Adsorption mechanisms, spectroscopic

Weissmahr, KW, Haderlein SB, Schwarzenbach RP, Hany R, Nuesch R (1997) In situ spectroscopic investigations of adsorption mechanisms of nitroaromatic compounds at clay minerals. Environ Sci Technol 31 240-247... 

Berrodier, I., Farges, F., Benedetti, M., Winterer, M., Brown Jr, G. E., Deveughele, M. (2004). Adsorption mechanisms of trivalent gold on iron- and aluminum-(oxy)hydroxides. Part 1 X-ray absorption and Raman scattering spectroscopic studies of Au(III) adsorbed on ferrihydrite, goethite, and boehmite. Geochimica et Cosmochimica Acta, 68(14), 3019-3042. doi 10.1016/j.gca.2004.02.009... 

Arai,Y. Sparks, D.L. (2001) ATR-ETIR spectroscopic investigation on phosphate adsorption mechanisms at the ferrihydrite-water interface. J. Coll. Int. Sd. 241 317—326 Araki, S. Hirai, H. Kyuma, K. (1986) Phosphate absorption of red and/or yellow colored soil materials in relation to the characteristics of free oxides. Soil Sd. Plant Nutr. 32 609-616... 

Elaboration of a new mathematical software for the kinetic steady- and non-steady-state experiments in particular, the reliable provision for the primary interpretation of kinetic data, new methods (program-adaptive and completely adaptive) of performing informative steady-state kinetic experiments and radically new methods of carrying non-steady-state experiments oriented for the establishment of reaction mechanisms. Finally, it is the development of complex methods involving a combination of kinetic and physical (adsorptive, isotopic, spectroscopic) studies. 

The most powerful methods for the study of adsorption mechanism of nitroaromatic compounds on clay minerals have become in situ spectroscopic investigations. Handerlein et al. [152, 153] and Weissmahr et al. [154-156] have investigated the adsorption of NACs particularly on illites, montmorillonites and homoionic kaolinites. The substituted nitrobenzenes on the surface of smectites were investigated by Boyd et al. [157, 158], The main focus in the experimental study of adsorption of NACs on the surface of clay minerals is the influence of the type of clay mineral, the effect of exchangeable cation of the mineral, the effect of the structure and the kind of substituents of NAC compound on the position and orientation of NACs to the surface of mineral, the character of interaction between NACs and the surface of mineral, the adsorption energy. 

Aral, Y., and Sparks, D. L. (2001). ATR-FTIR spectroscopic investigation on phosphate adsorption mechanisms at the ferrihydrite-water interface. J. Colloid Interface Sci. 241, 317-326. 

The other adsorption mechanism is that the solvation layers (at least the innermost Emim+ cation layer) remain on the silica surface but the disruption of the solvation layer(s) occurs as a result of the surfactant adsorption onto the solvation layer(s). If this is true, then the force-distance data shown in Figure 3.5 may result from the screening of the solvation layers by surfactant adsorption. The only difference between the two adsorption mechanisms is whether the RTIL solvation layers remain on the silica surface. It is suggested that the surfactant adsorption disrupts the structured solvation layers whatever the presence or absence of Emim ions (and EmimTFSI ion pairs) adsorbed on the silica surface. This depends on the adsorption affinity for the two species (Emim cation and BPS-20) with silica surface sites. The sum frequency vibrational spectroscopic (SFVS) data have demonstrated that the water molecules associated with EmimTFSI at the silica surface form hydrogen bonds with the IL anions and the structure of the imidazolium cations bound to the surface does not change significantly with water content [49]. This indicates the adherence to the imidazolium cations adsorbed onto the silica surface against the replacement by water (and even by the oxyethylene-type nonionic surfactant). 

The spectroscopic investigations performed by Sleight and coworkers [14-17] demonstrated that methanol chemisorption on M0O3 at room temperature results in a combination of molecular and dissociative adsorption mechanisms. The first mechanism can be considered as a physical adsorption since the methanol molecules adsorb intact on the surface. Dissociative adsorption is a chemisorption process that involves the formation of metal-methoxy (M-OCH3) groups. Further,... 

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 mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain. Several groups have reported that the voltammetry of Au in acidic electrolytes is straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966 Gibbs et al., 1977 Kita et al., 1985 Sun et al., 1999]. There is, however, no voltammetric evidence for the adsorption of CO on the Au surface, and spectroscopic studies indicate only a weak interaction of CO with poly crystalline Au surfaces in acidic solutions [Kunimatsu et al., 1986 Cuesta et al., 2003]. Moreover, there is little evidence for the formation of oxidizing species at the potential where the oxidation process is observed. Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly... 

The reaction mechanism of C02 reduction is still a subject of discussion, although, in general, the mechanisms proposed by Eyring and co-workers45 and Amatore and Saveant53 have proved acceptable for aqueous and nonaqueous solutions, respectively. In situ spectroscopic measurement techniques, by which intermediates and their adsorption behavior can be estimated, will become more and more important in better understanding each elementary step of the reaction pathway. 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

One should realize that adsorption isotherms are purely descriptions of macroscopic data and do not definitively prove a reaction mechanism. Mechanisms must be gleaned from molecular investigations (e.g., the use of spectroscopic techniques). Thus the conformity of experimental adsorption data to a particular isotherm does not indicate that this is a unique description of the experimental data, and that only adsorption is in operation. 

Adsorption of phosphate on Fe oxides involves a ligand exchange mechanism (Par-fitt and Russell, 1977 Sigg and Stumm, 1981) and appears to be promoted by increasing the ionic strength (Bowden et al., 1980). Spectroscopic studies have not provided an entirely consistent picture of the mode of phosphate adsorption, but the consensus from studies with a range of techniques is, that phosphate adsorbs on Fe oxides predominantly as a binuclear, bidentate complex. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

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