철-질소-탄소 촉매 내 산소 환원 반응 활성점 형성의 최적화를 위한 합성 설계

학위논문 (박사)-- 서울대학교 대학원 공과대학 화학생물공학부, 2017. 8. 성영은.고분자 전해질 연료전지는 수소와 산소의 화학적 에너지를 전기 에너지로 변환시켜주는 에너지 변환장치로써, 에너지 변환 효율이 높고 부산물로 공해 물질 없이 물만 배출하여 친환경 차세대 에너지 소자로 주목 받고 있다. 고분자 전해질 연료전지는 환원극에서의 느린 산소 환원 반응으로 인하여 전기화학 촉매를 필요로 하는데, 현재에는 백금 소재의 촉매가 상용적으로 사용되고 있다. 하지만 비싼 가격과 희소성으로 인하여 백금을 촉매 재료로 사용하는 것은 고분자 전해질 막 연료전지의 상용화에 대한 걸림돌로 작용하고 있다. 따라서 백금을 대체할 차세대 촉매 물질에 대한 연구의 진행이 활발하게 이루어지고 있다. 비백금계 촉매로는 금속 산화물, 금속-질소-탄소 복합체 촉매, 질소-탄소 복합체 촉매 등이 연구되고 있으며, 이 중에서도 철-질소-탄소 촉매들이 가장 우수한 성능을 보고하고 있다. 이러한 철-질소-탄소 촉매들은 일반적으로 철, 질소, 탄소 전구체의 혼합물을 고온에서 열처리함으로써 합성할 수 있다. 최근의 분광학 연구 결과들을 종합하여 보면, 철-질소-탄소 촉매의 활성점은 탄소의 미소 기공 내에 형성된 철-질소 배위 구조에 기반하고 있음을 알 수 있다. 하지만, 이러한 활성점을 형성하기 위한 포괄적인 합성 방법 혹은 가이드 라인은 아직 확립되지 못한 상황이다. 이에 본 연구에서는 체계적인 실험을 통해 철-질소-탄소 촉매 합성에 대한 전구체 및 탄소 담지체의 효과를 조사하고, 이를 통해 철-질소 활성점 형성의 최적화를 위한 합리적인 합성 설계를 제안하고자 하였다. 먼저 전구체 상태에서의 배위 결합의 유무가 철-질소-탄소 촉매에서의 활성점 형성에 미치는 영향을 확인하는 실험을 진행하였다. 질소의 치환 위치에 따른 구조 이성질체를 갖는 페난트롤린들을 질소 전구체로 사용할 때, 1,10-페난트롤린만이 철과 배위 결합을 하고, 1,7-페난트롤린과 4,7-페난트롤린은 배위 결합을 하지 않는 것이 자외선 가시광 흡수 분광법을 통해 확인할 수 있었다. 이 전구체들을 통하여 철-질소-탄소 촉매를 합성하였을 때, 배위 결합을 한 1,10-페난트롤린을 사용한 것이 산소 환원 반응 성능이 월등히 높았다. 투과 전자 현미경 및 엑스선 회절 분석을 통해 확인한 결과, 배위 결합을 한 전구체를 사용한 것은 철이 원자 단위로 잘 분산되어 있는 반면에 배위 결합을 하지 않은 전구체를 사용한 것은 철이 덩어리로 뭉치는 것을 확인하였다. 또한, 엑스선 광전자 분광법을 통해 분석한 결과, 배위 결합을 한 전구체를 사용한 경우에 피리디닉 질소의 비율이 크게 증가한 것을 확인하였다. 이를 통해 전구체내 배위 결합의 존재가 형성된 활성점의 질적 향상을 이루었음을 알 수 있었다. 다음으로 탄소 담지체의 기공 구조가 철-질소-탄소 촉매의 활성점 형성에 어떠한 영향을 미치는지를 확인하는 실험을 진행하였다. 이산화탄소 활성화 처리 시간으로 기공 구조가 조절된 탄소 담지체에, 동일한 양의 철과 질소 전구체를 함침한 후에 고온 열처리를 통해 촉매를 합성한 결과, 미소 기공률이 더 발달한 탄소 담지체를 사용한 경우에 철과 질소의 함량, 특히 피리디닉 질소의 비율이 더 커진 것을 확인할 수 있었다. 또한, 산소 환원 반응 활성 역시 미소 기공률이 발달한 탄소 담지체를 통해 만든 촉매가 더 높은 것을 확인하였다. 이를 통해서, 탄소 담지체의 미소 기공률이 높을수록 만들어진 촉매의 활성점의 양적 향상이 이루어졌음을 알 수 있었다. 위 두 가지 실험을 종합하여 볼 때, 전구체내 배위 결합 형성을 통해 활성점의 질적 향상을, 탄소 담지체의 미소 기공률 증대를 통한 활성점의 양적 향상을 가져올 수 있음을 확인할 수 있다. 본 연구의 결과를 바탕으로 철-질소-탄소 촉매에 대한 합리적인 합성 설계를 구상할 수 있을 것이며, 최적의 성능을 갖는 철-질소-탄소 촉매를 실제로 합성하는 데에도 도움이 될 것으로 기대한다.Polymer electrolyte fuel cells (PEFCs), which covert chemical energy of hydrogen and oxygen to electrical energy without any pollutant emission, have been received great attention as sustainable and clean energy conversion devices. Due to the sluggish oxygen reduction reaction (ORR) kinetics at the cathode side, Pt-based electrochemical catalysts have been commercially utilized in PEFC systems. However, excessive cost and scarcity of Pt hinder the economic feasibility of commercial PEFC devises. Therefore, various studies have been conducted for replacing Pt with cheap and abundant materials, such as heteroatom doped carbon, metal and nitrogen codoped carbon (M-N-C), metal chalcogenide, and metal oxide. Among those materials, Fe-N-C catalysts have shown the best performance with reaching the level of Pt-based catalyst. These Fe-N-C catalysts are generally synthesized through the pyrolysis of a mixture of Fe, N and C precursors. Based on recent spectroscopic studies, the active site of these catalysts are proposed to have Fe-N4 coordination structure within the micropores of carbon matrix. However, there are no established guideline or general strategy yet for Fe-N-C catalyst synthesis with optimizing the Fe-N4 active site formation. In this thesis, I investigated the precursor and carbon support effect on Fe-N-C catalyst synthesis with systematic experiments, and suggested the rational synthesis design for optimizing the Fe-N4 ORR active sites. In the precursor study, the Fe-N coordination precursor effect on the