Nitrogen probe molecule

The Nitrogen Availability Assay [376] consists of growth tests under defined conditions using mineral salts medium and organonitrogen compounds as sources of carbon and/or nitrogen. Probe molecules include quinoline, pyridine, carbazole, and porphyrin. Growth tests are performed using six conditions ... 

The apparent acidities of zeolite catalysts are characterized by Av0h values induced by adsorption of hexane (Av0h.C6) under the same conditions than those applied during separate catalytic experiments. The Avoh,c6 values for the different zeolite samples shown in fig. 2 were determined as Figure 1. DRIFT spectra measured in the above for the nitrogen probe molecule. vOH region before (solid lines) and after (dashed lines) contacting the samples with N2 at 298 K and 9 bar equilibrium pressure. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

Many adsorbents, particularly the amorphous adsorbents, are characterized by their pore size distribution. The distribution of small pores is usually determined by analysis, using one of several available methods, of a cryogenic nitrogen adsorption isotherm, although other probe molecules are also used. The distribution of large pores is usually determined by mercury porisimetry [Gregg and Sing, gen. refs.]. 

When protonation of a probe leads to dramatic effects on the electronic absorption and emission as described above, one can expect that cations other than protons might be capable of inducing similar effects if these ions can be made to bind to the basic atom of the probe molecule. A simple but effective method to do so exists in the formal replacement of the amino substituents ofchromophores by aza crowns. In the resulting chromoionophores the amine nitrogens possess simultaneously an electron-donor... 

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

The acid sites strength can be determined by measuring the heats of adsorption of basic probe molecules. The basic probes most commonly used are NH3 (pTTa = 9.24, proton affinity in gas-phase = 857.7 kJ/mol) and pyridine (pTTa = 5.19, proton affinity in gas-phase = 922.2 kJ/mol). The center of basicity of these probes is the electron lone pair on the nitrogen. When chemisorbed on a surface possessing acid properties, these probes can interact with acidic protons, electron acceptor sites, and hydrogen from neutral or weakly acidic hydroxyls. 

As examples of probe molecules directly introduced onto solid acids, we consider ketones and aldehydes, amines and other nitrogen-containing compounds, phosphines, and molecules that form multiple hydrogen bonds. 

The coordinative and/or dissociative adsorption of various probe molecules has been used to characterize the surface properties of Ti02) which finds applications as a catalyst, photocatalyst, and sensor. Among the molecules used as probes, we mention CO (37, 38, 563-576), C02 (563, 565, 577), NO (578,579), water (580,581), pyridine (582,583), ammonia (584,585), alcohols (586, 587), ethers (including perfluoroethers) (588), ozone (589), nitrogen oxide (590), dioxygen (591), formic acid (592-594), benzene (584), benzoic acid (595), and chromyl chloride (596). 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

It should also be noted that nitrogen is not an unbiased probe adsorbate. Obviously the surface accessibility for irregular materials depends on the size of the probe molecule a large probe cannot foUow the irregularity of the surface. Analyte molecules are usually larger than nitrogen molecules, and may not be able to penetrate aU pores. Thus only a fraction of the surface area is involved in analyte retention. To fulfill lUPAC recommendations for surface characterization methods [9], the most suitable method depends on the specific application. Recently an approach that involves IPR ion adsorption proved effective. The best probe to determine the packing area... 

The active site on the surface of selective propylene anmioxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an CC-H abstraction component such as Bi3+, Sb3+, or Te4+ an olefin chemisorption and oxygen or nitrogen insertion component such as Mo6+ or Sb5+ and a redox couple such as Fe2+/Fe3+ or Ce3+/ Ce4+ to enhance transfer of lattice oxygen between the bulk and surface of the catalyst. The surface and solid-state mechanisms of propylene ammoxidation catalysis have been determined using Raman spectroscopy (40,41), neutron diffraction (42—44), x-ray absorption spectroscopy (45,46), x-ray diffraction (47—49), pulse kinetic studies (36), and probe molecule investigations (50). 

As alluded to above, probe molecules such as organic phosphates can also be used to probe Lewis acid sites. However, the most common probe molecules employed are nitrogen-containing species, in parhcular ammonia, alkylamines and pyridine. For the latter two either or N NMR techniques may be applied however, N NMR is generally preferred as the resultant spectra appear less complex and, as adsorphon to the surface occurs via the nitrogen atom, the observed resonance shifts are larger than they would be for spectra. The development of this method to investigate Lewis acidity of aluminas and siUca-aluminas has previously been discussed in detail by Eckert [196] and by Maciel and Ellis [183] and will only briefly be recapped here. 

