5. L IGAND E XCHANGE R EACTION OF A TOMICALLY D ISPERSED C ATALYSTS TO
5.4. C ONCLUSION
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6
D ISSERTATION S UMMARY AND O UTLOOK
6.1.SUMMARY
This dissertation displays the development of preparation methods for the atomically dispersed catalysts and investigation of their catalytic trends and origins for the oxygen reduction reaction (ORR).
For the ORR, non-precious metal-based atomically dispersed M–N/C catalysts, where metal and nitrogen are co-doped on carbon, were tremendously investigated owing to their high ORR activity in the 2010s. However, at that time, the concept of atomically dispersed catalysts has not yet been firmly established, and it was at the beginning of the spread of transmission electron microscopy which can resolve at an atomic scale, and thereby there has been some controversy related to active sites of M–
N/C catalysts. To resolve controversy related to the active sites of non-precious metal-based atomically dispersed Fe–N/C catalyst, we prepared model catalysts which selectively contain atomically dispersed Fe–Nx, metallic iron and/or iron carbide encapsulated within graphitic carbon shell (Fe–Fe3C@C), and N-doped carbon sites through iron oxide conversion method. Utilizing the model catalysts, we demonstrated that the atomically dispersed Fe–Nx sites efficiently catalyze the ORR via 4 e− pathway, playing a major role for superb ORR activity, whereas the Fe–Fe3C@C sites mainly promote 2 e− oxygen reduction and sequential 2-electron peroxide reduction (2 e− × 2 e− pathway), playing an auxiliary role for the ORR.
Compared to non-precious metal-based atomically dispersed catalyst, usually M–N/C catalyst, it has been more difficult to fabricate precious-metal-based atomically dispersed catalysts. Because, the agglomeration during thermal activation is a major problem in the preparation of atomically dispersed catalysts, but in case of M–N/C catalyst, through simple acid leaching the agglomerated inactive metallic species could be removed with preserving single sites. However, for the precious metal, the metallic species are difficult to remove by acid treatment, and even if they are eliminated easily with a simple process, such manner is not appropriate to expensive and scarce precious metal-based catalysts.
For these reasons, the development of the general preparation strategy to precious-metal-based atomically dispersed catalysts has been challenging.
We overcome such difficulties by establishing “trapping-and-immobilizing” synthetic processes.
The synthetic strategy consists of “trapping” precious metal precursor molecules, individually, on a
heteroatom-doped carbonaceous layer coated on a carbon support and then “immobilizing” them during thermal activation via coating with a SiO2 protective layer. The strong interaction between the metal precursors and the support provides stable, isolated anchoring of metal precursors during the impregnation step and retards precursor decompositions during the activation step, which assists the conversion of the precursor into an isolated single-atom site. The SiO2 protective layer could mitigate the agglomeration of the isolated metal sites during the thermal activation step, thus maximizing the density of atomically dispersed sites.
The “trapping-and-immobilizing” method enabled the preparation of various atomically dispersed precious metals catalysts (Os, Ru, Rh, Ir, and Pt), which served as model catalysts for investigating the universal catalytic trends of atomically dispersed catalysts for the ORR, differing from those of nanoparticle (NP)-based catalysts. The atomically dispersed catalysts exhibited higher H2O2
selectivities and lower ORR activities compared with their NP counterparts. Among atomically dispersed catalysts, Rh showed the best ORR activity and Pt exhibited the highest H2O2 selectivity.
Combining DFT calculations and experimental results, we found that the optimal binding energy of atomically dispersed catalysts is important to their ORR activity as NP-based catalysts and relative binding energies of *OOH and *O species were identified as key parameters for determining the selectivity for H2O2 production of atomically dispersed catalysts. In addition, we observed a trade-off relationship between activity and selectivity in the 2 e– ORR pathway. Consequently, the unique ORR activity and selectivity trends of the atomically dispersed precious metal catalysts originate from their abnormally weak oxygen binding energies in conjunction with their geometric configurations.
The full dispersion of metal atoms on supports does not always guarantee superb catalytic properties. Like molecular catalysts, proper coordination environments and oxidation states are necessary to exhibit outstanding catalytic performance. Thus, it is essential to understand the optimal coordination structure of atomically dispersed catalysts at a molecular-level. Unfortunately, owing to the structural characteristics of atomically dispersed catalysts where metal atoms are coordinated with complex support surface sites, it is difficult to figure out and control the coordination environment of the catalytic sites, hindering the investigation of their structure-activity correlation. In order to control the coordination environment of atomically dispersed catalysts precisely, we performed a ligand exchange reaction on atomically dispersed Rh catalysts in a gas phase with NH3 and CO, respectively.
