Study on mode-filtered light sensor for methane detection

A novel mode-filtered light gas sensor has been reported.It was constructed by inserting an optical fiber deposited by a thin silicone cladding of cryptophane A into a fused-silica capillary.When different concentration of methane gas was introduced to the sensor,the phenomenon that mode-filtered light intensity decreased with the increasing concentration of methane was observed. And a linear relationship was obtained within the methane concentration range of 0.0-16.0%(v/v).The detection limit was 0.06% ...     

Rh/SiO_2催化剂上甲烷部分氧化制合成气的反应机理

采用原位Raman光谱技术,在原料气中的O2未完全耗尽的条件下,对CH4部分氧化制合成气反应的Rh/SiO2催化剂床层前部贵金属物种的化学态以及由CH4解离所生成的碳物种进行了表征.在此基础上采用脉冲反应和同位素示踪技术,比较了CH4的部分氧化及其与H2O和CO2的重整等反应对催化剂床层氧化区内CO和H2生成的相对贡献,并将实验结果与Ra-man光谱表征结果进行了关联.结果表明,在600°C下将还原后的4%Rh/SiO2催化剂切入CH4:O2:Ar=2:1:45原料气,催化剂床层前部未检测到铑氧化物的Raman谱峰,但可清晰检测到源于CH4解离的碳物种;在700°C和接触时间小于1ms的条件下,催化剂床层的氧化区内已有大量CO和H2生成,在相同的实验条件下,CH4与H2O或CO2重整反应对氧化区内合成气生成的贡献则很小;以CH4:16O2:H218O:He=2:1:2:95为原料气的同位素示踪实验结果表明,在原料气中16O2未完全耗尽的情况下,反应产物中C16O的含量占CO生成总量的92.3%,表明CO主要来自CH4的部分氧化反应.上述结果均表明,在O2存在下Rh/SiO2催化剂上CO和H2可以通过CH4直接解离和部分氧化机理生成.     

Development of a new gas sensor for binary mixtures based on the permselectivity of polymeric membranes: Application to carbon dioxide/methane and carbon dioxide/helium mixtures

Membrane-based gas sensors were developed and used for determining the composition on bi-component mixtures in the 0-100% range, such as oxygen/nitrogen and carbon dioxide/methane (biogas). These sensors are low cost and are aimed at a low/medium precision market.The paper describes the use of this sensor for two gas mixtures: carbon dioxide/methane and carbon dioxide/helium. The membranes used are poly(dimethylsiloxane) (PDMS) and Teflon-AF hollow fibers. The response curves for both sensors were obtained at three different temperatures. The results clearly indicate that the permeate pressure of the sensors relates to the gas mixture composition at a given temperature. The data is represented by a third order polynomial. The sensors enable quantitative carbon dioxide analysis in binary mixtures with methane or helium. The response of the sensors is fast (less than 50 s), continuous, reproducible and long-term stable over a period of 2.3×10⁷ s (9 months). The absolute sensitivity of the sensors depends on the carbon dioxide feed concentration ranging from 0.03 to 0.13 MPa.     

基于SnO2-CuO纳米纤维的薄膜型H2-S传感器研究

采用同轴静电纺丝技术制备了SnO2-CuO复合纳米纤维,采用提拉法将SnO2-CuO纳米纤维涂覆于印有梳状Au电极的氧化铝陶瓷管表面形成敏感薄膜,设计了一种新型薄膜式H2 S传感器。采用 X 射线衍射、扫描电子显微镜和X-射线光电子能谱( XPS)表征SnO2-CuO纳米纤维的相组成和微观形貌,分析了敏感薄膜成分配比和厚度对硫化氢敏感机理和电化学特性。采用WS-30A型气敏元件分析仪测试了H2 S传感器敏感特性、温度特性、湿度特性、动态响应、抗干扰特性和稳定性。结果表明,以C50纳米纤维为敏感薄膜(膜厚为70 nm)的H2 S传感器,在温度为25℃, H2 S气体浓度为10~60 mg/L时,传感器线性度和灵敏度分别为92.3％和98.2％,响应最大值为1080,承受的最大相对湿度为95％,动态响应和恢复时间分别为4和12 s。此传感器对CO, NO2, SO2, NH3, CO2, CH4和H2等有害气体具有较好的抗干扰性。在矿井中连续使用12月后,响应衰减了9.2％,响应正常时间为10.9月。     

