Particle-Size Effects with Supported Metals

Particle-Size Effects with Supported Metals 

The structure-sensitivity of alkane hydrogenolysis catalysed by platinum was first established by Oles Poltorak many years ago since then, comparatively little work has been done on this system. The TOP for cyclopentane decreased about five-fold on Pt/A Os catalysts of 7 to 65% dispersion at 573 K, this dependence being similar to that of the stepwise exchange of methane, but less than that for multiple exchange. The trends of isomerisation and hydrogenolysis selectivities with particle size observed with n-butane (Section 13.3) have 

Values of TOP for hydrogenolysis of cyclopentane and MCP on Rh/Al203 at 353 Kpassed through maxima at 20-30% dispersion (H/Rh 0.4), lowest values occurring at high dispersion (Pigure 14.9) MCP was much the less 

The motivation for these studies is fairly obvious it is to limit parasitic reactions such as hydrogenolysis and carbon deposition when the target reactions are skeletal isomerisation, cyclisation or aromatisation. What is to be described will amplify the information contained in Sections 12.5.3,13.7, and 13.8. 

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Particle-Size Effects with Supported Metals... 

The results can be understood in terms of the influence of an intrinsic particle size effect (independent of the support) and a support-induced particle size effect. For both reactions and both supported metals, the intrinsic effect manifests itself in a decrease in activity with decreasing particle size below about 3 nm. 

As is shown in Figure 6 (experiments) and Table 4 (Monte-Carlo analysis), a general trend is that Pt catalysts with supports of higher acidity lead to a higher contribution of the a-T)1 (Dl) and di-o-T)2 (D2) intermediates. As the ASA and LTL supports have similar metal particle sizes, this cannot be explained by particle size effects. Apparently, acidic supports enhance... 

It is almost impossible to distinguish clearly between those physical or catalytic effects that are intrinsically dependent on particle size from those which are conditioned by contact with the support, because at least in the context of catalysis small particles are necessarily employed, and their utility depends on their being supported. Furthermore, the smaller the particle, the greater will be the fraction of atoms directly in contact with the support and therefore influenced by it, while at the same time the fraction of coor-dinatively unsaturated surface atoms also increases, and this changes the physical properties of the whole particle. It is therefore virtually impossible to draw a clear distinction between intrinsic particle size effects and those that are due to metal-support interactions. In Section 3.4.2 we noted some effects of size on structure in systems where the influence of the support was likely to be minimal now we must examine effects of size in conjunction with the metal-support interaction. 

It is quite challenging to rmderstand in what way the zeolite influences the metal compared to other supports. The electronic changes that could be induced by the pore system are quite subtle and metal particle size effects may overrule these changes [200]. hi comparison to metal-support interactions on macroporous oxides, the interaction between metal particles and the supporting zeolite matrix seems more pronounced. This may be because the metal particles interact with the zeolite lattice over a substantial fraction of their surfece. It has also been suggested that in addition to the intrinsic electronic effects due to the small size of the metal particles in the zeolite cage, a modification of the electronic structure of the metal by the acidic zeolite framework has to be considered [201,202]. 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

In the early 80 s, Bachelier et al. [9] demonstrated that catalytic efficiency depends on the Mo loading of the catalyst. We have studied the effect of molybdenum loading on HDS activity and found an optimum metal loading of about 6 wt% for a selected y-alumina support (Figure 1). Such a behaviour has been rationalized by a change in the molybdenum sulfide particle size [9]. With the addition of cobalt, the activity per molybdenum atom increases at low cobalt content and then reaches a plateau as shown in Figure 2. Such a result confirms earlier works done by Bachelier et al. on NiMo catalysts [10, 12]. Today, the preferred interpretation is that cobalt atoms decorate the molybdenum sulfide particles. This hypothesis was predicted by a geometrical model [8] and confirmed experimentally [12],... 

This latter was accepted as evidence for possible aggregation, since some literature data indicate that hydrogenolysis, being a structure-sensitive reaction, involves multiple adsorption (/58-161). However, two recent review articles on particle size effects on metal catalysts (162, 163) emphasize the inconsistency of the rather scarce data on nickel and warn of the difficulties connected with this problem. These data, although scarce, support the conclusion that the structure in the precrystallization state is the most favorable for catalytic activity, but detailed understanding of the phenomenon requires further clarification. 

On the basis of the results obtained on Ru/C, the lower selectivity to geraniol + nerol observed on samples F and G, with respect to the carbon supported catalysts, cannot be ascribed to a different metal particle size. Therefore a support effect which modifies the cataljrtic properties of Ru and/or the presence of impurities are likely to be responsible for the observed behaviour. Within this contest, it should be noted that the carbon used as support contains about 1000 ppm of Fe whereas the amount of Fe on the alumina support is < 200 ppm. The beneficial effect of adding Fe to promote C=0 bond hydrogenation is well known [9,10]. 

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