General Introduction
Methane Conversion in Heterogeneous Catalysts
- C–H activation of methane
- Oxidative methane conversion
- Dry reforming of methane
The standard Gibbs free energy of reactions at 1073 K represents the change in oxidation trends depending on the oxidants. Methane consumption rate (b) as a function of time on stream and (c) amount of Ni surface. d) Ratio of band intensities D and G (ID/IG) and temperature for coke elimination of spent catalysts depending on Ni and Fe ratios.
Thermally Stable Nanocatalysts
- Nanocatalysts
- Synthetic strategies for stable catalysts
In addition, catalytic activity and stability according to the surface morphology108 and the confinement effect109,110 in the pore system of the support are also studied. CeO2–Pt@mSiO2 nanoparticles without Co particles on the surface showed no activity of the reaction.
Outline of This Dissertation
It can also be developed for tandem catalysis, where the catalyst is carried out continuously in the core and shell. To solve this problem, an Al2O3 shell was introduced on the surface of the nanocatalyst in the same way as in Chapter 3.
Bell, Investigation of the elementary processes involved in the selective oxidation of methane over MoOx/SiO2. Comparison of the performance of vanadium-supported catalysts for methane oxidation with formaldehyde reported in the literature. The conversion of CH4 and CO2 varied depending on the thickness of the Al2O3 shell in the Ni/CeO2/Al2O3 catalyst.
The change in the oxidation state of the Ni/CeO2/Al2O3 catalysts was investigated using XPS.
Methane Oxidation to Formaldehyde over Vanadium Oxide Supported on Various
Experimental Methods
- Preparation of m-SiO 2 (SBA-15 and MCF-17)
- Fabrication of VO x /m-SiO 2 catalysts
- Characterization
- Methane oxidation
Subsequently, 84 ml of tetraethylorthosilicate (Sigma-Aldrich, 98%) in a separatory funnel was added to the solution mixture and kept at 35 °C overnight without stirring. Before the hydrothermal process at 80 °C, 0.46 g of ammonium fluoride (Sigma-Aldrich, 98%) was added as a mineralizing agent to the solution. For DI, VO(acac)2 and m-SiO2 were mixed according to the metal loading in a mortar.
The reaction temperature was detected by a K-type thermocouple closely attached to the inside of the catalyst bed covered by the furnace.
Results and Discussion
- Structural characterizations of VO x /m-SiO 2 catalysts
- Active vanadium phases of VO x /m-SiO 2 catalysts
- Methane oxidation to formaldehyde
The surface areas and pore diameters of the VOx/MCF-17 and VOx/SBA-15 catalysts prepared by the DI method have similar results to those of the WI method (Figure 2.2c and Table 2.1). However, VOx/MCF-17(DI) catalysts with a higher surface area show distinct peaks at 1040 cm-1, revealing the presence of VO4 species regardless of the vanadium content. Depending on the synthesis method, the distributions of surface vanadium species are different in VOx/m-SiO2. a) Catalyst results of methane oxidation to formaldehyde over VOx/m-SiO2 catalysts prepared by (a) WI and (b) DI methods.
The Raman spectroscopy in Figure 2.7 shows a clear V=O bond pathway of the isolated species in all VOx/MCF-17(DI) catalysts that recorded high CH4 conversion regardless of vanadium loading.
Conclusion
This leads to a mixture of monomeric VO4 species and bulk structure despite the low vanadium loading in the calcination process, and most of the vanadium loading can be agglomerated. On the other hand, in the DI method, heat treatment continues immediately after solid mixing. In particular, VOx/m-SiO2 prepared by using MCF-17 via the DI method has been shown to be the best catalyst for the direct conversion of methane oxidation to formaldehyde.
Abrupt changes were also found in other SiO2@V2O5@Al2O3 nanostructures with different Al2O3 shell thickness (Figure 3.9). The highly dispersed Td vanadium species with a V–O–Al bond of the SiO2@V2O5@Al2O3 core@shell nanostructures was not seen in the conventional V2O5/Al2O3 catalysts. Schematic representation of (a) rotating chamber-type ALD reactor for the preparation of Ni/CeO2/Al2O3 catalysts with controlled Al2O3 layers and (b) one cycle of the ALD process.
