Sensor ammonia

Further work by Spetz et al (21) showed that the thick contact electrode did not have to be made from palladium. Their initial theory of operation was therefore modified to postulate that the hydrogen atoms, generated from the catalytic decomposition of the ammonia, caused an interfacial dipole layer to be formed at the platinum (or iridium)/silicon dioxide interface, not at the palladium/silicon dioxide interface. 

Ross et al (22) presented results on hydrogen and ammonia sensitivity of thin platinum-gate MOSFET devices. They concluded that the mechanism of response of these devices was as previously suggested, and cited literature on the dissociation of ammonia on silicon dioxide supported platinum catalysts as additional evidence. Lundstrom et al (23) presented further results on the two-metal capacitors. They outlined three possible modes of response of these devices, the first the same as the hydrogen sensitivity on palladium-gate devices (which they immediately ruled out), the second as already described. 

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Malins C., Doyle A., MacCraith B.D., Kvasnik F., Landl M., Simon P., Kalvoda L., Lukas R., Pufler K., Babusik I., Personal ammonia sensor for industrial environments, J. Environ. Monitor., 1999 1 417-422. 

Lobnik A., Wolfbeis O.S., Sol-gel based sensor for dissolved ammonia, Sensors and Actuators B 1998 51 203. 

Ammonia sensor can be designed in a similar method. Only here, the basic gas can directly interact with the indicator (reaction 8), thus no internal solution is needed, the reaction can be a single step. 

Waich K, Borisov SM, Mayr T, Klimant I (2009) Dual lifetime referenced trace ammonia sensors. Sens Actuators B 139 132-138... 

S. Kar and M. A. Arnold, Fiber-optic ammonia sensor for measuring synaptic glutamate and extracellular ammonia, Anal. Chem. 64, 2438-2443 (1992). 

The largest influence on the response speed is found to be the temperature. Wingbrant et al. has investigated the temperature-dependence of the response speed for a large number of different devices, with gate metals of 100-nm Pt + 10-nm TaSi, porous Pt, and porous Ir of different thicknesses used as both hydrogen and ammonia sensors [2] (Figure 2.18). 

SCR is a process by which NO gases in diesel exhausts can be reduced to levels that will meet future legislation. SCR is based on the reduction of NO in the catalytic converter by the injection of ammonia or urea into the exhaust gases before they enter the catalytic converter, where NO and NHj react to form and H O. The ammonia injection process may be controlled by measuring either the ammonia or nitric oxide slip after the catalytic converter. Such an ammonia sensor should be able to tolerate contaminants such as particles in the exhaust gases and should show very low cross sensitivity to NO and HC. Typical diesel exhaust contains 3-9% CO 50-250 ppm CO, 6-12% O, 200-1,000 ppm NO, and 130-260 ppm HC. Furthermore, the response to NH3 should have a time constant in the order of 1 second. 

Zinc-5,10,15,20-tetraphenylporphyrin (ZnTPP) has been used as a coating material in ammonia sensors by immobilizing it on the surface of silicone rubber. Absorbance and fluorescence emission were the modes of detection. A spectral change is caused by the coordination of NH3 molecules to the Zn11 ion in the immobilized metalloporphyrins. Sensing films made from the ZnTPP immobilized in silicone rubber were found to be the most sensitive for NH3 sensing (20). 

X. L. Liu, and C. W. Zhou, Surface treatment and doping dependence of ln2C>3 nanowires as ammonia sensors , Journal of Physical Chemistry B 107, 12451 (2003). 

Ammonia sensors based on metal oxide nanostructures... 

Wire that senses pH changes such as antimony metal can be coated with urease (65) and used to determine urea in pure blood (66). Covering the antimony metal with a gas perm-selective membrane (67) improves the selectivity. The microsensors respond from 0.1 to 10 mM urea in 30-45 s. This ammonia sensor has a faster baseline recovery than commercial gas membrane electrodes. 

Gas-sensing electrodes are examples of multiple membrane sensors these contain a gas-permeable membrane separating the test solution from an internal thin electrolyte film in which an ion-selective electrode is immersed. For example, for the ammonia sensor, the pH of the recipient layer is determined by the Henderson-Hasselbach equation [Eq. (18)], derived from the chemical equilibrium between solvated ammonia and ammonium ions ... 

H. Nojiri and M. Uchiyama, Bias-dependent etching of silicon in aqueous ammonia, Sensors Actuators A34, 167, 1992... 

While the majority of enzyme electrodes fabricated have been rather large devices, there have been some recent reports concerning the development of miniaturized and even microsensors. For example, MeyerhoflF (M5) prepared an essentially disposable urea sensor (tip diameter 3 mm) by immobilizing urease at the surface of a new type of polymer-membrane electrode-based ammonia sensor (see Fig. 4). Alexander and Joseph (Al) have also prepared a new miniature urea sensor by immobilizing urease at the surface of pH-sensitive antimony wire. Similarly, lannello and Ycynych (II) immobilized urease on a pH-sensitive iridium dioxide electrode. In these latter investigations, ammonia liberated from the enzyme-catalyzed reaction alters the pH in the thin film of enzyme adjacent to the pH-sensitive wire. 

Edwards J Photonic Sensors, Atlanta, Georgia Development of an optical ammonia sensor to measure agricultural emissions. This sensor is capable of detecting ammonia concentrations at the 100 ppb range. Cooperative Agreement... 

Another ammonia sensor speciHcally designed for use in bioliquids is based on the evanescent wave technique and can be applied to the vapor-phase determination of ammonia above blood and serum [136]. It utilizes the ninhydrin reaction occurring in the polymer coating of the fiber, and the resulting color change is monitored by total internal reflection. The probe is applicable to clinical determinations normally carried out in the vapor phase, but works irreversibly. A linear relationship exists between absorbance and ammonia concentration in the clinically useful range of 0-4.0 pg mL. Comparison with the reference method showed a correlation coefficient of 0.92. 

Ammonia slip t)T)ically occurs when overstoichiometric amoimts of urea/ammonia are injected. Due to the ammonia storage capacity of the catalyst this is not emitted directly so if an appropriate control action is taken, ammonia slip can be avoided. However, up to today an inexpensive and fast NOx and/or ammonia sensor is not available to accommodate such a control action. Solutions have been proposed in the use of engine maps for predicting the NOx output [9] and/or an additional oxidation catalyst for avoiding ammonia slip [3]. The latter is detrimental to the sulfate emissions and as most of the oxidized ammonia will leave the system as NO, the overall NOx removal efficiency will be lowered. Therefore, it would be convenient if a possible overdose of ammonia will leave the system in the form of nitrogen (N2)- This would solve the SCR control problem. 

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