基于草木灰修饰Fe基载氧体的化学链燃烧实验

采用草木灰对Fe基载氧体进行修饰,并在流化床反应器上进行了气体燃料化学链燃烧实验。研究了草木灰修饰对提高Fe基载氧体还原活性的可行性,讨论了草木灰种类、草木灰的无机组分对载氧体活性的影响。结果表明,草木灰修饰能有效提高Fe载氧体的反应活性,载氧体的反应活性由草木灰中K和Si的含量共同决定。随着修饰草木灰中的K含量提高,载氧体的活性逐渐提高;但在Fe3O4/FeO的转化阶段中,同时存在碱金属K对还原反应的催化作用和低熔点碱金属硅酸盐对还原反应的抑制作用。循环实验表明,草木灰中碱金属K对载氧体活性的提高效果始终明显,载氧体中负载的K在循环过程中出现了流失现象,而生成的碱金属硅酸盐类化合物,可抑制碱金属K的流失。     

铁基载氧体的还原特性研究

采用热重分析仪和质谱仪联用对使用机械混合法制备Fe2O3和Al2O3载体的还原反应过程进行研究。还原反应中使用3种10%还原气体(CH4,H2,CO),氧化反应中使用5%氧气以避免较大的温升。从载体的还原失重曲线中可明显地看出铁基载体的还原过程分为3个阶段,且反应速率各不相同。还原的3个阶段中第一阶段的反应速率最快,且燃料能够完全被氧化生成CO2,随着反应进行速率降低,燃料不完全转化程度增加。通过XRD(X射线衍射)分析各个还原阶段的产物,发现与以前认识的载氧体活性相与惰性相不同,Al2O3在反应过程中会参与反应,生成新的化合物FeAl2O4,而此化合物不稳定能够进一步分解,被还原成Fe。3种还原气体中,H2的还原反应速率最快,并且无积碳,而CH4的还原反应中存在较为严重的积碳现象。     

化学链中Cu、Fe基复合氧载体各组分间相互作用的研究进展

化学链燃烧是一种新型的燃烧方式,既可以避免氮氧化物的产生,又可以实现二氧化碳的内分离,近年来受到了国内外研究人员的广泛关注.氧载体的反应性、循环稳定性以及载氧能力等性能,是化学链燃烧系统的重要影响因素.添加惰性载体和制备多活性组分复合氧载体都是可以有效提高氧载体性能的主要方式.为了解决氧载体中各组分之间反应降低氧载体性能的问题,本文就近年来国内外有关Cu基和Fe基复合氧载体的研究进行了总结和分析,发现TiO2和SiO2会与Fe2 O3形成尖晶石,不适合做Fe2 O3的载体;添加活性组分虽然可以使氧载体释氧速率和循环反应性增强,但也会和Fe2 O3或者CuO反应产生复杂物相,使氧载体反应活性下降或者烧结.在此基础上针对活性组分与惰性组分以及活性组分与活性组分之间的相互作用两个方面进行了综述和展望.     

镍修饰的铁基载氧体甲烷化学链制氢实验

化学链制氢工艺作为一种新型能源转化技术,能在实现CO_2内分离的同时,生产高纯氢气.为了提高铁基载氧体与甲烷的反应活性,本文添加第二活性组分NiO对铁基载氧体进行修饰,使用机械混合法制备载氧体材料.经NiO修饰后,载氧体的主要物相为Fe_2O_3、Al_2O_3与NiFeAlO_4.CH_4-TPSR实验表明,载氧体的还原反应分为两步:第一步为游离态的Fe_2O_3被还原为Fe_3O_4;第二步为Fe_3O_4与NiFeAlO_4被还原为单质态Fe与(Fe,Ni)合金.在TGA反应器中研究各载氧体的反应活性,结果表明,添加NiO修饰后,生成了高反应活性的NiFeAlO_4,显著提高了铁基载氧体与甲烷的反应活性.NiO修饰量分别为0、2.5%、5%、10%,在V_(CH_4)∶V_(CO_2)=2∶1条件下,各铁基载氧体的平均表观固相转化速率依次为2.6%/min、5.4%/min、7.2%/min、8.2%/min.固定床化学链制氢实验中,添加10%NiO的载氧体反应活性最高,且产氢性能良好.若进一步提高NiO修饰量,将降低铁基载氧体的产氢潜能,不利于化学链制氢工艺.     

