Adsorption study

This section refers to adsorption studies that have been directed to the characterization of perovskites or to the determination of the role of adsorbed species on the catalytic activity. These include mainly equilibrium and kinetics of adsorption, successive or simultaneous adsorption of two gases, infrared spectroscopy (IR), and temperature-programmed desorption (TPD). Physical adsorption has been discussed in Section III because of the dependence of the textural characteristics of these oxides on the preparation method. 

The interaction of oxygen with perovskites has been studied mainly because of the importance of these materials as oxidation-reduction catalysts. Data of oxygen adsorption on LaM03 (M = Cr, Mn, Fe, Co, Ni) oxides were reported by Kremenid et al. (136). The adsorption profile at 25°C showed two maxima for Mn and Co (Fig. 12) that coincide with the maxima observed by Iwamoto et al. (137) for the respective simple oxides 

Total (open symbols) and reversible (filled symbols) adsorption profiles of 02 on LaM03 oxides on a clean surface (circles) or on a surface with preadsorbed isobutene (triangles) (P = 2 x 104 Pa T = 25°C). (Reprinted by permission from Ref. 136.) 

The relative constancy of NO adsorption with temperature and the strength of its bond with perovskite surfaces have suggested the use of this molecule over CO for determining surface metallic centers (107b, 

151-154). However, the IR evidence indicates that NO does not show any particular specificity for adsorption on metallic or oxide ions (151, 152, 154) therefore, the assumption of a 1 1 correspondence between adsorbed NO molecules and adsorption centers, as that reported for simple oxides (155) may not give a proper estimate of the number of surface transition-metal ions. Ulla et al. (158) used the poisoning effect of NO adsorption in ethylene hydrogenation at -20°C for the estimation of metallic centers in reduced LaCo03. Active-site concentration was found to be lower by one order of magnitude than the theoretical concentration of metallic cobalt. This was assumed to be due to the fact that only a small fraction of the metallic sites are active for hydrogenation. 

Virgin Deposition Virgin Deposition Virgin Deposition dpt-Ru (nm) 

Source Data from ref. 154, with permission from Elsevier. 

CARBON MATERIALS AS SUPPORTS FOR FUEL CELL ELECTROCATALYSTS 

X-ray diffraction analysis is used routinely by every catalyst manufacturer to determine the phase composition of the catalysts produced and the size of coherently scattering domains, and hardly needs a detailed description. An aspect that we would like to emphasize concerns the influence of the enviromnent on the oxidation state of carbon-supported metal nanoparticles. Quite often, authors try to correlate electrochemical performance with the phase composition of as-prepared samples. It has, however, been demonstrated convincingly in a number of publications by both x-ray diffraction [155] and x-ray absorption spectroscopy [156] that as-prepared fuel cell catalysts and samples stored under ambient conditions are often in the form of metal oxides but are reduced under the conditions of PEMFC or DMFC operation. The most dramatic changes are observed for samples with high metal dispersions, while larger particles are affected only marginally [17]. One should keep in mind, however, that the extent of the particle oxidation depends critically on the preparation procedure. 

To tackle the problem outlined above and obtain information on the structure and composition of fuel cell catalysts under relevant conditions, a number of authors have proposed in situ XRD or XAS cells where samples were (1) subjected to a controlled gas atmosphere (H2, CO, etc.) at specified temperatures [17,157-160], (2) characterized in model electrochemical cells filled with liquid electrolytes [160-164], or (3) studied in operating PEMFCs and DMECs [165-169]. Both XRD [17] and XAS [158] measurements confirm that Pt and Ru oxides are reduced upon heating at 373 to 423 K in a hydrogen atmosphere. On the contrary, Roth et al. [158] have shown that in a CO atmosphere, ruthenium oxides remain relatively stable, their susceptibility to reduction depending on the Pt-to-Ru site distribution. It has been suggested that Pt in contact with Ru acts as a catalyst for the reduction of ruthenium oxides and strengthens the Ru-CO bond, favoring it over Ru-0. Reduction also occurs in electrochemical cells 

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Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

B. Surface Energy and Free Energy Changes from Adsorption Studies... 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

The specific surface area of a solid is one of the first things that must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Such a determination can be made through adsorption studies themselves, and this aspect is taken up in the next chapter there are a number of other methods, however, that are summarized in the following material. Space does not permit a full discussion, and, in particular, the methods that really amount to a particle or pore size determination, such as optical and electron microscopy, x-ray or neutron diffraction, and permeability studies are largely omitted. 

