (FEG) are used as electron source.
When a filament is heated by Joule heat, electrons are released from the filament if theelectrons obtain higher energy than w (= E0  Ef). These electrons are called thermal
electrons. Here, the E0 is ground state energy, Ef is Fermi level energy, and w is called
work function. Therefore, the filament with smaller work function can effectively release
thermal electrons. In high electrical field, electrons are emitted from the surface ofmaterial by passing over the Schottky barrier. The current density of electrons is given by,
3⁄2

𝑗 = 𝑎𝑉 exp(−

𝑏𝑤 2
𝑉

),

(Equation 3.12)

where, V is voltage applied between filament and anode, and a and b are constant decided
by material and shape of filament, respectively.
2) Electron lens
The cross section of a magnetic field type electron lens are shown in figure 3.10.
Copper wire is coiled in area 1. Magnetic field are generated in the center of the lens by
the coil. When electron beam is entered into the magnetic fields, the electron beam is49rotated and refracted by the force corresponding to Fleming’s left-hand rule. After passing
through in the magnetic field the electrons are collected at the focal point. This behavior
is the same as optical convex lens. Therefore, when the distance between specimen andlens is a, between lens and image is b, and the focal length is f, the relationship of theseparameters can be expressed as,
1
𝑎

1

1

+ 𝑏 = 𝑓.

(Equation 3.13)

In addition, the magnification (M) can be given by,𝑏𝑏𝑎

𝑀 = 𝑎 ≈ 𝑓 = 𝑎−𝑓.

(Equation 3.14)

Therefore, the magnification can be enhanced by reducing the focal length (f), andbringing specimen closer to focal point (a  f). There are problems about the aberration
like the optical lens. The electron beam incident in the outside of lens is more strongly
refracted, resulting shifting of focal point (figure 3.11), which is called sphericalaberration. The spherical aberration can be reduced by increasing the accelerating voltage.
Recently, TEM with spherical aberration correction has been developing. In addition, if
the incident electrons have the different kinetic energy, the chromatic aberration occurs
because the refractive indexes of these electrons are different. Therefore, it is required touse the electron beam with the aligned energy.
3) Vacuum system
TEM must be working under vacuum condition to increase the mean free path of theelectrons. In addition to above purpose, the vacuum system has the advantages forstabilizing the high applied voltage, prolonging the life of filament, and so on. Ion pump50

and turbomolecular pump are usually used for evacuation in TEM equipment.14

Figure 3.10 Cross section of a magnetic field type electron lensFigure 3.11 Shift of the focal point by spherical aberration cited from reference [15].15
51

Procedure
The morphology and the local chemical state of TiGraphite and TiAcetone were analyzed byusing scanning transmission electron microscope (STEM, FEI Titan3 G2 60-300) with anacceleration voltage of 200 kV. The sample was dispersed on a quantifoil Mo TEM grid
(EM Japan Co. Ltd., R2/1 type, 200 mesh). The sample was transferred from glove box
to TEM using the atmos defend holder (Mel-build corporation) without exposing to air.

52

3.2.4 Powder X-ray diffraction (XRD)
Principle
Powder X-ray diffraction technique is used for identification of crystal structure. If Xray irradiates an atom, electrons of the atom are vibrated by alternating electric field of
the X-rays. When the electrons are vibrating, electromagnetic wave with the same
frequency as that of the vibration is released from the vibrated electron, then theelectromagnetic wave expands to sphere direction from the atom. This phenomenon iscalled Thomson scattering. Electromagnetic wave emitted by Thomson scattering has thesame wavelength as one of the incident X-ray. Since atoms have more than 1 electron, all
the scattered electromagnetic waves are released associating with each wave. Therefore,
the ability of X-ray scattering is higher with larger number of electrons in atom (larger
atomic number). Figure 3.12 shows atomic scattering factors, which means the X-ray
scattering ability of atoms. This figure indicates that the value is lower with decreasing
atomic number or with increasing sin /. Here,  is angle of scattered X-ray and  is the
wavelength of X-ray. Therefore, the scattering ability depends on the detection angle aswell as the atomic number. Basically, this fact causes that the intensity of diffracted Xray is lower in higher angle. In addition, the atoms with low atomic number like hydrogen
do not affect the XRD profile, indicating it is impossible to determine the position ofhydrogen by XRD profile.
The scattered X-rays are detected as peak when the X-ray satisfies Bragg’s law. Figure
3.13 shows the schematic of X-ray diffraction from lattice plane. Where, n is number oflattice plane from upper plane,  is wavelength of incident X-ray, d is distance between
lattice planes, and  is the angle between plane and diffracted X-ray. When the twoparallel X-rays of the same wavelength incident to point D on the upper plane and point53

B of the next plane, the X-rays are scattered like figure 3.13. In this case, an opticaldistance of X-rays for n = 2 against n = 1 is AB+BC. This length can be written by 2dsin.
If 2dsin is an integer multiple of wavelengths (), the X-rays are in phase with eachother, and then the X-rays intensify. Therefore, the Bragg’s law can be written as follows,n = 2dsin (n = 1, 2, 3).

(Equation 3.15)

Because the angle and intensity of X-ray diffraction depend on the crystal structure andthe atomic number of atoms, the crystal information can be identified by analysis of XRD
pattern. Therefore, the crystal structure can be identified by analysis of X-ray diffraction
pattern.
Apparatus
1) X-ray sources
A schematic of X-ray tube are shown in figure 3.14. The inside of tube has to be
evacuated. When the current was supplied in a filament, the temperature of filament isincreased due to Joule heat. At the same time, if voltage is supplied between the filament(cathode) and target (anode), thermal electrons are released from the filament to the target.
The electrons are accelerated by the applied voltage and collide with the target. If thebombarded electron has higher energy than the binding energy of the electrons in innershell of atom in the target, the electron is released from inner shell of atom, leading to avacancy generation. Another electron at higher energy levels drops into the vacancy withemitting the excess energy as an X-ray. The energy of the difference between L shell andK shell is emitted as the characteristic X-ray. The processes of the characteristic X-ray
emission are shown in figure 3.15. Since the energy difference between L shell and K
shell is fixed by each element, the wavelengths of characteristic X-rays depend on the54

targets as shown in table 3.2. In addition to the characteristic X-rays, the continuous Xray is also emitted due to bremsstrahlung interaction between the irradiated electrons andatoms. Finally, the emitted X-ray makes a spectrum like figure 3.16. The intensity of Xray released from the target is proportional to the acceleration voltage (V), atomic numberof the target (Z), and the current in the filament (i) as follows,

I  V2 Z i.

(Equation 3.16)

According to equation 3.16, V and i can be controlled whereas Z is fixed for a given target.
2) Detector
Scintillation counter is conventionally used for the detection of X-ray. Fluorescent
substance, such as anthracene and Tl-doped NaI, is used as scintillator. When the
scintillator is excited by X-ray, it emits violet light. Then, the violet light collides withphotocathode, and photon is emitted. The photon is amplified by photomultiplier tube,
and finally converted into electric signals. As the other options, there are proportionalcounter and p-type semiconductor detector. These counters detect X-rays by the similar
mechanism. When X-ray passes through the counter, electrons and cations (or holes) are
generated. The electrons are collected to anode, and the cations are collected to cathode,resulting in the supply of current in the electric circuit. Then the number of photons for
X-ray is estimated directly by the current. The accepter of the proportional counter isgaseous, while that of the semiconductor detector is solid. The density of substance of thesemiconductor type is much higher than one of the proportional counters, indicating Xray is effectively changed into electric signal by semiconductor. The time resolution andthe energy resolution are important factors for X-ray detector. The time resolution is thetime required for the transformation from 1 photon to signal. The energy resolution is the55minimum distinguishable energy difference. These resolutions of the detectors are shownin table 3.3. The semiconductor detector is better than scintillation type detector. Since
the time resolution of the scintillation counter is less, the ratio of intensities with eachpeak is sometimes different from database due to miscounting. The miscounting occurs
at more than 30000 ~ 50000 counts per second.16 Therefore, if X-ray with the highintensity is used as X-ray source, this problem should be carefully taken in to account.
ProcedureIn this work, XRD equipment (Rigaku RINT 2500V) was used for phase identification.
Cu target was used as X-ray source (Cu-K:  = 1.54 Å). The current load of filamentand accelerating voltage of thermal electrons were 200 mA and 40 kV, respectively.
Silicon semiconductor (Rigaku D/teX Ultra) was used as detector. For XRD measurement,a sample was spread on hand made XRD measurement glass plate. The sample was fixedby grease (Apiezon® H grease, Leef Energy Co. Ltd.). To minimize influence ofoxidation due to air during measurement, sample was sealed by using polyimide sheet(Kapton®, Du Pont-Toray Co. Ltd.). The XRD sample plate is shown in figure 3.17. The
XRD patterns were analyzed by comparing to databases [ICDD, JICST, COD2019] using
the PDXL software.