Mode-filtered light methane gas sensor based on cryptophane A

A mode-filtered light sensor has been developed for methane (CH₄) gas determination at ambient conditions. The proposed chemosensor consisted of an annular column which was constructed by inserting an optical fiber coated with a thin silicone cladding of cryptophane A into a fused-silica capillary. When CH₄ was introduced to the sensor, selective inclusion of CH₄ into the silicone layer would cause a change in the local refractive index of the cladding, resulting in the change of mode-filtered light that emanated from the fiber. Three detection windows were set alongside the capillary to propagate the light to a charge-coupled device (CCD). The changes of mode-filtered light on exposure to various concentrations of CH₄ were thus simultaneously monitored. The mode-filtered light intensity decreased with the increase in concentration of CH₄. The dynamic concentration range of the sensor for CH₄ was 0.0-16.0% v/v with a detection limit of 0.15% v/v. The highest sensitivity was found at the channel furthest away from the excitation light source. The response time (t_95%) was about 5 min. The reproducibility was good with a relative standard deviation (RSD) of less than 7% from evaluating six cryptophane A-coated fibers. Oxygen, hydrogen and carbon dioxide showed very little interference on detection but interferences from dichloromethane and carbon tetrachloride were observed. The proposed mode-filtered light sensor has been successfully applied to determine CH₄ samples and the accuracy was good. Our work offers a promising approach for CH₄ detection.     

基于ZnO-SnO2纳米纤维的化学链燃烧式甲烷传感器研究

采用双喷嘴静电纺丝技术制备了ZnO-SnO2纳米纤维,将制备的ZnO-SnO2纳米纤维均匀涂覆于铂热敏电阻表面形成催化薄膜,设计了一种新型化学链燃烧式甲烷传感器。采用 X 射线衍射仪、扫描电子显微镜、全自动程序化学吸附仪和X-射线光电子能谱仪,表征了ZnO-SnO2纳米纤维的相组成和微观形貌,讨论了催化薄膜表面的电化学性能对甲烷化学链燃烧反应的影响。采用DL07-YJ108D型电压测量仪测试了甲烷传感器灵敏响应性、温湿特性、选择性和长期稳定性。结果表明,以Zn50纳米纤维为催化薄膜的甲烷传感器,在温度为350℃,甲烷浓度为0.1~60μg/L时,传感器线性度和灵敏度最大值分别为99.4%和0.12 V/(μg/L),最大响应为8.2 V,动态响应和恢复时间分别为5.4和10.8 s,承受的最大相对湿度为95%。在矿井中连续使用6个月后,响应下降了2.0%。     

Methane Decomposition on the Surface of Molybdenum Nanoparticles at Room Temperature

The decomposition of methane on molybdenum nanoparticles was studied experimentally at room temperature. The molybdenum nanoparticles were synthesized in the gas phase using UV laser photolysis of Mo(CO)6 vapor in a flow reactor. The working part of the flow reactor was equipped with quartz windows for introducing the radiation from a pulsed Nd:YaG laser operating at the fourth harmonic (266 nm) at a frequency of 10 Hz. Methane was used as a carrier gas. As a result of irradiation of a mixture of methane with Mo(CO)6 vapors in the gas phase at room temperature, nanoparticles with sizes of 2–50 nm were synthesized. The phase composition of the nanoparticles included pure molybdenum, molybdenum carbide Mo2C, and molybdenum oxide MoO3. During the reaction, the hydrogen yield was measured with a VG-7 highly sensitive hydrogen analyzer based on a semiconductor metal–dielectric sensor. The measured H2 concentration varied from 5 to 25 ppm depending on the concentration of Mo(CO)6. The possibility of methane decomposition on molybdenum nanoparticles at room temperature was discussed based on the obtained data.     

