Modification of Kinetics and Thermodynamics by Various Oxides Addition and Composite Formation for Magnesium-Hydrogen System

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5. Conclusion

I focus on the kinetic and thermodynamic modification of the Mg-H system. The results in this

work are concluded as follows,

The catalytic behaviors of single phase of TiO2, ZrO2, HfO2, V2O5, Nb2O5, Ta2O5, CrO3, MoO3,

and WO3 (Nb2O5 and its neighboring oxide) with the Mg-MgH2 system were investigated. Finally,

the samples are categorized into two group; group A: MgH2 + TiO2, MgH2 + V2O5, MgH2 + Nb2O5,

MgH2 + CrO3, MgH2 + ZrO2, MgH2 + MoO3 (without initial oxide peaks) and group B: MgH2 +

HfO2, MgH2 + Ta2O5 and MgH2 + WO3 (with initial oxide peaks). The dispersion state expected

from the XRD patterns is an important factor because almost the samples with no diffraction peaks

of oxides showed high catalysis. On the other hand, the catalysis of MoO3 cannot be explained by

the dispersion state. The hydrogen desorption temperature for MgH2 + TiO2, MgH2 + V2O5, MgH2

+ Nb2O5, MgH2 + ZrO2 and MgH2+CrO3, shifted around 200 °C compared to pristine MgH2

(425 °C) categorized as Group A. The thermodynamics reduction route for each oxide was

calculated and proposed using a possible equation. The thermodynamic estimation and XPS

analyses clarified that the high catalytic activity was obtained when the intermediate oxidation

states such as +2 and +3 can be formed. The study on effective factors of oxide catalysts for the

Mg-H system reveals that the dispersion state of oxide is one of the important factors. Furthermore,

the chemical state of oxide is also an important factor to realize a highly active state as catalysis,

in which the intermediate oxidation state especially for +2 and +3 is responsible for the catalytic

activity of oxides.

The low, as well as extremely high catalysis, was not suitable for practical application,

former requires excess energy input and the latter is dangerous due to the high activity. The purpose

to search the newest and best catalysts using tuning of catalysis of Mg-H system by single phase

195

of ternary oxide formation using combining the different metal elements. Synthesis of ternary

oxides ensued using single phases such as V2O5 (active) and Ta2O5 (inactive), likewise others.

Catalytic tuning was achieved using the ternary oxides to control the catalysis as per our aim. The

hydrogen desorption peak temperature for MgH2 with synthesized TaVO5 ternary oxide was found

around 250 °C which is in the middle of MgH2+V2O5 peak temperature 213 °C and Ta2O5 peak

temperature 288 °C respectively. On the other hand, for Nb9VO25 ternary oxide, the hydrogen

desorption peak temperature for MgH2 with synthesized Nb9VO25 oxide was found around 250 °C

which is comparatively higher compared to MgH2+V2O5 peak temperature 220 °C and Nb2O5 peak

temperature 225 °C, respectively. Similarly, for Nb0.6Cr0.4O2 ternary oxide, the hydrogen

desorption peak temperature for MgH2 with synthesized Nb0.6Cr0.4O2 oxide was found around 303

°C which is comparatively higher compared to MgH2+Nb2O5 peak temperature 222 °C and

MgH2+CrO3 peak temperature 206 °C respectively. Therefore, the synthesis of ternary oxide is

useful to tune the catalysis of magnesium hydride.

In this work, various types of amorphous Nb and Ta oxides are synthesized by simple sol-gel

methods. These oxides are dispersed on the MgH2 surface by the ball-milling for 2 h, which is ten

times shorter than that of the previous synthesis process using the ball-milling. All the oxides reveal

catalysis for the hydrogen desorption and absorption reactions of Mg. Especially, the hydrogen

absorption can proceed around room temperature, suggesting that the high activation of Mg can be

achieved. The gel oxides include -OH groups, which would exist in the network between Nb and

O, namely they are regarded as precursors of stable Nb2O5 state formed during the heat treatment

at 500 °C. The metastable oxides are easily converted to the catalytic active state, which is a reduced

state generated due to the mechanochemical reaction with MgH2 during the ball-milling. Therefore,

the gel oxides are recognized as a suitable precursor to produce the highly activated Mg by simple

and mild conditions.

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The purpose is to achieve the thermodynamic modification of MgH2 using CaH2 by composite

formation. The thermodynamic properties of the 2MgH2-CaH2 composite have been investigated

with and without catalyst. The uniform distribution of catalyst pretended improvement in the

kinetics of hydrogenation-dehydrogenation of the composites. Interestingly, the peak shift in the

case of ZrCl4 could be possible, if ZrCl4 is electrochemically interacting with MgH2 and as

consequence, the 3d electron is in the diffused state. On the other hand, the peak shift of Zr0 could

be due to the doping of zirconium over the MgH2-CaH2-ZrCl4 sub-surface. The dehydrogenated

product has shown the formation of Mg2Ca intermetallic which favored the hydrogenation dehydrogenation process. Thermodynamic modification of MgH2 was achieved by introducing

CaH2 in the ratio of 2:1 respectively by the formation of Mg2Ca intermediate stage.