active site formation was investigated. To clarify this effect, three phenanthroline isomers were used as N precursor, because only 1,10-phenanthroline forms coordination bonds with Fe and other two phenanthrolines doesnt form these bonds. I found that these coordination bonds suppressed the gasification of phenathroline precursor during high temperature pyrolysis, while it was completely gasified without Fe-N coordination. This suppression of precursor gasification due to coordination bonds resulted in an even distribution of Fe without particle aggregation. During heat treatment, furthermore, the Fe-N coordination helped N to be located in the pyridinic N position, which is more favorable for active site formation than other N positions. Due to the above effects, Fe-N-C catalysts derived from 1,10-phenanthroline precursor showed much superior ORR activity compared to catalysts from other phenanthrolines. This results suggest that the utilization of Fe-N coordinated precursors can be a good strategy to design an optimized Fe-N-C catalysts with increasing the possibility of Fe-N4 active site formation. In carbon support study, the role of pre-defined microporosity of carbon supports in ORR catalytic site formation was investigated. The microporosity of the carbon supports are initially controlled by using hot CO2 treatment. Then, Fe and N are doped into these supports by precursor impregnation and subsequent pyrolysis. In the synthesized Fe–N–C catalysts, the more developed microporosity in the parent carbon supports facilitates more iron and nitrogen contents, especially pyridinic nitrogen, and Fe–N–Cs derived from carbon supports with higher microporosities show enhanced ORR activity, strongly suggesting that a high catalytic site density can be achieved by utilizing carbon supports with well-developed microporosities. This results indicate that development of microporosity of carbon supports can be another strategy to get highly active Fe-N-C catalysts with enlarging the Fe-N4 active site density.Chapter 1 Introduction 1 1.1 Fuel Cells 1 1.1.1 Principle of Fuel Cell 2 1.1.2 Types of Fuel Cells 4 1.2 Polymer Electrolyte Fuel Cells (PEFCs) 6 1.2.1 Components of PEFC 8 1.2.2 Overpotentials in PEFC 10 1.2.3 Catalysts for Oxygen Reduction Reaction (ORR) 14 1.3 Fe-N-C ORR Catalysts 14 1.3.1 Synthesis Approaches of Fe-N-C Catalysts 14 1.3.2 Nature of Active Sites in Fe-N-C Catalysts 18 1.3.3 Challenges in Fe-N-C Catalyst Synthesis 23 1.4 Subjects of This Thesis 25 1.4.1 Precursor Study 25 1.4.2 Carbon Support Study 26 Chapter 2 Experimental 28 2.1 Precursor Study 28 2.1.1 Chemicals and Materials 28 2.1.2 Precursor Variation and Preparation of Fe-N-C Catalysts 28 2.1.3 Material Characterizations 31 2.1.4 Electrochemical Measurements 32 2.2 Carbon Support Study 34 2.2.1 Chemical and Materials 34 2.2.2 Preparation of Carbon Supports 34 2.2.3 Preparation of Fe-N-C Catalysts 35 2.2.4 Material Characterizations 37 2.2.5 Electrochemical Measurements 37 Chapter 3 Results and Discussion 40 3.1 Precursor Study 40 3.1.1 Precursor: Coordination State 40 3.1.2 Physicochemical Characterizations of Fe-N-C Catalysts 44 3.1.3 Electrochemical Measurements 56 3.1.4 Precursor Coordination Effect on Active Site Formation 66 3.2 Carbon Support Study 68 3.2.1 Carbon Support: Pore Structure 68 3.2.2 Physical Characterizations of Fe-N-C Catalysts 75 3.2.3 Electrochemical Measurements 88 3.2.4 Carbon Pore Structure Effect on Active Site Formation 97 Chapter 4 Conclusions 101 References 104Docto

