Chemical adsorption measurements

Fig. XVn-6. Schematic of gravimetric apparatus for adsorption measurements. (From Ref. 30. Reprinted with permission from American Chemical Society, copyright 1995.)...

A detailed study of the physical and chemical adsorption of water on three xerogels, ferric oxide, alumina and titania, as well as on silica (cf. p. 272) has been carried out by Morimoto and his co-workers. Each sample was outgassed at 600°C for 4 hours, the water isotherm determined at or near 20°C, and a repeat isotherm measured after an outgassing at 30 C. The procedure was repeated on the same sample after it had been evacuated at a... 

Physical and Chemical Adsorption for the Measurement of Solid Surface Areas... 

The chemical compositions of the samples were obtained by ICP in a Varian 715-ES ICP-Optical Emission Spectrometer. Powder X-ray diffraction was performed in a Philips X pert diffractometer using monochromatized CuKa. The crystallinity of the zeolites was obtained from the intensity of the most intense reflection at 23° 20 considering the parent HZ5 sample as 100% crystalline. Textural properties were obtained by nitrogen physisorption at -196°C in a Micromeritics ASAP 2000 equipment. Surface areas were calculated by the B.E.T. approach and the micropore volumes were derived from the corresponding /-plots. Prior to the adsorption measurements the samples were degassed at 400°C and vacuum overnight. 

The results of chemical analysis show that all samples have similar values of Si/Al ratio and confirm that the recrystallization procedure doesn t lead to any significant changes in chemical composition. On the contrary, the adsorption measurements point to remarkable changes in zeolite texture (Fig.l). 

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

In order to accurately determine the chemisorbed amount from the overall adsorption isotherm, the sample can be further outgassed at the same temperature to remove the physically adsorbed amount, after which a new adsorption procedure is carried out to obtain isotherm II. The difference between the first and second isotherm gives the extent of irreversible adsorption ( ) at a given temperature (Figure 13.5b), and can be considered as a measurement of the amount of strong sites in the catalyst. However, in the first approximation, the magnitude of the heat of adsorption can be considered as a simple criterion to distinguish between physical and chemical adsorption. 

Catalysts were characterized by means of X-ray diffraction (Phillips diffractometer PW3710, with CuKa as radiation source), UV-Vis-DR spectroscopy (Perkin-Elmer Lambda 19) and chemical analysis. Measurements of surface acidity were carried out by recording transmission FT-IR spectra of samples pressed into self-supported disks, after adsorption of pyridine at room temperature, followed by stepwise desorption under dynamic vacuum at increasing temperature (Perkin-Elmer mod 1700 instrument). The procedure for chemical analysis is described in detail in ref. (13). 

George R. Hill In the low temperature physical solution process the surface area would probably be that determined by BET adsorption measurements. In the high temperature process, apparently the coal structure is opened up, and the surface would be the total surface of all the molecular units. This occurs, as the dissolution proceeds, by a combination of chemical bond breaking and solvent action with hydrogen transfer to the free radicals produced. 

When silica with the developed specific surface (> 100m2/g) is used, the number of active sites per gram of material reaches 1018-1019. This amount is sufficient for their reliable detection by different methods EPR, IR and optical spectroscopy, microcalorimetry, and adsorption measurements. Using the thermo chemical method, one can activate the surface of powdered and semitransparent film Si02 samples with a thickness of several tens of microns obtained by pressing the high-dispersity silica powder (aerosil). These film samples are suitable for quantitative optical studies in the UV, visible, and IR regions. 

Perez-Mendoza M, Schumacher C, Suarez-Garcfa F, Almazan-Almazan MC, Domingo-Garcia M, Lopez-Garzon FJ, and Seaton NA. Analysis of the microporous texture of a glassy carbon by adsorption measurements and Monte Carlo simulation. Evolution with chemical and physical activation. Carbon, 2006 44(4) 638-645. 

