이온-전자 혼합 전도성 지지체를 이용한 철산화물의 산화환원 반응성 향상

다국어 초록 (Multilingual Abstract)

Chemical-Looping process for hydrogen production efficiently converts carbonaceous fuels into hydrogen with the simultaneous capture of concentrated CO2 and forms heat to produce electricity by steam turbine. This process consists of reduction reactor, steam reactor and oxidation reactor and uses an oxygen carrier.
The oxygen carrier typically consists of an metal oxide and support. In the reduction reactor, carbonaceous fuels such as methane, syngas, and coal react with oxygen carriers. Reduced oxygen carriers are oxidized with steam and produce hydrogen in steam reactor. Partial oxidized oxygen carriers are completely oxidized with air in oxidation reactor. In this process, oxygen carriers are recirculated among the reactors through multiple redox reaction.
This process efficiency and the heat/mass balance largely depend on the selection of the oxygen carrier. The iron oxide redox pair (Fe2O3 ↔ Fe) is suitable for conversion of methane into syngas of hydrogen from the thermodynamic equilibrium and catalytic activity. The use of pure iron oxide as an oxygen carrier suffers from the decrease in the reactivity and the defluidization resulting from the sintering of iron oxide during the repeated redox cycling. In this paper, inert materials such as α-Al2O3, ZrO2, SiO2 and MgO were used as support to afford high dispersion of metal oxides. These materials enhanced the reactivity by providing macro- and micro inner pores for inner gas diffusion. Mixed ionic-electronic conductors (MIECs) are widely used in semiconductors, electrochemical energy storage materials, electrodes of fuel cells and batteries, separation membranes and catalysts with various requirement for chemical, electrical, thermal and mechanical properties. The enhanced reactivity was mainly ascribed to the induced migration of charging species (ions in the other one, electrons in one direction) inside the material without any electrical connection. The MIECs largely consist of perovskite-based and fluorite-based materials. The MIECs based on YSZ is expected to show high mixed conductivity and structural stability because YSZ exhibits good thermal and chemical stability, high oxide ion conductivity, and mechanical strength at high temperature. The addition of TiO2 to YSZ has been reported to enhance the electrical conductivity under the reducing conditions because of electron hopping between Ti4+ and Ti3+ species via a small polaron mechanism. Therefore, the introduction of n-type conductor TiO2, into YSZ for use as oxygen carriers is a very promising strategy.
Herein, we prepared a TiO2 doped YSZ promoted iron-based oxygen carrier (Y0.2Ti0.15Zr0.65O2-δ and Y0.2Ti0.18Zr0.62O2-δ) with high mixed conductivity and redox stability, in which TiO2 is the electron conductor and YSZ is the oxygen-ion conductor. For the composite mixed conductor, mixed conductivity can be controlled by tuning the fraction of constituents. To determine the effects of each composition on the redox properties, the fraction of titanium was varied from 0.15 to 0.18. To compare structural stability and redox performance, various types of supporting materials were prepared such as ZrO2, YSZ, TiO2, Y0.2Ti0.15Zr0.65O2-δ and Y0.2Ti0.18Zr0.62O2-δ as the inert support, pure ionic support, MIECs support, respectively. The prepared oxygen carrier particles were subjected to 100 redox cycling by alternating carbon monoxide reduction step and the subsequent air oxidation step in a fixed bed reactor.
The morphology and crystalline structure were compared between fresh and reacted particles using various techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The reactivity of the particles was measured by TGA and fixed bed reactor. The solid conversion rates were determined by varying the reducing gases (10% CH4, CO and balanced with Ar) and oxidizing gases (10% H2O and balanced with Ar) in TGA experiments.

목차 (Table of Contents)

• Ⅰ. 서 론 1
• Ⅱ. 이론적 배경 4
• 2.1. 열화학적 수소제조 4
• 2.2. 매체순환 수소제조공정 9
• 2.3. 산소공여입자 선택 12
• 2.4. 이온 전도성 및 전자 전도성 지지체의 특성 14
• 2.5. 활성 금속 산화물의 p-type 및 n-type 반응 특성 19
• Ⅲ. 실 험 21
• 3.1. 산소공여입자 제조 21
• 3.2. 산소공여입자 활성화 25
• 3.3. 환원 및 물분해 반응성 분석 27
• 3.4. 온도 시차 환원 특성 분석 29
• 3.5. 표면 및 단면 형상 분석 31
• 3.6. 결정구조 분석 31
• Ⅳ. 결과 및 고찰 32
• 4.1. 산소공여입자의 형상 및 결정구조 특성 32
• 4.1.1. 표면 및 단면 형상 특성 32
• 4.1.2. 결정구조 특성 50
• 4.2. 산소공여입자의 반응 특성 55
• 4.2.1. 비 등온 수소 환원 반응 55
• 4.2.2. 등온 메탄 환원 반응 59
• 4.2.3. 등온 일산화탄소 환원 반응 63
• 4.2.4. 등온 물분해 반응 67
• Ⅴ. 결 론 71
• Ⅵ. 참고 문헌 73
• ABSTRACT 77