Supported metals crystal structure change

On the other hand the carbon materials are widely known supports of metal catalysts from of old. It has been shown, that the carbon supports increase the dehydrogenative properties of the metal catalysts due to the epitaxial changing of metal crystal structure providing their structural relevance with reacting molecules [8],... 

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

Is it known that the rate of hydrogenolysis reactions are extremely sensitive to effects of alloying, surface contamination, poisoning, etc. Consequently, in all cases where supported metals are used there must be concern as to whether apparent particle size effects are due to structure sensitivity or to some minor contamination effect. In the few cases where clean single crystal surfaces have been used there is evidence of a structure effect.338 However, the maximum change in activity between different crystal faces seems to be about a factor of 10. For Ni single crystals the (100) surface is more active than the (111) surface. A similar conclusion has been reached for oriented Ni powder samples.339... 

Amorphous carbon is characterized by a highly imperfect structure and high reactivity. This shows by a considerable amount of mobile carbon atoms at a surprisingly low temperature. Besides, a vast number of defects and small sizes of graphene sheets make the carbon matrix very labile. As a result, it may be deformed under the action of adsorbates. For example, granules of amorphous carbon swell [88,89] in water with concomitant changes in the carbon substructure and porosity [90,91]. These properties of the support weaken rapidly as its crystal structure becomes more perfect. The labile structure of amorphous carbon is responsible for at least two mechanisms of blocking of the surface of supported metal particles. 

The thin films responsible for passivity are often amorphous, and since the extent of solid solubility is dependent on the crystal structure, the rigid compositions associated with the crystalline state are not necessarily operative within these thin films. It seems possible, therefore, that with films that are predominantly oxide a certain concentration of hydroxyl ions could be present, and likewise, films that are predominantly hydroxide could contain a certain proportion of oxygen ions. This view is supported by the corrosion behavior of such metals as aluminum and the stainless steels, where different degrees of passivity are obtained by alloying or by slight changes of concentration of the corroding solution. 

One can use practically the same catalyst preparation methods with carbon blacks as with activated carbon supports to give comparable results. In fact, some precious metal compounds reduce upon contact with furnace black, resulting in a change of the furnace black structure around the metal particle [39]. It has been observed [40] that carbon blacks can be oxidized by the metal crystals supported on them at rather low temperatures (from 398 to 468 K). In fact, some metals may form carbides [41], and this type of metal-support interaction will clearly affect the performance of the catalyst. 

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