Introduction

Metal nanoparticles show unique physical properties such as lower melting point and chemical properties such as high catalytic activity different from those of bulk metals owing to the change in lattice parameters and the higher ratio in the number of surface atoms compared to that of overall atoms. In the study of my Ph.D course, I focused on the hydrogen storage properties of metal nanoparticles which will be different from those of the bulk metals because both the natures of surface layer and internal metal core are crucial to the process to absorb hydrogen from dihydrogen gas. Hydrogen is expected to be employed for clean energy carrier and various metals and alloys have been investigated as dihydrogen storage materials. In this work, I studied hydrogen storage properties of palladium nanoparticles with network structure with tetrakis(terpyridine) ligands as bridging spacers and vanadium-palladium alloy nanoparticles.

1. Palladium nanoparticles with network structure using tetrakis(terpyridine) bridging ligand

Palladium metal is known for hydrogen storage metal and I have been studying palladium nanoparticles for their high stability and facile synthesis. In my master course, I studied metal-organic framework (MOF) type of palladium nanoparticle assemblies. It was aimed that hydrogen storage by metal nanoparticles and MOF were generated at the same time. For the synthesis of MOF type of nanoparticle assemblies, I connected palladium nanoparticles through ligand on their surface, and constructed network structure of nanoparticles. Previously, I used bis or tris(terpyridine) ligands as the bridging ligand to construct network structure. However, the porosity in the networks could not be controlled probably due to plane structure of ligands. Based on the results, I employed a new tetrahedral bridging ligand 1 in this study.

Initially, we designed and synthesized tetrakis(terpyridine) bridging ligand 1 by Suzuki coupling. The ligand has tetrahedral structure and terpyridine group on the each end. Tetrahedral structure helps construction of network structure effectively because of its three-dimensional structure. Then, to prepare the nanoparticle with network structure, ligand exchange reaction of the bridging ligand 1 with 1-pentyl isocyanide on palladium nanoparticles was carried out (Figure 1). Aggregation of the nanoparticles was confirmed by the TEM observations and the distance between nanoparticles was consistent with the value estimated from the size of the ligand. These results have confirmed the formation of the expected network structure of palladium nanoparticles. The hydrogen storage volume of nanoparticles with network structure showed 0.26 mass% at rt and10 MPa.

Next, we designed tetrakis(terpyridine) ligand 2 to enlarge porous in network structure. This ligand has four methoxy groups on aromatic ring to improve solubility of ligand because the ligand 1 (without methoxy groups) hardly dissolved in solvent. The ligand 2 was synthesized by Suzuki coupling reaction and ligand exchange reaction with the bridging ligand 2 was carried out. The hydrogen storage volume of nanoparticles bridged by ligand 2 showed 0.33 mass% at rt and 10 MPa.

As another strategy to enlarge the pore size, bis(terpyridine)-metal complexation on palladium nanoparticle surface was employed (Figure 2). First, we examined the conditions to isolate palladium nanoparticles protected by ligand 1. Dispersed single palladium nanoparticles were obtained when more than 12 times of ligand 1 in mol ratio compared with palladium nanoparticles was employed. Addition of Fe(BF4)2 to the dispersion of ligand 1 protected palladium nanoparticles in CHCl3 caused aggregation of nanoparticles, and the formation of porous nanoparticles network was confirmed by TEM. From the result of nitrogen adsorption measurement, the pore size of the network bridged by the bis(terpyridine)iron complexes was bigger than that of the previous ligand 2-bridged network. Hydrogen storage volume showed 0.25 mass% at rt and 10 MPa.

2. Palladium nanoparticles with larger size and tetrahedral structure

Most of metals in nanoparticles are sited at their surface and sites for hydrogen storage in metal decreased. Synthesis of larger nanoparticles is effective for improvement of hydrogen storage volume. To synthesize pentyl isocyanide protected palladium nanoparticles with larger size, I examined synthesis by polyol method. I prepared palladium nanoparticles by reduction of PdCl2(MeCN)2 with ethylene glycol in the presence of 1-pentyl isocyanide as surfactant From the observation of TEM image, synthesized nanoparticles have tetrahedral structure with 28 nm (Figure 3). The size of tetrahedral nanoparticles was 10 times bigger than that of previously prepared sphere ones. The hydrogen storage volume showed 0.34 mass% at 10 MPa and rt.

3. Vanadium-palladium nanoparticles

Vanadium stored six times hydrogen compared to palladium. Because vanadium metal on surface is oxidized easily, synthesis of vanadium nanoparticles is difficult. I examined stabilization of vanadium by doping vanadium to palladium nanoparticles. Synthesized nanoparticles contained 10 % of vanadium and formed alloy structure from the observation of lattice spacing in TEM image. Hydrogen storage volume showed 0.47 mass% at rt and 10 MPa.

