Chemical adsorption, surface area measure

Physical adsorption isotherms involve measuring the volume of an inert gas adsorbed on a material s surface as a function of pressure at a constant temperature (an isotherm). Using nitrogen as the inert gas, at a temperature close to its boiling point (near 77K), such isotherms are used to determine the amount of the inert gas needed to form a physisorbed monolayer on a chemically unreactive surface, through use of the Brunauer, Emmett, and Teller equation (BET). If the area occupied by each physisorbed N2 molecule is known (16.2A ), the surface area can then be determined. For reactive clean metals, the area can be determined using chemisorption of H2 at room temperature. Most clean metals adsorb one H atom per surface metal atom at room temperature (except Pd, which forms a bulk hydride), so if the volume of H2 required for chemisorption is measured, the surface area of the metal can be determined if the atomic spacings for the metal is known. The main use of physical adsorption surface area measurement is to determine the surface areas of finely divided solids, such as oxide catalyst supports or carbon black. The main use of chemisorption surface area measurement is to determine the particle sizes of metal powders and supported metals in catalysts. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

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

In one of the earliest reports of the use of clean evaporated alloy films in surface studies, Stephens described the preparation and characterization of Pd-Au films and presented some results for the adsorption of oxygen on them 46). Films of pure Pd and 60% Au were evaporated directly from wires, while films of 80% Au and pure Au were evaporated from a pre-outgassed tungsten support wire. The films were evaporated in a UHV system and the pressure was kept below PC8 Torr during evaporation. After evaporation, the films were stabilized by cycling between —195° and 30°C four times. They w ere characterized by X-ray diffraction and chemical analysis surface areas were measured by the BET method using krypton adsorption. 

Crystal structures of the bismuth molybdate and of the mixed iron and cobalt solid solution molybdate samples were controlled by X-ray diffraction (10). The chemical compositions of the samples were determined by atomic absorption and their surface areas measured by nitrogen adsorption using the BET method. 

Analyses of insertion electrodes include structural analysis by XRD, neutron diffraction, HRTEM with electron diffraction, chemical analysis by EDAX, XPS and dissolution followed by ICP, morphological analysis by electron microscopy, surface area measurements by gas adsorption, and electrochemical analysis by voltammetry chronopotentiometry (primary techniques) and fine electrochemical tools such as EIS, PITT, GITT, and... 

Five Ni-Mo/y-Al203 catalysts with wide variations in pore size distribution were used in the present study. They were eill in the form of extrudates. The catalysts were characterized according to standard procedures. A mercury porosimeter (Quantachrome - Model-Autoscan 60) was used to determine pore size distribution. A Quantasorb adsorption unit was used for BET surface area measurements. The chemical composition and physical dimensions of various catalysts used in the present study were not appreciably different. 

The field of chemistry concerned with the interrelation of electrical and chemical effects, especially the study of chemical changes caused by an electric current and the electrical energy production by chemical reactions, is termed electrochemistry [5]. While electrochemistry encompasses a huge array of different phenomena applied in a variety of technologies, applications, and characterization techniques, such as the surface area measurement by hydrogen adsorption discussed in Sect. 4.3.8, the main emphasis here will be focused on electrodeposition and devices based on electrochemistry, such as electrochemical supercapacitors and electrochromic displays. 

They found that the average pore size and the total pore volume are not adequate measures to predict the CO2 uptake of microporous carbon sorbents, the pore volume of micropores strongly governs the amount of adsorbed CO2 [161]. Neither high surface area CDC after chemical activation (surface area 3,101 m g ) nor high pore volume nano-TiC-CDC (Vtotai 1-61 cm g ) correspond with the highest CO2 adsorption capacity. At ambient pressure, the CO2 uptake closely follows a linear correlation with the volume of pores smaller or equal to a diameter of 1.5 mn. Pores smaller than 0.5 mn contribute to the amount of adsorbed CO2, but the best correlation is found for pore volume smaller than 0.8 mn (Fig. 2.29). The correlation between the amount of adsorbed CO2 at low partial pressures and volume of smaller pores is the basis for the well-known application of CO2 sorption as a method to calculate the pore characteristics of microporous materials. Subatmospheric pressures are of particular interest for industrial applications, where partial pressure of CO2 is below 1 bar, and here, the best prediction of the CO2 uptake capacity at 0.1 bar would be based on the volume of pores smaller or equal to a diameter of 0.5 nm (Fig. 2.29). This correlation can be used to design better CO2 sorbents and CCS devices. 

The surface area and the dimensions and volume of the pores can be determined in many ways. A convenient method is based on measurement of the capacity for adsorption. The experimental techniques do not differ from those used for chemisorption (see Section 3.6.3). The fundamental difference between physi.sorption and chemisorption is that in chemisorption chemical bonds are formed, and, as a consequence, the number of specific sites is measured, whereas in physisorption the bonds are weak so that non-chemical properties, in particular the surface area, are determined. 

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. 

Figure 1.6 Top Low-temperature nitrogen adsorption ( ) and desorption (x) isotherms measured on a calcined SBA-15 mesoporous silica solid prepared using an EO20PO70EO20 block copolymer [54]. Bottom Pore size distribution derived from the adsorption isotherm reported at the top [54]. A high surface area (850 m2/g), a uniform distribution of cylindrical nanopores (diameter —90 A), and a large pore volume (1.17 cm3/g) were all estimated from these data. These properties make this material suitable for use as support in the preparation of high-surface-area solid catalysts. (Reproduced with permission from The American Chemical Society.)...

Aside from N2 adsorption, Kr or Ar adsorption can be used at low temperatures to determine low (<1 m2/g) surface areas [46], Chemically sensitive probes such as H2, Oz, or CO can also be employed to selectively measure surface areas of specific components of the catalyst (see below). Finally, mercury-based porosimeters, where the volume of the mercury incorporated into the pores is measured as a function of increasing (well above atmospheric) pressures, are sometimes used to determine the size of meso- and macropores [1]. By and large, the limitations of all of the above methods are that they only provide information on average pore volumes, and that they usually lack chemical sensitivity. 

Comparative methods may be effectively used for measurements of partial surface areas, Ac, of components in porous composites, for example for active surface area in supported catalysts. The traditional methods of Ac measurements are based on chemisorption of H2, 02, CO, NOr. and some other gases that chemisorb on an active component, and have negligible adsorption on a support [5,54], The calculation of Ac is fulfilled by an equation similar to Equation 9.18 assuming some values of w and atomic stoichiometry of chemisorption [54]. But, unfortunately chemisorption is extremely sensitive to insignificant variations of chemical composition and structure of surface, which alters the results of the measurements. 

Table 5.1 Effect of swelling on the surface area of cotton as measured by chemical (thallation) and physical (N2 adsorption) methods. (Source G.A. Roberts, Accessibility of Cellulose , in Paper Chemistry , ed. J.C. Roberts, ch. 2, Blackie, Glasgow, 1991).

Materials in a colloidal state are frequently preferred in industrial processing operations because their large surface areas per unit volume enhance chemical reactivity, adsorptive capacity, heat transfer rates, and so on. Therefore, one cannot overlook the importance of the flow behavior and properties of colloids since they exert a significant influence on the performance, efficiency, and economy of the process. Note that some examples of this (e.g., ceramic processing, electrophoretic display devices, and food colloids) were mentioned in the vignettes presented in Chapter 1. In addition, one often uses the flow properties and behavior of the products as measures of the microstructure (or, morphology ) of the products and as a means of quality control (e.g., printing inks, toners, paints, skin creams, blood substitutes,... 

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