Aluminas differential heats

The surface of alumina presents strong acid and basic sites, as demonstrated by the differential heats of adsorption of basic probe molecules such as ammonia [169- 171] and pyridine [169,172] or of acidic probe molecules such as SO2 [169,171] and CO2 [173,174]. Table 13.2 presents a survey of microcalorimetric studies performed for AI2O3. 

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... 

Yoshizumi et al. (70) determined acid strength distributions on silica-alumina catalyst calorimetrically by measuring the heat adsorption of n-butylamine from benzene solution. They found that the differential heat of adsorption of n-butylamine ranged from 3.7 kcal/mole (weak acid sites) to 11.2 kcal/mole (strongest acid sites). 

Figure 9.6 represents the differential heats of NH3 and SOj adsorption as a function of coverage for silica, magnesia and y-alumina samples [15]. 

Tables XIII I76-I79), XIV (I80-I83), and XV present a survey of micro-calorimetric studies performed for silica, alumina, and silica-alumina, respectively. Silica displays relatively low heats of adsorption for both basic probe molecules (e.g., ammonia, triethylamine, n-butylamine, pyridine, and trimethylamine) and acidic probe molecules (e.g., hexafluoroisopropanol), indicating that the surface sites on silica are both weakly acidic and basic. Most of the adsorption over silica is considered mainly to be due to hydrogen bonding and van der Waals interaction. Infrared and gravimetric adsorption measurements of pyridine adsorbed on SiO at 423 K have shown that more than 98% of the pyridine adsorbed was hydrogen bonded (62). The differential heats of ammonia 18, 74, 85, 105, 140, 147) and triethylamine (18, 71, 94. 105, 176) on silica show a considerable decrease as the adsorption temperature is increased.

When these bases are compared in terms of their respective proton affinities, the order of basic strength is ammonia < n-butylamine < pyridine < trimethylamine < piperidine < triethylamine, which is the same order observed with microcalorimetric measurements. In fact, plots of the initial differential heat of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and on silica as a function of the proton affinity give linear correlations, as can be seen in Fig. 7 (18, 105). 

A comparison of acidic properties of a-Fe203 (Table XX), ZnO (TableXXI), and Ti02 (Table XXII) with the other oxides discussed above is difficult because extensive data are not available for these solids. For instance, the differential heats of benzene adsorption on 0t-Fe2O3 at 298 K 120) are similar to those for y-Al203. However, the adsorption of water at the same temperature 113, 114) seems to indicate that i -alumina is considerably weaker. The... 

Fig. la. Differential heats of ammonia adsorption versus coverage for the alumina and the supported gallium oxides. 

Fig. la shows the differential heats of adsorption of NH3 at 423 K for the alumina support and for the two differently prepared supported gallium oxides. The alumina and Ga-Al (i) sample show similar and very high initial heats of adsorption around... 

The differential heats of adsorption of ammonia and sulfur dioxide on amphoteric alumina show the coexistence of strong acid sites and basic sites on its surface [94,95]. The heats of ammonia adsorption on alumina are typical of a strong acidic surface the initial heat increases and the adsorption capacity decreases with increasing pretreatment temperature [11]. On the other hand, magnesia, which is a basic oxide, displays only strong basic sites and no acidity. 

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]. 

Silica-aluminas (amorphous aluminosilicates) are widely used as catalyst supports due to their high acidity and surface area. The behaviour of silica-alumina surfaces is similar to that of zeolites, concerning the initial differential heats of ammonia and pyridine, but the total number of acidic sites varies with the preparation method and the Si/Al ratio. The basicity of silica-alumina surfaces, as determined by CO2 adsorption [94,95], appears to be weaker than that of pure alumina. 

About lOmg of intercalated compound, montmorilonite and a mixture of DPD and alumina powder were examined by differential thermal analy-sis(DTA) and thermo-gravimetry (TG). The measurements were made in 200cm /min. of nitrogen flow, reference was alumina and heating rate was 5K/min. 

