Adsorption microcalorimetry measurements

Figure 7.5. Sticking coefficients along with differential heats of adsorption as measured by microcalorimetry for ethylene and acetylene on Rh(lOO). [Adapted from R. Kose, W.A. Brown and D.A. King, Chem. Rhys. Lett. 311 (1999) 109.]...

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

The methods for measuring the acidity of nanoporous aluminosilicates such as MCM41 have been reviewed by Zheng et al. [243], including microcalorimetry measurements of probe molecules adsorption. 

Calorimetric measurements of adsorption of CO2 at 303 K on different titania samples have provided evidence of their surface heterogeneity, as expected for oxides, with heats of adsorption ranging from -100 to 30 kj moT. Acidity measurements by ammonia adsorption microcalorimetry on the same samples gave rise to adsorption heats ranging between 150 and 60 kJ moT [47]. 

Flow adsorption microcalorimetry has been used to measure the heats of adsorption of ammonia in a nitrogen carrier on the H and Na forms of a Y zeolite [21]. The calorimeter was linked to a thermal conductivity detector in which the rates of adsorption and desorption and the associated rates of heat evolution or absorption were measured simultaneously at atmospheric pressure. The authors found that, as surface coverage increased, the sites covered first were not necessarily those with the highest molar heats of adsorption. 

Pigure 1.7. Two operational modes for adsorption microcalorimetry (a) the closed mode, (b) the open mode. The gas supply is in A, connected through a small tube (or otherwise) B to the volume C containing the adsorbent. In mode (a) the opening of the obstruction leads to adsorption. The heat evolved in the total system is now determined (A+B+C). In mode (b) the adsorbent Is in open connection with the supply. By moving the piston down, the adsorption can be Increased In small steps and only the heat evolved in part C Is measured. 

Ammonia accessibility to the porosity of several activated carbons measured by flow adsorption microcalorimetry... 

Rouquerol et al. (11, 12) have recently described the experimental determination of entropies of adsorption by applying thermodynamic principles to reversible gas-solid interactions. Theoretically, the entropy change associated with the adsorption process can only be measured in the case of reversible heat exchange. The authors showed how isothermal adsorption microcalorimetry can be used to obtain directly and continuously the integral entropy of adsorption by a slow and constant introduction of adsorbate under quasi-equilibrium conditions (11) or by discontinuous introduction of the adsorbate in an open system (12). 

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. 

We again point out that there are a number of experimental values of Eads for CO atop buUc Pt(l 11) suii ce. Early studies measured the Eads to be 1.43 eV [42], 1.40 [43] and 1.55 eV [45]. King et al. [46], by single crystal adsorption microcalorimetry, have recently measured a value of 1.90 0.07 eV for the low-coverage adsorption of CO on Pt(l 10) surface. Although this value refers to a different metal bulk surfece, adsorption energies of many small molecules on Pt do not vary much with facial structure. The calculated Eads (= 1-92 eV) of CO on Pt o cluster is closer to the most recent experimental value [46]. 

Karge, H.G., and L.C. Jozefowicz, 1994, A comparative study of the acidity of various zeolites using the differential heats of ammonia adsorption as measured by high-vacuum microcalorimetry, in Zeolites and Related Microporous Materials State of the Art 1994, eds J. Weitkamp, H.G. Karge, H. Pfeifer and W. Holderich, Vol. 84 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 685-692. 

The present paper highlights the influence of molecular sized micropores on the ordering of the adsorbed phase within AIPO4-11. A range of simple probe molecules was used including Ar, Kr, CH4, O2, N2 and CO. Their adsorption properties were studied by adsorption microcalorimetry at 77 K and 87 K as well as by neutron scattering measurements in the temperature range from 20 to 100 K. 

It is often difficult to determine the nature of the adsorbed species, or even to distinguish between the different kinds of adsorbed species from the calorimetric data. In many cases this technique fails to distinguish between cations md protonic sites due to the insufficient selectivity of the adsorption. For example, the differential heats of NH3 adsorption on strong Lewis centres and strong Brdnsted sites are relatively close to each other. This can make it difficult in some cases to discriminate Lewis and Bronsted sites solely by adsorption microcalorimetry of basic probe molecules if no complementary techniques are used. Because no exact information can be obtained regarding the nature of the acid centres from the calorimetric measurements, suitable IR, MAS NMR, and/or XPS [36] investigations are necessary to identify these sites. However,... 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

The acid-base properties of zeolites or oxides are often studied by measuring the selectivities to the different products in the decomposition of alcohols and particularly isopropanol. The rate of propene formation can very often be correlated to the number of acidic sites determined by ammonia adsorption. A relationship has been found between the strength of the acid sites of bulk oxides, as determined by ammonia adsorption microcalorimetry [95], and the activation energy of dehydration, while the activation energy of dehydrogenation was independent of the strength of the sites [149]. 

Llewellyn and Maurin (2005) demonstrated that gas (nitrogen and argon) adsorption microcalorimetry can be used a powerful technique for depth examination of the surface state of adsorbents and a minute following of adsorption mechanisms such as phase changes and transitions. The use of this technique in parallel to the measurements (and appropriate analysis) of the adsorption isotherms of the same gases, and DSC and/or NMR cryoporometry measurements can provide deeper insight into the interfacial phenomena over a broad temperature range. 

A calorimetric and IR study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [192]. The active sites for both N2O and CO are Cu(I) ions, which are present as a result of the pre-treatment in vacuum at 673 K. The measured amounts of chemisorbed species in the investigated systems and the values of differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mol ) and carbon monoxide (between 140 and 40 kJ mor ) demonstrate the dependence of the adsorption properties on the copper content. The samples were additionally characterized by ammonia adsorption microcalorimetry at 423 K [192]. 

Microcalorimetry measurements are combined with Grand Canonical Monte Carlo simulations in order to understand more deeply the interactions between methane and two types of faujasite systems. The modelling study, based on newly derived force fields for describing the adsorbate/adsorbate and adsoibate/adsorbent interactions, provide isotherms and evolutions of the differential enthalpy of adsorption as a fimction of coverage for DAY and NaX which are in very good accordance with those obtained experimentally. The influence of the location of the extra-framework cations within the supercages on these thermodynamics properties is also pointed out. Furthermore, the microscopic mechanisms of CH4 adsorption is then carefully analysed in each faujasite system which are consistent with the trend observed for the differential enthalpies of adsorption. 

For such ambitious applications, it is first necessary to understand more deeply the interactions between methane and the microporous adsorbent surface. The enthalpy of CH4 adsorption has been evaluated in various zeolite systems by using isosteric methods via the Clapeyron equation or performed using microcalorimetry measurements which allow direct... 

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