Chemisorption techniques

It is true that these X-ray procedures are much less sensitive to sample preparation them chemisorption techniques. Nonetheless, it is desirable to use them in conjunction with such methods. In analysis of chemisorption data, it is necessary to make an assumption as to the number of gas molecules that attach to each atom in the catalyst. 

Another way of investigating structure is through the classical method on metals of varying catalyst particle size. The key to this method is to measure active catalyst surface areas in order to determine changes in turnover rates with ensemble size. In recent years several chemisorption techniques have been developed to titrate surface metal centers on oxides (25). In this volume Rao and Narashimha and Reddy report on the use of oxygen chemisorption to characterize supported vanadium oxide. 

Thus ammonia chemisorption technique can be used for the estimation of active sites as well as monolayer capacity of Ti02 (anatase) supported vanadla catalysts. 

The fresh and spent catalysts were characterized with the physisorption/chemisorption instrument Sorptometer 1900 (Carlo Erba instruments) in order to detect loss of surface area and pore volume. The specific surface area was calculated based on Dubinin-Radushkevich equation. Furthermore thermogravimetric analysis (TGA) of the fresh and used catalysts were performed with a Mettler Toledo TGA/SDTA 851e instrument in synthetic air. The mean particle size and the metal dispersion was measured with a Malvern 2600 particle size analyzer and Autochem 2910 apparatus (by a CO chemisorption technique), respectively. 

As has been discussed above, knowledge of the dispersion of the catalyst is extremely difficult to obtain, especially in the promoted systems. This is the main obstacle to clear differentiation between the promoter models proposed and remains so today. Chemisorption techniques to count the active sites present on the sulfided surfaces have had limited success. 02 chemisorption has been associated with edge vacancies (active sites) on pure M0S2 (100). However, the same authors showed that it was impossible to correlate activity and amount of 02 chemisorbed on Co- or Ni-promoted molybdenum sulfide (101). The use of other test molecules was disputable, particularly NO, which can strongly modify the structure of the surface during the measurement (102). 

The high degree of dispersion of platinum on alumina has also been confirmed by the hydrogen chemisorption measurements of Keavney and Adler (Kl), and by the chemisorption studies of Gruber (G2). In addition, high dispersion of platinum on silica-alumina has been observed by Hughes and associates, using a carbon monoxide chemisorption technique (H10). 

A common method for determining the existence and the densities of different active sites is selective chemisorption. However, ignorance of the nature of the active sites makes it impossible to choose suitable probe molecules. Instead, a variation of the selective chemisorption technique can be used that makes use of the reactivity of the active sites. 

Examination of automotive catalysts by various chemisorption techniques has shown that a loss in noble metal surface area caused by higher temperatures correlates monotonically with various activity indices (62, 63). Moreover, Dalla Betta and co-workers (64) were able to separate the additional effect of poisons on the surface of the precious metal by painstaking attention to detail. They developed techniques for accurately measuring the crystallite-size distribution in used automotive catalysts by... 

As the chemisorption technique is very convenient, this layer is widely used for optical and optoelectronic devices. Among a number of chemisorption layer techniques, the use of compounds with carboxyl functional group is most prevalent for preparation of the chemisorption layer of probe molecules on the surface of anodic oxidized aluminum. As the probe molecules are arranged on the solid surface directly by using this technique, the chemisorption layer may possess a lower diffusion barrier for oxygen. Thus, highly sensitive devices for PSP can be accomplished by using a chemisorption layer. In this section, the fluorescence probes for PSP based on the chemisorption layer are introduced. 

FIGURE 16 Chemisorption on a metal surface. A, Chemisorption isotherm showing approach to monolayer coverage B, typical data from a pulsed chemisorption technique. 

