Oxides monolayer-dispersed

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... 

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

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. 

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. 

Monolayer-dispersed oxide or salt should give a Raman spectrum different from its crystalline counterpart. This statement has been confirmed by recent work in several laboratories (39, 45-50). 

Because a monolayer-dispersed oxide or salt on a support with a highly specific surface usually is present in considerable quantity, the sensitivity of extended X-ray absorption fine structure (EXAFS) analysis is good... 

The active components of many commercial supported heterogeneous catalysts are oxides or salts. Even for many metal catalysts, the precursors of metallic particles are also oxides or salts in some dispersed form. Hence the preparation of heterogeneous catalysts is deeply concerned in one way or another about the dispersion of oxides or salts on support surfaces. Furthermore, promoters or additives added to heterogeneous catalyst systems are also oxides or salts. Therefore, the spontaneous monolayer dispersion of oxides or salts on supports with highly specific surfaces as a widespread phenomenon will find extensive application in heterogeneous catalysis. Examples illustrative of this viewpoint are cited in the following sections. 

B. Monolayer-Dispersed Oxides and Salts as Surface Modifiers... 

C. Preparation of Supported Metal Particles from Monolayer-Dispersed Oxide... 

As was stated previously, metal cannot disperse as a monolayer onto catalyst supports. However, oxide precursors of metals can monolayer disperse on supports, and supported metal particles can be prepared from the monolayer-dispersed oxide by reduction. 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

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