Coatings zeolite sensors

Since the adsorption of a gas is able to modify the dielectric constant of zeolites, chemical sensors based on interdigital capacitors (IDCs) using zeolites layers as sensitive coatings offer a wide field of applications depending on the type, modification, and working temperature of the coated IDC sensor. 

In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coatcd sensor device, thus resulting in an increase of the sensor selectivity towards ethanol. 

In the middle of the last century, the original form of zeolite membranes were synthesized by dispersing the zeolite crystals in polymer membrane matrixes, which were used for gas separation and pervaporative alcohol/water separations. In the last few decades, the researches of polycrystalline zeolite membranes that supported on ceramic, glass, or metal substrates have grown into an attractive and abundant field. Their applications for gas separation, pervaporation, membrane reactors, sensors, low-k films, corrosion protection coatings, zeolite modified electrodes, fuel cells, heat pumps et al. have been wildly explored. In the following text, the applications of supported polycrystalline zeolite membranes for energy and fuels will be presented. 

Apart from the above described core-shell catalysts, it is also possible to coat active phases other than zeolite crystals, like metal nanoparticles, as demonstrated by van der Puil et al. [46]. More examples of applications on the micro level are given in Section 10.5, where microreactors and sensor apphcations are discussed. 

A very recent example of the first case is presented by Vilaseca et al. [71] where an LTA coating on a micromachined sensor made the sensor much more selective to ethanol than methane. Moos et al. [72, 73] report H-ZSM5 NH3 sensor based on impedance spectroscopy using the zeolite as active sensing material. At elevated temperatures (>673 K) NH3 still adsorbs significantly in contrast to CO2, NO,... 

When NOj levels are measured electrochemicaUy, NO and NO2 can lead to opposing signals because NO is oxidized and NO2 tends to be reduced. Moreover, it is preferred to obtain a total NO, measurement instead of only one of the constituents. The latter can be achieved by catalytically equilibrating the feed with oxygen before contact with the sensor by coating an active zeolite layer on top or placing a active catalyst bed in front of the sensor. Both approaches have been demonstrated successfully with a Pt-Y zeohte as active catalyst [74, 75]. The additional advantage of the filter bed is a reduction in the cross-sensitivity with CO due to CO oxidation above 673 K. 

Zhang, J. Tang, X. Dong, J. Wei, T. Xiao, H., Zeolite thin film coated long period fiber grating sensor for measuring trace chemical, Opt. Express 2008, 16, 8317 8323... 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

Another state-of-the-art detection system contains a surface acoustic wave (SAW) device, which is based on a piezoelectric crystal whose resonant frequency is sensitive to tiny changes in its mass—it can sense a change of 10-1° g/cm2. In one use of this device as a detector it was coated with a thin film of zeolite, a silicate mineral. Zeolite has intricate passages of a very uniform size. Thus it can act as a molecular sieve, allowing only molecules of a certain size to pass through onto the detector, where their accumulation changes the mass and therefore alters the detector frequency. This sensor has been used to detect amounts of methyl alcohol (CH3OH) as low as 10 9 g. 

FIGURE 10.25 Sn02 sensors modified with zeolitic filters (a) schematic representation of the zeolite-coated sensor and (b) (See color insert following page 588.) front and back view of the as-received Sn02 sensors. 

FIGURE 10.30 Scanning electron microscopy (SEM) images of cross section of a commercial optical fiber coated with a NaA zeoUte thick layer (a) total cross section and (b) magnification view of the NaA zeolite layer. (From Lopez, J., Pina, M.P., Coronas, J., Pelayo, J., and Santamaria, J., A novel optical device for gas sensor applications based on zeolitic materials. Books of abstracts of the 1st NanoSpain Workshop, San Sebastian, 2004.)... 

Thus, zeolite-coated IDCs have been tested for sensing n-butane [317] and also, NH3, NO, and CO [318,319] on Na-Y and NaPtY zeolite-based sensors at temperatures high enough to where chemical reactions may also occur (above 200°C). The response time is of the order of seconds and the cross-sensitivity to water is small at high temperatures, at which no water condensation occurs in the zeolite-pore system. Under certain conditions, selectivity of these reactive chemical sensors is remarkable. Thus, the detection of 10 ppm of n-butane with a NaPtY interdigitated capacitor with no response to CO and H2 has been reported [318]. Similarly, Moos et al. [320] described a ZSM-5 based capacitor sensor with on-chip heating for temperatures up to 450°C capable of detecting NH3 with no cross-sensitivity to CO, hydrocarbons, and O2. 

