Calorimetry, surface acidity

Calorimeters, see Microcalorimeters Calorimetry, surface acidity, 27 121 Calvet microcalorimeter, 22 197-201, 38 172-... 

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. 

A comparison between pulsed flow and conventional pulsed static calorimetry techniques for characterizing surface acidity using base probe molecule adsorption has been performed by Brown and coworkers [20, 21]. In a flow experiment, both reversible and irreversible probe adsorption occurring for each dose can be measured, and the composition of the gas flow gas can be easily modified. The AHads versus coverage profiles obtained from the two techniques were found to be comparable. The results were interpreted in terms of the extent to which NH3 adsorption on the catalyst surface is under thermodynamic control in the two methods. 

The surface acid-base properties of polycrystalline MgO surfaces have been assessed by means of thermogravimetry and DSC of desorption of pyridine and CO2 in the room temperature to 400 °C temperature range [44]. The endotherms and corresponding AH of desorption were discussed in relation with results determined previously using differential adsorption calorimetry and taking into account the structure, surface area and defects of the studied surfaces. 

The measurement of the acidity of solid acid surfaces has been the focus of a vast number of studies. The most commonly used techniques are Hammett titrations, chemisorption of bases and TPD. Extensive discussions of diese methods and their shortcomings are available in the literature [4], The use of adsorption calorimetry makes it possible to determine quantitatively the surface acidity and the acid-strength distribution of solid acids. Surface acid-base properties of catalytic solids can also be studied by base desorption using TG [71]. 

The surface acidity of three commercial mordenite (MOR) zeolites with Si/Al ratios of 10, 60, and 80 has been evaluated by adsorption micro calorimetry at 423 K, using pyridine as a probe molecule [210]. As could be expected from the Si/Al ratios, the total pyridine uptakes varied in the order MOR-10 >MOR-60 >MOR-80. The initial differential heats of adsorption were in the range 215-220 kJ mol After a sudden drop, Qdiff changed slightly and stepwise over a relatively wide range of pyridine uptake (at least for MOR-10) and then steeply decreased. The site-energy distributions and the thermokinetic parameters versus coverage were also determined [210]. 

Let us also mention a combined micro calorimetry and adsorption study aimed at characterizing the surface acidity of a series of MCM-41 aluminosilicates (referred to as SiAlxC , where x is the molar Si Al ratio and n the chain length of the surfactant template). With the exception of H - SiAl32Ci4 and SiAlsCu, all samples were found to present low surface acidity [281]. The behavior of these materials in liquid phase (adsorption of 1-butanol and immersion in various solvents) has also been studied [282],... 

The effect of the Si/Al ratio of H-ZSM-5 zeoUte-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption calorimetry of ammonia and tert-butylamine. The observed increase in light olefin selectivity and decrease in methanol conversion with increasing Si/Al ratio can be explained by a decrease in total acidity [189]. 

S.P. FeRx, C.S. Jowitt, D.R. Brown et aL, Base adsorption calorimetry for characterising surface acidity a comparison between pulse flow and conventional static techniques. Thermochim. Acta 433, 59 5 (2005)... 

The thermal techniques most commonly used to investigate the acid-base or redox character of soHd surfaces are differential thermal analysis (DTA), thermogravi-metric (TG) and differential thermogravimetric (DTG) methods, differential scanning calorimetry (DSG) and calorimetry. These techniques can be used either by themselves, or in conjunction with other techniques (for instance, TG-DSC, calorimetry-volumetry, DSG-chromatography, etc.) [5, 6]. 

Mo-V-Te and Mo-V-Te-Nb mixed-metal oxide catalysts have been characterized by means of C3H8-TPR and NH3 adsorption calorimetry. All samples were strongly heterogeneous, with initial adsorption heats of = 100-80 kJ moT for the Mo-V-Te samples. Introducing an Nb component into the catalysts slightly decreased the initial adsorption heats to = 60 kJ moT but drastically increased the surface density of weak acid sites (<30kJ moT ) [83]. 

All these studies have demonstrated the usefulness of calorimetry for studying the surface characteristics of vanadia-based catalysts. Among the surface properties of vanadia catalysts of relevance to their catalytic activity in selective oxidation reactions, acidity is one of the most significant. 

In addition to calorimetry, information to establish the mode of adsorption is often obtained spectroscopically. Changes in the optical properties of (groups on) the surface or the adsorptive may be monitored. As an Illustration of the former. Rochester studied adsorption from the gas euid liquid phase on rutile (TiOj) and used infrared spectroscopy to distinguish between attachment at different surface hydroxyls. As an example of an ESR study, McBride investigated the adsorption of fatty acids on amorphous alumina from methanol by labelling them with a spin probe. Relevant information could be... 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

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