Figure 3.12 Atomic scattering factors cited from [16].16
56

Figure 3.13 X-ray diffraction by crystalline plane

Figure 3.14 X-ray tube

Figure 3.15 Processes of the characteristic X-ray emission
57

Figure 3.16 The characteristic X-ray and the continuous.

Figure 3.17 Plate for XRD measurement

Table 3.2 Targets for X-ray source and the wavelength of the characteristic X-ray

58

Table 3.3 The resolution of X-ray detectors reported by Shimazu et al.1759

3.2.5 X-ray photoelectron spectroscopy (XPS)
Principle
X-ray photoelectron spectroscopy (XPS) is a technique used to investigate the chemical
state of elements existing at surface layer (several nm). When X-ray is irradiated on to amaterial surface, photoelectron is released from the material by the photoelectric effect.
The energy and its distribution of photoelectrons are measured by XPS measurement. The
kinetic energy (Ek) of photoelectron can be expressed by following equation,

Ek = h  Eb  .

(Equation 3.17)

Where, h is energy of incident X-ray, Eb is binding energy between electron and atomic
nucleus, and  is work function. The energy level is shown in figure 3.18. The binding
energy is defined as the energy from the electron level of each orbital to Fermi level. The
work function is defined as the energy difference between Fermi level and vacuum level,
and it depends on several factors such as element, shape of surface, lattice plane. However,
it is not necessary to calculate the work function of the sample for evaluating the binding
energy. When the sample is electrically connected with the spectrometer, it can beassumed that their Fermi levels become identical as shown in figure 3.18. Therefore, if
the work function of the spectrometer (sp) is known, the binding energy can be estimatedby the equation,

Ek’ = h  Eb  sp.

(Equation 3.18)

Here, Ek’ is kinetic energy evaluated by equipment. sp depends on each equipment, which
is decided by measuring a standard sample such as Ag film. The binding energy ofelectron is affected by the attractive force with nucleus and repulsive force with other60

electrons. Therefore, the binding energy is shifted by changing the environment ofelectrons. Electrons with lower binding energy, which is approximately less than 50 eV,
are called valence electrons, and they are strongly affected by surrounded atoms. When aphotoelectron is generated from a surface, the photoelectron moves in the surface withrepeating elastic scattering and inelastic scattering. If inelastic scattering occurred before
the release from the surface, the photoelectron cannot be detected as a peak. In otherwords, the photoelectron without inelastic scattering can be detected as photoelectron
peak. Tanuma et al. have calculated the inelastic mean free path (IMFP) as shown infigure 3.19. The figure indicates that electron can move the average of 3 nm in the surfaceof elements. Thus, the XPS spectrum reflects the electronic state of surface part only.
Apparatus
1) X-ray source
The mechanism of the X-ray generation for the X-ray source is the same as that of the
XRD equipment, although the energy range of X-ray is different. In the case of Cu K
(1.54 Å) used as incident X-ray for XRD, the energy of X-ray is 8.05 keV. On the other
hand, the energy of X-ray for XPS measurement is 1.49 keV (Al K) or 1.25 keV (Mg
K). In the XPS measurement, the discussion of chemical shift with 0.1~1 eV is necessary
to evaluate the valence state. However, the natural width of X-ray is 0.8~0.6 eV. To reduce
the width of X-ray, monochromator is often used.
2) Separator
Concentric Hemispherical Analyzer (CHA) is usually used to separate electrons having
different energy. Figure 3.20 shows the schematic drawing of CHA. Before the electron
goes into CHA, the kinetic energy of electrons is reduced to a constant energy (pass61energy) by electrostatic lens. As explain in the principle of TDMS, if the kinetic energyof electrons and the Lorentz force of separator are balanced, the electron can reach thedetector. When the radii of inner and outer hemispheres are r1 and r2, respectively, thevoltage gradient V(r) can be expressed as:𝑟 𝑟2𝑉(𝑟) = −𝑉p 𝑟 2 (𝑟1 −𝑟
.
)
0

2

1

(Equation 3.19)

Here, Vp is the applied voltage between r1 and r2. r0 is the radius of center between bothhemispheres (figure 3.20). The kinetic energy E0, which pass through on the r0, is called
pass energy. The pass energy can be expressed as,𝑒𝑉p

𝐸0 = 𝑟2 𝑟1 .
( − )(Equation 3.20)

𝑟1 𝑟2

And the energy resolution (EA) of CHA is
𝑤𝐸𝐸A = 2𝑟 0 .
0(Equation 3.21)

Where, w is width of incident and outgoing slits. This equation suggests that the energy
resolution is increased by decreasing width of slits and pass energy, and increasing r0. On
the other hand, the decrease of width of slits and pass energy leads to the lower count ofelectrons.
Procedure
The XPS measurements were carried out using two apparatuses of ESCA lab 250Xi(Thermo fisher), and JPS-9200 (Japan Electro Optical Laboratory Co. Ltd.) with Al-K
(1486.6 eV) to analyze chemical states of surface. The samples were fixed on a sample
holder by using carbon tape. To avoid charge up, flood gun was used during the XPS
measurements. The spectra were calibrated by C1s (285.0 eV) of each sample. The
samples were transferred into XPS equipment from glove box using transfer vessel.
62

Figure 3.18 Model of energy level when analyzer and sample are electrically connected.

Figure 3.19 Inelastic mean free path (IMFP) of elements calculated by Tanuma et al.18
63Figure 3.20 Schematic of concentric hemispherical analyzer (CHA).

64

3.2.6 Raman scattering spectroscopy
Principle
Raman scattering spectroscopy is a spectroscopy, which focus on the energy range
between 10 and 1000 cm1. Rotational and vibrational modes of atoms have the energy
in this region. Therefore, the chemical bonding states between atoms can be determinedby Raman spectroscopy. In the case of diatomic molecule, the molecule can be considered
as mass points connected by a spring as shown in figure 3.21. Therefore, the vibrationalenergy can be expressed by harmonic oscillator model as shown follows,1𝐸 = ℎ0 ( + 2)

( = 0, 1, 2, ). (Equation 3.22)

Where, h is Plank constant, 0 is frequency of vibration at ground state, and  is quantum
number of vibrations. According to Hooke’s law, the frequency of the vibration 0 is givenby,𝑘(Equation 3.23)

𝑚 𝑚(Equation 3.24)

10 = 2𝜋 √𝑚′ ,
𝑚′ = 𝑚 1+𝑚2 ,12

where, k is the force constant, and m’ is effective mass, and m1 and m2 are mass of eachmass point. Here, the force constant between both points represents the strength ofbinding.
When electromagnetic wave irradiates atoms, the electric field interacts with the atoms.
Then the atomic nucleus is vibrated by the electric field as shown in figure 3.22. Energy
transformation between incident light and atom occurred during the interaction, resulting
in the transformation of vibration energy levels. The induced dipole moment  is givenby,65(Equation 3.25)

𝜇 = 𝛼𝐸,

where,  is polarizability, and E is electric field of incident light. These parameters canbe expressed as,
𝐸 = 𝐸0 cos 2𝜋𝑖 𝑡,𝜕𝛼(Equation 3.26)

𝛼 = 𝛼0 + (𝜕𝑄) 𝑄,(Equation 3.27)

𝑄 = 𝑄 0 cos2𝜋𝑖 𝑡,(Equation 3.28)

0here, 0 is polarizability in equilibrium position, E0 is amplitude of electric field, Q isdisplacement of atomic nucleus, and Q0 is maximum displacement of atomic nucleus.
Therefore,  can be expressed as: = 0E0 cos 2 it+ 1/2 (/Q)0Q0E0[cos 2(i + )t + cos 2(i  )t].

