Probe molecules quantitative adsorption

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

Adsorption calorimetry consists of the coupling of a heat flow calorimeter with a system able to monitor the adsorption of a probe molecule by determining the amount of probe gas that has reacted with the solid under study. It is probably the most direct method for describing in detail both the quantitative and energetic features of surface sites. The adsorption of a probe molecule is an exothermic phenomenon (AHj s < 0), while desorption processes are associated with endothermic peaks (AHdes > 0). Heats of reduction are generally associated with an... 

Kaneko et al attempted to apply He adsorption at 4.2 K for evaluation of ultramicropores in activated carbon. [34] Although He adsorption at 4.2 K is efficient for detection of presence of ultramicropores, quantitative evaluation of ultramicroporosity is still difficult a kind of quantum effect is speculated. Johnson et al pointed that quantum effect is predominant in hydrogen adsorption in SWNT from their theoretical studies. [35] We need a small probe molecule for ultramicropore characterization. At the same time, contribution by quantum effect must be understood in order to establish nanopore characterization using the small probe molecule. 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Sorption in micropores can be described by the Dubinin-Radushkevic formalism that has been adapted by Stoeckli et al. This is a largely empirical approach and it should be emphasized that the use of a combination of Langmuir types isotherm leads to similar quantitative results. For evaluation of the distribution of micropores, one can either rely on high-resolution measurements of mostly nitrogen adsorption as suggested by Horvath and Kawazoe or use a combination of probe molecules of different minimum kinetic diameter. More recently, approaches based on density functional theory are put forward. 

It must be concluded that the quantitative determination of micropore size is still an ambiguous problem new theories, models, mechanisms and simulations are still under study [56-58]. Therefore isotherm interpretations must be used carefully and can be considered as useful mainly for qualitative studies. No reliable method has been developed for the determination of the micropore size distribution. At present the most promising approach appears to be that of pre-adsorption linked with the use of various probe molecules of known size and shape [59-61]. For example, this approach has been applied successfully for silica compacts characterisation in [61] using spherical symmetrical inert molecules, such as neopentane and trimethylsiloxysilane [(CH3)3SiO]4Si with diameters of 6.5 and 11.5 A respectively. In general the limited availability of volatile probe molecules with diameters extending above 10 A puts a restriction on the applicability of this method. Furthermore effective pore sizes determined by this technique depend on the kinetic and thermodynamic properties of the... 

CO/H7 and H7yCO coadsorption. The presence/absence of multiple-vacancy Pt sites in the absence/presence of Ce02 in the support has been also checked through the competitive adsorption of other probe molecules, among which H2 (normally used to test the dispersion of supported Pt) and CO. Some quantitative aspects of the competitive adsorption of these probes was reported in table 2 here the spectroscopic aspects of their coadsorption will be dealt with. 

Microcalorimetric NH3 adsorption is one of the powerful techniques for energetic characterization of solid surfaces and provides a direct and accurate method for the quantitative determination of the number of acid sites of different strengths. Microcalorimetry invplves the measurement of differential heats evolved updn adsorption of smeill quantities (micromoles) of basic probe molecules on to the catalysts. Such measurement yields information about the acid strength distribution i.e., the number of sites having the particular heat of adsorption for the basic probe molecule. 

These methods suffer from the lack of complementarity, and thus the significance of results provided by any of them is limited. A standard practice to detect the Bronsted or Lewis character of surface sites is pyridine adsorption combined with FTIR measurements the number of Lewis or Bronsted sites is more difficult to count, however. Other titration methods use either color indicators and acid or base titrants in nonpolar solvents or the adsorption of gaseous acidic or basic probes. They do not, in general, give consistent quantitative information about the number of acid or base sites even when applied to the same sample. There are several reasons the applicability of titration methods is limited Either the state of the surface is different for different methods or adsorption equilibrium is not always achieved. Another more serious source of discrepancies between titration methods is that probe molecules of different basicities "see" different surface sites. The lack of a uniquely defined thermodynamic scale of acid strength of surface sites makes difficult any correlation between results obtained with different probe molecules. The use of standard catalytic tests for probing the so-called catalytic acidity is not always a better approach, because the mechanistic assumptions involved are neither straightforward nor subject to experimental proof. 

The most promising approach to this problem is the use of suitable probe molecules for the quantitative characterization of site density and strength by means of adsorption microcalorimetry. The best-known appUcations of chemisorption involve the use of bases such as NH3 or pyridine to probe the acidity of zeoUtes. Moreover, it is well known that adsorption influences all phenomena depending on surface properties, since it constitutes the primary step for every catalytic reaction involving sohd catalysts. Adsorption is generally exothermic (AH < 0) and the heat evolved is called heat of adsorption. This heat is related to the ability of the sites to interact with the probe molecule, i.e. to their basic or acidic character. 

NTR suggest that these mesopores are probably closed. In order to confort this hypothesis, it could be interesting to p orm experimental methods such as liquid intrusion and thermoporosity [17], Gas adsorption study has revealed a TS deposits microporosity and its quantitative characterization has been done using Dubinin-Asthakov and Stoeckli theory. To valid these results, we will extend the number of probe molecules and compare our experimental results with DFT a statistical method [18]. Finally, this observed multiscale porosity can play a role in diffusion and retention of hydrogen, studies are on progress to put in evidence this effect. 

Quantitative Adsorption of Probe Molecules from Gas and Liquid Phases... 

In this preliminary Chapter it has been illustrated that adsorption microcalorimetry is very fruitfully employed in describing quantitatively the processes occurring at the gas-solid interface. The population of the surface sites active towards suitably chosen probe molecules is evaluated through the volumetric adsorption isotherms the... 

NO (reactant) and CO (product) molecules were used as IR probes of the copper oxidation state in H-Cu-ZSM-5 catalysts. CO adsorption is specific to Cu sites. Its characteristic band at 2158 cm provides quantitative results on integrating its molar extinction-coefficient (e) NO decomposes oxidizing Cu to Cu. Propane and oxygen in a special IR reactor cell always yielded chemisorbed CO. Use of e indicates the NO influence on Cu state. 

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