Adsorption, calorimetric measurements studies

A solid surface interacts with its surrounding molecules (in the gas or liquid phase) in varying degrees. For example, if a solid is immersed in a liquid, the interaction between the two bodies will be of interest. The interaction of a substance with a solid surface can be studied by measuring the heat of adsorption (besides other methods). The information one needs is whether the process is exothermic (heat is produced) or endothermic (heat is absorbed). This leads to the understanding of the mechanism of adsorption and helps in the application and design of the system. Calorimetric measurements have provided much useful information. When a solid is immersed in a liquid (Figure 5.10), in most cases there is a liberation of heat ... 

The simplest case to study is hydrogen. Earlier papers, where heats of adsorption were measured calorimetrically (59, 60), reported a decrease of the heat of adsorption when Ni was alloyed with Cu. The qualitatively same result was obtained later in a very detailed study by Prinsloo and Gravelle... 

In this paper, the chemical adsorption of NH3, using pulses, has been studied by combining the results of calorimetric measurement of heat released (in a differential scanning calorimeter) with the measurement of desorbed amount of base (by FTIR analysis of desorbed gases). In this way, the differential adsorption heat, representative of the aridity strength distribution of the deactivated catalyst, is obtained and the restrictions inherent to other techniques, which are affected by the measurement of coke degradation products, are avoided. 

As noted above, many investigations have been made of the adsorption of water on non-porous silicas. Less attention has been given to the dehydroxylation of porous silicas. An early study by Dzhigit et al. (1962) of the adsorption water vapour on a mesoporous silica gel involved both isotherm and calorimetric measurements. It was found that at very low surface coverage the adsorption enthalpy was not significantly affected by dehydroxylation, but a large difference became apparent as the surface coverage increased. A slow uptake of water vapour, which occurred after dehydroxylation, was attributed to chemisorption. 

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

Table VII presents a summary of calorimetric measurements of the differential heat of adsorption of ammonia, water, and carbon dioxide on the sodium form of ZSM-5 zeolite. Ammonia adsorption at 416 K (97.147) shows that NaZSM-5 zeolite is weakly acidic, whereas CO adsorption (147) indicates that in addition there are some weak basic sites. It should be noted that of the two samples studied with ammonia adsorption one was 70% H exchanged and the sodium content of the other was not given. Water adsorption on NaZSM-5 displayed unusual behavior, with a steep increase in the differential heat of adsorption at high surface coverages (166). An adsorption mechanism was proposed to explain these findings in which adsorption occurs first on the hydrophilic sites, consisting of sodium cations and framework anions where water molecules are bound by dipole-field interactions. Further adsorption takes place near these sites through weak interaction with zeolite surfaces, and when the number of water molecules close to these sites exceeds a certain value, they tend to reorient by forming clathrate-like struc-...

In a different study, Masuda et al. (140) found two different linear relationships between the activity for cumene cracking at 623 K and the total acidity as determined from calorimetric measurement of NH3 adsorption at room temperature. One correlation was for amorphous silica-alumina with different loadings and treatments, and the other correlation was for zeolitic silica-alumina with similar loadings and treatments. Other catalysts did not follow these correlations. The apparent lack of correlation between the different sets of catalysts is probably due to the fact that the total acidity measured includes both active and nonactive sites. It is likely that for a set of... 

Other paraffins have also been used for measuring acidity. Neopentane is an attractive compound since protonation of a C-C bond is the preferred primary step for cracking. Corma and coworkers and Guisnet and coworkers used n-heptane in studies of H-Y. Klyachko et al. used octane to characterize the acidities of mordenites and ZSM-5 zeolites. The catalytic activity correlated well with their acidities as determined by calorimetric measurements of the heats of adsorption of ammonia. Higher paraffins such as hexadecane have also been used, but their utility is questionable due to the increased number of secondary reactions that can occur. ... 

Pore size distributions are determined from the hysteresis loop in gas adsorption/desorption isotherms and from calorimetric measurements by the shift in the melting (or freezing) peak for a phase transition of water inside the pores. The determination of the fractional rejection properties is done by permeation experiments of a macromolecular solute with a broad molecular weight distribution (MWD). The MWD of permeate and feed are compared and translated into a fractional rejection curve. The comparison of results obtained from these three independent methods for some characteristic membranes gives an indication of the strength and weakness of each of the methods studied. 

Muris et al. [31] used adsorption isotherms and calorimetric measurements, to study CH4 (and Kr) adsorbed on as-received carbon nanotubes produced by arc discharge, also from Montpellier. This study provided the first complete monolayer isotherm for any gas adsorbed on nanotube bundles. They found that for both CH4 and Kr there were two substeps present in the first-layer data. These two substeps indicate that in the first layer adsorption occurs on two... 

Layer thickness and polymer concentration is obtained directly by ellipsometry, that is, the change in elliptically polarized light after reflection from a surface covered by an adsorbed layer. The number of adsorbed segments is accessible via infrared spectroscopic studies as well as via calorimetric adsorption enthalpy measurements. 

The hydrophilicity of nanooxides, which plays a very important role in their applications and affects many of their properties, was analyzed using calorimetry (oxides were degassed at 473 K at low pressures for several hours) and H NMR spectroscopy (oxides were equilibrated in air) methods applied to samples after different pretreatments. This characteristic is linked to the possibility of the formation of strong hydrogen and donor-acceptor bonds or/and dissociative adsorption of water. The treatments before the calorimetric measurements resulted in desorption of intact water and a portion of dissociatively adsorbed water (=MOH, M 0(H)M"=, where M=Si, Al, or Ti) from both surface and volume of oxide nanoparticles. However, in the case of the NMR measuranents, surface and volume water was readsorbed from air. Therefore, one could expect that the heat effects on the adsorption of water on the calorimetric measuranents should be stronger than that on the NMR measurements. This is typically observed for the samples studied with the exception of SA8 and ST20 (Table 2.12). 

Chappuis measured at a constant temperature adsorption of ammonia on charcoal and asbestes and that of sulphur dioxide, carbon dioxide and air on charcoal depending on the pressure of the gas taken up. He also made the first calorimetric measurements of heat evolved during wetting of adsorbents with liquids. This problem was also studied by Pouillet [80], Junck [81], Fitzgerald [82], Lagergren [83], Gaudechon [84] and Dewar [85]. 

The study of Piper and Morrison [48] is a clear example of the advantages of performing direct calorimetric measurements for isosteric or adsorption heats, instead of calculating it from... 

The choice of the system, aUcylammonium vermiculites in butanol-water, was guided by earher results of adsorption and calorimetric studies on graphite, which demonstrated the importance of the dilution term in interpreting the calorimetric measurements [34,36-39,42,43]. 

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

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