Sub-monolayers

The cross sections are sufficiently large that one can detect sub-monolayers of most heavy mass elements on... 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Figure 6-25. Fluorescence spectra at 4.2 K of T,. thin lilms with morphology characterized by a) grains b) layers c) islands d) sub-monolayer. The lower density ol aggregate states in the sub-monolayer architecture decreases the energy transfer efficiency to low energy stales and the fluorescence acquires dominant excitonie character (sec Section 6.6.2.2J.

Here we shall consider a different concept, which has an interesting potential, particularly in liquid phase reactions used for the production of fine chemicals. The concept is schematically illustrated in Fig. 3. The modification of the metal catalysts is achieved by very small quantities (usually a sub-monolayer) of adsorbed auxiliaries (modifiers), which are either simply added to the reaction mixture (in-situ), or brought onto the catalyst surface in a... 

Static SIMS, used for sub-monolayer elemental analysis. At the lowest current densities and hence the lowest rates of erosion, a monolayer on the surface has a lifetime of many hours. The surface is essentially unchanging during the experiment, but a vacuum system at a pressure of 10 10mbar is needed to allow adequate time to complete the analysis. In favourable cases, as little as 0.1% of a monolayer of material can be detected. 

In principle, the analysis of molecules, ions and adsorbed intermediates is possible if they survive the emersion (no potential control) and UH V conditions (elimination of most of the solvent). The use of ex situ methods for the analysis of sub-monolayer quantities of oxygen-sensitive substances requires an extremely inert atmosphere when the electrode is emersed. In order to check whether a given adsorbate survives the experimental conditions, a control experiment must be carried out, as we describe here for adsorbed CO on Pt. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

In this paper we shall review the various experimental parameters that can be used to characterize the adsorbate layer in the sub-monolayer to few-monolayers range. 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

IETS is well suited for investigating the adsorption of organofunctional-alkoxy silanes on alumina. Surface sensitivity at the sub-monolayer level, and orientational selectivity makes this technique particularly attractive, especially when used in conjunction with other techniques such as FT-IR, and SER spectroscopies. 

The maximum amount of species O and R at the surface rM is independent of the applied potential. This condition implies that no interactions between the surface-confined molecules are considered [45], that is, a low amount of species is at the surface (i.e., I < I m)- In other words, low values of the ratio FJI a are considered, with i = O or R (this situation has been denoted sub-monolayer, with the monolayer corresponding to a totally covered electrode surface). 

The experimental verification of the theoretical behavior of DSCVC responses is presented in [4] for two systems which have been adsorpted on a mercury electrode forming stable sub-monolayers the disodium salt of the 2,6-Anthraquinonedisulfonic acid (AQDS) 1.0 pM + HCIO4 0.5 M, which behaves as reversible (Fig. 7.45), and the 4-PhenylazoPhenol 5 pM + 0.5 M KNO3 (pH = 8.0), which behaves as quasi-reversible (Fig. 7.46). 

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