(Equation 3.29)

The first term on the right side of equation represents the same frequency of vibration as
incident light. On the other hand, (i + ) and (i  ) appear in second term, whichindicate that the frequency is increased or decreased, respectively. Therefore, thescattering lights include the electromagnetic wave with higher and lower energy
compared with incident light as shown in figure 3.23, for which the energy difference iscorresponding to the difference of vibration energy level. This phenomenon is called
Raman scattering, and these lights are denoted as Stokes and anti-Stokes scattering,
respectively. According to the equation 29, the selection rule of Raman scattering is(/Q)0  0.
This indicates that the polarizability of molecule must be changed.
66(Equation 3.30)

Procedure
The Raman spectroscopy was performed to analyze chemical bonding in the sample.
The light source was an Ar ion laser (Spectra-physics, Inc., Stabilite 2017) operated at
488.0 nm with an output power of 4 mW. The scattered light was analyzed by a triple
monochromator (JASCO, TRS-600) with a liquid-N2 cooled charge coupled device(CCD) detector (Princeton Instruments, Inc., model LN/CCD-1100-PB). The Ti sample
was pressed using pellet maker with 10mm diameter at 180 MPa for 5 min to make apellet for Raman spectroscopy. The pellet sample was fixed on the sample stage using
carbon tape. The sample stage was put into hand-made vessel, which can avoid air
contamination during the measurement.

Figure 3.21 Schematic view of stretching vibration for diatomic molecule

Figure 3.22 Induced dipole moment
67

Figure 3.23 Energy diagram of Raman scattering. (a) Stokes scattering (b) anti-Stokes
scattering.

68References
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Barkhordarian, G., Klassen, T. & Bormann, R. Fast hydrogen sorption kinetics ofnanocrystalline Mg using Nb2O5 as catalyst. Scr. Mater. 49, 213–217 (2003).

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Denis, A., Sellier, E., Aymonier, C. & Bobet, J. L. Hydrogen sorption propertiesof magnesium particles decorated with metallic nanoparticles as catalyst. J. Alloys
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Wang, C. et al. XPS study of the deoxidization behavior of hydrogen in TiH2
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Wang, C., Pan, L., Zhang, Y., Xiao, S. & Chen, Y. Deoxidization mechanism ofhydrogen in TiH2 dehydrogenation process. Int. J. Hydrogen Energy 41, 14836–
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Imamura, H., Kitazawa, I., Tanabe, Y. & Sakata, Y. Hydrogen storage incarbon/Mg nanocomposites synthesized by ball milling. Int. J. Hydrogen Energy
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Ivanov, E., Konstanchuk, I., Stepanov, A. & Boldyrev, V. Magnesium mechanicalalloys for hydrogen storage. J. Less Common Met. 131, 25–29 (1987).

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70

4 Results and discussion
4.1 The reaction between Ti and H2
In order to determine the dehydrogenation conditions of TiH2 to prepare Ti metal, thehydrogen desorption profile of pristine TiH2 was obtained. TG-DTA-MS result of thepristine TiH2 measured under 0.1 MPa Ar flow up to 650 C at 5 C/min is shown infigure 4.1.1. Three endothermic peaks with weight loss were observed from 400 C to650 C. In the MS measurement, the intensity of m/z = 2 corresponding to hydrogen (H2)increased with the weight loss, indicating that TiH2 was decomposed in the above
temperature region. Figure 4.1.2 shows the XRD results of TiH2 before and after the TGDTA-MS measurement. The diffraction peaks of the pristine TiH2 before the heating wereassigned to the TiH2 phase, whereas the diffraction pattern after the TG-DTA-MS
measurement was ascribed to the Ti phase. Thus, XRD results suggested the TiH2transformation in to Ti during the TG-DTA-MS measurement. From these results, theappropriate temperature to prepare dehydrogenated TiH2 was determined to 580 C,which was corresponding to the third peak temperature of the MS profile. The XRD resultof the dehydrogenated TiH2 through heat treatment is also shown in figure 4.1.2, which
confirmed that Ti phase was generated by the heat treatment at 580 C, indicating thehydrogen desorption was completed.
After the dehydrogenation treatment, the hydrogen absorption measurement of thedehydrogenated TiH2 was carried out under 0.1 MPa H2 flow at a temperature from 40 to
400 C. The hydrogenation curves of the dehydrogenated TiH2 and the pristine Ti as areference sample are shown in figure 4.1.3. About 4 wt.% of weight gain was observed
during the all TG measurements, and this value is close to the theoretical hydrogencapacity of TiH2 (4.2 wt.%). It was confirmed by the XRD measurements that Ti is
71converted to TiH2, as shown in figure 4.1.4. Therefore, the weight gain during TG
measurement is corresponding to hydrogen absorption by Ti. The hydrogenation of thepristine Ti occurred at 355 C, namely the thermal activation is required to realize thereaction. On the other hand, when the hydrogen absorption measurement of thedehydrogenated TiH2 was conducted immediately after the dehydrogenation treatment,
H2 was absorbed even at 40 C (just after switching carrier gas). This result suggests that
the Ti should have an active surface to hydrogen just after dehydrogenation, and thenthere was no need of any catalysts and/or heat-activation. However, the hydrogenation
temperature was increased to 125 and 225 C after keeping the sample in the glove box
for 1 and 7 days after the sample preparation. It was considered that surface oxidation ofTi proceeded by the small amount of oxygen (less than 5 ppm) and water (less than 2ppm) included in the glovebox. In other words, the surface of Ti just after
dehydrogenation had a clean surface, and it was easily inactivated under the conditions
with very low O2 and H2O concentration because of the high reactivity. As a result,hydrogenation temperature was increased for the Ti samples kept for long time even in
the glovebox. Ti 2p XPS spectra for the pristine Ti, TiH2, and the dehydrogenated TiH2
are shown in figure 4.1.5. For the pristine Ti, two peaks were observed at 459.0 eV and
464.8 eV, respectively. These peaks were assigned to the doublet of Ti2p3/2 (459.0 eV)
and Ti2p1/2 (464.8 eV) of Ti4+ (TiO2).1 Considering the detectable depth of XPS
measurement, the thickness of the TiO2 layer was roughly estimated to be more than 10
nm. Thus, it was concluded that the surface of the pristine Ti was covered by the TiO2
layer and the hydrogenation was inhibited by the surface oxide layer. The spectrum of thepristine TiH2 is similar to the one of the pristine Ti, suggesting TiH2 was also covered by
the TiO2 layer. On the other hand, the highest intensity peak in the Ti 2p XPS spectrum72

of the dehydrogenated TiH2 was observed at 454.0 eV. This peak can be assigned to theTi2p3/2 peak of Ti0 (metallic Ti).1 This result indicated that the fresh surface of Ti was
generated by the dehydrogenation of TiH2. The peak with low intensity was also observedin the area of Ti3+~Ti4+. The fresh Ti surface could be partially oxidized because themetallic Ti is quite active for O2 and H2O as discussed above. It is reported that the
reduction reaction of TiO2 formed on the TiH2 surface during H2 desorption. Wang et al.
suggested the following reaction,2,3
TiO2 + 4H (atom) → Ti + 2H2O (gas).

(4.1.1)

Before desorbing H2, H atoms are recombined on the Ti surface to form H2. Because the
H atoms are unstable, the Gibbs free energy change of above reaction is negative in thetemperature range of H2 desorption, thermodynamically (figure 4.1.6). Thus, Ti with the
fresh surface (TiFresh) was obtained by the dehydrogenation of TiH2, and it was
experimentally clarified that TiFresh can react to H2 even at around room temperature
without any catalysts and surface modification, if the TiO2 layer could be removed fromthe Ti surface.
To understand the difference of surface properties between Ti and Mg, thehydrogenation properties of Mg with fresh surface were also investigated. Although it isthough that the surface of Mg should be clean to absorb hydrogen4, there is no report for
the reaction between Mg with fresh surface and H2 at room temperature. Figure 4.1.7 isthe hydrogen desorption curve of the pristine MgH2. The hydrogen desorption wasobserved between 400 and 450 C. In addition, the phase change from MgH2 to Mg was
confirmed by XRD measurements (figure 4.1.8). The pristine MgH2 was dehydrogenatedusing TG-DTA-MS apparatus, and then the carrier gas was changed from Ar to H2 after73

cooling the TG-DTA-MS apparatus to investigate the hydrogen absorption properties.
However, the hydrogen absorption did not proceed even by heating up to 250 C as shownin figure 4.1.9. This result must be due to the difference in surface properties between Ti
and Mg. Ti has d-electrons and it is active for H2 without catalysts, whereas Mg does nothave d-electrons and then a catalyst for hydrogen dissociation is required for room
temperature hydrogenation.
Thus, the H2 absorption properties of TiFresh could be understood. Two kinds ofstrategies for surface modification of Ti have been considered. One is to add typical solid
catalysts to make an active site for hydrogen absorption. Another is to synthesize a special
surface in order to prevent oxidation and keep the active Ti surface. These methods were
investigated and the results are placed in next section. Considering the results of TiFresh,
the hydrogen absorption properties were evaluated after keeping for the sample more than
1 day in the glovebox to clarify the surface modification effects.