Adsorption Measurement. The capacities of the molecular sieves to adsorb vapor phase 1,3,5-triisopropylbenzene (97 %, Aldrich) and 1,2,4-triisopropylbenzene (99%, Camegie-Mellon University) were measured at 373 K using a McBain-Bakr balance. The adsorption temperature was chosen such that no chemical reactions of the adsorbates were observed. Prior to the adsorption experiment, the NH4+-forms of the solids (except SAPO-37) were dehydrated at 573 K under a vacuum of 10" 2 Torr. The as-made SAPO-37 was calcined at 793 K in an oxygen flow of 6 L/h in-situ in the adsorption system for removal of organic species and dehydration. The vapor pressure at 296 K of 1,3,5- and 1,2,4-triisopropylbenzene is approximately 0.45 Torr. The adsorption experiments were conducted at this pressure. 

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. 

With simple probe molecules, such as H2, information about the number of surface metal atoms is readily obtained by using adsorption measurements. However, even with such simple probe molecules further information about the heterogeneity of a surface may be obtained by performing temperature-programmed desorption measurements. With probe molecules which are chemically more specific (e.g., NH3 and organic amines, H2S and organic sulfides) it may be possible to obtain information about the number and nature of specific types of surface sites, for example, the number and strength of Lewis or Bronsted acid sites on oxides, zeolites or sulfides. 

The samples used were a standard Na-A zeolite and five nitrogeneous types of zeolite A, or N-A.(18) The N-A zeolites are siliceous analogues of zeolite A, synthesized with tetramethyl-ammonium cation. The Si Al ratio varied from 0.94 (NaA) to 3.54 for the most siliceous N-A sample.t The ratios were determined by wet chemical analysis, and the structure type and absence of impurity phases were confirmed by X-ray powder diffraction techniques. Adsorption measurements (oxygen, -183°C) showed a zeolite A content of greater than 90%. 

In natural environments, Mn and Fe oxides often coexist as mixtures, making it important to determine if their trace metal adsorption properties are altered by interactions with each other. To test this hypothesis we measured Pb adsorption to mixtures of biogenic Mn oxide and colloidal Fe oxide. The mixtures were prepared in two ways. First, previously prepared biogenic Mn oxide was mixed in suspension with previously precipitated amorphous Fe(III) oxide. Second, Mn(II) was biologically oxidized in the presence of a previously prepared Fe(III) oxide suspension (Nelson et al., 2002). As above, all adsorption measurements were made in a chemically defined medium (Nelson et al., 1999b). In both cases, observed Pb adsorption by the Fe/Mn oxide mixtures was similar to that predicted using Langmuir... 

For adsorption from the gas phase onto nonporous, impermeable surfaces, phys-isorption and nonactivated chemisorption are governed largely by gas-phase kinetics and are instantaneous on the time scale of chemical sensor measurements. 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

Catalysts Preparation. The silicoaluminophosphate (SAPO) molecular sieves employed in this study were synthesized in the laboratory of Professor Mark Davis in the Department of Chemical Engineering of the Virginia Polytechnic Institute, following the methods reported in U.S. Patent 4,440,871. The three different samples, distinguished by their microscopic structure, were the wide-pore SAPO-5, medium-pore SAPO-11, and the narrow-pore SAPO-34. Verification of their microscopic structure (through x-ray diffraction) and micropore diameters (by argon adsorption measurements) was performed at VPI. The SAPO molecular sieves were provided in the ammonium cation form. Ex situ calcination at 873 K for one hour in oxygen was performed on the SAPO samples prior to their use as catalysts for the propylene conversion. 

We will provide a succinct introduction to the main textural characterisation techniques for catalysts. As a heterogeneous catalyst comprises a support and an active phase, we will distinguish between techniques intended for studying the support, which will be presented in a first section (Physisorption isotherms and mercury porosimetry) and techniques used to characterise the active phase, in the strict sense of the term, shown in a second section (Chemical adsorption). For each technique, we will show the theoretical principle, the way in which the measurement is carried out and the equipment used. Finally, examples will be used to illustrate the type of response that can be given using these characterisation techniques. 

For practical purposes all adsorptions can be classified as one of two types. It can involve merely the van der Waals interaction between the substrate and the catalyst, a process that is termed physical adsorption or physisorption. Alternately, it can involve the formation of catalyst-substrate bonds as discussed above. This is termed chemical adsorption or chemisorption. While the latter is the basis for the chemistry of catalysts, physisorption is the basis for the BET procedure which is commonly used to measure the surface area of solids. ... 

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