After the reaction, the Cl ligands of Rh chloride precursor (RhCl3) were substituted with NH3 and CO, respectively, accompanying changes in the oxidation state of Rh. In a comparison of their catalytic properties for the ORR, CO-treated atomically dispersed Rh catalysts exhibited approximately 30-fold higher ORR activity and one-third lower H2O2 selectivity than NH3-treated catalysts. The ligand exchange reaction can also take place reversibly. When CO gas was treated to NH3-ligated catalysts, ORR activity was enhanced, and again when NH3 gas was delivered, ORR activity was deteriorated.
Such alternation of ORR performance is followed by the changes in the oxidation states of Rh. From
these results, we demonstrated that the oxidation state of atomically dispersed catalysts is a determining factor for their ORR activity and selectivity.
6.2.SUGGESTIONS FOR FUTURE WORKS
We suggest some future works to understand the catalytic properties of atomically dispersed catalysts in deeper. The high H2O2 selectivity of precious-metal-based atomically dispersed catalysts has been explained by the lack of ensemble sites.1 However, from non-precious metal-based atomically dispersed M–N/C catalysts, very low H2O2 selectivities have been shown.2 Considering such distinct ORR catalytic behavior between precious and non-precious-metal-based atomically dispersed catalysts, we hypothesize that the geometric effect is not dominating for determining ORR catalytic properties of atomically dispersed catalysts. The electronic effect which derive the abnormally-weak oxygen binding strength of precious-metal-based atomically dispersed catalysts may govern the ORR reactivity.3 The preparation of ensemble catalysts which consist of two or three metal atoms is expected to decouple the geometric and electronic effect on the ORR reactivity.
There has been a perception where the thermal stability of atomically dispersed catalysts is inferior compared to that of NP-based catalysts.4 In the state of impregnated metal precursor, which is not strongly coordinated with supports, the metal sites can be unstable. After thermal activation, however, metal atoms partially or fully embedded in the surface of supports.5 The embedded metal atoms can exhibit high thermal stability, sometimes even better than NPs. In fact, the metal sites of atomically dispersed catalysts can be considered as solutes in solid solution. On carbon supports, metal sites are placed as dopants. For metal or metal oxide supports, the structure of random alloy is formed.
Regarding these rigid structures of atomically dispersed catalysts, we hypothesize that well-prepared atomically dispersed catalysts can exhibit much better thermal stability compared to NP-based catalysts, which infer that atomically dispersed catalysts are appropriate to the reaction operated in high- temperature. As a preliminary result, we found that the atomically dispersed Pt catalysts prepared by the “trapping-and-immobilizing” method are not agglomerated until 900 °C in the inert gas condition.
If the high-performed catalytic reaction which operate in high temperature and reduction reaction condition where carbon support is stable can be found from the atomically dispersed Pt catalysts, it can be one of good example breaking the stereotype related to thermal stability of atomically dispersed catalysts.
6.3.REFERENCES
(1) Kim, J. H.; Kim, Y.-T.; Joo, S. H. Curr. Opin. Electrochem. 2020, 21, 109–116.
(2) Kim, J. H.; Sa, Y. J.; Jeong, H. Y.; Joo, S. H. ACS Appl. Mater. Interfaces 2017, 9, 9567–9575.
(3) Kim, J. H. Shin, D.; Lee, J.; Baek, D. S.; Shin, T. J.; Kim, Y.-T.; Jeong, H. Y.; Kwak, J. H.; Kim, H.; Joo, S. H. ACS Nano 2020, 14, 1990–2001.
(4) Su, Y.-Q.; Wang, Y.; Liu, J.-X.; Filot, I. A. W.; Alexopoulos, K.; Zhang, L.; Muravev, V.;
Zijlstra, B.; Vlachos, D. G.; Hensen, E. J. M. ACS Catal. 2019, 9, 3289–3297.
(5) Samantaray, M. K.; D’Elia, V.; Pump, E.; Falivene, L.; Harb, M.; Chikh, S. O.; Cavallo, L.;
Basset, J.-M. Chem. Rev. 2020, 120, 734–813.
