Adsorbate bands

All of the information on the structure of adsorbed molecules, which has been discussed previously, has been obtained from consideration of bands due to bonds within the adsorbed molecules. In that work, bands due to bonds between the surface and the adsorbed molecule were not observed. The failure to observe the latter bands was due primarily to experimental difficulties caused by the fact that silica and alumina have poor transmission in the regions where the adsorbent-adsorbate bands are expected. 

Criteria to Distinguish Between Solution and Adsorbate Bands. 139... 

The use of s- and p-polarized light to discriminate between solution and adsorbate bands under thin layer conditions, is possible only with careful consideration of the influence of the angle of incidence and the wavenumber on the differential reflectance. 

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

A common feature of the adsorbate band for these metals is the shift of the C-N mode to higher wavenumbers in the adsorbed state in relation to the free ion in solution. This positive shift is also observed for cyanide complexes, and has been attributed to the fact that CN is a better o-donor than it is a -acceptor. Due to the weak antibonding character of the 5 a orbital, c-donation produces a bond strengthening effect [115]. These facts were the subject of theoretical calculations [117, 118]. 

Besides this theoretical controversy, it is interesting to observe that the band intensity for silver (see Fig. 32a) and copper (Fig. 33) is potential-dependent. This suggests that the interaction of CN with the metal surface presents some electrostatic contribution, as expected for ionic bonding. Contrary to this, a nearly constant intensity of the CN adsorbate band is observed on gold [114] (Fig. 32b) and a bipolar band was observed in SNIFTIRS experiments [113]. These facts suggest a more irreversible adsorption of CN on gold than on Ag and Cu. Irreversible adsorption is expected for bonds with a higher covalent than ionic character. 

Evidence of a relatively flat orientation for adsorbed SCN is also given by the studies by Weaver and coworkers [113, 135] (Fig. 41), using SNIFTIRS, where the loss of thiocyanate from solution (positive band) can be observed simultaneously with the gain of adsorbed thiocyanate (negative band). Since both quantities must be equal, the smaller intensity of the adsorbate band indicates that the absorptivity for adsorbed SCN is smaller than for the solution species. This could account for the proposed flat orientation. The S-coordination of the SCN ion leads to a considerable decrease in the dynamic dipole moment [135]. 

Results on the adsorption of azide (N3 ) and cyanate (CNO ) ions have been reported by Corrigan et al. [113,135]. For adsorbed azide ion on a silver electrode only one potential-dependent band has been reported between 2074 and 2083 cm . As discussed by Corrigan and Weaver [135], at low potentials a loss of azide ions in solution is observed (band at 2048 cm in Fig. 44) without a corresponding adsorbate gain. As the potential is made more positive a weak adsorbate band is developed (2074 cm ). The most probable interpretation, according to Corrigan and Weaver, is that at low potentials the linear N3 ion is flat-adsorbed. As the degree of... 

A solution of 20 mg of this complex in 10 mL of water is poured into a column containing Dowex 50W-X8 resin (200-400 mesh, 2.5 cm x 34.5 cm, Na+ form). The adsorbed band is eluted with a 0.3 mole/L potassium antimonyl-( + )-tartrate solution. When the adsorbed band descends to four fifths of the... 

The general aim of chromatographic separation is the segregation of individual compounds from a sample mixture, as in Fig. 2-1. The relative success of this operation in the case of two compounds that migrate adjacently through the adsorbent bed is determined by the relative overlap of the two bands at the end of separation. Figure 2-6 portrays this situation for an adjacent pair of adsorbed bands. The resolution of the two bands is determined by two factors the relative separation of the band centers (d — d ) and the widths of the individual bands (cTj and a ). We will define the resolution function equal to (d — d f2 a -f ffj). 

To illustrate how different cations may be separated let us consider a column which is packed with a cation exchanger in the hydrogen form. As a solution containing M ions flows through the column the M ions will replace H" " ions on the resin according to the value of K. If only a small amount of the solution is used and it is washed down the column with pure water, all the M ions will eventually replace hydrogen ions, and will form a stationary adsorbed band. The distribution of the ions within this band will depend on the value of if large, the band will be narrow and concentrated if small, wide and diffuse. Thus, if a few ml of 0.1 moldm sodium chloride are placed on a column which is then washed with distilled water, the sodium ions will remain in a more or less narrow band near the top of the column and an equivalent amount of hydrochloric acid will be liberated to be eluted from the column. 

In order to make the adsorbed band of ions (M ) move down the column, water is ineffective and it is obviously necessary to elute with a solution of an acid (or a solution containing another cation), so that exchange can take place. The ions will then be washed out of the column, leaving it in its original form. The rate at which the band of ions moves will depend on the pH of the eluting acid and the value of K. Thus, two ions having different affinities for the resin will move at different rates down the column and a separation will be achieved. 

To deduce from the spectra the absolute amounts adsorbed or desorbed, calibrations were carried out. This was achieved by measuring the absorbances of the typical IR adsorbate bands as a function of the barometrically determined (admitted or removed) amounts of adsorbate. The thus-obtained calibration curves were almost linear in the range of coverage of interest (see, e.g., Vol. 4, Chap. 1 of this series, p. 37). Uptake of the adsor-... 

By far the most convincing explanation of how effective medium theory could be used to predict the enhancement and shape of adsorbate bands in SEIRA spectra was made by Su et al. who studied both the Maxwell-Garnett (MG) and Bruggeman representations of EMT. The MG representation of is by far the simplest of the Bergman, Maxwell-Garnett and Bruggeman formalisms... 

In the second paper, the authors extended the in situ FTIR studies of the C0/Ru(0001) system to 10 and 50 C. As was observed at room temperature, both linear (COl) and threefold-hollow (COh) binding CO adsorbates (bands at 2000-2045 and 1768-1805 cm , respectively) were detected on the Ru(OOOl) electrode at 10 and 50 °C. However, the temperature of the Ru(OOOl) electrode had a significant effect upon the structure and behavior of the CO adlayer. 

The simplest method of removing the zones containing the separated compounds (after allowing solvent to evaporate from the plate) is to hold the plate vertically, its side resting on a sheet of paper and to scrape off the desired zone with a spatula. For substances which are not sensitive to oxidation, the zones may be sucked from the layer directly into an extraction thimble by using a small vacuum cleaner [3] (Figure 5.16). Each separated adsorbent band can then be bottled off in 5 ml polythene tubes and retained for further examination. 

Extraction of Pure Polymer Additives from Separated Adsorbent Bands... 

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