Adsorbates 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. 

C s = eoneentration of the most abundant surfaee intermediate nh3S = eoneentration of adsorbed ammonia... 

The concentration of adsorbed ammonia from Equation 3-344 is CsPnh3... 

N(ls) spectra have provided very clear evidence for the nature of adsorbed ammonia and hydrazine (47-47c, 52, 64) whether adsorption is molecular, or if dissociated, the extent of the dissociation, i.e., whether the surface species are NH2, NH, or N. 

This empirical rate expression considers the active sites of the catalyst as only a fraction of the total adsorption sites for ammonia and is consistent vfith the presence of a reservoir of ammonia adsorbed species which can take part in the reaction. The ammonia reservoir is likely associated vfith poorly active but abundant W and Ti surface sites, which can strongly adsorb ammonia in fact, nhs roughly corresponds to the surface coverage of V. Once the ammonia gas-phase concentration is decreased, the desorption of ammonia species originally adsorbed at the W and Ti sites can occur followed by fast readsorption. When readsorption occurs at the reactive V sites, ammonia takes part in the reaction. Also, the analysis of the rate parameter estimates indicates that at steady state the rate of ammonia adsorption is comparable to the rate of its surface reaction with NO, whereas NH3 desorption is much slower. Accordingly, the assumption of equilibrated ammonia adsorption, which is customarily assumed in steady-state kinetics, may be incorrect, as also suggested by other authors [55]. 

The surface nethoxyl groups on the modified catalyst were measured by i.r. spectroscopy and their thermal stabilities were studied by Temperature-Programmed Decomposition (TPDE) in Ar. The surface acidity was measured by TPD of irreversibly adsorbed ammonia and by pyridine adsorption by dynamic method and i.r. spectroscopy. 0.10 g pretreated catalyst was used to measure the amount of irreversibly adsorbed pyridine. The irreversibly adsorbed ammonia was... 

Figure 3. TPD of irreversibly adsorbed ammonia on untreated (-) and modified (-.-) HZSM-5 in Ar.

The number of acid sites on pillared clays was determined by means of temperature programmed desorption (TPD) of ammonia. In each TPD experiment, a sample weighing about 0.5 g was treated in vacuo for 1 h at a given temperature in the range 400 - 600°C. Ammonia was adsorbed at a desired temperature (100-300°C) for 30 min and evacuated for 30 min. This sample was heated to 700°C at a rate of 10°C/min and desorbed ammonia was monitored by thermal conductivity detector. As water was desorbed simultaneously with ammonia, the ammonia TPD spectrum was obtained by point-by-point subtraction of the water desorption spectrum obtained with the sample which had not adsorbed ammonia. 

Fig. 4. The cracking activity of Al-mont( Q) and Al-sapo( A) as a function of the concentration of acid sites adsorbing ammonia at 200°C.

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

The experiment in Fig. 36 has been analyzed according to the usual plug-flow model of the test microreactor. In this case, the ammonia mass balance equations were modified in order to include the oxidation reaction R5 in Table V, which was considered to proceed via adsorbed ammonia NH3. Moreover, the mass balance for gaseous nitrogen was introduced... 

The following reactions were included in the kinetic model NH3 adsorption (R3 in Table V), NH3 desorption (R4 in Table IV), NH3 oxidation (R5 in Table IV) and standard SCR (R6 in Table V). Mass balances for adsorbed ammonia and nitrogen now include the standard SCR reaction. Moreover, the mass balance of gaseous NO was introduced, too... 

A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded to a carbon atom causes activation of the bonds from a /3-carbon atom to the substituents. In this way, the splitting of the Cp—Cy bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in /3-positions to the position where a carbonium ion might be formed was observed early and named the /3-rule by Thomas [2], The question remains open as to which type of acidic centre is able to activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [240], viz. 

From the reaction mechanism above, derive an expression for the adsorbed ammonia site fraction, 6 = [NH3(ad)]/T. Make the simplifying assumption that kip(NH3) + k-1 >> 2[Si(s)] this is equivalent to assuming that reaction 1 is at equilibrium, and that reaction 2 makes a negligible difference in the adsorbed ammonia concentration. Determine the NH3(ad) site fraction for an ammonia pressure of 124 Pa. 

Chromium triphenyl, (CgH Cr,1 is formed when chromium triphenyl iodide in liquid ammonia is electrolysed using a platinum cathode, or a solution of the iodide in liquid ammonia is treated with sodium in the same solvent. The product isolated by the former process often contains chemically combined or adsorbed ammonia. 

Filimonov et al. (174) first reported infrared spectra of ammonia adsorbed on chromium oxide. Recently, Eley et al. (179) have shown that ammonia is H bonded by surface OH groups and coordinately held by two distinct types of Lewis acid sites. Oxidation of adsorbed ammonia molecules at 100°C with the production of water that was chemisorbed on the surface was also observed (179). 

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