Ammonia thermal desorption

The adsorption capacity of the calcinated samples for nitrogen and for m-xylene decreased when the zinc content increased, which shows there was a partial blockage of the pore access. However no change was observed in the crystallinity (similar X Ray Diffraction Spectra) nor in the acidity of the zeolite (The ammonia thermal desorption spectra of HZSM5, 1.3 and 1.6 ZnHZSMS were almost identical (Figure 1)). 

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

Relatively few descriptions of direct mass spectral analysis of plastics compounds have appeared in the literature [22,37,63,240,243], Additives in PP were thermally desorbed into a heated reservoir inlet for 80 eV EI-MS analysis [240], Analysis of additives in PP compounds via direct thermal desorption ammonia CI-MS has been described [269] and direct mass spectrometric oligomer analysis has been reported [21],... 

Figure 5.60 Thermal desorption of ammonia from Mg(NH3)6Cl2.

Others. Attempts were made to develop general methods for mer-captans, amines, alcoholamines, and nitroalkanes. However, results were not satisfactory. We did not use a single collection medium for amines. The media used for various amines included silica gel with and without SO, a porous polymer with thermal desorption, and a bubbler. The preferred method is collection of silica gel followed by acidification of the sample with dilute HC1 immediately after collection. This method should be successful for ammonia and most aliphatic amines. Ion chromatography was used successfully for the analysis of methyl amine. This analytical method may be... 

The technique has been fruitfully used to characterize acid and basic sites in many catalysts, in particular for zeoHtes and metal oxides [143]. It has also been applied for POMs [144]. It consists of measuring the differential heats of adsorption when adsorbing successive increments of a basic probe molecule such as ammonia or pyridine for acidity characterization or of an acid probe molecule such as GO2 or SO2 to characterize basicity. The technique produces a histogram of the acid-base strength as a function of coverage, in particular when heterogeneity in strength exists. The data should then be compared with ammonia or pyridine desorption data from IR and thermal desorption experiments (see above). 

Pyridine has also been used extensively as an adsorbate in thermal desorption measurements. It has the advantage of being a weaker base than ammonia, thus selectively sorbing only on the stronger acid sites. Its larger size, however, has been shown to restrict its accessibility into small pores. ... 

NH3TPD Thermal desorption of ammonia was used to characterize the acid strength of the support and catalysts. A McBain microbalance was employed using 1 mg of sample. Ammonia was adsorbed at room temperature and the total mol of NH3 10 /m adsorbed were measured. Then the sample was heated using a ramp of 6 °C/min and the remaining amount of NH3W calculated at three temperatures (200/300/400 °C). See Table 3. 

The adsorption of gaseous ammonia can therefore be used to measure the number of acid sites. Because adsorption of ammonia is more difficult to measure than desorption, temperature-programmed desorption of ammonia is often employed to measure the number of aetive sites of solid-acid catalysts. Integration of the thermal desorption plots gives the total number of acid sites and the desorption temperature indicates their strength. Ion-exchangers cannot, however, readily be heated to the temperatures required for complete desorption of ammonia without being decomposed. 

Figure 1. Amnonia thermal desorption amount of ammonia desorbed at different temperatures —) HZSM5 (-—) 1.6 ZnHZSMS.

Surface areas of some representative clays, namely Zn(ll), Fe(lll), Ti(IV), Zr(IV) and Ce(lll), were calculated from BET isotherms determined at 77 K. In all cases the values obtained fall within the range of 220-240 m /g, with the exception Zr-clay whose values are around 190 m2/g. The number of acid sites was determined by stepwise thermal desorption of ammonia at 373 K, then swept by a flow of dry nitrogen while the temperature was raised by steps of 50 K. The amount of ammonia evolved from the solid was monitored by conductimetry. X-ray diffraction patterns were recorded on a Phillips computer-driven X-ray diffractometer using CuKai radiation. 

The levels of ammonia in the ambient air inside semiconductor plants, particularly in clean areas used for deep ultraviolet (DUV) photoresist and lithographic processes, are important since ammonia and l-methyl-2-pyrrolidone can affect the acid catalysis of photoresist formulations. Ion mobility spectrometry has provided the necessary analytical capabilities to monitor both compounds at low part-per-billion levels. Compared with gas chromatography (GC), the time required for a measurement with the ion mobility spectrometer was significantly shorter than the 10 min required for thermal desorption/GC. ... 

The states of similarly adsorbed nitrogen were studied by thermal desorption technique on the surface of Mo-W and Re-W. The desorption heat of adsorbed nitrogen measured were 41kJ mol for 7 states on Mo (100), 364kJ-mol (Ref. 82) for / states on Mo (100), and 364 kJ mol for / state on Mo (110). The / state on Re single crystal was considered to be an intermediate for ammonia synthesis. 

Recombination of adsorbed hydrogen atoms leads to the reverse process of thermal desorption whose rate is governed by Ead + E, where E is the activation energy for dissociative adsorption. If a hydrogen-covered Fe surface is heated up in vacuo, desorption will be completed at 500 Under the conditions of ammonia synthesis (>700 K) this step will hence be so rapid that the steady-state coverage of Had will be determined by the adsorption-desorption equilibrium H2+ 2Had, where the concentration of free surface sites is, of course, affected by the presence of all other surface intermediates. 

Figure 9.15. Comparison of the total ammonia adsorption of coated and extruded V2O5/WO3—Ti02 catalysts. Catalyst volume = 7 cm3. Model gas for loading 10% 02, 5% H20, NH3 = 1000ppm, and balance N2. GHSV = 52000h 1. Model gas for temperature-programmed desorption (TPD) experiment 10% 02, 5% H20, NO = 1000 ppm, NH3 = 1000 ppm, and balance N2. NH3 desorbed is calculated as the sum of thermally desorbed NH3, directly measured at the catalyst outlet, and chemically desorbed NH3, measured by the reduction of the NO concentration due to the SCR reaction.

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