Monolayer-dispersed catalysts, highly

A. Preparation of Highly Active Monolayer-Dispersed Catalysts... 

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. 

Rendering the active component into the monolayer-dispersed state is an important measure to undertake to enhance the activity of a catalyst. A striking example is the highly active catalyst system for... 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

For the present purpose, we take the term ultrathin to refer to an evaporated metal film where the concentration of metal on the substrate is low enough for the film to consist of small isolated metal crystals. If the average concentration of metal atoms on the substrate is of the order of a monolayer or less, the metal crystals are small enough for ultrathin films to serve as models for highly dispersed metal catalysts, but where surface cleanliness and catalyst structure can be better controlled. 

A comparison of XRD patterns a and a or f and f in Fig. 1 raises the question as to what has become of the active component, the quantity of which is about 10% by weight in a catalyst. First of all, the active components no longer exist in the crystalline state. It is also unlikely that they have become an amorphous mass. Later on we shall provide further evidence to verify that the active component in each case has not formed stoichiometric compound with the support nor has dissolved in the support to form solid solution. It is then reasonable to think that the active component has dispersed as a monolayer on the surface of the support. In view of the high surface area of the supports and the heavy metals in the active components of these catalysts, a monolayer may still amount to a considerable percentage, usually more than 10% by weight, of an active... 

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