1,721,030 research outputs found
Surface Properties of Al2O3 for Understanding Metal-Support Interaction and Catalytic Properties of Al2O3-based Catalysts
Full text linkDepartment of Chemical Engineering??-Al2O3 is one of the most important heterogeneous catalytic materials that has been used as a support for a wide range of applications in petroleum chemistry and automobile emission control. Thanks to its intrinsic acidity, it can also participate in acid-catalyzed reactions such as alcohol dehydration. For decades, numerous studies have been devoted to understanding the fundamental nature of Al2O3 owing to its practical importance in catalysis. The catalytic properties of oxides, such as activity, selectivity, and distribution of active phases on the support, are significantly affected by their surface characteristics because chemical processes mainly occur on the surface. However, understanding the surface properties of aluminas have been challenging due to unresolved structures, mixed effects of various factors (morphology, phases, impurities, and additives), and difficulties of characterization. Therefore, comprehensive approaches should include the preparation of well-defined model aluminas and sensitive characterization tools. This thesis focuses on understanding the surface properties of aluminas and their role in the interaction with the active phases (Pt) and the corresponding catalytic behavior of Al2O3-based catalysts. By preparing the model aluminas for morphology, phase, additive and using ethanol TPD as a surface characterization tool, we studied the surface properties of Al2O3 with various modification and further investigated the interaction with Pt and its reduction behavior on alumina surfaces.
First, we synthesized a series of well-defined platelet ??-Al2O3 with the systematic change of (100) facets and investigated the catalytic role of (100) facets by ethanol dehydration. By controlling pH during the preparation of boehmite (??-AlOOH), precursor for ??-Al2O3, (001) facet of boehmite increased under acidic condition. Topotactic transformation of boehmite into ??-Al2O3 maintained their morphologies, leading to platelet ??-Al2O3 with increased (100) facets of ??-Al2O3. Ethylene formation increased with increasing (100) facets, clearly demonstrating the critical role of (100) facets as active sites for ethanol dehydration on ??-Al2O3.
In chapter 3, We investigated the effect of morphology, phases, and additives on the surface properties of Al2O3 and which factors are the most important for surface properties of Al2O3 by preparing model aluminas with various modifications and using ethanol TPD. Ethanol TPD showed the desorption temperature (at a maximum rate of ethylene desorption, Td) of dissociative ethanol was significantly dependent on morphology, crystalline phase, and additives. Here, the additives affected the desorption temperature of ethylene most significantly. Ethanol dehydration tests showed that ethylene formation rates, normalized with respect to the amount of dissociative ethanol (quantified by ethanol TPD), exhibited an inverse correlation with Td on Al2O3 with various morphologies, crystalline phases, and additives, which suggests that Td can be used as a descriptor for surface properties of Al2O3, irrespective of modification origins. The activities and activation barriers of various commercial Al2O3 on ethanol dehydration were also consistent with our empirical model.
Based on understanding for surface properties of Al2O3, we study how these surface properties of aluminas affect the active phases (Pt) and the catalytic behavior of Pt/Al2O3. We chose two sets of model alumina having different number of sites with the identical properties and different properties of sites with the same number based on ethanol TPD. With two model aluminas, how the number and properties of specific sites on alumina surfaces affect the specific interaction between Pt and alumina was investigated. Pt showed higher dispersion with increasing number of sites and interaction strength because the Pt atoms can interact with specific sites on alumina in greater numbers and more strongly. However, these Pt dispersion changes do not represent the gradual size change, but the relative population change of small (10 nm). The number of highly dispersed Pt clusters increased with increasing number of sites and interaction strength. When Pt showed higher dispersion from more number of sites and stronger interaction on alumina, Pt/Al2O3 showed higher activity for benzene hydrogenation due to more available highly dispersed Pt.