74

DTA curve (arb. unit)

TG curve (wt. %)

0
-1
-2
-3

endothermic
reaction

MS (arb. unit)-4m/z = 2
100

200

300
400
500
Temperature (C)

600

Figure 4.1.1 TG-DTA-MS result of the pristine TiH2 measured under 0.1 MPa Ar flow up
to 650 C at 5 C/min.

75

Intensity (arb. units)

after heat treatment at 580 Cafter TG-DTA-MS measurement

as purchased TiH2

Ti 00-001-1197

TiH2 01-071-4961

20

40

60

80

2 (degree)
Figure 4.1.2 XRD patterns of the pristine TiH2 before and after heating. The TG-DTAMS measurement performed under Ar flow up to 650 C at 5 C/min and top figure is
XRD pattern of the dehydrogenated TiH2 at 580 C under dynamic vacuum for 4 h.

76

Temp.

(a) As pristine Ti

5
4

300

3

200

2
1

100

0
(b) Dehydrogenated TiH2

5

Temp.

4

400

Temperature (C)

TG curves (wt.%)

400

300

3

200

2
immediately
after 1 day
after 7 days

1
0
0

10

20

30 40 50
Time (min)

60

100

70

Figure 4.1.3 TG curves of (a) as pristine Ti and (b) dehydrogenated TiH2 performed under
0.1 MPa H2 flow.

77

(a) As pristine Ti
after TG measurement

Intensity (arb. units)

before TG measurement(b) Dehydrogenated TiH2

TiH2 01-071-4961

Ti 00-001-119720

40

2 (degree)60

80

Figure 4.1.4 XRD patterns of the pristine Ti and the dehydrogenated TiH2 before and after
TG measurement. These TG measurements were carried out under 0.1 MPa H2 flow up
to 400 C.

78

Intensity (arb. units)

as purchased Ti

as purchased TiH2

TiH2_vac 4 h @580 C

466

464

462 460 458 456
Binding Energy (eV)

454

452

Figure 4.1.5 Ti2p XPS results of the pristine Ti, TiH2, and the dehydrogenated TiH2.

79

Figure 4.1.6 Standard Gibbs free energy changes in the reaction for hydrogen reduction
with different Ti oxides presented by Wang et al.380

TG curve (wt.%)-2-4
endothermic
reaction

MS (arb. units)-6

DTA curve (arb. unit)0m/z = 2

100

200
300
400
Temperature (C)

500

Figure 4.1.7 TG-DTA-MS curve of the pristine MgH2 measured under 0.1 MPa Ar flow
up to 500 C at 5 C/min.

81

Intensity (arb. units)

after TG under H2after TG under Ar

as purchased MgH2
Mg 00-004-0770

MgH2 01-076-2727

20

40

60

80

2 (degree)
Figure 4.1.8 XRD patterns of the pristine MgH2 before and after the TG-DTA-MS
measurement, performed under Ar flow up to 650 C at 5 C/min, and after hydrogen
absorption measurement.

82

dehydrogenated MgH2

5

Temp.

250
200

3
150
2
100

1

Temperature (C)

TG curve (wt.%)

4

50

0
0

10

20

30 40 50
Time (min)60

70

Figure 4.1.9 TG curve of the dehydrogenated MgH2 performed under 0.1 MPa H2 up to250 C. This measurement was carried out just after dehydrogenation.

834.2 Surface modification by transition metals and oxides to make active sites
In order to make an active site for hydrogen absorption of Ti, the additives weredispersed on the TiH2 surface by ball-milling. As solid additives, Ni, Pd, Nb2O5, and V2O5
were chosen, which are well known as catalysts for hydrogenation of Mg.5,6 To determine
the dehydrogenation conditions for preparing the Ti samples from TiH2 with each solid
additive, H2 desorption profiles of the prepared TiH2 were obtained using TG-DTA-MS
apparatus, and the results are shown in figure 4.2.1. The MS intensity of H2 was found to
be increased continuously from 100 C for all the ball-milled samples. The results indicatethat the ball-milled samples desorbed H2 with increasing temperature from low
temperature, whereas the H2 desorption of the pristine TiH2 could be started from 400 C.
These results suggested that the ball-milling affected the H2 desorption properties of TiH2
significantly. It is known that the crystallite size and defects of material are importantfactors for discussion of the hydrogen desorption kinetics. Crystallite size can be
estimated by the full width at half maximum (FWHM) of the XRD peaks. The XRD
patterns of the ball-milled samples are shown in figure 4.2.2. The peaks were broadened
after ball-milling compared with the peaks of the pristine TiH2, suggesting that the
crystallite size was decreased by the ball-milling. In addition to the effects of thecrystallite size, Suzuki et al. reported that stress and deformation also affect thedestabilization of H atoms in TiH2. It leads to low temperature hydrogen desorption.7 No
significant effects of additives were observed on the H2 desorption of Ti. The results
indicated that the activation energy for hydrogen desorption does not change by theadditives. Basically, typical additives provide active sites for the surface reaction such as
the recombination of the H atoms.8 These results indicated that the rate-controlling stepof the hydrogen desorption of TiH2 was not surface reaction. It is considered that the84

activation energy originated in the large enthalpy change for dehydrogenation of TiH2
because TiH2 is quite stable. After the heating, Ti was observed by XRD measurement(figure 4.2.2). According to the H2 desorption profiles, the dehydrogenating treatment
was conducted at 550 C (corresponding to the peak temperature of hydrogen desorption)
under dynamic vacuum. The XRD results of the ball-milled TiH2 samples after thedehydrogenation treatment are shown in figure 4.2.3. The diffraction pattern of Ti phase
was mainly observed in all the heat-treated samples. By using the dehydrogenatedsamples, hydrogen absorption properties were investigated to evaluate the effects of solid
additives for hydrogenation of Ti. Figure 4.2.4 shows the TG curves of the preparedsamples performed under 0.1 MPa H2 flow. All the Ti samples showed weight gain due
to hydrogenation at more than 200 C, which was higher than that of the Ti kept in the
glove box for 1 day (figure 4.1.3b). Since these were prepared by the dehydrogenation ofTiH2 samples, these samples should have an active surface for H2, which is similar to thedehydrogenated pristine TiH2. However, the higher temperatures required forhydrogenation, indicated that inactivation proceeded more easily than that of thedehydrogenated pristine TiH2 due to surface oxidation. The enhanced oxidation might bedue to increase of the surface area during ball-milling. Since remarkable additive effects
could not be achieved, it was expected that the influence of surface oxidation was muchlarger than the effects of those additives. Light element based hydrogen storage materials
such as Mg require the effective catalysts to improve the hydrogenation kinetics becauseof poor H2 dissociation ability of the metallic Mg surface. On the other hand, it wasclarified that Ti essentially possesses the active surface for dissociation of H2 molecules,considering the results shown in figure 4.1.3b. When the oxide layer was formed on theTi surface, hydrogenation is inhibited because hydrogen cannot penetrate into the oxide85layer at room-temperatures. In fact, oxidation of the Ti surface might be accelerated by
the additives and the increased surface area. In order to control the kinetics of Ti, surface
modification to prevent oxidation is important rather than activation for the reaction with
H2. Therefore, the characteristic Ti surface with selective reactivity of H2 and protection
against O2 and H2O were required.

86

As purchased TiH2
m\z = 2

Intensity (arb. units)

Ball milled for 20 h

+ 1 mol% Ni+ 1 mol% Pd+ 1 mol% V2O5

+ 1 mol% Nb2O5

100

200

300
400
Temperature (C)

500

600

Figure 4.2.1 MS curves of H2 for the TiH2 samples, measured under 0.1 MPa Ar flow up
to 650 C at 5 C/min.

87

after the ball-millingball-milled+ 1 mol% Ni+ 1 mol% Pd+ 1 mol% V2O5

Intensity (arb. units)+ 1 mol% Nb2O5

as purchased TiH2

after the measurementball-milled+ 1 mol% Ni+ 1 mol% Pd+ 1 mol% V2O5

+ 1 mol% Nb2O5

as purchased TiH2
TiH2 01-071-4961
Ti 00-001-119720

40

2 (degree)60

80

Figure 4.2.2 XRD patterns of the TiH2 samples. These measurements were performed
before and after the heating under 0.1 MPa Ar flow up to 650 C at 5 C/min.

