Temperature ammonia response

Figure 2.4 The ammonia response versus temperature for an MISiC-FET sensor with (a) 25-nm Pt and (b) 60-nm Iras the gate contact metai. NHj concentration 12.5, 25, 50,100, 200, and 250 ppm in 10% Oj/Nj. (From [23]. 2003 iEEE. Reprinted with permission.)...

When ammonia was added continuously, which would be the normal mode of operation, the response curve was generally like that in Figure 2, even at low temperatures. Figure 5 demonstrates that equilibrium response was linear with the NO level up to NO/NH3 = 0.5 at 220°C and 1 ftVhr. Equilibrium values for continuous ammonia addition appeared to be the same as those for premixed ammonia and nitric oxide provided a correction was applied for ammonia response at elevated temperatures. With continuous ammonia addition, this ammonia effect was nullified electronically. 

The available data on temperature coefficients of conductance of the solutions can also receive adequate qualitative explanation from the unified model. The temperature coefficient of 2 per cent per degree in dilute solutions of sodium in ammonia is larger than the temperature coefficient of the viscosity which is 1.1 per cent per degree. In addition, for dilute potassium-ammonia solutions the temperature coefficient of the conductance is larger than in sodium-ammonia solutions and is found to be 2.9 per cent per degree. These two observations quite definitely indicate that some other factor than the decrease in viscosity with temperature is responsible for the observed temperature coefficient. For dilute solutions this factor is the expected increased dissociation of cluster monomers with... 

High temperature steam reforming of natural gas accounts for 97% of the hydrogen used for ammonia synthesis in the United States. Hydrogen requirement for ammonia synthesis is about 336 m /t of ammonia produced for a typical 1000 t/d ammonia plant. The near-term demand for ammonia remains stagnant. Methanol production requires 560 m of hydrogen for each ton produced, based on a 2500-t/d methanol plant. Methanol demand is expected to increase in response to an increased use of the fuel—oxygenate methyl /-butyl ether (MTBE). 

Bacterial catabolism of oral food residue is probably responsible for a higher [NHj] in the oral cavity than in the rest of the respiratory tract.Ammonia, the by-product of oral bacterial protein catabolism and subsequent ureolysis, desorbs from the fluid lining the oral cavity to the airstream.. Saliva, gingival crevicular fluids, and dental plaque supply urea to oral bacteria and may themselves be sites of bacterial NH3 production, based on the presence of urease in each of these materials.Consequently, oral cavity fNTi3)4 is controlled by factors that influence bacterial protein catabolism and ureolysis. Such factors may include the pH of the surface lining fluid, bacterial nutrient sources (food residue on teeth or on buccal surfaces), saliva production, saliva pH, and the effects of oral surface temperature on bacterial metabolism and wall blood flow. The role of teeth, as structures that facilitate bacterial colonization and food entrapment, in augmenting [NH3J4 is unknown. 

This investigation was undertaken to establish the ionic mechanism responsible for exchange reactions occurring at pressures ranging from 0.85 to 0.98 atm. in irradiated deuterium, hydrocarbon and deuterium, ammonia gaseous mixtures at 25 °C. and lower temperatures. New tech-... 

The enzyme systems responsible for fixing atmospheric N2 to form ammonia are known as the nitrogenases. These enzymes function at field temperatures and 0.8 atm N2 pressure, whereas the industrial Haber-Bosch process requires high temperatures (300-400°C) and high pressures (200-300 atm) in a capital-intensive process that relies on burning fossil fuel. Small wonder, then, that the chemistry of the nitrogenases has attracted considerable attention for many years. 

Co-adsorption experiments show a complex role of the nature and concentration of chemisorbed ammonia species. Ammonia is not only one of the reactants for the synthesis of acrylonitrile, but also reaction with Br()>nsted sites inhibits their reactivity. In particular, IR experiments show that two pathways of reaction are possible from chemisorbed propylene (i) to acetone via isopropoxylate intermediate or (ii) to acrolein via allyl alcoholate intermediate. The first reaction occurs preferentially at lower temperatures and in the presence of hydroxyl groups. When their reactivity is blocked by the faster reaction with ammonia, the second pathway of reaction becomes preferential. The first pathway of reaction is responsible for a degradative pathway, because acetone further transform to an acetate species with carbon chain breakage. Ammonia as NH4 reacts faster with acrylate species (formed by transformation of the acrolein intermediate) to give an acrylamide intermediate. At higher temperatures the amide may be transformed to acrylonitrile, but when Brreform ammonia and free, weakly bonded, acrylic acid. The latter easily decarboxylate forming carbon oxides. 