The first evidence for 5HTP as an intermediate in serotonin formation was the demonstration of potent 5HTP decarboxylase activity in tissues. This enzyme was purified from guinea pig and hog kidney and shown to be separable from 3,4-dihydroxyphenylalanine(dopa) decarboxylase. By procedures involving differential heat inactivation, salt fractionation, and adsorption on alumina it was possible to change the ratio of dopa decarboxylase to 5HTP decarboxylase by as much as twentyfold (Clark el al., 1954). 

In Fig. 1.21a, the differential heats of adsorption of CO on H—BEA zeolite and on MFI-Silicalite are reported as a function of the adsorbed amounts. Volumetric isotherms are illustrated in the figure inset. In both cases the adsorption was fully reversible upon evacuation of the CO pressure, as typical of both physical and weak, associative chemical adsorption. For H-BEA a constant heat plateau at 60kJ mol was measured. This value is typical of a specific interaction of CO with coordinative unsaturated Al(III) atoms, as it was confirmed by combining adsorption microcalorimetry and molecular modeling [73, 74, 78, 89] Note that the heat value was close to the heat of adsorption of CO at cus Al(III) sites on transition catalytic alumina, a typical Lewis acidic oxide [55, 73], Once saturated the Al(III) defects, the heat of adsorption started decreasing down to values typical of the H-bonding interaction of CO with the Br0nsted acidic sites (- 30 kJ mol , as reported by Savitz et al. [93]) and with polar defects, either confined in the zeolite nanopores or at the external surface. 

Silica-aluminas (amorphous aluminosilicates) are widely used as catalyst supports due to their high acidity and surface area. The behavior of silica-alumina surfaces is similar to that of zeolites concerning the initial differential heats of ammonia and... 

Fig. 8.7 Differential heat of ammonia adsorption versus coverage for a series of zeolites (H-ZSM-5, H-BETA, and H-MCM-22) and two silica-aluminas (SAH and SAG) (from Ref. [33])...

The obtained differential heats of adsorption allow the estimation of adsorption affinity toward the gas-phase pollutant the amounts adsorbed are also available. This possibility is illustrated by example presented in Fig. 10.6 the values of differential heats and the amounts adsorbed reveal that alumina-supported tin oxide expresses... 

Fig. 10.6 Differential heats vs. adsorbate coverage obtained for alumina-supported tin oxide [37]...

The CO adsorption microcalorimetry was also used to explain the promoting effect of Pt in bimetallic Ni-Pt catalysts supported on alumina nano-flbre (Alnf) tested for the liquid phase reforming of sorbitol to produce hydrogen [51]. The differential heat of adsorption for Ni-Pt/Alnf reduced to around 111 kJ/mol, which was 12 and... 

Figure 1. Differential spectra of CO chemisorbed on alumina-supported Rh particles before and after heating to 420 K in hydrogen. One of the three species of chemisorbed CO remains after heating and can be identified by a bending mode at 478 cm 1, a stretching mode at 586 cm 1, and a stretching mode at 1937 cmr1 as a linear CO species. The other CO species react and/or desorb while producing hydrocarbons on the Rh particles. The dominant species formed has been identified...

Figure 2. Differential spectra of CO chemisorbed on alumina-supported Co particles both before and after heating in hydrogen to 415 K. The chemisorbed CO is seen to react and form hydrocarbons in the tunnel junction. This hydrocarbon species is distinct from that formed on Rh as seen by vibrational modes near 1600...

Figure 3. Differential spectra of CO chemisorbed on alumina-supported Fe particles shown before and after two heatings in hydrogen to 420 K. Some CO reacts to form hydrocarbons on the Fe particles. The rising background seen at low frequencies indicates the formation of magnetic particles, either through sintering or the desorption of CO. The formation of OH in the junction upon heating does not correlate with the formation of a C-O bond nor with the formation of the C-H...

Figure 4. Differential spectra of CO chemisorbed on alumina-supported Ni particles both before and after heating to 425 K. Very little surface hydrocarbon is seen to form on the Ni particles. This lack of surface hydrocarbon reflects the selectivity of such catalysts for methanation over Fisher-Tropsch synthesis.

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