Group VIII metal particles

M. Current evidence Indicates that the effect may be ascribed to the formation of a TIO film on the surface of the group VIII metal particles (6,7) Whatever the cause of the SMSI state. It Is certainly the case that It has altered site energetics and possibly their density on the catalysts. The extraction of site... 

Organometallic Compounds with Supported or Unsupported Group VIII Metals Particles 57... 

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

The process of using organometallic compounds to prepare bimetallic particles is quite general, and we will review below the chemistry of organometallics reagents with metal surfaces, which are limited so far mainly to group VIII metals. 

As Ponec has pointed out, infrared measurements should be more sensitive than UPS in differentiating various types of surface sites, and hence particle size effects. When infrared measurements are made, it is found that the characteristic frequency of the adsorbed CO molecule at a constant surface coverage is dependent on the particle shape.2280 As the particle shape is changed from flat (film or single crystal) to curved (small particles) there is a shift to higher (for Cu) or to lower (for group VIII metals) frequency. Ponec has reviewed possible reasons for these changes in vibrational frequency.208... 

Den Hartog AJ, Deng M, Jongerious F, Ponec V (1990) Hydrogenation of acetylene over various group VIII metals - Effect of particle size and carbonaceous deposits. J Mol Catal 60 99... 

In this chapter, we will focus on core-shell structures in which the core is a metal but the shell is ceria or some mixed oxide of ceria. For catalytic applications, the oxide shell should probably be considered at least somewhat porous, so that reactant molecules can access sites on the metal core however, there have been suggestions that the presence of a group VIII metal in core-shell particles or metal cations within the ceria can modify the properties of the ceria. More generally, there are two primary reasons to consider core-shell nanoparticles in which the shell is an oxide ... 

Interestingly, tin addition not only suppresses the multiple hydrogenolysis of C-C and C-0 bonds, but also enhances the rate of alcohol formation. This improvement in selectivity can be interpreted by the superficial dilution of group VIII metal by inactive metal atoms of tin (10). Otherwise, to account for the increase in activity, the modification of the nature of the active site has been considered, but not precisely described (11). Furthermore, if the overall composition of the metallic particles is mastered and known, the misunderstanding of the superficial structure and of the nature of the active phase hinders to etablish accurate relations with the catalytic properties. 

The catalysts used are themselves complexes produced by interaction of alkyls of metals in Groups l-IIl of the Periodic Table with halides and other derivatives of Groups IV-VIII metals. Although soluble co-ordination catalysts are known, those used for the manufacture of stereoregular polymers are usually solid or adsorbed on solid particles. 

The results obtained with nickel raised the question whether the relation found between rate of exchange and particle size holds also for other metals of group VIII. We therefore carried out the benzene-D2 reaction on some iridium catalysts widely differing in particle size. We chose iridium because we knew from earlier experiments that iridium black gives a very characteristic cyclohexane isotopic distribution pattern with a maximum for C6H4Ds, whereas the patterns of Ni, Ru, Pd, and Pt show a maximum for the d6 compound. 

The outcome of the experiments on nickel and on iridium shows that there is a marked contrast between these metals as regards the influence of particle size upon the course of the reaction of benzene with deuterium. Clearly, further experiments should be done along the same lines with other metals of group VIII of the periodic system. 

Fig, 6.7. a Site time yield in the processes described in the figure as a function of radius of the metal cluster involved in the catalytic processes. No particle size effect is observed down to small clusters, b Particle size effect for Pt clusters dispersed on Si02 in the recombination reaction of oxygen and hydrogen, c Particle size effect for Fe clusters dispersed on MgO for the ammonia synthesis at atmospheric pressure and 570° K. d Relationship between catalytic activity, dispersion of the metal cluster on support and bulk metal character of the cluster for metals of the group VIII... 

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