Finally, specific interaction between Pt and alumina strongly affected the reduction behavior of oxidized Pt on alumina. We studied the reduction of three-dimensional (3D) PtO2, two-dimensional (2D) PtO2, and atomically dispersed Pt on Pt/Al2O3. Under oxidizing atmosphere, morphologies and sizes of PtO2 on Pt/Al2O3 are determined by the specific interaction between Pt and Al2O3 through Pt-O-Al bond, leading to highly dispersed Pt as ~1 nm 2D-raft PtO2 and atomically dispersed Pt on Al2O3. When all the Pt atoms can???t interact with anchoring sites on support under high Pt loading or SiO2 which has very weak metal-support interaction, 3D PtO2 was formed. Due to weak interaction with support, 3D PtO2 was reduced earlier (-20~-60 oC) than 2D PtO2 (~110 oC) and atomically dispersed Pt (>300 oC). Furthermore, when Pt/Al2O3 was calcined at 500-700 oC, PtO2 was reduced without a reducing agent (auto-reduction) with Pt sintering. Interaction strength also influences auto-reduction when calcination temperature increases. 3D PtO2 was auto-reduced after 550-600 oC calcination, but 2D PtO2 was more difficult to be reduced (50 oC higher). So, the interaction strength with the support determines how long Pt oxide can maintain as Pt oxide rather Pt metal. These morphologies of Pt oxide also affect their sintering behavior. Because 3D PtO2 is more easily auto-reduced than 2D PtO2 and atomically dispersed Pt, 3D metallic Pt clusters become less mobile than 2D PtO2 and atomically dispersed Pt (still, oxidized Pt). So, 3D PtO2 showed more sinter-resistant behavior than 2D PtO2 and atomically dispersed Pt. The metal-support interaction between Pt and alumina is important for the reduction of oxidized Pt on Pt/Al2O3, suggesting the guideline about a careful activation for the efficient utilization of metallic Pt for catalytic reactions.clos
Fundamental understanding on catalytic behavior of single-atom catalysts supported on CeO2
No full textSchool of Energy and Chemical Engineering (Chemical Engineering)ope
Understanding the catalytic behavior of supported Pd catalysts as an oxidation catalyst
No full textSchool of Energy and Chemical Engineering (Chemical Engineering)clos
Understanding the Catalytic Behavior of Pt-based Bimetallic Catalysts
Full text linkSchool of Energy and Chemical Engineering (Chemical Engineering)Pt-based catalysts have been well known for its high activities in various reactions. However, due to its scarcity in Earth's crust, high cost, and poor thermal stability, various ways for an effective utilization of platinum have been developed and investigated. One of the well-known methods is adding the secondary metal to Pt-based catalyst like alkali, alkali earth, and transition metals etc.., leading to the improved catalytic activity and stability of Pt supported catalysts. Each added the secondary metal has various roles in the catalytic behavior like changing the physio-chemical properties of loaded metal and support, changing metal-support interaction, or providing oxygen. Therefore, it???s important to choose the right additive for the desired purpose of catalytic behavior based on the understanding the catalyst??? properties in both monometallic and bimetallic catalysts. And thanks to many devotes to develop the Pt-based bimetallic catalysts for the desired purpose by showing an improved activity and durability, the bimetallic catalysts have been widely used in commercial including propane dehydrogenation (PDH), reforming, natural gas vehicle (NGV), and diesel oxidation catalyst (DOC).
In this thesis, various Pt-based bimetallic catalysts were prepared by the addition of secondary metals that Sn, Mn, and Pd to Pt/Al2O3 catalysts. And PDH, HC/CO/NO oxidation, and CH4 oxidation were conducted as a model reaction with PtSn/Al2O3, Pt/Mn-Al2O3, and Pt-Pd/Al2O3 catalysts, respectively. I tried to correlate the properties of Pt-based bimetallic catalyst with the catalytic behavior in each reaction. In order to investigate the surface properties of the bimetallic catalysts, I used diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) study with CO as a probe molecule. Through DRIFTS study by editing adsorption procedure and controlling pretreatment condition, the roles of alumina surface properties, interfacial sites, and the surface composition of bimetals were elucidated in each chapter, which were the decisive properties of each catalyst for determining the catalytic behavior in each reaction.
In chapter 2, we studied the effect of acid-base properties of alumina surface on metal sintering and coke deposition in PDH using PtSn/Al2O3 catalysts. For this, two different kinds of ??-Al2O3 that synthesized from ammonium aluminum carbonate hydroxide (A750) or commercial (P200) were used, which exhibited significantly different surface properties, despite the same bulk properties. Then, 0.5 wt.% of Pt and 0.9 wt.% of Sn were loaded on these aluminas (PtSn/A750 and PtSn/P200) and PDH was conducted on these catalysts. Two PtSn/Al2O3 catalysts showed totally different catalytic behavior in metal sintering and coke deposition, which were affected by the surface properties of ??-Al2O3. PtSn/A750 showed a significantly improved stability by inhibiting metal sintering and coke deposition due to the stronger but less amounts of Lewis acid sites on A750 after metal loading than those on P200. Finally, through IR spectra of adsorbed CO on PtSn/Al2O3 catalyst obtained at -150 ??C after saturation of the metal surface, we clearly demonstrated that coke precursors were initially induced on Lewis acid sites on the alumina surface. Strong but less amount of residual Lewis acid sites on alumina surface are needed to inhibit the metal sintering and reduce coke deposition for designing stable PtSn/Al2O3 catalyst.
In chapter 3, we report significantly improved activities of Mn modified Pt/Al2O3 catalyst in HC/CO/NO oxidation. Mn-modified Pt/Al2O3 showed much lower light-off temperatures in the oxidation of HC, CO, and NO than those of the unmodified Pt/Al2O3 catalyst, by as much as 30 ??C. Through XRD and TEM analysis, the size of Pt particles was estimated, resulting in reduced Pt particle size after Mn modification. The interaction between CO and Pt on Mn-doped catalysts was also weakened confirmed by CO-TPD. Furthermore, DRIFTS study demonstrated weakly adsorbed CO on Pt was readily oxidized even at room temperature, which was induced at Pt-Mn interfacial sites through provided oxygen nearby manganese oxide. The results give an insight towards the improvement of the activity of Pt-based DOCs used for HC, CO, and NO oxidation, by enhancing the fundamental understanding of the Pt nature promoted with Mn.