88

after the dehyrogenationball-milled+ 1 mol% Ni+ 1 mol% Pd+ 1 mol% V2O5

Intensity (arb. units)+ 1 mol% Nb2O5
as purchased TiH2

after the hydrogenationball-milled+ 1 mol% Ni+ 1 mol% Pd+ 1 mol% V2O5
+ 1 mol% Nb2O5

as purchased TiH2
TiH2 01-071-4961
Ti 00-001-119720

40

2 (degree)60

80

Figure 4.2.3 XRD patterns of the TiH2 samples. These measurements were carried out
after heat treatment under dynamic vacuum up to 550 C for 4 h, and after TG
measurements under 0.1 MPa H2 flow up to 400 C.

89

no additives
1 mol% Ni
1 mol% Pd
1 mol% V2O5
1 mol% Nb2O5

5

TG (wt.%)

400

300

3
200

2
1

Temperature (C)

4

Temp.

100

0
0

10

20

30 40 50
Time (min)60

70

Figure 4.2.4 TG curves of TiH2 ball-milled without additive and with each additive
measured under 0.1 MPa H2 flow.

904.3 Surface modification by organic solvents and graphite to prevent the surface
oxidation
Organic solvents
As another surface modification technique, Ti was ball-milled with various kinds oforganic solvents. It was expected that solvents might react with Ti during ball-milling,
and the characteristic surface might be generated on the Ti surface. The XRD analyses
shown in figure 4.3.1 suggested that the phase transformation from Ti to TiH2 occurred
during TG measurement, performed under 0.1 MPa H2 up to 400 C. Figure 4.3.2 shows
the TG curves of ball-milled Ti with each organic solvent. TiXylene absorbed H2 evenaround room temperature, suggesting that the effective surface for hydrogenation of Ti
could be formed by the ball-milling with xylene. Although the significant effects oforganic solvent for TiFe alloy have been reported, it was speculated that the solvents
including oxygen atoms in the molecule accelerate oxidation and inactivation of Ti due
to its highly active nature towards O2. However, in this work, TiAcetone and TiTHF could
absorb H2 at lower temperature compared with that of TiDry, TiCyclohexane, and TiHexane. It isnoteworthy here that the hydrogen was absorbed at around room temperature by TiAcetone.
Thus, the Ti surface could be modified by the reaction with acetone during the ballmilling process. As mentioned earlier, all the hydrogen absorption properties of thesamples were evaluated after keeping the samples in the glovebox for more than 1 day.
Therefore, the influence of O2 and H2O was suppressed by acetone as well as xylene
compared to the other samples. In other words, O2 and H2O molecules could not passthrough the modified Ti surface, but H2 selectively penetrated into the surface and reactedwith Ti. This phenomenon is quite similar to membrane separation of H2.

91

after the ball-milling
TiDry
TiAcetone

TiCyclohexane
TiHexane

Intensity (arb. units)

TiTHF
TiXyleneafter the hydrogenation
TiDry
TiAcetone

TiCyclohexane
TiHexane
TiTHF
TiXyleneTi 00-001-1197
TiH2 01-071-4961

2040

2 (degree)60

80

Figure 4.3.1 XRD profile of ball-milled Ti with and without additives after ball-milling
and after TG measurement under 0.1 MPa H2 flow.

92

TiDryTiAcetone
TiCyclohexane

5

TiTHF
TiHexane
TiXylene

Temp.

TG (wt.%)

300

3
200

2
1

Temperature (C)

4

400

100

0
0

10

20

30 40 50
Time (min)60

70

Figure 4.3.2 TG curves of ball-milled TiH2 with and without solvents measured under 0.1
MPa H2 flow.
Xylene and acetone
In order to understand the details of the effective factors for surface modification, 2methyl-4-pentanone and cis-1,2-dimethylcyclohexane, whose structure are similar toacetone and xylene, respectively, were investigated in addition to TiAcetone and TiXylene.
Figure 4.3.3 shows TG curves of the prepared samples, performed under 0.1 MPa H2 flowat around 40 C. These measurements were carried out every 24 h after the sample
preparation. A weight gain of approximately 3 wt.% was observed in all the samples evenaround 40 C during TG measurement, performed just after the sample preparation. The
93

results indicated that all the as-prepared samples were active in the hydrogenation
reaction. However, Ticis-1,2-Dimethylcyclohexane and Ti2-Methyl-4-pentanone could not absorb H2 after
keeping these in glove box for few days, indicating that cis-1,2-dimethylcyclohexane and2-methyl-4-pentanone did not show significant effects of surface modification. On theother hand, TiAcetone and TiXylene could absorb H2 even after several days, indicating theirpositive effect as additives to modify the Ti surface. The degradation of H2 absorption
properties of TiAcetone and TiXylene with time elapsed under glove box was different. The
amount of absorbed H2 by TiAcetone was decreased gradually, while it was decreased
suddenly after 3 days for TiXylene. These results indicated that the inactivation mechanism
is different in both the samples. Several kinds of mechanism for reaction selectivity havebeen considered. One possibility was that the modified surface possess the similar
properties as of metal membrane. As an example, palladium (Pd) thin film dissociates H2molecules on the surface, and then the H atoms diffuse in the Pd film. 9 These metallic
membrane can separate H2 with high selectivity from mixed gas via solution-diffusion
mechanism. If the functional surface having metal membrane properties could be formedon the Ti sample, the reactivity of Ti can be kept without inactivation because oxygen andwater cannot reach to Ti. However, the Ti samples were turned to inactivated state evenin the case of TiAcetone and TiXylene. As a second possibility, the modified surface has similarfunction as that of porous inorganic membrane such as silica10,11, through which the gases
can permeate in the surface with different permeation rates. In this case, oxygen can also
permeate into Ti bulk, indicating Ti would be gradually oxidized. The inactivation
behavior of TiAcetone is similar to this phenomenon. Third possibility was that the modified
surface has a function of adsorbent for oxygen and water similar to activated carbon.12
When the adsorption sites are fully occupied, the gases reach to the Ti surface, leading to94

an inactivation at once. The inactivation tendency of TiXylene is similar to the adsorbent
model.
To estimate the amount of carbon in modified surface indirectly, TG-DTA measurement
was carried out up to 650 C under 0.1 MPa of Ar flow. After this measurement, TiC wasobserved by XRD measurement in all the samples (figure 4.3.4). The TiC/Ti ratio wasestimated using (101) plane of Ti, and (200) plane of TiC obtained by XRD measurement
shown in figure 4.3.4 without considering the mass absorption coefficient. The calculated
TiC/Ti ratio was listed in table 4.3.1. If it is assumed that carbon in modified surface waschanged to TiC by heating, the amount of carbon in the Ticis-1,2-Dimethylcyclohexane surface
should be quite small compared to that of TiAcetone and TiXylene. Therefore, it wasconsidered that TiC precursor should affect H2 absorption properties. However, the H2absorption properties of TiAcetone, TiXylene, and Ti4-Methyl-2-pentanone were clearly different
even though the TiC/Ti ratio of each sample is close. Therefore, it was concluded that theamount of TiC precursor had no direct influence on H2 absorption properties.
To characterize the chemical state of TiC precursor, Raman scattering spectrometry
was carried out just after the sample preparation and after the several days elapsed in
special vessel to investigate the degradation with time (figure 4.3.5). Here, noise peaks
were observed from the apparatus at 1300 and 1550 cm1. Two characteristic peaks wereclearly observed in 1300 to 1400 cm1 and 1500 to 1675 cm1 for the spectrum of TiAcetone.
These peak were assigned to D band and G band, respectively.13,14 The peaks
corresponding to sp3-carbon and defects of hexatomic rings would appear in the D bandarea, while those in the G band correspond to local sp2 -bonded carbon. However, it was
surprising that D band and G band appeared in the spectrum of Ti Acetone even thoughacetone does not have hexatomic rings. From this result, it was considered that acetone95was reformed by reaction with Ti, suggesting that carbon was dissolved in the Ti surface.
Namely, TiCx was generated on the TiAcetone surface as a TiC precursor. The results of
TEM observation for TiAcetone are shown in figure 4.3.6. From the FFT patterns, themetallic Ti was observed with high intensity. On the other hand, other spots were detected,
which could not be assigned to metallic Ti, and these yellow spots were assigned to
TiCxHy such as TiC0.5H0.21 and TiC0.47H0.18. The IFFT image obtained from the spots ofmetallic Ti and TiCxHy, respectively. In this technique, the existence areas originated the
spots of compounds become lightened. These images revealed that TiCxHy (originatedyellow spots) exists in the outside of the particle. Thus, it could be considered that TiCxHy
could be considered as one of the possible precursors. The peak intensity in the Raman
spectra for TiAcetone was found to be decreased day-by-day. This result indicated that TiCx
was metastable state, and the surface was degraded with time. The degradation properties
were consistent to the H2 absorption properties. On the other hand, Ticis-1,2-Dimethylcyclohexane
and Ti4-Methyl-2-pentanone, which did not improve the hydrogenation properties of Ti, did notshow clear peaks. Thus, it was clarified that TiCx (or TiCxHy) formed on the TiAcetone
affected the hydrogenation properties of Ti. In other words, the chemical state of TiC
precursor was important for the reaction selectivity. Although the peak intensity of TiXylene
was lower than TiAcetone, the Raman spectrum was similar to one of TiAcetone. However, thepeak intensity did not decrease with time even after loss of the reactivity with H2. This
result suggested that the state of carbon observed in TiXylene surface was different from
TiCx observed in TiAcetone. The stable G band peak was originated in the molecular
structure of xylene with hexatomic ring. Considering the H2 absorption properties, thesurface of TiXylene had the effects of adsorbent. If xylene-based product with high surface
area like graphite is formed, the adsorbent effects can be understood. The spectrum of96

