Platinum group metals introduction

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Particular interest has been shown in the catalytic oxidation of polyols with dioxygen using supported platinum-group metals as the catalysts. The most studied metals have been palladium and platinum which are, however, often affected by deactivation problems [2]. The introduction of cocatalysts such as bismuth or lead represents an enhancement in the use of these catalysts having the double effect of Increasing catalytic activity and improving catalyst life [3]. 

Transformation of chemicals in highly reduced state, functionalization (introduction of heteroatoms like N, S, O, halogens, etc. into aromatic compounds), organometaUic catalysts using platinum group metals (Pt, Ru, Rh, Pd, etc.), multistep synthetic routes... 

The catalysts based on ruthenium undoubtedly have priority in the TH reactions of ketones due to their excellent, sometimes approaching enzyme-like, efficiencies, selectivities, and their rich chemistry which allows the introduction of diverse types of chiral ligands. However, the possibility of using other platinum-group metal catalysts have been demonstrated as a valid alternative to ruthenium systems. Since the first reports of Ir-based ATH of ketones, for example by Graziani and coworkers in 1982 [87], interest in iridium catalysts, which have often been successfully used in TH of olefins, has been growing [88]. 

The most exploited materials in catalysis are the metals from platinum group, but with the introduction of nanotechnology some other elements that in bulk state did not attract a lot of attention, either due to their lack of reactivity toward some anal3d es or due to their high costs in production, are now emerging. 

The introduction in catalysis of bimetallic formulations created an important area of application of microanalysis in transmission electron microscopy. In particular, with selective hydrogenation and postcombustion catalysts, where the metallic particle sizes are several nanometres, the STEM can be used to determine the composition particle by particle and thus confirm the success of the preparation. Figure 9.16 shows the analysis of individual particles in a bimetallic preparation. It is easy to detect the existence of genuinely bimetallic particles and others containing only platinum. It should, however, be noted that this analysis, obtained on a few nanometer sized particles, concerns only a very small quantity of the catalyst (in the present case approximately 10" g of metal ). As we have noted, it is dangerous to extrapolate only one result of this type to the solid as a whole. A statistical analysis of the response of a very large number of particles, in addition to a preliminary study of the chemical composition at different scales, can be used to confirm that this case indeed concerns two groups of particles. 

Besides the effects of the typical carbon functional groups, the role of nitrogen and sulfur functionalities, introduced on carbons by chemical and thermal treatments, on the electrochemical performance of Pt catalysts for oxygen reduction in direct methanol fuel cells was investigated [47]. Once again, the metal-support interaction influences the size and chemical state of platinum particles and, as a consequence, the electrocatalytic activity. The introduction of nitrogen and sulphur functionalities was reported to improve the catalytic activity, but this result was mainly ascribed to the Pt particle size. 

Under alkaline conditions, sixteen Pb(II) ions link the secondary hydroxyl side of y-CyD to form a metal-ion-mediated head-to-head dimer [217]. All the secondary hydroxyl groups are deprotonated and coordinated to bind Pb(II) ions forming a hexadecanuclear lead(II) alkoxide. Introduction of ionic substituents on CyDs enhances their metal-binding ability. Two amino groups introduced on the primary hydroxyl side of jS-CyD can chelate a platinum ion [64]. Imidazole-appended yS-CyD forms a ternary complex with a Cu(II) ion and L-tryptophanate [61]. The 6-amino and imidazolyl groups of the host molecule and the carboxyl and amino groups of L-tryptophanate are coordinated to the Cu(II) ion. 

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