Chemisorbed ammonia

Allyl alcohol + NHs- The coadsorption experiments were made by first putting the catalyst into contact with NH3 at 200°C followed by removal of gaseous NH3 at r.t. and then putting the catalyst with the chemisorbed ammonia into contact with the allyl alcohol at r.t. Using procedures for the coadsorption tests like those used for propylene + NH3, less resolved spectra were obtained. The more remarkable differences in comparison with the analogous spectra for propylene + NH3 (Fig. 3) or acrylic acid + NH3 coadsorption (see below) are that (i) the band in-... 

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 chemisorbed ammonia on uncovered surface of TiO is not high as adsorbed ammonia at higher temperatures desorbs with... 

The infrared study of chemisorbed ammonia by Mapes and Eischens (55) was the first to demonstrate the power and utility of the infrared spectroscopic method for determination of surface acidity. These investigators demonstrated that IR spectra of ammonia chemisorbed on cracking catalyst contained H—N—H-bending bands that arose from NH4+ and coordinated NH3 (Fig. 6), a finding that constituted direct evidence for the existence of Br0nsted and Lewis acids on the surface of silica-alumina catalyst. Parry (23) subsequently suggested the use of pyri-... 

Fig. 6. Infrared spectra of silica-alumina catalyst A, unheated B, dried at 500°C under vacuum C, dried catalyst with chemisorbed ammonia (35). (Reprinted with permission of the American Chemical Society.)...

Spectrum A of Fig. 15 is due to ammonia chemisorbed on the cracking catalyst which had been dried under vacuum for 70 hrs. at 500° C. Chemisorbed ammonia was defined as that retained by the catalyst after evacua-... 

When H2S is chemisorbed on platinum or water adsorbed on silica, the S—H and 0—H stretching bands appear at positions close to those observed for H2S and H20 in the gaseous state. These results, together with those found for chemisorbed ammonia, indicate that the stretching vibrations of the single bonds to hydrogens are not markedly affected, by formation of coordinate bonds between a surface and the central atom of the molecule. 

As an application of the pressed-salt method to catalysis, French and co-workers studied ammonia on cracking catalysts (57, 58). Other workers (83) have shown that this was an unfortunate choice because ion exchange occurs between the cracking catalyst and the halide salt. As a result of this ion exchange, spectra are observed which are due to ammonium halide rather than chemisorbed ammonia. 

This ion exchange is shown in Fig. 28, where A is the spectrum of Alon-C, B is due to Alon-C plus chemisorbed ammonia obtained by direct observation, and C was obtained after the Alon-C with chemisorbed ammonia was pressed into a KBr disk (S3). In B the ammonia is in the NHt form because the sample had been exposed to the atmosphere. It is seen that the direct method shows the NHt band at 6.84 p, while the pressed-salt method shows this band at 7.14 p. Although this difference in wavelength is not large, it is important because it shows that the pressed-salt method cannot be used to study adsorption systems where ion exchange is likely to be encountered. 

Figure 12.5 Irreversibly adsorbed and/or chemisorbed ammonia on silica gel as a Junction of the reaction temperature, with and without TCS modification.

A similarly strict conclusion cannot be drawn from the experimental data for chemisorbed ammonia, which give rise to a behavior intermediate between those of the two limiting cases. One should take into account, however, that due to its smaller size, an ammonium ion situated in a channel intersection does not lead to a simultaneous blockage of all four adjacent... 

The acidity of the catalyst arises due to interaction of the components (e.g., silica and alumina) during preparation. Pure silica and pure alumina have little or no cracking activity, but the presence of only a few hundredths of a per cent of alumina in silica is sufficient to produce an active catalyst (138,148,320,321). Activity and acidity both increase, up to a certain point, with increased alumina content (320,324). Infrared spectra of ammonia chemisorbed on silica-alumina catalyst indicate that most of the chemisorbed ammonia is in the NHs form, with only a relatively small amount of NH4+ (204). From this evidence it is concluded that most of the catalyst acid is of the Lewis type since, in reacting with a Bronsted acid or a hydrated Lewis acid, the ammonia would be converted to an ammonium ion. 

In the context of this model we thus expect that the addition of ammonia to an iron cluster will lower its ionization threshold. The magnitude of the IP decrease will be dependent on the number of ammonia molecules chemisorbed and on the size of the cluster. We except that if the ionization thresholds and reactivities toward hydrogen were measured for Fe (NH3) or Fe H2 (n variable, m constant), an IP-ln (rate constant) anticorrelation would be found. Expieriments to date have shown that, upon ammoniation, the minimum in reactivity of iron clusters toward hydrogen shifts to smaller cluster sizes and that the rate constant for hydrogen chemisorption for these ammoniated clusters is about a factor of 10 lower than that for the bare clusters. However, the number of chemisorbed ammonias is different for each cluster. Experiments involving metal clusters with a fixed number of chemisorbed ammonias is a needed probe of the detailed interaction between NH3 and a cluster. 

Chen and Yang [88] concluded from their studies that the active sites for SCR reaction over sulphated titania are Bronsted sites and the mechanism is Eley-Rideal. 52% of the chemisorbed ammonia was active in the SCR reaction while 48% of the chemisorbed ammonia was inactive. 

The sulfated samples show a feature at 168.5 eV which was assigned to S2p of S . Catalysts which were subjected to the SCR reaction show two peaks for N1 s in the XPS spectrum, namely at 399.9 and 401.7 eV. These peaks were assigned to the ammonia chemisorbed on Lewis sites and ammonia chemisorbed on Bronsted sites, respectively. This was confirmed by means of an intensity at 1401 cm in the FTIR spectrum which was assigned to chemisorbed ammonia on Bronsted sites. 

Do the framework A1 account for the total quantity of strongly chemisorbed ammonia ... 

The NH adsorption isotherms are not perfectly rectangular, thus the precise estimation of the quantities of strongly chemisorbed ammonia is somewhat difficult. We have retained for this estimation a pressure of 1 torr which corresponds to tlj pseudo-plateau region of the isotherms. The calculated NH../A1 ratios are close to 0.80 (Table II) and a nice linear correlation... 

Pliskin, W. A., and R. P. Eischens, 1955. The effect of using the pressed-salt technique to obtain the spectrum of chemisorbed ammonia. J. Phys. Chem. 59 1156. 

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