In chapter 4, we report partially oxidized palladium (PdOx) in Pd and PtPd bimetallic catalysts shows a linear correlation with CH4 oxidation activity. We characterized the amount of surface PdOx through DRIFTS study using CO as a probe molecule. With a careful consideration that the redox cycle of Pd particles occurs continuously during CH4 combustion, the Pd/Al2O3 catalysts were re-oxidized before obtaining IR spectra of CO adsorption to minimize the discrepancy between catalytically relevant phase and the characterized surface composition. From IR spectra of CO adsorption on re-oxidized Pd/Al2O3 catalysts, we quantified surface PdOx, of which peak appears at 2135~2145 cm-1, and correlate with the steady-state CH4 oxidation activities at 300 ??C. Furthermore, by applying this method to Pt-Pd bimetallic catalysts, we could generalize the correlation between the surface PdOx and CH4 oxidation activity irrespective of composition, preparation method, and support. These results suggest that CH4 oxidation takes place on PdOx through redox cycle and the amount of surface PdOx gives a critical role of determining CH4 oxidation activity rather than electronic properties of Pd. By mimicking the catalytically relevant phase of Pd and PtPd bimetallic particles, we could obtain the general correlation between surface PdOx and CH4 oxidation activity. This work can greatly help the fundamental understanding of CH4 oxidation reaction on Pd-based catalysts and further the development of Pd-based catalysts in methane combustion with better activity.clos
NMR을 이용한 구리 이온 교환된 제올라이트의 특성 연구
No full text학위논문(석사) - 한국과학기술원 : 화학과, 1993.2, [ vi, 45 p. ] NMR and xenon adsorption methods have been used for characterization of NaX zeolite exchanged with ion. The chemical shift in NMR spectrum for the adsorbed xenon appeared to decrease gradually as the ion exchange level of was increased, without a significant change in the pressure dependence. This result suggests that the copper ion was located in a site which was inaccessible for xenon collision. Due to a paramagnetic center at the ion, the NMR line width was increased with the Cu loading. Because of two ions became to substitute for one copper ion, the adsorbed xenon quantity seemed to decrease linearly with increasing the copper ion exchange level. Magic angle spinning (MAS) solid state NMR has also been used for investigation of ion exchange site in faujasitetype zeolites. When the ion was exchanged as aqueous complex, the paramagnetic ion locally affected the line width in the MAS NMR spectrum. This line broadening was very selective and characteristic of the ion exchange level. The site preference for ion was etc. On the contrary, if ion was exchanged as ammine form, the line width variation was not selective. The result was due to the ion exchange of at a site which was composed of two tetrahedra bridged by a Si atom, and therefore the paramagnetic effect was concentrated at the silicon atom. However, the ammine form seemed to lose the selectivity because it was exchanged as an octahedral complex.한국과학기술원 : 화학과
티타늄 알루미노실리케이트 제올라이트의 합성과 특성에 대한 연구
No full text학위논문(박사) - 한국과학기술원 : 화학과, 1996.2, [ x, 101 p. ]Titanium containing molecular sieves are important catalytic materials with remarkable activity for partial oxidation of organic compounds. Since, Taramasso et al. reported the discovery of TS-1 in 1983, various efforts have been made to understand the origin of the catalytic activity and to obtain new molecular sieves containing titanium in the framework. However, only a few other molecular sieves with silica frameworks have been found to have the catalytic activity so far. Previous works have revealed that the loss of the catalytic activity with other synthesis attempt was due to the formation of a separate domain during the synthesis under experimental conditions, high levels of alkali metal cations.
In the present work, a new synthesis procedure for titanium-containing aluminosilicate zeolites has been obtained using a clear colloidal dispersion (sol) which was prepared with titanium isopropoxide, water, hydrochloric acid and colloidal silica, Ludox HS-40. Various titanium aluminosilicates such as NaY, KL, offretite, mordenite, ZSM-5 and silicalite have been synthesized from the titania-silica sol thus obtained, following conventional hydrothermal crystallization procedures. All the Ti-zeolites were characterized by X-ray diffraction patterns, infra-red spectroscopy, X-ray absorption spectroscopy and partial oxidation reaction.
Infrared spectra of the Ti-ZSM-5 and Ti-silicalite showed a band upon ion exchange with and subsequent evacuation at 673 K. The X-ray absorption near edge structure (XANES) obtained at the Ti K-edge of Ti-zeolites showed a pre-edge peak at 4967 eV, which was characteristic of tetra- or penta-coordinated titanium in the framework. The curve fitting for extended X-ray absorption fine structure (EXAFS) gave the Ti-O coordination number of 4.5±0.4 with a distance of 0.190±0.005nm. The Ti-zeolites in the present work showed remarkable catalytic activity for partial oxidation of cyclohexene and 2-butanol usin...한국과학기술원 : 화학과
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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