The development of an SCR system for vehicle applications requires precise calibration of the amount of urea injected as a function of the quantity of NO emitted by the engine, exhaust temperature and catalyst characteristics. Although model simulations can help in the control, it is necessary to use specific NO sensors which, however, still have problems of sensitivity and transient response. Installing a clean-up catalyst for ammonia would provide more latitude and obtain higher NO conversion ratios without re-emission of ammonia into the atmosphere. 

Sanchez-BaUesta MT, Lafuente MT, Zacarias L, Granell A (2000) Involvement of phenylalanine ammonia-lyase in the response of fortune mandarin fruits to cold temperature. Physiol Plant 108(4) 382-389... 

Lietti and co-workers studied the kinetics of ammonia adsorption-desorption over V-Ti-O and V-W-Ti-O model catalysts in powder form by transient response methods [37, 52, 53[. Perturbations both in the ammonia concentration at constant temperature in the range 220-400 °C and in the catalyst temperature were imposed. A typical result obtained at 280 °C with a rectangular step feed of ammonia in flowing He over a V2O5-WO3/TiO2 model catalyst followed by its shut off is presented in Figure 13.5. Eventually the catalyst temperature was increased according to a linear schedule in order to complete the desorption of ammonia. 

The observed dependence of the N 2 selectivity on temperature may suggest that the reduction of stored nitrates by H2 occurs via an in-series two-step pathway. The first step is fast even at low temperatures and is responsible for the consumption of hydrogen and for the formation of ammonia. The second step is slower and implies the reduction of residual nitrates with ammonia to form nitrogen this reaction occurs to a significant extent only at higher temperatures. 

The reactivity of other gas molecules such as ammonia and CO also shows a strong temperature dependence [24]. The response to ammonia as a function of temperature is shown in Figure 2.4 for the case of two different catalytic metals, as previously described. It is seen that the temperature profile of the response is also dependent on the type of catalytic metal and the concentration of NH3. This provides the opportunity to tailor-make the sensor to fit the application by choosing appropriate catalytic metals and measurement temperatures. 

This device has shown stable gas sensitivity to hydrogen, ammonia, hydrocarbons, and CO up to temperatures of 500°C. Detection of hydrocarbons up to a temperature of 775°C has been demonstrated [65], but a slow drift was detected in the response at temperatures above 600°C. 

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). 

Sensations of irritation, pain and temperature, commonly referred to as trigeminal responses, are also experienced in the nose. Typically strong stimulants include CO2, menthol, ammonia and acids however, most odorants do have some chemesthetic component They have shorter response latencies than in the oral cavity, consistent with a shorter distance required to penetrate to the basal layer containing die nerve endings the stimulants also increase in intensity with repeated exposures (sniffs) (28), Irritants reduce the intensity of odorants the reverse is also true. The interaction is at the central processing level because an odorant presented in one nostril will be affected by an irritant given in the other nostril. 

Figure 37A (symbols) displays selected step-response TRM experiments performed with 2% 02 at different temperatures, namely at 200, 225, 250 and 275 °C, in terms of NH3, NO and N2 outlet concentrations vs. time. In the run performed at T — 250°C (squares), upon NH3 step feed at t — 0 s the NH3 outlet concentration trace exhibited a dead time ( 250 s) and then slowly grew with time on stream, eventually approaching a steady-state value of about 200 ppm, that is much lower than the ammonia feed concentration level (1,000 ppm). In correspondence to the NH3 admittance to the reactor, a sudden drop of the NO outlet concentration was observed together with a mirror-like increment of the N2 concentration, associated with the start-up of the SCR reaction. The levels of NH3, NO and N2 at steady state were in fact consistent... 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

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