Several metal-organic framework (MOF) materials were under investigated to test their applicability as sensor materials for impedimetric gas sensors. The materials were tested in a temperature range of 120 °C - 240 °C with varying concentrations of O2, CO2, C3H8, NO, H2, ethanol and methanol in the gas atmosphere and under different test gas humidity conditions. Different sensor configurations were studied in a frequency range of 1 Hz -1 MHz and time-continuous measurements were performed at 1 Hz. The materials did not show any impedance response to O2, CO2, C3H8, NO, or H2 in the gas atmospheres, although for some materials a significant impedance decrease was induced by a change of the ethanol or methanol concentration in the gas phase. Moreover, pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity, with a linear response curve at 120 °C. Further investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect.

Several materials in the class of metal-organic frameworks (MOF) were investigated to determine their sorption characteristics for sulfur compounds from fuels. The materials were tested using different model oils and common fuels such as low-sulfur gasoline or diesel fuel at room temperature and ambient pressure. Thiophene and tetrahydrothiophene (THT) were chosen as model substances. Total-sulfur concentrations in the model oils ranged from 30 mg/kg (S from thiophene) to 9 mg/kg (S from tetrahydrothiophene) as determined by elementary analysis. Initial sulfur contents of 8 mg/kg and 10 mg/kg were identified for lowsulfur gasoline and for diesel fuel, respectively, by analysis of the common liquid fuels. Most of the MOF materials examined were not suitable for use as sulfur adsorbers. However, a high efficiency for sulfur removal from fuels and model oils was noticed for a special copper-containing MOF (copper benzene-1,3,5-tricarboxylate, Cu-BTC-MOF). By use of this material, 78 wt % of the sulfur content was removed from thiophene containing model oils and an even higher decrease of up to 86 wt % was obtained for THT-based model oils. Moreover, the sulfur content of low-sulfur gasoline was reduced to 6.5 mg/kg, which represented a decrease of more than 22 %. The sulfur level in diesel fuel was reduced by an extent of 13 wt %. Time-resolved measurements demonstrated that the sulfur-sorption mainly occurs in the first 60 min after contact with the adsorbent, so that the total time span of the desulfurization process can be limited to 1 h. Therefore, this material seems to be highly suitable for sulfur reduction in commercial fuels in order to meet regulatory requirements and demands for automotive exhaust catalysis-systems or exhaust gas sensors. IntroductionThe reduction in the level of sulfur compounds in commercial gasoline and diesel fuels is a major concern of the petrol, automotive and power generation industries. This aim is a necessity not only to meet regulatory requirements but also to enhance the life-time of exhaust gas aftertreatment systems and sensors or fuel cell components. Consequently, there is an increasing demand for ultra-low sulfur fuels (preferably down to 0.1 ppmw S ) [1].As a legal guideline, the European Union has mandated the reduction of sulfur levels (S-levels) in gasoline and diesel fuels to 10 mg/kg S (10 ppmw S ) by 2009 (directive 2003/17/EC). In addition, S-levels in diesel fuel for traction engines and heating purposes was reduced from the original value of 2000 mg/kg to 1000 mg/kg in 2008 [2]. Analog regulations of the federal government of the US and the EPA (U.S. Environmental Protection Agency) are proposing the implementation of ultralow sulfur diesel fuel with 15 ppmw S for 2010 [3, 4]. Similar to the EU proposals, the German government has also enacted the introduction of sulfur-free fuel (with a maximum tolerable level of sulfur of 10 mg/kg) in 2009 (Order 10. BImSchV) [5]. In order to meet these worldwide demands, oil refiners have been endeavoring to promote sulfur r...

No abstract

The preparation of gas diffusion electrodes and their use in an amperometric enzyme biosensor for the direct detection of a gaseous analyte is described. The gas diffusion electrodes are prepared by covering a PTFE membrane (thickness 250 mm, pore size 2 mm, porosity 35%) with gold, platinum, or a graphite/PTFE mixture. Gold and platinum are deposited by e-beam sputtering, whereas the graphite/PTFE layer is prepared by vacuum filtration of a respective aqueous suspension. These gas diffusion electrodes are exemplarily implemented as working electrodes in an amperometric biosensor for gaseous formaldehyde containing NAD-dependent formaldehyde dehydrogenase from P. putida [EC. 1.2.1.46] as enzyme and 1,2-naphthoquinone-4-sulfonic acid as electrochemical mediator. The resulting sensors are compared with regard to background current, signal noise, linear range, sensitivity, and detection limit. In this respect, sensors with gold or graphite/PTFE covered membranes outclass ones with platinum for this particular analyte and sensor configuration.

An enzymatic sensor detecting the analyte formaldehyde directly from the gas phase is under investigation. In contrast to existing systems, it enables the quantification of the analyte without prior sampling or accumulation and thus can be used as an online system to monitor the formaldehyde concentration in ambient air. The amperometric sensor depends on the enzymatic conversion of the analyte using formaldehyde dehydrogenase from P. putida [EC. 1.2.1.46] as the recognition element. It shows a linear response curve up to 15 ppm, with a detection limit of 0.03 pm (S/N ¼ 3). In order to optimize the sensor performance the electrolyte composition within the sensor was varied with respect to pH value, buffer concentration and the addition of Ca 2þ and Mg 2þ ions. To elucidate the influence of the mediator and the enzyme on the sensor performance the stability and activity of the electrochemical mediator and the enzyme alone was examined separately in these different electrolytes.