Adsorption of ammonia

Calculate the rotational contribution to the entropy of adsorption of ammonia on silica at -30°C, assuming (n) that the adsorbed ammonia retains one degree of rotational freedom and (b) that it retains none. In case (n) assume that the nitrogen is bonded to the surface. 

Step 1 represents adsorption of ammonia and step 2 its activation. The irreversible step 3 is obviously not elementary in nature, but unfortunately much information on the level of elementary steps is not available. Step 4 describes water formation and step 5 is the reoxidation of the site. Step 6 describes the blocking of sites by adsorption of water. The model thus relies on partially oxidized sites and vacancies on an oxide, similarly to the hydrodesulfurization reaction described in Chapter 9. The reactions are summarized in the cyclic scheme of Fig. 10.15. 

As was the case for water, the adsorption of ammonia also affects the intensity and the spectral position of the 960 cm band. A clear upward shift of the maximum of the band was observed by Bordiga et al. upon NH3 dosage [64]. However, in the case of NH3, the molecule interacts directly with Ti even at the lowest Pnhs. reflecting a much higher affinity toward Ti with respect to water. 

A molecular oxygen state is the most likely to be involved, it would require a barrier of only 67 k.f mol 1 and is exothermic a hydroperoxide state is formed together with NH2(a). When the heats of adsorption of ammonia and oxygen are taken account of, then according to Neurock44,45 there is no apparent activation barrier to N-H activation. 

Acidity of both zeolites was investigated by adsorption of ammonia, pyridine, d3-acetonitrile and pivalonitrile ((CH3)3CCN) used as probe molecules followed by FTIR spectroscopy. All samples were activated in a form of self-supporting wafers at 450 °C or 550 °C under vacuum for 1 h prior to the adsorption of probe molecules. 

Pyridine sorption studies on EDTA-dealuminated Y zeolites at various temperatures (54,58), as well as measurements of differential heats of adsorption of ammonia on aluminum-deficient Y zeolites (57,59) have led to the conclusion that aluminum-deficient Y zeolites have stronger acid sites than the parent zeolite. 

Figure 3 Variations with coverage of the differential heats of adsorption of ammonia on H-ZSM-5 (sample 1) measu-red at 150°C, (A), 200°C, (a), 250°C, (fe 300°C (0) and 400°C (+) The sample was outgassed at 400°C prior NH3 adsorp-tion. The meaning of the arrows is explained in the text.

Figure 17.4. Equilibrium data for the adsorption of ammonia on charcoal1 10) (a) Adsorption isotherm (b) Adsorption isobar (c) Adsorption isostere...

Cu(NH3)2BTC2/3 and finally copper hydroxide in the presence of water. The formation of the BTC salts was supported by the collapse of the structure after interaction of ammonia with unsaturated copper centers. The release of BTC and copper oxide centers provides sites for reactive adsorption of ammonia during the course of the breakthrough experiments. Interestingly, even though the structure collapses, some evidence of the structural breathing of the resulting materials caused by reactions with ammonia was found, based on the ammonia adsorption at equilibrium and the analysis of the heat of interactions [51]. 

Petit C, Mendoza B, Bandosz TJ. Reactive adsorption of ammonia on Cu-based MOF/graphene composites, Langmuir 26 (2010) 15302-15309. 

Bandosz TJ, Petit C. On the reactive adsorption of ammonia on activated carbons modified by impregnation with inorganic compounds,). Coll. Interface Science 2009, 338, 329-345. 

Petit C, Bandosz TJ Enhanced adsorption of ammonia on Metal-Organic Framework / graphite oxide composites analysis of surface interactions, Adv. Fund. Mater. 2009,19,1-8. 

Petit C, Huang L, Jagiello J, Kenvin J, Gubbins KE, Bandosz TJ. Toward understanding reactive adsorption of ammonia on Cu-MOF/graphite oxide nanocomposites, Langmuir 2011, 27, 13043-13051... 