The Potential Role of an Adjunctive Real-Time Locating System in Preventing Secondary Transmission of SARS-CoV-2 in a Hospital Environment: Retrospective Case-Control Study

Background: There has been an increasing demand for new technologies regarding infection control in hospital settings to reduce the burden of contact tracing. Objective: This study aimed to compare the validity of a real-time locating system (RTLS) with that of the conventional contact tracing method for identifying high-risk contact cases associated with the secondary transmission of SARS-CoV-2. Methods: A retrospective case-control study involving in-hospital contact cases of confirmed COVID-19 patients, who were diagnosed from January 23 to March 25, 2022, was conducted at a university hospital in South Korea. Contact cases were identified using either the conventional method or the RTLS. The primary endpoint of this study was secondary transmission of SARS-CoV-2 among contact cases. Univariate and multivariable logistic regression analysis comparing test positive and versus negative contact cases were performed. Results: Overall, 509 and 653 cases were confirmed by the conventional method and the RTLS, respectively. Only 74 contact cases were identified by both methods, which could be attributed to the limitations of each method. Sensitivity was higher for the RTLS tracing method (653/1088, 60.0%) than the conventional tracing method (509/1088, 46.8%) considering all contact cases identified by both methods. The secondary transmission rate in the RTLS model was 8.1%, while that in the conventional model was 5.3%. The multivariable logistic regression model revealed that the RTLS was more capable of detecting secondary transmission than the conventional method (adjusted odds ratio 6.15, 95% CI 1.92-28.69; P=.007). Conclusions: This study showed that the RTLS is beneficial when used as an adjunctive approach to the conventional method for contact tracing associated with secondary transmission. However, the RTLS cannot completely replace traditional contact tracing.ope

