Ammonia adsorption energy

The activation energy for ammonia desorption was found to be dose to zero and accordingly a non-activated ammonia adsorption process was considered ( a = 0) ... 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

To describe catalytic reactions on a metal surface, adsorption energies of the reactants, intermediates and products are essential and so are the activation energies separating different intermediate steps. Figure 4.15 illustrates a full potential energy diagram for a catalytic reaction the synthesis of ammonia N2+3H2 — 2NH3. 

In this section we shall discuss the results of quantum-chemical calculations of the chemisorption interaction of the surface terminal hydroxyl groups in silicas with water and ammonia molecules. Two types of one-center coordination of H20 molecules by a hydroxyl group of Si02 are presented in Fig. 3. CNDO/BW calculations using these clusters gave adsorption energies... 

Large E contributions were also reported by Barrer (1978, p. 188) for the adsorption of carbon dioxide, ammonia and water vapour on NaX. Indeed, in the case of water, over 90% of the low-coverage adsorption energy was attributed to tp. With these highly polar molecules it is likely that the cation-adsorbate interaction provides a major contribution to E. ... 

The Study of ammonia adsorption was carried out in a flow adsorption microcalorimeter under dynamic conditions. Some of the parameters from these experiments are collected in Table 2.. In all cases the amount desorbed was much smaller than the adsorbed one, and so was the absolute value of the heat evolved. This clearly points out that ammonia adsorption consists of two different components. One is related to chemisorption (irreversible adsorption) and the other one (more labile or reversible) to physisorption in the pores. Another important feature about these experiments is that heat was still released long afterwards the NH3 uptake was negligible. This heat evolution, already reported in other experimental systems [4,7,8], is due to diffusion of adsorbed ammonia from low energy sites to higher energy sites having low accessibility. The lack of further uptake of NH3 may be due to irreversibly adsorbed molecules on the borders of micropores blocking NH3 towards the end of the adsorption process. 

Tab. 22.2 Proton transfer energy, A pj, deprotonation energy, E yp, hypothetical binding energy of NH4 on the deprotonated zeolite surfaces, E p(SH ), and energy of ammonia adsorption, ad( 3) ( J for Bronsted sites in different zeolite frameworks [13].

This kinetic behavior indicates a rate-determining surface reaction of weakly adsorbed nitrous oxide on a surface which adsorbs ammonia according to Langmuir s isotherm. The adsorption energy of ammonia on the active surface was found to be 2.2 kcal./mol. 

As you move from left to right across the periodic table the adsorption energy for a stable molecular species, such as ammonia, generally decreases due to the increase in the Fermi level, the filling of the metal d-band, and Pauli repulsion. 

In the ammonia oxidation system shown in Fig. 18, ammonia is adsorbed in the presence of oxygen on Cu(l 11). At lower surface coverages, atomic oxygen increases the adsorption energy of ammonia by 30 kJ/mol. The adsorption energy for ammonia in the presence of oxygen is -48... 

Figure 7.23. Ammonia TPD from clean Fe(lll) and K/Fe(lll) surfaces. The desorption temperature of ammonia from Fe(111) is lowered in the presence of potassium. Thus potassium lowers the adsorption energy of ammonia on the iron surface [59].

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions The changes in the apparent reaction order dependence in ammonia partial pressure suggest that to elucidate the effects of potassium on both iron single crystals and the industrial catalyst, it is necessary to understand the readsorption of gas-phase ammonia on the catalyst surface during ammonia synthesis The fact that the rate of ammonia synthesis is negative order in ammonia synthesis. Once adsorbed, the ammonia has a certain residence time (t) on the catalyst which is determined by its adsorption energy on iron [t cx tq exp (AH /RT)]... 

There is an additional effect of the potassium promoter to be discussed. It was found that its effect is more pronounced at higher reaction rates, that is, higher stationary concentrations of adsorbed ammonia. Adsorbed NH3 blocks sites for the adsorption of the reactants. Its dipole moment has, however, the opposite sign of that of a-N2, so the presence of K will lower its adsorption energy and hence reduce its site-blocking effect [27], as illustrated in Fig. 6.6b. 

Hydrogen is dissociatively adsorbed with high sticking coefficient with adsorption energies around 100 kj/mol [28], which is not significantly affected by the presence of potassium. At the high temperatures of ammonia s)mthesis, the lifetime of adsorbed H atoms will be very short, and they may be considered as a highly mobile two-dimensional gas. 

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