Solid ammonia adsorption

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

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... 

The alumina or silica-alumina supports used in bifunctional catalysts have been shown to be acidic in nature. The acidic properties are readily demonstrated by the affinity of these solids for adsorption of basic compounds such as ammonia, trimethylamine, re-butylamine, pyridine, and quinoline (01, R5). Furthermore, adsorption of certain acid-base indicators such as butter yellow gives a coloration similar to that observed in acid media (B3, B4). With regard to the origin of the acidity, Tamele (Tl) has suggested in the case of silica-alumina that aluminum atoms replace silicon atoms in the surface of the silica structure, giving rise to surface sites of the form... 

Increasing the pre-treatment temperature modifies the surface acidity of the solids. For y-alumina, there are numerous surface models, and various acid sites having different strengths are formed on the surface during dehydration. The influence of the pre-treatment temperature, between 573 and 1073 K, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry. The number and strength of the strong sites, which should be... 

DRIFT spectra were recorded after ammonia adsorption over the solids previously activated in air at 450°C, and cooled at 250°C, followed with a purge of helium. As illustrated in Fig. 5 some differences are observed for the catalysts with 2.5 and 7.5 wt% sulfate. These samples, as seen in Fig. 6, exhibit the lowest and highest catalytic activity, respectively. 

Sulfated titania has been investigated much less extensively than sulfated zirconia. Desmartin-Chomel et al. [97] have studied the acidic properties of sulfated titania using ammonia adsorption calorimetry and FTIR spectroscopy. The number of acid sites on the sulfated catalyst was noticeably increased, and dependent on the surface area of the original titania. The dispersion of the initial oxide controls the amount of sulfur retained by the solid and the thermal stability of the resulting sulfate. Ammonia adsorption is commonly used to determine the acidity of sulfated oxides however, it is also well-known that NH3 is a powerful reductmt, and that the acidity of sulfated zirconia is decreased by reduction. At low ammonia coverage, sulfated titanias exhibit a much lower heat of adsorption, and the IR study of NH3 adsorption showed that the first doses of NH3 dissociate at the surface with the formation of OH species. The lower heat of adsorption was then attributed to the contribution of NH3 dissociation to the differential heat of adsorption. This phenomenon has been observed for sulfated aluminas [109]. 

The number of published articles that include characterization of surface acidity by pyridine adsorption is immense. Therefore, we are not able to present a comprehensive analysis of the available data on the pyridine—solid interaction. In the next paragraphs, we shall briefly review the main conclusions that were obtained through the use of pyridine as a probe and will contrast the results of pyridine adsorption with those of ammonia adsorption. 

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... 

Tronconi and co-workers (98,116) have validated against experiment a more complex, heterogeneous, transient model, accovmting also for diffusion and reaction of NO and NH3 inside the porous walls of extruded honeycomb SCR catalysts. The model equations are presented in Table 5 x and z are the intraporous and axial coordinate, respectively is the ammonia adsorption capacity of the catalyst 6 is the NH3 surface coverage is the effective intraporous diffiisiv-ity s is the monolith wall half-thickness i is the gas velocity in the monolith channels are gas-solid mass transfer coefficients and dh is the hydraulic diameter of the monolith channels. Notably, a pseudo-steady-state assumption... 

An interesting index of total acidity of solid surfaces is provided by dividing the heat of ammonia adsorption from its 0.1 N solution by the total surface area estimated from the heats of adsorption of n-butanol from water as given in Table 4. 

Holland GP, Cherry BR, Alam TM (2004) N-15 solid-state NMR characterization of ammonia adsorption environments in 3A zeolite molecular sieves. J. Phys. Chem. B 108 16420-16426... 

Ammonia adsorption calorimetry has been apphed to investigate the acid properties of MWW-based catalysts with different framework topologies and crystallinities [222]. It was estabhshed that the acidic properties of the MCM-22 family depend mainly on the Al-content of the solid (the Si/Al ratio varied from 9.1 to 46.0). Delamination of the MCM-22 precursor, which yields ITQ-2, resulted in a decrease of the total acidity and an increase of the concentration of intermediate add sites, while the pillaring process, which yields MCM-36, affected mainly the total concentration of add sites, the acid strength distribution being similar to the corresponding MCM-22 zeoUte. 

The hydrogen stream is water scrubbed to remove solids, ammonia and hydrogen chloride and to saturate it with water vapor. The CO converts to H2 via the shift reaction, and the Rectisol process selectively removes the sour gases (H2S and CO2). Pressure Swing Adsorption (PSA) removes residual impurities (CO, nitrogen, etc.) to produce H2 with a purity of 99.85%. 

Water-gas shift conversion is included primarily as a means of reducing the concentration of water vapor in the gas. A low water concentration improves both ammonia adsorption efficiency and sulfur conversion in subsequent steps. Ammonia is removed by adscoption on an appropriate solid such as an acid zeolite and may be recovered by desorption at a higher temperature. 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

V-Mo-Zeolite catalysts prepared by solid-state ion exchange were studied in the selective catalytic reduction of NOx by ammonia. The catalysts were characterized by chemical analysis, X-ray powder diffraction, N2 adsorption (BET), DRIFT, UV-Vis and Raman, spectroscopy and H2 TPR. Catalytic results show that upon addition of Mo to V-ZSM-5, catalytic performance was enhanced compared to V-ZSM-5. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

Figure 13.5 Adsorption-desorption of ammonia at 280 " C on a model V2O5—WO3/TiO2 catalyst Dashed lines, inlet NH3 concentration triangles, outlet NH3 concentration solid lines, model fit with Temkin-type coverage dependence. Adapted from ref. [3].

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