Ammonia adsorption differential heats

Pyridine sorption studies on EDTA-dealuminated Y zeolites at various temperatures (54,58), as well as measurements of differential heats of adsorption of ammonia on aluminum-deficient Y zeolites (57,59) have led to the conclusion that aluminum-deficient Y zeolites have stronger acid sites than the parent zeolite. 

Differential heats of NH adsorption were measured for the samples outgassed at different temperatures ranging from 400 to 800°C. Ammonia was chosen as a basic probe because its size is small, which may limitate diffusion effects in small pore zeolite materials. The variations of the differential heats of adsorption are plotted in fig. 3 as a function of the successive pulses of... 

Figure 3 Variations with coverage of the differential heats of adsorption of ammonia on H-ZSM-5 (sample 1) measu-red at 150°C, (A), 200°C, (a), 250°C, (fe 300°C (0) and 400°C (+) The sample was outgassed at 400°C prior NH3 adsorp-tion. The meaning of the arrows is explained in the text.

Figure 1.10 Differential heats of adsorption as a function of coverage for ammonia on H-ZSM-5 (o) and H-mordenite ( ) zeolites [70], In both cases, the heats decrease with the extent of NH3 uptake, indicating that the strengths of the individual acidic sites on each catalyst are not uniform. On the other hand, the H-ZSM-5 sample has a smaller total number of acidic sites. Also, the H-mordenite sample has a few very strong sites, as manifested by the high initial adsorption heat at low ammonia coverage. These data point to a significant difference in acidity between the two zeolites. That may account for their different catalytic performance. (Reproduced with permission from Elsevier.)...

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... 

The distribution of surface acidity strength has been studied by measuring the differential adsorption heat of ammonia. The differential scanning calorimetry (Setaram DSC 111) and the FTIR spectrophotometry (Nicolet 740) have been simultaneously used in order to measure the heat associated with the neutralization of the acidic sites and the amount of chemisorbed base, respectively. Once the sample is saturated at 250 °C and the total acidity measurement is obtained as the amount of base used in the titration, the measurement of the total acidity has been verified by desorption of the base at programmed temperature (TPD). A ramp of 5 °C/min between 250 °C and 500 °C has been followed, with a He flow of 20 cm3/min and using the FTIR spectrophotometry for the measurement of the desorption products. 

Figure 9.4 Regions in a typical curve of differential heats of adsorption versus adsorbed amount. All regions (a, b, c, d) can be observed for zeolite samples presenting both Lewis and Bronsted acid sites, as probed by ammonia adsorption. For oxides presenting only Lewis acid sites, the regions a, c and d are observed.

Figu re 9.5 Variation with the activation temperature of the differential heats of adsorption versus ammonia coverage. 

Niobium oxide surfaces are also very dependent on the dehydration temperature. An illustration is given in Figure 9.5, which represents the differential heats of NH3 adsorption versus ammonia coverage for a niobium oxide from CBMM pre-treated at 423, 523, 623, 723 and 823 K [41]. 

Figure 9.6 Differential heats of adsorption of ammonia and sulfur dioxide on Si02, y-AhOs and MgO.

Figure 9.11 Differential heats of ammonia adsorption versus coverage for various mixed oxides.

Figure 9.15 Differential heat of ammonia adsorption versus ammonia uptake for various HPA samples.

A natural clay has been pillared with mixed solutions containing both A1 and Fe, Ti or Cr. The intercalation-generated solids distribution of acid strengths measured by calorimetric adsorption of ammonia is comparable to that of zeoUtes. The surfaces appear as heterogeneous and show initial adsorption heats close to 150-160 kJ moT if one excludes the first point of the differential heat versus coverage curves, which is much higher (=190kJ mol" ) [109]. 

Figure 9.19 Differential heats of ammonia adsorption over a silica support and silica-supported vanadia catalysts prepared by ALD (filled symbols) and Impregnation (open symbols). VS-A6 and VS-16 on one hand, and VS-A12 and VS-llO on the other hand, have comparable vanadia contents.

Another way to characterize acidity is to study the differential heat of adsorption of a basic probe compound, such as ammonia or pyridine, by microcalorimetry as a function of uptake. This technique yields the distribution of acid strength relative to coverage, but unfortunately does not differentiate between Br0nsted and Lewis... 

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). 

Figure 2. Differential heats of adsorption of ammonia on zeolite NaCaA at 4 temperatures...

For decationated mordenites, an increase in the ammonia adsorption temperature caused a significant increase in the differential heat of adsorption at low coverages, the largest increase of the initial heat being from 130 to 170 kJ mol" , corresponding to Lewis acid sites on a 90% decationated mordenite that was dehydroxylated at 923 K (88-90). The effect of increasing the... 

Auroux et al. (82, 91-93) observed a behavior similar to that seen for HY and HM zeolites above for the adsorption of ammonia over samples of HZSM-5 and HZSM-11 in the temperature range 416 to 673 K. The differential heat versus coverage curves had the same shape and decreased slightly with increasing temperature. Two exceptions in this behavior were noted. First, the differential heats for the samples at 673 K were unusually low. The initial heat for one sample of HZSM-5 at 673 K was lower than 100 kJ moP, whereas it was near 150 kJ mol at 523 K. The adsorption process seemed to be almost completely reversible at this temperature and could not be used to characterize the strong acidity of the solids. 

The second exception was specific for HZSM-5 that had been acidified with ammonium chloride and which had large particle sizes. The differential heat curve at 416-423 K for these samples passed through a maximum at relatively low coverages. This behavior could be explained by the combination of three independent phenomena immobile adsorption, mass-transfer limitations, and preferential location of the most energetic acid sites in the internal pores of the zeolite structure. Apparently, the strongest sites were not accessible to ammonia when the first doses were introduced but became progressively covered when further ammonia was added. Electron paramagnetic resonance studies (93) provided data to support this hypothesis. 

The results at 303 K indicate that NaY zeolite is only weakly acidic, displaying heats of adsorption between 94 kJ mol for ammonia (17, 57, 81) and 124 kJ mor for pyridine (81). This material also contains weak basic sites as determined by the heat of COj adsorption at 298 K (747). Increasing the temperature from 298 to 573 K confirms the effect of adsorption temperature on weakly acidic samples previously discussed in Section IH,A. As the adsorption temperature increases, the initial differential heat of ammonia adsorption increases slightly from 84 (35) to 94 kJ mol" (17,57,81) at 303 K before decreasing continuously to 65 kJ mol" at 573 K (55). Increasing... 

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