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Chapter 7 Conclusions
Supported Pd catalysts are key materials for reducing the environmental load caused by air
pollution. Exhaust gas emission regulations to prevent air pollution are becoming stricter.
Furthermore, the price of Pd has skyrocketed due to the rapid increase in demand and current
social conditions, and reducing the amount of Pd used is required. Therefore, the development
of highly active Pd catalysts is required. Investigation of the influence of sulfur species is
necessary for designing highly active catalysts. In this study, the effects of support, transition
metal addition, and sulfur species on the catalytic properties of Pd-supported catalysts were
investigated to develop high-performance Pd catalysts. The main research contents and results
are as follows:
Chapter 1 outlines the research background of this study and the fundamentals of catalytic
chemistry.
In Chapter 2, the effect of the support on the CO oxidation reaction under dilute O2
conditions was examined, and Pd/CeO2 showed higher catalytic activity in the low-temperature
region below 180 °C. STEM images and XAFS measurements revealed that Pd particles on
CeO2 were more highly dispersed than those on TiO2 and Al2O3. FTIR measurements showed
that Pd on CeO2 exists as PdO in an atmosphere and is reduced to Pd0 under CO flow.
Furthermore, oxidative desorption of CO species adsorbed on Pd/CeO2 is faster than that of CO
species on Pd/TiO2 and Pd/Al2O3.
In Chapter 3, the effect of SO2 on the catalytic structure of Pd/CeO2 was investigated by in
situ XAFS measurements to observe the changes under SO2 flow. in situ XAFS measurements
revealed that SO2 treatment at 500 °C sulfurized the surface lattice of CeO2 to form sulfate and
Ce3+ species. SO2 treatment did not form Pd sulfides, and the entire Pd/CeO2 catalyst surface
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was sulfurized. Furthermore, Pd loading on the CeO2 promoted sulfurization of the support
CeO2.
In Chapter 4, morphology-controlled c-CeO2 was prepared, and the effect of SO2 on the
CeO2 structure of Pd/CeO2 was analyzed at the atomic level using STEM-EELS measurements.
Pd was highly dispersed on c-CeO2. STEM-EELS measurements revealed that SO2 treatment
at 200 °C did not change the structure of the support CeO2, while SO2 treatment at 400 °C
reduced the support c-CeO2 and increased the number of oxygen vacancies. Moreover, it was
found that sulfurization by SO2 is enhanced at the Pd-CeO2 interface.
In Chapter 5, the effect of Cu addition on the CO oxidation activity of Pd/CeO2 catalysts
under dilute conditions was investigated. The addition of Cu to Pd/CeO2 improved the activity
at 90–250 °C, and Pd-Cu/CeO2 exhibited the highest activity in all temperature ranges. PdCu/CeO2 exhibited higher activity for CO oxidation than Cu/CeO2. The addition of Cu did not
affect the electronic state of Pd and did not form Pd-Cu bimetallic particles; however, it reduced
the Pd particle size and improved the dispersion of Pd.
In Chapter 6, the effect of SO2 on VOC oxidation over Pd/Al2O3 was discussed. In this
chapter, I found that the benzene oxidation activity of Pd/-Al2O3, a widely used catalyst for
VOC oxidation, was improved by SO2 treatment after the activity was reduced by sintering due
to high-temperature treatment. Pd species existed as coarsened PdO and did not form sulfides
or sulfites. DRIFT studies showed that SO2 treatment suppressed the formation of byproduct
compounds on the catalyst, resulting in increased activity for benzene oxidation.
In this study, I systematically investigated the effects of (1) the catalyst support, (2) the
transition metal addition, and (3) the sulfur species on the catalytic properties of Pd catalysts.
To improve catalytic activity, I investigated the effects of catalyst support and transition metal
additions. The Pd-Cu/CeO2 catalyst showed high activity in the CO oxidation reaction under
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dilute O2 conditions. In addition, the factors that contribute to activity enhancement were
clarified. Furthermore, the effects of sulfur species on catalyst structure and activity were
investigated in detail using various spectroscopic techniques to design catalysts highly resistant
to sulfur poisoning. Traditionally, elucidating the mechanism of sulfur poisoning has been
challenging due to the complexity of catalytic reactions and the difficulty of observing the local
structure of actual catalysts. This study significantly contributed to elucidating the mechanism
of sulfur poisoning by enabling analysis at the atomic level and under actual reaction conditions.
The findings in this study are expected to be applied not only to exhaust gas purification but
also to all catalytic reactions under residual sulfur, especially in petroleum refining and
hydrogen production from fossil fuels.
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Acknowledgements
This thesis is based on experimental work at Einaga-Hojo Laboratory, Department of
Molecular and Material Sciences, Graduate School of Engineering Sciences, Kyushu University.
I would like to express my gratitude to my supervisor Professor Hisahiro Einaga for his
continuous guidance.
I want to extend my thanks to Professor Hajime Hojo for his constant advice.
I thank my thesis committee, Professor Kengo Shimanoe and Professor Shinji Kudo, for
their helpful discussions and guidance with my thesis.
I appreciate the support given to me by Professor Takeharu Sugiyama of the Kyushu
Synchrotron Light Research Center in the XAFS measuring.
I would like to thank our secretary Ms. Akiko Nishioka for her support in our research life.
I thank all former and present members of the Einaga-Hojo Laboratory for their help and
support.
I would like to thank the staff of Green Asia Course for their help.
Last but not least, I must express my sincere appreciation to my family and friends for their
constant and continued support and patience.
Saki Shigenobu
February 2022
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