Adsorption coordination

Adsorption (coordination) of the sulfur compound to the active site... 

Fig. 20. Cluster models of two structural types of trigonal aluminum LASs in aluminosilicates. State 1 in structure (b) corresponds to the structural coordination, whereas state 2 represents the adsorption coordination.

Table 1 Comparison of adsorption coordination mode of reduced COj on Pd, Pt, and Ni...

At the next stage of adsorption coordinate complexes are formed with cis-position of water molecule relatively to hydroxyl group, those are characterized by the adsorption near 3550 cm . ... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

FIG. 16-1 Isotherms (left) and isosteres (right). Isosteres plotted using these coordinates are nearly straight parallel hnes, with deviations caused by the dependence of the isosteric heat of adsorption on temperature and loading. 

FIG. 5 (a) Fraction of adsorbed monomers (i.e., those with z-coordinate less than 6) vs ejk T for four different chain lengths, (b) The same for the second Legendre polynomial P2 cosd). (c) Scaling plot of and P2 cos9) vs distance from the adsorption threshold, using = —1.9 [13]. 

The data obtained while studying the role of aluminumorganic compounds during polymerization by TiCh (157-159) show that an aluminum-organic co-catalyst can be a reversible coordination inhibitor by itself. The decrease in the number of propagation centers by the addition of aluminumorganic compounds to titanium dichloride seems to be caused by the reversible adsorption of the aluminumorganic compound on the titan-... 

As a result the polymer chain growth seems to stop due to the impossibility of the monomer coordination on the titanium ion. The propagation centers are nonuniform and the adsorption of the aluminumorganic compound takes place mostly at sterically more accessible nonstereospecific active centers. 

The processes of reversible adsorption of the coordination" inhibitors (including the adsorption of organometallic compounds) result in an increase in the lifetime of the transition metal-carbon bond. It is possible that due to this, in the case of propylene polymerization by two-component catalysts based on TiCU, at low temperatures a long-term increase of molecular weight with time was observed (192,193). 

The effect induced by different electronegative additives is more pronounced in the case where the additive adatoms occupy the most coordinated sites forming ordered structures (e.g Cl addition onNi(lOO)). In this case (Fig. 2.28) one modifier adatom affects 3-4 CO adsorption sites and complete disappearance of the CO p2-peak is observed above modifier coverages of -0.25 or less. The lack of ordering and the tendency of the modifier to form amorphous islands (e.g. P on Ni(100)) diminishes the effect. Thus in the case of P on Ni(100) the disappearance of the CO p2-peak is observed at P coverages exceeding 0.6. 

The reaction coordinate that describes the adsorption process is the vibration between the atom and the surface. Strictly speaking, the adsorbed atom has three vibrational modes, one perpendicular to the surface, corresponding to the reaction coordinate, and two parallel to the surface. Usually the latter two vibrations - also called frustrated translational modes - are very soft, meaning that k T hv. Associative (nondissociative) adsorption furthermore usually occurs without an energy barrier, and we will therefore assume that A = 0. Hence we can now write the transition state expression for the rate of direct adsorption of an atom via this transition state, applying the same method as used above for the indirect adsorption. 

The other extreme is direct adsorption, in which the molecule lands immediately at its final adsorption site without the possibility of moving over the surface. In this case the only degrees of freedom the molecule has in the transition state are vibrational, among which the vibration between the molecule and the surface represents the reaction coordinate. This leaves us with the following expression, which immediately indicates that the rate constant is small ... 

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