A
CKNOWLEDGEMENTS지난 8년간 연구란 무엇이고 올바른 연구자의 방향성은 어떤 것인지 가르침을 주신 주상훈 지도교수님께 깊은 감사의 마음을 전합니다. 연구뿐만 아니라 인생에 대한 조언도 아낌없이 주신 덕에 8년간의 연구실 생활은 앞으로의 제 삶의 큰 밑거름이 될 것 같습니다. 귀한 시간을 내주셔서 저의 학위 논문과 디펜스 발표를 심사해주시고 앞으로 제가 어떻게 발전하면 좋을지 코멘트를 주신 안광진 교수님, 강석주 교수님, 장지욱 교수님, 권영국 교수님께 큰 감사의 말씀을 드립니다.
학위 기간 동안 많은 분들의 도움을 받아 연구 및 과제를 수행하였습니다. TEM 분석에 도움을 주신 정후영 교수님, XAS 측정을 도와주신 신태주 교수님, 계산 화학을 통해 많은 정보를 주시는 KAIST 김형준 교수님, MS 분석을 통해 깊이를 더해주신 곽자훈 교수님, 최근 EXAFS Fitting에 도움을 주신 이국승 박사님, 매 논문마다 날카로운 코멘트를 주시는 문회리 교수님, 과제 진행의 좋은 예를 보여주시는 POSTECH 김용태 교수님께 감사의 말씀을 전합니다.
긴 시간 함께 연구실 생활을 하며 많은 도움을 주시고 큰 힘이 되어 주신 랩 동기 및 선후배분들께도 정말 감사하다는 말을 전합니다. 천재영 박사님, 사영진 박사님, 서보라 박사님께서 실험실 초창기부터 고생하시면서 셋업을 해 주신 덕에 저는 많은 부분을 쉽게 배울 수 있었습니다.
또 저의 이상한 질문이나 터무니없는 말에도 선배들이 늘 귀 기울여 잘 들어주시고 디스커션 해주셔서 재미나게 연구할 수 있었습니다. 항상 쾌활한 분위기를 만들어주고 저의 부족한 부분을 채워준 우리 랩 메이트들 호영이, 두산이 형, 진우, 태정이 형, 준성이에게도 감사의 말을 전합니다.
항상 도움은 많이 못 주고, 받기만 한 것 같아 미안하고 고마운 마음이 같이 듭니다.
학부 1학년부터 축구라는 공통 관심사로 모여 거의 10년간 동고동락한 우리 구엘 멤버 승세, 규진이, 지홍이 세원이, 태양이 덕분에 학위 과정동안 정말 큰 힘이 됐고, EA 친구들 지정이, 정우, 맹기형, 혁준이, 종민이, 동훈이, 훈기, 우진이 덕분에 재밌게 학교 생활 보낼 수 있었습니다.
멀리서도 항상 응원해준 대구 친구들 성연이, 대걸이, 기정이, 영욱이, 현욱이, 광훈이에게도 감사의 마음을 전합니다.
9년간 항상 저의 곁을 지켜주며 긴 장거리의 시간 동안에도 끝까지 저를 믿어주고 지지해준 사랑하는 여자친구 명진이에게도 감사의 말을 전합니다.
끝으로 큰 지지와 격려를 보내주신 가족분들과 늘 동생 아껴주는 우리 누나, 그리고 저를 사랑으로 길러주시고 언제나 저의 선택을 존중해주시는 부모님께 깊은 감사의 마음을 전합니다.
Jae Hyung Kim
C
URRICULUMV
ITAEE
DUCATION 2015–2020 M.S.-Ph.D. Combined Program in Chemical Engineering, Ulsan National Institute ofScience and Technology (UNIST) (Advisor: Prof. Sang Hoon Joo) 2011–2015 B.S. Degree in Chemical Engineering, UNIST, February 2015.
A
WARDS& H
ONORS 2019 Excellent Oral Presentation Award, Korean Institute of Chemical Engineers2019 Excellent Poster Award, Korean Chemical Society 2019 Best Poster Award, Nano Korea
2016 Best Poster Award, Nano Korea
2016 Excellent Poster Award, Korean Institute of Chemical Engineers
2015 Best Poster Award, KCS Yeongnam Regional Meeting, Korean Chemical Society
R
ESEARCHI
NTERESTS – Design of novel catalysts for electrocatalysis and their practical applications (fuel cells and electrolyzer) – Understanding of structure-activity correlation in heterogeneous catalysis– Development of new nano-materials
I
NTERNATIONALP
UBLICATION [15] Jae Hyung Kim, Yong-Tae Kim*, and Sang Hoon Joo*“Electrocatalyst Design for Promoting Two-Electron Oxygen Reduction Reaction: Isolation of Active Site Atoms”
Curr. Opin. Electrochem. 21, 109–116 (2020).
