Monolayer dispersion

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

Langmuir model, 38 164-166 polynomial model, 38 167 theoretical background, 38 150-175 thermodynamics, 38 150-163 mobile, 26 360 modes, hydrogenolysis, 30 44 monolayer dispersion, 37 33-34 of NHj, 34 171... 

Monocoordination, versus dicoordination, BOC-MP, 37 125-127 Monolayer-dispersed, 37 2-4 adsorption, 37 33-34 capacities, 37 13-14... 

Spontaneous Monolayer Dispersion of Oxides and Salts onto Surfaces of Supports Applications to Heterogeneous Catalysis... 

This article provides a review of various aspects of the phenomenon of spontaneous monolayer dispersion, namely, its nature, effects, and applications. It is based in the main on the work that has been carried out in our laboratory in the last 16 years. Relevant data and results from other laboratories have been included for discussion. 

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. 

We have extended our investigation to a great many systems of oxides and salts on supports with highly specific surfaces (II-14, 18-21). They all display the phenomenon of spontaneous monolayer dispersion. In Table I these systems are given along with the temperature and the period of time for a suitable heat treatment. 

Monolayer dispersion is a spontaneous process. Thermodynamics would require that a spontaneous process should proceed with diminishing free enthalpy G or AG < 0. Normally, a process that disperses a substance in a crystalline state as a monolayer or submonolayer, if not as a multilayer, onto the surface of a support would gain in entropy. If this process is energetically not so unfavorable as to reverse its trend, the free enthalpy would decrease and so occurs the spontaneity. Otherwise, the process of a crystalline substance dispersing as monolayer onto the surface of a support would not proceed at all. 

Now we can talk about the role played by the heat treatment and the mechanism of monolayer dispersion. If the process is to proceed to its... 

The phenomenon of monolayer dispersion described above may well be attributable to solid/solid adsorption. However, for the phenomenon of monolayer dispersion, we wonder whether the analogy between liquid/ solid and solid/solid is so close as to justify borrowing a term such as wetting (27-31). 

Oxides or salts in a monolayer state and in their crystalline state behave differently in many respects. Effects of monolayer dispersion show up in spectra as well as in the properties of the oxides and salts. 

Samples of Mo03/Ti02 prepared by an impregnation method using a solution of (NH4)2Mo207 have been studied recently by Quincy et al. (39). They obtained a similar plot, which allows us to obtain the same monolayer dispersion capacity, namely, 0.12 g MoO3/100 m2 of Ti02 surface. 

Fig. 17. XPS spectra of valence band Cu3d. (a) Crystalline CuCl (b) 0.12 g CuCl monolayer dispersed on 1.0 g -y-AljOj of a specific surface (343 m2/g) B.E., binding energy.

Fig. 18. From a sample of 0.12 g CuCI monolayer dispersed on 1 g -y-Al203 of a specific surface (343 m2/g) SSIMS signal is found to decline exponentially with time. Ar+ ion beam 1 keV and 0.6 nA.

Similar results were reported for the same sample heated in a dry oxygen stream, and also for the system Mo03/Ti02 heated under 450°C either in a moist or dry oxygen stream. All these ISS studies are consistent with the spontaneous monolayer dispersion model. 

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