Monolayer dispersion support

Recently, ultrathin evaporated films have been used as models for dispersed supported metal catalysts, the main object being the preparation of a catalyst where surface cleanliness and crystallite size and structure could be better controlled than in conventional supported catalysts. In ultrathin films of this type, an average metal density on the substrate equivalent to >0.02 monolayers has been used. The apparatus for this technique is shown schematically in Fig. 8 (27). It was designed to permit use under UHV conditions, and to avoid depositing the working film on top of an outgassing film. ... 

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. 

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. 

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. 

This catalyst was prepared by impregnating y-Al203 with a Ni(N03)2 and La(N03)3 solution. By drying and calcinating the impregnated support we then obtained a system of monolayer-dispersed NiO and La203 on... 

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. 

It is worth mentioning that spontaneous monolayer dispersion is also a very useful scientific basis underlying the process of regeneration of deactivated metal catalysts. Supported metal catalysts may sinter during use at elevated temperatures. Sintering will cause the metal catalyst to lose initial activity, and in order to recover it one has to find an effective way to redisperse the metal on the catalyst support. Applying what we have learned from our studies on spontaneous monolayer dispersion to... 

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. 

Spontaneous monolayer dispersion of compounds on supports is a widespread phenomenon and displays many unique effects. The principles involved have applications not only to heterogeneous catalysis, but also to materials science and other related fields. The theoretical and practical aspects of this phenomenon appear to offer prospects that should not be overlooked. Studies in connection with this phenomenon are continuing in our laboratory. 

The authors acknowledge China s National Natural Science Foundation for generous support of this work, which is a part of the major project Structural Chemistry and Molecular Design. We are grateful to all our colleagues who have contributed toward a better understanding of the phenomenon of spontaneous monolayer dispersion. Thankful acknowledg-... 

Another important consideration in preparing mixed-oxide catalysts is the spontaneous monolayer dispersion of oxides and salts onto surfaces of support substrates on calcination. Both temperature and duration of calcination are important here, as discussed in the reviews by Xie and Tang [63] and by Knozinger and Taglauer [64]. If this dispersion step is inadequate or incomplete, the resulting oxide layer, and any reduced metal surface from it, will not be reproducible from the same catalyst system therefore, one can then have different catalysts prepared at different times and, of course, from one laboratory to another. Spreading and wetting phenomena in preparation of supported catalysts is discussed in Section A.2.2.1.3. 

Y. C. Xie and Y. Q. Tang, Spontaneons monolayer dispersion of oxides and salts onto surfaces of supports, Appl. Heterogen. Catal., 37, 1-25 (1990). 

Xie, Y.C. Tang, Y.Q. Spontaneous monolayer dispersion of oxides and salts onto surface of supports applications to heterogeneous catalysis. Adv. Catal. 1990, 37, 1 3. 

Different methods were used to prepare well-dispersed or monolayer type materials on various carriers. A desirable dispersion can be obtained by the first method developed in our experiments as long as certain conditions are carefully chosen and controlled for the different systems. The location, coordination and M-O bond strength of those surface-dispersed ferric ions are strongly determined by the nature of the supports and the preparative methods. Significant changes in oxidation-reduction properties and catalytic performances for the most well-dispersed or monolayer dispersed samples suggest that there is a strong interaction between the support and supported component. 

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