Equivalent monolayer coverage

Results are often converted to an equivalent monolayer coverage for direct comparison with field emission gun scanning transmission electron microscopy (FEGSTEM) EDX measurements of grain boundary segregation. The calculation is straightforward for binary systems, but the complexity rapidly increases as the number of elements increases. The equivalent monolayer coverages (O) for P and C in Fe are summarised in Table 92 ... 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

The measured NMR signal amplitude is directly proportional to the mass of adsorbate present, and the NMR signal versus pressure (measured at a fixed temperature) is then equivalent to the adsorption isotherm (mass of adsorbate versus pressure) [24-25]. As in conventional BET measurements, this assumes that the proportion of fluid in the adsorbed phase is significantly higher than the gaseous phase. It is therefore possible to correlate each relaxation time measurement with the calculated number of molecular layers of adsorbate, N (where N = 1 is monolayer coverage), also known as fractional surface coverage. 

The total coverage 0 = 2 0(z) may well exceed one (or even several) equivalent monolayers. For physisorbed layers of a homopol5nmer (as in fig. 3.18c), the treiin layer of segments in contact with the air gives the leading contribution to r, as we shall show below. However, for brushes (fig. 3.18d) the contribution of the entire layer must be considered we shall discuss brushes in subsec. 3.4j. 

Figure 20 The integrated TPD area (P2 state) plotted as a function of the Cu coverage in equivalent monolayers. The average sizes of the Cu clusters are also indicated. (From Ref. 58.)...

Figure 6.5 shows typical adsorption isotherms plotted as surface coverage (in equivalent monolayers) versus polymer volume fraction 0 in bulk solution (( was taken to vary between 0 and 10 which is the normal experimental range). The results in Figure 6.6 show the effect of increasing the chain length r and effect of solvency (athermal solvent with = 0 and theta solvent with = 0.5). The... 

Blondeau et used radiochemical measurements in conjunction with Raman scattering. They find that the amount of adsorbed pyridine is the equivalent of several layers (based on the assumption of a coverage of 1.5 x 10 mol cm, i.e., 9 x 10 " molecules cm ). Even without almost any electrochemical treatment they see about two to three layers, while at typical conditions for SERS (25 mC cm ) the equivalent of eight layers is seen. This, of course, decreases the values calculated for the enhancement factor based on monolayer coverage. For cyanide only a monolayer of Ag(CN) ion is seen, emphasizing the large enhancement factors evaluated for this ion. 

Fig. 5 XRD patterns of the support (a), CLD catalysts on the support dried at 393 K (b) and 573 K (c), and IMP catalysts (d). The amount of added vanadium was equivalent to monolayer coverage. Thick arrows, anatase dotted arrows, rutile and vertical dotted lines, V2O5.

Many surfaces exhibit a different periodicity than expected from the bulk lattice, as is most readily seen in the diffraction patterns of LEED often additional diffraction features appear which are indicative of a superlattice. This corresponds to the formation of a new two-dimensional lattice on the surface, usually with some simple relationship to the expected ideal lattice [5]. For instance, a layer of adsorbate atoms may occupy only every other equivalent adsorption site on the surface, in both surface dimensions. Such a lattice can be labelled (2x2) in each surface dimension the repeat distance is doubled relative to the ideal substrate. In this example, the unit cell of the original bulk-like surface is magnified by a factor of two in both directions, so that the new surface unit cell has dimensions (2 x 2) relative to the original unit cell. For instance, an oxygen overlayer on Pt (111), at a quarter-monolayer coverage, is observed to adopt an ordered (2 X 2) superlattice this can be denoted as Pt (111) -i- (2 x 2)-0, which provides a compact description of the main crystallographic characteristics of this surface. This particular notation is that of the Surface Structure Database [141 other equivalent notations are also common in the literature, such as Pt (111)-(2 x 2)-0 or Pt... 

Figure Bl.21.7. Top and side views of the best-fit structure of the Mo(100)-c (4 x 2)-3S surface structure (with a 3/4-monolayer coverage of sulfur), as determined by LEED [32]. A c (4 x 2) unit cell is outlined in the top view. The sulfur sizes (small black and dark grey circles) have been reduced from covalent for clarity, while the molybdenum atoms (large circles) are drawn with touching radii. The same cross-hatching has been assigned to molybdenum atoms that are equivalent by symmetry in the topmost two metal layers. Two-thirds of the sulfur atoms are displaced away from the centre of the hollow sites in which they are bonded these displacements by 0.13 A are drawn exaggerated. Arrows in the top view also indicate the directions and relative magnitudes of molybdenum atom displacements (these substrate atoms are drawn in their undisplaced positions, except for the buckling seen in the second molybdenum layer in the side view). The bulk interlayer spacing in Mo(lOO) is 1.575 A.

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