The general type of approach, that is, the comparison of an experimental heat of immersion with the expected value per square centimeter, has been discussed and implemented by numerous authors [21,22]. It is possible, for example, to estimate sv - sl from adsorption data or from the so-called isosteric heat of adsorption (see Section XVII-12B). In many cases where approximate relative areas only are desired, as with coals or other natural products, the heat of immersion method has much to recommend it. In the case of microporous adsorbents surface areas from heats of immersion can be larger than those from adsorption studies [23], but the former are the more correct [24]. 

Because of their prevalence in physical adsorption studies on high-energy, powdered solids, type II isotherms are of considerable practical importance. Bmnauer, Emmett, and Teller (BET) [39] showed how to extent Langmuir s approach to multilayer adsorption, and their equation has come to be known as the BET equation. The derivation that follows is the traditional one, based on a detailed balancing of forward and reverse rates. 

M. Suzuki, ed.. Fundamentals of Adsorption, Studies in Surface Science and Catalysis, B. Delman and J. T. Yates, eds., Elsevier, New York, 1993. 

Golander C G, Lin Y S, Fllady V and Andrade J D 1990 Wetting and plasma-protein adsorption studies using surfaoes with a hydrophobieity gradient Colloids Surf. 49 289-302... 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

Many different forms of the energy balance have been used in fixed-bed adsorption studies. The form chosen for a particular study depends on the process considered (e.g., temperature swing adsorption or pressure swing adsorption) and on the degree of approximation that is appropriate. 

The lack of a well-defined specular direction for polycrystalline metal samples decreases the signal levels by 10 —10, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven useful for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously for a number of metal oxides and compounds, and also semicon-... 

At the time of writing, in all papers published on adsorption studies on oxides surfaces, spectra have been reported of samples held at the ambient temperature of the sample compartment. It is obvious that when dealing with very volatile adsorbates, low temperature sample cells may be required to increase adsorption and also to prevent rapid desorption of the adsorbed species. In some instances, it is also desirable to record the spectra of species held at elevated temperatures for better correlation with industrial catalytic systems. It should be noted that there are only a few infrared spectra reported in the literature for high temperature studies of catalytic reactions. Sample emission at elevated temperature is a significant experimental complication in investigations of this type. 

It is desirable that the oxide chosen for an adsorption study has a high surface area. This would potentially allow a greater number of adsorbate molecules to be adsorbed and consequently more intense spectra would be obtained. In general, the observed spectra of adsorbed molecules at low coverages are weak. Further, some adsorbates (e.g. H2O) give rise to inherently weak Raman spectra even at high coverage. 

TABLE 16 75°C Adsorption Studies of Olefinsulfonates Effect of Hydrophobe Carbon Number and Structures ... 

Parsons-Zobel plot for NaF solutions was linear (Table 12). The value of Cf"0,determined by the extrapolation of the Cl, Q1 curve to Cjl = 0 and corrected by the value of /pz, has been obtained (Cf 0 - 0.32 F m 2). Adsorption studies of (C l at a polished pc-Pb show splitting of the adsorption-desorption peaks, which can be explained by the energetic inhomogeneity of the surface. The difference between Ea=Q values of various Pb faces has been estimated to be on the order of50-60 mV.604... 

Greece, thus far, has shown a wealth of electrochemical talent, e.g., in the work of Nikitas in adsorption studies, and the hope is that the excellent contributions of Professor Vayenas and his colleagues will continue to flourish and expand. Given this abundance of expertise and the improvement the new method makes to chemical catalysis, I can only hope that Patras will continue to gamer the support that it so richly deserves from the rest of the world. 

Simultaneous adsorption studies on supported metals and on their support, where both have been treated with reactants In exactly the same manner. 

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