The data reveal that the extra Al2O3 shell of the Ni/CeO2/Al2O3 catalysts has a negative effect on the DRM reaction because Al2O3 covers the Ni active sites. The changes in the structure and surface state of the Ni/CeO2/Al2O3-(x) catalysts were investigated, depending on the thickness of the Al2O3 shell. The coke resistance of the Ni/CeO2/Al2O3 catalysts up to 800 °C was confirmed by TGA.
Experimental Methods
- Preparation of SiO 2 @V 2 O 5 nanostructures
- Preparation of SiO 2 @V 2 O 5 @Al 2 O 3 -(x) (x = 10, 30, 40, 50, 70, and 100) core@shell
- Preparation of mesoporous silica-supported V 2 O 5 catalysts
- Characterization
- Methane oxidation
Results and Discussion
- Preparation of SiO 2 @V 2 O 5 @Al 2 O 3 -(x) core@shell nanostructures
- Thermal stability of V 2 O 5 species in core@shell nanostructures
- Catalytic oxidation of methane to formaldehyde
- Characterization of SiO 2 @V 2 O 5 @Al 2 O 3 -(x) core@shell nanostructures
The surface morphology of SiO2@V2O5@Al2O3-(50) core@shell nanostructures was investigated by SEM, STEM and EDS. The shell thickness of the SiO2@V2O5@Al2O3 core nanostructures increased with the number of ALD cycles (Figure 3.5a). However, much weaker XRD peaks were observed for the SiO2@V2O5@Al2O3 core@shell nanostructures (Figure 3.5b), since the Al2O3 shell was not crystalline.
The calculated particle sizes of SiO2@V2O5 and SiO2@V2O5@Al2O3-(50) core@shell nanostructures derived from in situ XRD along the (110) peaks.
Conclusion
The combined results show that the V2O5 nanoparticles formed on SiO2 beads were identified as V2O5 crystalline species, and the additional Al2O3 SiO2@V2O5@Al2O3-(50) shells were shown to provide new surfaces to generate highly dispersed monomeric Td species by promoting interactions between Al2O3 and V2O5 nanoparticles during the ALD process (Figure 3.19). Based on the fact that the AlVO4 phase was present in the SiO2@V2O5@Al2O3 core@shell nanostructures at elevated temperature, the interaction between the Al2O3 and V2O5 nanoparticles created V–O–Al bridging bonds in AlVO4. As a result, the newly formed monomeric vanadium species Td linked by the V–O–Al bond facilitated methane oxidation at 600 °C and achieved high methane conversion.
To obtain a coke-resistant catalyst, the thickness of the Al2O3 layers coated on the Ni/CeO2 NPs is controlled by the ALD process with different numbers of cycles. To evaluate the stability and durability of the Ni/CeO2/Al2O3-(1) catalyst, the DRM reaction was carried out at 700 °C for 60 h under the same gas conditions. A slight increase in the weight of the Ni/CeO2 catalyst was observed at approx. 450 °C due to the oxidation of Ni to NiO,57 whereas no significant weight change was observed for the Ni/CeO2/Al2O3 catalysts.
Although the reduction of the Ni NPs was limited by Al2O3, the relative fraction of Ce3+ in the Ni/CeO2/Al2O3-(x) catalysts increased due to the formation of oxygen vacancies in CeO2.
Experimental Methods
- Preparation of Ni/CeO 2 NP catalysts
- Preparation of Ni/CeO 2 /Al 2 O 3 -(x) (x = 1, 2, 10) catalysts
- Characterization
- Dry reforming of methane
After adding 4 ml of trioctylphosphine (Sigma-Aldrich, 97%), the mixture was heated to 220 °C and held for 2 h. After cooling to room temperature, the black colloid was washed with 5 ml n-hexane and 50 ml isopropanol and precipitated by centrifugation. CeO2 NRs were synthesized by a hydrothermal method.71 Briefly, 1.74 g of cerium nitrate hexahydrate (Aldrich, 99%) and 19.2 g of sodium hydroxide were dissolved in 40 mL of H2O.