TiXylene was similar to amorphous carbons with a mixture of sp2 and sp3 hybrid orbitals.13
Therefore, it is possible that the amorphous carbons formed on the TiXylene surface played
the role of adsorbent for water and oxygen. Thus, it was clarified that the mechanism ofreaction selectivity is different between TiAcetone and TiXylene. However, the details of TiC
precursor such as composition and structure were not identified. Thus, further
experiments are necessary to understand the details of TiC precursor as future works.
Here, D band and G band in the Raman spectrum were often discussed for graphite basedmaterials. In other words, graphite has large amount of carbon originated in D band andG band structure. Therefore, it was considered that the functional surface like TiAcetone and
TiXylene can be obtained by ball-milling with graphite. Thus, the effects of graphite wasinvestigated and the results are summarized in next section for further improvement of
the hydrogen absorption properties of Ti.

97

immediately
1 day
2 daysTiAcetone

3 days4 days43210TiXylene43
20

Ti4-Methyl-2-pentanone

432

TG curves (wt.%)1

1
0
Ticis-1,2-Dimethylcyclohexane

4
3
2
1
0

0

2

4
6
Time (min)810Figure 4.3.3 TG curves of TiAcetone, TiXylene, Ti2-Methyl-4-pentanone, and Ticis-1,2-Dimethylcyclohexane,
measured under 0.1 MPa H2 flow around 40 C. These measurement were carried out
every 24 h from sample preparation.
98

TiAcetone
after TG under Ar up to 650 Cafter ball-milling

Intensity (arb. units)

TiXyleneTi4-Methyl-2-pentanone
after TG under H2 up to 400 C

Ticis-1,2-Dimethylcyclohexane

TiH2 01-071-4961
TiC 00-001-1222
Ti 00-001-1197

20

30

40
2 (degree)

50

60

Figure 4.3.4 XRD patterns of TiAcetone and TiXylene before and after TG measurement. TG
measurement was performed under Ar atmosphere up to 650 C.

99Table 4.3.1 TiC/Ti ratio of each sample heated up to 650 C under Ar flow. Ti : C (carbon)
was the initial molar ratio of ball-milling. The mole of C was estimated by 20 wt.%
solvents. The peak areas of Ti and TiC were calculated using (101) plane for Ti, and (200)plane for TiC without considering the mass absorption coefficient, respectively.
Sample nameTi : CArea ratio of TiC/Ti

TiAcetone

1 : 0.5

0.06

TiXylene

1 : 0.7

0.07

Ticis-1,2-Dimethylcyclohexane

1 : 0.6

0.03

Ti4-Methyl-2-pentanoen

1 : 0.7

0.08

100

TiAcetone

immediately

after 1 day

Intensity (arb. units)

after 2 days
after 3 days

TiXylene

immediately

after 3 days

Ticis-1,2-Dimethylcyclohexane
immediately

Ti4-Methyl-2-pentanone

1200

1300

immediately

1400

1500

1600

1700

1

Raman shift (cm )Figure 4.3.5 Raman spectra of the prepared samples. The measurement was performed
just after sample preparation and after several days.

101Figure 4.3.6 TEM observation results of TiAcetone. (a) TEM bright-field and (b) highresolution TEM (HRTEM) images. The Fast Fourier Transform (FFT) patterns wereacquired from (c) area 1 and (d) area 2. Inverse FFT images from (e) blue spots and (f)yellow spots in area 2.

Graphite
From the discussion for the Ti ball-milled with organic solvents, it was concluded thatgraphite structure could have effects of surface modification. Therefore, Ti ball-milledwith graphite (TiGraphite) was synthesized and investigated. Figure 4.3.7 shows XRD
results of TiGraphite with various treatment. After the ball-milling, only Ti phase wasobserved without graphite peaks. This result suggested that graphite structure was broken
and it was dispersed on the Ti surface. The TG curves performed under 0.1 MPa H2 flowat around 40 C are shown in figure 4.3.8. A weight gain of approximately 3.4 wt.% was102

observed during the TG measurements, and the phase change from Ti to TiH2 was clearly
confirmed by XRD measurement. Therefore, hydrogenation occurred during the TG
measurement around 40 C. While the reactivity of TiAcetone and TiXylene with H2 was lost
after keeping these in glove box for several days, TiGraphite could absorb H2 around 40 Ceven after 7 days. This result indicated that graphite has superior effects for selective H2absorption of Ti. Furthermore, even though the H2 absorption temperature was increased
after exposing TiGraphite to the air for 1 day, it showed lower reaction temperature comparedto Ti without any surface modification and high stability without burning in air (figure
4.3.9). The amount of adsorbed H2 (3.4 wt.%) was close to the value estimated from the
mixture of Ti and graphite without any reaction between both materials (3.7 wt.%),indicating excess graphite remained in the sample. Here, Wei Ye et al. reported H2absorption properties of Ti/graphene composite with Kubas-type interaction between 100
and 300 C.15 Although the theoretical H2 capacity of pure Ti is 4.2 wt%, the Ti/graphene
composite (20 wt% graphene) absorbed 4.3 wt% H2, which was believed due to Kubastype interaction at 300 °C. On the other hand, H2 capacity of TiGraphite was close to thevalue of the mixture as mentioned above, indicating TiGraphite did not show the Kubas-type
interaction. The TiC/Ti ratio was 0.82, which was estimated by XRD patterns with heating
up to 650 C. This value was much higher than that of the Ti ball-milled with organic
solvents. Therefore, it was suggested that carbon was effectively doped on the Ti surface
by graphite.
In order to characterize the carbon on the TiGraphite surface, Raman spectrometry was
carried out. Figure 4.3.10 shows the Raman spectra of the pristine graphite and Ti Graphite.
The measurements of TiGraphite were performed just after sample preparation and after 3
days. Here, it should be note that the spectrum of Ti Graphite includes residual graphite as103