The direct titration of the parent carbon black which also carries significant amounts of functional groups is not possible. From this, it can be seen that the two alternative chemisorption techniques both have their relevance in carbon chemistry. 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

Wayne Goodman commented as follows The development of chemisorption techniques for the characterization of active catalytic surfaces has provided a mechanism for defining specific catalytic activity, thus separating true catalytic activity from the ambiguities of catalytic activity derived from catalyst formulation. As a consequence, chemisorption has allowed the development of relationships between catalyst structure and activity/selectivity. Furthermore, chemisorption has provided a framework which has allowed the surface science of supported metal particles and the surface science of single crystal catalysts to be interrelated. 

The effect of Si substitution on the turnover frequency for WGS is shown in Figure 11. The turnover frequencies plotted in this figure were based on the magnetite surface area as determined by the NO chemisorption technique. The turnover frequencies shown for unsupported Fe O indicate that the factor of 10 decline in activity for the silica-supported catalysts is not a particle size effect, but instead is a consequence of the substitution of Si into the lattice. However, when the adsorption of CO/COo at 663 K was used to titrate the surface sites instead of NO, the resulting turnover frequencies were essentially constant as shown in Figure 12. Accordingly, the CO/CO2 mixture apparently titrates the sites active for WGS. Clearly, the number of active sites is decreased markedly as the particle size decreases in the silica-substituted magnetite catalysts. 

The turnover number is a useful concept, but is limited by the difficulty of determining the true number of active sites. The situation is somewhat easier for metals, as a chemisorption technique could be used to measure the exposed surface area. 

Surface analysis investigations (XPS) were performed on a Leybold equipment already described in [16]. The fresh catalyst samples were stored imder argon prior to the catalytic tests and the surface analysis. The aged catalyst samples were also handled under argon. The precious metal dispersion was determined for some of the catalysts by a pulsed CO chemisorption technique [16]. 

The CO-methanation technique, like conventional chemisorption techniques, is subject to imcertainty over CO/metal-atom stoichiometries. However, for our objective of characterizing aging effects in large numbers of automotive catalysts, we are interested in measuring relative changes in dispersion/surface area rather... 

It has often been observed that catalytic samples having the same qualitative composition (for instance, platinum on alumina) but differing in preparation mode, show pronounced dissimilarities in catalytic behavior. Different methods of preparation will yield catalysts differing in crystallite size, crystallite size distribution and shape of the metal crystals. Since 1969, an important work on the statistics of surface atoms and surface sites on metal crystals was undertaken by van Hardeveld and Hartog [14]. In the mean time, a considerable amount of work started to determine the specific rates, rates per unit surface area of metal, for various reactions and in parallel, improvement of chemisorption techniques to determine the surface area of a metal component was achieved. 

To a certain extent, chemisorption techniques are also applicable to oxides [33]. 

Effect of oxide deposition on alumina OH bands, applicability of CO2 chemisorption technique for determining monolayer coverage of alumina supported metal oxides Specification After thermal treatment... 

The total concentration of active sites Ct is independent from the operating conditions and is typically determined from independent measurements. For acid catalysis, NH3 TPD measurements are at hand to titrate the number of active sites, whereas for metallic catalysts H2 and/or O2 pulse chemisorption techniques may be used. However, in view of the uncertainties as to the nature and, hence, the real concentration of active sites, Ct is often incorporated in the rate coefficients of the rate expression. 

Table 1 lists such properties of catalysts as average particle size, surface, silver content. On the whole, we observe a satisfactory coincidence of the average sizes determined by TEM and oxygen chemisorption techniques. Note that the silver lattice parameters found with the X-ray analysis correspond with a good accuracy to the reference values for the bulk silver. Thus, the silver atoms in the bulk of the particles (himdreds of Angstroms in size) are identical to those in the structure of the bulk metal. Moreover, according to HREM, there are no data proving metal-support interactions. 

It is commonly used and convenient to define in a catalyst the surface area of the free active sites. Considering as example a metal supported catalyst, we can define as total surface area the surface of the support that can have contact with the external fluid mass. When an active phase (i.e. noble metal) is deposited on the support, only part of the available support surface can be covered. The chemisorption techniques permit to evaluate selectively the surface area of the active phase that is usually smaller than the total catalyst surface area. 

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