Seredych M, Bandosz TJ, Adsorption of ammonia on graphite oxide / aluminum polycation and graphite oxide/zirconium polyoxycations composites,/ Colloid Interface Sci. 2008, 324 25-35. 

TPD-NH3 curves were obtained in a temperature range of 120 to 600°C, at a rate of 15°C/min. The adsorption of ammonia onto the sample was carried out at 25°C. Subsequently, the removal of ammonia was performed at 500 or 550°C for 1 h by purging air or pure nitrogen. Blank runs were carried out under the same conditions but with no NH3 adsorbed. The TPD-NH3 curves were obtained after subtraction of the blank run. 

The surface of silica turns hydrophobic on treatment with organo-silicon chlorides. Water vapor adsorption isotherms measured by Stober (219) showed a very marked decrease in reversible adsorption. Less than 0.3 primary adsorption centers per 100 A remained in the surface after covering with the organosiloxane layer. Similar effects were observed in the adsorption of ammonia. About 2.2 silanol groups per 100 A had not reacted with the trimethylsilyl chloride. Nevertheless, the greater part of these had become unaccessible for water vapor. Apparently, they were hidden underneath a trimethylsilyl umbrella. ... 

The results confirm that the adsorption of ammonia is very fast and that ammonia is strongly adsorbed on the catalyst surface. The data were analyzed by a dynamic isothermal plug flow reactor model and estimates of the relevant kinetic parameters were obtained by global nonlinear regression over the entire set of runs. The influences of both intra-particle and external mass transfer limitations were estimated to be negligible, on the basis of theoretical diagnostic criteria. 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

The following data indicate the close agreement between the data obtained by Titoff and those calculated with the aid of this equation for the heat of adsorption of ammonia by charcoal at 0° G. is calculated and determined in calories per c.c. of ammonia (at N.T.P.) adsorbed, x is measured in c.c. per gm. at N.T.P. 

The variation in the heat of adsorption of ammonia by meerschaum has been likewise examined by Ohappuis (Wied. Ann. XIX. 21,1883) over an extended range of pressures with the following results ... 

Acidic properties of zeolite L were observed to correlate well with its structural disorders. The Si-MAS-NMR spectrum of zeolite L having a Si/AI ratio different from 3 revealed that Al distribution deviated from the ideal and suggested the presence of six different boat-shaped 8-ring patterns. Differential molar heats of adsorption of ammonia changed step-wise with the adsorbed amount, which reflects the difference in the acid strength of protons located in structurally different 8-rings. 

In this study, we analyze this situation using Si-MAS-NMR spectroscopy and high-temperature ammonia-adsorption calorimetry. The acid strength will be determined from the heat of adsorption of ammonia. On adsorption of ammonia, the reaction. 

Heats of adsorption of ammonia were measured with a twin-conduction-type microcalorimeter equipped with a volumetric vacuum line. The details and procedures have been described previously [6-8], Prior to calorimetric measurements, samples were activated by calcination under 1 mPa pressure on increasing the temperature at a rate of 3 K min and at the final temperature, in general 723 K, for 10 h. Adsorption of ammonia was carried out at 473, 573 and 623 K. The Si-MAS-NMR spectra were taken using a JEOL GX-270. 

Adsorption of ammonia on the Au(lll) surface at V4 and V9 of the monolayer coverage has been theoretically modeled [85]. 

There is considerable evidence that surface acidity influences the catalytic activity of iron molybdate [254]. It was found by studying the adsorption of ammonia using infrared spectroscopy that, under reaction conditions, the acidity is due to Lewis sites. The conclusion is that surface acidity is a necessary, but not a sufficient, property. 

The adsorption of ammonia and amines has been studied many times as a method of estimation of the acidity of solid surfaces. Some of the results are pertinent to the mechanism of amine transformation on these catalysts. Depending on the structure of the catalyst surface, several types of adsorbed species have been observed. 

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