Graphite surfaces, interaction metal particles

A previous study (12) on the interaction of fine metal particles on graphite surfaces demonstrated that particles can catalyze surface reactions and lead to the generation of elongated pores of the type shown in Figure 4. The similarities of the pore geometry and the presence of mineral particulates led to the speculation that the pores observed in the exinite were the consequence of a reaction catalyzed by the mineral particles. The source of the particles is believed to be the mineral-rich granular inertinite that typically surrounds the spore exine. However, further studies revealed that all particles analyzed to date contain only calcium as the heavy atom, which leads one to question the absence of... 

In general, encapsulated metal particles were observed on all graphite-supported catalysts. According to Ref. [4] it can be the result of a rather weak metal-graphite interaction. We mention the existence of two types of encapsulated metal particles those enclosed in filaments (Fig. 1) and those encapsulated by graphite. It is interesting to note that graphite layers were parallel to the surface of the encapsulated particles. 

Basically, the effect of the surface nanotexture on the strength of metal-carbon bonding may occur as a result of epitaxy or interdiffusion of atoms in the contact region of a metal crystallite and carbon support. However, information concerning these aspects of the metal-carbon interaction is scarce. Graphite-supported Pd and Pt crystallites are oriented their 202 for Pd [19] and 111 or 110 for Pt [20-22] planes parallel to the basal plane of graphite substrate, but this epitaxial interaction is relatively weak [19-21,23]. In contrast, Pd particles supported on amorphous carbons are in random orientation [19,25]. Hence, heterogeneous support surfaces comprise structurally different sites for metal-particle stabilization. 

This example shows that a nonoxidic support can give rise to interesting properties in active particles. The catalytic performance is not simply correlated to size distributions. The experiments presented in this section reveal how few indisputable facts are yet known concerning the metal-support interaction for carbon substrates. Interactions of the type (metal d-states)-(carbon sp )-(carbon sp2) mediated via amorphous accommodation particles (Fig. 32) of intermediate layers should be considered in the prevailing picture of a yet unproved epitaxy between transition metals and graphite (001) surfaces. 

Baker [27] observed mobilization of small particles of several metal oxides on graphite at a temperature (the so-called mobility temperature) that was identical to the Tammann temperature. Thus, at least in systems exhibiting relatively weak interactions between active phase and support surface, particle mobility may be induced at this temperature. The particle migration may perhaps be described as a floating of the active phase particle on the liquid-like surface layer. 

Such higher residual activity could be in accordance with the model proposed by Sachtler, which assumes that S is preferentially adsorbed on Re sites in bimetallic Pt-Re particles [20]. Indeed, as the Pt-Re interaction is the highest, sulfur ought to divide the Pt-Re surface into very small Pt ensembles. Consequently, the reorganization of the carbonaceous overlayer into pseudo-graphitic entities, which are detrimental to the metallic function, could be impeded. 

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