Cataract subtype risk factors identified from the Korea National Health and Nutrition Examination survey 2008–2010

BACKGROUND: To assess the socio-demographic and health-related risk factors associated with cataract subtypes in Korea. METHODS: A total of 11,591 participants (aged ≥40 years) were selected from the Korean National Health and Nutrition Examination Survey between 2008 and 2010. The Korean Ophthalmologic Society conducted detailed ophthalmologic examinations on these participants based on the Lens Opacity Classification System III. Risk factors for developing any type of cataract, and its subtypes (nuclear, cortical, posterior subcapsular and mixed), were identified from univariate and multivariate logistic regression analysis. RESULTS: The prevalence of cataracts was 40.1% (95% CI, 37.8 - 42.3%) in participants over 40 years old. Older age, lower monthly household income, lower education, hypercholesterolemia, hypertension, and diabetes mellitus (DM) were independent risk factors for development of any cataract. Older age, lower monthly household income, lower education, hypercholesterolemia, and DM were independent risk factors for development of pure cortical cataracts. Older age, lower education, metabolic syndrome, and DM were independent risk factors for development of pure nuclear cataracts. Older age and DM were independent risk factors for development of pure posterior subcapsular cataracts. Older age, lower monthly household income, lower education, and DM were independent risk factors for development of mixed cataracts. CONCLUSION: Although socioeconomic disparities are related to cataract development, this study identified several "modifiable" risk factors that may help to lower the incidence of cataracts and associated vision loss. Improved control of blood pressure, blood, glucose, and cholesterol may help to reduce the incidence of cataracts in the general Korean population.ope

Description of antibiotic treatment in adults tested for Clostridioides difficile infection: a single-center case-control study

Background: Diagnosing Clostridioides difficile infection (CDI) is complicated. There have been reports on effects of compliance with anti-C. difficile prescription guidelines on patient outcomes. However, the causes of non-adherence and their impact on outcomes have rarely been explored. Therefore, an investigation on the risk factors for non-adherence with treatment guidelines and their influence on recurrence is important. Methods: This case-control study was conducted with patients with a positive C. difficile culture from March 2020 to April 2021. We conducted analysis based on treatment categories using factors associated with recurrent CDI as variables. Univariate and multivariable analyses were conducted to identify risk factors for non-adherence with treatment guidelines. Results: In total, culture positive stool samples from 172 patients were analyzed. Having positive glutamate dehydrogenase antigen (GDH Ag), negative toxin enzyme immunoassay (EIA), and positive nucleic acid amplification test (NAAT) (GDH+/toxin EIA-/NAAT +) results were associated with both under- (adjusted odds ratio [aOR] 3.49 [95% CI 1.62-7.51], p = 0.001) and over-treatment (aOR 0.17 [95% CI 0.06-0.48], p = 0.001). Patients with refractory diarrhea were over treated (aOR 2.71 [95% CI 1.02-7.20], p = 0.046). Patients with an increased risk of CDI recurrence were not over treated. Conclusions: Our results suggest that non-adherence with CDI treatment guidelines depends on the duration of symptoms and rapid EIA test results. Patients with an increased risk of recurrence were neglected.ope

Lessons Learned from an Experience with Vancomycin-Intermediate Staphylococcus aureus Outbreak in a Newly Built Secondary Hospital in Korea

