In this study, I aimed to systematically investigate the support effect of MOFs for the development of a composite catalyst of metal NPs and MOF supports. The support effect is an approach to enhance the catalytic activity due to an effect from the support. In the chapter 1, I described about the conventional support effect, especially “electronic interaction between metal NPs and a support” and “substrate adsorption by a support” with their current research.
Then, the unique properties of MOFs and thier application for the catalytic support were introduced by reference to reports on MOF’s support effects, i.e., “molecular sieving effect”.
The important thing is a systematic investigation of the two support effect i.e., “electronic interaction between metal NPs and a support” and “substrate adsorption by a support”, of MOFs had been not reported. Therefore, I aimed to do the systematic investigation of the two support effect of the MOFs.
In the chapter 2, I successfully prepared 12 types of MOFs as catalytic supports. The Pt deposition was conducted by an arc plasma deposition method without any chemical reagents.
Characterization through measurements of XRD, STEM, STEM-EDS and ICP, it was revealed that homogeneously loading of Pt NPs was performed with almost the same diameters and amounts (about 2.0 nm and 0.5 wt%) on the seven types of MOFs (Pt/MIL-125-NH2, Pt/UiO-66-NH2, Pt/HKUST-1, Pt/MIL-101, Pt/Zn-MOF-74, Pt/Mg-MOF-74, Pt/MIL-121). This is the first report of homogeneously loading of the same metal NPs on different types of MOFs. As a result of this chapter, I succeeded in preparation of the catalysts with almost the same Pt NPs which make it possible to do accurate evaluation of an effect from the type of support.
In the chapter 3, I aimed to do the systematic investigation of “an electronic interaction between metal NPs and MOFs” and focused on CO oxidation reaction as a target reaction.
The prepared Pt/MOFs (Pt/Zn-MOF-74, Pt/Mg-MOF-74, Pt/HKUST-1, and
Pt/UiO-66-98
NH2) were applied in this study. To evaluate the electronic properties of these catalysts, theoretical calculations, UPS and XPS measurements were conducted. The calculations and experiments revealed that the MOF supports have different electron-donating abilities. As the results of the XPS measurement, it was demonstrated that the electronic states of the loaded Pt NPs are different, which is originating from the electronic interaction between Pt NPs and the MOF supports. Then, CO oxidation reaction was performed using the Pt/MOFs and I clarified that the catalytic activities of the loaded Pt NPs significantly depend on the electronic properties of the MOF supports. This is the first demonstration to show that the electronic band energy of the MOF supports apparently affects the activity of the loaded metal NPs for heterogeneous catalysis due to the support effect of “electronic interaction between metal NPs and MOFs”.
In the chapter 4, to study about the support effect of “substrate adsorption by a MOF”, acetic acid hydrogenation reaction was chosen as a model reaction. I examined the stability for acetic acid on the 12 types of MOFs (DUT-5, HKUST-1, MIL-101, MIL-121, MIL-125, MIL-125-NH2, Mg-MOF-74, Zn-MOF-74, UiO-66, UiO-66-NH2, ZIF-8 and ZIF-67). From a result of acetic acid–exposure experiments using the method, seven types of MOFs (MIL-125-NH2, UiO-66-NH2, HKUST-1, MIL-101, Zn-MOF-74, Mg-MOF-74 and MIL-121) were found out as promising catalytic supports for the acetic acid hydrogenation reaction.
I characterized the acetic acid adsorption properties of MOFs by determination of acetic acid–
desorption temperature. For the evaluation of the temperature, acetic acid molecules were precisely introduced into MOFs which were applied for TPD-MS measurement. The obtained acetic acid–desorption temperatures of the MOFs definitely depend on the type of MOFs.
Although Zn-MOF-74, Mg-MOF-74, MIL-121 did not show any desorption peaks, MIL-125-NH2 and UiO-66-NH2 did desorption peaks at high temperature, i.e. around 100C.
Pt NPs were loaded on the MOFs as chapter 2. In addition, Pt/TiO2 and Pt/Al2O3 were also prepared. The Pt/TiO2 is known as one of the best catalyst for acetic acid hydrogenation reaction.
99
The catalytic performances of the prepared Pt catalysts (Pt/MIL-125-NH2, Pt/UiO-66-NH2,
Pt/HKUST-1, Pt/MIL-101, Pt/Zn-MOF-74, Pt/Mg-MOF-74, Pt/MIL-121, Pt/TiO2 and Pt/Al2O3) on the acetic acid hydrogenation were examined. The Pt/MIL-125-NH2, Pt/UiO-66-NH2 and Pt/TiO2 showed the high catalytic activities. Especially, the production yield on ethanol of Pt/MIL-125-NH2 was higher than those on the others in all temperature region. From these results, I successfully demonstrated that the support effect of “substrate adsorption by a MOF”.
Through this study, I have accomplished three things: preparation of Pt/MOFs with almost the same Pt NPs, systematic investigation of unveiled MOF’s support effect, i.e. “electronic interaction between metal NPs and MOFs” and “substrate adsorption by a MOF”. The preparation method for the composites of various MOFs with the same metal NPs can help us to do a systematic comparison of the effect of MOFs. Therefore, the clarifications of the two support effects definitely have much potential to enhance various catalysis which have a relationship with the electronic states or the substrate adsorption effects. Conversely, an unrecognized support effect may turn to be detectable by utilization of the result of this thesis.
Finally, I earnestly wish that the demonstration will give good insights to deepen understandings of heterogeneous catalysis and widen applications of a composite of metal NPs and MOFs.
100
Acknowledgements
The presented thesis is the summary of my research from April 2015 to March 2020. This work was supervised by Professor Miho Yamauchi (Department of Chemistry, Faculty of Science, Kyushu University).
I would like to express my acknowledgement to Prof. Miho Yamauchi for giving me tremendous support to make this thesis. She provided me with an optimum environment in which I could conduct an active research. Through my academic life, I have learned very much from her to be a researcher. This thesis couldn’t be completed without her enthusiastic guidance and support for me.
I am deeply grateful to Lecturer Masaaki Sadakiyo (Department of Applied Chemistry, Faculty of Science, Tokyo University of Science) who was an assistant professor of Yamauchi laboratory. He thoroughly taught me how to work on the research and took a lot of time to discuss with me. Most of my basic skill and knowledge for my research were given by him.
This was carried out well with his tremendous support.
I express my acknowledgement to Assoc. Prof. Nobutaka Maeda of Yamauchi laboratory for giving me a lot of knowledge and the opportunity to perform the MES-IR measurement, which strongly supported my research.
I would like to thank Assoc. Prof. Aleksander Staykov (International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu university) for collaboration on the theoretical calculation of MOFs for this work.
The author is deeply grateful to Dr. Kenichi Kato, research scientist at the Riken Spring-8 Center. He provided me with the tremendous support for XRD measurement using a synchrotron.
Thanks are given to all members of laboratory of Professor Miho Yamauchi for their many