Fe2O3负载MgO反应活性及表面形貌研究

铁镁载氧体实验中,随着MgO的添加,铁基载氧体中铁镁氧化物逐渐增多.Fe2O3负载MgO后,CO-TPR出峰时间变短,说明MgO能够提高Fe2O3与CO的反应活性.Fe8Mg2载氧体相对于实验中其他铁镁载氧体,TPR峰总面积最大,Fe8Mg2载氧体还原能力最强,还原效果最好.通过对铁镁载氧体形貌图对比研究,发现MgO的...     

钙基复合载氧体反应性能的研究

采用沉淀法制备了3种钙基复合载氧体,并在固定床试验台架上对复合载氧体与CO的反应性能进行了研究.对反应前后的载氧体进行了X射线衍射(XRD)和场发射扫描电镜(FSEM)表征分析.结果表明:3种添加物均能促进CaSO4与CO的还原反应,提高载氧体的反应速率和转化率;3种添加物对COS的释放具有抑制作用,La添加物对SO2的释放有一定的抑制作用,而Ti、Ni添加物促进了SO2的释放;添加Ni的载氧体具有较高的反应活性,表现出良好的循环特性;载氧体在1次循环反应后,固相中出现了少量的CaO;在6次循环反应后,CaO的峰值比第1次循环后的峰值高,载氧体表面由平整光滑变得疏松多孔,且分布均匀.     

Fe2O3为载氧体的煤/秸秆化学链燃烧循环特性研究

对煤、秸秆与Fe2O3以不同质量掺混比混合后化学链燃烧过程中载氧体还原/再生的多循环反应特性进行了研究,重点分析了固体燃料带入的灰分对化学链反应速率的影响以及秸秆的掺入对化学链反应的改善.结果表明：载氧体Fe2O3质量掺混比的增大有利于化学链反应的进行,燃烧起始反应温度降低;Fe2O3作为载氧体受灰分积累的影响较大,其可持续循环能力较差;煤中掺入秸秆改善了煤的化学链燃烧特性,提高了燃烧反应速率和载氧体的再生反应速率.     

Ni基和Co基金属载氧体的持续循环能力研究

化学链燃烧技术是一种新颖的二氧化碳分离技术,其中金属载氧体持续循环反应能力的优劣直接关系到该技术的实际应用和推广。以Ni基和Co基金属载氧体为研究对象,用热重分析仪(TGA)、扫描电镜(SEM)和X射线衍射仪(XRD)等工具研究了二者的持续循环能力。结果表明,添加惰性载体后载氧体的反应速率和持续循环能力均有大幅的提升,且Ni基载氧体比Co基载氧体表现出更好的反应特性和持续循环能力。     

甲烷化学链重整制备合成气技术中的钙钛矿型载氧体制备及动力学特性

采用燃烧法、微乳液法、共沉淀法和溶胶-凝胶法4种方法制备钙钛矿型氧化物La Fe O3作为载氧体用于甲烷化学链重整制备合成气过程,利用X射线衍射(XRD)、扫描电镜(SEM)、H2程序升温还原(H2-TPR)、比表面积分析(BET)等技术对载氧体进行表征,在固定床反应装置上考察4种方法制备的La Fe O3与甲烷的反应性能,寻求适用于甲烷化学链重整过程载氧体的最佳制备方法,然后通过H2-TPR的多速率升温过程探讨La Fe O3的还原动力学。结果表明,4种方法制备得到的载氧体均形成钙钛矿结构,溶胶-凝胶法和燃烧法制备的La Fe O3纯度和结晶度均更好,无杂相生成;从CH4转化率、n(H2)/n(CO)、CO和H2选择性等方面综合考虑,燃烧法制备得到的载氧体反应性能最好,用于甲烷化学链重整制备合成气的生成效果最好。H2-TPR的动力学计算表明,La Fe O3载氧体的低温吸附氧还原活化能为97.001 k J/mol,高温晶格氧还原活化能为30.388 k J/mol。     

Cu-Mn复合载氧体释氧反应性及动力学研究

载氧体作为化学链转换技术的核心与关键,其性能决定了反应过程的经济性和稳定性。采用石墨成孔浸渍法制备了4种不同含量Cu修饰的Cu-Mn复合载氧体,对其进行了表征和抗磨损性能实验。在热重分析仪上研究了载氧体的释氧特性和循环反应特性,并采用Model-free method分析了载氧体的释氧动力学特性。结果表明,载氧体制备过程中活性组分CuO和Mn2O3并未与惰性载体MgO发生化学反应。Cu修饰对载氧体具有拓孔作用,可以显著提高载氧体比表面积和总释氧量,降低释氧所需温度,缩短释氧时间,降低释氧活化能。但随着Cu含量的增加,载氧体抗烧结性能和抗磨损性能逐渐降低,且随着循环次数增加,载氧体循环反应性能降低。     