Conclusion

I synthesized porous palladium nanoparticle assemblies using tetrakis(terpyridine) bridging ligands. Nanoparticles aggregated with the distance estimated by the size of the ligand between nanoparticles. I succeeded to control the network structure of palladium nanoparticles by bridging ligands.

Next, I synthesized isocyanide protected palladium tetrahedral nanoparticles with larger size. The size of nanoparticles was 10 times compared to previous ones and hydrogen storage volume was larger than small ones. In addition, the tetrahedral structure is unique structure and expected possibilities for bridging structure.

Finally, I synthesized vanadium-palladium alloy nanoparticles. It formed alloy structure and hydrogen storage volume was 0.47 mass% which was larger than that of palladium nanoparticles. If I prepared alloy nanoparticles with larger ratio of vanadium, the hydrogen storage properties would be changed compared to this result.

Figure 1. Synthesis of palladium nanoparticles with network structure

Figure 2. Synthesis of network structure using iron ion for bridging and their TEM images

Figure 3. TEM image of tetrahedral palladium nanoparticles

Figure 4.vanadium-palladium alloy nanoparticle

本論文は6章からなり、第1章は研究の概論と背景、第2章はテトラキステルピリジン配位子で架橋したパラジウムナノ粒子の合成、第3章は錯形成反応を利用したパラジウムナノ粒子架橋体の合成と水素吸蔵特性、第4章は四面体構造を有するパラジウムナノ粒子の合成と架橋構造体の構築、第5章はパラジウム-バナジウム合金ナノ粒子の合成とその水素吸蔵特性、第6章は研究成果の総括を述べている。以下に各章の概要を示す。

第1章では、本研究の概論と背景について述べた。内容としては、金属ナノ粒子、水素吸蔵金属及びmetal-organic frameworkについて述べた。

第2章では、2種類のテトラキステルピリジン配位子を用いてパラジウムナノ粒子の架橋化を行い、ナノ粒子のネットワーク構造の構築に関する研究を述べた。電子顕微鏡による観察から、パラジウムナノ粒子はダイヤモンド構造をとって架橋していることが確認された。窒素吸着測定により、架橋構造は0.03 cm3 / gの細孔量を有していた。水素吸蔵測定では、ナノ粒子集合体に約0.3 mass%の水素が吸蔵されたものの、ほとんどがナノ粒子への吸蔵であり、細孔への水素吸蔵は確認されなかった。これは、作製した構造体内の細孔量が不十分であり、最大でも0.03 mass%の水素吸蔵量に止まることが原因であった。

第3章では、テルピリジン-鉄イオン結合を用いてパラジウムナノ粒子の架橋を行い、ナノ粒子のネットワーク構造の構築に関する研究を述べた。テルピリジン配位子は第二章のテトラキステルピリジン配位子を用い、4つのテルピリジン基のうち1つはナノ粒子に、残りは鉄イオンを介してテルピリジン基同士を結合させた。窒素吸着測定により、ナノ粒子集合体は0.05 cm3 / gの細孔を有しており、二章で作製した架橋体より大きな細孔を構築することに成功した。水素吸蔵測定により、ナノ粒子集合体に0.25 mass%の水素が吸蔵されたものの、細孔による水素吸蔵量は最大で0.05 mass%であり、依然として細孔への水素吸着量は僅かであった。

第4章では、四面体構造を有するパラジウムナノ粒子の合成とその架橋体の構築について述べた。ポリオール法を用いることで、四面体構造を有するパラジウムナノ粒子の合成法を見出した。合成条件の最適化を行なうことで、四面体構造をメイン構造として作り出すことに成功した。その架橋体は、0.1 cm3 / gの細孔を有しており、今回作製したパラジウムナノ粒子集合体の中で最大の細孔量を有する構造体を構築することに成功した。

第5章では、パラジウム-バナジウム合金ナノ粒子の合成について述べた。合成はポリオール法を用いて行い、10 %のバナジウムを含む合金ナノ粒子の合成に成功した。水素吸蔵測定により、合金ナノ粒子は0.47 mass%と今回報告したナノ粒子の中で一番大きな値を示した。

第6章では、今回の論文の総括を記した。

以上、本論文では、水素吸蔵材料への応用を目指したパラジウムナノ粒子の集合体の構築について述べた。本研究を通してパラジウムナノ粒子集合体によって構築された細孔の大きさを制御することに成功し、パラジウムナノ粒子の合成条件を調整することで、四面体型の形状やバナジウムとの合金を作製することに成功している。本論文の研究を基礎とし、金属ナノ粒子の種類や架橋構造の設計を改良していくことで、実用に向けた水素吸蔵材料への応用が開けると期待される。なお、本論文は蓑田 愛、宮地麻里子、山野井慶徳、大島伸司、小堀良浩、西原 寛とのとの共同研究であり、一部はすでに学術雑誌として出版されたものであるが、論文提出者が主体となって実験、解析を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。