197

Acknowledge

It is a great honor for me to express the heartiest gratitude and thanks to my Ph.D. supervisor, Dr.

Hiroki Miyaoka for kind guidance, support, enthusiastic interest, vital discussions, suggestions, and

kind cooperation throughout the research work. Their timely help, constructive criticism, and

conscientious effort made it possible to present this thesis. His cool and polite nature is a chapter

for the coming generation and society.

I am also very much thankful to Prof. Takayuki Ichikawa for their valuable guidance, time,

continuous support in conducting the experiments during my research work. His kind suggestions

are fruitful for the in-depth knowledge of the research work.

A special thanks to my lovely Guru Prof. Sanjay Kumar (Senior Scientist, BARC, HBNI) who

provided me with proper guidance and very much flexibility during my project work at BARC,

Mumbai. He also taught me a lot of technical tricks and tips which are very useful in research. I am

very much thankful and will always be for their love and for forcing me to learn more and more in

life.

I am also very much thankful to Prof. Yoshitsugu Kojima for his continuous guidance, suggestions,

kind help, and support throughout my research work. As a part of my research work, Dr. Yoshitsugu

Kojima has provided me with this opportunity to work here in Japan at Hiroshima University. I am

thankful and will always be for his kind love, support, and teaching.

I am sincerely obliged to Dr. Sanjib Majumdar (Prof. HBNI, Mumbai & Head, HTMS, MP & CED,

BARC, Mumbai, India), Dr. Tammana S.R.Ch. Murthy (Senior Scientist, BARC, Mumbai), Dr.

Bhaskar Paul (Senior Scientist, BARC, Mumbai) for their encouragement to learn and grow. I am

sincerely obliged to and for his support in several ways, including opportunities to visit different

facilities in BARC, Mumbai, India.

I am also very much thankful to Dr. Ankur Jain for his guidance during my Ph.D. work.

I also sincerely thank my colleague Dr. Keita Nakajima, Dr. Hiroki Uesato, Dr. Masakuni

Yamaguchi for extending their help and technical support. I sincerely thank Mr. Hiroyuki Gi for

his valuable cooperation and experimental support.

I am also thankful to Dr. Keita Shinzato, Dr. Fanquine Guo, and Dr. Khushboo Sharma for their

kind suggestions at different places of research work. I immensely enjoyed their company and

intellectual discussions.

I am also very much thankful to a few of my other friends, especially Dr. Shiv Kumar (HiSOR,

Hiroshima University), who always supported me here during my stay in several ways.

I am also very much thankful to a few other Indians staying in Japan for their company Dr. Ajay

Tiwari (Scientist, Micron, Japan), Dr. Yash Sharma (Scientist, Micron, Japan), and Mr. Gajendra

Kumar.

I am deeply indebted to Dr. Anamika Singh and Mr. Shreyansh Singh, particularly for their

encouragement in educational life.

I would like to thank group members Mrs. Saroi Inagaki San, Mr. Ichikawa San, Mr. Harada, Mr.

Yao, Mr. Tsunematsu, and others for their kind cooperation and helping me at various stages.

I deeply acknowledge my respected and beloved parents and all my family members, my sisters

Mrs. Shashi Singh & Mrs. Usha Singh, my Jeeja Mr. Subba Singh. A lot of love and blessings to

my little champ Rohit, Shatrughan, Vandana, Vijay, Anjali, Priyanshu, Shudhanshu, Sheshnath.

Without their love and encouragement, this work would not have evolved and been accomplished

in its present form. I express my praise to all of them.

Last but not least, I express my deepest thanks to the Almighty, who have given me this wonderful

life and whose blessings have paved my way ahead through all ordeals.

II

බ⾲ㄽᩥ

(Articles)

(1) Synthesis of Highly Activated Magnesium by Niobium and Tantalum Gel Oxide Catalyst

P. K. Singh, H. Gi, K. Shinzato, K. Katagiri, H. Miyaoka, T. Ichikawa, Y. Kojima

Materials Transactions, 62 (2), 284-289 (2021).

(2) Development of Ca-Mg-H2-ZrCl4 composite for hydrogen storage applications

P. K. Singh, A. Singh, V. Kain, Y. Kojima, H. Miyaoka, S. Kumar

International Journal of Hydrogen Energy, 46, 34362-34368, (2021).

III

ཧ⪃ㄽᩥ

(Thesis Supplements㸧

(1) The catalytic effect of ZrCl4 on thermal dehydrogenation LiAlD4

S. Kumar, A. Singh, P. K. Singh, H. Miyaoka, V. Kain, Y. Kojima

International Journal of Hydrogen Energy, 45, 14413-14417, (2020).

(2) Systematic Study on Nitrogen Dissociation and Ammonia Synthesis by Lithium and

Group 14 Element Alloys,

K. Shinzato, K. Tagawa, K. Tsunematsu, H. Gi, P. K. Singh, T. Ichikawa, H. Miyaoka,

Submitted, Advanced Materials, Under Review

IV

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