Carbon nanotubes supported Fe-based compounds as the electrode catalyst for oxygen reduction reaction

An amorphous Fe-based catalyst supported on polypyrrole-modified carbon nanotubes is synthesized by a chemical method. The microstructure, surface composition and morphology are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The synthesized amorphous Fe-based catalyst is composed of amorphous FeOOH and microcrystalline Fe₂O₃. Compared with a crystalline FeOOH catalyst, the amorphous Fe-based catalyst demonstrates higher electrocatalytic activity toward the oxygen reduction reaction (ORR), due to its amorphous structure and large specific surface area. It is considered that amorphization of transition metal compounds could be one of the methods used to improve their catalytic activity toward the ORR.     

Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier

The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H₂/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H₂/CO, CO₂ yield and carbon conversion efficiency gradually increased, while the yield of H₂, CO and CH₄ and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H₂, CH₄ and CO₂, H₂/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 °C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H₂, H₂/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/C of 1.5, 800 °C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H₂ content of 35.64%, syngas content of 56.40%, H₂/CO ratio of 1.72 and LHV of 12.25 MJ/Nm³.     

Development and performance of a K–Pt/γ-Al2O3 catalyst for the preferential oxidation of CO in hydrogen-rich synthesis gas

Proton exchange membrane fuel cells are widely employed in micro combined heat and power cogeneration (micro-CHP) systems, and the feed to them should be essentially free of CO. CO preferential oxidation is an effective method for the thorough removal of CO from synthesis gas. A series of K–Pt/γ-Al₂O₃ catalysts are prepared and tested for their CO cleaning capabilities. The catalyst is prepared from potassium nitrate acid, chloroplatinic acid and γ-Al₂O₃ powder by normal or ultrasonic impregnation. The catalyst performance is investigated in a micro-reactor system. The effects of K loading, Pt loading, ultrasonic processing, space velocity, O₂-to-CO ratio and operation temperature on catalyst performance are studied. A CO concentration of less than 10 ppm is achieved when the CO concentration in the feed gas is 0.45%. It was found that both ultrasonic processing and the addition of K promote the catalyst performance. The 15K1.0Pt/Al–U catalyst exhibits the best performance.     

Sulfur behavior in chemical looping combustion with NiO/Al2O3 oxygen carrier

Chemical looping combustion (CLC) is a novel technology where CO₂ is inherently separated during combustion. Due to the existence of sulfur contaminants in the fossil fuels, the gaseous products of sulfur species and the interaction of sulfur contaminants with oxygen carrier are a big concern in the CLC practice. The reactivity of NiO/Al₂O₃ oxygen carrier reduction with a gas mixture of CO/H₂ and H₂S is investigated by means of a thermogravimetric analyzer (TGA) and Fourier Transform Infrared spectrum analyzer in this study. An X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to evaluate the phase characterization of reacted oxygen carrier, and the formation mechanisms of the gaseous products of sulfur species are elucidated in the process of chemical looping combustion with a gaseous fuel containing hydrogen sulfide. The results show that the rate of NiO reduction with H₂S is higher than the one with CO. There are only Ni and Ni₃S₂ phases of nickel species in the fully reduced oxygen carrier, and no evidence for the existence of NiS or NiS₂. The formation of Ni₃S₂ is completely reversible during the process of oxygen carrier redox. A liquid phase sintering on the external surface of reduced oxygen carriers is mainly attributed to the production of the low melting of Ni₃S₂ in the nickel-based oxygen carrier reduction with a gaseous fuel containing H₂S. Due to the sintering of metallic nickel grains on the external surface of the reduced oxygen carrier, further reaction of the oxygen carrier with H₂S is constrained, and there is no increase of the sulfidation index of the reduced oxygen carrier with the cyclical reduction number. Also, a continuous operation with a syngas of carbon monoxide and hydrogen containing H₂S is carried out in a 1 kW_th CLC prototype based on the nickel-based oxygen carrier, and the effect of the fuel reactor temperature on the release of gaseous products of sulfur species is investigated.     

The catalytic properties of the sputtered iron on carbon nanotubes for polymer electrolyte membrane fuel cells

Pure iron metal target was sputtered onto carbon nanotube grown on carbon paper to fabricate iron-based catalysts for the oxygen reduction reaction (ORR). The carbon nanotube-supported Fe-based catalysts have active sites which are believed to include iron cations coordinated by pyridinic nitrogen functionalities between the graphitic sheets. A Fe-based electrocatalyst treated at 950 °C displayed the highest mass activity. The treated sample at lower temperature could not form the Fe/N/CNT sufficiently. On the other hand, the formed Fe/N/CNTs were degraded thermally at higher temperature. Cyclic voltammetry of the Fe-based electrocatalysts showed similar trends with mass activity which is the largest value at 950 °C. Even though the catalytic activity is not comparable with that of Pt/C catalysts yet, sputtered Fe-based electrocatalysts are promising to explore the non-precious metal electrocatalysts.     