A vancomycin-intermediate Staphylococcus aureus (VISA) outbreak occurred in an intensive care unit (ICU) in South Korea. We aimed to investigate the condition that led to the VISA outbreak and seek measures to prevent further spread of the multidrug-resistant organism. A total of three VISA isolates were obtained from two patients and a health care worker (HCW) in a newly built 450-bed secondary hospital. Extensive screening of close contacts for VISA in terms of space sharing and physical contact, irrespective of contact time, was performed. Furthermore, multilocus sequence type, staphylococcal cassette chromosome mec type, and spa type profiles were determined for all VISA isolates. The relationship between vancomycin use and the minimum inhibitory concentration (MIC) of S. aureus was also investigated. Molecular typing showed that the strains of the three VISA isolates were identical, indicating horizontal hospital transmission. We assumed that VISA colonised in the HCW could have transmitted to the two patients, which resulted in one infection and one colonisation. The affected HCW was excused from work and was decolonised with mupirocin. Five weeks after the interventions, no additional VISA isolates were identified. No relationship between vancomycin use and MIC of S. aureus was identified. Extensive screening of contacts in addition to decolonisation is crucial in preventing the further spread of VISA.ope

Mathematical Modeling of HIV Prevention Measures Including Pre-Exposure Prophylaxis on HIV Incidence in South Korea

BACKGROUND: Multiple prevention measures have the possibility of impacting HIV incidence in South Korea, including early diagnosis, early treatment, and pre-exposure prophylaxis (PrEP). We investigated how each of these interventions could impact the local HIV epidemic, especially among men who have sex with men (MSM), who have become the major risk group in South Korea. A mathematical model was used to estimate the effects of each these interventions on the HIV epidemic in South Korea over the next 40 years, as compared to the current situation. METHODS: We constructed a mathematical model of HIV infection among MSM in South Korea, dividing the MSM population into seven groups, and simulated the effects of early antiretroviral therapy (ART), early diagnosis, PrEP, and combination interventions on the incidence and prevalence of HIV infection, as compared to the current situation that would be expected without any new prevention measures. RESULTS: Overall, the model suggested that the most effective prevention measure would be PrEP. Even though PrEP effectiveness could be lessened by increased unsafe sex behavior, PrEP use was still more beneficial than the current situation. In the model, early diagnosis of HIV infection was also effectively decreased HIV incidence. However, early ART did not show considerable effectiveness. As expected, it would be most effective if all interventions (PrEP, early diagnosis and early treatment) were implemented together. CONCLUSIONS: This model suggests that PrEP and early diagnosis could be a very effective way to reduce HIV incidence in South Korea among MSM.ope

Clinical Implication and Prognostic Value of Artificial-Intelligence-Based Results of Chest Radiographs for Assessing Clinical Outcomes of COVID-19 Patients

This study aimed to investigate the clinical implications and prognostic value of artificial intelligence (AI)-based results for chest radiographs (CXR) in coronavirus disease 2019 (COVID-19) patients. Patients who were admitted due to COVID-19 from September 2021 to March 2022 were retrospectively included. A commercial AI-based software was used to assess CXR data for consolidation and pleural effusion scores. Clinical data, including laboratory results, were analyzed for possible prognostic factors. Total O2 supply period, the last SpO2 result, and deterioration were evaluated as prognostic indicators of treatment outcome. Generalized linear mixed model and regression tests were used to examine the prognostic value of CXR results. Among a total of 228 patients (mean 59.9 ± 18.8 years old), consolidation scores had a significant association with erythrocyte sedimentation rate and C-reactive protein changes, and initial consolidation scores were associated with the last SpO2 result (estimate −0.018, p = 0.024). All consolidation scores during admission showed significant association with the total O2 supply period and the last SpO2 result. Early changing degree of consolidation score showed an association with deterioration (odds ratio 1.017, 95% confidence interval 1.005–1.03). In conclusion, AI-based CXR results for consolidation have potential prognostic value for predicting treatment outcomes in COVID-13 patients.ope

A Case of Infective Endocarditis Caused by Community-Acquired Acinetobacter baumannii