Fukui K and Nishida S. CO gas sensor based on Au-La203 added Sn02 ceramics with siliceous zeolite coat. Sens Actuators B 1997 45(2) 101-106. 

Single-layer zinc-phosphate zeolite crystals were grown with more than 90% of their (111) faces oriented to a gold-coated silicon surface. Sudi oriented zeolite films might find application as membrane catalysts or as specific chemical sensors [66]. 

Zeolite membranes have been demonstrated for many applications. Applications such as separation membranes, membrane reactors, adsorption, and catalysis have been covered in several reviews. In this entry, we focus on new applications including sensors, low-dielectric constant (low-k) films, corrosion resistant coatings, hydrophilic coatings, heat pumps, and thermoelectrics. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

Further examples of acoustic sensors modified with zeolites include a QCM sensor with silver-exchanged ZSM-5 that responds selectively to acetone (in diabetic s breath) in the ppm-range,ll 16] principal component analysis of multiple QCM-sensor responses (with LTA, MFI, SOD) for the detection of NO/SO2 mixtures,[117] MFI-zeolite-coated microcantilevers with ppm-sensitivity for Freon detection [118,119] and other zeolite-coated cantilevers for humidity sensing.[120]... 

The nature and mobility of ions and solvent in zeolite cages will affect the ac-impedance of the material. This effect can be utilized for zeolite-based sensor concepts where a zeolite film is coated on interdigitated electrodes. For example, it was shown that the impedance of a film of proton-conducting H-ZSM-5 is influenced by the presence of ammonia (Figs. 15, 16).[126,127] The ammonia is protonated in the zeolite, thus producing much larger ammonium ions with different mobilities in the zeolite that can be detected by impedance spectroscopy. The detection of ammonia is of interest for automotive applications where the selective catalytic reduction of NOx by ammonia is envisioned. 

Fig. 15. Schematic presentation of the sensor (a) measuring electrode design coated with zeolite film (top) and heater electrodes (bottom), (b) cross section. The resistance of the Pt heater is also used as temperature sensor. [ 1271...

Fig. 16. Resistance changes of a zeolite-coated (H-ZSM-5) sensor exposed to different ammonia concentrations. The resistance was derived from an RC equivalent circuit at 1 MHz measurement frequency. [127]...

Finally, we mention a novel transduction concept based on the heal evolved from a reaction such as combustion. Microcalorimetric devices can now be made using lithographic techniques. One of the two sensitive areas of such a device (were evolved heat can be measured) was coated with a thin film of CoA1P04-5, the other was kept open as a reference 135] The additional benefit of a zeolite with catalytic activity for such a device is the molecular sieving effect that can be combined in the response of the sensor (a molecule too big to enter the catalytically active interior of the zeolite should only show a weak response). The change in temperature was measured with a meandering Pt-wire resistor. This device was examined in the detection of CO and cyclohexane, and sensitivity and selectivity in the low ppm-range was observed. 

Vilaseca M, Coronas J, Cirera A, Comet A, Morante J, Santamarfa J. Development and application of micromachined Pd/SnOj gas sensors with zeolite coatings. Sens Actuators B Chem 2008 133(2) 435 1. 

It was observed that the orf/io-methyl pyridyl-substituted porphyrinic materials demonstrated approximately twice the catalytic activity of the meta- and para-substituted systems.A Cu(TPP)/zeolite Y material was coated onto an electrode as part of a carbon paste to be used as an electrochemical sensor. No catalytic activity was noted when glutathione was used as a substrate for electroxidation, and cysteine was observed to block electroxidation, but efficient activity for the electrocatalytic oxidation of hydrazine was observed. The authors cited this as an example of conferred selectivity due to the presence of the Cu(TPP) / zeolite material (Figure 107). ... 

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