Quantitative analysis of NO2 in the presence of CO using a single tungsten oxide semiconductor sensor and dynamic signal processing

We demonstrate that NO2 can be quantitatively analysed in the presence of CO using a single tungsten oxide based resistive gas sensor. The working temperature of the sensor was modulated between 190 and 380 degrees C and its dynamic response to different concentrations of CO, NO2, and CO + NO2 mixtures was monitored. Either the fast Fourier transform (FFT) or the discrete wavelet transform (DWT) was used to extract important features from the sensor response. These features were then input to different (statistical and neural) pattern recognition methods. The species considered can be discriminated with a success rate higher than 90% using a Fuzzy ARTMAP or a radial basis function neural network. The concentrations of the gases studied can be accurately predicted, by using the DWT coupled to partial least squares (PLS) models. The correlation coefficients of the predicted versus real concentrations were 0.923, 0.870 and 0.866 for CO, NO2, and NO2 in CO + NO2 mixtures, respectively.     

Planar Integrated Optical Waveguide Sensor for Isopropyl Alcohol in Aqueous Media

An attenuated total reflection (ATR) sensor for water-miscible organic solvents was constructed using a combination of sol-gel processing and integrated optical waveguide (IOW) technologies. The sensor consisted of single-mode, sol-gel based planar waveguide coated with a 40 nm thick, porous sol-gel indicator layer prepared from methyltriethoxysilane and doped with methyl red. The response of the senor to aqueous isopropyl alcohol (IPA) was investigated. Solvation of the indicator dye by IPA causes the absorbance spectrum to undergo a blue shift coupled with an increase in molar absorptivity. IPA was detected by measuring changes in ATR of the guided mode at 488 nm. A response curve extending from 1 to 100% (v/v) IPA in water was constructed for the sensor, from which a detection limit of 0.7% (v/v) IPA/water was estimated. Response and reversal times were typically less than one minute, making this sensor potentially attractive for on-line monitoring applications. The rapid response characteristics are attributable to relatively weak, reversible interactions between the indicator and analyte.     

Co-Generation of C2 Hydrocarbons and Synthesis Gases from Methane and Carbon Dioxide： a Thermodynamic Analysis

This paper deals with thermodynamic chemical equilibrium analysis using the method of direct minimization of Gibbs free energy for all possible CH4 and CO2 reactions. The effects of CO2/CH4 feed ratio, reaction temperature, and system pressure on equilibrium composition, conversion, selectivity and yield were studied. In addition, carbon and no carbon formation regions were also considered at various reaction temperatures and CO2/CH4 feed ratios in the reaction system at equilibrium. It was found that the reaction temperature above 1100 K and CO2/CH4 ratio=1 were favourable for synthesis gas production with H2/CO ratio unity, while carbon dioxide oxidative coupling of methane （CO2 OCM） reaction to produce ethane and ethylene is less favourable thermodynamically. Numerical results indicated that the no carbon formation region was at temperatures above 1000 K and CO2/CH4 ratio larger than 1.     

Ni/ZrO2催化剂上甲烷水蒸气重整反应的研究

研究了Ni/ZrO2催化剂对甲烷水蒸气重整制合成气的反应性能。考察了催化剂的还原温度、载体焙烧温度以及反应温度、原料配比和空速等对催化剂性能的影响。利用XRD、TEM、XPS等手段对催化剂的织构形貌进行了表征。研究表明，Ni/ZrO2催化剂用于甲烷水蒸气重整制合成气不仅具有较高的活性，也具有较好的稳定性。水蒸气比增加，CH4转化率增大、CO选择性下降。CH4转化率及CO选择性均随空速增大而下降。使用10%Ni/ZrO2催化剂，在650 ℃、空速1.984×104 h－1、原料气配比H2O∶CH4∶N2=2∶1∶2.67的条件下，获得CH4转化率85%、CO选择性70%的结果。     

气相色谱法分析氦中氧气、氮气、一氧化碳、二氧化碳及氙气混合气体标准物质

建立利用气相色谱热导检测器分析气体激光器用氦中氧气、氮气、一氧化碳、二氧化碳及氙气混合气体标准物质的方法。试验比较了不同极性、不同类型的色谱柱,对柱箱温度、载气流量进行了优化,最终确定以HP–PLOT/分子筛色谱柱分离氧气、氮气、一氧化碳、氙气,柱箱温度保持40℃,以HP–PLOT Q柱分离二氧化碳,柱箱温度保持60℃,载气流量均为2 m L/min。在混合气体标准物质量值范围内,该分析方法测定结果的相对标准偏差不大于1%(n=6),对重量法配制得标准气体进行分析比对,测量误差不大于1%。     

Kinetics and stability of CH4-CO2 mixed gas hydrates during formation and long-term storage.

