NH3 probes

Data on the effect of sodium hydroxide were measured by means of an NH3 probe supplied by Orion Research Company which operates in a manner analogous to a pH probe except that a membrane is used through which only the NH3 permeates. Thus the response of the probe is proportional to the activity (or partial pressure) of ammonia. 

The data in Table 24 on the effect of sodium acetate were measured in the same manner as data in Tables 1 and 2, so no assumptions about the ammonia probe behavior were necessary. This was done because readings from the NH3 probe became erratic after... 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

This observation evidences that the NO-NH3 probe mixture titrates both v5+=o and V +-OH active surface sites and that two vanadium sites are involved... 

The second principal problem is the reactivity of the anilinium ion (Ar — NH3) as a nucleophilic partner in an electrophilic substitution. Originally it was assumed that the unsubstituted ammonio group -NH3 and alkylated ammonio groups such as -N(CH3)3 are prototypes of substituents without electron-donating power. Since the 1960s, however, a large mass of data obtained with several completely different probes (for examples see Reynolds and Topsom, 1984) indicates that these ammonio... 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

The effect of platinum in a bacterial cell is to act in a very selective way — on cell division or causing lysis of lysogenic bacteria. It is likely that these changes are due to site specific attack on particular proteins or on particular bases in RNA or in DNA. It is necessary now to describe this attack in detail and to develop new probes for following the site in vivo. This exercise can be followed by a parallel examination of how cis- [Pt (NH3) 2CI2] acts as an anti-tumour agent. Here we only point to some interesting observations. 

Figure8. (a) Pump-probe spectra of (NH3)2NH+ through the A (v= 0,1,2 corresponding to 214, 211, 208 nm, respectively) states the data reveal the influence of the vibrational level probed in the experiments, (b) Pump-probe spectrum of (NH3hH+ and (NH3)sH+ with pump pulses at 208 nm and probe pulses at 312 nm A (v = 2) of the ammonia molecule. The role of cluster size is evident. The delay time is the interval between the pump and probe laser, (a) Taken with permission from ref. 65 (b) Taken with permission from ref. 68.

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

The number of sites titrated by NH3 and pyridine are similar except for sample Al-SBA-15(15) which means a good accessibility of pyridine in the solid pores without any steric hindrance. On the contrary, the integral heats of adsorption are higher when using pyridine due to its higher protonic affinity in gas phase compared to NH3 and the way in which probe molecules bind on the solid surface [6, 7]. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

Ammonia also forms clusters in the gas phase and the reactions of ammonia clusters with bare metal ions have been studied (61). The ammonia clusters probed by electron impact as [(NH3) H]+ showed a monotonic decrease in intensity with increasing value of n, but the metal complex ions [M(NH3) ]+ showed intensity gaps. Thus for most of the metals the [M(NH3)2]+ ion was much more intense than the [M(NH3) ]+ ions, where n 2, and so the coordination number 2 was reported to be the favored coordination number in the first coordination sphere. The favored ions M(NH3)m]+ were n = 2 for Cr+, Mn+, Fe+, Co+, Ni+, and Cu+, and n = 4 for V+. The non-transition metal Mg+ and Al+ had the favored coordination number of 3. 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

Completely isotropic motion of the probe in the interlayer would produce AB= A and 9 = 54.7°, the "magic angle". Thus, hectorite has the greatest anisotropy of motion, with a tendency of the z-axis of the nitroxide to tilt toward the z axis. This orientation may facilitate direct surface-methyl group interaction as well as close approach of the -NH3+ group to surface charge sites. 

After calibration the probe was inserted into the flask shown in Figure 2. A concentrated solution of NH3-H2S-CO2-H2O of measured density was then pipeted into the flask and after temperature and pH equilibration the pH was read. This normally took a period of five minutes for the equilibration process. 

The data in Tables 4 to 13 at 25°C plot nearly as vertical lines independent of the wt % NH3 in solution except for pure ammonia. At low concentrations the curves tend to deviate because of the ionization of NH3 and water. If the amount of ammonia in solution is approximately known, these curves can be used to estimate the moles of acid gas per mole of NH3 with fairly good accuracy. At 80°C the curves tend to slant more so the amount of ammonia in solution would have to be known more accurately before an estimate of the ratio of acid gas to ammonia could be made. Also we do not recommend the use of a pH probe at 80°C as a control indicator because the response of the probe is more erratic, and precise data are difficult to obtain. The use of an indicator probe at 25°C seems more logical because the output is more stable. 

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