Saturation coverage

The coverage of adsorbates on a given substrate is usually reported in monolayers (ML). Most often, 1 ML is defined as the number of atoms in the outemiost atomic layer of the umeconstmcted, i.e. bulk-tenuinated, substrate. Sometimes, however, 1 ML is defined as the maximum iiumber of adsorbate atoms that can stick to a particular surface, which is temied the saturation coverage. The saturation coverage can be much smaller... 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Rgure 5 NEXAFS spectra above the C K-edge for a saturation coverage of pyridine C5H5N on Pt(111), measured at two different polarisation angles with the X-ray beam at normal incidence and at 20° to the sample surface. 

Figure 2.12. Effect of K coverage on the total saturation coverage of CO on Pt(lll).42 Reprinted with permission from Elsevier Science.

A general conclusion regarding H2 adsorption on alkali modified metal surfaces is that alkali addition results in a pronounced decrease of the dissociation adsorption rate of hydrogen as well as of the saturation coverage. 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

NO is now chemisorbed on the Rh particles at a temperature where it does not adsorb on the AI2O3. The saturation coverage of NO on Rh(lOO) corresponds to one NO molecule per two rhodium surface atoms, with NO sitting in a c(2x2) surface structure. After having saturated the catalyst with NO, a temperature-programmed desorption experiment (TPD) is performed with a heating rate of 2 K min". NO is seen to desorb with a maximal rate at 460 K. The total NO gas that desorbs amounts to 18.5 mL per gram catalyst (P = 1 bar and T = 300 K). It can be assumed that NO does not dissociate on the Rh(lOO) surface. 

Figure 1. Temperature programmed reaction spectra of C2H4 and H2 following saturation coverage of C2H4 on the clean and oxygen covered Pd(lOO) surface at 100 K. Atomic oxygen was generated on the surface by exposure to O2 at 300 K.

Figure 2. Electron energy loss spectra for saturation coverage of C2H4 on clean (a) and oxygen covered (b) Pd(lOO) at 80 K. Oxygen was dosed as in fig. 1 to a coverage of 0.18 ML.

Figure 3. Temperature-programmed reaction spectra morutoring m/e = 73 (a cracking fragment of trimethylsilane) after adsorbing a saturation coverage of methyl groups on CusSi surfaces prepared by ion bombardment at 160 K (dotted curve) and 330 K (solid curve).

The dependence of methane formation on methyl coverage is shown in fig. 3. The yield of methane increases as the coverage increases. At coverages below 0.05 ML, little meAane is formed. Nearly all of the methyl groups decompose. At saturation coverage, about 0.14 ML of the methyl groups form methane, and roughly 0.27 ML decompose... 

X-ray diffraction and surface area measurements suggest that these W-atom surface densities correspond to saturation coverages, which markedly inhibit zirconia sintering and tetragonal to monoclinic transformations at high temperatures. Zr02 surface areas after 1073 K calcination are 4 m g" and increase to an asymptotic value of 51 m g for W surface densities above 5-6 W-atoms nm (Figure 4). Similarly,... 

W0x-Zr02 samples with surface densities above 5-6 W-atoms nm contain only the metastable tetragonal phase (Figure 4), but pure Zr02 contains only the thermodynamically stable monoclinic phase. This apparent saturation coverage is higher than... 

Fig. 6 IR spectra of adsorbed CO on 1 ML Co deposited at room temperature on the alumina fflm as a function of CO coverage. Spectra were taken at 44 K. Black trace second up from the bottom shows the spectrum resulting from saturation coverage of a 50 50 mixture of i CO i CO. Overlaid gray trace is artificially created by adding the saturation coverage spectra CO and CO scaled by a factor 1/2. Lowest black trace shows the spectrum of CO saturation coverage on particles grown hy 1 MLCo + 0.05 MLPd. Corresponding grey trace is the pure Co spectrum for comparison. Right inset shows TPD spectrum of 1 ML Co [64]...

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