mentioned in the discussion for TG results. D band (1300 ~ 1400 cm1) and G band (1500~ 1675 cm1) were detected in the pristine graphite, then the D/G ratio was increased afterball-milling. In addition, the spectrum of TiGraphite was similar to turbostratic carbon.16
These results suggested that graphite structure was partially broken by the ball-milling.
The results were consistent to the XRD results. In addition, the peak intensities of D band
and G band in Raman spectra were not changed after 3 days. This result indicated that the
carbon existed on the TiGraphtie was stable. For the H2 separation using turbostratic carbon,
the gaseous molecules permeate into turbostratic carbon through its pore or cracks. The
permeability of molecules into carbon was suggested to be dependent on the kinetic
diameter. Therefore, the permeation of oxygen was inhibited due to larger size than H2.
In addition, Wollbrink et al. reported that the permeation of water could be prevented due
to hydrophobic characteristics of carbon.16 Therefore, it was indicated that turbostratic
carbon can prevent the permeation of water as well as oxygen. Although it was difficult
to know the chemical state of Ti species on the TiGraphite surface by Raman spectroscopy,
turbostratic graphite existed in the Tigraphite.
XPS measurements were also conducted to understand the chemical state of the
TiGraphite surface, and the results are shown in figure 4.3.11. The measurements were
performed by using the apparatus of JPS-9200. As discussed in the TiFresh part, the pristine
Ti was covered by the TiO2 layer (Ti4+). In the case of the pristine TiC, the low-valence
Ti was detected at 454.8 eV (and 460.8 eV) with higher intensity, which was consistent
with typical Ti2p3/2 (and Ti2p1/2) peak of TiC.17 TiO2 peak was also observed in thepristine TiC with lower intensity. The spectrum of TiC indicated that the oxide layer on
the TiC particle was thinner than one of the pristine Ti. The spectrum of Ti Acetone wassimilar to the pristine TiC, indicating TiCx existed on the TiAcetone. This result was104consistent with the results of the Raman spectrum and TEM observation for TiAcetone.
TiGraphite also showed a peak at 454.8 eV in the Ti 2p XPS spectrum, indicating TiCx wasformed by the ball-milling of Ti and graphite. It is noteworthy that the intensity was quite
low compared with one of TiAcetone, indicating carbon with a thickness of several nm
covered Ti particle. Considering the results of the Raman spectrum and XPS, it was
concluded that turbostratic graphite covered the Ti surface.
For the further characterization of TiGraphite, TEM observation was carried out and theresults are shown in figure 4.3.12. Figure 4.3.12a and 4.3.12b are high-angle annular darkfield scanning TEM (HAADF-STEM) image of TiGraphite and the energy-dispersive X- rayspectrometry (EDS) mapping of Ti, C, and O elements. For the HAADF-STEM image,electron-rich areas were observed as the bright part. Because Ti element was detected by
EDS mapping, the bright part of HAADF-STEM image was mainly corresponding to Ti.
EDS mapping of C showed that carbon surrounded the Ti particle. It was also confirmed
by HRTEM observation that turbostratic graphite was formed on the Ti surface, which isshown in the inset of figure 4.3.12c. These results were well-consistent with the resultsof Raman spectra and XPS. Line analyses of energy-loss spectroscopy (EELS) were
performed across the boundary between Ti and C. The measurement areas are expressed
as a square in figure 4.3.12d. The peaks were observed at 285 eV and in 287 to 310 eV(figure 4.3.12e), which were assigned to  and  bonding of carbon, respectively.18 These
peak positions and shapes were similar to one of amorphous carbon. Therefore, the mixed
carbon phase of amorphous and turbostratic carbons existed on the TiGraphite surface. This
result indicated that TiGraphite should have the effects of the adsorbent functions for oxygen
like TiXylene as well as hydrophobic effects for water like turbostratic carbon. For the
spectra of Ti L2,3-edge in the Ti bulk (areas 1-4), two peaks were observed at 457 and 463
105

eV. These peaks were corresponding to metallic Ti.19 The peaks of Ti L2,3-edge obtainedat the interface between Ti and graphite were shifted to ~1 eV higher energy than metallic
Ti (Fig. 4.3.12f). This spectrum was similar to TiCx reported before.20 It has been reported
that the peaks were gradually shifted to 1.4 eV higher energy with increasing N atom in
TiNx (x = 0 to 1).19 The presence of TiCx was already confirmed by XPS measurements(figure 4.3.11). These results indicated that TiCx was formed in the interface between Ti
and graphite. Therefore, it was considered that Ti Graphite should have inorganic porous
membrane effects by the TiCx layer, which had a similar function to that of TiAcetone. Thus,
the inactivation of TiGraphite was prevented by the mixture of turbostratic and amorphous
carbons with “hydrophobic adsorbent effects” and the TiCx layer with inorganic porous
membrane effects.

106

TiGraphite

Intensity (arb. units)

after TG under 0.1 MPa H2 up to 400 Cafter TG under 0.1 MPa Ar up to 650 Cafter ball-milling

TiH2 01-071-4961
TiC 00-001-1222

Ti 00-001-119720

40

2 (degree)60

80

Figure 4.3.7 XRD patterns of TiGraphite before and after TG measurements. The TG
measurements were performed under H2 or Ar flow up to 400 or 650 C at 5 C/mink,
respectively.

107

4
3
2
immediately
1 day
2 days3 days1

4 days
5 days6 days7 days

0
0

2

6

4

8

10

Figure 4.3.8 TG curves of TiGraphite measured under 0.1 MPa H2 flow around 40 C. The
measurements were performed every 24 h from sample preparation.

108

Figure 4.3.9 (a) TG curves of TiFresh and TiGraphite before and after exposing in air for 1day, performed under 0.1 MPa H2 flow. (b) XRD patterns of both samples.

109

TiGraphite

Intensity (arb. units)

immediately

after 3 days

Graphite

1300

1400

1500

1600

1700

1

Raman shift (cm )Figure 4.3.10 Raman spectra of the pristine graphite and TiGraphite. The measurements of
TiGraphite were carried out just after sample preparation and after 3 days.

110Intensity (arb. units)

Ti 2pgraphite Ti
acetone Ti
as pristine TiC
as pristine Ti
465

460
455
Binding Energy (eV)Figure 4.3.11 Ti2p XPS spectra of TiGraphite, TiAcetone, as purchased TiC, and Ti.

111

450

Figure 4.3.12 TEM observation results of TiGraphite. (a) HAADF-STEM image and (b)EDS mapping of Ti, C, O elements. (c) HRTEM image in the interface between Ti andgraphite. Inserted figure is high magnification image of the graphite area. (d) STEMEELS mapping of Ti, and spectra of (e) C K-edge and (f) TiL2,3-edge at each area.
112

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Milošev, I., Kosec, T. & Strehblow, H. H. XPS and EIS study of the passive film
formed on orthopaedic Ti-6Al-7Nb alloy in Hank’s physiological solution.
Electrochim. Acta 53, 3547–3558 (2008).

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Wang, C., Pan, L., Zhang, Y., Xiao, S. & Chen, Y. Deoxidization mechanism ofhydrogen in TiH2 dehydrogenation process. Int. J. Hydrogen Energy 41, 14836–
14841 (2016).

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Wang, C. et al. XPS study of the deoxidization behavior of hydrogen in TiH2
powders. Powder Technol. 302, 423–425 (2016).

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Shang, C. X. & Guo, Z. X. Effect of carbon on hydrogen desorption and absorptionof mechanically milled MgH2. in Journal of Power Sources vol. 129 73–80(Elsevier, 2004).

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Barkhordarian, G., Klassen, T. & Bormann, R. Fast hydrogen sorption kinetics ofnanocrystalline Mg using Nb2O5 as catalyst. Scr. Mater. 49, 213–217 (2003).

6.

Denis, A., Sellier, E., Aymonier, C. & Bobet, J. L. Hydrogen sorption propertiesof magnesium particles decorated with metallic nanoparticles as catalyst. J. Alloys
Compd. 476, 152–159 (2009).

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Suzuki, H., Taniguchi, H., Hanada, N., Takai, K. & Hagihara, Y. Effects ofdeformation on hydrogen absorption and desorption properties of titanium. in
Journal of Alloys and Compounds vol. 509 S759–S762 (Elsevier, 2011).

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Hu, M., Xie, X., Chen, M., Zhu, C. & Liu, T. TiCX-decorated Mg nanoparticles
confined in carbon shell: Preparation and catalytic mechanism for hydrogenstorage. J. Alloys Compd. 817, 152813 (2020).

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Uemiya, S. et al. Separation of hydrogen through palladium thin film supported ona porous glass tube. J. Memb. Sci. 56, 303–313 (1991).

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de Vos, R. M., Maier, W. F. & Verweij, H. Hydrophobic silica membranes for gas
separation. J. Memb. Sci. 158, 277–288 (1999).

11.

Lee, H. R., Kanezashi, M., Shimomura, Y., Yoshioka, T. & Tsuru, T. Evaluationand fabrication of pore-size-tuned silica membranes with tetraethoxydimethyl
disiloxane for gas separation. AIChE J. 57, 2755–2765 (2011).

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Hao, P., Shi, Y., Li, S. & Cai, N. Hydrophobic activated carbon for elevated113

temperature pressure swing adsorption. Adsorption (2020) doi:10.1007/s10450020-00223-6.
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Zhang, L. & Koka, R. V. A study on the oxidation and carbon diffusion of TiC in
alumina–titanium carbide ceramics using XPS and Raman spectroscopy. Mater.
Chem. Phys. 57, 23–32 (1998).

14.

Mermoux, M., Chang, S., Girard, H. A. & Arnault, J.-C. Raman spectroscopy studyof detonation nanodiamond. Diam. Relat. Mater. 87, 248–260 (2018).

15.

Ye, W., Chi, Q., Zhou, H. & Gao, P. Ball-milling preparation of titanium/graphene
composites and its enhanced hydrogen storage ability. Int. J. Hydrogen Energy 43,
19164–19173 (2018).

