Shifts upon adsorption

Even potentially more important is the observation that the stretching frequency of molecular hydrogen adsorbed at cationic sites is also strongly affected by positive polarizing helds. From the few low-temperature experiments reported (and those documented in this review), it is emerging clearly that f (H-H) is red shifted upon adsorption of H2 at positive centers and that the Af /i ratio is much larger than that found for CO and, a fortiori, for N2. This result suggests that H2 may be used much more as an efficient probe of surface helds. 

The redox potential for the dye can shift upon adsorption from solution due to coulombic or stronger covalent interactions with the solid substrate. This potential change can amount to several hundreds of millivolts. While n-type semiconductors cannot be used generally to measure oxidation potentials of adsorbed dye sensitizers by conventional cyclic voltammetry, reduction potential i°(S/S ) is often more accessible. Assuming oxidation and reduction potentials of the dye ground state on the surface are linked by a constant relation ... 

The catalytically active sites of isomorphous substituted MFI structures have been characterized by inirared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH, adsorption, the NH, TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from Al>Fe>In> >silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample. 

Wavenumbers (in cm ) of the Bronsted OH stretching, v(OH), and OD bending modes, 6(OD), and their shifts upon adsorption of CDjCN... 

Two groups have independently addressed the issue of how aUcah metal doping of nanotubes affects the Hj-nanotube interaction [77, 78]. Both observe an enhancement of the binding. Dubot and Cenedese [77] predict that the H2 stretching mode will shift upon adsorption and that this shift should be observable experimentally. Froudakis [78] predicts a binding energy of about 150 meV that decreases as the number of H2 molecules per metal atom is increased. 

Several types of cells have been constructed and are commercially available that allow an in situ measurement or have a movable holder to evacuate and heat the sample. With these cells it is also possible to admit gas or vapor to subsequently measure the frequency shift upon adsorption. This technique can reveal additional information about the surface properties of the material. Carbon monoxide, for instance, has been applied as a probe to detect hydroxyl groups at the silica surface. Upon adsorption of CO at 77 K, the band due to free hydroxyl groups shifts to a lower frequency by 78 or 93 cm-1 (35, 36, 37). 

An interesting analysis consists in determining the v OH shift upon adsorption of weaHy basic probe molecules as H2S, CO, C2H4 or C6Hg as shown in table 2, the shift increasing with acid strength. 

Excitation Spectra and Fluorescence Lifetimes. In this section, we further examine adsorption onto colloidal polystyrene by investigating the excitation spectrum of the adsorbed species. In Figure 7a, we show the excitation spectra of Py-PEG-Py(4250) before and after the addition of polystyrene particles. The spectrum undergoes a substantial red shift upon adsorption the main peak moves approximately 4 nm. A similar effect is found in the complexation with PMAA, as shown in Figure 3, where the monomer spectrum is again shifted to higher wavelengths. From these spectral shifts, we infer a spectral sensitivity to the environment. This sensitivity is illus-... 

The far-left column in the cluster-adsorbate system. Adjacent three columns (steric/donation/back donation) give in the change in frequency due to each contribution, calculated with respect to the uncoordinated DFT frequency of 2095 cm" for CO. The main entries have been calculated in the order as given in the Table, i.e., steric-donation-back donation, whereas the entries within parentheses refer to the order steric-back donation-donation. The column rest refers to the residual frequency shift upon adsorption not accounted for by the sum of steric/donation/back donation. The far-right column gives the coordinated C-0 vibrational frequency. 

An interesting adsorption feature, which is also described by this simple model in the limiting case when m = and A(() = Atj), is the surface potential shift upon adsorption. Ax, defined from... 

Figure 6, Plots of 1/Ax vs.. 1/9 for the adsorption of acetonitrile ( , o), propionitrile (+, x) and butyronitrile (0, A) at the uncharged Hg solution ( , +, 0) and air solution (o, X, A) interfaces. The straight lines were calculated using a least squares procedure. Data were reprinted from J. Electroaml. Chem., 385, P. Nikitas and A. Pappa-Louisis, The Surface Potential Shift upon Adsorption at the Air/Solution and the Uncharged Hg/Solution Interfaces, p. 257, Copyright 1995, with permission from Elsevier Science.

The spectrum on n-butylamine on y-alumina is shown in the Figure 3a( ). There are six prominent resonances, two of which are readily assigned to the y-methylene and methyl carbons by direct comparison with the liquid phase spectrum. These resonances are only slightly shifted upon adsorption. The remaining four resonances arise from the two a and 3 methylene carbons, from which we conclude that at least two types of chemically different butylamine species are present on the surface. 

XRD is useful to determine cation location in the faujasite structure. For some light cations such as Li or for structures with many possible sites, far infrared spectroscopy may give valuable information. It has been applied mainly to faujasites (figure 13) (80-82) and quite less to A or ZSM-5 (82). In each structure the cations in the various sites give rise to bands at a given wavenumber (80) which shift upon adsorption of H2O (81,82) or other adsorbates such as H2S, C6H6... (81). These results clearly indicate that the interaction cation - adsorbate described above implies also a change in the interaction of the cation with the zeolite framework. The deconvolution of the spectra may lead to the evaluation of the cation population in each site. 

A similar interaction occurs with the very stable complexes of various cations such as Cu ", Ni " or Co ". At high ammonia concentration (pH > 8). [M(NH3)6] complexes dominate in. solution and are adsorbed on various substrates [3). The UV-visible characteristic absorption of the nickel amino complex is not shifted upon adsorption on silica [4]. Adsorption of these complexes does not cause a shift in the intersection point of the proton titration curves (i.e does not cause a shift in the PZC of the oxide [5]). The negatively charged surface of the oxide acts as the counterion of the cation complex (M(NH3)e1 at the most, to the formation of ion pairs ... 

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