To prepare the supported Ni/CeO2 NP catalysts, 0.5 g of CeO2 NRs were dispersed in 10 mL of 1-octadecene (Aldrich, 90%) and the as-prepared colloid of Ni NPs dispersed in n-hexane mixed in a flask.
Results and Discussion
- Preparation of Ni/CeO 2 /Al 2 O 3 -(x) catalysts
- Catalytic reaction of Ni/CeO 2 /Al 2 O 3 -(x) catalysts
- Coke formation on the catalysts
- Characterization
The HAADF-STEM image and the corresponding EDS mapping image of the Ni/CeO2/Al2O3-(1) catalyst confirmed the distribution of the Al2O3 layer on the Ni/CeO2 NPs (Figure 4.3). However, as the number of ALD cycles increased from 1 to 10, the Al2O3 shell thickness for Ni/CeO2/Al2O3 increased, while the conversion of both CH4 and CO2 decreased. TEM images of spent Ni/CeO2 catalyst showing nanoparticle agglomeration and carbon growth on Ni.
The spent Ni/CeO2 catalyst was sealed by massive amounts of coke, accompanied by sintering (Figure 4.7), while no coke was deposited on the Ni/CeO2/Al2O3 catalysts, regardless of the thickness of the Al2O3 layer.
Conclusion
Basset, Synergetic effects leading to coke-resistant NiCo bimetallic catalysts for dry reforming of methane, ChemCatChem. Kawi, Anti-coking Ni/SiO2 catalyst for dry reforming of methane: Role of oleylamine/oleic acid organic pair, Chemcatchem. Wang , Ni/La2O3 catalysts for dry reforming of methane: Insights into the factors that improve the catalytic performance ChemCatChem.
Noronha, Ni nanoparticles embedded in CeZrO2 as stable catalyst for dry methane reforming, Appl.
Summary and Suggestions for Future Works
Suggestions for Future Works
The carriers require properties such as high thermal stability, SMSI formation and providing oxygen atoms in oxygen vacancies or chemical intermediate phases that react with CO2. Studies aimed at producing monodisperse nanocarbon materials, called single-chirality, have been carried out using various carbon sources such as ethylene and ethanol.2 Methane sources are also grown on the catalyst surface in the form of CNTs in the CVD reactors. Potential candidates for CDM catalysts that can form carbides such as Fe, Co can be mentioned.
Ni on the support, or refractory metals of W, Mo, Re, V, Nb, Ru, Rh and Os.3,4 In addition, research into the direct conversion of raw materials such as natural gas or shale gas will be an important alternative to the exploitation of oil resources .
지난 6년 동안 대학원 생활을 하면서 연구뿐만 아니라 생활 속에서도 항상 모범적인 답변에 앞장서 주시는 지도교수님 안광진 교수님께 많은 것을 배울 수 있었던 것에 진심으로 감사드립니다. 나는 항상 닮고 싶은 세상입니다. 제가 지금까지 연구와 과제를 수행할 수 있었던 것은 공부하는 동안 많은 분들의 도움 덕분이었습니다. 메탄 연구에 큰 도움을 준 박은덕 교수, ALD 장비 연구에 큰 공헌을 한 정윤석 교수, 촉매 분석에 대해 심도 있는 조언을 해준 곽자훈 교수, 박사. 초기 연구에 많은 도움을 주신 이만식님. 메탄 열분해 반응에 대해 많은 연구를 해주신 이창영 교수님께 감사드리며, 메탄 개질반응 연구에 많은 도움을 주신 고창현 교수님께 감사드립니다.
같은 반 친구로 대학원을 다녔고, 이제 졸업하고 사업을 하게 된 큰형 신명형. 연구실 내 사실상 동창이자 현재 가장 열심히 일하고 있는 준경과 지현. 박사. 해외에서도 연구실의 유일한 의사로 열심히 일하고 있는 진투님, 큰 다툼 없이 오랫동안 함께해주셔서 저희 연구실의 최강자 호정님을 비롯해 모든 분들께 감사드립니다. .
