Ammonia as a probe

The influence of the nature of the aluminum source on the acidic properties of mesostructured materials (MCM41) has also been studied in the literature [244]. Microcalorimetry experiments using ammonia as a probe molecule have shown that Al insertion into the mesoporous silicate framework affected acid site strength and distribution in a manner controlled by the synthesis conditions (materials prepared... 

Ai (140) measured the acid site concentration by the adsorption of ammonia. No correlation was found between the P/V ratio, the acidity, and the catalytic activity. This result has been attributed to the use of ammonia as a probe molecule that cannot distinguish between Lewis and Br0nsted acidity. Comaglia et al. (137) measured the acid sites using pyridine and acetonitrile as probes. However, the pyridine results showed no correlation between the Lewis to Br0nsted acid site ratio or the Lewis acid site concentration and the activity and selectivity of the catalyst for MA formation. 

A drawback of using ammonia as a probe molecule is the large variety of possible adsorption forms. For example, an ammonium ion formed in the adsorption process could be bound to lattice oxygen ions through two, three, or four hydrogen atoms. A similar variety of interactions can also stabilize coordinated ammonia or ammonia H-bonded to weakly acidic hydroxyls. In the latter case, measurement of the hydroxyl acidity through the H-bond method should be made with another probe for which such... 

Unfortunately, the position of the 6as(NH4) modes of the ammonium ion strongly depends on the adsorption geometry and does not permit any definite conclusions on the acid strength. To determine the acid strength, one needs to combine IR spectroscopy with temperature-programmed desorption experiments. In conclusion, ammonia as a probe gives information on the number of acidic hydroxyls (that are able to pro-tonate it) and, by combination with other techniques, on the strength of these sites. 

Evidently, there is no straight correlation between the acidity of hydroxyls estimated by using ammonia as a probe and the acidity measured by the hydrogen-bond method. As a rule of thumb, hydroxyls experiencing a CO-induced red shift of the OH modes of more than approximately 150 cm are able to protonate ammonia. 

The active sites of isomorphously substituted MFl structures activated at 673 K have been characterized by janchen et al. [245] using microcalorimetric measurements carried out at 423 K with ammonia as a probe. Because of decreasing heats of NH3 adsorption, the Bronsted acid site strength of the modified MFl was reported to decrease in the sequence Al > Fe > In > silicalite. In... 

API materials have also been studied by microcalorimetry using ammonia as a probe molecule at 423 K by Yuen et al. [277]. The results indicated... 

Kosslick et al. [145] investigated surface acidity by using ammonia as a probe molecule. Adsorbed pyridine and solid state Ga MAS-NMR were also used for the same purpose [ 144,146]. Otero Arean and co-workers [ 147]... 

Studies of coadsorption at Cu(110) and Zn(0001) where a coadsorbate, ammonia, acted as a probe of a reactive oxygen transient let to the development of the model where the kinetically hot Os transient [in the case of Cu(110)] and the molecular transient [in the case of Zn(0001)] participated in oxidation catalysis16 (see Chapters 2 and 5). At Zn(0001) dissociation of oxygen is slow and the molecular precursor forms an ammonia-dioxygen complex, the concentration of which increases with decreasing temperature and at a reaction rate which is inversely dependent on temperature. Which transient, atomic or molecular, is significant in chemical reactivity is metal dependent. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

The same types of catalysts used for HDS are also used for hydrodenitrogena-tion, and ammonia is used as a probe molecule to characterize these catalysts. An IINS investigation of NH3 on partially desulfided RuS2 (25) showed that the chemisorption was dissociative, forming NH2 groups on the coordinatively unsaturated ruthenium sites. 

Arp, D. J., and Zumft, W. G. (1983). L-Methionine-SR-Sulfoximine as a probe for the role of glutamine synthetase in nitrogenase switch-off by ammonia and glutamine in Rkodopseudomonas palustris. Arch. Microbiol. 134, 17—22. 

In a recent paper, Guimon et al. [42] advocate the use of NHj as a probe molecule. Interestingly working with commercial NH4Y samples they also find two peaks at 402.7 and 401.2 eV (referencing to Cjj = 284.6) when the samples are calcined at 400°C and a third one close to 399 eV when the calcination temperature is 700°C. The N/Al ratio never reaches the value of 1, even at a desorption temperature of 100°C in spite of the fact that ammonia is a stronger base than pyridine. 

The difference in size of the two probe molecule does not explain these results. Even though ammonia, as a small molecule, is able to penetrate through narrow windows of the zeolite structure which might be inaccessible for pyridine or for the diol, the acidity of these sites should not be very different from the sites in the larger cavities. 

Temperature Programmed Desorption. Ammonia has been used as a probe molecule in a number of studies of crystalline borosilicate molecular sieve (22,33,45). It has been shown that the ammonia is desorbed from the borosilicate samples at low temperature, 465°K, indicating the weak acidity of the hydroxyls (33). The hydroxyls have been shown to have higher acidity than silanol groups and lower acidity than those of ZSM-5 (45). The results are consistent with IR and calorimetric data for NH3 adsorption/desorption (35.). ... 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

Ammonia, which possesses a large dipole moment, has been used extensively as a probe molecule for the characterisation of both Lewis and Bronsted acidic sites. Figure 22 shows the significant difference in the FR data between ammonia in zeohte crystals and in pellets. The FR spectra of ammonia in zeolite crystals demonstrated that the rate of the ammonia adsorption on different acidic sites in the crystals controls the overall dynamics of the processes occurring in the systems, hi the case of pellets, the rate-controlhng step was found to be macropore diffusion with (Fig. 22a,2,b,2) or without (Fig. 22c,2) surface resistances [77]. 

Early applications of IR spectroscopy in zeolite research go back to studies by Szymanski et al. [201],Bertsch and Habgood [202], Tsitsishvili [203], Watanabe andHabgood [204] and especially the pioneering works of Uytterhoeven,Christ-ner and Hall [205] and Cant and Hall [206] who, in particular, investigated the formation of OH groups on the external and internal surface of Y-type zeolites as, somewhat later. Ward [207] also did. Hall s group also studied the adsorption of small molecules such as ammonia [205] and ethylene [208-210] and employed pyridine as a probe to discriminate Bronsted and Lewis acid sites (cf. 

Substituted amines are characterized by even higher basicity than ammonia and can thus be protonated by hydroxyls that are not able to form NH4. For example, Bronsted acidity has been found on titania when using trimethylamine (TMA) as a probe (455). TMA has also been successfully used for revealing the heterogeneity of the bridging hydroxyls in FAU zeolites (456,457). 

The number of published articles that include characterization of surface acidity by pyridine adsorption is immense. Therefore, we are not able to present a comprehensive analysis of the available data on the pyridine—solid interaction. In the next paragraphs, we shall briefly review the main conclusions that were obtained through the use of pyridine as a probe and will contrast the results of pyridine adsorption with those of ammonia adsorption. 

Apart from using ammine complexes as a probe for adsorption mechanisms, there is a need to develop treatment procedures for ammonia-containing metal wastes. Ammonia-containing metal wastes are encountered in, for example, mining by-products, which may be subject to ammoniacal leaching either directly or after treatment [51,58,59]. Leather tannery effluent also contains high levels of Cr(III), often in the presence of ammonia, the pollutant that causes the most difficulty in Cr(IIl) effluent treatment [8]. 

---