Enhancing oxygen reduction reaction performance in acidic media via bimetal Fe and Cr synergistic effects

Currently, it is a great challenge to improve the stability of Fe-based oxygen reduction reaction (ORR) catalysts in acidic medium, because the Fenton reaction between Fe-based catalyst and H₂O₂ will reduce the stability of the catalyst. In this study, Fe, Cr and nitrogen-doped carbon (FeCr-N-C) was synthesized via co-impregnation of biomass walnut shells with metal precursor solutions. The FeCr-N-C catalyst has an onset potential of 0.88 V vs. RHE and outperforms the Fe-N-C catalyst in acidic media. Moreover, FeCr-N-C shows negligible activity decay (ΔE_1/2 = 14 mV) in 0.1 M HClO₄ after 20 000 cycles. The experimental results proved that the bimetal synergism can produce low yield of H₂O₂ (<2%), which might attribute to variations of local electronic structure. The reactive oxygen species of the catalyst were analyzed by Ultraviolet-visible (UV-Vis) absorption spectra. It was proved that the presence of bimetal inhibited the Fenton reaction between Fe²⁺/Fe³⁺ and H₂O₂, and further improved the stability of the catalysts. Hence, this study proposes an efficient strategy to facilitate the practical application of Fe-based catalysts in acidic media.     

Reactivity investigation on biomass chemical looping conversion for syngas production

Chemical looping gasification (CLG) is regarded as an innovative and promising technology for producing syngas. In this work, CLG of straw was conducted in a fixed bed reactor with Fe₂O₃ as the oxygen carrier, whose results led to conclusions that Fe₂O₃, the oxygen carrier, proved advantageous to the secondary gasification reaction and the formation of CO and CO₂. It was also found that CO was further oxidized to CO₂ at high Fe₂O₃/C molar ratio, which resulted in a decreased gasification efficiency and low heat value of syngas. Therefore, a conclusion was drawn that the most optimized Fe₂O₃/C molar ratio was 0.2. In addition, the alkali metals in the biomass evaporated as chlorine salts into gas phase and retained as alkali metal oxide at high temperature, resulting in coking, slagging and heating surface corrosion. In the mean time, the oxygen carrier mainly converted to Fe and sintering phenomenon was serious at high temperature despite the fact that high temperature promoted gas yield, carbon conversion efficiency and gasification efficiency. Therefore, the most optimized temperature was set to 800 °C in order to maximize gas yield and gasification efficiency.     

Efficient tuning of the Pt nano-particle mono-dispersion on Vulcan XC-72R by selective pre-treatment and electrochemical evaluation of hydrogen oxidation and oxygen reduction reactions

The effect of chemical pre-treatment of the carbon support used for deposition of Pt nano-particles is reported. Data on particle size, distribution and their electocatalytic activity toward hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) are reported. Vulcan XC-72R carbon was pre-treated with 5% HNO₃, 0.07 M H₃PO₄, 0.2 M KOH and 10% H₂O₂. The properties of carbon supports were studied by N₂ adsorption and X-ray photoelectron spectroscopy (XPS). Chemical reduction with ethylene glycol (EG) was used to synthesize Pt on carbon supports and the differences in catalyst morphology were characterized using CO chemisorption, X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. The electrocatalytic activity of Pt/C catalysts toward HOR and ORR was examined by cyclic voltammetry (CV) on a rotating ring-disk electrode (RRDE) and compared with E-Tek Pt/C. The ORR was predominantly involved via four-electron process with the first electron transfer being the rate-determining step. However, the specific activity and mass activity were greatly influenced by the pre-treatment employed.     

Study of the synergism effect of a binary carbon system in the nanostructure of the gas diffusion electrode (GDE) of a proton exchange membrane fuel cell

We report on the use of dual carbon supports activated charcoal (AC) and Vulcan XC-72R (VC) as catalysts for the fabrication of a gas diffusion electrode. The electrocatalytic properties for the oxygen reduction reaction were evaluated in a sulfuric acid electrolyte using polarization curves and electrochemical impedance spectroscopy. The water uptake and oxygen permeability were also obtained, as were the performances of electrodes in a membrane electrode assembly. A binary support electrode exhibits better performance than the usual single support electrode, with the best performance being obtained when the mass ratio of the two carbons is 50:50.