The formation of CH4-CO2 mixed gas hydrates was observed by measuring the change of vapor-phase composition using gas chromatography and Raman spectroscopy. Preferential consumption of carbon dioxide molecules was found during hydrate formation, which agreed well with thermodynamic calculations. Both Raman spectroscopic analysis and the thermodynamic calculation indicated that the kinetics of this mixed gas hydrate system was controlled by the competition of both molecules to be enclathrated into the hydrate cages. However, the methane molecules were preferentially crystallized in the early stages of hydrate formation when the initial methane concentration was much less than that of carbon dioxide. According to the Roman spectra, pure methane hydrates first formed under this condition. This unique phenomenon suggested that methane molecules play important roles in the hydrate formation process. These mixed gas hydrates were stored at atmospheric pressure and 190 K for over two months to examine the stability of the encaged gases. During storage, CO2 was preferentially released. According to our thermodynamic analysis, this CO2 release was due to the instability of CO2 in the hydrate structure under the storage conditions.     

A fast-response catalytic sensor for flammable gases

A fast-response flammable-gas sensor has been developed, which has a typical response time of less than 1 s; this is approximately an order of magnitude less than that of a conventional diffusion-operated sensor. The reduction has been achieved by flowing the sampled gas directly over the sensing element and by operating the sensing element at a constant temperature, independent of reaction. The performance of the sensor has been evaluated in methane—air and butane—air mixtures using porous (poison-resistant) and non-porous (‘pellistor’-type) sensing elements.     

Low temperature H2S sensor based on copper oxide/tin dioxide thick film

Nanostructured tin dioxide (SnO2) powders were prepared by a sol-gel dialytic process and and the doping of CuO on it was completed by a deposition-precipitation method. The thick film sensors were fabricated from the CuO/SnO2 polycrystalline powders. Sensing behavior of the sensor was investigated with various gases including CO, H2, NH3, hexane, acetone, ethanol, methanol and H2S in air. The as-synthesized gas sensor had much better response to H2S than to other gases. At the same time, the CuO/SnO2 sensor had enough sensitivity, together with fast response and recovery, to distinguish H2S from those gases at 160 and 210 ℃. Therefore, it might have promising applications in the future.     

Efficient CO2 capture on low-cost silica gel modified by polyethyleneimine

In this work,a series of polyethyleneimine(PEI) functionalized commercial silica gel were prepared by wet impregnation method and used as CO 2 sorbent.The as-prepared sorbents were characterized by N 2 adsorption,FT-IR and SEM techniques.CO 2 capture was tested in a fixed bed reactor using a simulated flue gas containing 15.1% CO 2 in a temperature range of 25-100 C.The effects of sorption temperature and amine content on CO 2 uptake of the adsorbents were investigated.The silica gel with a 30 wt% PEI loading manifested the largest CO 2 uptake of 93.4 mg CO 2 /g adsorbent(equal to 311.3 mg CO 2 /g PEI) among the tested sorbents under the conditions of 15.1%(v/v) CO 2 in N 2 at 75 C and atmospheric pressure.Moreover,it was rather low-cost.In addition,the PEI-impregnated silica gel exhibited stable adsorption-desorption behavior during 5 consecutive test cycles.These results suggest that the PEI-impregnated silica gel is a promising and cost-effective sorbent for CO 2 capture from flue gas and other stationary sources with low CO 2 concentration.     