16.

Wollbrink, A. et al. Amorphous, turbostratic and crystalline carbon membranes
with hydrogen selectivity. Carbon N. Y. 106, 93–105 (2016).

17.

El Mel, A. A., Angleraud, B., Gautron, E., Granier, A. & Tessier, P. Y. XPS studyof the surface composition modification of nc-TiC/C nanocomposite films underin situ argon ion bombardment. Thin Solid Films 519, 3982–3985 (2011).

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Jin, T. et al. Mechanical polishing of ultrahard nanotwinned diamond via transition
into hard sp2-sp3 amorphous carbon. Carbon N. Y. 161, 1–6 (2020).

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Abdallah, I., Dupressoire, C., Laffont, L., Monceau, D. & Vande Put, A. STEMEELS identification of TiOXNY, TiN, Ti2N and O, N dissolution in the Ti2642S
alloy oxidized in synthetic air at 650°C. Corros. Sci. 153, 191–199 (2019).

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Woltersdorf, J., Feldhoff, A. & Lichtenberger, O. The Complex Bonding ofTitanium Nitride Layers in C/Mg Composites Revealed by ELNES Features. Cryst.
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114

5 Conclusion
In this study, the essential H2 absorption properties of Ti with fresh surface, and thesurface modification techniques to improve the H2 absorption kinetics were investigated.
(1) The reaction between Ti and H2
The Ti with the fresh surface prepared from TiH2 by dehydrogenation. This Ti absorbed
H2 even at around 40 °C without any catalysts and heat-activation. On the other hand, Mg
with fresh surface did not react with H2 even heating up to 250 C. These results indicatedthe difference of the surface properties between Ti and Mg. This difference is consistent
with the general understanding, where d-electron is active for H2 dissociation. However,
the Ti surface was easily inactivated even in the glove box after 1 day because of its highreactivity. As a result, the hydrogen absorption was inhibited by the oxide layer, and heatactivation was required to realize the reaction after formation of the oxide layer. Thus, toimprove the hydrogen absorption properties of Ti, the suitable surface modification isrequired to realize the selective reaction for H2 without the degradation of surface activity
due to oxygen and water.
(2) Surface modification by transition metals and oxides to make active sites
Various kinds of solid additives, which were typical metal and oxide catalysts forhydrogen dissociation, were dispersed on the Ti surface to make an active site for thehydrogenation, and the hydrogenation properties were evaluated. However, these samples
absorbed H2 at more than 200 °C. Thus, it was found that the influence of surface
oxidation on H2 absorption by Ti was much larger than effects of the additives. The above
results also suggested that the special surface modification to prevent oxidation was
required for selective reaction of Ti with H2.

115

(3) Surface modification by organic solvents and graphite to prevent the surface oxidation
Ball-milling with organic solvents was performed as another method for modifying theTi surface. Hydrogen could be absorbed by TiAcetone and TiXylene at around room
temperature even after keeping those samples for 3-4 days in the glove box. These results
indicated that H2 could selectively penetrate into the surface and react with Ti. The
degradation tendencies of hydrogenation reactivity of TiAcetone and TiXylene were different.
The reactivity of TiAcetone with H2 was degraded gradually, and this tendency could be
explained by the porous inorganic membrane model. And, TiXylene degraded suddenly
after 3 days, which could be understood by the adsorbent model. Raman spectroscopy
revealed that the chemical state of carbon formed on the Ti surface was important for theselective reaction. In addition, the TiCx layer was observed in TiAcetone by Raman
spectroscopy and TEM observation. On the other hand, the Raman spectrum of TiXylene
indicated the formation of amorphous carbon on the TiXylene surface. Furthermore, thehydrogenation properties of Ti could be more improved by graphite. The reactivity of
TiGraphite was kept at least 7 days, indicating that graphite showed the superior surface
modification effects for Ti. To understand the effective factors of surface modification forimprovement of the hydrogenation properties of Ti, the characterization of the TiGraphite
surface was carried out by Raman spectrometry, XPS measurement, and TEM observation
as well as TG and XRD measurements. The TG and XRD measurements suggested thatthe amount of TiC formed on the Ti surface by heating was much larger than that of Ti
modified by organic solvents, namely carbon was effectively doped on the Ti surface.
Raman spectroscopy showed the transformation of crystallite graphite into turbostratic
carbon by ball-milling with Ti. Further, the XPS results indicated that TiCx was formed
and the turbostratic carbon covered on the TiGraphite surface. These facts were also116

confirmed by TEM observation. TEM results indicated the existence of amorphous
carbon on the TiGraphite surface, and the existence of TiCx layer at the interface between
the carbons and Ti bulk. As a summary, it was clarified that the selective reaction with H2
by TiGraphite was achieved by the mixed carbon phase (turbostratic carbon and amorphous)with the hydrophobic adsorbent effects for water and oxygen and the TiCx layer with themolecular sieving effects.
From the above study, the fundamental reaction between Ti and H2 could be understood.
It can react with H2 even without any catalyst and heat-activation. The influence of thesurface oxidation on the hydrogenation of Ti was larger than the effects of solid additives.
The functional surface could be synthesized by ball-milling with organic solvents. It wasthought that the mixed turbostratic/amorphous carbon phase and the TiCx layer have H2
separation effects. These carbons are effectively generated from graphite, and the superiorhydrogen absorption properties of Ti was achieved by ball-milling with graphite. By
understanding the essential hydrogenation ability of Ti, it was proposed that the
suppression of the surface oxidation to keep the active surface and simultaneously
selective reaction with H2 were important for Ti.

117

Acknowledgements
I would like to extend my sincere thanks to my supervisor, Professor Dr. Takayuki
Ichikawa for his helpful guidance, suggestions. Thanks to him, I had lot of great
experience in my doctoral course. I would like to special thanks to Professor Dr.
Yoshitsugu Kojima for a lot of valuable discussion, comments, and suggestions. I learned
many things about research from him. I would like to express my gratitude to Associate
Professor Dr. Hiroki Miyaoka for his helpful guidance, suggestions. I always discussedwith him when I was in trouble with my research, writing paper, presentation. I am special
thanks to co-authors of my research paper, Assistant professor Dr. Yuki Nakagawa in
Hokkaido University, Professor Dr. Norio Ogita in Hiroshima University, Mr. Yuya
Hayashi in Hokkaido University, Associate Professor Dr. Shigehito Isobe in Hokkaido
University, and Professor Dr. Tamaki Shibayama in Hokkaido University for theircooperation. Finally, I would like to grate thanks to my colleagues of laboratory, Mr.
Tomoyuki Ichikawa, Ms. Misao Mukoda, Ms. Saori Inagaki, Mr. Hiroyuki Kurihama,
Associate Professor Dr. Ankur Jain, Assistant Professor Dr. Machi Kanna, AssistantProfessor Dr. Rini Singh, Dr. Rajesh Kumar, Dr. Shotaro Yamaguchi, Dr. Keita Nakajima,Dr. Hiroki Uesato, Dr. Yasuhiro Matsumura, Mr. So Hamamoto, Ms. Pratibha Pal, Ms.
Fangqin Guo, Mr. Pankaj Kumar Singh, Mr. Fernando Cano-Banda, Mr. Masakuni
Yamaguchi, Mr. Hiroyuki Gi for their kind help in my doctoral course.

118

Published Papers[1] Room-Temperature Hydrogen Absorption of Titanium with Surface Modification by
Organic Solvents
K. Shinzato, S. Hamamoto, H. Miyaoka, T. Ichikawa
The Journal of Physical Chemistry C, 123 (32), pp 19269-19274 (2019.7.10).

[2] Metal Hydrides and Related Materials. Energy Carriers for Novel Hydrogen andElectrochemical Storage
A. El Kharbachi, E.M. Dematteis, K. Shinzato, S.C. Stevenson, L.J. Bannenberg, M.
Heere, C. Zlotea, P.Á. Szilágyi, J.-P. Bonnet, W. Grochala, D.H. Gregory, T. Ichikawa,
M. Baricco, B.C. Hauback
The Journal of Physical Chemistry C, 124 (14), pp 7599-7607 (2020.4.9).

[3] Surface modification effects of graphite for selective hydrogen absorption by titanium
at room temperature
K. Shinzato, Y. Nakagawa, S. Hamamoto, Y. Hayashi, H. Miyaoka, S. Isobe, T.
Shibayama, N. Ogita, T. Ichikawa
Chemical Communications, 56 (53), pp 7237-7240 (2020.5.26)

119