Monolayer dispersion coverage

According to a simple model based on the assumption that the anions of oxide or salt form a close-packed monolayer on the surface of the support and the cations occupy the interstices left over by anions, one can figure out the close-packed monolayer capacity for oxide or salt on a unit area of the support. We estimate it at 0.10 g/100 m2 or higher for various active components (see later, Table II). The specific surface of the support is about 200 m2/g for y-Al203, 300 m2/g for silica gel, and 1000 m2/g for active carbon. Although each of the catalysts in Fig. 1 contains a considerable amount of active component, its content is still lower than that estimated on the basis of a close-packed monolayer. Therefore, the monolayer dispersion in many of these catalysts does not correspond to the full coverage of the support surface, and more precisely is known as submonolayer dispersion. 

V—0-Support (930cm ) and V—O—V (625 cm ) bonds. Similar distributions of monomeric and polymeric surface VO4 species are found on other oxide supports with the exception of Si02 [30]. For the supported V20s/Si02 catalyst system, only isolated surface VO4 species are present below the maximum dispersion limit (<3 V atoms/nm ). For all supported vanadium oxide catalysts, crystalline V2O5 NPs are also present above the monolayer surface coverage or maximum dispersion limit [31]. 

As discussed in Section 2.2, the maximum sensitivity in IRRAS is achieved with p-polarized radiation at grazing angles of incidence. With a single reflection, the method can detect CO adsorbed on Ir at 0.002-monolayer (ML) coverage [90]. A SNR on the order of 1000 can be reached, even with a conventional doublebeam dispersive spectrophotometer [91]. This is crucial for studies of catalysis... 

The net pH at PZC for a supported metal oxide catalyst possessing monolayer surface coverage is dependent on the pH at PZC value of the oxide support substrate and the pH at PZC value of the pure metal oxide that is in the dispersed metal oxide phase ... 

Both the oxide support and the dispersed oxide exert an influence on the net pH at PZC because the thin aqueous film is in contact with both components (especially when clusters of the surface metal oxides are present). Below monolayer surface coverage, the pH at PZC is a function of the surface coverage of the dispersed oxide and monotonically decreases from the value of the oxide support to the value at monolayer coverage given by Equation (1.1) with increasing surface coverage. The individual values of the pH at PZC for pure oxides at room temperature are well documented in the literature [36-38] and are presented in Table 1.1 for typical oxides encountered as oxide supports and active metal oxide phases. 

The influence of the surface coverage of different surface metal oxides on the net pH at PZC for a series of AI2O3 supported metal oxides is shown in Figure 1.1. The pH at PZC of the AI2O3 support is 8.8 and continuously decreases as the surface coverage of metal oxides with low values of pH at PZC is increased. Note that at monolayer surface coverage the net pH at PZC of the different supported metal oxide catalysts asymptotically reaches values intermediate between that of the pure alumina support and the pure dispersed metal oxide phase. 

Deposition of Co2(CO)g from the gas phase under a CO or N2 atmosphere on mesoporous high surface-area MCM-41 material has been reported [148]. Under CO, a Co2(CO)g monolayer coverage of up to 21 wt% cobalt was obtained. Although treatment at circa 150°C under N2 produced total decarbonylation, the surface area and pore size of the sample did not change and the presence of metalUc cobalt species could not be determined from the XRD patterns of decarbonylated samples these facts could indicate a good metal dispersion and capabilities for catalytic uses in hydrogenation reactions [148]. 

Figure 2. Dependence of surface pressure on monolayer coverage for polymeric particles 113 nm in diameter without dispersant (A) and with dispersant ( ) according to Wolert et al 36 theoretical calculations used Eq. (14) with model parameters given in Table 2.

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