All of the monoliths produced were dried at 150°C and fractions subsequently heat-treated at either 500°C or 850°C in a nitrogen atmosphere. The static and dynamic adsorption capacities of these monolithic composites at 30°C were determined using o-DCB as a probe molecule. This molecule was chosen since it may be considered as approximately corresponding to half a molecule of tetrachloro-dibenzene dioxin (TCDD), the most toxic isomer of the dioxin family. 

The adsorption on zeolites of a basic probe molecule containing nitrogen can be used to monitor changes in external acidity. The acid sites determined in this way are the first to come into play during contact between catalyst and charge and may have a decisive influence on the activity and selectivity of these solids (Fig. 5.10). 

Nitrogen and carbon monoxide are both candidates as small probe molecules which may interact only with strong acid sites in zeolites and which can be observed by infrared spectroscopy. As an illustration of this method, consider the recent work of Wakabayashi et al. on N2 adsorbed in H-mordenite [29], HY and HZSM-5 [30], References to infrared spectra of adsorbed CO include [31-33]. 

Another interesting aspect lies in the capacity of nitrogen containing molecules such as basic NH3 and pyridine and acidic (amphotenic) pyrrole, to probe respectively acidic and basic sites in zeolites. This allows among other things to monitor acidic and basic sites in the outer surface of the crystals, a region in which the control of the density and strength of such sites is often critical for catalysts selectivity and deactivation. 

In order to more precisely differenciate the acid sites, adsorption of pyridine (pKa=5.25), 3,5-dimethylpyridine (pKa=6.15) and 2,6-dimethylpyridine (pKa=6.72) was carried out at 353 K on the samples. These three basic probes display a lower pKa than ammonia (pKa=9.25) and should titrate less weak acid sites. 2,6-lutidine (2,6-DMP) is supposed to adsorb on Bronsted sites preferently to 3,5-lutidine (3,5-DMP) which should adsorb, as pyridine, on both Lewis and Bronsted sites. This behavior can be explained by the steric hindrance due to the methyl groups, the nitrogen atom being less accessible. For example. Figure 4 shows the differential heats of adsorption of the three probe molecules on the sample with Ti=249 pmol/g pretreated at 773 K. All the curves show a sharp decrease till... 

In the original paper one of the major advantages put forward in favour of the Os method over the contenq)ary t method was that it allows a similar type of analysis of adsorption isotherms of other adsorptives, besides nitrogen, to be made. This is of particular importance in the case of activated carbons where it is customary to make use of a range of probe molecules of different size, shape, polarizability and polarity in order to carry out a more complete characterization. A number of authors have since demonstrated the general feasability of doing this and reference data for the adsorption of neopentane and butane, for... 

HOC Solubilization. Carbon-14 labeled DDT (2,2-bis(4-chlorophenyl)-l,l/l-trichloroethane) or pyrene were used as HOC probes to study the micelle formation phenomena. The probe dissolved in toluene was added to a culture tube, and the solvent evaporated under nitrogen. Ten milliliters of each HA or FA solution were added to the tube which was then capped with a teflon-lined lid. The tube was transferred to a water bath (25.0 °C 0.5 °C) where the solutions were equilibrated for 24 hours with periodic shaking. After equilibration the tubes where centrifuged to minimize the amount of suspended or particulate probe molecule. An aliquot of the supernatant was then withdrawn and transferred to a scintillation vial. Scintillation fluid was added to the vial and the p-emission counted at a 0.5% counting error. Additional methodological details can be found elsewhere (13). 

Sorption in micropores can be described by the Dubinin-Radushkevic formalism that has been adapted by Stoeckli et al. This is a largely empirical approach and it should be emphasized that the use of a combination of Langmuir types isotherm leads to similar quantitative results. For evaluation of the distribution of micropores, one can either rely on high-resolution measurements of mostly nitrogen adsorption as suggested by Horvath and Kawazoe or use a combination of probe molecules of different minimum kinetic diameter. More recently, approaches based on density functional theory are put forward. 

The microporous gel may have a fraction of micropores so narrow that nitrogen molecules cannot enter and cover the total silica particle surface. This would result in the smaller apparent specific surface area than the real area because we use nitrogen molecule as the probe. This shows that the surface area estimated by the gas adsorption method is dependent on the size of the probe molecule even in case of nitrogen. 

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