[14] Jae Hyung Kim, Dongyup Shin, Jaekyoung Lee, Du San Baek, Tae Joo Shin, Yong-Tae Kim, Hu Young Jeong, Ja Hun Kwak, Hyungjun Kim*, and Sang Hoon Joo*
“A General Trapping-and-Immobilizing Strategy to Atomically Dispersed Precious Metal Catalysts for Unravelling Their Catalytic Trends for the Oxygen Reduction Reaction”
ACS Nano 14, 1990–2001 (2020).
[13] Taejung Lim, Gwan Yeong Jung, Jae Hyung Kim, Sung O Park, Jaehyun Park, Yong-Tae Kim, Seok Ju Kang, Hu Young Jeong, Sang Kyu Kwak*, and Sang Hoon Joo*
“Atomically Dispersed Pt−N4 Sites as Efficient and Selective Electrocatalysts for the Chlorine Evolution Reaction”
Nat. Commun. 11, 412 (2020).
[12] Myohwa Ko, Le Thanh Mai Pham, Young Jin Sa, Jinwoo Woo, Trang Nguyen, Jae Hyung Kim,
Dongrak Oh, Pankaj Sharma, Jungki Ryu, Tae Joo Shin, Sang Hoon Joo*, Yong Hwan Kim*, and Ji-Wook Jang*
“Unassisted Solar Lignin Valorisation Using a Compartmented Photo-Electro-Biochemical Cell”
Nat. Commun. 10, 5123, (2019).
[11] Chinh Nguyen-Huy, Jihyeon Lee, Ji Hui Seo, Euiseob Yang, Jaekyoung Lee, Keunsu Choi, Hosik Lee, Jae Hyung Kim, Man Sig Lee, Sang Hoon Joo, Ja Hun Kwak, Jun Hee Lee*, and Kwangjin An*
“Structure-Dependent Catalytic Properties of Mesoporous Cobalt Oxides in Furfural Hydrogenation”
Appl. Catal. A 583, 117125 (2019).
[10] Young Jin Sa, Jae Hyung Kim, and Sang Hoon Joo*
“Active Edge Site-Rich Carbon Nanocatalysts with Enhanced Electron Transfer for Efficient Electrochemical Hydrogen Peroxide Production”
Angew. Chem. Int. Ed. 58, 1100–1105 (2019).
[9] Sungeun Jeoung, In Tae Ju, Jae Hyung Kim, Sang Hoon Joo*, and Hoi Ri Moon*
“Hierarchically Porous Adamantane-Shaped Carbon Nanoframes”
J. Mater. Chem. A 6, 18906–18911 (2018).
[8] Jinwoo Woo, Young Jin Sa, Jae Hyung Kim, Hyun-Wook Lee, Chanho Pak, and Sang Hoon Joo*
“Impact of Textural Properties of Mesoporous Porphyrinic Carbon Electrocatalysts on Oxygen Reduction Reaction Activity”
ChemElectroChem 5, 1928–1936 (2018).
[7] Bora Seo, Gwan Yeong Jung, Jae Hyung Kim, Tae Joo Shin, Hu Young Jeong, Sang Kyu Kwak*, and Sang Hoon Joo*
“Preferential Horizontal Growth of Tungsten Sulfide on Carbon and Insight into Active Sulfur Site for the Hydrogen Evolution Reaction”
Nanoscale 10,3838–3848 (2018).
[6] Young Jin Sa, Jae Hyung Kim, and Sang Hoon Joo*
“Recent Progress in the Identification of Active Sites in Pyrolyzed Fe‒N/C Catalysts and Insights into Their Role in Oxygen Reduction Reaction”
J. Electrochem. Sci. Technol. 8, 169–182 (2017).
[5] Jae Hyung Kim, Young Jin Sa, Hu Young Jeong, and Sang Hoon Joo*
“Roles of Fe–Nx and Fe–Fe3C@C Species in Fe–N/C Electrocatalysts for Oxygen Reduction Reaction”
ACS Appl. Mater. Interfaces 9, 9567–9575 (2017).
[4] Sungeun Jeoung, Sun Hye Sahgong, Jae Hyung Kim, Soo Min Hwang,* Youngsik Kim and Hoi Ri Moon*
“Upcycling of nonporous coordination polymers: controllable-conversion toward porosity-tuned N- doped carbons and their electrocatalytic activity in seawater batteries”
J. Mater. Chem. A 4, 13468–13475 (2016).