Acinetobacter baumannii has become an important pathogen that causes healthcare-associated infections, including pneumonia, bacteremia, and urinary tract infection. Only sporadic cases of infective endocarditis (IE) have been reported worldwide, and in particular, community-acquired IE caused by A. baumannii is very rare. To our knowledge, no case has ever been reported in Korea. We report a 30-year-old woman who recovered from native-valve community-acquired IE caused by A. baumannii. She did not have any underlying diseases and presented with left-sided weakness, dysarthria, and fever. On investigation of acute cerebral infarction, a vegetation on her anterior mitral leaflet was found in echocardiography, and A. baumannii was cultured from the blood. In this case, A. baumannii showed intermediate resistance to cefotaxime and was sensitive to all other antibiotics. She underwent antibiotic therapy with cefoperazone/sulbactam for 5 weeks, without surgery. Consequently, the vegetation was absent in follow-up echocardiography, and no complications related to IE developed.ope

Vaccine Effect on Household Transmission of Omicron and Delta SARS-CoV-2 Variants

Background: We evaluated the household secondary attack rate (SAR) of the omicron and delta severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants, according to the vaccination status of the index case and household contacts; further, in vaccinated index cases, we evaluated the effect of the antibody levels on household transmission. Methods: A prospective cross-sectional study of 92 index cases and 197 quarantined household contacts was performed. Tests for SARS-CoV-2 variant type and antibody level were conducted in index cases, and results of polymerase chain reaction tests (during the quarantine period) were collected from contacts. Association of antibody levels in vaccinated index cases and SAR was evaluated by multivariate regression analysis. Results: The SAR was higher in households exposed to omicron variant (42%) than in those exposed to delta variant (27%) (P = 0.040). SAR was 35% and 23% for unvaccinated and vaccinated delta variant exposed contacts, respectively. SAR was 44% and 41% for unvaccinated and vaccinated omicron exposed contacts, respectively. Booster dose immunisation of contacts or vaccination of index cases reduced SAR of vaccinated omicron variant exposed contacts. In a model with adjustment, anti-receptor-binding domain antibody levels in vaccinated index cases were inversely correlated with household transmission of both delta and omicron variants. Neutralising antibody levels had a similar relationship. Conclusion: Immunisation of household members may help to mitigate the current pandemic.ope

Antibody Level Predicts the Clinical Course of Breakthrough Infection of COVID-19 Caused by Delta and Omicron Variants: A Prospective Cross-Sectional Study

Background: Omicron variant viruses spread rapidly, even in individuals with high vaccination rates. This study aimed to determine the utility of the antibody against spike protein level as a predictor of the disease course of coronavirus disease 2019 (COVID-19) in vaccinated patients. Methods: Between December 11, 2021, and February 10, 2022, we performed a prospective observational cohort study in South Korea, which included patients infected with Delta and Omicron variants. A multivariable logistic regression analysis to determine the association between antibody levels and outcomes was conducted. The relationship between antibody levels and cycle threshold (Ct) values was confirmed using a generalized linear model. Results: From 106 vaccinated patients (39 Delta and 67 Omicron), the geometric mean titers of antibodies in patients with fever (≥37.5°C), hypoxia (≤94% of SpO2), pneumonia, C-reactive protein (CRP) elevation (>8 mg/L), or lymphopenia (<1100 cells/μL) were 1201.5 U/mL, 98.8 U/mL, 774.1 U/mL, 1335.1 U/mL, and 1032.2 U/mL, respectively. Increased antibody levels were associated with a decrease in the occurrence of fever (adjusted odds ratio [aOR], 0.23; 95% CI, 0.12-0.51), hypoxia (aOR, 0.23; 95% CI, 0.08-0.7), CRP elevation (aOR, 0.52; 95% CI, 0.29-0.0.94), and lymphopenia (aOR, 0.57; 95% CI, 0.33-0.98). Ct values showed a positive correlation between antibody levels (P = .02). Conclusions: Antibody levels are predictive of the clinical course of COVID-19 in vaccinated patients with Delta and Omicron variant infections. Our data highlight the need for concentrated efforts to monitor patients with severe acute respiratory syndrome coronavirus 2 infection who are at risk of low antibody levels.ope