Properties of oxygen permeation and partial oxidation of methane in La0.6Sr0.4CoO3−δ (LSC)–La0.7Sr0.3Ga0.6Fe0.4O3−δ (LSGF) membrane

The oxygen separation membrane having perovskite structure for the partial oxidation of methane to synthesis gas was prepared. La_0.7Sr_0.3Ga_0.6Fe_0.4O_3−δ (LSGF) perovskite membrane coated with La_0.6Sr_0.4CoO_3−δ (LSC) (M1), and the one side of M1 membrane coated with NiO (M2) was prepared to examine the partial oxidation of methane. The single oxygen permeations of the LSC + LSGF (M1) membrane and NiO coated membrane (M2) were measured. The oxygen permeation flux in M1 membrane was higher than that of M1 membrane at 850 °C.

The partial oxidation experiment of methane using the prepared membranes was examined at 850 °C. The value of CH₄ conversion and CO selectivity of M2 membrane was higher than that of M1 membrane.

NiO/NiAl₂O₄ catalyst was used to improve the methane conversion, and the partial oxidation experiment of methane with M1 membrane was examined at 850 °C. The CH₄ conversion was 88%, and CO selectivity was 100%.     

Gas chromatographic sensing on an optical fiber by mode-filtered light detection

A chemical sensor for gas phase measurements is reported which combines the principles of chemical separation and fiber optic detection. The analyzer incorporates an annular column Chromatographic sensor, constructed by inserting a polymer-clad optical fiber into a silica capillary. Light from a helium-neon laser is launched down the fiber, producing a steady intensity distribution within the fiber, but a low background of scattered light. When sample vapor is introduced to the sensor, and an analyte-rich volume interacts with the polymer cladding, Chromatographic retention is observed simultaneously with a change in the local refractive index of the cladding. An increase in cladding refractive index (RI) causes light to be coupled out of the fiber, with detection at a right-angle to the annular column length to provide optimum S/N ratio. This detection mechanism is called mode-filtered light detection. We report a gas Chromatographic separation on a 3.1 m annular column (320 microm i.d. silica tube, 228 microm o.d. fiber with a 12 microm fluorinated silicone clad) of methane, benzene, butanone and chlorobenzene in 6 min. The annular column length was reduced to 22 cm to function as a sensor, with selected organic vapors exhibiting unique retention times and detection selectivity. The detection selectivity is determined by the analyte RI and the partition coefficient into the cladding. The calculated limit of detection (LOD) for benzene vapor is 0.03% by volume in nitrogen, and several chlorinated species had LOD values less than 1%. For binary mixtures of organic vapors, the detected response appears to be the linear combination of the two organic standards, suggesting that the annular column may be useful as a general approach for designing chemical sensors that incorporate separation and optical detection principles simultaneously.     

Simplification and evaluation of a gold-deposited SPR optical fiber sensor.

The structure of the sensing element of a gold-deposited optical fiber sensor was simplified and quantitative analyses of various alcohols with the sensor showed improvement of the performance. The sensor uses surface plasmon resonance (SPR) at the interface of a sample solution and a thin (10 - 70 nm) gold film deposited on half of the exposed core of the optical fiber. The sensor with a film thickness of 45 nm can detect a small change of 5.6 x 10(-5) refractive index (RI) units in the refractivity. The response time is less than 0.5 min and the relative standard deviation for measurements is less than or equal to 1%. A straight line with a correlation coefficient of 0.9995 was obtained below 10%, v/v in the calibration curve for methanol solutions of benzyl alcohol. The minimum of the response curve due to the maximum excitation of SPR in the refractivity range from 1.33 to 1.44 RI units shifts to a lower refractivity as the film becomes thicker. The response curves of the sensors were calculated from SPR theoretical equations while considering of the distribution in the thickness of the deposited gold films. The improvement in the performance of the sensor is discussed.     

Low temperature H_2S sensor based on copper oxide/tin dioxide thick film

Nanostructured tin dioxide (SnO2) powders were prepared by a sol-gel dialytic process and and the doping of CuO on it was completed by a deposition-precipitation method.The thick film sensors were fabricated from the CuO/SnO2 polycrystalline powders.Sensing behavior of the sensor was investigated with various gases including CO,H2,NH3,hexane,acetone,ethanol,methanol and H2S in air.The as-synthesized gas sensor had much better response to H2S than to other gases.At the same time,the CuO/SnO2 sensor had enough sensitivity,together with fast response and recovery,to distinguish H2S from those gases at 160 and 210 ?C.Therefore,it might have promising applications in the future.